NIST Special Publication 1019 Sixth Edition Fire Dynamics Simulator User’s Guide Kevin McGrattan Simo Hostikka Randall McDermott Jason Floyd Craig Weinschenk Kristopher Overholt http://dx.doi.org/10.6028/NIST.SP.1019 VTT Technical Research Centre of Finland NIST Special Publication 1019 Sixth Edition Fire Dynamics Simulator User’s Guide Kevin McGrattan Randall McDermott Fire Research Division Engineering Laboratory Gaithersburg, Maryland, USA Simo Hostikka Aalto University Espoo, Finland Jason Floyd Craig Weinschenk Jensen Hughes Baltimore, Maryland, USA Kristopher Overholt Continuum Analytics Austin, Texas, USA http://dx.doi.org/10.6028/NIST.SP.1019 N T OF C O M M IT E D ER UN ICA E D EP E TM C ER AR September 11, 2016 FDS Version 6.5.2 Revision: FDS0-151-ga97f4c3-dirty ST ATES OF AM U.S. Department of Commerce Penny Pritzker, Secretary National Institute of Standards and Technology Willie May, Under Secretary of Commerce for Standards and Technology and Director Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the entities, materials, or equipment are necessarily the best available for the purpose. National Institute of Standards and Technology Special Publication 1019 Natl. Inst. Stand. Technol. Spec. Publ. 1019, 292 pages (October 2013) CODEN: NSPUE2 FDS Developers The Fire Dynamics Simulator and Smokeview are the products of an international collaborative effort led by the National Institute of Standards and Technology (NIST) and VTT Technical Research Centre of Finland. Its developers and contributors are listed below. Principal Developers of FDS Kevin McGrattan, NIST Simo Hostikka, Aalto University Randall McDermott, NIST Jason Floyd, Jensen Hughes, Baltimore, Maryland, USA Craig Weinschenk, Jensen Hughes, Baltimore, Maryland, USA Kristopher Overholt, Continuum Analytics, Austin, Texas, USA Principal Developer of Smokeview Glenn Forney, NIST Principal Developer of FDS+Evac Timo Korhonen, VTT Contributors Daniel Haarhoff, Jülich Supercomputing Centre, Germany Susan Kilian, hhpberlin, Germany Vivien Lecoustre, University of Maryland, USA Anna Matala, VTT William Mell, U.S. Forest Service, Seattle, Washington, USA Topi Sikanen, VTT Ben Trettel, The University of Texas at Austin, USA Julio Cesar Silva, National Council for Scientific and Technological Development, Brazil i ii About the Developers Kevin McGrattan is a mathematician in the Fire Research Division of NIST. He received a bachelor of science degree from the School of Engineering and Applied Science of Columbia University in 1987 and a doctorate at the Courant Institute of New York University in 1991. He joined the NIST staff in 1992 and has since worked on the development of fire models, most notably the Fire Dynamics Simulator. Simo Hostikka is an associate professor of fire safety engineering at Aalto University School of Engineering, since January 2014. Before joining Aalto, he worked as a Principal Scientist and Team Leader at VTT Technical Research Centre of Finland. He received a master of science (technology) degree in 1997 and a doctorate in 2008 from the Department of Engineering Physics and Mathematics of the Helsinki University of Technology. He is the principal developer of the radiation and solid phase sub-models within FDS. Randall McDermott joined the Fire Research Division at NIST in 2008. He received a B.S. from the University of Tulsa in Chemical Engineering in 1994 and a Ph.D. from the University of Utah in 2005. His research interests include subgrid-scale models and numerical methods for large-eddy simulation, adaptive mesh refinement, immersed boundary methods, and Lagrangian particle methods. Jason Floyd is a Senior Engineer at Jensen Hughes, in Baltimore, Maryland. He received a bachelor of science and a doctorate in the Nuclear Engineering Program of the University of Maryland. After graduating, he was awarded a National Research Council Post-Doctoral Fellowship at the Building and Fire Research Laboratory of NIST. He is a principal developer of the combustion, control logic, and HVAC sub-models within FDS. Craig Weinschenk is an engineer at Jensen Hughes, in Baltimore, Maryland. He worked in the Fire Research Division at NIST as a National Research Council Postdoctoral Research Associate in 2011. He received a B.S. from Rowan University in 2006 in Mechanical Engineering. He received an M.S. in 2007 and a doctorate in 2011 from The University of Texas at Austin in Mechanical Engineering. His research interests include numerical combustion, fire-structure interaction, and human factors research of fire-fighting tactics. Kristopher Overholt is a software engineer at Continuum Analytics, developers of the Anaconda Python distribution. He received a B.S. in Fire Protection Engineering Technology from the University of Houston-Downtown in 2008, an M.S. in Fire Protection Engineering from Worcester Polytechnic Institute in 2010, and a Ph.D. in Civil Engineering from The University of Texas at Austin in 2013. He worked in the Fire Research Division at NIST from 2013 to 2015, where he was central to the development of the FDS continuous integration framework, Firebot. He also worked on aspects of FDS related to verification and validation and quality metrics. His research interests include inverse fire modeling problems, soot deposition in fires, and the use of fire models in forensic applications. iii Glenn Forney is a computer scientist in the Fire Research Division of NIST. He received a bachelor of science degree in mathematics from Salisbury State College and a master of science and a doctorate in mathematics from Clemson University. He joined NIST in 1986 (then the National Bureau of Standards) and has since worked on developing tools that provide a better understanding of fire phenomena, most notably Smokeview, an advanced scientific software tool for visualizing Fire Dynamics Simulation data. Timo Korhonen is a Senior Scientist at VTT Technical Research Centre of Finland. He received a master of science (technology) degree in 1992 and a doctorate in 1996 from the Department of Engineering Physics and Mathematics of the Helsinki University of Technology. He is the principal developer of the evacuation sub-model within FDS. Daniel Haarhoff did his masters work at the Jülich Supercomputing Centre in Germany, graduating in 2015. His thesis is on providing and analyzing a hybrid parallelization of FDS. For this, he implemented OpenMP into FDS 6. Susan Kilian is a mathematician with numerics and scientific computing expertise. She received her diploma from the University of Heidelberg and received her doctorate from the Technical University of Dortmund in 2002. Since 2007 she has been a research scientist for hhpberlin, a fire safety engineering firm located in Berlin, Germany. Her research interests include high performance computing and the development of efficient parallel solvers for the pressure Poisson equation. Vivien Lecoustre is a Research Associate at the University of Maryland. He received a master of science in Aerospace Engineering from ENSMA (France) in 2005 and a doctorate in Mechanical Engineering from the University of Maryland in 2009. His research interests include radiation properties of fuels and numerical turbulent combustion. Anna Matala is a Research Scientist at VTT Technical Research Centre of Finland and a Ph.D. candidate at Aalto University School of Science. She received her M.Sc. degree in Systems and Operations Research from Helsinki University of Technology in 2008. Her research concentrates on pyrolysis modelling and parameter estimation in fire simulations William (Ruddy) Mell is an applied mathematician currently at the U.S. Forest Service in Seattle, Washington. He holds a B.S. degree from the University of Minnesota (1981) and doctorate from the University of Washington (1994). His research interests include the development of large-eddy simulation methods and sub-models applicable to the physics of large fires in buildings, vegetation, and the wildland-urban interface. Topi Sikanen is a Research Scientist at VTT Technical Research Centre of Finland and a graduate student at Aalto University School of Science. He received his M.Sc. degree in Systems and Operations Research from Helsinki University of Technology in 2008. He works on the Lagrangian particle and liquid evaporation models. Ben Trettel is a graduate student at The University of Texas at Austin. He received a B.S. in Mechanical Engineering in 2011 and an M.S. in Fire Protection Engineering in 2013, both from the University of Maryland. He develops models for the transport of Lagrangian particles for the Fire Dynamics Simulator. Julio Cesar Silva is a Guest Researcher in the Fire Research Division of NIST from National Council for Scientific and Technological Development, Brazil. He received a M.Sc. in 2010 and a doctorate in 2014 from Federal University of Rio de Janeiro in Civil Engineering. His research interests include fire-structure interaction and he develops coupling strategies between FDS and finite-element codes. iv Preface This Guide describes how to use the Fire Dynamics Simulator (FDS). Because new features are added periodically, check the current version number on the inside front jacket of this manual. Note that this Guide does not provide the background theory for FDS. A four volume set of companion documents, referred to collectively as the FDS Technical Reference Guide [1], contains details about the governing equations and numerical methods, model verification, experimental validation, and configuration management. The FDS User’s Guide contains limited information on how to operate Smokeview, the companion visualization program for FDS. Its full capability is described in the Smokeview User’s Guide [2]. v vi Disclaimer The US Department of Commerce makes no warranty, expressed or implied, to users of the Fire Dynamics Simulator (FDS), and accepts no responsibility for its use. Users of FDS assume sole responsibility under Federal law for determining the appropriateness of its use in any particular application; for any conclusions drawn from the results of its use; and for any actions taken or not taken as a result of analysis performed using these tools. Users are warned that FDS is intended for use only by those competent in the fields of fluid dynamics, thermodynamics, heat transfer, combustion, and fire science, and is intended only to supplement the informed judgment of the qualified user. The software package is a computer model that may or may not have predictive capability when applied to a specific set of factual circumstances. Lack of accurate predictions by the model could lead to erroneous conclusions with regard to fire safety. All results should be evaluated by an informed user. Throughout this document, the mention of computer hardware or commercial software does not constitute endorsement by NIST, nor does it indicate that the products are necessarily those best suited for the intended purpose. vii viii Acknowledgments The Fire Dynamics Simulator, in various forms, has been under development for almost 25 years. It was first released to the public in 2000. Since then, continued improvements have been made to the software based largely on feedback from its users. Included below are some who made important contributions related to the application of FDS. • At NIST, Dan Madrzykowski, Doug Walton, Bob Vettori, Dave Stroup, Steve Kerber, Nelson Bryner, and Adam Barowy have used FDS and Smokeview as part of several investigations of fire fighter line of duty deaths. They have provided valuable information on the model’s usability and accuracy when compared to large scale measurements made during fire reconstructions. • Bryan Klein of Thunderhead Engineering assisted in adding cross-referencing functionality to this document, making it easier to view electronically. He also designed the on-line services for revision control, bug reporting, and general discussion of topics related to FDS. • At VTT, Joonas Ryynänen implemented and documented the FED/FIC routine. • The US Nuclear Regulatory Commission has provided financial support for the verification and validation of FDS, along with valuable insights into how fire models are used as part of probabilistic risk assessments of nuclear facilities. Special thanks to Mark Salley and Dave Stroup. • The Society of Fire Protection Engineers (SFPE) sponsors a training course on the use of FDS and Smokeview. Chris Wood of ArupFire, Dave Sheppard of the US Bureau of Alcohol, Tobacco and Firearms (ATF), and Doug Carpenter of Combustion Science and Engineering developed the materials for the course, along with Morgan Hurley of the SFPE. • David McGill of Seneca College, Ontario, Canada has conducted a remote-learning course on the use of FDS, and he has also maintained a web site that has provided valuable suggestions from users. • Paul Hart of Swiss Re, GAP Services, and Pravinray Gandhi of Underwriters Laboratories provided useful suggestions about water droplet transport on solid objects. • François Demouge of the Centre Scientifique et Technique du Bâtiment (CSTB) in France assisted with implementation of synthetic turbulence inflow boundary conditions. • Max Gould, Summer Undergraduate Research Fellow, assisted in the testing and verification of nonstandard boundary treatment methods. Finally, on the following pages is a list of individuals and organizations who have volunteered their time and effort to “beta test” FDS and Smokeview prior to its official release. Their contribution is invaluable because there is simply no other way to test all of the various features of the model. ix FDS 6 Beta Testers Mohammed Assal Choon-Bum Choi William J. Ferrante Emanuele Gissi Timothy M. Groch Georges Guigay Simon J. Ham Chris Lautenberger Tim McDonald Dave McGill Adrian Milford Luca Nassi Stephen Olenick Natalie Ong Chris Salter Joakim Sandström Julio Cesar Silva Boris Stock Csaba Szilagyi Giacomo Villi Andreas Vischer Christopher Wood CFD Algeria Building and Tunnel Technologies Inc. (BNTTEK), Korea Roosevelt Fire District, Hyde Park, New York, USA Corpo nazionale dei Vigili del Fuoco, Italy Engineering Planning and Management, Inc., Framingham, Massachusetts, USA Mannvit Engineering, Iceland Fire Safety Engineering Consultants Limited, UK Reax Engineering, Berkeley, California, USA Endress Ingenieurgesellschaft mbH, Germany Seneca College, Ontario, Canada Sereca Fire Consulting Ltd., British Columbia, Canada National Fire Department, Italy Combustion Science and Engineering, Inc., Columbia, Maryland, USA Arup Fire Singapore Hoare Lea and Partners, UK LTU/Brandskyddslaget, Sweden Federal University of Rio de Janeiro, Brazil BFT Cognos GmbH, Aachen, Germany OPTOMM Ltd., Budapest, Hungary Università di Padova, Italy Wijnveld//Ingenieure, Osnabrück, Germany FireLink, LLC, Tewksbury, Massachusetts, USA x Contents FDS Developers i About the Developers iii Preface v Disclaimer vii Acknowledgments ix Contents xi List of Figures xix List of Tables xxi I The Basics of FDS 1 Introduction 1.1 Features of FDS . . . . . . . . . . . . 1.2 What’s New in FDS 6? . . . . . . . . 1.3 Changes to Input Parameters in FDS 6 1.4 A Note on Longer Run Times in FDS 6 2 3 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Getting Started 2.1 How to Acquire FDS and Smokeview . . . . . . . . . . . . . . 2.2 Computer Hardware Requirements . . . . . . . . . . . . . . . 2.3 Computer Operating System (OS) and Software Requirements . 2.4 Installation Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Running FDS 3.1 A Brief Primer on Computer Hardware . . . . . . . . . . . . . . . . . . . . . . 3.2 Starting an FDS Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Single or Multiple Mesh Simulation with OpenMP Parallel Processing 3.2.2 Multiple Mesh Simulation with MPI Parallel Processing . . . . . . . . 3.2.3 Using MPI and OpenMP Together . . . . . . . . . . . . . . . . . . . 3.2.4 Efficiency of an MPI Calculation . . . . . . . . . . . . . . . . . . . . 3.2.5 Running Very Large Jobs . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Monitoring Progress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3 4 6 9 . . . . 11 11 11 12 12 . . . . . . . . 13 13 14 15 17 19 19 20 21 4 User Support 23 4.1 The Version Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 4.2 Common Error Statements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 4.3 Support Requests and Bug Tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 II Writing an FDS Input File 5 The Basic Structure of an Input File 29 5.1 Naming the Input File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 5.2 Namelist Formatting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 5.3 Input File Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 6 Setting the Bounds of Time and Space 6.1 Naming the Job: The HEAD Namelist Group (Table 17.7) . . . . . . . . . . . . . . . . . 6.2 Simulation Time: The TIME Namelist Group (Table 17.28) . . . . . . . . . . . . . . . . 6.2.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 Special Topic: Controlling the Time Step . . . . . . . . . . . . . . . . . . . . . 6.2.3 Special Topic: Steady-State Applications . . . . . . . . . . . . . . . . . . . . . 6.3 Computational Meshes: The MESH Namelist Group (Table 17.13) . . . . . . . . . . . . . 6.3.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.2 Two-Dimensional and Axially-Symmetric Calculations . . . . . . . . . . . . . 6.3.3 Multiple Meshes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.4 Mesh Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.5 Mesh Stretching: The TRNX, TRNY and TRNZ Namelist Groups (Table 17.29) . 6.3.6 Mesh Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Miscellaneous Parameters: The MISC Namelist Group (Table 17.14) . . . . . . . . . . . 6.4.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.2 Special Topic: Mean Forcing and Data Assimilation . . . . . . . . . . . . . . . 6.4.3 Special Topic: Specified Force Field . . . . . . . . . . . . . . . . . . . . . . . 6.4.4 Special Topic: Stopping and Restarting Calculations . . . . . . . . . . . . . . . 6.4.5 Special Topic: Initializing a 3D Velocity Field . . . . . . . . . . . . . . . . . . 6.4.6 Special Topic: Turning off the Flow Field . . . . . . . . . . . . . . . . . . . . 6.4.7 Special Topic: Defying Gravity . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.8 Special Topic: The Baroclinic Vorticity . . . . . . . . . . . . . . . . . . . . . . 6.4.9 Special Topic: Large Eddy Simulation Parameters . . . . . . . . . . . . . . . . 6.4.10 Special Topic: Numerical Stability Parameters . . . . . . . . . . . . . . . . . . 6.4.11 Special Topic: Flux Limiters . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Initial Conditions: The INIT Namelist Group (Table 17.10) . . . . . . . . . . . . . . . . 6.6 The Pressure Solver: The PRES Namelist Group (Table 17.18) . . . . . . . . . . . . . . 6.6.1 Parameters Related to the Solution of Poisson Equation for Pressure . . . . . . 6.6.2 Pressure Considerations in Long Tunnels . . . . . . . . . . . . . . . . . . . . . 6.6.3 Parameters Related to the Background Pressure when Breaking Pressure Zones 6.7 Setting Limits: The CLIP Namelist Group (Table 17.2) . . . . . . . . . . . . . . . . . . 7 27 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 33 33 33 34 34 35 35 35 35 37 39 41 42 42 42 43 43 44 45 45 46 46 47 50 50 52 52 54 56 56 Building the Model 57 7.1 Bounding Surfaces: The SURF Namelist Group (Table 17.26) . . . . . . . . . . . . . . . . 57 xii 7.2 7.3 7.4 7.5 8 Creating Obstructions: The OBST Namelist Group (Table 17.16) . . . . . . . . . 7.2.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2 Thin Obstructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.3 Overlapping Obstructions . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.4 Preventing Obstruction Removal . . . . . . . . . . . . . . . . . . . . . 7.2.5 Transparent or Outlined Obstructions . . . . . . . . . . . . . . . . . . . 7.2.6 Creating Holes in Obstructions: The HOLE Namelist Group (Table 17.8) Applying Surface Properties: The VENT Namelist Group (Table 17.30) . . . . . . 7.3.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.2 Special Vents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.3 Controlling Vents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.4 Trouble-Shooting Vents . . . . . . . . . . . . . . . . . . . . . . . . . . Coloring Obstructions, Vents, Surfaces and Meshes . . . . . . . . . . . . . . . . 7.4.1 Colors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.2 Texture Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Repeated Objects: The MULT Namelist Group (Table 17.15) . . . . . . . . . . . . Fire and Thermal Boundary Conditions 8.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Surface Temperature and Heat Flux . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Specified Solid Surface Temperature . . . . . . . . . . . . . . . . . . 8.2.2 Special Topic: Convective Heat Transfer Options . . . . . . . . . . . 8.2.3 Special Topic: Adiabatic Surfaces . . . . . . . . . . . . . . . . . . . 8.3 Heat Conduction in Solids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.1 Structure of Solid Boundaries . . . . . . . . . . . . . . . . . . . . . . 8.3.2 Thermal Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.3 Back Side Boundary Conditions . . . . . . . . . . . . . . . . . . . . 8.3.4 Initial and Back Side Temperature . . . . . . . . . . . . . . . . . . . 8.3.5 Walls with Different Materials Front and Back . . . . . . . . . . . . . 8.3.6 Special Topic: Specified Internal Heat Source . . . . . . . . . . . . . 8.3.7 Special Topic: Non-Planar Walls and Targets . . . . . . . . . . . . . . 8.3.8 Special Topic: Solid Phase Numerical Gridding Issues . . . . . . . . 8.4 Simple Pyrolysis Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.1 A Gas Burner with a Specified Heat Release Rate . . . . . . . . . . . 8.4.2 Special Topic: A Radially-Spreading Fire . . . . . . . . . . . . . . . 8.4.3 Solid Fuels that Burn at a Specified Rate . . . . . . . . . . . . . . . . 8.5 Complex Pyrolysis Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.1 Reaction Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.2 Reaction Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.3 Shrinking and swelling materials . . . . . . . . . . . . . . . . . . . . 8.5.4 Multiple Solid Phase Reactions . . . . . . . . . . . . . . . . . . . . . 8.5.5 The Heat of Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.6 Special Topic: The “Threshold” Temperature . . . . . . . . . . . . . 8.5.7 Liquid Fuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.8 Fuel Burnout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6 Testing Your Pyrolysis Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6.1 Simulating the Cone Calorimeter . . . . . . . . . . . . . . . . . . . . 8.6.2 Simulating Bench-scale Measurements like the TGA, DSC, and MCC xiii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 57 58 58 59 59 59 60 60 61 63 63 64 64 64 66 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 69 70 70 70 71 72 72 73 74 74 74 76 76 76 77 77 77 78 79 79 81 84 85 85 85 86 87 89 89 92 9 Ventilation 9.1 Simple Vents, Fans and Heaters . . . . . . . . . . . . . . . . . . . . 9.1.1 Simple Supply and Exhaust Vents . . . . . . . . . . . . . 9.1.2 Total Mass Flux . . . . . . . . . . . . . . . . . . . . . . . 9.1.3 Heaters . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1.4 Louvered Vents . . . . . . . . . . . . . . . . . . . . . . . 9.1.5 Specified Normal Velocity Gradient . . . . . . . . . . . . 9.1.6 Species and Species Mass Flux Boundary Conditions . . . 9.1.7 Tangential Velocity Boundary Conditions at Solid Surfaces 9.1.8 Synthetic Turbulence Inflow Boundary Conditions . . . . . 9.1.9 Random Mass Flux Variation . . . . . . . . . . . . . . . . 9.2 HVAC Systems: The HVAC Namelist Group (Table 17.9) . . . . . . 9.2.1 HVAC Duct Parameters . . . . . . . . . . . . . . . . . . . 9.2.2 HVAC Dampers . . . . . . . . . . . . . . . . . . . . . . . 9.2.3 HVAC Node Parameters . . . . . . . . . . . . . . . . . . 9.2.4 HVAC Fan Parameters . . . . . . . . . . . . . . . . . . . 9.2.5 HVAC Filter Parameters . . . . . . . . . . . . . . . . . . 9.2.6 HVAC Aircoil Parameters . . . . . . . . . . . . . . . . . . 9.2.7 Louvered HVAC Vents . . . . . . . . . . . . . . . . . . . 9.3 Pressure-Related Effects: The ZONE Namelist Group (Table 17.30) . 9.3.1 Specifying Pressure Zones . . . . . . . . . . . . . . . . . 9.3.2 Leaks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3.3 Special Topic: Stack Effect . . . . . . . . . . . . . . . . . 9.4 Pressure Boundary Conditions . . . . . . . . . . . . . . . . . . . . . 9.5 Special Flow Profiles . . . . . . . . . . . . . . . . . . . . . . . . . 9.6 Atmospheric Stratification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 95 95 96 96 97 97 98 98 99 100 101 101 104 104 105 107 108 109 110 110 113 116 117 119 120 10 User-Specified Functions 123 10.1 Time-Dependent Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 10.2 Temperature-Dependent Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 10.3 Spatially-Dependent Velocity Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 11 Chemical Species 11.1 Specifying Primitive Species . . . . . . . . . . . . . . . . 11.1.1 Basics . . . . . . . . . . . . . . . . . . . . . . . 11.1.2 Specifying Gas and Liquid Species Properties . . 11.1.3 Air . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Specifying Lumped Species (Mixtures of Primitive Species) 11.2.1 Combining Lumped and Primitive Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 127 128 129 133 134 136 12 Combustion 12.1 Single-Step, Mixing-Controlled Combustion . . 12.1.1 Simple Chemistry Parameters . . . . . 12.1.2 Heat of Combustion . . . . . . . . . . 12.1.3 Special Topic: Turbulent Combustion 12.1.4 Special Topic: Flame Extinction . . . 12.2 Complex Stoichiometry . . . . . . . . . . . . . 12.2.1 Complex Fuel Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 137 137 139 141 141 143 144 xiv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.2.2 Multiple Chemical Reactions . . . . . . . . . . . . . . 12.2.3 Special Topic: Using the EQUATION input parameter . 12.3 Finite Rate Combustion . . . . . . . . . . . . . . . . . . . . . . 12.4 Special Topic: Chemical Time Integration . . . . . . . . . . . . 12.5 Special Topic: Aerosol Deposition . . . . . . . . . . . . . . . . 12.5.1 Example Case: Soot Deposition from a Propane Flame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 148 149 151 151 151 13 Radiation 13.1 Basic Radiation Parameters: The RADI Namelist Group . . . . . . . . . . . . . 13.1.1 Radiative Fraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.1.2 Spatial and Temporal Resolution of the Radiation Transport Equation 13.2 Radiative Absorption and Scattering . . . . . . . . . . . . . . . . . . . . . . . 13.2.1 RadCal Considerations . . . . . . . . . . . . . . . . . . . . . . . . . 13.2.2 Radiative Absorption and Scattering by Particles . . . . . . . . . . . . 13.2.3 Wide Band Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153 153 153 154 155 155 155 156 14 Particles and Droplets 14.1 Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.2 Massless Particles . . . . . . . . . . . . . . . . . . . . . . . . . . 14.3 Liquid Droplets . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.3.1 Thermal Properties . . . . . . . . . . . . . . . . . . . . 14.3.2 Radiative Properties . . . . . . . . . . . . . . . . . . . . 14.3.3 Size Distribution . . . . . . . . . . . . . . . . . . . . . 14.3.4 Secondary Breakup . . . . . . . . . . . . . . . . . . . . 14.3.5 Fuel Droplets . . . . . . . . . . . . . . . . . . . . . . . 14.4 Solid Particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.4.1 Basic Geometry and Boundary Conditions . . . . . . . . 14.4.2 Drag . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.4.3 Splitting Particles . . . . . . . . . . . . . . . . . . . . . 14.4.4 Gas Generating Particles . . . . . . . . . . . . . . . . . 14.4.5 Vegetation . . . . . . . . . . . . . . . . . . . . . . . . . 14.4.6 Firebrands . . . . . . . . . . . . . . . . . . . . . . . . . 14.4.7 Porous Media . . . . . . . . . . . . . . . . . . . . . . . 14.4.8 Screens . . . . . . . . . . . . . . . . . . . . . . . . . . 14.4.9 Electrical Cable Failure . . . . . . . . . . . . . . . . . . 14.5 Particle Insertion . . . . . . . . . . . . . . . . . . . . . . . . . . . 14.5.1 Particles Introduced at a Solid Surface . . . . . . . . . . 14.5.2 Particles or Droplets Introduced at a Sprinkler or Nozzle 14.5.3 Particles or Droplets Introduced within a Volume . . . . 14.6 Special Topic: Suppression by Water . . . . . . . . . . . . . . . . 14.6.1 Velocity on Solid Surfaces . . . . . . . . . . . . . . . . 14.6.2 Reduction of the Burning Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 157 157 158 158 158 159 161 161 163 163 164 164 164 166 167 168 169 170 171 172 173 173 175 175 176 15 Devices and Control Logic 15.1 Device Location and Orientation: The DEVC Namelist Group (Table 17.5) 15.2 Device Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.3 Special Device Properties: The PROP Namelist Group (Table 17.20) . . . 15.3.1 Sprinklers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 179 180 181 181 xv . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.4 15.5 15.6 15.7 15.3.2 Nozzles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.3.3 Special Topic: Specified Entrainment (Velocity Patch) . . . . . 15.3.4 Heat Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . 15.3.5 Smoke Detectors . . . . . . . . . . . . . . . . . . . . . . . . 15.3.6 Beam Detection Systems . . . . . . . . . . . . . . . . . . . . 15.3.7 Aspiration Detection Systems . . . . . . . . . . . . . . . . . . Basic Control Logic . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.4.1 Creating and Removing Obstructions . . . . . . . . . . . . . . 15.4.2 Activating and Deactivating Vents . . . . . . . . . . . . . . . Advanced Control Functions: The CTRL Namelist Group . . . . . . . . 15.5.1 Control Functions: ANY, ALL, ONLY, and AT_LEAST . . . . 15.5.2 Control Function: TIME_DELAY . . . . . . . . . . . . . . . . 15.5.3 Control Function: DEADBAND . . . . . . . . . . . . . . . . . 15.5.4 Control Function: RESTART and KILL . . . . . . . . . . . . 15.5.5 Control Function: CUSTOM . . . . . . . . . . . . . . . . . . 15.5.6 Control Function: Math Operations . . . . . . . . . . . . . . 15.5.7 Control Function: PID Control Function . . . . . . . . . . . . 15.5.8 Combining Control Functions: A Pre-Action Sprinkler System 15.5.9 Combining Control Functions: A Dry Pipe Sprinkler System . 15.5.10 Example Case: activate_vents . . . . . . . . . . . . . . . . . . Controlling a RAMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15.6.1 Changing the Independent variable . . . . . . . . . . . . . . . 15.6.2 Freezing the Output Value, Example Case: hrr_freeze . . . . . Visualizing FDS Devices in Smokeview . . . . . . . . . . . . . . . . . 15.7.1 Devices that Indicate Activation . . . . . . . . . . . . . . . . 15.7.2 Devices with Variable Properties . . . . . . . . . . . . . . . . 15.7.3 Objects that Represent Lagrangian Particles . . . . . . . . . . 16 Output 16.1 Output Control Parameters: The DUMP Namelist Group . . . . . . . 16.2 Device Output: The DEVC Namelist Group . . . . . . . . . . . . . . 16.2.1 Single Point Output . . . . . . . . . . . . . . . . . . . . . 16.2.2 Linear Array of Point Devices . . . . . . . . . . . . . . . 16.2.3 Quantities at Certain Depth . . . . . . . . . . . . . . . . . 16.2.4 Back Surface Temperature . . . . . . . . . . . . . . . . . 16.3 Profiles of Quantities: The PROF Namelist Group . . . . . . . . . . 16.4 Animated Planar Slices: The SLCF Namelist Group . . . . . . . . . 16.5 Animated Boundary Quantities: The BNDF Namelist Group . . . . . 16.6 Animated Isosurfaces: The ISOF Namelist Group . . . . . . . . . . 16.7 Plot3D Static Data Dumps . . . . . . . . . . . . . . . . . . . . . . . 16.8 SMOKE3D: Realistic Smoke and Fire . . . . . . . . . . . . . . . . 16.9 Particle Output Quantities . . . . . . . . . . . . . . . . . . . . . . . 16.9.1 Liquid Droplets that are Attached to Solid Surfaces . . . . 16.9.2 Solid Particles on Solid Surfaces . . . . . . . . . . . . . . 16.9.3 Droplet and Particle Densities and Fluxes in the Gas Phase 16.9.4 Coloring Particles and Droplets in Smokeview . . . . . . . 16.9.5 Detailed Properties of Solid Particles . . . . . . . . . . . . xvi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 185 186 187 188 189 191 191 192 193 194 195 195 196 196 197 197 197 198 199 200 200 200 202 202 204 206 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 209 210 210 211 212 212 213 213 214 214 215 215 216 216 216 217 218 218 16.10 Special Output Quantities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.10.1 Heat Release Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.10.2 Visibility and Obscuration . . . . . . . . . . . . . . . . . . . . . . . . . . 16.10.3 Layer Height and the Average Upper and Lower Layer Temperatures . . . . 16.10.4 Thermocouples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.10.5 Heat Flux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.10.6 Adiabatic Surface Temperature . . . . . . . . . . . . . . . . . . . . . . . . 16.10.7 Special Topic: Detailed Spray Properties . . . . . . . . . . . . . . . . . . . 16.10.8 Useful Solid Phase Outputs . . . . . . . . . . . . . . . . . . . . . . . . . . 16.10.9 Fractional Effective Dose (FED) and Fractional Irritant Concentration (FIC) 16.10.10 Spatially-Integrated Outputs . . . . . . . . . . . . . . . . . . . . . . . . . 16.10.11 Temporally-Integrated Outputs . . . . . . . . . . . . . . . . . . . . . . . . 16.10.12 Statistical Outputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.10.13 Wind and the Pressure Coefficient . . . . . . . . . . . . . . . . . . . . . . 16.10.14 Near-wall Grid Resolution . . . . . . . . . . . . . . . . . . . . . . . . . . 16.10.15 Dry Volume and Mass Fractions . . . . . . . . . . . . . . . . . . . . . . . 16.10.16 Aerosol and Soot Concentration . . . . . . . . . . . . . . . . . . . . . . . 16.10.17 Gas Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.10.18 Enthalpy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.10.19 Computer Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.10.20 Output File Precision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.10.21 A Posteriori Mesh Quality Metrics . . . . . . . . . . . . . . . . . . . . . . 16.10.22 Extinction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16.11 Extracting Numbers from the Output Data Files . . . . . . . . . . . . . . . . . . . . 16.12 Summary of Frequently-Used Output Quantities . . . . . . . . . . . . . . . . . . . . 16.13 Summary of Infrequently-Used Output Quantities . . . . . . . . . . . . . . . . . . . 16.14 Summary of HVAC Output Quantities . . . . . . . . . . . . . . . . . . . . . . . . . 17 Alphabetical List of Input Parameters 17.1 BNDF (Boundary File Parameters) . . . . 17.2 CLIP (Clipping Parameters) . . . . . . . 17.3 CSVF (Comma Separated Velocity Files) 17.4 CTRL (Control Function Parameters) . . 17.5 DEVC (Device Parameters) . . . . . . . 17.6 DUMP (Output Parameters) . . . . . . . 17.7 HEAD (Header Parameters) . . . . . . . 17.8 HOLE (Obstruction Cutout Parameters) . 17.9 HVAC (HVAC System Definition) . . . . 17.10 INIT (Initial Conditions) . . . . . . . . 17.11 ISOF (Isosurface Parameters) . . . . . . 17.12 MATL (Material Properties) . . . . . . . 17.13 MESH (Mesh Parameters) . . . . . . . . 17.14 MISC (Miscellaneous Parameters) . . . 17.15 MULT (Multiplier Function Parameters) . 17.16 OBST (Obstruction Parameters) . . . . . 17.17 PART (Lagrangian Particles/Droplets) . 17.18 PRES (Pressure Solver Parameters) . . . 17.19 PROF (Wall Profile Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218 218 219 220 221 221 222 224 226 227 228 231 231 232 232 233 233 234 234 234 235 235 239 239 241 245 247 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 250 250 250 250 251 252 253 254 254 255 256 256 257 257 259 260 261 262 262 17.20 17.21 17.22 17.23 17.24 17.25 17.26 17.27 17.28 17.29 17.30 17.31 III PROP (Device Properties) . . . . . . . . . . . . . RADI (Radiation Parameters) . . . . . . . . . . . RAMP (Ramp Function Parameters) . . . . . . . . REAC (Reaction Parameters) . . . . . . . . . . . SLCF (Slice File Parameters) . . . . . . . . . . . SPEC (Species Parameters) . . . . . . . . . . . . SURF (Surface Properties) . . . . . . . . . . . . . TABL (Table Parameters) . . . . . . . . . . . . . TIME (Time Parameters) . . . . . . . . . . . . . TRNX, TRNY, TRNZ (MESH Transformations) VENT (Vent Parameters) . . . . . . . . . . . . . . ZONE (Pressure Zone Parameters) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . FDS and Smokeview Development Tools 263 264 264 265 266 266 267 269 270 270 270 271 273 18 The FDS/Smokeview Repository 275 19 Compiling FDS 277 19.1 FDS Source Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 20 Output File Formats 20.1 Diagnostic Output . . . . . . . . . . . . 20.2 Heat Release Rate and Related Quantities 20.3 Device Output Data . . . . . . . . . . . 20.4 Control Output Data . . . . . . . . . . . 20.5 CPU Usage Data . . . . . . . . . . . . . 20.6 Gas Mass Data . . . . . . . . . . . . . . 20.7 Slice Files . . . . . . . . . . . . . . . . 20.8 Plot3D Data . . . . . . . . . . . . . . . 20.9 Boundary Files . . . . . . . . . . . . . . 20.10 Particle Data . . . . . . . . . . . . . . . 20.11 Profile Files . . . . . . . . . . . . . . . 20.12 3-D Smoke Files . . . . . . . . . . . . . 20.13 Geometry, Isosurface Files . . . . . . . . 20.14 Geometry Data Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 279 280 280 280 281 281 281 282 282 283 284 284 285 286 289 xviii List of Figures 3.1 3.2 OpenMP timing study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 MPI scaling study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 An example of a multiple-mesh geometry. . . . . . . . . . . . . . Rules governing the alignment of meshes . . . . . . . . . . . . . . Piecewise-linear mesh transformation . . . . . . . . . . . . . . . . Polynomial mesh transformation . . . . . . . . . . . . . . . . . . Snapshot of the helium_2d_isothermal test case . . . . . . . . Results of the duct_flow test case . . . . . . . . . . . . . . . . . Results of the dancing_eddies test cases . . . . . . . . . . . . . Reduction of pressure iterations in the dancing_eddies test cases Convergence test for the tunnel_demo test case . . . . . . . . . . 7.1 7.2 Results of the circular_burner test case . . . . . . . . . . . . . . . . . . . . . . . . . 63 An example of the multiplier function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 8.1 8.2 8.3 8.4 8.5 8.6 Simple demonstration of the pyrolysis model . . . . . . . Results of the couch test case . . . . . . . . . . . . . . . A more complicated demonstration of the pyrolysis model Results of the water_ice_water test case . . . . . . . Results of the box_burn_away test cases . . . . . . . . Sample results of a tga_analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 84 86 87 90 93 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10 9.11 9.12 9.13 9.14 9.15 Results of the volume_flow test cases . . . . . . The tangential_velocity test case . . . . . . Synthetic Eddy Method vent profiles . . . . . . . An example of simplifying a complex duct . . . . Example of fan curves . . . . . . . . . . . . . . . Example of a jet fan . . . . . . . . . . . . . . . . Results of the HVAC_aircoil case . . . . . . . . Results of the pressure_rise test case . . . . . Results of the zone_break test cases . . . . . . . Results of the zone_shape test case . . . . . . . Results of the door_crack test case . . . . . . . Results of the stack_effect test case . . . . . . Snapshots of the pressure_boundary test case Results of the parabolic_profile test case . . Boundary layer profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 97 100 103 107 107 109 111 112 113 115 117 118 119 120 xix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 38 39 39 46 53 53 54 56 11.1 Results of the gas_filling test case . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 12.1 Results of the pvc_combustion test case . . . . . . . . . . . . . . . . . . . . . . . . . . 146 12.2 HRR for energy_budget_adiabatic_two_fuels test case . . . . . . . . . . . . . . . 148 12.3 Wall soot deposition for the propane_flame_deposition test case . . . . . . . . . . . 152 14.1 14.2 14.3 14.4 14.5 14.6 14.7 14.8 14.9 14.10 Droplet size distributions . . . . . . . . . . . . . . . . . Results of the spray_burner test case . . . . . . . . . . Examples of particle splitting . . . . . . . . . . . . . . . Example of specified gas mass production from particles . Example of burning vegetation . . . . . . . . . . . . . . Mass generation of firebrands in the dragon_5a test case Results of the particle_flux test case . . . . . . . . . Results of the bucket_test_3 case . . . . . . . . . . . Example of water cascading over solid obstructions . . . Results of the e_coefficient test case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 162 165 166 168 169 172 175 176 177 15.1 15.2 15.3 15.4 15.5 15.6 15.7 Results of the bucket_test_2 case . . . . . . . Results of the flow_rate test case . . . . . . . . Results of the beam_detector test case . . . . . Results of the aspiration_detector test case Results of the control_test_2 case . . . . . . Snapshots of the activate_vents test case . . . Example of freezing the output of a RAMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 186 189 190 199 199 201 16.1 Results of the bucket_test_1 case . . . . . . . . . . . . . . 16.2 Results of the bucket_test_4 case . . . . . . . . . . . . . . 16.3 Results of the hallways test case . . . . . . . . . . . . . . . . 16.4 Results of the adiabatic_surface_temperature test case 16.5 Examples of the measure of turbulence resolution . . . . . . . 16.6 Haar mother wavelet . . . . . . . . . . . . . . . . . . . . . . . 16.7 Haar wavelet transforms on four typical signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 217 219 223 236 237 238 xx . . . . . . . . . . . . . . . . . . . . . . . . . . . . List of Tables 1.1 1.2 List of changes to input parameters for FDS 6 . . . . . . . . . . . . . . . . . . . . . . . . List of changes to input parameters for FDS 6 (continued) . . . . . . . . . . . . . . . . . . 5.1 Namelist Group Reference Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 6.1 6.2 Turbulence model options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Flux limiter options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 7.1 A sample of color definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 10.1 Parameters used to control time-dependence . . . . . . . . . . . . . . . . . . . . . . . . . 125 11.1 Optional gas and liquid species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 13.1 Default Radiative Fraction based on FUEL . . . . . . . . . . . . . . . . . . . . . . . . . . 154 15.1 15.2 15.3 15.4 15.4 15.5 15.5 15.5 15.6 15.6 Suggested values for smoke detector model . . . . . . . . . . . . Control function types . . . . . . . . . . . . . . . . . . . . . . . Single frame static objects . . . . . . . . . . . . . . . . . . . . . Dual frame static objects . . . . . . . . . . . . . . . . . . . . . . Dual frame static objects (continued) . . . . . . . . . . . . . . . Dynamic Smokeview objects . . . . . . . . . . . . . . . . . . . Dynamic Smokeview objects (continued) . . . . . . . . . . . . . Dynamic Smokeview objects (continued) . . . . . . . . . . . . . Dynamic Smokeview objects for Lagrangian particles . . . . . . Dynamic Smokeview objects for Lagrangian particles (continued) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 194 202 203 204 204 205 206 206 207 16.1 16.2 16.3 16.4 16.5 Output quantities available for PDPA . . . . . . . . . . . . . . Coefficients used for the computation of irritant effects of gases Frequently used output quantities . . . . . . . . . . . . . . . . Infrequently used output quantities . . . . . . . . . . . . . . . HVAC output quantities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 227 242 245 247 17.1 17.2 17.3 17.4 17.5 17.6 17.7 17.8 Boundary file parameters (BNDF namelist group) . . . Clipping parameters (CLIP namelist group) . . . . . . Comma separated velocity files (CSVF namelist group) Control function parameters (CTRL namelist group) . Device parameters (DEVC namelist group) . . . . . . Output control parameters (DUMP namelist group) . . Header parameters (HEAD namelist group) . . . . . . Obstruction cutout parameters (HOLE namelist group) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 250 250 250 250 251 252 253 254 xxi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 8 17.9 17.10 17.11 17.12 17.13 17.14 17.15 17.16 17.17 17.18 17.19 17.20 17.21 17.22 17.23 17.24 17.25 17.26 17.27 17.28 17.29 17.30 17.31 HVAC parameters (HVAC namelist group) . . . . . . . . . Initial conditions (INIT namelist group) . . . . . . . . . Isosurface parameters (ISOF namelist group) . . . . . . . Material properties (MATL namelist group) . . . . . . . . Mesh parameters (MESH namelist group) . . . . . . . . . Miscellaneous parameters (MISC namelist group) . . . . . Multiplier function parameters (MULT namelist group) . . Obstruction parameters (OBST namelist group) . . . . . . Lagrangian particles (PART namelist group) . . . . . . . Pressure solver parameters (PRES namelist group) . . . . Wall profile parameters (PROF namelist group) . . . . . . Device properties (PROP namelist group) . . . . . . . . . Radiation parameters (RADI namelist group) . . . . . . . Ramp function parameters (RAMP namelist group) . . . . Reaction parameters (REAC namelist group) . . . . . . . Slice file parameters (SLCF namelist group) . . . . . . . . Species parameters (SPEC namelist group) . . . . . . . . Surface properties (SURF namelist group) . . . . . . . . . Table parameters (TABL namelist group) . . . . . . . . . Time parameters (TIME namelist group) . . . . . . . . . MESH transformation parameters (TRN* namelist groups) Vent parameters (VENT namelist group) . . . . . . . . . . Pressure zone parameters (ZONE namelist group) . . . . . 19.1 FDS source code files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 xxii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 255 256 256 257 258 260 260 261 262 262 263 264 264 265 266 266 267 269 270 270 270 271 Part I The Basics of FDS 1 Chapter 1 Introduction The software described in this document, Fire Dynamics Simulator (FDS), is a computational fluid dynamics (CFD) model of fire-driven fluid flow. FDS solves numerically a form of the Navier-Stokes equations appropriate for low-speed (Ma < 0.3), thermally-driven flow with an emphasis on smoke and heat transport from fires. The formulation of the equations and the numerical algorithm are contained in the FDS Technical Reference Guide [1]. Verification and Validation of the model are discussed in the FDS Verification [3] and Validation [4] Guides. Smokeview is a separate visualization program that is used to display the results of an FDS simulation. A detailed description of Smokeview is found in a separate User’s Guide [2]. 1.1 Features of FDS The first version of FDS was publicly released in February 2000. To date, about half of the applications of the model have been for design of smoke handling systems and sprinkler/detector activation studies. The other half consist of residential and industrial fire reconstructions. Throughout its development, FDS has been aimed at solving practical fire problems in fire protection engineering, while at the same time providing a tool to study fundamental fire dynamics and combustion. Hydrodynamic Model FDS solves numerically a form of the Navier-Stokes equations appropriate for lowspeed, thermally-driven flow with an emphasis on smoke and heat transport from fires. The core algorithm is an explicit predictor-corrector scheme, second order accurate in space and time. Turbulence is treated by means of Large Eddy Simulation (LES). It is possible to perform a Direct Numerical Simulation (DNS) if the underlying numerical mesh is fine enough. LES is the default mode of operation. Combustion Model For most applications, FDS uses a single step, mixing-controlled chemical reaction which uses three lumped species (a species representing a group of species). These lumped species are air, fuel, and products. By default the last two lumped species are explicitly computed. Options are available to include multiple reactions and reactions that are not necessarily mixing-controlled. Radiation Transport Radiative heat transfer is included in the model via the solution of the radiation transport equation for a gray gas, and in some limited cases using a wide band model. The equation is solved using a technique similar to finite volume methods for convective transport, thus the name given to it is the Finite Volume Method (FVM). Using approximately 100 discrete angles, the finite volume solver requires about 20 % of the total CPU time of a calculation, a modest cost given the complexity of radiation heat transfer. The absorption coefficients of the gas-soot mixtures are computed using the RadCal narrow-band model [5]. Liquid droplets can absorb and scatter thermal radiation. This is important in 3 cases involving mist sprinklers, but also plays a role in all sprinkler cases. The absorption and scattering coefficients are based on Mie theory. Geometry FDS approximates the governing equations on a rectilinear mesh. Rectangular obstructions are forced to conform with the underlying mesh. Multiple Meshes This is a term used to describe the use of more than one rectangular mesh in a calculation. It is possible to prescribe more than one rectangular mesh to handle cases where the computational domain is not easily embedded within a single mesh. Parallel Processing FDS employs OpenMP [6], a programming interface that exploits multiple processing units on a single computer. For clusters of computers, FDS employs Message Passing Interface (MPI) [7]. Details can be found in Section 3.2.2. Boundary Conditions All solid surfaces are assigned thermal boundary conditions, plus information about the burning behavior of the material. Heat and mass transfer to and from solid surfaces is usually handled with empirical correlations, although it is possible to compute directly the heat and mass transfer when performing a Direct Numerical Simulation (DNS). 1.2 What’s New in FDS 6? Many of the changes in FDS 6 are improvements to the various sub-models that do not affect the basic structure or parameters of the input file. Most of the changes listed below do not require additional input parameters beyond those used in FDS 5. Hydrodynamics and Turbulence • Conservative, total variation diminishing (TVD) scalar transport is implemented: Superbee (LES default) and CHARM (DNS default). These schemes prevent over-shoots and under-shoots in species concentrations and temperature. • Improved models for the turbulent viscosity are implemented: Deardorff (default), Dynamic Smagorinsky, and Vreman. These models provide more dynamic range to the flow field for coarse resolution and converge to the correct solution at fine resolution. • The conservative form of the sensible enthalpy equation is satisfied by construction in the FDS 6 formulation, eliminating temperature anomalies and energy conservation errors due to numerical mixing. • The baroclinic torque is included by default. • Improvements are made to the wall functions for momentum and heat flux. An optional wall heat flux model accounts for variable Prandtl number fluids. • Jarrin’s Synthetic Eddy Method (SEM) is implemented for turbulent boundary conditions at vents. Species and Combustion • Custom species mixtures (“lumped species”) can be defined with the input group SPEC. 4 • Turbulent combustion is handled with a new partially-stirred batch reactor model. At the subgrid level, species exist in one of two states: unmixed or mixed. The degree of mixing evolves over the FDS time step by the interaction by exchange with the mean (IEM) mixing model. Chemical kinetics may be considered infinitely fast or obey an Arrhenius rate law. • It is now possible to transport, produce, and consume product species such as CO and soot. Chemical mechanisms must be provided by the user and may include reversible reactions. • It is now possible to deposit aerosol species onto surfaces. • There are an increased number of predefined species that now include liquid properties. Lagrangian Particles • The functionality of Lagrangian particles has expanded to include the same heat transfer and pyrolysis models that apply to solid walls. In other words, you can now assign a set of surface properties to planar, cylindrical, or spherical particles much like you would for a solid surface. • More alternatives and user-defined option are available for the liquid droplet size distribution. • You can specify the radiative properties of the liquid droplets. • Drag effects of thin porous media (i.e., window screens) can be simulated using planes of particles. Solid Phase Heat Transfer and Pyrolysis • The basic 1-D heat transfer and pyrolysis model for solid surfaces remains the same, but there has been a change in several of the input parameters to expand functionality and readability of the input file. • The pyrolysis model allows for the surface to shrink or swell, based on the specified material densities. HVAC • Filters, louvered vents, and heating/cooling capability has been added for HVAC systems. • HVAC is now functional with MPI. Radiation • RadCal database has been extended to include additional fuel species. • In cells with heat release, the emission term is based on a corrected σ T 4 such that when this term is integrated over the flame volume the specified radiative fraction (default 0.35) is recovered. This differs from FDS 5 and earlier where the radiative fraction times the heat release rate was applied locally as the emission term. Multi-Mesh Computations • By default, FDS now iterates pressure and velocity at mesh and solid boundaries. You can control the error tolerance and maximum number of iterations via parameters on the PRES line. 5 Control Functions • CTRL functions have been extended to include math operations. • The evaluation of RAMPs and DEVCs can be stopped, freezing their value, based upon the activation of a device or control function. Devices and Output • Multiple pipe networks can be specified for sprinklers for reduction of flow rate based on the number of operating heads. • The numerical value of a control function can be output with a DEVC. • A line of devices can be specified using a number of POINTS on one DEVC line. • Statistical outputs for RMS, covariance, and correlation coefficient are available. 1.3 Changes to Input Parameters in FDS 6 This section describes the changes in the input parameters between FDS version 5 and version 6. Table 1.1 lists in alphabetical order parameters from FDS 5 that have changed. Note that this table does not list new parameters in FDS 6. There has been a limited amount of backward compatibility programmed into FDS 6. In other words, several commonly used parameters and conventions from previous versions still work, but you are encouraged to gradually modify your input files to conform to the new conventions. Gradually, obsolescent features will be removed. Some of the more notable changes in FDS 6 are: • If you want to model a fire, you must include a REAC line with a specified FUEL. See Chapter 12 for details. • The output quantity ’MIXTURE_FRACTION’ has been replaced with ’MIXTURE FRACTION’ and is only usable if there is a single REAC input of the form A + B → C and INITIAL_UNMIXED_FRACTION=0. • There is no longer a STATE_FILE because there is no longer a simple mixture fraction model. • PRESSURE_CORRECTION has been eliminated. See Section 6.6 for ways to improve the performance of the pressure solver. • Species mass and volume fraction outputs are no longer invoked using QUANTITY=’species name’. Use QUANTITY=’MASS FRACTION’ or QUANTITY=’VOLUME FRACTION’ along with SPEC_ID instead. Also note that all predefined species (Table 11.1) are now referenced with all uppercase letters. • The output quantity ’SOOT VOLUME FRACTION’ is now ’AEROSOL VOLUME FRACTION’ along with SPEC_ID to identify the name of the species. 6 7 BNDF CLIP CLIP DUMP DUMP INIT MATL MATL MATL MATL MATL MISC MISC MISC MISC MISC MISC MISC MISC MISC MISC OBST PART PART PART PART PART PART Namelist RECOUNT_DRIP MAXIMUM_MASS_FRACTION MINIMUM_MASS_FRACTION MAXIMUM_DROPLETS STATE_FILE NUMBER_INITIAL_DROPLETS NU_FUEL NU_GAS NU_RESIDUE NU_WATER RESIDUE BACKGROUND_SPECIES CONDUCTIVITY CO_PRODUCTION CSMAG MW PRESSURE_CORRECTION RADIATION EVAC_SURF_DEFAULT SURF_DEFAULT VISCOSITY SAWTOOTH FUEL HEAT_OF_VAPORIZATION H_V_REFERENCE_TEMPERATURE MELTING_TEMPERATURE NUMBER_INITIAL_DROPLETS PARTICLES_PER_SECOND FDS 5 Parameter MISC SPEC PRES RADI SURF SURF SPEC SURF PART SPEC SPEC SPEC INIT PROP INIT MATL MATL MATL MATL MATL SPEC SPEC DUMP Namelist SPEC_ID=’[FUEL]’ HEAT_OF_VAPORIZATION H_V_REFERENCE_TEMPERATURE MELTING_TEMPERATURE N_PARTICLES PARTICLES_PER_SECOND Eliminated C_SMAGORINSKY MW VELOCITY_TOLERANCE RADIATION EVAC_DEFAULT DEFAULT VISCOSITY New procedure N_PARTICLES, N_PARTICLES_PER_CELL NU_SPEC + SPEC_ID NU_SPEC + SPEC_ID NU_MATL NU_SPEC + SPEC_ID MATL_ID BACKGROUND=.TRUE. CONDUCTIVITY Eliminated MAXIMUM_PARTICLES FDS 6 Parameter Eliminated Eliminated Eliminated Table 1.1: Changes to input parameters, FDS version 5 to 6. Section 14.5.3 Section 8.5 Section 8.5 Section 8.5 Section 8.5 Section 8.5 Section 11 Section 11 Section 12.3 Section 6.4.9 Section 11 Section 6.6 Section 13.1 Section 7.1 Section 7.1 Section 11 Section 9.1.7 Section 14.3.5 Section 14.3.1 Section 14.3.1 Section 14.3.1 Section 14.5.3 Section 14.5.2 Section 6.7 Section 6.7 Section 16.1 Notes Namelist PART PART PART PROP PROP PROP PROP PROP PROP PROP PROP PROP RADI RADI REAC REAC REAC REAC REAC REAC SPEC SURF SURF SURF TIME VENT SURF TIME MISC SPEC SURF REAC REAC REAC SPEC MISC PROP PROP SPEC SPEC PART Namelist Eliminated VOLUME_FLOW T_END New procedure VISIBILITY_FACTOR RADCAL_ID HEAT_TRANSFER_COEFFICIENT New procedure RADIATIVE_FRACTION A FUEL MASS_EXTINCTION_COEFFICIENT MAXIMUM_VISIBILITY New procedure Eliminated PARTICLES_PER_SECOND PARTICLE_VELOCITY New procedure New procedure New procedure New procedure New procedure New procedure SPECIFIC_HEAT_LIQUID VAPORIZATION_TEMPERATURE SPEC_ID=’WATER VAPOR’ FDS 6 Parameter Table 1.2: Changes to input parameters, FDS version 5 to 6 (continued). FDS 5 Parameter SPECIFIC_HEAT VAPORIZATION_TEMPERATURE WATER CABLE_DIAMETER CABLE_FAILURE_TEMPERATURE CABLE_JACKET_THICKNESS CABLE_MASS_PER_LENGTH CONDUIT_DIAMETER CONDUIT_THICKNESS DROPLETS_PER_SECOND DROPLET_VELOCITY DT_INSERT CH4_BANDS RADIATIVE_FRACTION BOF ID MASS_EXTINCTION_COEFFICIENT MAXIMUM_VISIBILITY OXIDIZER VISIBILITY_FACTOR ABSORBING H_FIXED POROUS VOLUME_FLUX TWFIN MASS_FRACTION Notes Section 14.3.1 Section 14.3.1 Section 14.1 Section 14.4.9 Section 14.4.9 Section 14.4.9 Section 14.4.9 Section 14.4.9 Section 14.4.9 Section 14.5.2 Section 15.3.1 Section 14.5 Section 13.1.1 Section 12.3 Section 12.1.1 Section 16.10.2 Section 16.10.2 Section 12.2 Section 16.10.2 Section 11.1.2 Section 8.2.2 Section 9.2.4 Section 9.1.6 Section 6.2 8 1.4 A Note on Longer Run Times in FDS 6 A number of changes made in FDS 6 are aimed at improving the robustness and accuracy of the simulations. However, these improvements come at increased cost in both CPU time and memory usage. Some of this increased cost is offset by increasingly faster computers and improved parallel processing. In particular, starting with FDS 6.1.0, the default released version of FDS will employ OpenMP [6] by default. OpenMP is a programming interface that enables FDS to exploit multiple processing units on a given computer. Most Windows-based personal computers now come with multi-core processors, but past versions of FDS could only exploit a single core for a given calculation. With this new release and the increasingly faster processors available on the market, FDS 6 ought to maintain and eventually surpass the computing speed of past versions. Listed below are suggested ways to decrease CPU time, but these options should be considered very carefully. The default parameter settings are designed to address a wide range of fire scenarios, but there are scenarios for which approximations used in past versions of FDS may still be appropriate. The best way to determine if one or more of these time-saving assumptions is appropriate, run identical simulations with and without the assumption to determine if the difference in results is acceptable. 1. The improved turbulence model in FDS 6 has been found to produce comparable results to older versions of FDS using slightly less refined numerical grids. Section 6.3.6 introduces a dimensionless parameter, D∗ /δ x, that indicates the number of grid cells of dimension δ x that span the characteristic width of the fire, D∗ . A grid resolution study should be performed to determine the loss of accuracy caused by a reduced value of D∗ /δ x. 2. One reason for the increased CPU cost of FDS 6 is the more precise treatment of gas species properties. Previous versions of FDS assumed that the specific heat of a gas species is solely dependent on its molecular weight, and that the ratio of specific heats, c p /cv , is equal to 1.4, a value appropriate for a diatomic gas like nitrogen, N2 . Section 11.1.2 provides more details. FDS 6 now assumes that gas species are temperature-dependent, and this assumption increases the cost of the calculation in a number of different routines, in particular the calculation of the divergence. If you set CONSTANT_SPECIFIC_HEAT_RATIO=.TRUE. together with STRATIFICATION=.FALSE. on the MISC line, you can once again assume that the gas species are all diatomic. For scenarios where the overall compartment temperature does not approach flashover conditions, this assumption might be appropriate. 3. In situations where you are simulating a relatively small fire in a relatively large space and you are not interested in heat fluxes to surrounding structures, it might be reasonable to turn off the radiation transport calculation by setting RADIATION equal to .FALSE. on the RADI line. FDS will still assume that a fixed fraction of the fire’s energy is radiated away, only now the energy is simply removed from the calculation. 4. In situations where the heat transfer conditions are stationary or change only gradually, you can reduce the cost of the radiation solution by reducing the temporal resolution. More details in Section 13.1.2. A sensitivity study should be performed to determine the loss of accuracy. 9 10 Chapter 2 Getting Started FDS is a computer program that solves equations that describe the evolution of fire. It is a Fortran program that reads input parameters from a text file, computes a numerical solution to the governing equations, and writes user-specified output data to files. Smokeview is a companion program that reads FDS output files and produces animations on the computer screen. Smokeview has a simple menu-driven interface. FDS does not. However, there are various third-party programs that have been developed to generate the text file containing the input parameters needed by FDS. This guide describes how to obtain FDS and Smokeview and how to use FDS. A separate document [2] describes how to use Smokeview. 2.1 How to Acquire FDS and Smokeview Detailed instructions on how to download executables, manuals, source-code and related utilities, can be found at the project home page: http://firemodels.github.io/fds-smv/ The typical FDS/Smokeview distribution consists of an installation package or compressed archive, which is available for MS Windows, Mac OS X, and Linux. If you ever want to keep an older version of FDS and Smokeview, copy the installation directory to some other place so that it is not overwritten during the updated installation. 2.2 Computer Hardware Requirements The only hard requirement to run the compiled versions of FDS and Smokeview is a 64 bit Windows, Linux, or Mac OS X operating system. The single computer or compute cluster ought to have fast processors (CPUs), and at least 4 GB RAM per processor. The CPU speed will determine how long the computation will take to finish, while the amount of RAM will determine how many mesh cells can be held in memory. A large hard drive is required to store the output of the calculations. It is not unusual for the output of a single calculation to consume more than 1 GB of storage space. Most computers purchased within the past few years are adequate for running Smokeview with the caveat that additional memory (RAM) should be purchased to bring the memory size up to at least 2 GB. This is so the computer can display results without “swapping” to disk. For Smokeview it is also important to obtain a fast graphics card for the PC used to display the results of the FDS computations. 11 The MPI version of FDS requires shared disk access to each computer where cases will be run. On Windows systems this involves a domain network with the ability to share folders. On a Linux or Mac OS X system this involves NFS cross mounted files systems with ssh keys setup for passwordless login. For Multi-Mesh calculations, the MPI version of FDS can operate over standard 100 Mb/s networks. A gigabit (1000 Mb/s) network will further reduce network communication times improving data transfer rates between instances of MPI FDS running the parallel cases. 2.3 Computer Operating System (OS) and Software Requirements The goal of making FDS and Smokeview publicly available has been to enable practicing engineers to perform fairly sophisticated simulations at a reasonable cost. Thus, FDS and Smokeview have been designed for computers running Microsoft Windows, Mac OS X, and various implementations of Unix/Linux. MS Windows An installation package is available for Windows operating system. It is not recommended to run FDS/Smokeview under any version of MS Windows released prior to Windows 7. Mac OS X Pre-compiled executables are installed into a user selected directory using an installation script. Mac OS X 10.4.x or better is recommended. You can always download the latest version of FDS source and compile FDS for other versions of OS X (See Appendix 19 for details). Linux Pre-compiled executables are installed into a user selected directory using an installation script. If the pre-compiled FDS executable does not work (usually because of library incompatibilities), the FDS Fortran source code can be downloaded and compiled (See Appendix 19 for details). If Smokeview does not work on the Linux workstation, you can use the Windows version to view FDS output. Unix There are no pre-compiled versions of FDS for the various flavors of Unix. However, the advice for Linux applies equally as well to Unix. 2.4 Installation Testing If you are running FDS under a quality assurance plan that requires installation testing, a test procedure is provided in Appendix B of the FDS Verification Guide [3]. This guide can be obtained from the FDS-SMV website. 12 Chapter 3 Running FDS Each FDS simulation is controlled by a single text-based input file, typically given a name that helps identify the particular case, and ending with the file extension .fds. This input file can be written directly with a text editor or with the help of a third-party graphical user interface (GUI). The simulation is started directly via the command prompt or through the GUI. The creation of an input file is covered in detail in Part II. This chapter describes how the simulation is run once the input file is written. If you are new to FDS and Smokeview, it is strongly suggested that you start with an existing input file, run it as is, and then make the appropriate changes to the file for your desired scenario. By running a sample case, you become familiar with the procedure, learn how to use Smokeview, and ensure that your computer is up to the task before embarking on learning how to create new input files. Sample input files are included as part of the standard installation. A good case for a first time user is located in the subfolder called Fires within the folder called Examples. Find the file called simple_test.fds and copy it to a folder on your computer that is not within the installation folder. The reason for doing this is to avoid cluttering up the installation folder with a lot of output files. Follow the instructions in Section 3.2.1 to run this simple single mesh case. The simulation should only take a few minutes. Once the simulation is completed, use Smokeview to examine the output. In this way, you will quickly learn the basics of running and analyzing simulations. 3.1 A Brief Primer on Computer Hardware The following brief definitions were copied from the Indiana University Knowledge Base, which was developed with support from (U.S) National Science Foundation grant OCI-1053575. Cores: Recent developments in computational architecture can lead to confusion concerning what a microprocessor is. Since the advent of multi-core technology, such as dual-cores and quad-cores, the term “processor” has been used to describe a logical execution unit or a physical chip. A multi-core chip may have several cores. With the advent of multi-core technology, the term “processor” has become context-sensitive, and is largely ambiguous when describing large multi-core systems. Essentially a core comprises a logical execution unit containing an L1 cache and functional units. Cores are able to independently execute programs or threads. Supercomputers are listed as having thousands of cores. Chips: A chip refers to a physical integrated circuit (IC) on a computer. A chip in the context of this document refers to an execution unit that can be single- or multi-core technology. Sockets: The socket refers to a physical connector on a computer motherboard that accepts a single physical chip. Many motherboards can have multiple sockets that can in turn accept multi-core chips. 13 Processes: A process is an independent program running on a computer. A process has a full stack of memory associated for its own use, and does not depend on another process for execution. MPI (Message Passing Interface) processes are true processes because they can run on independent machines or the same machine. Threads: A thread is essentially a process that does not have a full stack of memory associated for it. The thread is tied to a parent process, and is merely an offshoot of execution. Typically thread processes must run on the same computer, but can execute simultaneously on separate cores of the same node. OpenMP parallelism uses threads for child processes. Hyper-threading: Hyper-threading is an Intel technology that originally preceded multi-core systems, and was used to make a single core appear logically as multiple cores on the same chip. Intel abandoned hyper-threading briefly during the advent of multi-core processors but reintroduced the technology in 2008. Since then, Intel has used it extensively to improve the performance of parallel computations in its multi-core processors. Hyper-threading improves performance by sharing the computational workload between multiple cores whenever possible, allowing the operating system to schedule more than one process at a time. N-ways: Multi-core compute nodes can be described by the number of execution units, or cores. A computer with 8 cores would be described as an 8-way node. This machine can have 8 independent processes running simultaneously. A 32-core system would be called a 32-way node. Processors: As explained above, a processor could describe either a single execution core or a single physical multi-core chip. The context of use will define the meaning of the term. 3.2 Starting an FDS Calculation FDS can be run on a single computer, using one or more cores, or it can be run on multiple computers. Starting with FDS version 6.2.0, for each supported operating system (Windows, Linux, Mac OS X) there is a single executable file called fds (with an .exe file extension on Windows). Previous releases of FDS contained two executables, one that ran on a single processor and one that ran on multiple processors. Starting with FDS 6.2.0, these two executables have been combined into one, and it can run either in serial or parallel mode. There are two ways that FDS can be run in parallel; that is, exploit multiple cores on a single computer or multiple processors/cores distributed over multiple computers on a network or compute cluster. The first way is OpenMP (Open Multi-Processing) [6] which allows a single computer to run a single or multiple mesh FDS simulation on multiple cores. The use of OpenMP does not require the computational domain to be broken up into multiple meshes, and it will still work with cases that have multiple meshes defined. The second way to run FDS in parallel is by way of MPI (Message Passing Interface). Here, the computational domain must be divided into multiple meshes and typically each mesh is assigned its own process. These processes can be limited to a single computer, or they can be distributed over a network. MPI and OpenMP can also be used together. For example, 4 MPI processes can be assigned to 4 different computers, and each MPI process can be supported by, say, 8 OpenMP threads, assuming each computer has 8 cores. Most of the speed up is achieved by the MPI. For a reasonably fast network, you can expect 4 MPI processes to speed up the computation time by a factor of about 0.9 times 4. The OpenMP can provide an extra factor up to about 2, regardless of the number of cores used beyond about 4. 14 3.2.1 Single or Multiple Mesh Simulation with OpenMP Parallel Processing If your simulation involves only one mesh, you can only run it on one computer, but you can exploit its multiple processors or cores using OpenMP. When you install FDS, it will query your computer to determine the number of available cores. By default, FDS will use approximately half of the available cores1 on a single computer. This is done for two reasons: (1) so as not to take over your entire machine when you run a simulation, and (2) because using all cores for a single simulation may not minimize the run time. OpenMP works best when exploiting multiple (logical) cores associated with a single (physical) processor or “socket”. For example, if your computer has two processors, each with 4 cores, it may not be worthwhile to use all 8 cores in an OpenMP simulation. You need to experiment with your own machine to determine the strategy that is best for you. To change the number of cores that are available for a given FDS simulation, you can set an environment variable called OMP_NUM_THREADS. The way to do this depends on the operating system and will be explained below. When the job is started, FDS will print the number of cores that will be used for that job. Note that this setting only applies until you log out or restart your machine. To set the default value of available cores upon startup, the OMP_NUM_THREADS environment variable can also be set in the startup configuration scripts on the machine. Refer to the documentation for your operating system for more information on how to configure environment variables upon startup. To confirm the speedup for the OpenMP version of the code, a series of test cases are run for two problem sizes (643 and 1283 ) varying the number of OpenMP threads. The setup is a simple channel flow carrying two extra species to mimic the scalar transport performed in typical fire problems (see the Timing_Benchmarks/openmp_test series in the FDS verification suite). The results are shown in Fig. 3.1. Generally, users can expect a factor of 2 speedup using 4 cores (default setting). FDS0−86−g80cff4e Relative clock time (%) 100 643 1283 80 60 40 20 0 1 2 3 4 5 6 Number of OpenMP threads 7 8 Figure 3.1: Benchmark timing comparison for the OpenMP test cases. The computer that ran these jobs has 2 (physical) sockets, and each socket has 4 (logical) cores. This explains the decrease in efficiency beyond 4 OpenMP threads. Details of how to run the simulation are included below for the different operating systems are as follows: 1 To determine the number of cores used by OpenMP, just type fds at the command prompt. 15 MS Windows Open up a Command Prompt window (click Start, then Run, then type “cmd”), and change directories (“cd”) to where the input file for the case is located. Decide how many cores you want to devote to the simulation. For example, if you have 4 cores available, type the following at the command prompt: set OMP_NUM_THREADS=4 Then run the simulation by typing: fds job_name.fds The progress of the simulation is indicated by diagnostic output that is written out onto the screen. Detailed diagnostic information is automatically written to a file job_name.out. Screen output can be redirected to a file via the alternative command: fds job_name.fds > job_name.err Note that it is also possible to associate the .fds extension with the FDS executable directly, thereby making FDS run by double-clicking on the input file. Be careful not to accidentally double-click on the input file when trying to edit it. This action will cause previously generated output files to be over-written. Mac OS X, Unix, Linux The installer for Mac OS X, Unix, and Linux versions of FDS sets the PATH variable allowing one to invoke FDS without a full path reference to the executable. To specify 4 OpenMP threads on a Mac OS X or Linux computer running a Bash shell, type the following at the command prompt: export OMP_NUM_THREADS=4 To run FDS from the command line type: fds job_name.fds The input parameters are read from the file job_name.fds, and error statements and other diagnostics are written out to the screen. To run the job in the background: fds job_name.fds >& job_name.err & Note that in the latter case, the screen output is stored in the file job_name.err and the detailed diagnostics are saved automatically in a file job_name.out. It is preferable to run jobs in the background so as to free the console for other uses. To see output while an FDS job is running type: tail -f job_name.err or tail -f job_name.out 16 3.2.2 Multiple Mesh Simulation with MPI Parallel Processing FDS uses MPI (Message-Passing Interface) [7] to allow multiple computers, or multiple cores on one computer, to run a single multi-mesh FDS job. The main idea is that you must break up the FDS domain into multiple meshes, and then the flow field in each mesh is computed as a different process. Note the subtle difference between these terms – a process does not have the same meaning as a processor. The process can be thought of as a “task” that you would see in the Windows Task Manager or by executing the “top” command on a Linux/Unix machine. The processor refers to the computer hardware. A single processor may run multiple processes, for example. The computation on a given FDS mesh is thought of as an individual process, and MPI handles the transfer of information between these processes. Usually, each mesh is assigned its own process in an MPI calculation, although it is also possible to assign multiple meshes to a single process. In this way, large meshes can be computed on dedicated processors, while smaller meshes can be clustered together in a single process running on a single processor, without the need for MPI message passing between themselves. Also note that FDS refers to its meshes by the numbers 1, 2, 3, and so on, whereas MPI refers to its processes by the numbers 0, 1, 2, and so on. Thus, Mesh 1 is assigned to Process 0; Mesh 2 to Process 1, and so on. You do not explicitly number the meshes or the processes yourself, but error statements from FDS or from MPI might refer to the meshes or processes by number. As an example, if a FDS case with five meshes, the first printout (usually to the screen unless otherwise directed) is: Mesh Mesh Mesh Mesh Mesh 1 2 3 4 5 is is is is is assigned assigned assigned assigned assigned to to to to to MPI MPI MPI MPI MPI Process Process Process Process Process 0 1 2 3 4 This means that 5 MPI processes (numbered 0 to 4) have started and that each mesh is being handled by its own process. The processes may be on the same or different computers. Each computer has its own memory (RAM), but each individual MPI process has its own independent memory, even if the processes are on the same computer. There are different implementations of MPI, much like there are different Fortran and C compilers. Each implementation is essentially a library of subroutines called from FDS that transfer data from one process to another across a fast network. The format of the subroutine calls has been widely accepted in the community, allowing different vendors and organizations the freedom to develop better software while working within an open framework. For Mac OS X and Linux, we use Open MPI, an open source implementation that is developed and maintained by a consortium of academic, research, and industry partners (www.openmpi.org). For Windows, we use Intel MPI 2 . Intel MPI for Windows The files needed to run an MPI calculation across a Windows domain network are bundled in with the FDS download. There is no need to install Intel MPI or its redistributable libraries. The following procedure is intended for a Windows domain network; that is, a network where user accounts are centrally managed such that any user can log in to any machine using the same credentials. To run the MPI version of FDS, you need to have administrator privileges on the machine that launches the job, but not necessarily on the machines that run the job. We have designed the FDS installation script to set up the necessary firewall exceptions and system environment variables automatically so that you need 2 Prior to FDS version 6.1.2, the Windows version of FDS used MPICH, a free implementation of MPI developed by Argonne National Laboratory. The MPICH developers have announced that they are no longer supporting the Windows version. 17 only install FDS as you normally would to make this work. You do not have to download and install the MPI software separately. Everything you need is in the FDS installation package. First, uninstall the FDS-SMV package on all of the machines you plan to use. The new version will overwrite all existing versions unless they are moved or renamed. If you wish to run FDS on more than one core on a single machine, then type the following command: mpiexec -n 4 fds job_name.fds where 4 indicates the number of processes that will be created. Note that as a default each process will use 4 OpenMP threads. It is advisable to set the environmental variable OMP_NUM_THREADS so that the total number of threads (n × OMP_NUM_THREADS) is less than or equal to the total number of cores on the machine If you wish to run FDS on more than one machine, the following steps should be taken: 1. Open up a command prompt by right-clicking on the command prompt icon and choosing “Run as administrator”. Type the following command: mpiexec -hosts 2 <my_machine> 1 <other_machine> 1 test_mpi If this command returns a “Hello World” message from your machine and the other machine on your network, proceed to the next step. If this command fails, check that you can “see” the other machine by pinging it, and check that the other machine can “see” your machine as well. 2. Share (with both read and write privilege) a working directory on your machine. Do not put this directory within the Program Files path. Share the working directory with everybody so that all other machines can see it. Note how this directory is defined on the other machines. Sometimes it is \\<my_machine>\<my_shared_directory>\ and sometimes it is defined via the numerical IP address, like \\129.6.129.87\<my_shared_directory>\. The definition depends on the way your domain name server (DNS) works. In any case, do not leave blank spaces within any directory or file names. We have found that blanks create all sorts of trouble. Unless you are a DOS/Windows expert, avoid them. 3. Within the command prompt, cd to the working directory. Find or create within the working directory a relatively small, simple, two mesh FDS input file. At the command prompt, type: mpiexec -hosts 2 m1 1 m2 1 -wdir \\...\... fds job_name.fds where m1 and m2 are the names of two computers on your network. 4. If successful, you should see the usual FDS printout indicating the processes being assigned to the machines. If unsuccessful, try running the case on your own machine: mpiexec -hosts 1 m1 2 -wdir \\...\... fds job_name.fds If this is not successful, check with your network administrator or monitor the FDS help forums for advice. Open MPI for Linux and Mac OS X The release versions of FDS for Linux and Mac OSX are built with Open MPI, an open source MPI implementation that is developed and maintained by a consortium of academic, research, and industry partners. With Open MPI, FDS is run using the command: mpirun -np 5 fds -hostfile my_hosts.txt job_name.fds 18 where the 5 indicates that 5 processes are to be used. In this case, the executable fds is located in the working directory, but you can also provide the full path name to the installation directory. The file my_hosts.txt might look like this: comp1 slots=2 comp2 slots=1 comp3 slots=2 where slots indicate the number of available cores on that computer. To make the process run in the background, use the command: mpirun ... >& job_name.err & The file job_name.err contains what is normally printed out to the screen. 3.2.3 Using MPI and OpenMP Together Because it more efficiently divides the computation, MPI is the better choice for multiple mesh simulations. However, it is possible to combine MPI and OpenMP in the same simulation. If you have multiple computers at your disposal, and each computer has multiple cores, you can assign one MPI process to each computer, and use multiple cores on each computer to speed up the processing of a given mesh using OpenMP. For example, on a Windows Domain Network, if you have two computers that are available and each computer has four cores, you can run a two mesh simulation as follows: mpiexec -hosts 2 m1 1 m2 1 -wdir \\...\... -env OMP_NUM_THREADS 4 fds job\_name.fds The use of OpenMP in this instance will probably speed the calculation by a factor of 2, but now you will be using 8 cores rather than 2. It might be better to divide the computational domain into 8 meshes and simply use MPI to process them. This all depends on your particular OS, hardware, network traffic, and so on. You should choose a good test case and try different meshing and parallel processing strategies to see what is best for you. 3.2.4 Efficiency of an MPI Calculation At the end of a calculation, FDS prints out a file called CHID_cpu.csv that records the amount of CPU time that each MPI process spends in the major routines. For example, the column header VELO stands for all the subroutines related to computing the flow velocity; MASS stands for all the subroutines related to computing the species mass fractions and density. The column header MAIN represents all of the CPU time that is not explicitly accounted for; that is, time spend in the main control loop. Ideally, this ought to be a few percent of the overall CPU time usage. There are two basic approaches to assessing the efficiency or scalability of an MPI (parallel) computation. The first is known as “weak scaling,” in which the amount of work done by each MPI process stays the same and additional processes are added to solve a larger problem. For example, if you are simulating the flow of air over a patch of terrain, and you keep adding more and more meshes of the same physical and numerical dimension, assigning each new mesh to its own MPI process, so as to simulate a larger and larger patch of terrain, then you would expect that the overall time of the simulation would not increase significantly with each additional mesh. The efficiency of such a calculation is given by the following expression: Ew = 19 t1 tN (3.1) where t1 is the CPU time for the case with 1 mesh (MPI process), and tN is the CPU time for the case with N meshes (MPI processes). The left hand plot of Fig. 3.2 shows the results of a weak scaling study of FDS. Meshes with dimension 50 by 50 by 50 are lined up side by side, ranging from 1 to 288 meshes. Ideally, the CPU time ought to be about the same for all cases, because each MPI process is doing the same amount of work. Only mesh to mesh communication should lead to inefficiencies. However, notice in the figure that the efficiency of the 1, 2, 4, and 8 mesh cases is greater than those with more MPI processes. The reason for this is that on most compute clusters, each node has multiple cores, and typically jobs run faster when a node is less than completely full. These test cases were run at NIST, where there is a compute cluster with 8 cores per node, and one with 12 cores per node. The second way to assess MPI efficiency is known as “strong scaling.” Here, you simulate a given scenario on a single mesh, and then you divide the mesh so that the cell size and the overall number of cells does not change. Ideally, if you divide a given mesh into two and run the case with two MPI processes instead of one, you would expect your computation time to decrease by a factor of two. But as you increase the number of MPI processes, you increase the amount of communication required among the processes. You also increase the overall number of boundary cells to compute, even though the overall number of gas phase cells remains the same. The efficiency of such a set of calculations is given by: Es = t1 N tN (3.2) In the strong study demonstrated here, a single mesh of dimension 180 by 160 by 80 is divided into a range of smaller meshes, with the smallest partitioning being 288 meshes of dimension 20 by 20 by 20. The resulting decrease in the CPU time of the entire calculation and the major subroutines is shown in the right hand plot of Fig. 3.2. Ideally, the CPU time should be inversely proportional to the number of meshes (MPI processes); that is, the relative CPU times ought to follow the black dotted lines. The one notable exception to this rule is for “COMM” or COMMunications. This curve represents the time spent in communicating information across the network. Git−r10−645−ga05e5aa Git−r10−645−ga05e5aa 0 1.2 10 Strong Scaling Test Weak Scaling Test Relative Wall Clock Time 1 Efficiency 0.8 0.6 0.4 0.2 0 −1 10 Total DIVG MASS VELO PRES COMM RADI MAIN −2 10 −3 10 −4 10 FDS Ideal 1 10 100 MPI Processes 1000 1 10 100 MPI Processes 1000 Figure 3.2: Example of a weak (left) and strong (right) scaling study. 3.2.5 Running Very Large Jobs Most FDS simulations reported in the literature use one to several dozen meshes, and MPI is the method of choice to parallelize these jobs. Usually the meshes are mapped to MPI processes in a one to one manner and the meshes contain a comparable number of grid cells. However, it is possible to run FDS jobs that 20 involve thousands of meshes. In 2016, the FDS developers at NIST were given access to the Oak Ridge Leadership Computing Facility at Oak Ridge National Laboratory in Tennessee. The facility provides users access to compute clusters with very large numbers of processors connected via a high speed network. FDS simulations were performed using up to approximately 10,000 MPI processes. If you have access to facilities such as this one, here are a few pointers: 1. Use MPI only. OpenMP will probably not speed up the run time appreciably, and it will consume cores that could be put to better use running more MPI processes. 2. For jobs using thousands of meshes/processes, add the parameter SHARED_FILE_SYSTEM=.FALSE. to the MISC line. This directs FDS to break up the Smokeview (.smv) file according to MPI process. This will greatly speed up the preliminary part of the simulation because the Smokeview file is written serially, not in parallel. For a modest number of meshes, this serial write is not a problem, but for thousands of meshes, the initialization routines can take hours. When the job completes, you can reconstruct the Smokeview file by appending the numbered files, CHID_n.smv, to the main Smokeview file, CHID.smv. 3. Set DT_CPU to some convenient time interval on the DUMP line. This parameter directs FDS to periodically write out a file (CHID_cpu.csv) that records the wall clock time that each MPI process consumes in the major subroutines. This can help you determine if any of the MPI processes spend an inordinate amount of time idling. 4. Run your job for a short amount of time to estimate the time required for the full job. Most large compute clusters will limit you to a certain amount of wall clock time, after which your job is simply stopped. If you have to use the restart feature in FDS, practice first with a short job to make sure that the job can be continued properly. 5. Do a strong scaling study for your particular case. That is, run the job a fixed number of time steps with the least number of meshes that can fit within the machine’s memory. Then divide the mesh by factors of 2, 4, or 8 until you reach a point where the increased number of meshes/processes does not provide a significant speed up. 3.3 Monitoring Progress Diagnostics for a given calculation are written into a file called CHID.out. The current simulation time and time step is written here, so you can see how far along the program has progressed. At any time during a calculation, Smokeview can be run and the progress can be checked visually. By default, the diagnostics in the CHID.out file are verbose. When running large MPI jobs it may be advantageous to quiet this output, which is all written by MPI process 0. To do this, add &DUMP SUPPRESS_DIAGNOSTICS=.TRUE. / Be aware the output file will not monitor mesh boundary velocity errors in this case; it will echo only the simulation time and time step. You could still output a BNDF of QUANTITY=’VELOCITY_ERROR’, if necessary. To stop a calculation before its scheduled time, either kill the process, or preferably create a file in the same directory as the output files called CHID.stop. The existence of this file stops the program gracefully, causing it to dump out the latest flow variables for viewing in Smokeview. Since calculations can be hours or days long, there is a restart feature in FDS. Details of how to use this feature are given in Section 6.4.4. Briefly, specify at the beginning of calculation how often a “restart” file 21 should be saved. Should something happen to disrupt the calculation, like a power outage, the calculation can be restarted from the time the last restart file was saved. It is also possible to control the stop time and the dumping of restart files by using control functions as described in Section 15.5. 22 Chapter 4 User Support It is not unusual over the course of a project to run into various problems, some related to FDS, some related to your computer. FDS is an CPU and memory intensive calculation that can push your computer’s processor and memory to its limits. In fact, there are no hardwired bounds within FDS that prevent you from starting a calculation that is too large for your hardware. Even if your machine has adequate memory (RAM), you can still easily set up calculations that can require weeks or months to complete. It is difficult to predict at the start of a simulation just how long and how much memory will be required. Learn how to monitor the resource usage of your computer. Start with small calculations and build your way up. Although many features in FDS are fairly mature, there are many that are not. FDS is used for practical engineering applications, but also for research in fire and combustion. As you become more familiar with the software, you will inevitably run into areas that are of current research interest. Indeed, burning a roomful of ordinary furniture is one of the most challenging applications of the model. So be patient, and learn to dissect a given scenario into its constitutive parts. For example, do not attempt to simulate a fire spreading through an entire floor of a building unless you have simulated the burning of the various combustibles with relatively small calculations. Along with the FDS User’s Guide, there are resources available on the Internet. These resources include an “Issue Tracker” for reporting bugs and requesting new features, a “Discussion Group” for clarifying questions and discussing more general topics rather than just specific problems, and “Wiki Pages” that provide supplementary information about FDS-SMV development, third-party tools, and other resources. Before using these on-line resources, it is important to first try to solve your own problems by performing simple test calculations or debugging your input file. The next few sections provide a list of error statements and suggestions on how to solve problems. 4.1 The Version Number If you encounter problems with FDS, it is crucial that you submit, along with a description of the problem, the FDS version number. Each release of FDS comes with a version number like 5.2.6, where the first number is the major release, the second is the minor release, and the third is the maintenance release. Major releases occur every few years, and as the name implies significantly change the functionality of the model. Minor releases occur every few months, and may cause minor changes in functionality. Release notes can help you decide whether the changes should effect the type of applications that you typically do. Maintenance releases are just bug fixes, and should not affect code functionality. To get the version number, just type the executable at the command prompt without an input file, and the relevant information will appear, along with a date of compilation (useful to you) and a so-called Git hash tag (useful to us). The Git hash tag refers to the GitHub repository number of the source code. It allows us to go back in time and 23 recover the exact source code files that were used to build that executable. Get in the habit of checking the version number of your executable, periodically checking for new releases which might already have addressed your problem, and telling us what version you are using if you report a problem. 4.2 Common Error Statements An FDS calculation may end before the specified time limit. Following is a list of common error statements and how to diagnose the problems: Input File Errors: The most common errors in FDS are due to mis-typed input statements. These errors result in the immediate halting of the program and a statement like, “ERROR: Problem with the HEAD line.” For these errors, check the line in the input file named in the error statement. Make sure the parameter names are spelled correctly. Make sure that a / (forward slash) is put at the end of each namelist entry. Make sure that the right type of information is being provided for each parameter, like whether one real number is expected, or several integers, or whatever. Make sure there are no non-ASCII characters being used, as can sometimes happen when text is cut and pasted from other applications or word-processing software. Make sure zeros are zeros and O’s are O’s. Make sure 1’s are not !’s. Make sure apostrophes are used to designate character strings. Make sure the text file on a Unix/Linux machine was not created on a Windows machine, and vice versa. Make sure that all the parameters listed are still being used – new versions of FDS often drop or change parameters forcing you to re-examine old input files. Numerical Instability Errors: It is possible that during an FDS calculation the flow velocity at some location in the domain can increase due to numerical error causing the time step size to decrease to a point1 where logic in the code decides that the results are unphysical and stops the calculation with an error message in the file CHID.out. In these cases, FDS ends by dumping out one final Plot3D file giving you a hint as to where the error is occurring within the computational domain. Usually, a numerical instability can be identified by fictitiously large velocity vectors emanating from a small region within the domain. Common causes of such instabilities are mesh cells that have an aspect ratio larger than 2 to 1, high speed flow through a small opening, a sudden change in the heat release rate, or any number of sudden changes to the flow field. There are various ways to solve the problem, depending on the situation. Try to diagnose and fix the problem before reporting it. It is difficult for anyone but the originator of the input file to diagnose the problem. Inadequate Computer Resources: The calculation might be using more RAM than the machine has (you will see an error message like “ERROR: Memory allocation failed for ZZ in the routine INIT”) , or the output files could have used up all the available disk space. In these situations, the computer may or may not produce an intelligible error message. Sometimes the computer is just unresponsive. It is your responsibility to ensure that the computer has adequate resources to do the calculation. Remember, there is no limit to how big or how long FDS calculations can be – it depends on the resources of the computer. For any new simulation, try running the case with a modest-sized mesh, and gradually make refinements until the computer can no longer handle it. Then back off somewhat on the size of the calculation so that the computer can comfortably run the case. Trying to run with 90 % to 100 % of computer resources is risky. In fact, for a typical 32 bit Windows PC with 4 GB RAM, only 2 GB will be available to FDS, based on user feedback. If you want to run bigger cases, consider buying a computer 1 By default, the calculation is stopped when the time step drops below 0.0001 of the initial time step. via the TIME line by specifying the DT_LIMITING_RATIO. 24 This factor can be changed with a 64 bit operating system or break up the calculation into multiple meshes and use the MPI version of FDS. If you are using a Linux/Unix machine, make sure that the stacksize is unlimited, which will allow FDS to access as much of the RAM as possible. Changing the stacksize limit differs with each shell type, so it is best to do an on-line search to find out how to ensure that your stacksize is unlimited. Run-Time Errors: An error occurs either within the computer operating system or the FDS program. An error message is printed out by the operating system of the computer onto the screen or into the diagnostic output file. This message is most often unintelligible to most people, including the programmers, although occasionally one might get a small clue if there is mention of a specific problem, like “stack overflow,” “divide by zero,” or “file write error, unit=...” Sometimes the error message simply refers to a “Segmentation Fault.” These errors may be caused by a bug in FDS, for example if a number is divided by zero, or an array is used before it is allocated, or any number of other problems. Before reporting the error to the Issue Tracker, try to systematically simplify the input file until the error goes away. This process usually brings to light some feature of the calculation responsible for the problem and helps in the debugging. File Writing Errors: Occasionally, especially on Windows machines, FDS fails because it is not permitted to write to a file. A typical error statement reads: forrtl: severe (47): write to READONLY file, unit 8598, file C:\Users\...\ The unit, in this case 8598, is just a number that FDS has associated with one of the output files. If this error occurs just after the start of the calculation, you can try adding the phrase FLUSH_FILE_BUFFERS=.FALSE. on the DUMP line of the input file (see Section 16.1). This will prevent FDS from attempting to flush the contents of the internal buffers, something it does to make it possible to view the FDS output in Smokeview during the FDS simulation. On some Windows machines, you might encounter security settings that prevent command line programs such as FDS from writing to system folders that contain program files. In this case, try to rerun the case in a non-system folder (i.e., a location within your home directory). Poisson Initialization: Sometimes at the very start of a calculation, an error appears stating that there is a problem with the “Poisson initialization.” The equation for pressure in FDS is known as the Poisson equation. The Poisson solver consists of large system of linear equations that must be initialized at the start of the calculation. Most often, an error in the initialization step is due to a mesh IJK dimension being less than 4 (except in the case of a two-dimensional calculation). It is also possible that something is fundamentally wrong with the coordinates of the computational domain. Diagnose the problem by checking the MESH lines in the input file. 4.3 Support Requests and Bug Tracking Because FDS development is on-going, problems will inevitably occur with various routines and features. The developers need to know if a certain feature is not working, and reporting problems is encouraged. However, the problem must be clearly identified. The best way to do this is to simplify the input file as much as possible so that the bug can be diagnosed (i.e., create and submit a minimal working example). Also, limit the bug reports to those features that clearly do not work. Physical problems such as fires that do not ignite, flames that do not spread, etc., may be related to the mesh resolution or scenario formulation, and you need to investigate the problem first before reporting it. If an error message originates from the operating 25 system as opposed to FDS, first investigate some of the more obvious possibilities, such as memory size, disk space, etc. If that does not solve the problem, report the problem with as much information about the error message and circumstances related to the problem. The input file should be simplified as much as possible so that the bug occurs early in the calculation. Attach the simplified input file if necessary, following the instructions provided at the web site. In this way, the developers can quickly run the problematic input file and hopefully diagnose the problem. Note: Reports of specific bugs, problems, feature requests, and enhancements should be posted to the Issue Tracker and not the Discussion Group. 26 Part II Writing an FDS Input File 27 Chapter 5 The Basic Structure of an Input File 5.1 Naming the Input File The operation of FDS is based on a single ASCII1 text file containing parameters organized into namelist2 groups. The input file provides FDS with all of the necessary information to describe the scenario. The input file is saved with a name such as job_name.fds, where job_name is any character string that helps to identify the simulation. If this same string is repeated under the HEAD namelist group within the input file, then all of the output files associated with the calculation will then have this common prefix name. There should be no blank spaces in the job name. Instead use the underscore character to represent a space. Using an underscore characters instead of a space also applies to the general practice of naming directories on your system. Be aware that FDS will simply over-write the output files of a given case if its assigned name is the same. This is convenient when developing an input file because you save on disk space. Just be careful not to overwrite a calculation that you want to keep. 5.2 Namelist Formatting Parameters are specified within the input file by using namelist formatted records. Each namelist record begins with the ampersand character, &, followed immediately by the name of the namelist group, then a comma-delimited list of the input parameters, and finally a forward slash, /. For example, the line &DUMP NFRAMES=1800, DT_HRR=10., DT_DEVC=10., DT_PROF=30. / sets various values of parameters contained in the DUMP namelist group. The meanings of these various parameters will be explained in subsequent chapters. The namelist records can span multiple lines in the input file, but just be sure to end the record with a slash or else the data will not be understood. Do not add anything to a namelist line other than the parameters and values appropriate for that group. Otherwise, FDS will stop immediately upon execution. Parameters within a namelist record can be separated by either commas, spaces, or line breaks. It is recommended that you use commas or line breaks, and never use tab stops because they are not explicitly defined in the namelist data structure. Comments and notes can be written into the file so long as nothing comes before the ampersand except a space and nothing comes between the ampersand and the slash except appropriate parameters corresponding to that particular namelist group. 1 ASCII – American Standard Code for Information Interchange. 2A namelist is a Fortran input record. 29 There are 256 characters that make up the standard ASCII text. The parameters in the input file can be integers, reals, character strings, or logical parameters. A logical parameter is either .TRUE. or .FALSE. – the periods are a Fortran convention. Character strings that are listed in this User’s Guide must be copied exactly as written – the code is case sensitive and underscores do matter. The maximum length of most character input parameters is 60. Most of the input parameters are simply real or integer scalars, like DT=0.02, but sometimes the inputs are multidimensional arrays. For example, when describing a particular solid surface, you need to express the mass fractions of multiple materials that are to be found in multiple layers. The input array MATL_MASS_FRACTION(IL,IC) is intended to convey to FDS the mass fraction of component IC of layer IL. For example, if the mass fraction of the second material of the third layer is 0.5, then write MATL_MASS_FRACTION(3,2)=0.5 To enter more than one mass fraction, use this notation: MATL_MASS_FRACTION(1,1:3)=0.5,0.4,0.1 which means that the first three materials of layer 1 have mass fractions of 0.5, 0.4, and 0.1, respectively. The notation 1:3 means array element 1 through 3, inclusive. Note that character strings can be enclosed either by single or double quotation marks. Be careful not to create the input file by pasting text from something other than a simple text editor, in which case the punctuation marks may not transfer properly into the text file. Some text file encodings may not work on all systems. If file reading errors occur and no typographical errors can be found in the input file, try saving the input file using a different encoding. For example, the text file editor Notepad works fine on a Windows PC, but a file edited in Notepad may not work on Linux or Mac OS X because of the difference in line endings between Windows and Unix/Linux operating systems. The editor Wordpad typically works better, but try a simple case first. 5.3 Input File Structure In general, the namelist records can be entered in any order in the input file, but it is a good idea to organize them in some systematic way. Typically, general information is listed near the top of the input file, and detailed information, like obstructions, devices, and so on, are listed below. FDS scans the entire input file each time it processes a particular namelist group. With some text editors, it has been noticed that the last line of the file is often not read by FDS because of the presence of an “end of file” character. To ensure that FDS reads the entire input file, add &TAIL / as the last line at the end of the input file. This completes the file from &HEAD to &TAIL. FDS does not even look for this last line. It just forces the “end of file” character past relevant input. Another general rule of thumb when writing input files is to only add parameters that make change from the default value. That way, you can more easily distinguish between what you want and what FDS wants. Add comments liberally to the file, so long as these comments do not fall within the namelist records. The general structure of an input file is shown below, with many lines of the original validation input 3 file removed for clarity. &HEAD CHID='WTC_05', TITLE='WTC Phase 1, Test 5' / 3 The actual input file, WTC_05.fds, is part of the FDS Validation Suite 30 &MESH &TIME &MISC &DUMP IJK=90,36,38, XB=-1.0,8.0,-1.8,1.8,0.0,3.82 / T_END=5400. / TMPA=20. / NFRAMES=1800, DT_HRR=10., DT_DEVC=10., DT_PROF=30. / &REAC FUEL FYI C H CO_YIELD SOOT_YIELD = = = = = = 'N-HEPTANE' 'Heptane, C_7 H_16' 7. 16. 0.008 0.015 / &OBST XB= 3.5, 4.5,-1.0, 1.0, 0.0, 0.0, SURF_ID='STEEL FLANGE' / ... &SURF ID = 'STEEL FLANGE' COLOR = 'BLACK' MATL_ID = 'STEEL' BACKING = 'EXPOSED' THICKNESS = 0.0063 / ... &VENT MB='XMIN', SURF_ID='OPEN' / ... &SLCF PBY=0.0, QUANTITY='TEMPERATURE', VECTOR=.TRUE. / ... &BNDF QUANTITY='GAUGE HEAT FLUX' / ... &DEVC XYZ=6.04,0.28,3.65, QUANTITY='OXYGEN', ID='EO2_FDS' / ... &TAIL / End of file. Fire Pan It is recommended that when looking at a new scenario, first select a pre-written input file that resembles the case, make the necessary changes, then run the case at fairly low mesh resolution to determine if the geometry is set up correctly. It is best to start off with a relatively simple file that captures the main features of the problem without getting tied down with too much detail that might mask a fundamental flaw in the calculation. Initial calculations ought to be meshed coarsely so that the run times are less than an hour and corrections can easily be made without wasting too much time. As you learn how to write input files, you will continually run and re-run your case as you add in complexity. Table 5.1 provides a quick reference to all the namelist parameters and where you can find the reference to where it is introduced in the document and the table containing all of the keywords for each group. 31 Table 5.1: Namelist Group Reference Table Group Name BNDF CLIP CSVF CTRL DEVC DUMP HEAD HOLE HVAC INIT ISOF MATL MESH MISC MULT OBST PART PRES PROF PROP RADI RAMP REAC SLCF SPEC SURF TABL TIME TRNX VENT ZONE Namelist Group Description Boundary File Output Clipping Parameters Velocity Input File Control Function Parameters Device Parameters Output Parameters Input File Header Obstruction Cutout Heating, Vent., Air Cond. Initial Condition Isosurface File Output Material Property Mesh Parameters Miscellaneous Multiplier Parameters Obstruction Lagrangian Particle Pressure Solver Parameters Profile Output Device Property Radiation Ramp Profile Reaction Parameters Slice File Output Species Parameters Surface Properties Tabulated Particle Data Simulation Time Mesh Stretching Vent Parameters Pressure Zone Parameters 32 Reference Section 16.5 6.7 6.4.5 15.5 15.1 16.1 6.1 7.2.6 9.2 6.5 16.6 8.3 6.3 6.4 7.5 7.2 14 6.6 16.3 15.3 13.1 10 12 16.4 11 7.1 15.3.1 6.2 6.3.5 7.3 9.3 Parameter Table 17.1 17.2 17.3 17.4 17.5 17.6 17.7 17.8 17.9 17.10 17.11 17.12 17.13 17.14 17.15 17.16 17.17 17.18 17.19 17.20 17.21 17.22 17.23 17.24 17.25 17.26 17.27 17.28 17.29 17.30 17.31 Chapter 6 Setting the Bounds of Time and Space This chapter describes global input parameters that affect the general scope of the simulation, like the simulation time and the size and extent of the computational domain. Essentially, these parameters establish the spatial and temporal coordinate systems that are used by all other components of the simulation, which is why these parameters are usually listed at the top of the input file and why they are described here first. Naming the Job: The HEAD Namelist Group (Table 17.7) 6.1 The first thing to do when setting up an input file is to give the job a name. The name of the job is important because often a project involves numerous simulations in which case the names of the individual simulations should be meaningful and help to organize the project. The namelist group HEAD contains two parameters, as in this example: &HEAD CHID='WTC_05', TITLE='WTC Phase 1, Test 5' / CHID is a string of 30 characters or less used to tag the output files. If, for example, CHID=’WTC_05’, it is convenient to name the input data file WTC_05.fds so that the input file can be associated with the output files. No periods or spaces are allowed in CHID because the output files are tagged with suffixes that are meaningful to certain computer operating systems. If CHID is not specified, then it will be set to the name of the input file minus everything at and beyond the first period. TITLE is a string of 60 characters or less that describes the simulation. It is simply a descriptive text that is passed to various output files. 6.2 Simulation Time: The TIME Namelist Group (Table 17.28) TIME is the name of a group of parameters that define the time duration of the simulation and the initial time step used to advance the solution of the discretized equations. 6.2.1 Basics Usually, only the duration of the simulation is required on this line, via the parameter T_END. The default is 1 s. For example, the following line will instruct FDS to run the simulation for 5400 seconds. &TIME T_END=5400. / 33 If T_END is set to zero, only the set-up work is performed, allowing you to quickly check the geometry in Smokeview. If you want the time line to start at a number other than zero, you can use the parameter T_BEGIN to specify the time written to file for the first time step. This would be useful for matching time lines of experimental data or video recordings. Time-based RAMPs are evaluated using the actual time if the RAMP activation time is the same as T_BEGIN; otherwise, they are evaluated using the time from when the RAMP activates. Therefore, if you are setting T_BEGIN in order to test a time-based CTRL or DEVC that is ultimately linked to a RAMP, then you should set T_BEGIN to be slightly less than the time the RAMP will activate. For example if you are testing a VENT that is to open at 10 s whose SURF_ID uses a RAMP, T_BEGIN should be set slightly less than 10 s. 6.2.2 Special Topic: Controlling the Time Step The initial time step size can be specified with DT. This parameter is normally set automatically by dividing the size of a mesh cell by the characteristic velocity of the flow. During the calculation, the time step is adjusted so that the CFL (Courant, Friedrichs, Lewy) condition is satisfied. The default value of DT is 1 √ 5 (δ x δ y δ z) 3 / gH s, where δ x, δ y, and δ z are the dimensions of the smallest mesh cell, H is the height of the computational domain, and g is the acceleration of gravity. Note that by default the time step is never allowed to increase above its initial value. To allow this to happen, set RESTRICT_TIME_STEP=.FALSE. If something sudden is to happen right at the start of a simulation, like a sprinkler activation, it is a good idea to set the initial time step to avoid a numerical instability caused by too large a time step. Experiment with different values of DT by monitoring the initial time step sizes recorded in the output file job_name.out. Finally, if you want to prevent FDS from automatically changing the time step, set LOCK_TIME_STEP equal to .TRUE. on the TIME line, in which case the specified time step, DT, will not be adjusted. This parameter is intended for diagnostic purposes only, for example, timing program execution. It can lead to numerical instabilities if the initial time step is set too high. 6.2.3 Special Topic: Steady-State Applications Occasionally, there are applications in which only the steady-state solution (in a time-averaged sense) is desired. However, the time necessary to heat the walls to steady-state can make the cost of the calculation prohibitive. In these situations, if you specify a TIME_SHRINK_FACTOR of, say, 10, the specific heats of the various materials is reduced by a factor of 10, speeding up the heating of these materials roughly by 10. An example of an application where this parameter is handy is a validation experiment where a steady heat source warms up a compartment to a nearly equilibrium state at which point time-averaged flow quantities are measured. Note that when TIME_SHRINK_FACTOR is used a device with QUANTITY=’TIME’ or a device or control function with a DELAY will have those values adjusted by the value of TIME_SHRINK_FACTOR. For example if a 10 s DELAY is specified for a CTRL input with a TIME_SHRINK_FACTOR of 10, then FDS will adjust the DELAY to 1 s. 34 6.3 Computational Meshes: The MESH Namelist Group (Table 17.13) All FDS calculations must be performed within a domain that is made up of rectilinear volumes called meshes. Each mesh is divided into rectangular cells, the number of which depends on the desired resolution of the flow dynamics. MESH is the namelist group that defines the computational domain. 6.3.1 Basics A mesh is a single right parallelepiped, i.e., a box. The coordinate system within a mesh conforms to the right hand rule. The origin point of a mesh is defined by the first, third and fifth values of the real number sextuplet, XB, and the opposite corner is defined by the second, fourth and sixth values. For example, &MESH IJK=10,20,30, XB=0.0,1.0,0.0,2.0,0.0,3.0 / defines a mesh that spans the volume starting at the origin and extending 1 m in the positive x direction, 2 m in the positive y direction, and 3 m in the positive z direction. The mesh is subdivided into uniform cells via the parameter IJK. In this example, the mesh is divided into 10 cm cubes. It is best if the mesh cells resemble cubes; that is, the length, width and height of the cells ought to be roughly the same. If it is desired that the mesh cells in a particular direction not be uniform in size, then the namelist groups TRNX, TRNY and/or TRNZ may be used to alter the uniformity of the mesh (See Section 6.3.5). Any obstructions or vents that extend beyond the boundary of the mesh are cut off at the boundary. There is no penalty for defining objects outside of the mesh, and these objects will not appear in Smokeview. The pressure solver in FDS employs Fast Fourier Transforms (FFTs) in the y and z directions, and this algorithm works most efficiently if the number of cells in these directions (the J and K of IJK) can be factored into low primes, like 2, 3, and 5. The number of cells in the x direction (the I in IJK) is not affected by this restriction because the pressure solver does not use an FFT in the x direction. However, since the pressure solver uses less than 10 % of the total CPU time, the gains in using low prime dimensions are usually negligible. Experiment with different mesh dimensions to ensure that those that are ultimately used do not unduly slow down the calculation. 6.3.2 Two-Dimensional and Axially-Symmetric Calculations The governing equations solved in FDS are written in terms of a three dimensional Cartesian coordinate system. However, a two dimensional Cartesian or two dimensional cylindrical (axially-symmetric) calculation can be performed by setting the J in the IJK triplet to 1 on the MESH line. For axial symmetry, add CYLINDRICAL=.TRUE. to the MESH line, and the coordinate x is then interpreted as the radial coordinate r. No boundary conditions should be set at the planes y = YMIN=XB(3) or y = YMAX=XB(4), nor at r = XMIN=XB(1) in an axially-symmetric calculation in which r = XB(1)=0. For better visualizations, the difference between XB(4) and XB(3) should be small so that the Smokeview rendering appears to be in 2-D. An example of an axially-symmetric helium plume is given in Section 6.4.8. 6.3.3 Multiple Meshes The term “multiple meshes” means that the computational domain consists of more than one computational mesh, usually connected although this is not required. If more than one mesh is used, there should be a MESH line for each. The order in which these lines are entered in the input file matters. In general, the meshes should be entered from finest to coarsest. FDS assumes that a mesh listed first in the input file has precedence over a mesh listed second if the two meshes overlap. Meshes can overlap, abut, or not touch 35 Figure 6.1: An example of a multiple-mesh geometry. at all. In the last case, essentially two separate calculations are performed with no communication at all between them. Obstructions and vents are entered in terms of the overall coordinate system and need not apply to any one particular mesh. Each mesh checks the coordinates of all the geometric entities and decides whether or not they are to be included. To run FDS in parallel using MPI (Message Passing Interface), you must break up the computational domain into multiple meshes so that the workload can be divided among the computers. In general, it is better to run multiple mesh cases with the MPI version of FDS if you have the computers available, but be aware that two computers will not necessarily finish the job in half the time as one. For the MPI version to work well, there has to be a comparable number of cells in each mesh, or otherwise most of the computers will sit idle waiting for the one with the largest mesh to finish processing each time step. You can use multiple meshes even when running the non-MPI version of FDS, in which case one CPU will serially process each mesh, one by one. Usually in a MPI calculation, each mesh is assigned its own process, and each process its own processor. However, it is possible to assign more than one mesh to a single process, and it is possible to assign more than one process to a single processor. Consider a case that involves six meshes: &MESH &MESH &MESH &MESH &MESH &MESH ID='mesh1', ID='mesh2', ID='mesh3', ID='mesh4', ID='mesh5', ID='mesh6', IJK=..., IJK=..., IJK=..., IJK=..., IJK=..., IJK=..., XB=..., XB=..., XB=..., XB=..., XB=..., XB=..., MPI_PROCESS=0 MPI_PROCESS=1 MPI_PROCESS=1 MPI_PROCESS=2 MPI_PROCESS=3 MPI_PROCESS=3 / / / / / / The parameter MPI_PROCESS instructs FDS to assign that particular mesh to the given process. In this case, only four processes are to be started, numbered 0 through 3. Note that the processes need to be invoked in 36 ascending order, starting with 0. Why would you do this? Suppose you only have four processors available for this job. By starting only four processes instead of six, you can save time because ‘mesh2’ and ‘mesh3’ can communicate directly with each other without having to transmit data using MPI calls over the network. Same goes for ‘mesh5’ and ‘mesh6’. In essence, it is as if these mesh pairs are neighbors and need not send mail to each other via the postal system. The letters can just be walked next door. Additionally to the mesh assignment to individual MPI processes, the number of OpenMP threads for each MPI process may be specified. The parameter N_THREADS instructs FDS to set this number of threads and must be consistent for all meshes on the same MPI process, but may vary between them: &MESH &MESH &MESH &MESH &MESH &MESH ID='mesh1', ID='mesh2', ID='mesh3', ID='mesh4', ID='mesh5', ID='mesh6', IJK=..., IJK=..., IJK=..., IJK=..., IJK=..., IJK=..., XB=..., XB=..., XB=..., XB=..., XB=..., XB=..., MPI_PROCESS=0 / MPI_PROCESS=1, N_THREADS=2 / MPI_PROCESS=1, N_THREADS=2 / MPI_PROCESS=2, N_THREADS=4 / MPI_PROCESS=3 / MPI_PROCESS=3 / Notes: An unspecified value will result in the default number of threads (see 3.2.1) on the MPI process. Some cluster systems do not support an heterogenous distribution of resources, here computational cores, per MPI process. Therefore, the number of OpenMP threads may change, but not the amount of cores assigned to the MPI process. For cases involving many meshes, you might want to assign them colors using either the character string COLOR or the integer triplet RGB. You may also want to consider using the multiplying feature to easily create a 3-D array of meshes. See Section 7.5 for details. Some parallel computing environments do not have a centralized file system, in which case FDS must write the output files for each process to a separate disk. If your computing cluster does not have a SHARED_FILE_SYSTEM, then set this parameter to .FALSE. on the MISC line. This parameter is also handy for MPI jobs involving hundreds or thousands of processes, in which case writing a single Smokeview file is time-consuming. When SHARED_FILE_SYSTEM is set to .FALSE., the file that Smokeview reads is broken into pieces, one for each MPI process. By doing this, you avoid serially writing to the Smokeview file. One other useful parameter for larger MPI jobs is called VERBOSE on the MISC line. This logical parameter suppresses information that is printed to the diagnostic output files. By default, its value is .TRUE. for MPI jobs involving 50 or less processes, and .FALSE. for larger jobs. 6.3.4 Mesh Alignment Whether the calculation is to be run using MPI or not, the rules of prescribing multiple meshes are similar, with some issues to keep in mind. The most important rule of mesh alignment is that abutting cells ought to have the same cross sectional area, or integral ratios, as shown in Fig. 6.2. The following rules of thumb should also be followed when setting up a multiple mesh calculation: • Avoid putting mesh boundaries where critical action is expected, especially fire. Sometimes fire spread from mesh to mesh cannot be avoided, but if at all possible try to keep mesh interfaces relatively free of complicated phenomena since the exchange of information across mesh boundaries is not yet as accurate as cell to cell exchanges within one mesh. • In general, there is little advantage to overlapping meshes because information is only exchanged at exterior boundaries. This means that a mesh that is completely embedded within another receives information at its exterior boundary, but the larger mesh receives no information from the mesh embedded within. Essentially, the larger, usually coarser, mesh is doing its own simulation of the scenario and 37 This is the ideal kind of mesh to mesh alignment. This is allowed so long as there are an integral number of fine cells abutting each coarse cell. This is allowed, but of questionable value. This is not allowed. Figure 6.2: Rules governing the alignment of meshes. 38 is not affected by the smaller, usually finer, mesh embedded within it. Details within the fine mesh, especially related to fire growth and spread, may not be picked up by the coarse mesh. In such cases, it is preferable to isolate the detailed fire behavior within one mesh, and position coarser meshes at the exterior boundary of the fine mesh. Then the fine and coarse meshes mutually exchange information. • Be careful when using the shortcut convention of declaring an entire face of the domain to be an OPEN vent. Every mesh takes on this attribute. See Section 7.3 for more details. • If a planar obstruction is close to where two meshes abut, make sure that each mesh “sees” the obstruction. If the obstruction is even a millimeter outside of one of the meshes, that mesh does not account for it, in which case information is not transferred properly between meshes. Accuracy of the Multiple Mesh Calculation Experiment with different mesh configurations using relatively coarse mesh cells to ensure that information is being transferred properly from mesh to mesh. There are two issues of concern. First, does it appear that the flow is being badly affected by the mesh boundary? If so, try to move the mesh boundaries away from areas of activity. Second, is there too much of a jump in cell size from one mesh to another? If so, consider whether the loss of information moving from a fine to a coarse mesh is tolerable. 6.3.5 Mesh Stretching: The TRNX, TRNY and TRNZ Namelist Groups (Table 17.29) By default the mesh cells that fill the computational domain are uniform in size. However, it is possible to specify that the cells be non-uniform in one or two of the three coordinate directions. For a given co- 1.2 1.2 0.9 0.9 x 1.5 x 1.5 0.6 0.6 0.3 0.3 0 0 0.3 0.6 ξ 0.9 1.2 0 0 1.5 Figure 6.3: Piecewise-linear mesh transformation. 0.3 0.6 ξ 0.9 1.2 1.5 Figure 6.4: Polynomial mesh transformation. ordinate direction, x, y or z, a function can be prescribed that transforms the uniformly-spaced mesh to a non-uniformly spaced mesh. Be careful with mesh transformations! If you shrink cells in one region you must stretch cells somewhere else. When one or two coordinate directions are transformed, the aspect ratio of the mesh cells in the 3D mesh will vary. To be on the safe side, transformations that alter the aspect ratio 39 of cells beyond 2 or 3 should be avoided. Keep in mind that the large eddy simulation technique is based on the assumption that the numerical mesh should be fine enough to allow the formation of eddies that are responsible for the mixing. In general, eddy formation is limited by the largest dimension of a mesh cell, thus shrinking the mesh resolution in one or two directions may not necessarily lead to a better simulation if the third dimension is large. Transformations, in general, reduce the efficiency of the computation, with two coordinate transformations impairing efficiency more than a transformation in one coordinate direction. Experiment with different meshing strategies to see how much of a penalty you will pay. Here is an example of how to do a mesh transformation. Suppose your mesh is defined &MESH IJK=15,10,20, XB=0.0,1.5,1.2,2.2,3.2,5.2 / and you want to alter the uniform spacing in the x direction. First, refer to the figures above. You need to define a function x = f (ξ ) that maps the uniformly-spaced Computational Coordinate (CC) 0 ≤ ξ ≤ 1.5 to the Physical Coordinate (PC) 0 ≤ x ≤ 1.5. The function has three mandatory constraints: it must be monotonic (always increasing), it must map ξ = 0 to x = 0, and it must map ξ = 1.5 to x = 1.5. The default transformation function is f (ξ ) = ξ for a uniform mesh, but you need not do anything in this case. Two types of transformation functions are allowed. The first, and simplest, is a piecewise-linear function. Figure 6.3 gives an example of a piecewise-linear transformation. The graph indicates how 15 uniformly spaced mesh cells along the horizontal axis are transformed into 15 non-uniformly spaced cells along the vertical axis. In this case, the function is made up of straight line segments connecting points (CC,PC), in increasing order, as specified by the following lines in the input file: &TRNX CC=0.30, PC=0.50, MESH_NUMBER=2 / &TRNX CC=1.20, PC=1.00, MESH_NUMBER=2 / The parameter CC refers to the Computational Coordinate, ξ , located on the horizontal axis; PC is the Physical Coordinate, x, located on the vertical axis. The slopes of the line segments in the plot indicate whether the mesh is being stretched (slopes greater than 1) or shrunk (slopes less than 1). The tricky part about this process is that you usually have a desired shrinking/stretching strategy for the Physical Coordinate on the vertical axis, and must work backwards to determine what the corresponding points should be for the Computational Coordinate on the horizontal axis. Note that the above transformation is applied to the second mesh in a multiple mesh job. The second type of transformation is a polynomial function whose constraints are of the form dn f (CC) = PC dξ n Figure 6.4 gives an example of a polynomial transformation, for which the parameters are specified (assuming that this is the third mesh): &TRNX IDERIV=0, CC=0.75, PC=0.75, MESH_NUMBER=3 / &TRNX IDERIV=1, CC=0.75, PC=0.50, MESH_NUMBER=3 / df (0.75) = 0.5, or, in words, the function maps 0.75 which correspond to the constraints f (0.75) = 0.75 and dξ into 0.75 and the slope of the function at ξ = 0.75 is 0.5 . The transform function must also pass through the points (0,0) and (1.5,1.5), meaning that FDS must compute the coefficients for the cubic polynomial f (ξ ) = c0 + c1 ξ + c2 ξ 2 + c3 ξ 3 . More constraints on the function lead to higher order polynomial functions, so be careful about too many constraints which could lead to non-monotonic functions. The monotonicity of the function is checked by the program and an error message is produced if it is not monotonic. 40 Do not specify either linear transformation points or IDERIV=0 points at coordinate values corresponding to the mesh boundaries. This is done automatically by FDS. 6.3.6 Mesh Resolution A common question asked by new FDS users is, “What should my grid spacing be?” The answer is not easy because it depends considerably on what you are trying to accomplish. In general, you should build an FDS input file using a relatively coarse mesh, and then gradually refine the mesh until you do not see appreciable differences in your results. This is referred to as a mesh sensitivity study. For simulations involving buoyant plumes, a measure of how well the flow field is resolved is given by the non-dimensional expression D∗ /δ x, where D∗ is a characteristic fire diameter ∗ D = Q̇ √ ρ∞ c p T∞ g 25 (6.1) and δ x is the nominal size of a mesh cell1 . The quantity, Q̇, is the total heat release rate of the fire. If it changes over time, you should consider the corresponding change in resolution. The quantity D∗ /δ x can be thought of as the number of computational cells spanning the characteristic (not necessarily the physical) diameter of the fire. The more cells spanning the fire, the better the resolution of the calculation. It is better to assess the quality of the mesh in terms of this non-dimensional parameter, rather than an absolute mesh cell size. For example, a cell size of 10 cm may be “adequate,” in some sense, for evaluating the spread of smoke and heat through a building from a sizable fire, but may not be appropriate to study a very small, smoldering source. The FDS Validation Guide [4] contains a table of the values of D∗ /δ x used in the simulation of the validation experiments. The table is near the end of the chapter that describes all the experiments. These values range over two orders of magnitude and were chosen based on a grid resolution study and the particular attributes of the given fire scenario. It would be inappropriate to take any of these values as an “acceptable” minimum. There are a number of special output quantities that provide local measures of grid resolution. See Section 16.10.21 for details. 1 The characteristic fire diameter is related to the characteristic fire size via the relation Q∗ diameter of the fire. 41 = (D∗ /D)5/2 , where D is the physical Miscellaneous Parameters: The MISC Namelist Group (Table 17.14) 6.4 MISC is the namelist group of global miscellaneous input parameters. It contains parameters that do not logically fit into any other category. 6.4.1 Basics Only one MISC line should be entered in the data file. For example, the input line &MISC TMPA=25. / sets the ambient temperature at 25 ◦ C. The MISC parameters vary in scope and degree of importance. Here is a partial list of MISCellaneous parameters. Others are described where necessary throughout this guide. DNS A logical parameter that, if .TRUE., directs FDS to perform a Direct Numerical Simulation, as opposed to the default Large Eddy Simulation (LES). This feature is appropriate only for simulations that use mesh cells that are on the order of a millimeter or less in size, or for diagnostic purposes. GVEC The 3 components of gravity, in m/s2 . The default is GVEC=0,0,-9.81. NOISE FDS initializes the flow field with a very small amount of “noise” to prevent the development of a perfectly symmetric flow when the boundary and initial conditions are perfectly symmetric. To turn this off, set NOISE=.FALSE. To control the amount of noise, set NOISE_VELOCITY. Its default value is 0.005 m/s. OVERWRITE If .FALSE. FDS checks for the existence of CHID.out and stops execution if it exists. P_INF Background pressure (at the ground) in Pa. The default is 101325 Pa. TMPA Ambient temperature, the temperature of everything at the start of the simulation. The default is 20 ◦ C. U0, V0, W0 Computationally, these are the initial values of the gas velocity in each of the coordinate directions. Normally, these are all 0 m/s, but there are a few applications where it is convenient to start the flow immediately, like in an outdoor simulation involving wind. Physically, these values should be thought of a “far-field” values of the velocity components. Setting these values affects the pressure boundary condition for OPEN vents. See the FDS Tech Guide [1]. 6.4.2 Special Topic: Mean Forcing and Data Assimilation A situation that occurs often in atmospheric flows is that initial and boundary conditions are not well defined. Typically, you know only that the mean wind is 10 m/s in the northeast direction, for example. More generally, there may be weather stations located at specific locations within the domain which continuously gather wind speed and direction. The process of steering the solution of the mass, momentum, and energy equations to match the statistics of the data gathered at the weather stations is known as data assimilation. This branch of modeling is early in its development, but very sophisticated (translation: complicated) methods already exist [8] and are employed in operational weather forecasting models. In FDS, you may invoke the most rudimentary of data assimilation techniques, a method called nudging. In brief, we add a mean forcing term to the momentum equation to nudge the solution toward a desired result. Currently, FDS can only affect the mean flow velocities. To turn on this capability, set the logical MEAN_FORCING(1:3)=.TRUE. on MISC. When this is set, FDS will drive the mean velocity toward the 42 value of U0, V0, or W0 (also set on MISC). For example, to point the wind in the northeast direction (assuming that the positive x axis points eastward) at 10 m/s use &MISC MEAN_FORCING(1:2)=.TRUE.,.TRUE., U0=7.07, V0=7.07, DT_MEAN_FORCING=.1 / Note that you must also specify a time scale for relaxation, DT_MEAN_FORCING. For an outdoor flow, all other boundaries (except the ground) should be set to OPEN. You may omit a region of the flow domain from forcing using the HOLE feature. If a HOLE is used with MEAN_FORCING it is advisable to also set PROJECTION=.TRUE. on MISC; otherwise the combination of U0 in the bulk region and zero velocity in the HOLE region creates an initial divergence error that will take some time to wash out of the domain. 6.4.3 Special Topic: Specified Force Field Similar to the MEAN_FORCING feature, you may specify a constant and uniform force per unit volume by setting FORCE_VECTOR(1:3) on MISC. This is useful, for example, in specifying a mean pressure drop in a duct. In the absence of other forces, the force vector Fi affects the momentum equation by ∂ ui = −Fi /ρ (6.2) ∂t This feature is typically used together with periodic boundaries. To specify a mean pressure drop of 0.01 Pa/m in the x direction for a periodic duct, for example, use &MISC FORCE_VECTOR(1)=0.01 / &VENT MB='XMIN', SURF_ID='PERIODIC' / &VENT MB='XMAX', SURF_ID='PERIODIC' / 6.4.4 Special Topic: Stopping and Restarting Calculations An important MISC parameter is called RESTART. Normally, a simulation consists of a sequence of events starting from ambient conditions. However, there are occasions when you might want to stop a calculation, make a few limited adjustments, and then restart the calculation from that point in time. To do this, first bring the calculation to a halt gracefully by creating a file called CHID.stop in the directory where the output files are located. Remember that FDS is case-sensitive. The file name must be exactly the same as the CHID and ‘stop’ should be lower case. FDS checks for the existence of this file at each time step, and if it finds it, gracefully shuts down the calculation after first creating a final Plot3D file and a file (or files in the case of a multiple mesh job) called CHID.restart (or CHID_nn.restart). To restart a job, the file(s) CHID.restart should exist in the working directory, and the phrase RESTART=.TRUE. needs to be added to the MISC line of the input data file. For example, suppose that the job whose CHID is “plume” is halted by creating a dummy file called plume.stop in the directory where all the output files are being created. To restart this job from where it left off, add RESTART=.TRUE. to the MISC line of the input file plume.fds, or whatever you have chosen to name the input file. The existence of a restart file with the same CHID as the original job tells FDS to continue saving the new data in the same files as the old2 . If RESTART_CHID is also specified on the MISC line, then FDS will look for old output files tagged with this string instead of using the specified CHID on the HEAD line. In this case, the new output files will be tagged with CHID, and the old output files will not be altered. When running the restarted job, the diagnostic output of the restarted job is appended to output files from the original job. 2 By default, when a job is restarted, the spreadsheet output files will be appended at the time the job was restarted, not the time the job was stopped. If you want the output files to be appended without clipping off any existing data, even though some duplicate output will be left over, then set CLIP_RESTART_FILES to .FALSE. on the DUMP line. 43 There may be times when you want to save restart files periodically during a run as insurance against power outages or system crashes. If this is the case, at the start of the original run set DT_RESTART=50. on the DUMP line to save restart files every 50 s, for example. The default for DT_RESTART is 1000000, meaning no restart files are created unless you gracefully stop a job by creating a dummy file called CHID.stop. It is also possible to use the new control function feature (see Section 15.5) to stop a calculation or dump a restart file when the computation reaches some measurable condition such as a first sprinkler activation. Between job stops and restarts, major changes cannot be made in the calculation like adding or removing vents and obstructions. The changes are limited to those parameters that do not instantly alter the existing flow field. Since the restart capability has been used infrequently by the developers, it should be considered a fragile construct. Examine the output to ensure that no sudden or unexpected events occur during the stop and restart. 6.4.5 Special Topic: Initializing a 3D Velocity Field It may be useful to start a calculation from an established flow field. Usually this can be accomplished with the normal restart functionality. But in some circumstances restart may be fragile, or you may want to specify a profile throughout the entire domain. For such situations we have added the ability to read the velocity field information from a comma-separated value (.csv) file. You have the option of creating the velocity file using FDS or creating your own. To generate the velocity initialization file with FDS, in the input file add a DUMP line with a UVW_TIMER (time in seconds). The timer will accept up to 10 values, and will write velocity files for each mesh and each timer index to the working directory. For example, if you want to write the simulation velocity field at 10 minutes into the run, add the following: &DUMP UVW_TIMER(1)=600 / FDS will then write CHID_uvw_nn.csv for each time index and mesh. The format for this file is WRITE(LU_UVW) IMIN,IMAX,JMIN,JMAX,KMIN,KMAX DO K=KMIN,KMAX DO J=JMIN,JMAX DO I=IMIN,IMAX WRITE(LU_UVW,*) U(I,J,K),',',V(I,J,K),',',W(I,J,K) ENDDO ENDDO ENDDO You may read in the 3-D velocity field using a CSVF line. For example: &CSVF UVWFILE='my_velocity_field.csv' / If multiple meshes are involved, it is assumed that the CSVF lines are provided in the input file in the same order as the meshes. For two meshes you might have the following: &CSVF UVWFILE='CHID_t001_m001.csv' / &CSVF UVWFILE='CHID_t001_m002.csv' / &MISC PROJECTION=.TRUE. / It is recommended that you also specify PROJECTION=.TRUE. on MISC if the specified velocity field does not satisfy the divergence constraint to machine precision on a staggered grid (this may not even be true if an analytical solution to the Navier-Stokes equations is sampled at the staggered velocity component locations). 44 6.4.6 Special Topic: Turning off the Flow Field For certain types of diagnostic tests, it is useful to turn off the velocity field and exercise some other aspect of the model, like radiation of particle transport. To do this, set FREEZE_VELOCITY=.TRUE. on the MISC line. 6.4.7 Special Topic: Defying Gravity By default, gravity points in the negative z direction, or more simply, downward. However, to change the direction of gravity to model a sloping roof or tunnel, for example, specify the gravity vector on the MISC line with a triplet of numbers of the form GVEC=0.,0.,-9.81, with units of m/s2 . This is the default, but it can be changed to be any direction. There are a few special applications where you might want to vary the gravity vector as a function of time or as a function of the first spatial coordinate, x. For example, on board space craft, small motions can cause temporal changes in the normally zero level of gravity, an effect known as “g-jitter.” More commonly, in tunnel fire simulations, it is sometimes convenient to change the direction of gravity to mimic the change in slope. The slope of the tunnel might change as you travel through it; thus, you can tell FDS where to redirect gravity. For either a spatially or temporally varying direction and/or magnitude of gravity, do the following. First, on the MISC line, set the three components of gravity, GVEC, to some “base” state like GVEC=1.,1.,1., which gives you the flexibility to vary all three components. Next, designate “ramps” for the individual components, RAMP_GX, RAMP_GY, and RAMP_GZ, all of which are specified on the MISC line. There is more discussion of RAMPs in Section 10, but for now you can use the following as a simple template to follow: &MISC GVEC=1.,0.,1., RAMP_GX='x-ramp', RAMP_GZ='z-ramp' / &RAMP &RAMP &RAMP &RAMP ID='x-ramp', ID='x-ramp', ID='x-ramp', ID='x-ramp', X= 0., X= 50., X= 51., X=100., F=0.0 / F=0.0 / F=-0.49 / F=-0.49 / &RAMP &RAMP &RAMP &RAMP ID='z-ramp', ID='z-ramp', ID='z-ramp', ID='z-ramp', X= 0., X= 50., X= 51., X=100., F=-9.81 F=-9.81 F=-9.80 F=-9.80 / / / / Note that both the x and z components of gravity are functions of x. FDS has been programmed to only allow variation in the x coordinate. Note also that F is just a multiplier of the “base” gravity vector components, given by GVEC. This is why using the number 1 is convenient – it allows you to specify the gravity components on the RAMP lines directly. The effect of these lines is to model the first 50 m of a tunnel without a slope, but the second 50 m with a 5 % slope upwards. Note that the angle from vertical of the gravity vector due to a 5 % slope is tan−1 0.05 = 2.86◦ and that 0.49 and 9.80 are equal to the magnitude of the gravity vector, 9.81 m/s2 , multiplied by the sine and cosine of 2.86◦ , respectively. To check your math, the sum of the squares of the gravity components ought to equal 9.81. Notice in this case that the y direction has been left out because there is no y variation in the gravity vector. To vary the direction and/or magnitude of gravity in time, follow the same procedure but replace the X in the RAMP lines with a T. 45 6.4.8 Special Topic: The Baroclinic Vorticity The pressure term in the momentum transport equation solved by FDS is decomposed as follows: p̃ 1 1 ∇ p̃ = ∇ − p̃ ∇ ρ ρ ρ (6.3) The pressure term is written like this so that a separable elliptic partial differential equation can be solved for the “total” pressure, H ≡ |u|2 /2 + p̃/ρ, using a direct solver. The second term is calculated based on the pressure field from the previous time step, a slight approximation necessary to render the pressure equation separable. This term is sometimes referred to as the baroclinic torque, and it is responsible for generating vorticity due to the non-alignment of pressure and density gradients. In versions of FDS prior to 6, the inclusion of the baroclinic torque term was found to sometimes cause numerical instabilities. If it is suspected that the term is responsible for numerical problems, it can be removed by setting BAROCLINIC=.FALSE. on the MISC line. For example, in the simple helium plume test case below, neglecting the baroclinic torque changes the puffing behavior noticeably. In other applications, however, its effect is less significant. For further discussion of its effect, see Ref. [9]. Example Case: Flowfields/helium_2d_isothermal This case demonstrates the use of baroclinic correction for an axially-symmetric helium plume. Note that the governing equations solved in FDS are written in terms of a three dimensional Cartesian coordinate system. However, a two dimensional Cartesian or two dimensional cylindrical (axially-symmetric) calculation can be performed by setting the number of cells in the y direction to 1. An example of an axially-symmetric helium plume is shown in Fig. 6.5. &HEAD &MESH &TIME &MISC &RADI &SPEC &SURF &VENT &VENT &OBST &DUMP &SLCF &SLCF &TAIL CHID='helium_2d_isothermal',TITLE='Axisymmetric Helium Plume' / IJK=72,1,144 XB=0.00,0.08,-0.001,0.001,0.00,0.16, CYLINDRICAL=.TRUE. / T_END=5.0 / DNS=.TRUE. / RADIATION=.FALSE. / ID='HELIUM' / ID='HELIUM', VEL=-0.673, MASS_FRACTION(1)=1.0, TAU_MF(1)=0.3 / MB='XMAX' ,SURF_ID='OPEN' / MB='ZMAX' ,SURF_ID='OPEN' / XB= 0.0,0.036,-0.001,0.001,0.00,0.02, SURF_IDS='HELIUM','INERT','INERT' / PLOT3D_QUANTITY(1)='PRESSURE',PLOT3D_QUANTITY(5)='HELIUM' / PBY=0.000,QUANTITY='DENSITY', VECTOR=.TRUE. / PBY=0.000,QUANTITY='HELIUM' / / Figure 6.5: Simulation of a helium plume. 6.4.9 Special Topic: Large Eddy Simulation Parameters By default FDS uses the Deardorff [10, 11] turbulent viscosity, p (µLES /ρ) = Cν ∆ ksgs (6.4) where Cν = 0.1 and the subgrid scale (sgs) kinetic energy is taken from an algebraic relationship based on scale similarity (see the FDS Technical Reference Guide [1]). The LES filter width is taken as the geometric mean of the local mesh spacing in each direction, ∆ = (δ x δ y δ z)(1/3) . 46 Options for the TURBULENCE_MODEL on the MISC line are listed in Table 6.1. Note that the model used in FDS versions 1-5 is ’CONSTANT SMAGORINSKY’. The thermal conductivity and material diffusivity are related to the turbulent viscosity by: kLES = µLES c p Prt ; (ρD)LES = µLES Sct (6.5) The turbulent Prandtl number Prt and the turbulent Schmidt number Sct are assumed to be constant for a given scenario. Although it is not recommended for most calculations, you can modify Prt = 0.5, and Sct = 0.5 via the parameters PR, and SC on the MISC line. A more detailed discussion of these parameters is given in the FDS Technical Reference Guide [1]. Table 6.1: Turbulence model options. TURBULENCE_MODEL ’CONSTANT SMAGORINSKY’ ’DYNAMIC SMAGORINSKY’ ’DEARDORFF’ ’VREMAN’ ’RNG’ 6.4.10 Description Constant coefficient Smagorinsky model [12] Dynamic Smagorinsky model [13, 14] Deardorff model [10, 11] Vreman’s eddy viscosity model [15] Renormalization group eddy viscosity model [16] Coefficient(s) C_SMAGORINSKY None C_DEARDORFF C_VREMAN C_RNG, C_RNG_CUTOFF Special Topic: Numerical Stability Parameters FDS uses an explicit time advancement scheme; thus, the time step plays an important role in maintaining numerical stability and accuracy. Below we examine the constraints on the time step necessary for stability in the presence of advection, diffusion, and expansion of the velocity and scalar fields. In addition, there are additional constraints that ensure accuracy of various algorithms. The Courant-Friedrichs-Lewy (CFL) Constraint The well-known CFL constraint given by CFL = δt kuk <1 δx (6.6) places a restriction on the time step due to the advection velocity. The limits for the CFL are set by CFL_MIN (default 0.8) and CFL_MAX (default 1) on MISC. Physically, the constraint says that a fluid element should not traverse more than one cell within a time step. For LES, this constraint has the added advantage of keeping the implicit temporal and spatial filters consistent with each other. In other words, in order to resolve an eddy of size δ x, the time step needs to obey the CFL constraint. If one were to employ an implicit scheme for purpose of taking time steps say 10 times larger than the CFL limit, the smallest resolvable turbulent motions would then be roughly 10 times the grid spacing, which would severely limit the benefit of using LES. In most cases, if you want the simulation to run faster, a better strategy is to coarsen the grid resolution while keeping the CFL close to 1. The exact CFL needed to maintain stability depends on the order (as well as other properties) of the time integration scheme and the choice of velocity norm. Three choices for velocity norm are available in FDS (set on MISC): 47 CFL_VELOCITY_NORM=0 (LES default, least restrictive, corresponds to L∞ norm of velocity vector) |u| |v| |w| kuk = max , , δx δx δy δz (6.7) CFL_VELOCITY_NORM=1 (DNS default, most restrictive, corresponds to L1 norm of velocity vector) kuk |u| |v| |w| = + + δx δx δy δz (6.8) CFL_VELOCITY_NORM=2 (L2 norm of velocity vector) s u 2 v 2 w 2 kuk = + + δx δx δy δz (6.9) The Von Neumann Constraint The Von Neumann constraint is given by µ VN ≡ δt max , Dα ρ 1 1 1 + 2+ 2 2 δx δy δz < 1 2 (6.10) The Von Neumann stability check is invoked by setting CHECK_VN=.TRUE. on the MISC line (for DNS, CHECK_VN=.TRUE. by default). The limits for VN may be adjusted using VN_MIN (default 0.4) and VN_MAX (default 0.5) on MISC. We can understand this constraint in a couple of different ways. First, we could consider the model for the diffusion velocity of species α in direction i, Vα,iYα = −Dα ∂Yα /∂ xi , and we would then see that VN is simply a CFL constraint due to diffusive transport. We can also think of VN in terms of a total variation diminishing (TVD) constraint. That is, if we have variation (curvature) in the scalar field, we do not want to create spurious oscillations that can lead to an instability by overshooting the smoothing step. Consider the following explicit update of the heat equation for u in 1-D. Here subscripts indicate grid indices and ν is the diffusivity. δt ν n (u − 2uni + uni+1 ) δ x2 i−1 un+1 = uni + i (6.11) Very simply, notice that if δt ν/δ x2 = 1/2 then un+1 = (uni−1 + uni+1 )/2. If the time step is any larger we i overshoot the straight line connecting neighboring cell values. Of course, this restriction is only guaranteed to be TVD if the u field is “smooth”; otherwise, the neighboring cell values may be shifted in the opposite direction. Unfortunately, in LES there is no such guarantee and so the VN constraint can be particularly devilish in generating instabilities. For this reason, some practitioners like to employ implicit methods for the diffusive terms. Realizable Mass Density Constraint In an explicit Euler update of the continuity equation, if the time increment is too large the computational cell may be totally drained of mass, which, of course, is not physical. The constraint ρ n+1 > 0 therefore leads to the following restriction on the time step: δt < ρn n u · ∇ρ n + ρ n ∇ · un 48 (6.12) We can argue that the case we are most concerned with is when ρ n is near zero. A reasonable approximation to (6.12) then becomes −1 ρ ui = +∇·u (6.13) δt < δ xi u ρ−0 + ρ∇ · u i δ xi Eq. (6.13) basically adds the effect of thermal expansion to the CFL constraint and provides a reason to prefer CFL_VELOCITY_NORM=1 as the basis for the time step restriction. To be clear, the CFL constraint is now given by kuk CFL = δt + |∇ · u| (6.14) δx Stability of particle transport The movement of Lagrangian particles over the course of a time step is calculated using an analytical solution and remains stable regardless of the time step used by the flow solver. However, if the particle moves over the width of several grid cells in a single time step, the momentum transferred between the particle and the gas cannot be allocated properly to all of the affected cells. To overcome this problem, FDS subdivides the gas phase time step based on each particle’s velocity. For example, if the particle travels across two cells in a single gas phase time step, then its trajectory is calculated by subdividing the time step into two. In some cases with extremely fast particles, however, the stability of the overall flow behavior may require setting an additional parameter that limits the time step of the flow solver according to the speed of the fastest particle in the simulation. The actual value of of the constraint is set using PARTICLE_CFL_MAX on the MISC line. A value of 1 (default) means that the fastest moving particle can move a distance of one grid cell during the time step. Because very fast nozzle velocities can cause extremely small time steps and hence very long run times, the PARTICLE_CFL constraint is set to .FALSE. by default. Setting PARTICLE_CFL to .TRUE. on the MISC line activates this constraint. One additional parameter might be useful in some special cases. The numerical transport scheme for particles is first order accurate in time. If you want to improve the accuracy of the scheme, set SECOND_ORDER_PARTICLE_TRANSPORT to .TRUE. on the PART line. There is an increased computational cost incurred by this option, so check whether or not the extra cost is worth the extra accuracy. For most applications, given the relatively small time step of the gas phase solver, and the subdivision of the particle trajectory calculation to enforce a local particle CFL constraint, the improved accuracy is not worth the additional cost. Heat Transfer Constraint Note that the heat transfer coefficient, h, has units of W/(m2 · K). Thus, a velocity scale may be formed from h/(ρ c p ). Anytime we have a velocity scale to resolve we have a CFL-type stability restriction. Therefore, the heat transfer stability check loops over all wall cells to ensure δt ≤ δ x ρ c p /h. This check may be invoked by setting CHECK_HT=.TRUE. on the MISC line. It is .FALSE. by default. Adjusting the Time Step At the end of the first part of the explicit predictor-corrector time update, the time step is checked to ensure that it is within the appropriate stability bounds. If it is not, it is adjusted up or down by 10 % (or until it is within limits) and the predictor part of the time step is re-run. Resetting the stability parameters is not recommended except in very special circumstances, as they can lead to simulations failing due to numerical instabilities. If you want to prevent FDS from automatically changing the time step, set LOCK_TIME_STEP to .TRUE. on the TIME line, in which case the specified time step, DT, will not be adjusted. This parameter 49 is intended for diagnostic purposes only, for example, timing program execution. It can lead to numerical instabilities if the initial time step is set too high. 6.4.11 Special Topic: Flux Limiters FDS employs total variation diminishing (TVD) schemes for scalar transport. The default for LES is Superbee [17], so chosen because this scheme does the best job preserving the scalar variance in highly turbulent flows with coarse grid resolution. The default scheme for DNS is CHARM [18] because the gradient steepening used in Superbee forces a stair step pattern at high resolution, while CHARM is convergent. A few other schemes (including Godunov and central differencing) are included for completeness; more details can be found in the Tech Guide [19]. Table 6.2 below shows the integer codes which may be used to invoke the various limiter schemes. &MISC FLUX_LIMITER=1 / ! invoke Godunov (first-order upwind scheme) Table 6.2: Flux limiter options. Scheme Central differencing Godunov Superbee (LES default) MINMOD CHARM (DNS default) MP5 6.5 FLUX_LIMITER 0 1 2 3 4 5 Initial Conditions: The INIT Namelist Group (Table 17.10) Usually, an FDS simulation begins at time t = 0 with ambient conditions. The air temperature is assumed constant with height, and the density and pressure decrease with height (the z direction). This decrease is not noticed in most building scale calculations, but it is important in large outdoor simulations. There are some scenarios for which it is convenient to change the ambient conditions within some rectangular region of the domain using the namelist keyword INIT. Note that the rectangular regions defined by INIT may overlap, but in such cases, it is the second of the overlapping regions that takes precedence, including default conditions. That is, it is possible to overwrite the initial conditions explicitly specified by the first INIT line with the default initial conditions implied by the second INIT line. Species Species concentrations can be initialized using pairs of SPEC_ID(N) and MASS_FRACTION(N) where N is an ordinal index starting from 1. Note that N is not necessarily indicative of the order in which the species are listed in the input file. Also note that the background species cannot be specified when using MASS_FRACTION. The mass fraction of the background species will be set to account for any mass fraction not specified with other species. Make sure that you specify all species (components of MASS_FRACTION(N)) on the same INIT line for example, 50 &INIT XB=0.0,0.1,0.0,0.025,0.0,0.1, MASS_FRACTION(1)=0.21, SPEC_ID(1)='OXYGEN', MASS_FRACTION(2)=0.06, SPEC_ID(2)='PROPANE' / Here, within the region whose bounds are given by the sextuplet XB, the initial mass fractions of oxygen and propane will be initialized to 0.21 and 0.06, respectively. You must specify the gas species using the SPEC namelist group. See Section 11 for details. Temperature To modify the local initial temperature, add lines of the form, &INIT XB=0.0,0.1,0.0,0.025,0.0,0.1, TEMPERATURE=60. / This indicates that the temperature shall be 60 ◦ C instead of the ambient within the bounds given by XB. The INIT construct may be useful in examining the influence of stack effect in a building, where the temperature is different inside and outside. If you wanted to initialize both temperature and species in the same volume, both quantities would use the same INIT line, &INIT XB=0.0,0.1,0.0,0.025,0.0,0.1, MASS_FRACTION(1)=0.21, SPEC_ID(1)='OXYGEN', MASS_FRACTION(2)=0.06, SPEC_ID(2)='PROPANE', TEMPERATURE=60. / Density When specifying an initial density it is important to recognize the order in which FDS solves the governing equations. In the following example, initial species mass fractions, temperature, and density are all initialized in the same volume. &INIT XB=0.0,0.1,0.0,0.025,0.0,0.1, MASS_FRACTION(1)=0.21, SPEC_ID(1)='OXYGEN', MASS_FRACTION(2)=0.06, SPEC_ID(2)='PROPANE', TEMPERATURE=60., DENSITY=1.13 / This example is a case where we have over-defined the problem. Since the temperature is computed from the equation of state using the specified density, the specified temperature will not, in general, satisfy the equation of state, and FDS will overwrite the specified temperature. Heat Release Rate Per Unit Volume (HRRPUV) The INIT line may also be used to specify a volumetric heat source term. For example, &INIT XB=0.0,0.1,0.0,0.025,0.0,0.1, HRRPUV=1000. / indicates that the region bounded by XB shall generate 1000 kW/m3 . This feature is mainly useful for diagnostics, or to model a fire in a very simple way. 51 6.6 6.6.1 The Pressure Solver: The PRES Namelist Group (Table 17.18) Parameters Related to the Solution of Poisson Equation for Pressure FDS uses a low-Mach number formulation of the Navier-Stokes equations. One of the consequences of this is that the speed of sound is assumed infinite, and that the pressure throughout the computational domain is affected, instantaneously, by local changes in the flow field. A simple example of this is when air is pushed through a tunnel. If the tunnel has forced flow at one end and an opening at the other, the volume flow at the opening is the same as that which is forced at the other end. Without any heat addition, the air is assumed incompressible. Information is passed through the tunnel instantaneously in the model via a solution of a linear system of equations for the pressure. For a single mesh, the solution of this Poisson equation for the pressure is very accurate. However, for multiple meshes, there is potentially a delay in information passing throughout the domain because the Poisson equation is solved on each individual mesh, without any influence from the larger computational domain. The details of the numerical approach can be found in the FDS Technical Reference Guide. Another limitation of the pressure solver is that at solid surfaces that are not part of the boundary of the computational domain, the pressure solver enforces a no-flux boundary condition. However, it is not perfect, and it is possible to have a non-zero normal velocity at a solid surface. For most applications, this velocity is so small that it has a negligible effect on the solution. If either the error in the normal component of the velocity at a mesh interface or at a solid boundary is large, you can reduce it by making more than the default number of calls to the pressure solver at each time step. To do so, specify VELOCITY_TOLERANCE on the PRES line to be the maximum allowable normal velocity component on the solid boundary or the largest error at a mesh interface. It is in units of m/s. If you set this, experiment with different values, and monitor the number of pressure iterations required at each time step to achieve your desired tolerance. The default value is δ x/2, where δ x is the characteristic grid cell size. The number of iterations are written out to the file CHID.out. If you use a value that is too small, the CPU time required might be prohibitive. The maximum number of iterations for each half of the time step is given by MAX_PRESSURE_ITERATIONS, also on the PRES line. Its default value is 10. By default, FDS will suspend the pressure iterations if the error is not reduced by at least 25 % at each step. If you do not want FDS to suspend the pressure iteration, set SUSPEND_PRESSURE_ITERATIONS to .FALSE. on the PRES line. This will be done automatically if you change the default values of VELOCITY_TOLERANCE or MAX_PRESSURE_ITERATIONS. If .TRUE., the parameter CHECK_POISSON tells FDS to check that the left-hand and right-hand sides of the Poisson equation for H are equivalent (see the FDS Tech Guide [1]). The error is printed to the CHID.out file. Example Case: Pressure_Solver/duct_flow To demonstrate how to improve the accuracy of the pressure solver, consider the flow of air through a square duct that crosses several meshes. In the sample input file, duct_flow.fds, air is pushed through a 1 m2 duct at 1 m/s. With no thermal expansion, the volume flow into the duct ought to equal the volume flow out of the duct. Figure 6.6 displays the computed inflow and outflow as a function of time, and the number of pressure iterations required. The outflow does not match the inflow exactly because of inaccuracies at the solid and mesh boundaries. The VELOCITY_TOLERANCE has been set to 0.001 m/s with MAX_PRESSURE_ITERATIONS set to 1000 and the grid cell size is 0.2 m. 52 FDS0−86−g80cff4e FDS0−86−g80cff4e 1.5 100 Volume Flow (duct_flow) Pressure Iterations (duct_flow) 1 Iterations Volume Flow (m3/s) 80 60 40 0.5 20 Ideal (Flow) FDS (flow_in) FDS (flow_out) 0 0 10 20 30 Time (s) 40 50 0 0 60 10 20 30 Time (s) 40 50 60 Figure 6.6: (Left) Volume flow into and out of a square duct. (Right) The number of pressure iterations as a function of time. Example Case: Pressure_Solver/dancing_eddies In this example, air is pushed through a 30 cm long, two-dimensional channel at 0.5 m/s. A plate obstruction normal to the flow creates a Karman vortex street. The computational domain is divided into 4 meshes. Two simulations are performed, one in which the VELOCITY_TOLERANCE is set to a relatively small value, and one in which it is set to the default value. Figure 6.7 shows the downstream pressure histories for these two cases compared to a simulation that uses only one mesh. The case with the tighter tolerance produces a better match to the single mesh solution, but at a higher computational cost. FDS0−86−g80cff4e Pressure (dancing_eddies_default) 0.15 0.1 0.1 0.05 0.05 Pressure (Pa) Pressure (Pa) 0.15 FDS0−86−g80cff4e 0 −0.05 −0.1 −0.15 0 Figure 6.7: 0 −0.05 −0.1 −0.15 −0.2 −0.25 Pressure (dancing_eddies_tight) −0.2 FDS, 1 mesh FDS, 4 mesh, tol=0.0005 m/s 0.5 1 Time (s) −0.25 1.5 2 0 FDS, 1 mesh FDS, 4 mesh, tol=0.00001 m/s 0.5 1 Time (s) 1.5 2 (Top) Comparison of pressure traces in the channel for two different settings of VELOCITY_TOLERANCE, the default value on the left and a tighter tolerance on the right. (Bottom) A contour plot of the pressure after 2 s with the default tolerance. 53 One strategy for reducing the computational cost associated with a tightened tolerance on the normal velocity at mesh boundaries is to overlap the meshes. The idea is described in detail for a non-structured mesh in Ref. [20]. The basic idea is that a slight overlap of meshes can lead to faster convergence of the pressure solver towards the desired level of accuracy. In the dancing_eddies test case, the case called dancing_eddies_tight is rerun as dancing_eddies_tight_overlap where each mesh is extended an additional 10 grid cells in the longitudinal direction. This increases the work by approximately a factor of 85/75, but it reduces the number of iterations. Figure 6.8 shows the reduction in the number of pressure iterations (left) and the overall reduction in CPU time (right). FDS0−86−g80cff4e FDS0−86−g80cff4e 100 400 CPU Time (dancing_eddies) 350 CPU Time (dancing_eddies) 300 CPU Time (s) Pressure Iterations 80 60 No overlap Overlap 40 250 200 150 100 20 50 0 0 0.5 1 Time (s) 1.5 0 0 2 No overlap Overlap 0.5 1 Simulated Time (s) 1.5 2 Figure 6.8: The number of pressure iterations (left) and the total CPU time (right) for overlapped and nonoverlapped meshes. Special Case: True Periodic Boundaries In Section 7.3.2, we discuss how to set periodic boundaries. By default, in order to handle the most general case of a periodic domain with multiple meshes (imagine a periodic channel divided into several meshes), FDS treats the pressure boundary condition as what we call an “interpolated boundary.” This means that the matrix for the pressure Poisson equation is arranged for Dirichlet boundary conditions (the value of the solution is specified at the boundary). This can lead to small errors in the solution and sometimes it is desirable to use the true periodic matrix for the Poisson equation. But with the current solver (Fishpak) this is only possible for a single mesh. If you want to implement true periodic boundaries for a single mesh case, set the appropriate FISHPAK_BC value to zero on the PRES line. For example, &PRES FISHPAK_BC(1:3)=0,0,0 / 6.6.2 Pressure Considerations in Long Tunnels A common application of FDS is tunnel fires, but simulations of fires in long, relatively tight tunnels can have spurious fluctuations in the pressure field that can lead to numerical instabilities. The root cause of the problem is the way that FDS solves the equation for the pressure, p̃ (the tilde indicates that this is the perturbation pressure, or pressure above background). To solve for p̃, an elliptic PDE is formed by taking the divergence of the momentum equation, which contains the term ∇ p̃/ρ. Ideally, this PDE would be of the form: 1 ∇· ∇ p̃n = · · · (6.15) ρn 54 where n indicates the current time step. However, this equation cannot be solved efficiently because the matrix of the linear system of equations that is formed when discretizing it does not have constant coefficients because the density, ρ n , changes with each new time step, n. To get around this problem, the pressure term in the momentum equation is decomposed as follows: 1 p̃ 1 ∇ p̃ = ∇ − p̃ ∇ ρ ρ ρ (6.16) which leads to a Poisson equation that can be solved efficiently: 2 ∇ p̃n,k ρn = ∇ · p̃ n,k−1 1 ∇ ρn + ··· (6.17) The superscript k indicates that this equation is solved multiple times, and k indicates the iteration. With each iteration, the old and new values of p̃ are driven closer together. The iterations continue3 until the maximum value of the following expression in discretized form: n,k 1 1 n,k 2 p̃ n,k−1 ∇ p̃ − ∇ + ∇ · p̃ ∇ ε = max ∇ · n n i jk ρ ρ ρn k (6.18) drops below a specified tolerance. By default, the PRESSURE_TOLERANCE is 20/δ x2 where δ x is the characteristic grid cell size. If a numerical instability occurs in a simulation involving a tunnel, you could try reducing this value on the PRES line to alleviate the mismatch between old and new pressure fields. A simple test case, Pressure_Solver/tunnel_demo.fds, demonstrates some of the issues discussed in this section. An 8 MW fire is situated in a 4 m by 4 m by 128 m long tunnel that is sloped 10◦ , closed at the lower end and open at the upper end. The tunnel is divided into 8 meshes, all the same size, with a uniform grid of 20 cm. The case runs for a relatively short amount of time, and a diagnostic file4 containing information about the velocity and pressure errors is printed out. This file is generated by setting VELOCITY_ERROR_FILE to .TRUE. on the DUMP line. The name of the file is tunnel_demo_pressit.csv. Column 1 of the file is the time step iteration. Column 2 is the pressure iteration within the time step. Column 3 is the total number of pressure iterations for the entire simulation. Columns 4-7 contain the mesh and cell indices where the maximum velocity error occurs, which is listed in column 8. Columns 9-12 contain the mesh and cell indices where the maximum pressure error occurs, which is listed in column 13. Figure 6.9 shows the velocity and pressure errors over a short span of the simulation. For each quantity, there is a default error tolerance which may or may not be reached depending on whether or not the MAX_PRESSURE_ITERATIONS (default 10) has been reached, or whether or not the error is reduced by at least 25 % over the previous iteration. In some situations, the error decreases slowly, and these criteria have been added to avoid excessive iterations that lead to little improvement in accuracy. The errors shown in Fig. 6.9 are relatively large because fires in long, closed enclosures like tunnels can generate large fluctuations in pressure that challenge both the multiple mesh capability and the pressure solving algorithm discussed above. 3 Keep in mind that the pressure equation iterations continue until three criteria are satisfied. The first deals with the decomposition of the pressure term, the second deals with the normal component of velocity at internal solid surfaces, and the third deals with the mismatch of normal velocity components at mesh interfaces. 4 Caution: the velocity error file can be quite large. Use it only for relatively short simulations only. 55 4 FDS0−86−g80cff4e 2 10 Velocity Error (tunnel_demo) Pressure Error (tunnel_demo) 8 1.5 6 Velocity Error Tolerance FDS (Velocity Error) 4 Error (1/s2) Error (m/s) FDS0−86−g80cff4e x 10 Pressure Error Tolerance FDS (Pressure Error) 1 0.5 2 0 3000 3010 3020 3030 Iteration 3040 3050 0 3000 3010 3020 3030 Iteration 3040 3050 Figure 6.9: Reduction in velocity and pressure error due to iteration of the pressure solver. 6.6.3 Parameters Related to the Background Pressure when Breaking Pressure Zones There are two parameters on the PRES line that control iterative procedures related to the coupling of velocity and pressure. One is called RELAXATION_FACTOR and its default value is 1. When there is an error in the normal component of velocity at a solid boundary, this parameter dictates that the correction be applied in 1 time step. If its value were 0.5, the correction would be applied in 2 time steps. A similar parameter is the PRESSURE_RELAX_TIME. It controls the rate at which the pressures in adjacent compartments are brought into equilibrium following a breach. Its default value is 1 s, meaning that equilibrium is achieved in roughly a second. 6.7 Setting Limits: The CLIP Namelist Group (Table 17.2) The algorithms in FDS are designed to work within a certain range of values for density, temperature and mass fraction. To prevent unphysical results, there are bounds placed on these variables to prevent a single spurious value from causing a numerical instability. It also prevents out of range errors from calls to temperature-dependent look-up tables. By default, FDS determines the lowest and highest values of the variables based on your input, but it is not possible in all cases to anticipate just how low or high a given value might be. Thus, on rare occasions you might need to set upper or lower bounds on the density, temperature, or species mass fractions. Temperature and density bounds are input under the namelist group called CLIP. The parameters are listed in Table 17.2. You only need to set these values if you notice that one of them appears to be “cut off” when examining the results in Smokeview. For typical fire scenarios, you need not set these values, but if you anticipate relatively low or high values in an unusual case, take a look at the calculation results to determine if a change in the bounds is needed. It is possible for the species mass fractions to dip slightly below zero or increase slightly above 1. To force the species mass fractions to remain strictly between 0 and 1, set CLIP_MASS_FRACTION to .TRUE. on the MISC line. 56 Chapter 7 Building the Model A considerable amount of work in setting up a calculation lies in specifying the geometry of the space to be modeled and applying boundary conditions to the solid surfaces. The geometry is described in terms of rectangular obstructions that can heat up, burn, conduct heat, etc.; and vents from which air or fuel can be either injected into, or drawn from, the flow domain. A boundary condition needs to be assigned to each obstruction and vent describing its thermal properties. A fire is just one type of boundary condition. This chapter describes how to build the model. 7.1 Bounding Surfaces: The SURF Namelist Group (Table 17.26) Before describing how to build up the geometry, it is first necessary to explain how to describe what these bounding surfaces consist of. SURF is the namelist group that defines the structure of all solid surfaces or openings within or bounding the flow domain. Boundary conditions for obstructions and vents are prescribed by referencing the appropriate SURF line(s) whose parameters are described in this section. The default boundary condition for all solid surfaces is that of a smooth inert wall with the temperature fixed at TMPA, and is referred to as ’INERT’. If only this boundary condition is needed, there is no need to add any SURF lines to the input file. If additional boundary conditions are desired, they are to be listed one boundary condition at a time. Each SURF line consists of an identification string ID=’...’ to allow references to it by an obstruction or vent. Thus, on each OBST and VENT line that are to be described below, the character string SURF_ID=’...’ indicates the ID of the SURF line containing the desired boundary condition parameters. If a particular SURF line is to be applied as the default boundary condition, set DEFAULT=.TRUE. on the SURF line. 7.2 Creating Obstructions: The OBST Namelist Group (Table 17.16) The namelist group OBST contains parameters used to define obstructions. The entire geometry of the model is made up entirely of rectangular solids, each one introduced on a single line in the input file. 7.2.1 Basics Each OBST line contains the coordinates of a rectangular solid within the flow domain. This solid is defined by two points (x1 ,y1 ,z1 ) and (x2 ,y2 ,z2 ) that are entered on the OBST line in terms of the real sextuplet XB. In addition to the coordinates, the boundary conditions for the obstruction can be specified with the parameter SURF_ID, which designates which SURF line (Section 7.1) to apply at the surface of the obstruction. If the obstruction has different properties for its top, sides and bottom, do not specify only one SURF_ID. Instead, 57 use SURF_IDS, an array of three character strings specifying the boundary condition IDs for the top, sides and bottom of the obstruction, respectively. If the default boundary condition is desired, then SURF_ID or SURF_IDS need not be set. However, if at least one of the surface conditions for an obstruction is the inert default, it can be referred to as ’INERT’, but it does not have to be explicitly defined. For example: &SURF ID='FIRE', HRRPUA=1000.0 / &OBST XB=2.3,4.5,1.3,4.8,0.0,9.2, SURF_IDS='FIRE','INERT','INERT' / puts a fire on top of the obstruction. This is a simple way of prescribing a burner. In addition to SURF_ID and SURF_IDS, you can also use the sextuplet SURF_ID6 as follows: &OBST XB=2.3,4.5,1.3,4.8,0.0,9.2, SURF_ID6='FIRE','INERT','HOT','COLD','BLOW','INERT' / where the six surface descriptors refer to the planes x = 2.3, x = 4.5, y = 1.3, y = 4.8, z = 0.0, and z = 9.2, respectively. Note that SURF_ID6 should not be used on the same OBST line as SURF_ID or SURF_IDS. Obstructions may be created or removed during a simulation. See Section 15.4.1 for details. 7.2.2 Thin Obstructions Obstructions can have zero thickness. Often, thin sheets, like a window, form a barrier, but if the numerical mesh is coarse relative to the thickness of the barrier, the obstruction might be unnecessarily large if it is assumed to be one layer of mesh cells thick. All faces of an obstruction are shifted to the closest mesh cell. If the obstruction is very thin, the two faces may be approximated on the same cell face. FDS and Smokeview render this obstruction as a thin sheet, but it is allowed to have thermally thick boundary conditions. This feature is fragile, especially in terms of burning and blowing gas. A thin sheet obstruction can only have one velocity vector on its face, thus a gas cannot be injected reliably from a thin obstruction because whatever is pushed from one side is necessarily pulled from the other. For full functionality, the obstruction should be specified to be at least one mesh cell thick. Thin sheet obstructions work fine as flow barriers, but other features are fragile and should be used with caution. To prevent FDS from allowing thin sheet obstructions, set THICKEN_OBSTRUCTIONS=.TRUE. on the MISC line, or THICKEN=.TRUE. on each OBST line for which the thin sheet assumption is not allowed. Obstructions that are too small relative to the underlying numerical mesh are rejected. Be careful when testing cases on coarse meshes. 7.2.3 Overlapping Obstructions If the faces of two obstructions overlap each other, FDS will choose the surface properties of the obstruction that is specified second in the input file. If you do not want this, add OVERLAY=.FALSE. to the OBST line of the second obstruction, in which case the surface properties of the first obstruction will be applied. The default value of OVERLAY is .TRUE. When obstructions overlap, Smokeview renders both obstructions independently of each other, often leading to an unsightly cross-hatching of the two surface colors where there is an overlap. A simple remedy for this is to “shrink” the obstruction you do not wish to take precedence by slightly by adjusting its coordinates (XB) accordingly. Then, in Smokeview, toggle the “q” key to show the obstructions as you specified them, rather than as FDS rendered them. 58 7.2.4 Preventing Obstruction Removal Obstructions can be protected from the HOLE punching feature. Sometimes it is convenient to create a door or window using a HOLE. For example, suppose a HOLE is punched in a wall to represent a door or window. An obstruction can be defined to fill this hole (presumably to be removed or colored differently or whatever) so long as the phrase PERMIT_HOLE=.FALSE. is included on the OBST line. In general, any obstruction can be made impenetrable to a HOLE using this phrase. By default, PERMIT_HOLE=.TRUE., meaning that an obstruction is assumed to be penetrable unless otherwise directed. Note that if a penetrable obstruction and an impenetrable obstruction overlap, the obstruction with PERMIT_HOLE=.FALSE. should be listed first. If the obstruction is not to be removed or rejected for any reason, set REMOVABLE=.FALSE. This is sometimes needed to stop FDS from removing the obstruction if it is embedded within another, like a door within a wall. In rare cases, you might not want to allow a VENT to be attached to a particular obstruction, in which case set ALLOW_VENT=.FALSE. 7.2.5 Transparent or Outlined Obstructions Obstructions can be made semi-transparent by assigning a TRANSPARENCY on the OBST line. This real parameter ranges from 0 to 1, with 0 being fully transparent. The parameter should always be set along with either COLOR or an RGB triplet. It can also be specified on the appropriate SURF line, along with a color indicator. If you want the obstruction to be invisible, set COLOR=’INVISIBLE’. Obstructions are typically drawn as solids in Smokeview. To draw an outline representation, set OUTLINE equal to .TRUE. 7.2.6 Creating Holes in Obstructions: The HOLE Namelist Group (Table 17.8) The HOLE namelist group defines parameters that carve a hole out of an existing obstruction or set of obstructions. To do this, add lines of the form &HOLE XB=2.0,4.5,1.9,4.8,0.0,9.2 / Any solid mesh cells within the volume 2.0 < x < 4.5, 1.9 < y < 4.8, 0.0 < z < 9.2 are removed. Obstructions intersecting the volume are broken up into smaller blocks. If the hole represents a door or window, a good rule of thumb is to punch more than enough to create the hole. This ensures that the hole is created through the entire obstruction. For example, if the OBST line denotes a wall 0.1 m thick: &OBST XB=1.0,1.1,0.0,5.0,0.0,3.0 / and you want to create a door, add this: &HOLE XB=0.99,1.11,2.0,3.0,0.0,2.0 / The extra centimeter added to the x coordinates of the hole make it clear that the hole is to punch through the entire obstruction. When a HOLE is created, the affected obstruction(s) are either rejected, or created or removed at predetermined times. See Section 15.4.1 for details. To allow a hole to be controlled with either the CTRL or DEVC namelist groups, you will need to add the CTRL_ID or DEVC_ID parameter respectively, to the HOLE line. When the state of the HOLE evaluates to .FALSE., an obstruction will be placed in the HOLE. By default the obstruction filling the HOLE will take the color of the surrounding OBST that the HOLE was punched through. To make the obstruction filling the HOLE a different color than the original obstruction, set the 59 COLOR or integer triplet RGB on the HOLE line (see Section 7.4). If you want the obstruction filling the HOLE to be invisible, then set COLOR=’INVISIBLE’. Additionally, you may use the keyword TRANSPARENCY, real number from 0 to 1, to make the obstruction filling the HOLE transparent. See Section 15.4.1 for an example. If an obstruction is not to be punctured by a HOLE, add PERMIT_HOLE=.FALSE. to the OBST line. Note that a HOLE has no effect on a VENT or a mesh boundary. It only applies to OBSTstructions. It is a good idea to inspect the geometry by running either a setup job (T_END=0 on the TIME line) or a short-time job to test the operation of devices and control functions. 7.3 Applying Surface Properties: The VENT Namelist Group (Table 17.30) Whereas the OBST group is used to specify obstructions within the computational domain, the VENT group (Table 17.30) is used to prescribe planes adjacent to obstructions or external walls. Note that the label VENT is used for historical reasons – this group of parameters has evolved well beyond its initial role as simply allowing for air to be blown into, or sucked out of, the computational domain. 7.3.1 Basics The vents are chosen in a similar manner to the obstructions, with the sextuplet XB denoting a plane abutting a solid surface. Two of the six coordinates must be the same, denoting a plane as opposed to a solid. Note that only one VENT may be specified for any given wall cell. If additional VENT lines are specified for a given wall cell, FDS will output a warning message and ignore redundant VENT lines. The term “VENT” is somewhat misleading. Taken literally, a VENT can be used to model components of the ventilation system in a building, like a diffuser or a return. In these cases, the VENT coordinates form a plane on a solid surface forming the boundary of the duct. No holes need to be created through the solid; it is assumed that air is pushed out of or sucked into duct work within the wall. Less literally, a VENT is used simply as a means of applying a particular boundary condition to a rectangular patch on a solid surface. A fire, for example, is usually created by first generating a solid obstruction via an OBST line, and then specifying a VENT somewhere on one of the faces of the solid with a SURF_ID with the characteristics of the thermal and combustion properties of the fuel. For example, the lines &OBST XB=0.0,5.0,2.0,3.0,0.0,4.0, SURF_ID='big block' / &VENT XB=1.0,2.0,2.0,2.0,1.0,3.0, SURF_ID='hot patch' / specify a large obstruction (with the properties given elsewhere in the file under the name ’big block’) with a “patch” applied to one of its faces with alternative properties under the name ’hot patch’. This latter surface property need not actually be a “vent,” like a supply or return duct, but rather just a patch with different boundary conditions than those assumed for the obstruction. Note that the surface properties of a VENT over-ride those of the underlying obstruction. A VENT must always be attached to a solid obstruction. See Section 9.1 for instructions on specifying different types of fans that allow gases to flow through. An easy way to specify an entire external wall is to replace XB with MB (Mesh Boundary), a character string whose value is one of the following: ’XMAX’, ’XMIN’, ’YMAX’, ’YMIN’, ’ZMAX’ or ’ZMIN’ denoting the planes x = XMAX, x = XMIN, y = YMAX, y = YMIN, z = ZMAX or z = ZMIN, respectively. Like an obstruction, the boundary condition index of a vent is specified with SURF_ID, indicating which of the listed SURF lines to apply. If the default boundary condition is desired, then SURF_ID need not be set. Be careful when using the MB shortcut when doing a multiple mesh simulation; that is, when more than one rectangular mesh is used. The plane designated by the character string MB may be mistakenly applied 60 to more than one mesh, possibly leading to confusion about whether a plane is a solid wall or an open boundary. Check the geometry in Smokeview to assure that the VENTs are properly specified. Use color as much as possible to double-check the set-up. More detail on color in Section 7.4 and Table 7.1. Also, the parameter OUTLINE=.TRUE. on the VENT line causes the VENT to be drawn as an outline in Smokeview. 7.3.2 Special Vents There are three reserved SURF_ID’s that may be applied to a VENT – ’OPEN’, ’MIRROR’, and ’PERIODIC’. The term reserved means that these SURF_IDs should not be explicitly defined by you. Their properties are predefined. Open Vents The first special VENT is invoked by the parameter SURF_ID=’OPEN’. This is used only if the VENT is applied to the exterior boundary of the computational domain, where it denotes a passive opening to the outside. By default, FDS assumes that the exterior boundary of the computational domain (the XBs on the MESH line) is a solid wall. To create a totally or partially open domain, use OPEN vents on the exterior mesh boundaries. It is sometimes convenient to specify doors or windows that open out to the exterior of the computational domain by simply specifying it to be OPEN. However, keep in mind that the pressure boundary condition on such an opening is imperfect, and it is recommended that if the flow through the doorway or window is important, you should extend the domain a few meters rather than use an OPEN boundary. You would still have to use the OPEN boundary to open up one or more sides of the computational domain, but these openings would be far enough away from the modeled door or window that they would not affect the flow pattern. By default, it is assumed that ambient conditions exist beyond the ’OPEN’ vent. However, in some cases, you may want to alter this assumption, for example, the temperature. If you assume a temperature other than ambient, specify TMP_EXTERIOR along with SURF_ID=’OPEN’. You can modify the time history of this parameter using a ramp function, TMP_EXTERIOR_RAMP. Use this option cautiously – in many situations if you want to describe the exterior of a building, it is better to include the exterior explicitly in your calculation because the flow in and out of the doors and windows will be more naturally captured. See Section 9.3.3 for more details. If you want to specify a non-ambient pressure at the OPEN boundary, see Section 9.4. The OPEN pressure boundary condition is most stable for flows that are predominantly normal to the vent, either mostly in or mostly out. This is because the prescribed pressure at an OPEN boundary is illconditioned (a small perturbation to the input may lead to large change in the output) if the flow is parallel to the vent. Suppose, for example, that an outdoor flow is 10 m/s in the x direction and ±0.001 m/s in the z direction with an OPEN top boundary. The kinetic energy of this flow is roughly k = 50 m2 /s2 . When the vertical velocity is positive (+0.001 m/s) then the prescribed boundary condition for the stagnation pressure is set to H = k = 50 m2 /s2 . But when the vertical velocity is negative (-0.001 m/s) then H = 0 (see [19]). For this reason, OPEN vents should be used with care in outdoor applications. See Section 6.4.2 for an alternative approach. Vents to the outside of the computational domain (OPEN vents) can be opened or closed during a simulation. It is best done by creating or removing a thin obstruction that covers the OPEN VENT. See Section 15.4.2 for details. Mirror Vents A VENT with SURF_ID=’MIRROR’ denotes a symmetry plane. Usually, a MIRROR spans an entire face of the computational domain, essentially doubling the size of the domain with the MIRROR acting as a plane 61 of symmetry. The flow on the opposite side of the MIRROR is exactly reversed1 . From a numerical point of view, a MIRROR is a no-flux, free-slip boundary. As with OPEN, a MIRROR can only be prescribed at an exterior boundary of the computational domain. Often, OPEN or MIRROR VENTs are prescribed along an entire side of the computational domain, in which case the “MB” notation is handy. In conventional RANS (Reynolds-Averaged Navier-Stokes) models, symmetry boundaries are often used as a way of saving on computation time. However, because FDS is an LES (Large Eddy Simulation) model, the use of symmetry boundaries should be considered carefully. The reason for this is that an LES model does not compute a time-averaged solution of the N-S equations. In other words, for a RANS model, a fire plume is represented as an axially-symmetric flow field because that is what you would expect if you time-averaged the actual flow field over a sufficient amount of time. Thus, for a RANS model, a symmetry boundary along the plume centerline is appropriate. In an LES model, however, there is no time-averaging built into the equations, and there is no time-averaged, symmetric solution. Putting a MIRROR boundary along the centerline of a fire plume will change its dynamics entirely. It will produce something very much like the flow field of a fire that is adjacent to a vertical wall. For this reason, a MIRROR boundary condition is not recommended along the centerline of a turbulent fire plume. If the fire or burner is very small, and the flow is laminar, then the MIRROR boundary condition makes sense. In fact, in 2-D calculations, MIRROR boundary conditions are employed in the third coordinate direction (this is done automatically, you need not specify it explicitly). Periodic Vents A VENT with SURF_ID=’PERIODIC’ may be used in combination with another periodic vent on the boundary of the domain in any of the three coordinate directions. As an example, consider the following (valid for a single mesh case): &VENT MB='XMIN', SURF_ID='PERIODIC' / &VENT MB='XMAX', SURF_ID='PERIODIC' / In this example, the entire XMIN boundary is periodic with the XMAX boundary. For multi-mesh cases with PERIODIC boundaries the VENT planes must be specified using PBX, PBY, or PBZ. If xmin = 0 and xmax = 1, for example, use &VENT PBX=0, SURF_ID='PERIODIC' / &VENT PBX=1, SURF_ID='PERIODIC' / For additional information related to periodic boundaries and the pressure solver see Section 6.6.1. Periodic vents may not be used to connect offset vents or vents in different coordinate directions. For such cases, you must employ HVAC capabilities (see Section 9.2). Circular Vents Circular or semi-circular vents may be specified as the intersection of a rectangle with coordinates XB and a circle with center XYZ and radius RADIUS. The rectangular surface cells that are assigned the corresponding SURF_ID will be those whose centroid falls within the intersection. In the example case called Fires/circular_burner.fds, the following two lines create a circular vent that is 1 m in diameter and flows propane gas at a rate of 0.02 kg/m2 /s: &SURF ID='BURNER', MASS_FLUX(1)=0.02, SPEC_ID(1)='PROPANE', TAU_MF(1)=0.01 / 1 Note that the mirror image of a scene is not shown in Smokeview. 62 &VENT XB=-0.6,0.6,-0.6,0.6,0,0, XYZ=0,0,0, RADIUS=0.5, SURF_ID='BURNER', SPREAD_RATE=0.05 / The XB coordinates designate the orientation of the vent. In this case, the extent of the area specified by XB is large enough to contain the entire circle. Note also in this example that the parameter SPREAD_RATE causes the fire to spread outward at a rate of 0.05 m/s. The mass flux of propane through the vent is plotted in Fig. 7.1. Notice that the mass flux increases following a “t-squared” profile. This is what is expected of a fire which spreads radially at a linear rate. In this case, the fire reaches the RADIUS of the circle in 10 s, as expected. Note also that the parameter TAU_MF indicates that the fuel should ramp up quickly once the flame front reaches a given grid cell. In other words, TAU_MF controls the local ramp-up of fuel; the SPREAD_RATE controls the global ramp-up. Following the ramp-up, the fuel flows at a rate equal to the area of the circle times the mass flux of fuel per unit area. Even if the circle is crudely resolved on a coarse grid, the fuel flow rate will be adjusted to produce the desired value governed by the circular vent. FDS0−86−g80cff4e 0.02 Burning Rate (kg/s) Burning Rate (circular_burner) 0.015 0.01 0.005 Ideal (Mdot) FDS (MLR_FUEL) 0 0 5 10 Time (s) 15 20 Figure 7.1: Results of the circular_burner test case. 7.3.3 Controlling Vents VENT functionality can be controlled in some cases using “devices” and “controls,” specified via a DEVC_ID or a CTRL_ID. See Section 15.4.2 for details. 7.3.4 Trouble-Shooting Vents Unlike most of the entries in the input file, the order that you specify VENTs can be important. There might be situations where it is convenient to position one VENT atop another. For example, suppose you want to designate the ceiling of a compartment to have a particular set of surface properties, and you designate the entire ceiling to have the appropriate SURF_ID. Then, you want to designate a smaller patch on the ceiling to have another set of surface properties, like an air supply. In this case, you must designate the supply VENT first because for that area of the ceiling, FDS will ignore the ceiling properties and apply the supply properties. FDS processes the first VENT, not the second as it did in versions prior to FDS 5. Now, the rule for VENTs is “first come, first served.” Keep in mind, however, that the second VENT is not rejected entirely – only where there is overlap. FDS will also print out a warning to the screen (or to standard error) saying which VENT has priority. Smokeview can help identify where two VENTs overlap, assuming each has a unique COLOR. Because Smokeview draws VENTs on top of each other, areas of overlap will have a grainy, awkward appearance 63 that changes pattern as you move the scene. In situations where you desire the overlap for the sake of convenience, you might want to slightly adjust the coordinates of the preferred VENT so that it is slightly offset from the solid surface. Make the offset less than about a tenth of a cell dimension so that FDS snaps it to its desired location. Then, by toggling the “q” key in Smokeview, you can eliminate the grainy color overlap by showing the VENT exactly where you specified it, as opposed to where FDS repositioned it. This trick also works where the faces of two obstructions overlap. If an error message appears requesting that the orientation of a vent be specified, first check to make sure that the vent is a plane. If the vent is a plane, then the orientation can be forced by specifying the parameter IOR. If the normal direction of the VENT is in the positive x direction, set IOR=1. If the normal direction is in the negative x direction, set IOR=-1. For the y and z direction, use the number 2 and 3, respectively. Setting IOR may sometimes solve the problem, but it is more likely that if there is an error message about orientation, then the VENT is buried within a solid obstruction, in which case the program cannot determine the direction in which the VENT is facing. 7.4 Coloring Obstructions, Vents, Surfaces and Meshes It is useful when visualizing the results of a simulation to assign to objects a meaningful color or pattern. There are two ways to do this in FDS. You can either assign a single color, or you can assign a texture map, which is essentially an image of a your choosing. 7.4.1 Colors Colors for many items within FDS can be prescribed in two ways; a triplet of integer color values, RGB, or a character string, COLOR. The three RGB integers range from 0 to 255, indicating the amount of Red, Green and Blue that make up the color. If you define the COLOR by name, it is important that you type the name exactly as it is listed in the color tables. Color parameters can be specified on a SURF line, in which case all surfaces of that type will have that color, or color parameters can be applied directly to obstructions or vents. For example, the lines: &SURF ID='UPHOLSTERY', ..., RGB=0,255,0 / &OBST XB=..., COLOR='BLUE' / will color all UPHOLSTERY green and this particular obstruction blue. Table 7.1 provides a small sampling of RGB values and COLOR names for a variety of colors2 . It is highly recommended that colors be assigned to surfaces via the SURF line because as the geometries of FDS simulations become more complex, it is very useful to use color as a spot check to determine if the desired surface properties have been assigned throughout the room or building under study. Obstructions and vents may be colored individually, over-riding the color designated by the SURF line. The special case COLOR=’INVISIBLE’ causes the vent or obstruction not to be drawn by Smokeview. 7.4.2 Texture Maps There are various ways of prescribing the color of various objects within the computational domain, but there is also a way of pasting images onto the obstructions for the purpose of making the Smokeview images more realistic. This technique is known as “texture mapping.” For example, to apply a wood paneling image to a wall, add to the SURF line defining the physical properties of the paneling the line: 2A complete listing of all 500+ colors can be found by searching the FDS source code file data.f90. 64 Table 7.1: A sample of color definitions. Name AQUAMARINE ANTIQUE WHITE BEIGE BLACK BLUE BLUE VIOLET BRICK BROWN BURNT SIENNA BURNT UMBER CADET BLUE CHOCOLATE COBALT CORAL CYAN DIMGRAY EMERALD GREEN FIREBRICK FLESH FOREST GREEN GOLD GOLDENROD GRAY GREEN GREEN YELLOW HONEYDEW HOT PINK INDIAN RED INDIGO IVORY IVORY BLACK KELLY GREEN KHAKI LAVENDER LIME GREEN MAGENTA R 127 250 245 0 0 138 156 165 138 138 95 210 61 255 0 105 0 178 255 34 255 218 128 0 173 240 255 205 75 255 41 0 240 230 50 255 G 255 235 245 0 0 43 102 42 54 51 158 105 89 127 255 105 201 34 125 139 215 165 128 255 255 255 105 92 0 255 36 128 230 230 205 0 B 212 215 220 0 255 226 31 42 15 36 160 30 171 80 255 105 87 34 64 34 0 32 128 0 47 240 180 92 130 240 33 0 140 250 50 255 Name MAROON MELON MIDNIGHT BLUE MINT NAVY OLIVE OLIVE DRAB ORANGE ORANGE RED ORCHID PINK POWDER BLUE PURPLE RASPBERRY RED ROYAL BLUE SALMON SANDY BROWN SEA GREEN SEPIA SIENNA SILVER SKY BLUE SLATEBLUE SLATE GRAY SPRING GREEN STEEL BLUE TAN TEAL THISTLE TOMATO TURQUOISE VIOLET VIOLET RED WHITE YELLOW 65 R 128 227 25 189 0 128 107 255 255 218 255 176 128 135 255 65 250 244 84 94 160 192 135 106 112 0 70 210 0 216 255 64 238 208 255 255 G 0 168 25 252 0 128 142 128 69 112 192 224 0 38 0 105 128 164 255 38 82 192 206 90 128 255 130 180 128 191 99 224 130 32 255 255 B 0 105 112 201 128 0 35 0 0 214 203 230 128 87 0 225 114 96 159 18 45 192 235 205 144 127 180 140 128 216 71 208 238 144 255 0 &SURF ID='wood paneling',..., TEXTURE_MAP='paneling.jpg', TEXTURE_WIDTH=1., TEXTURE_HEIGHT=2. / Assuming that a JPEG file called paneling.jpg exists in the working directory, Smokeview should read it and display the image wherever the paneling is used. Note that the image does not appear when Smokeview is first invoked. It is an option controlled by the Show/Hide menu. The parameters TEXTURE_WIDTH and TEXTURE_HEIGHT are the physical dimensions of the image. In this case, the JPEG image is of a 1 m wide by 2 m high piece of paneling. Smokeview replicates the image as often as necessary to make it appear that the paneling is applied where desired. Consider carefully how the image repeats itself when applied in a scene. If the image has no obvious pattern, there is no problem with the image being repeated. If the image has an obvious direction, the real triplet TEXTURE_ORIGIN should be added to the VENT or OBST line to which a texture map should be applied. For example, &OBST XB=1.,2.,3.,4.,5.,7., SURF_ID='wood paneling', TEXTURE_ORIGIN=1.,3.,5. / applies paneling to an obstruction whose dimensions are 1 m by 1 m by 2 m, such that the image of the paneling is positioned at the point (1,3,5). The default value of TEXTURE_ORIGIN is (0,0,0), and the global default can be changed by added a TEXTURE_ORIGIN statement to the MISC line. 7.5 Repeated Objects: The MULT Namelist Group (Table 17.15) Sometimes obstructions, holes and vents are repeated over and over in the input file. This can be tedious to create and make the input file hard to read. However, if a particular set of objects repeats itself in a regular pattern, you can use a utility known as a multiplier. If you want to repeat an obstruction, for example, create a line in the input file as follows: &MULT ID='m1', DX=1.2, DY=2.4, I_LOWER=-2, I_UPPER=3, J_LOWER=0, J_UPPER=5 / &OBST XB=x1,x2,y1,y2,z1,z2, MULT_ID='m1' / This has the effect of making an array of obstructions according to the following formulae: x1’ = x1 + DX0 + i DX ; I_LOWER ≤ i ≤ I_UPPER x2’ = x2 + DX0 + i DX ; I_LOWER ≤ i ≤ I_UPPER y1’ = y1 + DY0 + j DY ; J_LOWER ≤ j ≤ J_UPPER y2’ = y2 + DY0 + j DY ; J_LOWER ≤ j ≤ J_UPPER z1’ = z1 + DZ0 + k DZ ; K_LOWER ≤ k ≤ K_UPPER z2’ = z2 + DZ0 + k DZ ; K_LOWER ≤ k ≤ K_UPPER In situations where the position of the obstruction needs shifting prior to the multiplication, use the parameters DX0, DY0, and DZ0. A variation of this idea is to replace the parameters, DX, DY, and DZ, with a sextuplet called DXB. The six entries in DXB increment the respective values of the obstruction coordinates given by XB. For example, the x coordinates are transformed as follows: x1’ = x1 + DX0 + n DXB(1) ; N_LOWER ≤ n ≤ N_UPPER x2’ = x2 + DX0 + n DXB(2) ; N_LOWER ≤ n ≤ N_UPPER Notice that we use N_LOWER and N_UPPER to denote the range of N. This more flexible input scheme allows you to create, for example, a slanted roof in which the individual roof segments shorten as they ascend to 66 Figure 7.2: An example of the multiplier function. the top. This feature is demonstrated by the following short input file that creates a hollowed out pyramid using the four perimeter obstructions that form the outline of its base: &HEAD &MESH &TIME &MULT &MULT &MULT &MULT &OBST &OBST &OBST &OBST &MULT &HOLE &TAIL CHID='pyramid', TITLE='Simple demo of multiplier function' / IJK=100,100,100, XB=0.0,1.0,0.0,1.0,0.0,1.0 / T_END=0. / ID='south', DXB=0.01,-.01,0.01,0.01,0.01,0.01, N_LOWER=0, N_UPPER=39 ID='north', DXB=0.01,-.01,-.01,-.01,0.01,0.01, N_LOWER=0, N_UPPER=39 ID='east', DXB=-.01,-.01,0.01,-.01,0.01,0.01, N_LOWER=0, N_UPPER=39 ID='west', DXB=0.01,0.01,0.01,-.01,0.01,0.01, N_LOWER=0, N_UPPER=39 XB=0.10,0.90,0.10,0.11,0.10,0.11, MULT_ID='south', COLOR='RED' / XB=0.10,0.90,0.89,0.90,0.10,0.11, MULT_ID='north', COLOR='BLUE' / XB=0.10,0.11,0.11,0.89,0.10,0.11, MULT_ID='west', COLOR='GREEN' / XB=0.89,0.90,0.11,0.89,0.10,0.11, MULT_ID='east', COLOR='CYAN' / ID='holes', DX=0.15, DZ=0.1, I_UPPER=1, K_UPPER=1 / XB=0.40,0.45,0.00,1.00,0.15,0.20, MULT_ID='holes' / / / / / / The end result of this input file is to create a pyramid by repeating long, rectangular obstructions at the base of each face in a stair-step pattern. Note in this case the use of N_LOWER and N_UPPER which automatically cause FDS to repeat the obstructions in sequence rather than as an array. 67 Note that the MULTiplication functionality works for MESH, OBST, HOLE, VENT, and INIT lines. For a MESH, it only applies to the bounds (XB) of the mesh, not the number of cells. 68 Chapter 8 Fire and Thermal Boundary Conditions This chapter describes how to specify the thermal properties of solid objects. This is the most challenging part of setting up the simulation. Why? First, for both real and simulated fires, the growth of the fire is very sensitive to the thermal properties of the surrounding materials. Second, even if all the material properties are known to some degree, the physical phenomena of interest may not be simulated properly due to limitations in the model algorithms or resolution of the numerical mesh. It is your responsibility to supply the thermal properties of the materials, and then assess the performance of the model to ensure that the phenomena of interest are being captured. 8.1 Basics By default, the outer boundary of the computational domain is assumed to be a solid boundary that is maintained at ambient temperature. The same is true for any obstructions that are added to the scene. To specify the properties of solids, use the namelist group SURF (Section 7.1). Solids are assumed to consist of layers that can be made of different materials. The properties of each material required are designated via the MATL namelist group (Section 8.3). These properties indicate how rapidly the materials heat up, and how they burn. Each MATL entry in the input file must have an ID, or name, so that they may be associated with a particular SURF via the parameter MATL_ID. For example, the input file entries: &MATL ID='BRICK', CONDUCTIVITY=0.69, SPECIFIC_HEAT=0.84, DENSITY=1600. / &SURF ID='BRICK WALL', MATL_ID='BRICK', COLOR='RED', BACKING='EXPOSED', THICKNESS=0.20 / &OBST XB=0.1,5.0,1.0,1.2,0.0,1.0, SURF_ID='BRICK WALL' / define a brick wall that is 4.9 m long, 1 m high, and 20 cm thick. Note that the thickness of the wall indicated by the OBST line is independent of the THICKNESS specified by the SURF line. The OBST line defines the geometry of the obstruction (i.e., how the obstruction is seen by the flow solver). The SURF line defines the heat transfer characteristics of the obstruction (i.e., how the obstruction is seen by the 1D solid phase solver). This allows an obstruction to snap to the local grid but still have the heat transfer solution reflect the actual thickness. 69 8.2 Surface Temperature and Heat Flux This section describes how to specify simple thermal boundary conditions. These are often used when there is little or no information about the properties of the solid materials. If the properties of the materials are known, it is better to specify these properties and let the model compute the heat flux to, and temperature of, the walls and other solid surfaces. 8.2.1 Specified Solid Surface Temperature Usually, the thermal properties of a solid boundary are specified via the MATL namelist group, which is in turn invoked by the SURF entry via the character string MATL_ID. However, sometimes it is convenient to specify a fixed temperature boundary condition, in which case set TMP_FRONT to be the surface temperature in units of ◦ C: &SURF ID='HOT WALL', COLOR='RED', TMP_FRONT=200. / Note that there is no need to specify a MATL_ID or THICKNESS. Because the wall is to be maintained at the given temperature, there is no need to say anything about its material composition or thickness. 8.2.2 Special Topic: Convective Heat Transfer Options This section is labeled as a special topic because normally you do not need to modify the convective heat transfer model in FDS. However, there are special cases for which the default model may not be adequate, and this section describes some options. Default Convective Heat Transfer Model In an LES calculation, the convective heat transfer coefficient, h, is based on a combination of natural and forced convection correlations: 1 k 00 2 q̇c = h (Tg − Tw ) W/m ; h = max C |Tg − Tw | 3 , Nu W/(m2 · K) (8.1) L where C is a empirical coefficient for natural convection (1.52 for a horizontal plate and 1.31 for a vertical plane or cylinder) [21], L is a characteristic length related to the size of the physical obstruction, and k is the thermal conductivity of the gas. The Nusselt number (Nu) depends on the geometric and flow characteristics. For many flow regimes, it has the form: Nu = C1 +C2 Ren Prm ; Re = ρ|u|L µ ; Pr = 0.7 (8.2) For planar surfaces, the default values are C1 = 0, C2 = 0.037, n = 0.8, m = 0.33, and L = 1 m. For cylindrical surfaces, the default values are C1 = 0, C2 = 0.683, n = 0.466, m = 0.33, and L = D, the diameter of the cylinder. For spherical surfaces, the default values are C1 = 2, C2 = 0.6, n = 0.5, m = 0.33, and L = D, the diameter of the sphere. Note that for a sphere, the coefficient for natural convection, C, is assumed to be zero. It is possible to change these values for a particular application, but it is not possible to find a set of parameters that is appropriate for the wide variety of scenarios considered. Various correlations for planes, cylinders, and spheres can be found in Refs. [21, 22]. You can change the values of the empirical coefficient for natural convection, C, by specifying C_HORIZONTAL and C_VERTICAL on the SURF line. The length scale, L, is specified by CONVECTION_LENGTH_SCALE on the SURF line. You can change the empirical coefficients for the forced 70 convection model by using C_FORCED_CONSTANT, C_FORCED_RE, C_FORCED_RE_EXP, and C_FORCED_PR_EXP for the constants C1 , C2 , n, and m in the Nusselt number correlation, all of which are input on the SURF line. Logarithmic Law of the Wall Near-wall treatments, such as wall models or wall functions, aim to mimic the sudden change from molecular to turbulent transport close to the walls using algebraic formulations without the need of resolving the otherwise computationally expensive region of flow-field. The main theory follows dimensional analysis based on the idea that shear at the wall is constant. Accordingly, the non-dimensional velocity u+ is calculated using a wall function [19]. By analogy, we define the non-dimensional temperature T + ≡ (Tg − Tw )/Tτ , where Tg is the gas temperature of the first off-wall grid cell and Tτ is defined with the wall heat flux, q̇00w , as Tτ = q̇00w /ρw uτ cp . The local heat transfer coefficient is then obtained from h= ρw cp uτ q̇00w = Tg − Tw T+ (8.3) Refer to the FDS Tech Guide [19] for further details of the formulation. To specify this heat transfer model for a particular surface, set HEAT_TRANSFER_MODEL equal to ’LOGLAW’ on the SURF line. Specified Convective Heat Transfer Coefficient If you want to specify the convective heat transfer coefficient, you can set it to a constant using HEAT_TRANSFER_COEFFICIENT on the SURF line in units of W/(m2 · K). Specifying the Heat Flux at a Solid Surface Instead of altering the convective heat transfer coefficient, you may specify a fixed heat flux directly. Two methods are available to do this. The first is to specify a NET_HEAT_FLUX in units of kW/m2 . When this is specified FDS will compute the surface temperature required to ensure that the combined radiative and convective heat flux from the surface is equal to the NET_HEAT_FLUX. The second method is to specify separately the CONVECTIVE_HEAT_FLUX, in units of kW/m2 , and the radiative heat flux. The radiative heat flux is specified by setting both TMP_FRONT and EMISSIVITY appropriately on the SURF line. Note that if you wish there to be only a convective heat flux from a surface, then the EMISSIVITY should be set to zero. If NET_HEAT_FLUX or CONVECTIVE_HEAT_FLUX is positive, the wall heats up the surrounding gases. If NET_HEAT_FLUX or CONVECTIVE_HEAT_FLUX is negative, the wall cools the surrounding gases. 8.2.3 Special Topic: Adiabatic Surfaces For some special applications, it is often desired that a solid surface be adiabatic, that is, there is no net heat transfer (radiative and convective) from the gas to the solid. For this case, all that must be prescribed on the SURF line is ADIABATIC=.TRUE., and nothing else. FDS will compute a wall temperature so that the sum of the net convective and radiative heat flux is zero. Specifying a surface as ADIABATIC will result in FDS defining NET_HEAT_FLUX=0 and EMISSIVITY=1. No solid surface is truly adiabatic; thus, the specification of an adiabatic boundary condition should be used for diagnostic purposes only. 71 8.3 Heat Conduction in Solids Specified temperature or heat flux boundary conditions are easy to apply, but only of limited usefulness in real fire scenarios. In most cases, walls, ceilings and floors are made up of several layers of lining materials. The MATL namelist group is used to define the properties of the materials that make up boundary solid surfaces. A solid boundary can consist of multiple layers1 of different materials, and each layer can consist of multiple material components. 8.3.1 Structure of Solid Boundaries Material layers and components are specified on the SURF line via the array called MATL_ID(IL,IC). The argument IL is an integer indicating the layer index, starting at 1, the layer at the exterior boundary. The argument IC is an integer indicating the component index. For example, MATL_ID(2,3)=’BRICK’ indicates that the third material component of the second layer is BRICK. In practice, the materials are often listed as in the following example: &MATL ID CONDUCTIVITY SPECIFIC_HEAT DENSITY &SURF ID MATL_ID COLOR BACKING THICKNESS = = = = = = = = = 'INSULATOR' 0.041 2.09 229. / 'BRICK WALL' 'BRICK','INSULATOR' 'RED' 'EXPOSED' 0.20,0.10 / Without arguments, the parameter MATL_ID is assumed to be a list of the materials in multiple layers, with each layer consisting of only a single material component. When a set of SURF parameters is applied to the face of an OBST, the first MATL_ID defines the first layer of solid material. The other MATL_IDs are applied in succession. If BACKING=’EXPOSED’, the last MATL_ID is applied to the opposite face of the OBST, assuming that the OBST is zero or one grid cells thick. If the OBST is thicker than one grid cell, then BACKING=’EXPOSED’ is not defined and it will be treated as if the condition BACKING=’VOID’ was set. If in the example above, BRICK WALL was applied to the entire OBST using SURF_ID, then when doing a heat transfer calculation from the +x face to the −x face, FDS would consider the OBST to be BRICK followed by INSULATOR and the same for a heat transfer calculation from the −x face to the +x face. To avoid this, specify a second SURF that has the reverse MATL_ID and use SURF_ID6 to apply the two SURF definitions to opposite faces of the OBST. Mixtures of solid materials within the same layer can be defined using the MATL_MASS_FRACTION keyword. This parameter has the same two indices as the MATL_ID keyword. For example, if the brick layer contains some additional water, the input could look like this: &MATL ID CONDUCTIVITY SPECIFIC_HEAT DENSITY = = = = 'WATER' 0.60 4.19 1000. / &SURF ID = 'BRICK WALL' MATL_ID(1,1:2) = 'BRICK','WATER' MATL_MASS_FRACTION(1,1:2) = 0.95,0.05 1 The maximum number of material layers is 20. The maximum number of material components is 20. 72 MATL_ID(2,1) COLOR BACKING THICKNESS = = = = 'INSULATOR' 'RED' 'EXPOSED' 0.20,0.10 / <--- for layers 1 and 2 In this example, the first layer of material, Layer 1, is composed of a mixture of brick and water. This is given by the MATL_ID array which specifies Component 1 of Layer 1 to be brick, and Component 2 of Layer 1 to be water. The mass fraction of each is specified via MATL_MASS_FRACTION. In this case, brick is 95 %, by mass, of Layer 1, and water is 5 %. It is important to notice that the components of the solid mixtures are treated as pure substances with no voids. The density of the mixture is !−1 Yi ρ= ∑ (8.4) i ρi where Yi are the material mass fractions and ρi are the material bulk densities defined on the MATL lines. In the example above, the resulting density of the wall would be about 1553 kg/m3 . The fact that the wall density is smaller than the density of pure brick may be confusing, but can be explained easily. If the wall can contain water, the whole volume of the wall can not be pure brick. Instead there are voids (pores) that are filled with water. If the water is taken away, there is only about 1476 kg/m3 of brick left. To have a density of 1600 kg/m3 for a partially void wall, a higher density should be used for the pure brick. 8.3.2 Thermal Properties For any solid material, specify its thermal CONDUCTIVITY (W/(m · K)), DENSITY (kg/m3 ), SPECIFIC_HEAT (kJ/(kg · K)), and EMISSIVITY (0.9 by default). Both CONDUCTIVITY and SPECIFIC_HEAT can be functions of temperature. DENSITY and EMISSIVITY cannot. Temperature-dependence is specified using the RAMP convention. As an example, consider marinite, a wall material suitable for high temperature applications: &MATL ID = EMISSIVITY = DENSITY = SPECIFIC_HEAT_RAMP = CONDUCTIVITY_RAMP = &RAMP ID='k_ramp', T= 24., &RAMP ID='k_ramp', T=149., &RAMP ID='k_ramp', T=538., &RAMP ID='c_ramp', T= 93., &RAMP ID='c_ramp', T=205., &RAMP ID='c_ramp', T=316., &RAMP ID='c_ramp', T=425., 'MARINITE' 0.8 737. 'c_ramp' 'k_ramp' / F=0.13 / F=0.12 / F=0.12 / F=1.172 / F=1.255 / F=1.339 / F=1.423 / Notice that with temperature-dependent quantities, the RAMP parameter T means Temperature, and F is the value of either the specific heat or conductivity. In this case, neither CONDUCTIVITY nor SPECIFIC_HEAT is given on the MATL line, but rather the RAMP names. The solid material can be given an ABSORPTION_COEFFICIENT (1/m) that allows the radiation to penetrate and absorb into the solid. Correspondingly, the emission of the material is based on the internal temperatures, not just the surface. 73 8.3.3 Back Side Boundary Conditions The layers of a solid boundary are listed in order from the surface. By default, if the obstruction is less than or equal to one cell thick, then the innermost layer will be exposed to the air temperature on the back side. If the obstruction is on the boundary of the domain or is more than one cell thick, it is assumed to back up to an air gap at ambient temperature. For example a thin steel plate (i.e. thickness less than or equal to the grid) would use the FDS predicted temperatures on either side of the plate for predicting heat transfer. There are other back side boundary conditions that can be applied. One is to assume that the wall backs up to an insulated material in which case no heat is lost to the backing material. The expression BACKING=’INSULATED’ on the SURF line prevents any heat loss from the back side of the material. Use of this condition means that you do not have to specify properties of the inner insulating material because it is assumed to be perfectly insulated. If the wall is assumed to back up to the room on the other side of the wall and you want FDS to calculate the heat transfer through the wall into the space behind the wall, the attribute BACKING=’EXPOSED’ should be listed on the SURF line. This feature only works if the wall is less than or equal to one mesh cell thick, and if there is a non-zero volume of computational domain on the other side of the wall. Obviously, if the wall is an external boundary of the domain, the heat is lost to an ambient temperature void. The same happens if the back side gas cell cannot be found (in which case, the wall would not be one cell thick). This is the default boundary conditions. If the wall is assumed to always back up to the ambient, then the attribute BACKING=’VOID’ should be set. The back side emissivity of the surface can be controlled by specifying EMISSIVITY_BACK on the SURF line. If not specified, the back side emissivity will be calculated during the simulations as a mass-weighted sum of the MATL emissivities. 8.3.4 Initial and Back Side Temperature By default, the initial temperature of the solid material is set to ambient (TMPA on the MISC line). Use TMP_INNER on the SURF line to specify a different initial temperature of the solid. The layers of the surface can have different initial temperatures. Also, the back side temperature boundary condition of a solid can be set using the parameter TMP_BACK on the SURF line. TMP_BACK is not the actual back side surface temperature, but rather the gas temperature to which the back side surface is exposed. This parameter has no meaning for surfaces with BACKING=’EXPOSED’ or BACKING=’INSULATED’. As an alternative to TMP_INNER one can also use RAMP_T_I to specify the name of a RAMP containing a depth vs. temperature profile for the surface. Note that the parameters TMP_INNER and TMP_BACK are only meaningful for solids with specified THICKNESS and material properties (via the MATL_ID keyword). 8.3.5 Walls with Different Materials Front and Back If you have an OBST that is one cell thick with gas cells on both sides (i.e., the obstruction is not at the edge of the domain) and you apply the attribute BACKING=’EXPOSED’, then FDS calculates the heat conduction through the entire THICKNESS, and it uses the gas phase temperature and heat flux on the front and back sides for boundary conditions. A redundant calculation is performed on the opposite side of the obstruction. FDS always applies a SURF to an obstruction by having the first layer be the exposed surface of the face and the last layer as the opposite face. Take for example the SURF definition below and assume that the grid spacing is 10 cm. On the -x side of the OBST, layer 1 will be MATERIAL A, layer 2 will be MATERIAL B, and layer 3 will be the last MATERIAL A. On the +x side the SURF will be applied in the same manner. 74 &OBST XB=0.1,0.2,...., SURF_ID='SYMMETRIC'/ &SURF ID = 'SYMMETRIC' COLOR = 'ANTIQUE WHITE' BACKING = 'EXPOSED' MATL_ID(1:3,1) = 'MATERIAL A','MATERIAL B','MATERIAL A' THICKNESS(1:3) = 0.1,0.2,0.1 / For example, take the SURF definition below and assume that the grid spacing is 10 cm. On the -x side of the OBST, layer 1 will be MATERIAL A, layer 2 will be MATERIAL B, and layer 3 will be MATERIAL C. On the +x side, the SURF will be applied in the same manner, layer 1 will be MATERIAL A, layer 2 will be MATERIAL B, and layer 3 will be MATERIAL C. This means that both sides of the OBST will compute heat transfer assuming MATERIAL A is the first layer. &OBST XB=0.1,0.2,...., SURF_ID='NON-SYMMETRIC'/ &SURF ID = 'NON-SYMMETRIC' COLOR = 'ANTIQUE WHITE' BACKING = 'EXPOSED' MATL_ID(1:3,1) = 'MATERIAL A','MATERIAL B','MATERIAL C' THICKNESS(1:3) = 0.1,0.2,0.1 / Therefore, if you apply the attribute BACKING=’EXPOSED’ on a SURF line that is applied to a zero or one-cell thick obstruction, you should be careful of how you specify multiple layers. If the layering is symmetric, the same SURF line can be applied to both sides. However, if the layering is not symmetric, you must create two separate SURF lines and apply one to each side. For example, a hollow box column that is made of steel and covered on the outside by a layer of insulation material and a layer of plastic on top of the insulation material, would have to be described with two SURF lines like the following: &SURF ID COLOR BACKING MATL_ID(1:3,1) THICKNESS(1:3) = = = = = 'COLUMN EXTERIOR' 'ANTIQUE WHITE' 'EXPOSED' 'PLASTIC','INSULATION','STEEL' 0.002,0.036,0.0063 / &SURF ID COLOR BACKING MATL_ID(1:3,1) THICKNESS(1:3) = = = = = 'COLUMN INTERIOR' 'BLACK' 'EXPOSED' 'STEEL','INSULATION','PLASTIC' 0.0063,0.036,0.002 / If, in addition, the insulation material and plastic are combustible, and their burning properties are specified on the appropriate MATL lines, then you need to indicate which side of the column would generate the fuel vapor. In this case, the steel is impermeable; thus you should add the parameter LAYER_DIVIDE=2.0 to the SURF line labeled ’COLUMN EXTERIOR’ to indicate that fuel vapors formed by the heating of the two first layers (’PLASTIC’ and ’INSULATION’) are to be driven out of that surface. You need to also specify LAYER_DIVIDE=0.0 on the SURF line labeled ’COLUMN INTERIOR’ to indicate that no fuel vapors are to driven into the interior of the column. In fact, values from 0.0 to 1.0 would work equally because the material ’STEEL’ would not generate any fuel vapors. By default, LAYER_DIVIDE is 0.5 times the number of layers for surfaces with EXPOSED backing, and equal to the number of layers for other surfaces. 75 8.3.6 Special Topic: Specified Internal Heat Source The condensed phase heat conduction equation has a source term that describes the internal sources and sinks of energy. There are three types of sources that contribute to this term: heats of reaction for the pyrolysis (see Section 8.5), internal absorption and emission of radiation (see Section 8.5.7), and the source specified by the user. An example of the case where specified heat source could be needed is the heating of electrical cables due to internal current. You can specify the internal source term for each layer of the surface using INTERNAL_HEAT_SOURCE on the SURF line. Its units are kW/m3 and the default value is zero. In the example below, the cylindrical surface describing a cable consists of an outer plastic layer and inner core of metal. The metal core is heated with a power of 300 kW/m3 . &SURF ID THICKNESS MATL_ID(1,1) MATL_ID(2,1) GEOMETRY LENGTH INTERNAL_HEAT_SOURCE 8.3.7 = = = = = = = 'Cable' 0.002,0.008 'PLASTIC' 'METAL' 'CYLINDRICAL' 0.1 0.,300. / Special Topic: Non-Planar Walls and Targets All obstructions in FDS are assumed to conform to the rectilinear mesh, and all bounding surfaces are assumed to be flat planes. However, many objects, like cables, pipes, and ducts, are not flat. Even though these objects have to be represented in FDS as “boxes,” you can still perform the internal heat transfer calculation as if the object were really cylindrical or spherical. For example, the input lines: &OBST XB=0.0,5.0,1.1,1.2,3.4,3.5, SURF_ID='CABLE' / &SURF ID='CABLE', MATL_ID='PVC', GEOMETRY='CYLINDRICAL', THICKNESS=0.01 / can be used to model a power cable that is 5 m long, cylindrical in cross section, 2 cm in diameter. The heat transfer calculation is still one-dimensional; that is, it is assumed that there is a uniform heat flux all about the object. This can be somewhat confusing because the cable is represented as an obstruction of square cross section, with a separate heat transfer calculation performed at each face, and no communication among the four faces. Obviously, this is not an ideal way to do solid phase heat transfer, but it does provide a reasonable bounding surface temperature for the gas phase calculation. More detailed assessment of a cable would require a two or three-dimensional heat conduction calculation, which is not included in FDS. Use GEOMETRY=’SPHERICAL’ to describe a spherical object. 8.3.8 Special Topic: Solid Phase Numerical Gridding Issues To compute the temperature and reactions inside the solids, FDS solves the one-dimensional heat transfer equation numerically. The size of the mesh cells on the surface of the solid is automatically chosen using a rule that makes the cell size smaller than the square root of the material diffusivity (k/ρc). By default, the solid mesh cells increase towards the middle of the material layer and are smallest on the layer boundaries. The default parameters are usually appropriate for simple heat transfer calculations but sometimes the use of pyrolysis reactions makes the temperatures and burning rate fluctuate. Adjustments may also be needed in case of extremely transient heat transfer situations. The numerical accuracy and stability of the solid phase solution may be improved by one of the following methods: 76 Make the mesh density more uniform inside the material by setting STRETCH_FACTOR(NL)=1. on the SURF line. This will generate a perfectly uniform mesh for layer number NL. (This happens automatically if the layer contains one or more reacting materials.) Values between 1 and 2 give different levels of stretching. Note that STRETCH_FACTOR needs to be specified for all the layers. Make the mesh cells smaller by setting CELL_SIZE_FACTOR less than 1.0. For example, a value of 0.5 makes the mesh cells half the size. The scaling always applies to all layers. Improve the time resolution by setting WALL_INCREMENT=1 on the TIME line. This forces the solid phase temperatures to be solved on every time step. Limit the number of cells in a layer by setting N_LAYER_CELLS_MAX. This array input has a default of 1000. If all the material components of the surface are reacting, and the pyrolysis reactions have no solid residue, the thickness of the surface is going to shrink when the surface reacts. Each of the shrinking layers will vanish from the computation when its thickness gets smaller than a prescribed limiting value. This value can be set on a SURF line via MINIMUM_LAYER_THICKNESS keyword, defaulting to 1 × 10−6 m. When all the material of a shrinking surface is consumed but BURN_AWAY is not prescribed, the surface temperature is set to TMP_BACK, convective heat flux to zero and burning rate to zero. See Section 8.6 for ways to check and improve the accuracy of the solid phase calculation. 8.4 Simple Pyrolysis Models FDS has several approaches for describing the pyrolysis of solids and liquids. The approach to take depends largely on the availability of material properties and the appropriateness of the underlying pyrolysis model. Note that all pyrolysis models in FDS require that you explicitly define the gas phase reaction. See Chapter 12 for details. 8.4.1 A Gas Burner with a Specified Heat Release Rate Solids and liquid fuels can be modeled by specifying their relevant properties via the MATL namelist group. However, if you simply want to specify a fire of a given heat release rate (HRR), you need not specify any material properties. A specified fire is basically modeled as the ejection of gaseous fuel from a solid surface or vent. This is essentially a burner, with a specified Heat Release Rate Per Unit Area, HRRPUA, in units of kW/m2 . For example &SURF ID='FIRE', HRRPUA=500. / applies 500 kW/m2 to any surface with the attribute SURF_ID=’FIRE’. See the discussion of time-dependent quantities in Chapter 10 to learn how to ramp the heat release rate up and down. An alternative to HRRPUA with the exact same functionality is MLRPUA, except this parameter specifies the Mass Loss Rate of fuel gas Per Unit Area in kg/(m2 · s). Do not specify both HRRPUA and MLRPUA on the same SURF line. Neither of them can be used if the model contains multiple reactions. 8.4.2 Special Topic: A Radially-Spreading Fire Sometimes it is desired that a fire spread radially at some specified rate. Rather than trying to obtain material properties to directly model the ignition and spread of the fire, you can specify the fire spread rate directly. First, you need to add a SURF line with a specified heat release rate, HRRPUA, and an optional time history 77 parameter, RAMP_Q or TAU_Q (see Section 10.1). Then, you must specify XYZ and SPREAD_RATE on either a VENT or the same SURF line. The fire is directed to start at the point XYZ and spread radially at a rate of SPREAD_RATE (m/s). The optional ramp-up of the HRR begins at the time when the fire arrives at a given point. For example, the lines &SURF &RAMP &RAMP &RAMP &RAMP &VENT ID='FIRE', HRRPUA=500.0, RAMP_Q='fireramp' / ID='fireramp', T= 0.0, F=0.0 / ID='fireramp', T= 1.0, F=1.0 / ID='fireramp', T=30.0, F=1.0 / ID='fireramp', T=31.0, F=0.0 / XB=0.0,5.0,1.5,9.5,0.0,0.0, SURF_ID='FIRE', XYZ=1.5,4.0,0.0, SPREAD_RATE=0.03 / create a rectangular area via the VENT line on which the fire starts at the point (1.5,4.0,0.0) and spreads outwards at a rate of 0.03 m/s. Each surface cell burns for 30 s as the fire spreads outward, creating a widening ring of fire. Note that the RAMP_Q is used to turn the burning on and off to simulate the consumption of fuel as the fire spreads radially. It should not be used to mimic a t-squared fire growth rate – the whole point of the exercise is to mimic this curve in a more natural way. Eventually, the fire goes out as the ring grows past the boundary of the rectangle. Some trial and error is probably required to find the SPREAD_RATE that leads to a desired time history of the heat release rate. If you desire that the fire spread over an area that is not confined to a flat plane, specify XYZ and SPREAD_RATE on the SURF line directly and then apply that SURF line to the obstructions or particles over which you want the fire to spread. This technique can be useful for simulating the spread of fire through a cluttered space when the detailed properties of the materials are unknown, or when the uncertainties associated with modeling the pyrolysis of the solid fuels directly are too great. If the starting time of the simulation, T_BEGIN, is not zero, be aware that the default start time of the radially spreading fire is T_BEGIN, not zero. This is also true of TAU_Q, but it is not true of RAMP_Q. Because this might be confusing, if you start the calculation at a time other than zero, do a quick test to ensure that the ramps or fire spread behave as expected. 8.4.3 Solid Fuels that Burn at a Specified Rate Real objects, like furnishings, office equipment, and so on, are often difficult to describe via the SURF and MATL parameters. Sometimes the only information about a given object is its bulk thermal properties, its “ignition” temperature, and its subsequent burning rate as a function of time from ignition. For this situation, add lines similar to the following: &MATL ID CONDUCTIVITY SPECIFIC_HEAT DENSITY = = = = 'stuff' 0.1 1.0 900.0 / &SURF ID COLOR MATL_ID HRRPUA IGNITION_TEMPERATURE RAMP_Q THICKNESS = = = = = = = 'my surface' 'GREEN' 'stuff' 1000. 500. 'fire_ramp' 0.01 / &RAMP ID='fire_ramp', T= 0.0, F=0.0 / &RAMP ID='fire_ramp', T= 10.0, F=1.0 / &RAMP ID='fire_ramp', T=310.0, F=1.0 / 78 &RAMP ID='fire_ramp', T=320.0, F=0.0 / An object with surface properties defined by ’my surface’ shall burn at a rate of 1000 kW/m2 after a linear ramp-up of 10 s following its “ignition” when its surface temperature reaches 500 ◦ C. Burning shall continue for 5 min, and then ramp-down in 10 s. Note that the time T in the RAMP means time from ignition, not the time from the beginning of the simulation. Note also that now the “ignition temperature” is a surface property, not material property. After the surface has ignited, the heat transfer into the solid is still calculated, but there is no coupling between the burning rate and the surface temperature. As a result, the surface temperature may increase too much. To account for the energy loss due to the vaporization of the solid fuel, HEAT_OF_VAPORIZATION can be specified for the surface. For example, when using the lines below, the net heat flux at the material surface is reduced by a factor 1000 kJ/kg times the instantaneous burning rate. &SURF ID COLOR MATL_ID HRRPUA IGNITION_TEMPERATURE HEAT_OF_VAPORIZATION RAMP_Q THICKNESS = = = = = = = = 'my surface' 'GREEN' 'stuff' 1000. 500. 1000. 'fire_ramp' 0.01 / The parameters HRRPUA, IGNITION_TEMPERATURE, and HEAT_OF_VAPORIZATION are all telling FDS that you want to control the burning rate yourself, but you still want to simulate the heating up and “ignition” of the fuel. When these parameters appear on the SURF line, they are acting in concert. If HRRPUA appears alone, the surface will begin burning at the start of the simulation, like a piloted burner. The addition of an IGNITION_TEMPERATURE delays burning until your specified temperature is reached. The addition of HEAT_OF_VAPORIZATION tells FDS to account for the energy used to vaporize the fuel. For any of these options, if a MATL line is invoked by a SURF line containing a specified HRRPUA, then that MATL ought to have only thermal properties. The MATL line should have no reaction parameters, product yields, and so on, like those described in the previous sections. By specifying HRRPUA, you are controlling the burning rate rather than letting the material pyrolyze based on the conditions of the surrounding environment. 8.5 Complex Pyrolysis Models This section describes the parameters that describe the reactions that occur within solid materials when they are burning. It is strongly recommended before reading this section that you read some background material on solid phase pyrolysis, for example “Thermal Decomposition of Polymers,” by Hirschler and Morgan, or “Flaming Ignition of Solid Fuels,” by Torero, both of which are in the 4th edition of the SFPE Handbook of Fire Protection Engineering. 8.5.1 Reaction Mechanism A solid surface in FDS may consist of multiple layers with multiple material components per layer. The material components are described via MATL lines and are specified on the SURF line that describes the structure of the solid. Each MATL can undergo several reactions that may occur at different temperatures. It may not undergo any – it may just heat up. However, if it is to change form via one or more reactions, designate the number of reactions with the integer N_REACTIONS. It is very important that you designate N_REACTIONS or else FDS will ignore all parameters associated with reactions. Note that experimental evidence of multi79 ple reactions does not imply that a single material is undergoing multiple reactions, but rather that multiple material components are undergoing individual reactions at distinct temperatures. Currently, the maximum number of reactions for each material is 10 and the chain of consecutive reactions may contain up to 20 steps. For a given material, the jth reaction can produce other solid materials whose names are designated with MATL_ID(i,j), and gas species whose names are designated with SPEC_ID(i,j). Note that the index, i, runs from 1 to the number of material or gaseous species. This index does not correspond to the order in which the MATL or SPEC lines are listed in the input file. For a given reaction, the relative amounts of solid or gaseous products are input to FDS via the yields: NU_MATL(i,j) and NU_SPEC(i,j), respectively. The yields are all zero by default. If NU_MATL(i,j) or NU_SPEC(i,j) is non-zero, then you must indicate what the solid residue is via MATL_ID(j), the ID of another MATL that is also listed in the input file. Ideally, the sum of the yields should add to 1, meaning that the mass of the reactant is conserved. However, there are times when it is convenient to have the yields sum to something less than one. For example, the spalling or ablation of concrete can be described as a “reaction” that consumes energy but does not produce any “product” because the concrete is assumed to have either fallen off the surface in chunks or pulverized powder. The concrete’s mass is not conserved in the model because it has essentially disappeared from that particular surface. For consistency, the HEAT_OF_COMBUSTION(j) can also be specified for each reaction, j. These values are used only if the corresponding heats of combustion for the gaseous species are greater than zero. In the example below, the pyrolysis of wood is included within a simulation that uses a finite-rate reaction instead of the default mixing-controlled model. Notice in this case that all of the gas species (except for the background nitrogen) are explicitly defined, and as a result, FDS needs to be told explicitly what gaseous species are produced by the solid phase reactions. In this case, 82 % of the mass of wood is converted to gaseous ’PYROLYZATE’ and 18 % is converted to solid ’CHAR’. &SPEC &SPEC &SPEC &SPEC ID ID ID ID = = = = 'PYROLYZATE', MW=53.6 / 'OXYGEN', MASS_FRACTION_0 = 0.23 / 'WATER VAPOR' / 'CARBON DIOXIDE' / &MATL ID EMISSIVITY CONDUCTIVITY SPECIFIC_HEAT DENSITY N_REACTIONS A(1) E(1) N_S(1) MATL_ID(1,1) NU_MATL(1,1) SPEC_ID(1:4,1) NU_SPEC(1:4,1) HEAT_OF_REACTION(1) HEAT_OF_COMBUSTION(1) = = = = = = = = = = = = = = = 'WOOD' 0.9 0.2 1.3 570. 1 1.89E10 1.51E5 1.0 'CHAR' 0.18 'PYROLYZATE','OXYGEN','WATER VAPOR','CARBON DIOXIDE' 0.82,0,0,0 430. 14500. / Note that the indices associated with the parameters are not needed in this case, but they are shown to emphasize that, in general, there can be multiple reactions with corresponding kinetic parameters and products. 80 8.5.2 Reaction Rates For each reaction that each material component undergoes you must specify kinetic parameters of the reaction rate. The general evolution equation for a material undergoing one or more reactions is: N N Nm r,i r,i dYs,i = − ∑ ri j + ∑ ∑ νs,i0 j ri0 j dt j=1 i0 =1 j=1 where ri j = n Ai j Ys,is,i j 0 Ei j nO ,i j XO2 2 exp − R Ts ; (i0 6= i) Ys,i = ρs,i ρs (0) (8.5) (8.6) The term, ri j , defines the rate of reaction at the temperature, Ts , of the ith material undergoing its jth reaction. The second term on the right of the equation (8.5) represents the contributions of other materials producing the ith material as a residue with a yield of νs,i0 j . This term is denoted by NU_MATL(:,j) on the i0 -th MATL line. ρs,i is the density of the ith material component of the layer, defined as the mass of the ith material component divided by the volume of the layer. ρs (0) is the initial density of the layer. Thus, Ys,i = ρs,i /ρs (0) is a quantity that increases if the ith material component is produced as a residue of some other reaction, or decreases if the ith component decomposes. If the layer is composed of only one material, then ρs,i /ρs (0) is initially 1. ns,i j is the reaction order and prescribed under the name N_S(j), and is 1 by default. If the value of ns is not known, it is a good starting point to assume it is 1. The pre-exponential factor, Ai j , is prescribed under the name A(j) on the MATL line of the ith material, with units of s−1 . Ei j , the activation energy, is prescribed via E(j) in units of kJ/kmol. Remember that 1 kcal is 4.184 kJ, and be careful with factors of 1000. For a given reaction, specify both A and E, or neither. Do not specify only one of these two parameters. Typically, these parameters only have meaning when both are derived from a common set of experiments, like TGA (thermo-gravimetric analysis). The fourth term of the reaction rate equation (8.6) can be used to simulate oxidation reactions. If the heterogeneous reaction order nO2 ,i j is greater than zero, the reaction rate is affected by the local oxygen volume fraction, XO2 . It is calculated from the gas phase (first grid cell) oxygen volume fraction XO2 ,g by assuming simultaneous diffusion and consumption so that the concentration profile is in equilibrium, and the concentration at depth x is given by XO2 (x) = XO2 ,g exp(−x/Lg ) (8.7) where Lg is the gas diffusion length scale. nO2 ,i j is prescribed under the name N_O2(j) on the MATL line of the ith material. It is zero by default. Lg is prescribed under the name GAS_DIFFUSION_DEPTH(j), and it is 0.001 m by default. Estimating Kinetic Parameters It is very important to keep in mind that the kinetic constants, A and E, are not available for most real materials. However, there is a way to model such materials using a simplified reaction scheme. The key assumption is that the material components can undergo only one reaction, at most. If, for example, the material undergoes three distinct reactions, there must be at least three material components, each of which undergoes one reaction. If there is an additional residue left over, then a fourth material component is needed. In lieu of specifying A and E, there are several parameters that can be used by FDS to derive effective values, the most important of which is the REFERENCE_TEMPERATURE (◦ C). To understand this parameter, consider the plot shown in Fig. 8.1. These curves represent the results of a hypothetical TGA experiment in which a single component material undergoes a single reaction that converts the solid into a gas. The 81 Mass Fraction (blue curve) is the normalized density of the material (Ys ) which decreases as the sample is slowly heated, in this case at a rate of 5 K/min. The Reaction Rate (green curve) is the rate of change of the mass fraction as a function of time (− dYs / dt). Where this curve peaks is referred to in FDS as the REFERENCE_TEMPERATURE.2 Note that the REFERENCE_TEMPERATURE is not the same as an ignition temperature, nor is it necessarily the surface temperature of the burning solid. Rather, it is simply the temperature at which the mass fraction of the material decreases at its maximum rate within the context of a TGA or similar experimental apparatus. Although you cannot specify an ignition temperature, you can specify a threshold temperature, see Section 8.5.6 for details. The kinetic constants for component i of a multi-component solid are given by3 : Ei,1 = 2 e rp,i R Tp,i Ys,i (0) Ṫ ; Ai,1 = e rp,i E/R Tp,i e Ys,i (0) (8.8) where Tp,i and rp,i /Ys,i (0) are the reference temperature and rate, respectively. The REFERENCE_RATE is the reaction rate, in units of s−1 , at the given REFERENCE_TEMPERATURE divided by the mass fraction, Ys,i (0), of material in the original sample undergoing the reaction. For a single component, single reaction material, Ys,1 (0) = 1. The HEATING_RATE (Ṫ ) is the rate at which the temperature of the TGA (or equivalent) test apparatus was increased. It is input into FDS in units of K/min (in the formula, it is expressed in K/s). Its default value is 5 K/min. In Fig. 8.1, the area under the green curve (Reaction Rate) is equal to the heating rate (in units of K/s). FDS0−86−g80cff4e dYs = −AYs exp(−E/RTs ) dt 2 1.6 0.6 1.2 0.4 0.8 0.2 0.4 −1 0.8 Reaction Rate (s ) × 10 Mass Fraction 3 1 Ys (0) = 1 Tp = 300 ◦ C rp = 0.002 s−1 Ṫ = 5 K/min νs = 0 0 0 50 100 150 200 250 300 350 0 400 Temperature (°C) Figure 8.1: The blue curve represents the normalized mass, Ys = ρs /ρs (0), of a solid material undergoing heating at a rate of 5 K/min. The green curve represents the reaction rate, − dYs / dt. The ordinary differential equation that describes the transformation is shown at right. Note that the parameters Tp , rp , and νs represent the “reference” temperature, reaction rate, and residue yield of the single reaction. From these parameters, values of A and E can be estimated using the formulae in (8.8). The full set of parameters for this case are listed in pyrolysis_1.fds. There are many cases where it is only possible to estimate the REFERENCE_TEMPERATURE (Tp ) of a particular reaction because micro-scale calorimetry data is unavailable. In such cases, an additional parameter can be specified to help fine tune the shape of the reaction rate curve, assuming some sort of measurement or estimate has been made to indicate at what temperature, and over what temperature range, the reaction takes place. The PYROLYSIS_RANGE (∆T ) is the approximate width (in degrees Celsius or Kelvin) of the green 2 The term “reference temperature” is used simply to maintain backward compatibility with earlier versions of FDS. formulas have been derived from an analysis that considers a first-order reaction. When using the proposed method, do not specify non-unity value for the reaction order N_S on the MATL line. 3 These 82 curve, assuming its shape to be roughly triangular. Its default value is 80 ◦ C. Using these input parameters, an estimate is made of the peak reaction rate, rp,i , with which Ei,1 , then Ai,1 , are calculated. rp,i 2 Ṫ = (1 − νs,i ) Ys,i (0) ∆T (8.9) The parameter, νs,i , is the yield of solid residue. When in doubt about the values of these parameters, just specify the REFERENCE_TEMPERATURE. Note that FDS will automatically calculate A and E using the above formulae. Do not specify A and E if you specify the REFERENCE_TEMPERATURE, and do not specify PYROLYSIS_RANGE if you specify REFERENCE_RATE. For the material decomposition shown in Fig. 8.1, the MATL would have the form: &MATL ID ... N_REACTIONS SPEC_ID(1,1) NU_SPEC(1,1) REFERENCE_TEMPERATURE(1) REFERENCE_RATE(1) HEATING_RATE(1) HEAT_OF_COMBUSTION(1) HEAT_OF_REACTION(1) = 'My Fuel' = = = = = = = = 1 '...' 1. 300. 0.002 5. ... ... / Note that the indices have been added to the reaction parameters to emphasize the fact that these parameters are stored in arrays of length equal to N_REACTIONS. If there is only one reaction, you need not include the (1), but it is a good habit to get into. Note also that if the default combustion model is used, you can denote that the reaction produces fuel gas using the appropriate SPEC_ID. Note also that the HEAT_OF_COMBUSTION is the energy released per unit mass of fuel gas that mixes with oxygen and combusts. This has nothing to do with the pyrolysis process, so why is it specified here? The answer is that there can be only one gas phase reaction of fuel and oxygen in FDS, but there can be dozens of different materials and dozens of solid phase reactions. To ensure that the fuel vapors from different materials combust to produce the proper amount of energy, it is very important to specify a HEAT_OF_COMBUSTION for each material. That way, the mass loss rate of fuel gases is automatically adjusted so that the effective mass loss rate multiplied by the single, global, gas phase heat of combustion produces the expected heat release rate. If, for example, the HEAT_OF_COMBUSTION specified on the REAC line is twice that specified on the MATL line, the mass of contained within wall cell will be decremented by that determined by the pyrolysis model, but the mass added to gas phase would be reduced by 50 %. A different value of heat of combustion can be specified for each reaction, j, via the parameter HEAT_OF_COMBUSTION(j). Modeling Upholstered Furnishings The example input files called Fires/couch.fds and Fires/room_fire.fds demonstrate a simple way to model upholstered furniture. In residential fires, upholstered furniture makes up a significant fraction of the combustible load. A single couch can generate several megawatts of energy and sometimes lead to compartment flashover. Modeling a couch fire requires a simplification of its structure and materials. At the very least, we want the upholstery to be modeled as a layer of fabric covering polyurethane foam. We need the thermal properties of each, along with estimates of the “reference” temperatures as described above. The foam might be described as follows: &MATL ID SPECIFIC_HEAT = 'FOAM' = 1.0 83 CONDUCTIVITY DENSITY N_REACTIONS SPEC_ID NU_SPEC REFERENCE_TEMPERATURE HEAT_OF_REACTION HEAT_OF_COMBUSTION = = = = = = = = 0.05 40.0 1 'FUEL' 1. 350. 1500. 30000. / Note that these properties are completely made up. Both the fabric and the foam decompose into fuel gases via single-step reactions. The fuel gases from each have different composition and heats of combustion. FDS automatically adjusts the mass loss rate of each so that the “effective” fuel gas is that which is specified on the REAC line. Figure 8.2 shows the fire after 10 min. Only the reaction zone of the fire is shown; the smoke is hidden so that you can see the fire progressing along the couch. Figure 8.2: Output of couch test case showing fire on the couch at 10 min. 8.5.3 Shrinking and swelling materials Many practical materials change in thickness during the thermal reactions. For example: • Non-charring materials will shrink as material is removed from the condensed phase to the gas phase. 84 • Porous materials like foams would shrink when the material melts and forms a non-porous layer. • Some charring materials swell, i.e., get thicker, when a porous char layer is formed. • Intumescent fire protection materials would swell significantly, creating an insulating layer. In FDS, the layer thickness is updated according to the ratio of the instantaneous material density and the density of the material in its pure form, i.e., the DENSITY on the MATL line. In cases involving several material components, the amount of swelling and shrinking is determined by the maximum and sum of these ratios, respectively. In mathematical terms, this means that in each time step the size of each condensed phase cell is changed according to the ratio δ ρs,i maxi ρs,i if max ≥1 i ρi ρi δ= (8.10) ρ ∑i ρs,i if maxi ρs,ii < 1 ρi For example, if the original material with a DENSITY of 500 kg/m3 is completely converted into a residue with a DENSITY of 1000 kg/m3 , the thickness of the material layer will be half of the original. You can prevent shrinking by setting ALLOW_SHRINKING to .FALSE. on the MATL line. You can prevent swelling by specifying ALLOW_SWELLING to .FALSE. on the MATL line. By default, these flags are true. Shrinking/swelling does not take place if any of the materials with non-zero density has the corresponding flag set to false. 8.5.4 Multiple Solid Phase Reactions The solid phase reaction represented by Fig. 8.1 is fairly simple – a single, homogeneous material is heated and gasified completely. In general, real materials are not so simple. First, they consist of more than one material component, each of which can react over a particular temperature interval, and some of which leave behind a solid residue. Some material components may even undergo multiple reactions that form different residues, like woods that form various amounts of tar, char, and ash, depending on the rate of heating. Figure 8.3 demonstrates a more complicated material than the one previously described. It is a hypothetical material that contains 10 % (by mass) water and 90 % solid material. The water evaporates in the neighborhood of 100 ◦ C and the solid pyrolyzes in the neighborhood of 300 ◦ C, leaving 20 % of its mass behind in the form of a solid residue. The full set of parameters for this case are listed in pyrolysis_2.fds. 8.5.5 The Heat of Reaction Equation (8.6) describes the rate of the reaction as a function of temperature. Most solid phase reactions require energy; that is, they are endothermic. The amount of energy consumed, per unit mass of reactant that is converted into reaction products, is specified by the HEAT_OF_REACTION(j). Technically, this is the enthalpy difference between the products and the reactant. A positive value indicates that the reaction is endothermic; that is, the reaction takes energy out of the system. Usually the HEAT_OF_REACTION is accurately known only for simple phase change reactions like the vaporization of water. For other reactions, it must be determined empirically (e.g., by thermo-gravimetric analysis). 8.5.6 Special Topic: The “Threshold” Temperature In FDS, the reaction rate expression in Eq. (8.6) includes an optional term: n Ei j ns,i j ri j = Ai j Ys,i exp − max 0, Sthr,i j (Ts − Tthr,i j ) t,i j R Ts 85 (8.11) FDS0−86−g80cff4e dYs,1 = −A1,1 Ys,1 exp(−E1,1 /RTs ) dt dYs,2 = −A2,1 Ys,2 exp(−E2,1 /RTs ) dt dYs,3 dYs,2 = −νs,2,1 dt dt 2 1.6 0.6 1.2 0.4 0.8 0.2 0.4 −1 0.8 Reaction Rate (s ) × 10 Mass Fraction 3 1 Tp,1,1 = 100 + 273 K rp,1,1 = 0.0016 s 0 0 50 100 150 200 250 300 350 0 400 −1 νs,1,1 = 0 Temperature (°C) Ys,1 (0) = 0.1 Ys,2 (0) = 0.9 Ys,3 (0) = 0.0 Tp,2,1 = 300 + 273 K rp,2,1 = 0.0012 s−1 νs,2,1 = 0.2 Ṫ = 5 K/min Figure 8.3: The blue curve represents the combined mass fraction, ∑ Ys,i , and the green curve the net reaction rate, − d/dt(∑ Ys,i ), for a material that contains 10 % water (by mass) that evaporates at a temperature of 100 ◦ C, and 90 % solid material that pyrolyzes at 300 ◦ C, leaving a 20 % (by mass) residue behind. Note that the numbered subscripts refer to the material component and the reaction, respectively. In this case, there are three material components, and the first two each undergo a single reaction. The third material component is formed as a residue of the reaction of the second material. The system of ordinary differential equations that governs the transformation of the materials is shown at right. Tthr,i j is an optional “threshold” temperature that allows the definition of non-Arrhenius pyrolysis functions and ignition criteria, and is prescribed by THRESHOLD_TEMPERATURE(j). Sthr,i j is the “threshold direction” that allows the triggering of reaction when temperature gets “above” Tthr,i j (Sthr,i j = +1) or “below” Tthr,i j (Sthr,i j = −1). nt, j is prescribed under the name N_T(j) and Sthr,i j under THRESHOLD_SIGN. By default, Tthr,i j is -273.15 degrees Celsius, nt, j is zero and Sthr,i j = +1; thus, the last term of Equation 8.11 does not affect the pyrolysis rate. The term can be used to describe a threshold temperature for the pyrolysis reaction by setting Tthr,i j and nt, j = 0. Then the term is equal to 0 at temperatures below Tthr,i j and 1 at temperatures above. The threshold temperature can be used to simulate simple phase change reactions, such as melting and freezing. To make the reaction rate controlled by available energy, i.e., not kinetics, another optional term should be included in the reaction rate formula ri j = A i j 1 Hr,i j ∆t n max 0, Sthr,i j (Ts − Tthr,i j ) t,i j (8.12) This form of reaction rate can be implemented by setting a logical parameter PCR(j)=.TRUE.. The preexponental factor Ai j should then be given a value that is close or slightly smaller than the specific heat (kJ/(kg · K)) of the material mixture at phase change temperature. The input file water_ice_water.fds demonstrates the use of the “threshold” temperature. A small amount of liquid water at 10 ◦ C is cooled down to -10 ◦ C in 10 min, and then heated up again to 10 ◦ C. The concentration of the liquid water as a function of temperature is plotted in Fig. 8.4. The cooling phase is indicated by the blue line and heating phase by the red line. 8.5.7 Liquid Fuels For a liquid fuel, the thermal properties are similar to those of a solid material, with a few exceptions. The evaporation rate of the fuel is governed by the mass transfer number (see FDS Technical Reference Guide for details). The properties of a liquid fuel are given on the MATL line: 86 FDS0−86−g80cff4e Liquid concentration (kg/m3) 1000 Cooling (freezing) Heating (melting) 900 800 700 600 500 400 300 200 100 0 −10 −8 −6 −4 −2 0 2 4 6 8 10 Temperature (°C) Figure 8.4: Freezing and melting of water. &MATL ID EMISSIVITY NU_SPEC SPEC_ID HEAT_OF_REACTION CONDUCTIVITY SPECIFIC_HEAT DENSITY BOILING_TEMPERATURE ABSORPTION_COEFFICIENT = = = = = = = = = = 'ETHANOL LIQUID' 1. 0.97 'ETHANOL' 836.98 0.17 2.4398 789. 78.5 1534.3 / The inclusion of BOILING_TEMPERATURE on the MATL line tells FDS to use its liquid pyrolysis model. It also automatically sets N_REACTIONS=1, that is, the only “reaction” is the phase change from liquid to gaseous fuel. Thus, HEAT_OF_REACTION in this case is the latent heat of vaporization. The gaseous fuel yield, NU_SPEC, is 0.97 instead of 1 to account for impurities in the liquid that do not take part in the combustion process. The thermal conductivity, density and specific heat are used to compute the loss of heat into the liquid via conduction using the same one-dimensional heat transfer equation that is used for solids. Obviously, the convection of the liquid is important, but is not considered in the model. Note also the ABSORPTION_COEFFICIENT for the liquid. This denotes the absorption in depth of thermal radiation. Liquids do not just absorb radiation at the surface, but rather over a thin layer near the surface. Its effect on the burning rate is significant. 8.5.8 Fuel Burnout The thermal properties of a solid or liquid fuel determine the length of time for which it can burn. In general, the burnout time is a function of the mass loss rate, ṁ00 , the density, ρs , and the layer thickness, δs : tb = ρs δs ṁ00 (8.13) However, each type of pyrolysis model handles fuel burnout in a slightly different way. These differences will be highlighted in the individual sections below. 87 Solid Fuel Burnout If a heat release rate RAMP function is not included for a solid fuel that burns at a specified rate, the surface will continue to burn at the specified rate indefinitely with no fuel burnout. If detailed heat release rate versus time data is not available, you can estimate the burnout time for a surface using the heat of combustion, ∆H, material density, ρs , material thickness, δs , and HRRPUA, q̇00f : tb = ρs δs ∆H q̇00f (8.14) Use the RAMP function to stop the burning once the calculated burnout time is reached. The burnout time of a reacting solid fuel is calculated automatically by FDS based on the layer THICKNESS, component DENSITY, and the calculated burning rate. Liquid Fuel Burnout The burnout time of a liquid fuel is calculated automatically based on the liquid layer THICKNESS, liquid DENSITY, and the calculated burning rate. Special topic: Making Fuels Disappear If a burning object is to disappear from the simulation once it is consumed, set BURN_AWAY=.TRUE. on the corresponding SURF line. The solid object disappears from the calculation cell by cell, as the mass contained within each solid cell is consumed either by the pyrolysis reactions or by the prescribed HRR. The following issues should be kept in mind when using BURN_AWAY: • For surfaces with prescribed HRR (HRRPUA) or prescribed mass loss rate (MLRPUA) AND thermally thick heat conduction, the mass flux is affected by the heats of combustion defined for the gas phase reaction and the first listed material (MATL) component. • Use BURN_AWAY parameter cautiously. If an object has the potential of burning away, a significant amount of extra memory has to be set aside to store additional surface information as the mesh cells disappear. • If BURN_AWAY is prescribed, the SURF should be applied to the entire object, not just a face of the object because it is unclear how to handle edges of solid obstructions that have different SURF_IDs on different faces. • If the volume of the obstruction changes because it has to conform to the uniform mesh, FDS does not adjust the burning rate to account for this as it does with various quantities associated with areas, like HRRPUA. • A parameter called BULK_DENSITY (kg/m3 ) can be applied to the OBST rather than the SURF line. This parameter is used to determine the combustible mass of the solid object. The calculation uses the user-specified object dimensions, not those of the mesh-adjusted object. This parameter overrides all other parameters from which a combustible mass would be calculated. Note that without a BULK_DENSITY specified, the total amount of mass burned will depend upon the grid resolution. The use of the BULK_DENSITY parameter ensures a specific fuel mass per unit volume that is independent of the grid resolution. Note that in the event that the solid phase reaction involves the production of solid residue (like char or ash), the BULK_DENSITY refers to the mass of the solid that is converted to gas upon reaction. 88 • The mass of the object is based on the densities of all material components (MATL), but it is only consumed by mass fluxes of the known species. If the sum of the gaseous yields is less than one, it will take longer to consume the mass. Simple examples demonstrating how solid fuels can be forced to disappear from the domain are labeled Fires/box_burn_away. These are examples of a solid block of solid material that is pyrolyzed until it is completely consumed. The heat flux is generated by placing hot surfaces around the box. There is no combustion. In the first example, box_burn_away1, the released gas is (’METHANE’), and in the second example, box_burn_away2, it is an additional species called ’GAS’. In the third and fourth examples box_burn_away3 and box_burn_away4, the released gas is fuel but the pyrolysis rate is specified. In the fourth case, the heat of combustion for the foam material is set different from that of the gas, with a ratio of 0.75. The properties of the solid material were chosen simply to assure a quick calculation. The objective of the test is to check that the released mass and the integrated burning rate are consistent with the material properties of the block. The block is 0.4 m on a side, with a density of 20 kg/m3 . The integrated densities of the pyrolysis product gases (written to box_burn_away#_devc.csv), as well as the integrated burning rate (written to box_burn_away#_hrr.csv) at the end of the 30 s calculation ought to be: (0.4)3 m3 × 20 kg/m3 = 1.28 kg (8.15) except for the fourth case, where the amount of released gas is affected by the ratio of heats of combustion 0.75 × 1.28 kg = 0.96 kg (8.16) The same case is tested in two dimensions (box_burn_away_2D and box_burn_away_2D_residue). In the latter case, only half of the mass is converted to fuel, leaving behind a residue that is 50 % of the original mass. The box is forced to burn away by setting the BULK_DENSITY to 10 kg/m3 . This is the combustible mass. These two cases exhibit a fictitious increase in solid mass when new unburned surfaces are exposed as entire mesh cells disappear. The increased mass is just an artifact of reporting the residual solid mass as the product of surface density and surface area. Both the final solid mass and the gaseous degradation products should match the expected values at the end of the simulation. 8.6 Testing Your Pyrolysis Model Modeling the burning of real materials can be very complicated. Undoubtedly, the SURF and MATL lines in the input file will consist of a combination of empirical and fundamental properties, often originating from different sources. How do you know that the various property values and the associated thermo-physical model in FDS constitute an appropriate description of the solid? For a full-scale simulation, it is hard to untangle the uncertainties associated with the gas and solid phase routines. However, it is easy to perform a simple check of any set of solid phase model by essentially turning off the gas phase. In the following sections, guidance is provided on how to perform a quick simulation of the cone calorimeter and benchscale measurements like thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), and micro-combustion calorimetry (MCC). 8.6.1 Simulating the Cone Calorimeter This section describes how to set up a simple model of the cone calorimeter or other similar apparatus. This is not a full 3-D simulation of the apparatus, but rather a 1-D simulation of the solid phase degradation under an imposed external heat flux. You can literally create a model of the cone heater and sample holder in FDS 89 FDS0−86−g80cff4e FDS0−86−g80cff4e 1.5 1.5 Pyrolyzed Mass (box_burn_away1) Pyrolyzed Mass (box_burn_away2) Mass (kg) 1 Mass (kg) 1 Ideal FDS (fuel) 0.5 0 0 Ideal FDS (GAS) 0.5 5 10 15 Time (s) 20 25 0 0 30 5 10 15 Time (s) FDS0−86−g80cff4e 20 25 30 FDS0−86−g80cff4e 1.5 1.5 Pyrolyzed Mass (box_burn_away3) Pyrolyzed Mass (box_burn_away4) Mass (kg) 1 Mass (kg) 1 Ideal FDS (fuel) 0.5 0.5 Ideal FDS (fuel) 0 0 5 10 15 Time (s) 20 25 0 0 30 5 10 15 Time (s) FDS0−86−g80cff4e 25 30 FDS0−86−g80cff4e 1.5 1.5 Pyrolyzed Mass (box_burn_away_2D_residue) Pyrolyzed Mass (box_burn_away_2D) Mass (kg) 1 Mass (kg) 1 Ideal (fuel) Ideal (solid) FDS (fuel) FDS(solid) 0.5 0 0 20 Ideal (fuel) Ideal (solid) FDS (fuel) FDS(solid) 0.5 5 10 15 Time (s) 20 25 0 0 30 5 10 Figure 8.5: Output of box_burn_away test cases. 90 15 Time (s) 20 25 30 to simulate the coupling of gas and solid phase phenomena, but before even attempting this, it is worthwhile to perform a quick simulation like the one described here to test the solid phase model only. 1. Create a trivially small mesh, just to let FDS run. Since the gas phase calculation is essentially being shut off, you just need 4 cells in each direction (IJK=4,4,4) for the pressure solver to function properly. 2. On the TIME line, set WALL_INCREMENT=1 to force FDS to update the solid phase every time step (normally it does this every other time step), and set DT to whatever value appropriate for the solid phase calculation. Since there is no gas phase calculation that will limit the time step, it is best to control this yourself. 3. Set HEAT_TRANSFER_COEFFICIENT=0 on the SURF line. This turns off the convective heat flux from gas to surface and vice versa. The heat flux to the solid is specified via EXTERNAL_FLUX4 (kW/m2 ) on the SURF line that is assigned to the solid surface. 4. Turn off all the gas phase computations by setting SOLID_PHASE_ONLY=.TRUE. on the MISC line. This will also speed up the computations significantly. If a REAC line is needed to define a fuel gas, you may turn off combustion by setting Y_O2_INFTY=0.01 on the MISC line. This sets the background oxygen mass fraction to 0.01, too low to support any burning. 5. Generate MATL lines, plus a single SURF line, as you normally would, except add EXTERNAL_FLUX to the SURF line. This is simply a “virtual” source that heats the solid. Think of this as a perfect radiant panel or conical heating unit. 6. Assign the SURF_ID to a VENT that spans the bottom of the computational domain. Create OPEN vents on all other faces. 7. Finally, add solid phase output devices to the solid surface, like ’WALL TEMPERATURE’, ’NET HEAT FLUX’, ’BURNING RATE’, ’GAUGE HEAT FLUX’, and ’WALL THICKNESS’ (assuming the solid is to burn away). Use these to track the condition of the solid as a function of time. In particular, make sure that the ’BURNING RATE’ is appropriate for the particular external heat flux applied. Make sure that the ’WALL TEMPERATURE’ is appropriate. Compare your results to measurements made in a benchscale device, like the cone calorimeter. Keep in mind, however, that the calculation and the experiment are not necessarily perfectly matched. The calculation is designed to eliminate uncertainties related to convection, combustion, and apparatus-specific phenomena. Below is an FDS input file that demonstrates how you can test a candidate pyrolysis model by running very short calculations. The simulation only involves the solid phase model. Essentially, the gas phase calculation is shut off except for the imposition of a 52 kW/m2 “external” heat flux. The solid in this example is a 8.5 mm thick slab of PMMA. For more details, see the FDS Validation Guide under the heading “FAA Polymers.” &HEAD &MESH &TIME &MISC &REAC &MATL CHID='FAA_Polymers_PMMA', TITLE='Black PMMA at 50 kW/m2' / IJK=3,3,4, XB=-0.15,0.15,-0.15,0.15,0.0,0.4 / T_END=600., WALL_INCREMENT=1, DT=0.01 / Y_O2_INFTY=0.01, SOLID_PHASE_ONLY=.TRUE. / FUEL='METHANE' / ID='BLACKPMMA' ABSORPTION_COEFFICIENT=2700. N_REACTIONS=1 A(1) = 8.5E12 4 You can control EXTERNAL_FLUX using either TAU_EF or RAMP_EF. This is useful if you want to ramp up the heat flux following ignition to account for the additional radiation from the flame. See Section 10.1 for more details about ramps. 91 &SURF &VENT &DUMP &DEVC &DEVC &DEVC &TAIL 8.6.2 E(1) = 188000 EMISSIVITY=0.85 DENSITY=1100. SPEC_ID='METHANE' NU_SPEC=1. HEAT_OF_REACTION=870. HEAT_OF_COMBUSTION=25200. CONDUCTIVITY = 0.20 SPECIFIC_HEAT = 2.2 ID='PMMA SLAB' COLOR='BLACK' BACKING='INSULATED' MATL_ID='BLACKPMMA' THICKNESS=0.0085 HEAT_TRANSFER_COEFFICIENT=0. EXTERNAL_FLUX=52 / XB=-0.05,0.05,-0.05,0.05,0.0,0.0, SURF_ID = 'PMMA SLAB' / DT_DEVC=5. / XYZ=0.0,0.0,0.0, IOR=3, QUANTITY='WALL TEMPERATURE', ID='temp' / XYZ=0.0,0.0,0.0, IOR=3, QUANTITY='BURNING RATE', ID='MLR' / XYZ=0.0,0.0,0.0, IOR=3, QUANTITY='WALL THICKNESS', ID='thick' / / Simulating Bench-scale Measurements like the TGA, DSC, and MCC There are a number of techniques to measure the thermo-physical properties of a solid material. Most of these involve heating a very small sample at a relatively slow, linear rate. In this way, thermal conduction is minimized and the sample can be considered thermally-thin. FDS has a special feature that mimics thermal-gravimetric analysis (TGA), differential scanning calorimetry (DSC), and micro-combustion calorimetry (MCC) measurements. To use it, set up your input file as you normally would. Then, add the flag TGA_ANALYSIS=.TRUE. to the SURF line you want to analyze. You can only analyze one SURF line at a time. Optionally, you can specify TGA_HEATING_RATE (K/min) and TGA_FINAL_TEMPERATURE (◦ C) to indicate the linear heating rate and the final temperature of the sample. The default values are 5 K/min and 800 ◦ C. The initial temperature is TMPA. Note that this feature is only appropriate for a SURF line that describes a thermally-thick sample consisting of a single layer with multiple reacting components. For example, the following SURF line describes a material that consists of three components: &SURF ID = 'Cable Insulation' TGA_ANALYSIS = .TRUE. TGA_HEATING_RATE = 60. THICKNESS = 0.005 MATL_ID(1,1) = 'Component MATL_ID(1,2) = 'Component MATL_ID(1,3) = 'Component MATL_MASS_FRACTION(1,1:3) A', B', C', = 0.26,0.33,0.41 / The two TGA entries will force FDS to perform a numerical version of the TGA, DSC and MCC measurements. The THICKNESS and other boundary conditions on the SURF line will be ignored. After running the analysis, which only takes a second or two, FDS will then shut down without running the actual simulation. To run the simulation, either remove the TGA entries or set TGA_ANALYSIS to .FALSE. The result of the TGA_ANALYSIS is a single comma-delimited file called CHID_tga.csv. The first and second columns of the file consist of the time and sample temperature. The third column is the normalized sample mass; that is, the sample mass divided by its initial mass. The fourth column is the mass loss rate, in 92 units of s−1 . The fifth column is the heat release rate per unit mass of the sample in units of W/g, typical of an MCC measurement. The sixth column is the heat absorbed by the sample normalized by its mass, also in units of W/g, typical of a DSC measurement. Results for a typical analysis of wood are shown in Fig. 8.6. In this case, a sample of wood containing about 10 % water by mass heats up and undergoes three reactions, including the evaporation of water. Note that the TGA plots include both fuel and water vapor, while the MCC results only show fuel. Details of the output quantities are discussed in Section 16.10.8. Further details on these measurement techniques and how to interpret them are found in the FDS Verification Guide [3]. −3 FDS0−86−g80cff4e 1.2 2 FDS0−86−g80cff4e x 10 TGA Results (tga_analysis) TGA Results (tga_analysis) Mass Loss Rate (1/s) Mass Fraction 1 0.8 0.6 0.4 1.5 1 0.5 0.2 0 0 100 200 300 400 Temperature (°C) 500 0 0 600 100 200 300 400 Temperature (°C) FDS0−86−g80cff4e 4 MCC Results (tga_analysis) 3.5 25 Heating Rate (W/g) Heat Release Rate (W/g) 600 FDS0−86−g80cff4e 30 20 15 10 5 0 0 500 DSC Results (tga_analysis) 3 2.5 2 1.5 1 0.5 100 200 300 400 Temperature (°C) 500 0 0 600 100 200 300 400 Temperature (°C) Figure 8.6: Sample results of a tga_analysis. 93 500 600 94 Chapter 9 Ventilation This chapter explains how to model a ventilation system. There are two ways to do this. First, if you only want to specify air flow rates into and out of compartments, read Section 9.1 for a description of simple velocity boundary conditions. However, if you want to model the entire HVAC system, read Section 9.2. 9.1 Simple Vents, Fans and Heaters The ventilation system of individual compartments within a building is described using velocity boundary conditions. For example, fresh air can be blown into, and smoke can be drawn from, a compartment by specifying a velocity in the normal direction to a solid surface. However, there are various other facets of velocity boundary conditions that are described below. 9.1.1 Simple Supply and Exhaust Vents The easiest way to describe a supply or exhaust fan is to specify a VENT on a solid surface, and designate a SURF_ID with some form of specified velocity or volume flow rate. The normal component of velocity is usually specified directly via the parameter VEL. If VEL is negative, the flow is directed into the computational domain, i.e., a supply vent. If VEL is positive, the flow is drawn out of the domain, i.e., an exhaust vent. For example, the lines &SURF ID='SUPPLY', VEL=-1.2, COLOR='BLUE' / &VENT XB=5.0,5.0,1.0,1.4,2.0,2.4, SURF_ID='SUPPLY' / create a VENT that supplies air at a velocity of 1.2 m/s through an area of nominally 0.16 m2 , depending on the realignment of the VENT onto the FDS mesh. Regardless of the orientation of the plane x = 5, the flow will be directed into the room because of the sign of VEL. In this example the VENT may not be exactly 0.16 m2 in area because it may not align exactly with the computational mesh. If this is the case then VOLUME_FLOW can be prescribed instead of VEL. The units are m3 /s. If the flow is entering the computational domain, VOLUME_FLOW should be a negative number, the same convention as for VEL. Note that a SURF with a VOLUME_FLOW prescribed can be invoked by either a VENT or an OBST, but be aware that in the latter case, the resulting velocity on the face or faces of the obstruction will be given by the specified VOLUME_FLOW divided by the area of that particular face. For example: &SURF ID='SUPPLY', VOLUME_FLOW=-5.0, COLOR='GREEN' / &OBST XB=..., SURF_ID6='BRICK','SUPPLY','BRICK','BRICK','BRICK','BRICK' / 95 dictates that the forward x-facing surface of the obstruction is to have a velocity equal to 5 m3 /s divided by the area of the face (as approximated within FDS) flowing into the computational domain. Note that VEL and VOLUME_FLOW should not be specified on the same SURF line. The choice depends on whether an exact velocity is desired at a given vent, or whether the given volume flux is desired. Note also that if the VENT or OBST crosses mesh boundaries, the specified VOLUME_FLOW will be recomputed in each mesh so that the desired volume flow is achieved. This was not the case in FDS version 6.3 and earlier. The sample cases called volume_flow_1.fds and volume_flow_2.fds in the Flowfields folder demonstrate that a VENT or an OBST can be divided among several meshes. In both cases, air is drawn from a 1 m by 1 m by 1 m box at a rate of 0.01 m3 /s. These cases also ensure that the VENT or OBST need not be aligned with the mesh to yield the desired flow rate. Figure 9.1 displays the volume flow drawn though an OPEN boundary on an opposite face of the box with either a VENT (left) or OBST (right) with a specified volume flow. FDS0−86−g80cff4e FDS0−86−g80cff4e 0.015 0.015 Flow Test (volume_flow_2) Volume Flow (m3/s) 3 Volume Flow (m /s) Flow Test (volume_flow_1) 0.01 0.005 0.01 0.005 Expected FDS 0 0 20 40 60 80 Expected FDS 0 0 100 Time (s) 20 40 60 80 100 Time (s) Figure 9.1: Flow rate of air drawn through a unit cube via a VENT (left) and OBST (right) with specified VOLUME_FLOW. 9.1.2 Total Mass Flux Most often, you specify a simple supply or exhaust vent by setting either a normal velocity or volume flux at a solid surface. However, you may wish to control the total mass flow rate per unit area (kg/(m2 · s)) via the parameter MASS_FLUX_TOTAL. This parameter uses the same sign convention as VEL above. In fact, the value entered for MASS_FLUX_TOTAL is converted internally into a velocity boundary condition whose value for an outflow is adjusted based on the local density. Note that MASS_FLUX_TOTAL should only be used for an outflow boundary condition; for inflow use MASS_FLUX which is discussed in Section 9.1.6. 9.1.3 Heaters You can create a simple heating vent by changing the temperature of the incoming air &SURF ID='BLOWER', VEL=-1.2, TMP_FRONT=50. / The VENT with SURF_ID=’BLOWER’ would blow 50 ◦ C air at 1.2 m/s into the flow domain. Making VEL positive would suck air out, in which case TMP_FRONT would not be necessary. 96 Note that if HRRPUA or solid phase reaction parameters are specified, no velocity should be prescribed. The combustible gases are ejected at a velocity computed by FDS. 9.1.4 Louvered Vents Most real supply vents are covered with some sort of grill or louvers which act to redirect, or diffuse, the incoming air stream. It is possible to mimic this effect, to some extent, by prescribing both a normal and the tangential components of the flow. The normal component is specified with VEL as described above. The tangential is prescribed via a pair of real numbers VEL_T representing the desired tangential velocity components in the other two coordinate directions (x or y should precede y or z). For example, the line in the example case Flowfields/tangential_velocity.fds &SURF ID='LOUVER', VEL=-2.0, VEL_T=3.0,0.0, TAU_V=5., COLOR='GREEN' / is a boundary condition for a louvered vent that pushes air into the space with a normal velocity of 2 m/s and a tangential velocity of 3 m/s in the first of the two tangential directions. Note that the negative sign of the normal component of velocity indicates that the fluid is injected into the computational domain. The tangential velocity of 3 m/s indicates that the flow is in the positive y direction. Both the normal and tangential velocity components are ramped up with either TAU_V or RAMP_V, as shown in Fig. 9.2. FDS0−86−g80cff4e 4 3.5 Velocity Components (tangential_velocity) Velocity (m/s) 3 2.5 2 1.5 1 Exact (u) Exact (v) FDS (u) FDS (v) 0.5 0 0 5 10 Time (s) 15 20 Figure 9.2: Normal and tangential velocity components at a louvered vent, compared to the ideal curve. In cases of limited mesh resolution, it may not be possible to describe a louvered vent or slot diffuser using VEL_T because there may not be enough mesh cells spanning the opening. In these cases, you might consider simply specifying a flat plate obstruction in front of the VENT with an offset of one mesh cell. The plate will simply redirect the air flow in all lateral directions. If the louvered vent is part of an HVAC system, see 9.2.7 for details on how to specify the louver. 9.1.5 Specified Normal Velocity Gradient It is sometimes desirable to specify a Neumann boundary condition (specified gradient) for the velocity in the direction normal to the boundary. For example, the following allows inflow and outflow along the top of the domain, but ∂ w/∂ z = 0. Note that FREE_SLIP=.TRUE. only sets ∂ u/∂ z = 0 and ∂ v/∂ z = 0. 97 &SURF ID = 'sky', COLOR = 'INVISIBLE', VEL_GRAD=0., FREE_SLIP=.TRUE. / &VENT MB='ZMAX', SURF_ID='sky' / 9.1.6 Species and Species Mass Flux Boundary Conditions There are two types of species boundary conditions (see Chapter 11 for a general discussion of gas species). By default, gas species do not penetrate solid surfaces and you need not specify anything if this is all you need. If the mass fraction of the species is to be some value at a forced flow boundary where VEL, VOLUME_FLOW, or MASS_FLUX_TOTAL is specified, set MASS_FRACTION(:) equal to the desired species mass fractions on the appropriate SURF line. If the mass flux of the species is desired, set MASS_FLUX(:) instead of MASS_FRACTION(:). If MASS_FLUX(:) is set, do not specify VEL, VOLUME_FLOW, or MASS_FLUX_TOTAL. These are automatically calculated based on the specified mass flux. The inputs MASS_FLUX(:) and typically MASS_FRACTION(:) should only be used for inflow boundary conditions. MASS_FLUX(:) should be positive with units of kg/(m2 · s). Also note that the background species cannot be specified when using MASS_FRACTION. The mass fraction of the background species will be set to account for any mass fraction not specified with other species. Here is an example of how to specify a surface that generates methane at a rate of 0.025 kg/(m2 · s): &SPEC ID='METHANE' / &SURF ID='METHANE BURNER', SPEC_ID(1)='METHANE', MASS_FLUX(1)=0.025 / &VENT XB=..., SURF_ID='METHANE BURNER' / Here is example of how to specify a surface that blows methane with a velocity of 0.1 m/s: &SPEC ID='METHANE' / &SURF ID='METHANE BLOWER', MASS_FRACTION(1)=1.0, SPEC_ID(1)='METHANE', VEL=-0.1 / &VENT XB=..., SURF_ID='METHANE BLOWER' / Note that specifying a combination of VEL and MASS_FRACTION can lead to inaccurate results if the specified velocity is small because diffusion will dominate the mass transport. To obtain an accurate species mass flux at a boundary, use MASS_FLUX. Alternatively, add CONVERT_VOLUME_TO_MASS=.TRUE. for velocity boundaries (VEL, VOLUME_FLOW, or MASS_FLUX_TOTAL), which converts volume flow to a mass flux based on the specified boundary composition (MASS_FRACTION) and temperature (TMP_FRONT): ṁ00α = ρZα u = p∞W Q̇ Zα RT A (9.1) where ρ is the density, Zα is the mass fraction of α, u is the velocity normal to the surface, p∞ is the ambient pressure, W is the mixture molecular weight, R is the ideal gas constant, T is the surface temperature, Q̇ is the volume flow rate, and A is the area of the vent. 9.1.7 Tangential Velocity Boundary Conditions at Solid Surfaces The no-slip condition implies that the continuum tangential gas velocity at a surface is zero. In turbulent flow the velocity increases rapidly through a boundary layer that is only a few millimeters thick to its “free-stream” value. In most practical simulations, it is not possible to resolve this boundary layer directly; thus, an empirical model is used to represent its effect on the overall flow field. For a DNS (Direct Numerical Simulation), the velocity gradient at the wall is computed directly from the resolved velocity near the wall (NO_SLIP=.TRUE. by default). For an LES (Large Eddy Simulation), a “log law” wall 98 model is applied. The surface roughness (in meters) is set by ROUGHNESS on SURF. See the FDS Technical Reference Guide [1] for wall model details. To force a solid boundary to have a free-slip condition, set FREE_SLIP=.TRUE.1 on the SURF line. In LES, to override the wall model and force a no-slip boundary condition, set NO_SLIP=.TRUE. on the SURF line. 9.1.8 Synthetic Turbulence Inflow Boundary Conditions Real flows of low-viscosity fluids like air are rarely perfectly stationary in time or uniform in space—they are turbulent (to some degree). Of course, the turbulence characteristics of the flow may have a significant impact on mixing and other behaviors so the specification of nominally constant and uniform boundary conditions may be insufficient. To address this issue, FDS employs a synthetic eddy method (SEM)2 . Refer to Jarrin [23] for a detailed description. In brief, “eddies” are injected into the flow at random positions on the boundary and advect with the mean flow over a short distance near the boundary equivalent to the maximum eddy length scale. Once the eddy passes through this region it is recycled at the inlet of the boundary with a new random position and length scale. The eddies are idealized as velocity perturbations over a spherical region in space with a diameter (eddy length scale) selected from a uniform random distribution. The selection procedures guarantee that prescribed first and second-order statistics (including Reynolds stresses) are satisfied. Synthetic turbulence is invoked by setting the number of eddies, N_EDDY, the characteristic eddy length scale, L_EDDY, and either the root mean square (RMS) velocity fluctuation, VEL_RMS, or the Reynolds stress tensor components, REYNOLDS_STRESS(3,3) on the VENT line. In Fig. 9.3 we show examples using SEM for flat, parabolic, atmospheric, and ramp profiles with 10 % turbulence intensity (see the sem_* test series in the Turbulence verification subdirectory). The input lines for the atmospheric case are (see Section 9.6 for further discussion of profile parameters). &SURF ID='inlet', VEL=-1, PROFILE='ATMOSPHERIC', Z0=0.5, PLE=0.3 / &VENT MB='XMIN', SURF_ID='inlet', N_EDDY=100, L_EDDY=0.2, VEL_RMS=.1 / Note that the Reynolds stress is symmetric and only the lower triangular part needs to bep specified. The RMS velocity fluctuation is isotropic (equivalent for each component). Thus, VEL_RMS ≡ 2k/3, where k ≡ h 12 u0i u0i i is the turbulent kinetic energy per unit mass. Below is an example illustrating the equivalence between the RMS velocity fluctuation and the diagonal components of the Reynolds stress. Note that if VEL_RMS is specified, this is equivalent to REYNOLDS_STRESS(1,1) = VEL_RMS**2 REYNOLDS_STRESS(2,2) = VEL_RMS**2 REYNOLDS_STRESS(3,3) = VEL_RMS**2 and all other components of REYNOLDS_STRESS are zero. If the fluctuations are not isotropic, then the Reynolds stresses must be specified componentwise. In Chapter 7 of Jarrin’s thesis [23], he introduces the Modified Synthetic Eddy Method in which the eddy length scales are anisotropic. This allows more realistic characterization of streamwise vortices in a turbulent boundary layer. To specify the length scales corresponding to the σi j values in Jarrin’s Eq. (7.1) use L_EDDY_IJ(3,3). Here is an example with random values for the eddy length scales and Reynolds 1 This parameter sets the wall stress to zero (removes viscous friction). It is not equivalent to the SAWTOOTH=.FALSE. functionality from versions of FDS prior to Version 6. 2 SEM only applies to velocity boundary conditions and so may not be used for HVAC vents, which are strict mass flux boundaries. 99 FDS0−86−g80cff4e FDS0−86−g80cff4e 1 1 Prescribed mean Prescribed rms FDS mean FDS rms 0.8 0.6 0.6 z (m) z (m) 0.8 0.4 0.4 0.2 0.2 0 0.4 0.6 0.8 u (m/s) 1 0 0 1.2 Prescribed mean Prescribed rms FDS mean FDS rms 0.2 0.4 0.6 u (m/s) FDS0−86−g80cff4e 1 1.2 FDS0−86−g80cff4e 1 1 Prescribed mean Prescribed rms FDS mean FDS rms 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0 0 0.2 0.4 0.6 0.8 u (m/s) Prescribed mean Prescribed rms FDS mean FDS rms 0.8 z (m) z (m) 0.8 1 0 0 1.2 0.2 0.4 0.6 u (m/s) 0.8 1 1.2 Figure 9.3: Synthetic Eddy Method vent profiles: flat (upper left), parabolic (upper right), atmospheric (lower left), and linear ramp (lower right). stress components: &VENT XB=... , SURF_ID='WIND', N_EDDY=500, L_EDDY_IJ(1,1)=21., L_EDDY_IJ(1,2)=6.22, L_EDDY_IJ(1,3)=4.23 L_EDDY_IJ(2,1)=2.35, L_EDDY_IJ(2,2)=5.66, L_EDDY_IJ(2,3)=2.50 L_EDDY_IJ(3,1)=5.42, L_EDDY_IJ(3,2)=0.78, L_EDDY_IJ(3,3)=1.01 REYNOLDS_STRESS(1,1)=2.16, REYNOLDS_STRESS(1,2)=0., REYNOLDS_STRESS(1,3)=-0.47 REYNOLDS_STRESS(2,1)=0., REYNOLDS_STRESS(2,2)=1.53, REYNOLDS_STRESS(2,3)=0. REYNOLDS_STRESS(3,1)=-0.47, REYNOLDS_STRESS(3,2)=0., REYNOLDS_STRESS(3,3)=4.259 / 9.1.9 Random Mass Flux Variation The current implementation of the Synthetic Eddy Method does not allow variation in mass flux defined on a SURF line. For example, HRRPUA and MASS_FLUX boundary conditions are by default taken as constant for a given SURF. In reality, of course, this is rarely the case—generally there is some level of random variation in the mass flux on the surface. In an attempt to accommodate this effect, we have implemented an experimental feature where the user may specify a mass flux variation as a fractional value with the parameter MASS_FLUX_VAR. For example, if you want a 10 % variation in a heat release rate per unit area of 100 kW/m2 then use 100 &SURF ID='burner', HRRPUA=100., MASS_FLUX_VAR=0.1 / It may be helpful to use the boundary file output quantity MASS FLUX WALL CELL to visualize the variation. For example, &BNDF QUANTITY='MASS FLUX WALL CELL', CELL_CENTERED=.TRUE. / HVAC Systems: The HVAC Namelist Group (Table 17.9) 9.2 There are occasions where simply defining fixed flow and fixed species boundary conditions is not sufficient to model the behavior of an HVAC (Heating, Ventilation, and Air Conditioning) system. If the ability to transport heat and combustion products through a duct network or the ability to fully account for the pressurization of a compartment due to a fire on the flows in a duct network is important, you can make use of a coupled HVAC network solver. The solver computes the flows through a duct network described as a mapping of duct segments and nodes where a node is either the joining of two or more ducts (a tee for example) or where a duct segment connects to the FDS computational domain. The current HVAC solver does not allow for mass storage in the duct network (i.e., what goes in during a time step, goes out during a time step). HVAC components such as fans and binary dampers (fully open or fully closed) can be included in the HVAC network and are coupled to the FDS control function capability. You can select from three fan models. The HVAC solver is invoked if there is an HVAC namelist group present in the input file. An HVAC network is defined by providing inputs for the ducts; duct nodes; and any fans, dampers, filters, or heating and coiling coils present in the system. Additionally you must define the locations where the HVAC network joins the computational domain. The basic syntax for an HVAC component is: &HVAC TYPE_ID='componenttype', ID='componentname', ... / TYPE_ID is a character string that indicates the type of component that the namelist group is defining. TYPE_ID can be DUCT, NODE, FAN, FILTER, AIRCOIL, or LEAK (see Section 9.3.2). ID is a character string giving a name to the component. The name must be unique amongst all other components of that type; however, the same name can be given to components of different types (i.e., a duct and a node can have the same name but two ducts cannot). A number of examples of simple HVAC systems are given in the HVAC folder of the sample cases and are discussed in the FDS Verification Guide. As described in the Technical Reference Manual, the HVAC pressure solution is not directly coupled to the FDS pressure solution. Rather their is implicit coupling using the wall boundary condition for HVAC vents. At times this can result in stability problems. If this occurs, setting the keyword DT_HVAC on the MISC line may help. When set, this will cause FDS to use the value of DT_HVAC in the HVAC solver rather than the current FDS time step (filter loading will use the FDS time step). In effect as DT_HVAC is increased, the HVAC solution will approach a steady-state solution for the current conditions in the domain. 9.2.1 HVAC Duct Parameters A typical input line specifying a duct is as follows: 101 &HVAC TYPE_ID='DUCT', ID='ductname', NODE_ID='node 1','node 2', AREA=3.14, LOSS=1.,1., LENGTH=2., ROUGHNESS=0.001, FAN_ID='fan 1', DEVC_ID='device 1' / where: AIRCOIL_ID is the ID of an aircoil located in the duct. The operation of the aircoil can be controlled by either a device or a control function. AREA is the cross sectional area of the duct in m2 . DAMPER is a logical parameter indicating the presence of a damper in the duct. The state of the damper is controlled by either a device or a control function (see Section 9.2.2). DIAMETER is the diameter of the duct in m. If only DIAMETER is specified, the AREA will be computed assuming a round duct. Do not specify both DIAMETER and PERIMETER. DEVC_ID is the ID of a DEVC for a damper, fan, or aircoil in the duct. An alternative is CTRL_ID. FAN_ID is the ID of a fan located in the duct. Instead of specifying a FAN_ID, you could specify the VOLUME_FLOW rate (m3 /s) through the duct. The operation of the fan can be controlled by either a device or a control function. LENGTH is the LENGTH of the duct in m. Note that LENGTH is not computed automatically as the difference between the XYZ of the duct’s endpoints. LOSS is a pair of real numbers giving the forward and reverse dimensionless "minor losses" loss coefficient (Kminor ) in the duct. Minor losses are pressure losses through components such as tees, valves and bends. However, you can use LOSS to represent wall friction losses if you want - in this case ensure you leave ROUGHNESS unset so that the HVAC solver does not compute a friction factor. The forward direction is defined as flow from the first node listed in NODE_ID to the second node listed in NODE_ID. MASS_FLOW is a fixed mass flow rate (kg/s) through the duct. Only specify one of MASS_FLOW or VOLUME_FLOW. You can change its value in time either using the characteristic time, TAU_VF, to define a tanh (TAU_VF>0) or t2 ramp (TAU_VF<0); or you can specify a RAMP_ID. MASS_FLOW should only be specified for con- ditions where the upstream node density will not change during the solution process. NODE_ID gives the IDs of the nodes on either end of the duct segment. Positive velocity in a duct is defined as flow from the first node to second node. PERIMETER is used along with AREA to specify a duct with non-circular cross-section. The DIAMETER will be computed as the hydraulic diameter. RAMP_LOSS If specified this RAMP is a multiplier for the LOSS. REVERSE is a logical parameter that when .TRUE. indicates that the specified FAN_ID or VOLUME_FLOW blows from the second node to the first. ROUGHNESS is the absolute roughness in m of the duct that is used to compute the friction factor for the duct. If ROUGHNESS is not set, the HVAC solver will not compute the friction factor and the wall friction will be zero - if this is the case you may want to account for wall friction losses in LOSS. "Perfectly smooth" ducts and pipes still have wall losses and therefore setting ROUGHNESS to zero will tell the HVAC solver to compute the minimum friction factor (which is non-zero) - this is not the same as leaving ROUGHNESS unset. 102 VOLUME_FLOW is a fixed flow rate (m3 /s) through the duct. Only specify one of MASS_FLOW or VOLUME_FLOW. If you specify VOLUME_FLOW, you can change its value in time either using the characteristic time, TAU_VF, to define a tanh (TAU_VF>0) or t2 ramp (TAU_VF<0); or you can specify a RAMP_ID. This cannot be controlled by a device or control function; however, a constant volume flow FAN can be. Note that only one of AIRCOIL_ID, DAMPER, or FAN_ID should be specified for a duct. Also note that if one of these is specified, but no device or control function is provided, then the item will be assumed to be on or open as appropriate. To reduce the computational cost of the HVAC solver, a duct should be considered as any length of duct that connects two items that must be defined as nodes (i.e., a connection to the FDS domain, a filter, or a location where more than two ducts join). That is, a duct should be considered as any portion of the HVAC system where flow can only be in one direction at given point in time (flow can reverse direction over time). For example the top of Figure 9.4 shows a segment of an HVAC system where flow from a tee goes through an expansion fitting, two elbows, an expansion fitting, and a straight length of duct before it terminates as a connection to the FDS domain. Figure 9.4: An example of simplifying a complex duct. This could be input as each individual fitting or duct with its associated area and loss as shown in the middle of the figure; however, this would result in five duct segments (one for each component) with six node connections resulting in eleven parameters (five velocities and six pressures) which must be solved for. This is not needed since whatever the flow rate is in any one segment of the duct, that same flow rate exists in all other segments; thus, the velocities in any segment can be found by taking the area ratios, v1 /v2 = A2 /A1 . Since flow losses are proportional to the square of the velocity, an equivalent duct can be constructed using 103 the total length of the duct, and a representative area (Aeff ) or diameter. The pressure losses associated with all the segments of the duct can be collapsed to a single effective loss (Keff ) by summing all of the fitting, K, losses through the duct as follows: Aeff Keff = ∑ Ki (9.2) Ai i where i is a fitting and Ai is the area associated with the fitting loss. 9.2.2 HVAC Dampers Dampers can be modeled in one of two ways. The first method is a simple binary (flow or no flow) damper. This can be placed in a duct by adding the keyword DAMPER along with either a CTRL_ID or DEVC_ID. When the control or device is .TRUE. the damper will be open, and when .FALSE. the damper will be closed and block 100 % of the duct area. The example below shows a duct with a damper that that is linked to a DEVC that closes the damper at 10 s. For further details see the HVAC_damper example case, which is documented in the Verification Guide. &HVAC TYPE_ID='DUCT',ID='EXHAUST 2',NODE_ID='TEE','EXHAUST 2',AREA=0.01, LENGTH=1.0,LOSS=0,0,DAMPER=.TRUE.,DEVC_ID='TIMER'/ &DEVC QUANTITY='TIME',ID='TIMER',SETPOINT=10,INITIAL_STATE=.TRUE.,XYZ=0,0,0/ The second method is to specify a RAMP_LOSS for the duct. This approach multiplies the LOSS array for the duct by the output of the RAMP. This allows for dampers that have leakage and/or dampers that have a variable position other than fully open or fully closed. An example is shown below where the LOSS in the duct changes from 1,1 at 10 s to 2000,2000 at 11 s. &HVAC TYPE_ID='DUCT',ID='EXHAUST 2',NODE_ID='TEE','EXHAUST 2',AREA=0.01, LENGTH=1.0,LOSS=1,1,RAMP_LOSS='LOSS RAMP'/ &RAMP ID='LOSS RAMP',T=10,F=1/ &RAMP ID='LOSS RAMP',T=11,F=2000/ 9.2.3 HVAC Node Parameters Below are three example duct node inputs representing a typical tee-type connection (multiple ducts being joined), a connection to the FDS domain, and a connection to the ambient outside the FDS domain. &HVAC TYPE_ID='NODE', ID='tee', DUCT_ID='duct 1','duct 2',..'duct n', LOSS=lossarray, XYZ=x,y,z / &HVAC TYPE_ID='NODE', ID='FDS connection', DUCT_ID='duct 1', VENT_ID='vent', LOSS=enter,exit / &HVAC TYPE_ID='NODE', ID='ambient', DUCT_ID='duct 1', LOSS=enter,exit, XYZ=x,y,z, AMBIENT=.TRUE. / where: AMBIENT is a logical value. If .TRUE., then the node is connected to the ambient (i.e., it is equivalent to the OPEN boundary condition on a SURF line). DUCT_ID gives the IDs of the ducts connected to the node. Up to 10 ducts can be connected to a node. FILTER_ID gives the ID a filter located at the node. A node with a filter can only have two connected ducts. 104 LOSS is an n by n array of real numbers giving the dimensionless loss coefficients for the node. LOSS(I,J) is the loss coefficient for flow from duct I to duct J expressed in terms of the downstream duct area (see discussion in 9.2.1 on how to adjust losses for area changes). For a terminal node (e.g., a node connected to the ambient or to a VENT) the LOSS is entered as a pair of numbers representing loss coefficient for flow entering the HVAC system and for flow exiting the HVAC system. VENT_ID is the name of the VENT where the node connects to the FDS computational domain. No two VENTs should be defined with the same VENT_ID. XYZ is a triplet of real numbers giving the coordinates of the node. This location is used to compute buoyancy heads. If the node is connected to the FDS domain, then do not specify XYZ. FDS will compute it as the centroid of the VENT. Note that if you do not specify an XYZ for an interior node, then FDS will use the default value of 0,0,0. A duct node must either have two or more ducts attached to it or it must have either AMBIENT=.TRUE. or a specified VENT_ID. When defining a VENT as a component of an HVAC system you must set SURF_ID to ’HVAC’ and you must set the VENT_ID. Note, do not split an HVAC VENT over multiple-meshes. It is permissible to have individual VENT lines for an HVAC system in different meshes, but any single HVAC VENT needs to be contained within a single mesh. This restriction does not apply to surfaces with leakage flows. 9.2.4 HVAC Fan Parameters Below are given sample inputs for the three types of fans supported by FDS. &HVAC TYPE_ID='FAN', ID='constant volume', VOLUME_FLOW=1.0, LOSS=2./ &HVAC TYPE_ID='FAN', ID='quadratic', MAX_FLOW=1., MAX_PRESSURE=1000., LOSS=2. / &HVAC TYPE_ID='FAN', ID='user fan curve', RAMP_ID='fan curve', LOSS=2. / where: LOSS is the loss coefficient for flow through the fan when it is not operational. MAX_FLOW is the maximum volumetric flow of the fan in m3 /s. This input activates a quadratic fan model. MAX_PRESSURE is the stall pressure of the fan in units of Pa. This input activates a quadratic fan model. RAMP_ID identifies the RAMP that contains a table of pressure drop across the fan (Pa) versus the volumetric flow rates (m3 /s) for a user-defined fan curve. TAU_FAN defines a tanh (TAU_FAN > 0) or t2 ramp (TAU_FAN < 0) for the fan. This is applied to the flow rate computed by any of the three types (constant flow, quadratic, or user-defined ramp) of fans. VOLUME_FLOW is the fixed volumetric flow of the fan (m3 /s). If you wish to have a time dependent flow use the VOLUME_FLOW input for a duct rather than for a fan. Note that only one set of fan model inputs (VOLUME_FLOW, RAMP_ID, or MAX_FLOW + MAX_PRESSURE) should be specified. Also note that FAN defines a class of fans rather than one specific fan. Therefore, more than one duct can reference a single FAN. 105 Fan Curves In Section 9.1 there is a discussion of velocity boundary conditions, in which a fan is modeled simply as a solid boundary that blows or sucks air, regardless of the surrounding pressure field. In the HVAC model, this approach to modeling a fan occurs when the fan is specified with a VOLUME_FLOW. In reality, fans operate based on the pressure drop across the duct or manifold in which they are installed. A very simple “fan curve” is given by: s |∆p − ∆pmax | V̇fan = V̇max sign(∆pmax − ∆p) (9.3) ∆pmax This simple “fan curve” is the “quadratic” fan model as the pressure is proportional to the square of the volume flow rate. The volume flow in the absence of a pressure difference, MAX_FLOW, is given by V̇max . The pressure difference, ∆p = p1 − p2 , indicates the difference in pressure between the downstream compartment, or “zone,” and the upstream. The subscript 1 indicates downstream and 2 indicates upstream. The term, ∆pmax , is the maximum pressure difference, MAX_PRESSURE, the fan can operate upon, and it is assumed to be a positive number. The flow through a fan will decrease from V̇max at zero pressure difference to 0 m3 /s at ∆pmax . If the pressure difference increases beyond this, air will be forced backwards through the fan. If the downstream pressure becomes negative, then the volume flow through the fan will increase beyond MAX_FLOW. More complicated fan curves can be specified by defining a RAMP. In the example inputs below, one fan of each type is specified. A constant volume flow fan with a VOLUME_FLOW of 10 m3 /s, a quadratic fan with MAX_FLUX=10 and MAX_PRESSURE=500, and a user-defined fan with the RAMP set to the values of the quadratic fan in 200 Pa increments, &HVAC TYPE_ID='FAN', ID='constant volume', VOLUME_FLOW=10.0/ &HVAC TYPE_ID='FAN', ID='quadratic', MAX_FLOW=10., MAX_PRESSURE=500./ &HVAC TYPE_ID='FAN', ID='user fan curve', RAMP_ID='fan curve'/ &RAMP &RAMP &RAMP &RAMP &RAMP &RAMP &RAMP &RAMP &RAMP &RAMP &RAMP ID='fan ID='fan ID='fan ID='fan ID='fan ID='fan ID='fan ID='fan ID='fan ID='fan ID='fan curve',T=-10.00,F= 1000/ curve',T= -7.75,F= 800/ curve',T= -4.47,F= 600/ curve',T= 4.47,F= 400/ curve',T= 7.75,F= 200/ curve',T= 10.00,F= 0/ curve',T= 11.83,F= -200/ curve',T= 13.42,F= -400/ curve',T= 14.83,F= -600/ curve',T= 16.12,F= -800/ curve',T= 17.32,F=-1000/ Figure 9.5 displays the fan curves for the inputs shown above. Additional examples can be found in the ashrae7 and fan_test example cases, which are documented in the Verification Guide. Jet Fans Fans do not have to be mounted on a solid wall, like a supply or an exhaust fan. If you just want to blow gases in a particular direction, create an obstruction OBST and apply to it VENT lines that are associated with a simple HVAC system. This allows hot, smokey gases to pass through the obstruction, much like a free-standing fan. See the example case jet_fan.fds which places a louvered fan (blowing diagonally down) near a fire (see Fig. 9.6). 106 Static Pressure (Pa) 1000 500 0 −500 −1000 −10 constant volume quadratic user fan curve −5 0 5 10 Volume Flow Rate (m3/s) 15 20 Figure 9.5: Fan curves corresponding to a constant fan with VOLUME_FLOW=10, a quadratic fan with MAX_FLOW=10 and MAX_PRESSURE=500, and a user-defined RAMP equivalent to the quadratic fan. You may also want to construct a shroud around the fan using four flat plates arranged to form a short passageway that draws gases in one side and expels them out the other. The obstruction representing the fan can be positioned about halfway along the passage (if a louvered fan is being used, place the fan at the end of the passage). Figure 9.6: Jet fan with a louvered output UVW=-1,0,-1. 9.2.5 HVAC Filter Parameters A sample input for a filter is given by: &HVAC TYPE_ID='FILTER', ID='filter 1', LOADING=0., SPEC_ID='SOOT', EFFICIENCY=0.99, LOADING_MULTIPLIER=1, CLEAN_LOSS=2., LOSS=100./ where: 107 CLEAN_LOSS is the dimensionless loss coefficient for flow through the filter when it is clean (zero loading). EFFICIENCY is an array of the species removal efficiency from 0 to 1 where 0 is no removal of that species and 1 is complete filtration of the species. The species are identified using SPEC_ID. LOADING is an array of the initial loading (kg) of the filter for each species being filtered. LOADING_MULTIPLIER is an array of the species multiplier, Mi , used in computing the total filter loading when computing the loss coefficient of the filter. LOSS invokes a linear loss coefficient model where the dimensionless loss coefficient, K, is given as a linear function of the total loading, KFILTER = KCLEAN_LOSS + KLOSS ∑ (Li Mi ), where Li is the species loading and Mi is a multiplier. Only one of LOSS or RAMP_ID should be specified. RAMP_ID identifies the RAMP that contains a table of pressure drop across the filter as a function of total loading (the summation term given in the definition of LOSS above). Only one of LOSS or RAMP_ID should be specified. SPEC_ID identifies the tracked species for the inputs of LOADING_MULTIPLIER and LOADING. A sample set of filter inputs is shown below. These lines define a filter that removes the species PARTICULATE with 100 % efficiency. The filter has an initial loss coefficient of 1 and that loss increases by a factor of 7332 for each kg of PARTICULATE captured by the filter. For further details see the sample case HVAC_filter, which is documented in the Verification Guide. &SPEC ID='PARTICULATE',MW=28.,MASS_FRACTION_0=0.001,SPECIFIC_HEAT=1./ &HVAC TYPE_ID='NODE',ID='FILTER',DUCT_ID='DUCT1','DUCT2',XYZ(3)=0.55, FILTER_ID='FILTER'/ &HVAC TYPE_ID='FILTER',ID='FILTER',CLEAN_LOSS=1.,SPEC_ID='PARTICULATE',EFFICIENCY=1., LOSS=7732.446,LOADING_MULTIPLIER=1./ Note that a filter input refers to a class of filters and that multiple ducts can reference the same filter definition. 9.2.6 HVAC Aircoil Parameters An aircoil refers to a device that either adds or removes heat from air flowing through a duct. In a typical HVAC system this is done by blowing the air over a heat exchanger (hence the term aircoil) containing a working fluid such as chilled water or a refrigerant. A sample input line is as follows: &HVAC TYPE_ID='AIRCOIL', ID='aircoil 1', EFFICIENCY=0.5, COOLANT_SPECIFIC_HEAT=4.186, COOLANT_TEMPERATURE=10., COOLANT_MASS_FLOW=1./ where: COOLANT_MASS_FLOW is the flow rate of the working fluid (kg/s). COOLANT_SPECIFIC_HEAT is the specific heat (kJ/(kg · K)) of the working fluid. COOLANT_TEMPERATURE is the inlet temperature of the working fluid (◦ C). EFFICIENCY is the heat exchanger efficiency, η, from 0 to 1. A value of 1 indicates the exit temperatures on both sides of the heat exchanger will be equal. 108 FDS0−86−g80cff4e FDS0−86−g80cff4e 60 Coil Heat Addition (HVAC_aircoil) 55 50 Exit Temperature (K) Heat Addition (kW) 55 60 45 40 35 30 25 20 0 50 45 40 35 30 25 Ideal Q FDS Q 0.2 Duct Exit Temperature (HVAC_aircoil) 0.4 0.6 Time (s) 0.8 20 0 1 Ideal T FDS T 0.2 0.4 0.6 Time (s) 0.8 1 Figure 9.7: (Left) Heat addition and (Right) duct exit temperature for the HVAC_aircoil case. FIXED_Q is the constant heat exchange rate. A negative value indicates heat removal from the duct. The heat exchange rate can be controlled by either RAMP_ID or by TAU_AC. TAU_AC defines a tanh (TAU_AC>0) or t2 ramp (TAU_AC<0) for the aircoil. This is applied to the FIXED_Q of the aircoil. Alternatively, a RAMP_ID can be given. Note that either FIXED_Q or the set COOLANT_SPECIFIC_HEAT, COOLANT_MASS_FLOW, COOLANT_TEMPERATURE, and EFFICIENCY should be specified. In the latter case, the heat exchange is computed as a two step process. First, the outlet temperature is determined assuming 100 % efficient (i.e., both fluids exit at the same temperature): Tfluid,out = c p,gas uduct Aduct ρduct Tduct,in + c p,fluid ṁfluid Tfluid,in c p,gas uduct Aduct ρduct + c p,fluid ṁfluid (9.4) Second, the actual heat exchanged is computed using the EFFICIENCY. q̇coil = η c p,fluid ṁfluid (Tfluid,in − Tfluid,out ) (9.5) Note that an aircoil input refers to a class of aircoils and that multiple ducts can reference the same aircoil definition. The sample input file HVAC_aircoil.fds demonstrates the use of the aircoil inputs. A constant flow duct removes air (defined as 28 g/mol with a specific heat of 1 kJ/(kg · K)) from the floor and injects in through the ceiling at a volume flow rate of 1 m3 /s. An aircoil is defined with a working fluid flowing at 10 kg/s and 100 ◦ C with a specific heat of 4 kJ/(kg · K). The aircoil has an efficiency of 50 %. Using the above equations the aircoil will add 45.2 kW of heat to the gas flowing through the duct resulting in a duct exit temperature of 332 K. These results are shown in Fig. 9.7. 9.2.7 Louvered HVAC Vents The HVAC system being modeled may either have louvers that redirect the flow leaving a vent or the orientation of the real vent may not lie along one of the axes in FDS. To define the flow direction for an HVAC outlet, you can use the keyword UVW on VENT. UVW is the vector indicating the direction of flow from the VENT. For example: 109 &OBST XB=1.0,2.0,0.0,1.0,0.0,1.0 / &VENT XB=1.0,1.0,0.0,1.0,0.0,1.0, SURF_ID='HVAC', ID='HVAC OUTLET', UVW=-1,0,1 / The above input defines a vent lying in the y-z plane facing in the −x direction. The flow vector indicates that the flow from this vent is in the −x direction with a 45 degree up angle (the x and z components are equal in size). FDS will set the tangential velocity of the vent to obtain the specified direction indicated by UVW. This will only be done if the vent is inputting gas into the domain. 9.3 Pressure-Related Effects: The ZONE Namelist Group (Table 17.30) FDS assumes pressure to be composed of a “background” component, p(z,t), plus a perturbation pressure, p̃(x,t). Most often, p is just the hydrostatic pressure, and p̃ is the flow-induced spatially-resolved perturbation. You can specify any number of sealed compartments within the computational domain that can have their own “background” pressures, and these compartments, or “pressure zones,” can be connected via leakage and duct paths whose flow rates are tied to the pressure of the adjacent zones. 9.3.1 Specifying Pressure Zones A pressure zone can be any region within the computational domain that is separated from the rest of the domain, or the exterior, by solid obstructions. There is currently no algorithm within FDS to identify these zones based solely on your specified obstructions. Consequently, it is necessary that you identify these zones explicitly in the input file. The basic syntax for a pressure ZONE is: &ZONE XB=0.3,1.2,0.4,2.9,0.3,4.5 / This means that the rectangular region, 0.3 < x < 1.2, 0.4 < y < 2.9, 0.3 < z < 4.5, is assumed to be within a sealed compartment. There can be multiple ZONEs declared. The indices of the zones, which are required for the specification of leaks and fans, are determined simply by the order in which they are specified in the input file. By default, the exterior of the computational domain is Zone 0. If there are no OPEN boundaries, the entire computational domain will be assumed to be Zone 1. There are several restrictions to assigning pressure zones. First, the declared pressure zones must be completely within a region of the domain that is bordered by solid obstructions. If the sealed region is not rectangular, FDS will extend the specified ZONE boundaries to conform to the non-rectangular region. It is possible to “break” pressure zones by removing obstructions between them. An example of how to break pressure zones is given below. Second, pressure zones can span multiple meshes, but it is recommended that you check the pressure in each mesh to ensure consistency. Also, if the ZONE does span multiple meshes, make sure that the specified rectangular coordinates do so as well. This allows FDS to determine the actual extent of the ZONE independently for each mesh. Note that if you plan to have one zone open up to another via the removal of an obstruction, make sure that the coordinates of the two zones abut (i.e., touch) even if one of the zones includes the solid obstruction that separates them. FDS recognizes that a zone boundary has been removed when two adjacent cells belonging to two different zones have no solid obstruction between them. It is recommended that you extend at least one of the zone boundaries into the solid obstruction separating the two zones. That way, when the obstruction is removed, the newly created gas phase cells will be assigned to one or the other zone and it will become obvious that two adjacent gas phase cells are of two different zones, at which point the zones will merge and no longer have distinct background pressures. For the special case where a zone has periodic boundaries (SURF_ID=’PERIODIC’ on VENT), you must add PERIODIC=.TRUE. on the ZONE line. 110 Example Case: Pressure Rise in a Compartment This example tests several basic features of FDS. A narrow channel, 3 m long, 0.002 m wide, and 1 m tall, has air injected at a rate of 0.1 kg/m2 /s over an area of 0.2 m by 0.002 m for 60 s, with a linear ramp-up and ramp-down over 1 s. The total mass of air in the channel at the start is 0.00718 kg. The total mass of air injected is 0.00244 kg. The domain is assumed two-dimensional, the walls are adiabatic, and STRATIFICATION is set to .FALSE. simply to remove the slight change in pressure and density with height. The domain is divided into three meshes, each 1 m long and each with identical gridding. We expect the pressure, temperature and density to rise during the 60 s injection period. Afterwards, the temperature, density, and pressure should remain constant, according to the equation of state. Figure 9.8 shows the results of this calculation. The density matches exactly showing that FDS is injecting the appropriate amount of mass. The steady state values of the pressure, density and temperature are consistent with the ideal values obtained from the first law of thermodynamics. FDS0−86−g80cff4e 70 6 x 10 5 50 4 Ideal (Temp) FDS (Temp 3) 40 30 20 10 0 FDS0−86−g80cff4e Pressure (pressure_rise) 60 Pressure (Pa) Temperature (°C) Temperature (pressure_rise) 4 Ideal (Pres) FDS (Pres 3) 3 2 1 100 200 300 Time (s) 400 500 0 0 600 100 200 300 Time (s) 400 500 600 FDS0−86−g80cff4e Density (kg/m3) 1.6 Density (pressure_rise) 1.5 1.4 Ideal (Dens) FDS (Dens 3) 1.3 1.2 1.1 1 0 100 200 300 Time (s) 400 500 600 Figure 9.8: Output of pressure_rise test case. Example Case: Breaking Pressure Zones In this example, three simple compartments are initially isolated from each other and from the ambient environment outside. Each compartment is a separate pressure zone. Air is blown into Zone 1 at a constant rate of 0.1 kg/s, increasing its pressure approximately 2000 Pa by 10 s, at which time Zone 1 is opened to Zone 2, decreasing the overall pressure in the two zones to roughly one-third the original value because the 111 volume of the combined pressure zone has been roughly tripled. At 15 s, the pressure is further decreased by opening a door to Zone 3, and, finally, at 20 s the pressure returns to ambient following the opening of a door to the outside. Figure 9.9 displays the pressure within each compartment. Notice that the pressures do FDS0−86−g80cff4e FDS0−86−g80cff4e 2500 2500 Pressure (zone_break_fast) Pressure (Pa) 2000 1500 1000 500 0 0 Ideal (Pres1) Ideal (Pres2) Ideal (Pres3) FDS (pres_1) FDS (pres_2) FDS (pres_3) Pressure (zone_break_slow) 2000 Pressure (Pa) Ideal (Pres1) Ideal (Pres2) Ideal (Pres3) FDS (pres_1) FDS (pres_2) FDS (pres_3) 1500 1000 500 5 10 15 Time (s) 20 25 0 0 30 5 10 15 Time (s) 20 25 30 Figure 9.9: Output of zone_break test cases. The figure on the left results from using a pressure relaxation time of 0.5 s. The figure on the right uses 1 s, the default. not come to equilibrium instantaneously. Rather, the PRESSURE_RELAX_TIME (on the PRES line) is applied to bring the zones into equilibrium over a specified period of time. This is done for several reasons. First, in reality doors and windows do not magically disappear as they do in FDS. It takes a finite amount of time to fully open them, and the slowing of the pressure increase/decrease is a simple way to simulate the effect. Second, relatively large pressure differences between zones wreak havoc with flow solvers, especially ones like FDS that use a low Mach number approximation. To maintain numerical stability, FDS gradually brings the pressures into equilibrium. This second point ought to be seen as a warning. Since pressure zones are defined using XB, it might not be possible to define a complexly shaped set of pressure zones using just the ZONE inputs. A work around for this is to define the complex zone as a series of zones in the same manner you would divide a domain into multiple meshes. The check for merged pressure zones only works if two zones were initially isolated at the start of the calculation (there must have been a wall that was removed). Therefore, define an obstruction at the boundary of the zones that is removed after the first time step. Do not use FDS to study the sudden rupture of pressure vessels! Its low Mach number formulation does not allow for high speed, compressible effects that are very important in such analyses. The zone breaking functionality described in this example is only intended to be used for relatively small pressure differences (<0.1 atm) between compartments. Real buildings cannot withstand substantially larger pressures anyway. Example Case: Irregularly Shaped Zone This example is similar to the ones in the previous section, except in this case, the pressure zone is L-shaped and split across two meshes. The objective is simply to ensure that the specification of the pressure zone is properly accounted for in the model. Figure 9.10 compares the predicted pressure in the compartment compared to an exact solution. Air is injected into the compartment for 5 s, after which the compartment is opened to a smaller compartment. At 15 s, the smaller compartment is opened to the outside. 112 FDS0−86−g80cff4e 4000 3500 Pressure (zone_shape) Pressure (Pa) 3000 2500 Ideal (Pres) FDS (pres_1) 2000 1500 1000 500 0 0 5 10 15 20 25 Time (s) Figure 9.10: Output of zone_shape test case. Shown is the pressure in an L-shaped compartment that is opened to another compartment at 5 s, and the outdoors at 15 s. 9.3.2 Leaks With a few notable exceptions, like containment buildings for nuclear power plants, real world construction is not air tight. Small gaps occur along windows and doors and where walls abut each other and the floors and ceilings. As a compartment is pressurized by a fire, air will escape through these small gaps. This is referred to as leakage. Leakage is inherently a sub-grid scale phenomenon because the leakage area is usually very small. In other words, it is not possible to define a leak directly on the numerical mesh. It is sometimes possible to “lump” the leaks into a single mesh-resolvable hole, but this is problematic for two reasons. First, the leakage area rarely corresponds neatly to the area of a single mesh cell-sized hole. Second, the flow speeds through the hole can be large and cause numerical instabilities. A better way to handle leakage is by exploiting the HVAC model. The compartment surface that is leaking can be thought of as a large HVAC vent that connects via a very small duct to the outside. This allows the leakage to be removed over a large area in the domain (just as it would be in reality) while correctly capturing the actual area of the leakage path. There are two approaches to this. The first approach is by exploiting only pressure zones. A pressure zone is a user-specified volume within the computational domain that is entirely surrounded by solid obstructions. For example, the interior of a closed room can be, and should be, declared a pressure zone. In this approach surfaces within a pressure zone are denoted as leaking and those surfaces can be considered an HVAC vent that connects to the outside via a tiny duct whose area is the leakage area. This leakage approach will prevent a compartment from seeing large pressure changes as fires grow and decay, but it cannot account for effects like exterior wind or the stack effect. The second approach is intended for leaks with well defined locations (a cracked open door where the crack size is subgrid) or for leaks where the stack effect is important. It uses the local pressure (which includes the zone pressure), which allows for leakage to vary in magnitude. Pressure Zone Leakage The pressure zone leakage approach is intended to capture the bulk leakage that occurs through walls. This approach assumes that the amount of leaking gas is very small and that it will exchange sufficient heat as it moves through a wall to be at the same temperature as the wall surface. With this approach the pressure between the source and destination zones is used to compute a leakage flow via the HVAC model. That flow is then uniformly imposed over all surfaces designated as part of the leakage path. The first step is to define pressure zones, leakage areas, and a description of the surfaces through which leaking occurs: 113 &ZONE &ZONE &SURF &SURF XB=..., LEAK_AREA(0)=0.0001 / XB=..., LEAK_AREA(1)=0.0002, LEAK_AREA(0)=0.0003 / ID='LEAKY EXTERIOR WALL',..., LEAK_PATH=1,0 / ID='LEAKY INTERIOR WALL',..., LEAK_PATH=1,2 / The first line designates a region of the computational domain to be Pressure Zone 1. Note that the order of the ZONE lines is important; that is, the order implicitly defines Zone 1, Zone 2, etc. Zone 0 is by default the ambient pressure exterior. In this example, a leak exists between Zone 1 and the exterior Zone 0, and the area of the leak is 0.0001 m2 (1 cm by 1 cm hole, for example). Zone 2 leaks to Zone 1 (and vice versa) with a leak area of 0.0002 m2 . Zone 2 also leaks to the outside with an area of 0.0003 m2 . Note that zones need not be physically connected for a leak to occur, but in each zone, except for the exterior, there must be some surface with a designated LEAK_PATH. Here, in Zones 1 and 2 there are surfaces defined by both ’LEAKY EXTERIOR WALL’ and ’LEAKY INTERIOR WALL’ so as to provide a surface over which to apply the leakage. Leakage is uniformly distributed over all of the solid surfaces assigned the LEAK_PATH. The order of the two pressure zones designated by LEAK_PATH is unimportant, and the solid ostructions where the leakage is applied need not form a boundary between the two zones. The volume flow, V̇ , through a leak of area AL is given by s |∆p| V̇leak = AL sign(∆p) 2 (9.6) ρ∞ where ∆p is the pressure difference between the adjacent compartments (in units of Pa) and ρ∞ is the ambient density (in units of kg/m3 ). The discharge coefficient normally seen in this type of formula is assumed to be 1. In a typical building as the interior pressure rises, the leakage area will grow as small gaps, cracks, and other leakage paths open up. Leakage tests performed according to test standards such as ASTM E 779 provide two additional data points to quantify this behavior. These are the LEAK_PRESSURE_EXPONENT and the LEAK_REFERENCE_PRESSURE. The use of these additional inputs are shown in the equation below as n and ∆pref respectively where AL,ref is given by LEAK_AREA. ∆p n−0.5 AL = AL,ref (9.7) ∆pref By default, n = 0.5 and ∆pref = 4 Pa, meaning that the leak area will not change with pressure unless you specify an exponent other than 0.5. The HVAC output quantities can be used to determine the leakage flows. FDS names the duct connecting Zone A with Zone B ’LEAK A B’ and the duct nodes ’LEAK A B’ for the Zone A side of the leak and ’LEAK B A’ for the Zone B side. Note that for the duct names, FDS will use the lower numbered zone as Zone A. FDS is limited by default to a maximum of 200 ZONE inputs. This can be increased if needed by the parameter MAX_LEAK_PATHS on the MISC line. Localized Leakage The local leakage approach is intended to represent leakage through a specific crack. For example, a cracked open door might have a opening that is too small to resolve with the grid. One would; however, still want to capture the fact that hot gases could escape the top of the crack and cold gases enter the bottom. The local leakage approach uses the local pressure rather than just the zone pressure. Therefore, one can define multiple leakage paths for different windows, over the height of a door, or over the height of a tall stairwell where stack effect might be important. To use this approach two VENT inputs are linked via an HVAC input 114 with TYPE_ID=’LEAK’. In the example below a 0.001 m2 leakage path is created between the VENT with ID=’VENT 1’ and the VENT with ID=’VENT 2’. Note that the SURF_ID for a VENT with localized leakage is not ’HVAC’. The input must correspond to a SURF input. Wall heat transfer will computed based upon the inputs on the referenced SURF input. &VENT XB=...,SURF_ID='SURF 1',ID='VENT 1'/ &VENT XB=...,SURF_ID='SURF 2',ID='VENT 2'/ &HVAC ID='LEAK1',TYPE_ID='LEAK',VENT_ID='VENT 1',VENT2_ID='VENT 2',AREA=0.001/ This will create a duct with the name ’LEAK1’ whose two nodes will be named VENT_ID and VENT2_ID. If the leakage path is to connect to the ambient outside the domain, then set VENT2_ID=’AMBIENT’. In this case the second node will be the first node name with AMB appended (e.g. ’VENT 1 AMB’). Note that each VENT must lie in one pressure zone; however, it may span more than one MESH. Unlike the zone leakage approach, this approach has the option to preserve the energy of the gas flowing through the leak. For example, for door crack using with pressure zone leakage, the outflowing gas at the top of the door would always be the same temperature as the outside of the door. To maintain hot gas flowing out of the leak, add add LEAK_ENTHALPY=.TRUE. to the HVAC input. For each outflowing wall cell, this will compute the enthalpy difference between the temperature of the leak flow and the temperature of the surface and add it as a source of heat to the adjacent gas cell. The default value is LEAK_ENTHALPY=.FALSE. This approach also has the option of changing the flow loss by specifying LOSS on the HVAC input. The default is LOSS=1; the same as for pressure zone leakage. Note, for this approach it is not required that one have pressure zones defined; however, it is recommended to do so if different pressure zones do in fact exist in the model. Example Case: door_crack This example involves a small compartment that contains a fan in one wall and a closed door with leakage at its bottom in the opposite wall. A small (160 kW) fire is added to the compartment. Initially, the pressure rises due to the heat from the fire and the fan blowing air into the compartment. Eventually the pressure rise inside the compartment exceeds the maximum pressure of the fan, at which point the compartment begins to exhaust from both the fan and the leakage. Pressure will continue to rise due to the fire until the pressure relief due to leakage and back flow through the fan equals the pressure increase from the fire. FDS0−86−g80cff4e FDS0−86−g80cff4e 1100 200 Pressure (door_crack) HRR (door_crack) Heat Release Rate (kW) Pressure (Pa) 1050 1000 950 900 150 100 50 850 800 0 500 1000 Time (s) 0 0 1500 500 1000 Time (s) Figure 9.11: Output of door_crack test case. Symbols are expected values. 115 1500 9.3.3 Special Topic: Stack Effect Tall buildings often experience buoyancy-induced air movement due to temperature differences between the interior and exterior, known as stack effect [24]. These temperature differences create flows within vertical shafts (stairwells, atriums, elevator shafts, etc.) due to leaks or openings at different levels. To simulate this phenomenon in FDS, you must include the entire building, or a substantial fraction of it, both inside and out, in the computational domain. It is important to capture the pressure and density decrease in the atmosphere based on the specified temperature LAPSE_RATE (◦ C/m) that is entered on the MISC line. For the case where the stack flow is through small leakage paths, divide the building into one or more pressure ZONEs. The leakage paths can be defined in terms of HVAC components. Note that the leakage model combines all leaky surfaces over the entire height of the building and as a result averages out the pressure gradients. For doing stack effect calculations individual leakage paths should be defined. A simple example is described next. Example Case: Atmospheric_Effects/stack_effect The stack_effect test case is a two-dimensional simulation of a 100 m tall building whose interior air temperature is slightly warmer than its exterior. The building has leakage paths at the top and ground floors only. Since the inside air temperature is slightly warmer, the inside air pressure is slightly higher as well, and it drives air out of the building and in turn draws air into the building at the ground level. The interior air temperature, Tb , is initially 20 ◦ C (293 K), and the exterior air temperature, T∞ , is 10 ◦ C (283 K). The LAPSE_RATE is set to 0 ◦ C/m; thus, T0 (z) = T∞ outside the building and T0 (z) = Tb inside the building. Two small leakage openings are defined 2.5 m above the ground floor and 2.5 m below the roof using the HVAC solver. Each opening is given an area of 0.01 m2 and a loss coefficient of 2 (e.g., an entrance loss into the leak path of 1 and and exit loss out of the leak path of 1 both of which represent a sharp edge opening). The initial density stratification inside and outside the building can be calculated using the relation: ρ0 (z) gW = exp − z (9.8) ρ∞ RT0 where R is the universal gas constant, g is the acceleration of gravity, and W is the average molecular weight of the air, z is the height above the GROUND_LEVEL, and T0 is the ambient temperature. Applying this formula to the external and internal locations at the lower and upper vents results in densities of 1.2412, 1.1989, 1.2272, and 1.1858 kg/m3 , respectively. Since the openings in the building are equally spaced over its height, the neutral plane should be close to its midpoint. This can be computed from: ∆H Tb = 1+ Hn T∞ (9.9) where Hn is the neutral plane height above the bottom vent and ∆H = 95 m is the distance between the leak points. This gives a neutral plane of 46.68 m above the lower vent or 49.18 m above the bottom of the building. Note that this is close to the midpoint value of 50 m above the bottom of the building. The pressure difference across the building’s wall is computed from 1 W p0 (z) g ∆z 1 ∆p = − (9.10) R T∞ Tb where ∆z is the distance from the leak point to the neutral plane. Using the neutral plane location, the ∆z values are -46.68 m for the lower vent and +48.32 m for the upper vent which respectively result in lower 116 FDS0−86−g80cff4e FDS0−86−g80cff4e 5 1.5 Leakage Velocity Density Density (kg/m3) Velocity (m/s) 4 3 2 Ideal Upper Exterior Ideal Lower Exterior Ideal Upper Interior Ideal Lower Interior FDS Upper Exterior FDS Lower Exterior FDS Upper Interior FDS Lower Interior 0.5 Ideal Upper Ideal Lower FDS Upper FDS Lower 1 0 0 1 20 40 60 80 0 0 100 20 Time (s) 40 60 80 100 Time (s) Figure 9.12: (Left) Velocity at the upper and lower vents for the stack_effect case. (Right) Upper and lower exterior and interior densities. and upper vent pressure differences of -19.4 Pa and +20.4 Pa. Using the loss of 2 and the pressure difference in the HVAC momentum equation results in a steady-state inflow velocity at the bottom of 3.95 m/s and an outflow velocity at the top of 4.15 m/s. Results for velocity and density are shown in Fig. 9.12. 9.4 Pressure Boundary Conditions In some situations, it is more convenient to specify a pressure, rather than a velocity, at a boundary. Suppose, for example, that you are modeling the interior of a tunnel, and a wind is blowing at one of the portals that affects the overall flow within the tunnel. If (and only if) the portal is defined using an OPEN vent, then the dynamic pressure at the boundary can be specified like this: &VENT XB=..., SURF_ID='OPEN', DYNAMIC_PRESSURE=2.4, PRESSURE_RAMP='wind' / &RAMP ID='wind', T= 0.0, F=1.0 / &RAMP ID='wind', T=30.0, F=0.5 / . . The use of a dynamic pressure boundary affects the FDS algorithm as follows. At OPEN boundaries, the hydrodynamic pressure (head) H is specified as H = DYNAMIC_PRESSURE/ρ∞ + |u|2 /2 H = DYNAMIC_PRESSURE/ρ∞ (outgoing) (incoming) (9.11) where ρ∞ is the ambient density and u is the most recent value of the velocity on the boundary. The PRESSURE_RAMP allows you to alter the pressure as a function of time. Note that you do not need to ramp the pressure up or down starting at zero, like you do for various other ramps. The net effect of a positive dynamic pressure at an otherwise quiescent boundary is to drive a flow into the domain. However, a firedriven flow of sufficient strength can push back against this incoming flow. The following lines, taken from the sample case, pressure_boundary, demonstrates how to specify a time-dependent pressure boundary at the end of a tunnel. The tunnel is 10 m long, 1 m wide, 1 m tall with a fire in the middle and a pressure boundary imposed on the right side. The left side (XMIN) is just an OPEN boundary with no pressure specified. It is assumed to be at ambient pressure. 117 &VENT &VENT &RAMP &RAMP &RAMP MB = 'XMIN' SURF_ID = 'OPEN' / MB = 'XMAX' SURF_ID = 'OPEN', DYNAMIC_PRESSURE=2.4, PRESSURE_RAMP='wind_ramp' / ID='wind_ramp', T= 0., F= 1. / ID='wind_ramp', T=15., F= 1. / ID='wind_ramp', T=16., F=-1. / Figure 9.13 shows two snapshots from Smokeview taken before and after the time when the positive pressure is imposed at the right portal of a tunnel. The fire leans to the left because of the preferential flow in that direction. It leans back to the right when the positive pressure is directed to become negative. Figure 9.13: Snapshots from the sample case pressure_boundary showing a fire in a tunnel leaning left, then right, due to a positive, then negative, pressure imposed at the right portal. 118 9.5 Special Flow Profiles By default, the air injected at a vent has a uniform or “top hat” velocity profile, but the parameter PROFILE on the SURF line can yield other profiles. Parabolic PROFILE=’PARABOLIC’ produces a parabolic profile with VEL (m/s) being the maximum velocity or VOLUME_FLOW (m3 /s) being the desired volume flow. As an example, the test case in the Flowfields examples folder called parabolic_profile.fds demonstrates how you can create a circular or rectangular vent, each with a parabolic inlet profile. The two VENT lines below create circular and rectangular inlets, respectively, each of which inject air (or the background gas) at a rate of 0.5 m3 /s into the compartment. &SURF ID='BLOW', VOLUME_FLOW=-0.5, PROFILE='PARABOLIC' / &VENT SURF_ID='BLOW', XB=-3,1,-3,1,0,0, RADIUS=2., XYZ=-1,-1,0 / &VENT SURF_ID='BLOW', XB= 3,5,-2,1,0,0 / The purpose of the test case is to ensure that the proper amount of gas (in this case nitrogen) is forced into the compartment, as confirmed by the pressure rise. Figure 9.14 displays a comparison of the calculated versus the exact pressure rise in a large compartment with these two parabolic vents. The pressure should rise according to the equation and analytical solution: dp γ V̇ = p dt V =⇒ γ V̇ p(t) − p0 = p0 e V t − 1 (9.12) where the ratio of specific heats, γ = 1.4, volume flow rate, V̇ = 1 m3 /s, volume, V = 4000 m3 , and ambient pressure, p0 = 101325 Pa. Note that to obtain this simple result, FDS was run with the option CONSTANT_SPECIFIC_HEAT_RATIO set to true. Git−r21−69−gfe82f22−dirty 2500 Pressure (parabolic_profile) Pressure (Pa) 2000 1500 1000 500 Exact (Pressure) FDS (pres) 0 0 10 20 30 Time (s) 40 50 60 Figure 9.14: Results of the parabolic_profile test case Atmospheric PROFILE=’ATMOSPHERIC’ produces a power law atmospheric wind profile of the form u = u0 (z/z0 ) p where z is the height above the ground. If an atmospheric profile is prescribed, also prescribe Z0 for z0 and 119 PLE for p. VEL specifies the reference velocity u0 . Note that z0 is not the ground, but rather the height above the ground where the wind speed is measured, like an elevated weather station. It is assumed that the ground is located at 0 m; to change this assumption, set GROUND_LEVEL on the MISC line to be the appropriate elevation. Be careful not to apply an atmospheric velocity profile (e.g. negative z) below GROUND_LEVEL or FDS will stop with an error. Boundary Layer (Circular Vent) PROFILE=’BOUNDARY LAYER’ may be used for circular vents created using RADIUS. By adding VEL_BULK on the SURF line together with VEL, FDS will produce a plug flow core profile, with max velocity given by VEL, and a quadradic profile in the boundary layer. The form of the profile is illustrated in Fig. 9.15 for a circular vent. The functional form of the velocity profile (here taken a vertical profile) is if r ≤ R − δ wmax 2 w(r) = r−(R−δ ) if R − δ < r ≤ R wmax 1 − δ (9.13) The bulk velocity is the volumetric flow rate divided by the circular flow area. VEL_BULK is negative pointing into the domain. The boundary layer thickness is δ . This feature is handy for dealing with the case where the maximum velocity is higher than the bulk velocity. Here is an example input file line: &SURF ID='JET', VEL=-69, VEL_BULK=-53, PROFILE='BOUNDARY LAYER', ... / ∂w =0 ∂r wmax d R Figure 9.15: Boundary layer profile. 9.6 Atmospheric Stratification Another useful parameter for outdoor simulations is the temperature lapse rate of the atmosphere. Typically, in the first few hundred meters of the atmosphere, the temperature decreases several degrees Celsius per kilometer. This small temperature change is important when considering the rise of smoke since the temperature of the smoke decreases rapidly as it rises. The LAPSE_RATE of the atmosphere can be specified on the MISC line in units of ◦ C/m. A negative sign indicates that the temperature decreases with height. This need only be set for outdoor calculations where the height of the domain is tens or hundreds of meters. The default value of the LAPSE_RATE is 0 ◦ C/m. 120 By default, FDS assumes that the density and pressure decrease with height, regardless of the application or domain size. For most simulations, this effect is negligible, but it can be turned off completely by setting STRATIFICATION=.FALSE. on the MISC line. 121 122 Chapter 10 User-Specified Functions Many of the parameters specified in the FDS input file are fixed constants. However, there are several parameters that may vary in time, temperature, or space. The namelist groups, RAMP and TABL, allow you to control the behavior of these parameters. RAMP allows you to specify a function with one independent variable (such as time) and one dependent variable (such as velocity). TABL allows you to specify a function of multiple independent variables (such as a solid angle) and multiple dependent variables (such as a sprinkler flow rate and droplet speed). 10.1 Time-Dependent Functions At the start of any calculation, the temperature is ambient everywhere, the flow velocity is zero everywhere, nothing is burning, and the mass fractions of all species are uniform. When the calculation starts temperatures, velocities, burning rates, etc., are ramped-up from their starting values because nothing can happen instantaneously. By default, everything is ramped-up to their prescribed values in approximately 1 s. However, you can control the rate at which things turn on, or turn off, by specifying time histories either with pre-defined functions or with user-defined functions. The parameters TAU_Q, TAU_T, and TAU_V indicate that the heat release rate (HRRPUA); surface temperature (TMP_FRONT); and/or normal velocity (VEL, VOLUME_FLOW), or MASS_FLUX_TOTAL are to ramp up to their prescribed values in TAU seconds and remain there. To prescribe different heat release rate ramps, the TAU_Q parameter can be defined as a negative value (t-squared growth rate) or a positive value (tanh growth rate), which results in a time-dependent heat release rate as Q̇0 t 2 if TAU_Q is negative τ Q̇(t) = (10.1) Q̇ · tanh t if TAU_Q is positive 0 τ where Q̇0 is the user-specified heat release rate. If the fire ramps up following a t-squared curve, then it remains constant after TAU_Q seconds. These rules apply to TAU_T and TAU_V as well. The default value for all TAUs is 1 s. If something other than a tanh or t-squared ramp up is desired, then a user-defined function must be input. To do this, set RAMP_Q, RAMP_T or RAMP_V equal to a character string designating the ramp function to use for that particular surface type, then somewhere in the input file generate lines of the form: &RAMP ID='rampname1', T= 0.0, F=0.0 / &RAMP ID='rampname1', T= 5.0, F=0.5 / &RAMP ID='rampname1', T=10.0, F=0.7 / 123 Here, T is the time, and F indicates the fraction of the heat release rate, wall temperature, velocity, mass fraction, etc., to apply. Linear interpolation1 is used to fill in intermediate time points. Note that each set of RAMP lines must have a unique ID and that the lines must be listed with monotonically increasing T. Note also that the TAUs and the RAMPs are mutually exclusive. For a given surface quantity, both cannot be prescribed. As an example, a simple blowing vent can be controlled via the lines: &SURF ID='BLOWER', VEL=-1.2, TMP_FRONT=50., RAMP_V='BLOWER RAMP', RAMP_T='HEATER RAMP' / &RAMP ID='BLOWER RAMP', T= 0.0, F=0.0 / &RAMP ID='BLOWER RAMP', T=10.0, F=1.0 / &RAMP ID='BLOWER RAMP', T=80.0, F=1.0 / &RAMP ID='BLOWER RAMP', T=90.0, F=0.0 / &RAMP ID='HEATER RAMP', T= 0.0, F=0.0 / &RAMP ID='HEATER RAMP', T=20.0, F=1.0 / &RAMP ID='HEATER RAMP', T=30.0, F=1.0 / &RAMP ID='HEATER RAMP', T=40.0, F=0.0 / Use TAU_T or RAMP_T to control the ramp-ups for surface temperature. The surface temperature at time t, Tw (t), is Tw (t) = T0 + f (t) (TMP_FRONT − T0 ) (10.2) where f (t) is the result of evaluating the RAMP_T at time t, T0 is the ambient temperature, and TMP_FRONT is specified on the same SURF line as RAMP_T. Use TAU_MF(N) or RAMP_MF(N) to control the ramp-ups for either the mass fraction or mass flux of species N. For example: &SURF ID='...', MASS_FLUX(1:2)=0.1,0.3, SPEC_ID(1:2)='ARGON','NITROGEN', TAU_MF(1:2)=5.,10. / indicates that argon and nitrogen are to be injected at rates of 0.1 kg/(m2 · s) and 0.3 kg/(m2 · s) over time periods of approximately 5 s and 10 s, respectively. Table 10.1 lists the various quantities that can be controlled by RAMPs. 10.2 Temperature-Dependent Functions Thermal properties like conductivity and specific heat can vary significantly with temperature. In such cases, use the RAMP function like this: &MATL ID FYI SPECIFIC_HEAT_RAMP CONDUCTIVITY_RAMP DENSITY = = = = = 'STEEL' 'A242 Steel' 'c_steel' 'k_steel' 7850. / &RAMP ID='c_steel', T= 20., F=0.45 &RAMP ID='c_steel', T=377., F=0.60 &RAMP ID='c_steel', T=677., F=0.85 / / / &RAMP ID='k_steel', T= 20., F=48. &RAMP ID='k_steel', T=677., F=30. / / 1 By default, FDS uses a linear interpolation routine to find time or temperature-dependent values between user-specified points. The default number of interpolation points is 5000, more than enough for most applications. However, you can change this value by specifying NUMBER_INTERPOLATION_POINTS on any RAMP line. 124 Table 10.1: Parameters for controlling the time-dependence of given quantities. Quantity Heat Release Rate Heat Flux Temperature Velocity Volume Flux Mass Flux Mass Fraction Mass Flux Particle Mass Flux External Heat Flux Pressure Flow Gravity Gravity Gravity Group Input Parameter(s) TAU RAMP ID SURF SURF SURF SURF SURF SURF SURF SURF SURF SURF VENT PROP MISC MISC MISC HRRPUA NET_HEAT_FLUX, etc. TMP_FRONT VEL VOLUME_FLOW MASS_FLUX_TOTAL MASS_FRACTION(N) MASS_FLUX(N) PARTICLE_MASS_FLUX EXTERNAL_FLUX DYNAMIC_PRESSURE FLOW_RATE GVEC(1) GVEC(2) GVEC(3) TAU_Q TAU_Q TAU_T TAU_V TAU_V TAU_V TAU_MF(N) TAU_MF(N) TAU_PART TAU_EF RAMP_Q RAMP_Q RAMP_T RAMP_V RAMP_V RAMP_V RAMP_MF(N) RAMP_MF(N) RAMP_PART RAMP_EF PRESSURE_RAMP FLOW_RAMP RAMP_GX RAMP_GY RAMP_GZ FLOW_TAU Note that for temperature ramps, as opposed to time ramps, the parameter F is the actual physical quantity, not just a fraction of some other quantity. Thus, if CONDUCTIVITY_RAMP is used, there should be no value of CONDUCTIVITY given. Note also that for values of temperature, T, below and above the given range, FDS will assume a constant value equal to the first or last F specified. Note also that the DENSITY of a material cannot be controlled with a RAMP function. 10.3 Spatially-Dependent Velocity Profiles Similar to using PROFILE=‘ATMOSPHERIC’ on SURF, it is possible to specify PROFILE=‘RAMP’ to generate 1D or 2D profiles of the normal component of velocity on a surface. The following code generates a u(z) profile on the ‘XMIN’ boundary. &SURF ID='inlet', VEL=-7.72, PROFILE='RAMP', RAMP_V_Z='u_prof' / &VENT MB='XMIN', SURF_ID='inlet'/ &RAMP &RAMP &RAMP &RAMP &RAMP &RAMP &RAMP &RAMP &RAMP &RAMP &RAMP &RAMP &RAMP &RAMP ID='u_prof', ID='u_prof', ID='u_prof', ID='u_prof', ID='u_prof', ID='u_prof', ID='u_prof', ID='u_prof', ID='u_prof', ID='u_prof', ID='u_prof', ID='u_prof', ID='u_prof', ID='u_prof', T=0., T=0.0098, T=0.01005, T=0.01029, T=0.01077, T=0.01174, T=0.01368, T=0.01562, T=0.01756, T=0.0195, T=0.02144, T=0.02338, T=0.02532, T=0.0588, F=0. / F=0. / F=0.2474 F=0.4521 F=0.6256 F=0.7267 F=0.8238 F=0.8795 F=0.9378 F=0.9663 F=0.9922 F=0.9987 F=1 / F=1 / / / / / / / / / / / 125 Note that V indicates the velocity component normal to the surface. You can also specify RAMP_V_X and RAMP_V_Y and add these to the SURF. Note that only profiles in the planar directions will affect a given surface. That is, if the surface is oriented in the y − z plane, only RAMP_V_Y and RAMP_V_Z apply. In the RAMP definition T is the independent variable and F is the dependent variable. In this example, T is the z coordinate in meters and F is the factor multiplying VEL on the SURF line. Two ramps may be applied to a surface. T does not need to be directly related to the FDS mesh. The velocity points will be interpolated linearly by the ramp function. In fact, this functionality is convenient for taking experimental data directly as a boundary condition to FDS. You basically just need to list T and F from the data (you can set VEL=-1 if you want). The results for this particular case can be seen in the section, “Flow Over a Backward Facing Step”, in the FDS Verification Guide [3]. In this problem, the step height is h = 0.0098 m, and the specification of the inlet profile is critical to correctly matching the reattachment point downstream of the step. 126 Chapter 11 Chemical Species FDS was designed primarily to study fire phenomena, and much of the basic chemistry of combustion is handled with a minimum of user inputs. However, there are many applications in which you might want to simulate the movement of gases in the absence of fire, or additional chemical species might be added to a simulation that involves fire. Gas species are defined with the input group SPEC. This input group is used to define both primitive gas species and lumped species (mixtures of one or more primitive SPEC). There are different roles that a gas species might play in a simulation. A gas species might be explicitly tracked. In other words, a transport equation is solved for it. A gas species might just serve as the “background” species. Note that no transport equation is needed for the background species as it is whatever mass remains after all other tracked species have been accounted for. Or, a gas species might be one component of a mixture of gases that are transported together. For example, FDS exploits the idea that the products of combustion from a fire mix and travel together; you only need to solve one transport equation for this “lumped species.” The default combustion model in FDS assumes that the reaction is mixing-controlled, and transport equations for only the lumped species—Fuel and Products—are solved (the lumped species Air is the default background). There is no reason to solve individual (and costly) transport equations for the major reactants and products of combustion—Fuel, O2 , CO2 , H2 O, N2 , CO and soot—because they are all pre-tabulated functions of the three lumped species. More detail on combustion is given in Chapter 12. For the moment, just realize that you need not, and should not, explicitly list the reactants and products of combustion using SPEC lines if all you want is to model a fire involving a hydrocarbon fuel. 11.1 Specifying Primitive Species The SPEC input is used to define a gas species in FDS. Once defined the species can be tracked as a single species (i.e., a primitive species) and/or the species can be used as part of one or more lumped species, also defined with SPEC. It is possible for a species to be both part of a lumped species and tracked separately. Often an extra gas introduced into a calculation is the same as a product of combustion, like water vapor from a sprinkler or carbon dioxide from an extinguisher. These gases are tracked separately. Thus, water vapor generated by the combustion is tracked via the Products lumped species variable and water vapor generated by evaporating sprinkler droplets is tracked via its own transport equation. If a species is only to be used as part of one or more lumped species, LUMPED_COMPONENT_ONLY=.TRUE. must be added to the SPEC line. This tells FDS not to allocate space for the species in the array of tracked gases. If a species is to be used as the background species, the parameter BACKGROUND=.TRUE. should be set on the SPEC line. This also tells FDS not to allocate space for the species in the array of tracked gases. A species with LUMPED_COMPONENT_ONLY=.TRUE. cannot be used as an individual species, and it cannot be used as the background species. However, a primitive species with BACKGROUND=.TRUE. can also be used 127 as part of a lumped species definition. Note that the default background species is AIR which is defined as a lumped species consisting of N2 , O2 , CO2 , and H2 O. If no background species is defined in the input file, then FDS will create the background species of AIR by internally creating the input lines shown in Example 2 in Section 11.2. Note that while you can define as many species using SPEC as you wish in an input file, any namelist input tied to a list of species, such as any SPEC_ID input or MASS_FRACTION input, is limited to using no more than 20 species. 11.1.1 Basics Each SPEC line should include at the very least the name of the species via a character string, ID. Once the extra species has been declared, you introduce it at surfaces via the parameters MASS_FRACTION(:) or MASS_FLUX(:) along with the character array SPEC_ID(:). A very simple example of how a gas—in this case hydrogen—can be introduced into the simulation is given by the simple input file called gas_filling.fds. The relevant lines are as follows: &SPEC &SURF &RAMP &RAMP &RAMP &RAMP &VENT &DUMP ID='HYDROGEN' / ID='LEAK', SPEC_ID(1)='HYDROGEN', MASS_FLUX(1)=0.01667, RAMP_MF(1)='leak_ramp' / ID='leak_ramp', T= 0., F=0.0 / ID='leak_ramp', T= 1., F=1.0 / ID='leak_ramp', T=180., F=1.0 / ID='leak_ramp', T=181., F=0.0 / XB=-0.6,0.4,-0.6,0.4,0.0,0.0, SURF_ID='LEAK', COLOR='RED' / MASS_FILE=.TRUE. / The hydrogen is injected through a 1 m by 1 m vent at a rate of 0.01667 kg/(m2 · s) and shut off after 3 min. The total mass of hydrogen at that point ought to be 3 kg (see Fig. 11.1). Notice that no properties were needed for the HYDROGEN because it is a species whose properties are included in Table 11.1. The background species in this case is assumed to be air. The mass flow rate of the hydrogen is controlled via the ramping parameter RAMP_MF(1). The parameter MASS_FILE=.TRUE. instructs FDS to produce an output file that contains a time history of the hydrogen mass. FDS0−86−g80cff4e 4 Hydrogen Mass (kg) 3.5 Hydrogen Mass (gas_filling) 3 2.5 2 1.5 1 0.5 0 0 50 100 150 Time (s) 200 250 300 Figure 11.1: Hydrogen mass vs. time for gas_filling test case. 128 Initial Conditions If the initial mass fraction of the gas is something other than zero, then the parameter MASS_FRACTION_0 is used to specify it. For example, if you want the initial concentration in the domain to be 90% background (air) diluted with 10% argon, use &SPEC ID='ARGON', MASS_FRACTION_0=0.1 / Specifying Humidity If you are using the default background lumped species AIR, then you can specify HUMIDITY on MISC to set the ambient mass fraction of water vapor. HUMIDITY is the relative humidity of water vapor in units of %. It is 40 % by default. If you are defining the primitive species of WATER VAPOR, then MASS_FRACTION_0 is independent of HUMIDITY. That is setting MASS_FRACTION_0 for the species WATER VAPOR will not change the ambient humidity, it will add additional water vapor. 11.1.2 Specifying Gas and Liquid Species Properties There are several options for specifying the properties of gas and liquid species. Option 1: FDS Defined Species Gases and liquids whose properties are tabulated within FDS are listed in Table 11.1. The physical properties of these species are known and do not need to be specified. When using one of these gases you need only specify the correct ID and provide, if needed, the initial mass fraction. FDS will then use precompiled data to compute the various thermophysical properties from 0 K to 5000 K. 129 Table 11.1: Optional gas and liquid species [25] Species ACETONE ACETYLENE ACROLEIN AMMONIA ARGON BENZENE BUTANE CARBON CARBON DIOXIDE CARBON MONOXIDE CHLORINE DODECANE ETHANE ETHANOL ETHYLENE FORMALDEHYDE HELIUM HYDROGEN HYDROGEN ATOM HYDROGEN BROMIDE HYDROGEN CHLORIDE HYDROGEN CYANIDE HYDROGEN FLUORIDE HYDROGEN PEROXIDE HYDROPEROXY RADICAL HYDROXYL RADICAL ISOPROPANOL LJ AIR METHANE METHANOL N-DECANE N-HEPTANE N-HEXANE N-OCTANE NITRIC OXIDE NITROGEN NITROGEN ATOM NITROGEN DIOXIDE NITROUS OXIDE OXYGEN OXYGEN ATOM PROPANE Mol. Wt. (g/mol) 58.07914 26.037280 56.063260 17.03052 39.948000 78.11184 58.122200 12.0107 44.009500 28.010100 70.906 170.33484 30.069040 46.068440 28.053160 30.025980 4.002602 2.015880 1.007940 80.911940 36.460940 27.025340 20.006343 34.014680 33.006740 17.007340 60.095020 28.854760 16.042460 32.041860 142.281680 100.201940 86.175360 114.228520 30.006100 28.013400 14.006700 46.05500 44.012800 31.998800 15.999400 44.095620 Formula C3 H6 O C2 H2 C3 H4 O NH3 Ar C6 H6 C4 H10 C CO2 CO Cl2 C1 2H2 6 C2 H6 C2 H5 OH C2 H4 CH2 O He H2 H HBr HCl HCN HF H2 O2 HO2 OH C3 H7 OH CH4 CH2 OH C10 H22 C7 H16 C6 H12 C8 H18 NO N2 N NO2 N2 O O2 O C3 H8 130 σ (Å) 4.6 4.033 4.549 2.9 3.42 5.349 4.687 2.94 3.941 3.690 4.217 4.701 4.443 4.530 4.163 3.626 2.551 2.827 2.31 3.353 3.339 3.63 3.148 3.02 3.02 2.66 4.549 3.711 3.758 3.626 5.233 4.701 5.949 4.892 3.492 3.798 2.66 3.992 3.828 3.467 2.66 5.118 ε/k (K) 560.2 231.8 576.7 558.3 124.0 412.3 531.4 74.8 195.2 91.7 316.0 205.78 215.7 362.6 224.7 481.8 10.22 59.7 123.6 449.0 344.7 569.1 330.0 106.5 106.5 92.1 576.7 78.6 148.6 481.8 226.46 205.75 399.3 231.16 116.7 71.4 92.1 204.88 232.4 106.7 92.1 237.1 Liquid Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y RadCal Surrogate MMA PROPYLENE MMA TOLUENE PROPANE CARBON DIOXIDE CARBON MONOXIDE N-HEPTANE ETHANE METHANOL ETHYLENE METHANOL Y Y Y Y Y Y METHANOL Y Y Y Y Y Y Y Y METHANE METHANOL N-HEPTANE N-HEPTANE N-HEPTANE N-HEPTANE Y Y Y Y PROPANE Table 11.1: Optional gas and liquid species (continued). Species PROPYLENE SOOT SULFUR DIOXIDE SULFUR HEXAFLUORIDE TOLUENE WATER VAPOR Mol. Wt. (g/mol) 42.079740 10.910420 64.063800 146.055419 92.138420 18.015280 Formula C3 H6 C0.9 H0.1 SO2 SF6 C6 H5 CH3 H2 O σ (Å) 4.678 3.798 4.112 5.128 5.698 2.641 ε/k (K) 298.9 71.4 335.4 146.0 480.0 809.1 Liquid Y RadCal Surrogate PROPYLENE SOOT Y Y Y TOLUENE WATER VAPOR Option 2: User-Specified Properties If the gas species is not included in Table 11.1, then you must specify its thermophysical properties. By using the inputs discussed below, you can also override the default properties for a pre-defined gas species. For a gas species not included in Table 11.1, its molecular weight, MW, should be specified on the SPEC line in units of g/mol, otherwise the molecular weight of nitrogen will be used. If the species is participating in a reaction, then the ENTHALPY_OF_FORMATION in units of kJ/mol must also be specified. Additional discussion on the enthalpy of formation can be found in Chapter 12. The remaining thermophysical properties of conductivity, diffusivity, enthalpy, viscosity, absorptivity (thermal radiation), and liquid properties are discussed below. Most properties can be defined as a constant value or as a temperature dependent look-up table using a RAMP. For the latter, if the temperature in a gas cell is above or below the endpoints of the RAMP, the endpoint value will be used. FDS will not extrapolate beyond the ends of the RAMP. Conductivity Conductivity can be specified in one of three ways: it can be defined as a constant using CONDUCTIVITY (W/(m · K)), it can be defined as a temperature vs. specific heat ramp using RAMP_K, or it can be computed by FDS using MW, PR_GAS on SPEC (default value is PR on MISC), and the Lennard-Jones potential parameters σ (SIGMALJ) and ε/k (EPSILONKLJ). If no inputs are specified, FDS will compute the conductivity using the MW and the Lennard-Jones parameters for nitrogen. Diffusivity Diffusivity is assumed to be the binary diffusion coefficient between the given species and the background species. Diffusivty can be specified in one of three ways: it can be defined as a constant using DIFFUSIVITY (m2 /s), it can be defined as a temperature vs. diffusivity ramp using RAMP_D, or it can be computed by FDS using MW and the Lennard-Jones potential parameters σ (SIGMALJ) and ε/k (EPSILONKLJ). If no inputs are specified, FDS will compute the diffusivity using the MW and the Lennard-Jones parameters for nitrogen. Enthalpy The enthalpy of the gas mixture is given by the following formula: h(T ) = h(Tref ) + Z T Tref c p (T 0 ) dT 0 (11.1) where c p is the SPECIFIC_HEAT (kJ/(kg · K)) with optional temperature dependence using RAMP_CP. The (optional) REFERENCE_TEMPERATURE, Tref (◦ C), is the temperature that corresponds to the 131 REFERENCE_ENTHALPY, h(Tref ) (kJ/kg). The default value of the REFERENCE_TEMPERATURE is 25 ◦ C. If SPECIFIC_HEAT is specified and the REFERENCE_ENTHALPY is not, the REFERENCE_ENTHALPY will be set to h(Tref ) = c p Tref . If no inputs for enthalpy are provided, then the specific heat of the gas will be calculated from its molecular weight using the relation: γ R c p,α = (11.2) γ − 1 Wα The ratio of specific heats, GAMMA, is 1.4 by default and can be changed on the MISC line. If you want all the gas specific heats to follow this relation, set CONSTANT_SPECIFIC_HEAT_RATIO=.TRUE. on the MISC line (note: this option also requires STRATIFICATION=.FALSE.). In this case the REFERENCE_ENTHALPY will be assumed to be 0 kJ/kg at a REFERENCE_TEMPERATURE of 0 K. The reference enthalpy can also be determined by defining the ENTHALPY_OF_FORMATION on the SPEC line with a reference temperature for all species given by H_F_REFERENCE_TEMPERATURE on the MISC line (default is 25 ◦ C). Note that ENTHALPY_OF_FORMATION will override any value given for REFERENCE_ENTHALPY. Viscosity The dynamic viscosity of the gas species can be specified in one of three ways: it can be defined as a constant using VISCOSITY, it can be defined as a temperature vs. viscosity ramp using RAMP_MU, or it can be computed using the Lennard-Jones potential parameters σ (SIGMALJ) and ε/k (EPSILONKLJ). If no viscosity inputs are provided, FDS will use the Lennard-Jones values for nitrogen. Radiative Properties Species that can absorb and emit thermal radiation are defined via the parameter RADCAL_ID on the SPEC line. Some of the predefined species have this parameter already defined as shown in Table 11.1. There are, however, many other species which are absorbing. For absorbing species not listed in Table 11.1, RADCAL_ID can be used to identify a RadCal [5] species to serve as a surrogate. For example: &SPEC ID='ETHANOL', RADCAL_ID='METHANOL' / would use the RadCal absorptivities for METHANOL when computing the absorptivity of ETHANOL. For absorbing species not present in RadCal, it is recommended to choose a RadCal surrogate with similar molecular functional groups and molecular mass. The infrared spectrum is greatly affected by the species molecular functional groups. Species molecular mass also affects the spectrum: a heavier species of a given molecular functional group tends to absorb and emit more infrared radiation than a lighter species of the same functional group. For simple chemistry, if the fuel is not present in Table 11.1 and no FUEL_RADCAL_ID is provided on the REAC line, then the absorption properties of methane will be used. Gibbs Energy If a reverse chemical reaction is specified using REVERSE=.TRUE. on a REAC input, FDS can use the forward reaction kinetics along with the equilibrium values of the reaction to determine the reverse kinetics. The equilibrium is determined by minimizing the Gibbs energy. The temperature vs. Gibbs energy (kJ/mol) for a species can be specified with RAMP_G_F. 132 Liquids If the species listed in Table 11.1 includes liquid properties, it can be applied to liquid droplets, in which case the following relationship should hold: hgas (Tboil ) = hliquid (Tboil ) + hv (11.3) More detail is included in Chapter 14. 11.1.3 Air There are two predefined species for air in FDS. The first predefined species is the default background species of AIR. This is a lumped species consisting of oxygen, nitrogen, carbon dioxide, and water vapor whose mass fractions are controlled by the Y_CO2_INFTY, Y_O2_INFTY, and HUMIDITY inputs. This lumped species is automatically defined by FDS if no other SPEC input is defined as the BACKGROUND. Note that ID=’AIR’ cannot be used on a SPEC input unless that input or some other SPEC input is defined as the BACKGROUND. The second predefined species is the primitive species of LJ AIR. This is an effective gas species whose molecular weight and enthalpy are defined based on the Y_CO2_INFTY and Y_O2_INFTY inputs, and whose other thermophysical properties use the Lennard-Jones parameters for air. For simulations without combustion, using LJ AIR as the BACKGROUND species will slightly reduce the computational cost. Specifying a Chemical Formula If you want FDS to compute the molecular weight of the gas species, you can input a FORMULA rather than the molecular weight, MW. This will also be used as the label for the gas species by Smokeview. FORMULA is a character string consisting of elements followed by their atom count. Subgroups bracketed by parentheses can also be given. The element name is given by its standard, case-sensitive, IUPAC1 abbreviation (e.g., C for carbon, He for helium). The following are all equivalent: &SPEC ID='ETHYLENE GLYCOL', FORMULA='C2H6O2'/ &SPEC ID='ETHYLENE GLYCOL', FORMULA='OHC2H4OH'/ &SPEC ID='ETHYLENE GLYCOL', FORMULA='C2H4(OH)2'/ Two Gas Species with the Same Properties In general only one species for a given ID can be defined; however, you may wish to model multiple inlet streams of a species and be able to identify how well the streams are mixing. This can be done by defining a new species with a single component. For example, the lines: &SPEC &SPEC &SPEC &DEVC &DEVC ID='CARBON DIOXIDE', LUMPED_COMPONENT_ONLY=.TRUE./ ID='CO2 1',SPEC_ID='CARBON DIOXIDE'/ ID='CO2 2',SPEC_ID='CARBON DIOXIDE'/ XYZ=..., QUANTITY='MASS FRACTION', SPEC_ID='CO2 1', ID='Device 1'/ XYZ=..., QUANTITY='MASS FRACTION', SPEC_ID='CO2 2', ID='Device 2'/ define two duplicate species, both of which are CARBON DIOXIDE. Both will use the built in property data for CO2 (note specifying one or more properties for a duplicate species will override the default properties). The ID for each duplicate species can then be used in the remainder of the input file. In this exmaple, the LUMPED_COMPONENT_ONLY was given so that FDS only tracks CO2 1 and CO2 2 and not CARBON 1 International Union of Pure and Applied Chemistry 133 DIOXIDE. Note that the ID you provide for a duplicate species cannot match the ID of any other primitive or lumped SPEC input. Also note that this feature can only be used to duplicate a predefined species (i.e. a species listed in Table 11.1). In the above example, the two DEVC lines refer to the IDs of the duplicate species. If instead the primitive species ID was used, then the output would sum the mass fractions over all species containing that primitive species. For example, if the output quantities in the example above are changed as shown below, then Device 1 would just output the mass fraction of CO2 1 and Device 2 would output the sum of both. &DEVC XYZ=..., QUANTITY='MASS FRACTION', SPEC_ID='CO2 1', ID='Device 1'/ &DEVC XYZ=..., QUANTITY='MASS FRACTION', SPEC_ID='CARBON DIOXIDE', ID='Device 2'/ Another application of this would be if you wanted to track the water that evaporated from sprinklers, separately from the water that resulted from combustion. The following inputs would allow you to do that: &REAC &SPEC &PART &DEVC &DEVC FUEL='PROPANE', ... / ID='WATER VAPOR SPK', SPEC_ID='WATER VAPOR' / ID='Sprinkler Droplets',SPEC_ID='WATER VAPOR SPK'/ XYZ=..., QUANTITY='MASS FRACTION', SPEC_ID='WATER VAPOR SPK', ID='Spr H2O'/ XYZ=..., QUANTITY='MASS FRACTION', SPEC_ID='WATER VAPOR', ID='All H2O'/ The gas species called WATER VAPOR SPK has the same properties as WATER VAPOR. The first device records only water that results from droplet evaporation, and the second device records water that originates from both sprinklers and combustion. By default, duplicate species are considered to be a lumped species and not a primitive species. That is, by default, a duplicate species cannot be used in a lumped species definition. Setting PRIMITIVE=.TRUE. will have FDS treat the duplicate species as a primitive species and allow it to be used in a lumped species definition. Note that when this is done, one can no longer aggregate SPEC_ID based outputs over the original and the duplicate species as the aggregation is done on the basis of the primitive species names. This is demonstrated in the example below. The species O2 and O3 are duplicate species of OXYGEN. O3 is defined as a primitive species and O2 is considered only a lumped species. The initial DEVC outputs in order would be 0.3 (0.1 for OXYGEN plus 0.2 for O2), 0.2 (the O2 initial value), and 0.3 (the O3 initial value). Note the OXYGEN output is not 0.6 since the O3 is defined as a new primitive species. &SPEC &SPEC &SPEC &SPEC ID='NITROGEN',BACKGROUND=.TRUE./ ID='OXYGEN',MASS_FRACTION_0=0.1/ ID='O2',SPEC_ID='OXYGEN',MASS_FRACTION_0=0.2/ ID='O3',SPEC_ID='OXYGEN',MASS_FRACTION_0=0.3,PRIMITIVE=.TRUE./ &DEVC XYZ=0.5,0.5,0.5,QUANTITY='MASS FRACTION',SPEC_ID='OXYGEN'/ &DEVC XYZ=0.5,0.5,0.5,QUANTITY='MASS FRACTION',SPEC_ID='O2'/ &DEVC XYZ=0.5,0.5,0.5,QUANTITY='MASS FRACTION',SPEC_ID='O3'/ 11.2 Specifying Lumped Species (Mixtures of Primitive Species) The SPEC namelist group also allows you to define species mixtures. The purpose of a species mixture is to reduce the number of species transport equations that are explicitly solved. For example, consider air. Air is composed of nitrogen, oxygen, water vapor, and carbon dioxide. If we define the four component species of air, we will have four total species. One of these, say nitrogen, can be set to be the background species, leaving three transport equations to solve. Alternatively, we can define a “lumped species” that represents the air mixture and save on CPU time because the three transport equations are no longer needed and the 134 “lumped” air becomes the background species. The following inputs define equivalent initial mixtures. But in the first example three species are explicitly tracked and in the second example only a background mixture is specified. Note that this example represents the background species created by FDS when no background is explicitly defined in the input file. It is also noted that any implicitly defined species (such as the nitrogen, oxygen, water vapor, and carbon dioxide for the air background species or the species in the lumped product species for simple chemistry, see 12.1.1), can be referenced by any of the outputs such as DEVC or SLCF. Example 1: All primitive species &SPEC &SPEC &SPEC &SPEC ID='NITROGEN', BACKGROUND=.TRUE. / Note: The ID='OXYGEN', MASS_FRACTION_0=0.23054 ID='WATER VAPOR', MASS_FRACTION_0=0.00626 ID='CARBON DIOXIDE', MASS_FRACTION_0=0.00046 background must be defined first. / / / Example 2: Defining a background species &SPEC &SPEC &SPEC &SPEC ID='NITROGEN', ID='OXYGEN', ID='WATER VAPOR', ID='CARBON DIOXIDE', LUMPED_COMPONENT_ONLY=.TRUE. LUMPED_COMPONENT_ONLY=.TRUE. LUMPED_COMPONENT_ONLY=.TRUE. LUMPED_COMPONENT_ONLY=.TRUE. &SPEC ID='AIR', BACKGROUND=.TRUE., SPEC_ID(1)='NITROGEN', SPEC_ID(2)='OXYGEN', SPEC_ID(3)='WATER VAPOR', SPEC_ID(4)='CARBON DIOXIDE', / / / / MASS_FRACTION(1)=0.76274, MASS_FRACTION(2)=0.23054, MASS_FRACTION(3)=0.00626, MASS_FRACTION(4)=0.00046 / The logical parameter LUMPED_COMPONENT_ONLY indicates that the species is only present as part of a lumped species. When .TRUE., FDS will not allocate space to track that species individually. The parameters to define a lumped species are: BACKGROUND Denotes that this lumped species is to be used as the background species. ID Character string identifying the name of the species. You must provide this. This cannot be the same as an ID of another SPEC input. SPEC_ID Character array containing the names of the primitive species that make up the lumped species. MASS_FRACTION The mass fractions of the components of the lumped species in the order listed by SPEC_ID. FDS will normalize the values to 1. Alternatively, VOLUME_FRACTION can be specified. Do not use both on an SPEC line. MASS_FRACTION_0 The initial mass fractions of lumped species. When defining a lumped species, either MASS_FRACTION or VOLUME_FRACTION must be used to define the component species. The addition of lumped species to FDS has changed the meaning of SPEC_ID on some FDS inputs. For INIT, MATL, PART, and SURF, SPEC_ID refers to either a tracked primitive species or a lumped species. For outputs and devices, DEVC, that require a SPEC_ID, the SPEC_ID input can refer to either a tracked primitive species, a lumped species, or a lumped species component that is not tracked. For REAC see the discussion on specifying reactions in Chapter 12. 135 11.2.1 Combining Lumped and Primitive Species There are cases where you may wish to have a single primitive species be both part of a lumped species and also a separately tracked species. For example, when using simple chemistry, Section 12.1.1, FDS will include water vapor in the product species and in the air background species. If you also wish to have sprinklers in the simulation, then you will need to track water vapor from the sprinklers separately from that in the air or products. This is simply done by adding the line: &SPEC ID='WATER VAPOR'/ This will override the implicitly created water vapor species defined with LUMPED_COMPONENT_ONLY=.TRUE. and cause FDS to track water vapor as a separate species. Note that in this case if you requested an output for the mass fraction of WATER VAPOR you would get the water vapor in the air and product lumped species as well as that which evaporated from sprinkler droplets. If you wanted in this case (where water vapor is implicitly defined), to be able to track the water vapor from sprinklers separately you could follow the example in Section 11.1.3 and define: &SPEC ID='SPRINKLER WATER VAPOR',SPEC_ID='WATER VAPOR'/ Using the species SPRINKLER WATER VAPOR for the sprinklers would allow you to track sprinkler generated water vapor separately. 136 Chapter 12 Combustion A common source of confusion in FDS is the distinction between gas phase combustion and solid phase pyrolysis. The former refers to the reaction of fuel vapor and oxygen; the latter the generation of fuel vapor at a solid or liquid surface. Whereas there can be many types of combustibles in an FDS fire simulation, in the simple chemistry, mixing-controlled combustion model there can only be one gaseous fuel. The reason is cost. It is expensive to solve transport equations for multiple gaseous fuels. Consequently, the burning rates of solids and liquids are automatically adjusted by FDS to account for the difference in the heats of combustion of the various combustibles. In effect, you specify a single gas phase reaction as a surrogate for all potential fuels. Combustion can be modeled in two ways. By default, the reaction of fuel and oxygen is infinitely fast and controlled only by mixing, hence the label mixing-controlled. The alternative is that the reaction is finite-rate. The latter approach usually requires very fine grid resolution that is not practical for largescale fire applications. This chapter describes both methods, with an emphasis on the more commonly used mixing-controlled model. The REAC namelist group contains the parameters for both modes of combustion. 12.1 Single-Step, Mixing-Controlled Combustion This approach to combustion, referred to below as the “simple chemistry” combustion model, considers a single fuel species that is composed primarily of C, H, O, and N that reacts with oxygen in one mixingcontrolled step to form H2 O, CO2 , soot, and CO. Information about the reaction is provided on the REAC line. Starting with FDS 6, you must specify a REAC line to model a fire. You are responsible for defining the basic fuel chemistry and the post-combustion yields of CO and soot. The default values are 0. 12.1.1 Simple Chemistry Parameters For the simple chemistry model, each reaction is assumed to be of the form: Cx Hy Oz Nv + νO2 O2 → νCO2 CO2 + νH2 O H2 O + νCO CO + νS Soot + νN2 N2 (12.1) You need only specify the chemical formula of the fuel along with the yields of CO and soot, and the volume fraction of hydrogen in the soot, XH . FDS will use that information and calculate the stoichiometric 137 coefficients automatically as follows: νO2 νCO2 = νH2 O = νCO = νs = νN2 νCO νH2 O z + − 2 2 2 x − νCO − (1 − XH ) νS y XH − νS 2 2 WF yCO WCO WF yS WS v 2 XH WH + (1 − XH )WC = νCO2 + = Ws = The following parameters may be prescribed on the REAC line when using the simple chemistry model. Note that the various YIELDs are for well-ventilated, post-flame conditions. There are options to predict various species yields in under-ventilated fire scenarios, but these special models still require the post-flame yields for CO, soot and any other species listed below. FUEL (Required) A character string that identifies fuel species for the reaction. When using simple chemistry, specifying FUEL will cause FDS to use the built-in thermophysical properties for that species when computing quantities such as specific heat or viscosity. Table 11.1 provides a listing of the available species. If the FUEL is in the table, then FDS will use the built-in formula to obtain the values of C, H, O, and N. If not listed in Table 11.1, FDS uses the gas thermophysical properties of ETHYLENE along with the molecular weight given by the FORLMULA or the values of C, H, O, and N. Either way, FDS will implicitly create a SPEC input for FUEL. This allows FUEL to be used as a SPEC_ID input elsewhere (for example as an initial condition or an output quantity). If you define FUEL yourself as a SPEC, any properties you specify will override the default values. FORMULA A character string that identities the chemical formula of the fuel species for the reaction. This input only has meaning when simple chemistry is being used and the formula can only contain C, H, O, or N. Specifying a formula means the individual inputs of C, H, O, and N do not need to be specified. See 11.1.2 for a description on how to input a FORMULA. ID A character string that identifies the reaction. Normally, this label is not used by FDS, but it is useful to label the REAC line if more than one reactions are specified. C, H, O, N The fuel chemical formula. All numbers are positive. One of either C or H must be specified. This input is not needed if FORMULA is specified or if the FUEL is in Table 11.1. CO_YIELD The fraction of fuel mass converted into carbon monoxide, yCO . Note that this parameter is only appropriate when the simple chemistry model is applied. (Default 0.) SOOT_YIELD The fraction of fuel mass converted into smoke particulate, ys . Note that this parameter is only appropriate when the simple chemistry model is applied. (Default 0.) SOOT_H_FRACTION The fraction of the atoms in the soot that are hydrogen. The default value is 0.1, equivalent to the input FORMULA=’C0.9H0.1’ (Section 11.1.3). Note that this parameter is only appropriate when the simple chemistry model is applied. 138 FUEL_RADCAL_ID RadCal species to be used for the fuel. The default is the default RadCal species for the fuel species or ’METHANE’ if there is no species default. See Section 13.2.1 for details. The ambient mass fractions for the constituents of air are specified on MISC using the inputs: Y_O2_INFTY Ambient mass fraction of oxygen (Default 0.232378) Y_CO2_INFTY Ambient mass fraction of carbon dioxide (Default 0.000595) HUMIDITY Relative humidity of the background air species, in units of %. (Default 40 %). A few sample REAC lines are given here. &REAC FUEL = 'METHANE' / In this case, there is no need for a FORMULA or atom count because the FUEL is listed in Table 11.1. It is assumed that the soot and CO yields are zero. FDS will compute the yields of product species and the heat of combustion based upon predefined values. &REAC FUEL SOOT_YIELD CO_YIELD HEAT_OF_COMBUSTION = = = = 'PROPANE' 0.01 0.02 46460. / In this case, the fuel species is again predefined. However, here the heat of combustion is specified explicitly rather than calculated. Additionally, minor species yields have been specified with the soot yield specified as 0.01 and the CO yield specified as 0.02. See Section 12.1.2 for more details on the heat of combustion. &REAC FUEL = 'MY FUEL' FORMULA = 'C3H8O3N4' HEAT_OF_COMBUSTION = 46124. / In this case, the fuel is not predefined. Therefore, either the FORMULA or the atom counts must be defined. This input defined the FORMULA. In this case, the heat of combustion is known and specified; however, if it weren’t FDS would compute it using EPUMO2 and the fuel chemistry. Note that simple chemistry can also be used for cases where the fuel is a lumped species so long as the defining primitive species contain only C, H, N, and O atoms. An example can be found in Section 12.2.1. When simple chemistry is being used, FDS will automatically create three lumped species: AIR, FUEL, and PRODUCTS. The actual name of the fuel species will be the name given on the REAC line (for example MY FUEL in the last sample above). FDS creates these lumped species in the same manner as you would in an input file. FDS first defines the primitive species and then defines the lumped species. In essence FDS internally creates input lines like those shown in Example 2 of Section 11.2. This means when doing simple chemistry, that even though you did not explicitly define oxygen in the input file, you can request an output for oxygen since it was implicitly defined by FDS. 12.1.2 Heat of Combustion The energy release per unit volume (kJ/m3 ) from a gas phase chemical reaction (or system of reactions) is found by taking the sum of the net change in mass for each species in a given time step multiplied by the respective species’ enthalpy of formation (kJ/kg). In this formulation, the enthalpy of formation for all participating species needs to be specified. If a reaction (simple chemistry or user-defined) contains only species defined in Table 11.1, then all of the enthalpies of formation are known. These values can be found 139 in the FDS source code in data.f90. For reactions with species that are not included in Table 11.1, there are several options to ensure that all of the enthalpies of formation are specified. Option 1: Specify Enthalpy of Formation You can specify unknown enthalpies on the SPEC line in units of kJ/mol: &SPEC ID = 'GLUCOSE', FORMULA = 'C62H12O6', ENTHALPY_OF_FORMATION=-1.297E3 / Option 2: Specify Heat of Combustion For a given reaction, if the only species missing an enthalpy of formation is the fuel, the missing value can be found if the heat of combustion is specified on REAC. Section 12.2.1 provides an example for which the fuel, polyvinyl chloride, has an unspecified enthalpy of formation but a specified heat of combustion on the REAC line. Option 3: Use of EPUMO2 (Simple Chemistry) If the enthalpy of formation of the fuel and heat of combustion are not specified, for simple chemistry cases only, the heat of combustion is assumed to be ∆h ≈ νO2 WO2 EPUMO2 νF WF kJ/kg (12.2) The quantity EPUMO2 (kJ/kg) is the amount of energy released per unit mass of oxygen consumed. Its default is 13,100 kJ/kg. Typically, a chemical reaction is balanced by setting the stoichiometric coefficient of the fuel νF to 1. In FDS, the stoichiometric coefficients of the chemical reaction are normalized by the stoichiometric coefficient of the fuel, effectively setting νF to 1. Note that if both EPUMO2 and HEAT_OF_COMBUSTION are specified that FDS will ignore the value for EPUMO2. From the HEAT_OF_COMBUSTION, FDS solves for the enthalpy of formation of the fuel. If heats of reaction have been specified on the MATL lines and the heats of combustion of the materials differ from that specified by the governing gas phase reaction, then add a HEAT_OF_COMBUSTION (kJ/kg) to the MATL line. With the simple chemistry combustion model, it is assumed that there is only one fuel. However, in a realistic fire scenario, there may be many fuel gases generated by the various burning objects in the building. Specify the stoichiometry of the predominant reaction via the REAC namelist group. If the stoichiometry of the burning material differs from the global reaction, the HEAT_OF_COMBUSTION is used to ensure that an equivalent amount of fuel is injected into the flow domain from the burning object. The heat of combustion can be determined in a couple of ways. One approach is to take the difference in the heats of formation for the products (assuming complete combustion) and the reactants. This is typically how values are tabulated for pure fuels (e.g., one species) in handbooks. This ideal heat of combustion does not account for the SOOT_YIELD or CO_YIELD that occurs in a real fire. Carbon and hydrogen that go to soot and CO rather than CO2 and H2 O result in a lower effective heat of combustion. Setting IDEAL=.TRUE. will reduce the HEAT_OF_COMBUSTION based upon the inputs for SOOT_YIELD and CO_YIELD. If EPUMO2 is specified instead of HEAT_OF_COMBUSTION, then the EPUMO2 will not be changed. The second approach to determining the heat of combustion is to burn a known mass of the material in a calorimeter and divide the heat release rate by the mass loss rate (known as the effective heat of combustion). In this approach, represented by IDEAL=.FALSE., the measured value of the heat release rate includes the effects of any CO or soot that is produced and no adjustment is needed. The default value is IDEAL=.FALSE. 140 Note: If you specify a heat of combustion on the REAC line, FDS will calculate the enthalpy of formation of the fuel such that the user-specified heat of combustion is maintained. This is important to recognize for cases where a heat of combustion is measured experimentally or if you want to model impurities in the fuel that would not be realized using the standard heat of formation of the fuel. If the reaction is under defined, FDS will return an error at the start of the calculation. 12.1.3 Special Topic: Turbulent Combustion Unless you are performing a Direct Numerical Simulation (DNS), the reaction rate of fuel and oxygen is not based on the diffusion of fuel and oxygen at a well-resolved flame sheet. Instead, semi-empirical rules are invoked by FDS to determine the rate of mixing of fuel and oxygen within a given mesh cell at a given time step. Each computational cell can be thought of as a batch reactor where only the mixed composition can react. The variable, ζ (t), denotes the unmixed fraction, ranging from zero to one and governed by the equation: dζ −ζ = (12.3) dt τmix Here, τmix is the mixing time scale. The change in mass of a species is found from the combination of mixing and the production/destruction rate found from the chemical reaction. If a cell is initially unmixed, ζ0 = 1 by default, then combustion is considered non-premixed within the grid cell. In this case, the fuel and air are considered completely separate at the start of the time step and must mix together before they burn. This means if the mixing is slow enough, that unburned fuel may exist at the end of the time step even if sufficient oxygen is present in the grid cell to burn all the fuel. If the cell is initially fully mixed, ζ0 = 0, then the combustion is considered premixed (e.g., equivalent to infinitely fast mixing). You can set the amount of mixing in each cell at the beginning of every time step using the parameter INITIAL_UNMIXED_FRACTION on the MISC line. Note that the MIXTURE FRACTION output quantity applies the Burke-Schumann solution (see [26]) to map species composition to mixture fraction (i.e., mixture fraction is not a primitive flow variable but rather is calculated from the local fuel and product mass fractions). This requires simple chemistry with a reaction of the type F + A → P with infinitely fast mixing and infinitely fast chemistry. Therefore, you must set INITIAL_UNMIXED_FRACTION=0 to output MIXTURE FRACTION. The Technical Reference Guide [1] contains more detailed information about the turbulent combustion model. 12.1.4 Special Topic: Flame Extinction Modeling suppression of a fire due to the introduction of a suppression agent like CO2 or water mist, or due to the exhaustion of oxygen within a compartment is challenging because the relevant physical mechanisms occur at length scales smaller than a single mesh cell. Flames are extinguished due to lowered temperatures and dilution of the oxygen supply. A simple suppression algorithm has been implemented in FDS that attempts to gauge whether or not combustion is viable based on the local energy release. There must be sufficient energy released to raise the cell temperature above the critical flame temperature for combustion to occur. The Technical Reference Guide [1] contains more details about how the mechanism works. The only parameter you can control is the CRITICAL_FLAME_TEMPERATURE (CFT) (the adiabatic flame temperature at the lower flammability limit) set on the REAC line. The default value is 1327 °C. This value represents combustion of typical hydrocarbon fuels. Note that if you are using an effective heat of combustion or other types of fuels, the CFT should be calculated (see below) to reflect your reaction system. To eliminate any gas phase suppression, set SUPPRESSION=.FALSE. on the MISC line. 141 If the mixing-controlled combustion model is used and flame extinction occurs, the unburned fuel gas can re-ignite if it mixes with sufficient oxygen somewhere else in the domain. To prevent this from happening, you can set the AUTO_IGNITION_TEMPERATURE (AIT), in °C, below which combustion will not occur. Note that if this parameter is used, then some form of heat/ignition source must be present in order for combustion to begin. Finding or Calculating Extinction Parameters For most fuels of interest, the AIT and CFT may be found in [27]. The AITs are tabulated for a range of fuels in Table 2-7.1 of the SFPE Handbook [28], “Summary of Limits of Flammability, Lower Temperature Limits (TL), and Minimum Autoignition Temperatures (AIT) of Individual Gases and Vapors in Air at Atmospheric Pressure.” Similarly, Table 2-7.3, “Thermodynamic Equilibrium Properties at Extinction,” provides CFT (listed as T(LFL), for temperature at lower flammability limit) for several common fuels. If the lower flammability limit (LFL) of the fuel mixture is known but the CFT is not, then Beyler’s Eq. (4) may be used to estimate the CFT. The formula is repeated here for convenience: LFL ∆Hc TCFT = T0 + (12.4) 100 nCp where ∆Hc = Heat of combustion of the fuel (J/kg) LFL/100 = Mole fraction of fuel n = Number of moles of products of combustion per mole of fuel/air mixture Cp = Heat capacity of products of combustion (J/(kg · K)) T0 = Initial temperature of the fuel/air mixture (ambient conditions) (K) TCFT = Adiabatic flame temperature of a lower flammability limit mixture (K) 142 12.2 Complex Stoichiometry The “simple chemistry” parameters described above can only be used when there is a single mixingcontrolled reaction and the fuel molecule contains only C, O, H, and N. For any other situation, you must specify the reaction stoichiometry in greater detail. This means that you must explicitly specify the gas species, or species mixtures, along with the stoichiometry of the reaction. The easiest way to explain this is by way of example. Consider a single reaction involving methane. When you specify the REAC line to be: &REAC FUEL='METHANE' / FDS assumes the following reaction: 1 (CH4 ) + 9.636 (0.2076 O2 + 0.7825 N2 + 0.0095 H2 O + 0.0004 CO2 ) −→ | {z } | {z } Fuel Air 10.636 (0.0944 CO2 + 0.1966 H2 O + 0.7090 N2 ) (12.5) {z } | Products By default, there are trace amounts of carbon dioxide and water vapor in the air, which, like the nitrogen, is carried along in the reaction. This is important, because the more complicated way to specify a single step reaction of methane is as follows: &SPEC &SPEC &SPEC &SPEC &SPEC ID='NITROGEN', ID='OXYGEN', ID='WATER VAPOR', ID='CARBON DIOXIDE', ID='METHANE' / LUMPED_COMPONENT_ONLY=.TRUE. LUMPED_COMPONENT_ONLY=.TRUE. LUMPED_COMPONENT_ONLY=.TRUE. LUMPED_COMPONENT_ONLY=.TRUE. &SPEC ID='AIR', BACKGROUND=.TRUE., SPEC_ID(1)='OXYGEN', SPEC_ID(2)='NITROGEN', SPEC_ID(3)='WATER VAPOR', SPEC_ID(4)='CARBON DIOXIDE', / / / / VOLUME_FRACTION(1)=0.2076, VOLUME_FRACTION(2)=0.7825, VOLUME_FRACTION(3)=0.0095, VOLUME_FRACTION(4)=0.0004 / &SPEC ID='PRODUCTS', SPEC_ID(1)='CARBON DIOXIDE', VOLUME_FRACTION(1)=0.0944, SPEC_ID(2)='WATER VAPOR', VOLUME_FRACTION(2)=0.1966, SPEC_ID(3)='NITROGEN', VOLUME_FRACTION(3)=0.7090 / &REAC FUEL='METHANE', SPEC_ID_NU='METHANE','AIR','PRODUCTS', NU=-1,-9.636,10.636, HEAT_OF_COMBUSTION=50000. / The reaction stoichiometry is specified using the stoichiometric coefficients, NU(N), corresponding to the tracked1 species, SPEC_ID_NU(N). There are several parameters on the REAC line that control the specification of the stoichiometry: CHECK_ATOM_BALANCE If chemical formulas are provided for all species that participate in a reaction, then FDS will check the stoichiometry to ensure that atoms are conserved. Setting this flag to .FALSE. will bypass this check. (Default .TRUE.) REAC_ATOM_ERROR Error tolerance in units of atoms for the reaction stoichiometry check. (Default 0.00001) REAC_MASS_ERROR Relative error tolerance computed as (mass of products - mass of reactants)/(mass of products) for the reaction stoichiometry mass balance check. (Default 0.0001) 1A “tracked” species is one for which LUMPED_COMPONENT_ONLY is .FALSE. 143 12.2.1 Complex Fuel Molecules For complex fuel molecules that contain only C, H, N, and O the simple chemistry reaction parameters can still be applied. Consider natural gas, which is often a mixture of several component gases. To build the fuel mixture, the primitive (component) species need be defined. Each primitive species will get a LUMPED_COMPONENT_ONLY designation, which means that each of these species will not be explicitly tracked as they are components to a mixture. Note that these lumped components can only contain C, H, N, and O atoms. The lumped fuel, ’natural gas’, can be created using the defined primitive components and subsequently the reaction can be defined using simple chemistry (Section 12.1.1): &SPEC &SPEC &SPEC &SPEC ID='METHANE', LUMPED_COMPONENT_ONLY=.TRUE./ ID='ETHYLENE', LUMPED_COMPONENT_ONLY=.TRUE./ ID='NITROGEN', LUMPED_COMPONENT_ONLY=.TRUE./ ID='CARBON DIOXIDE',LUMPED_COMPONENT_ONLY=.TRUE./ &SPEC ID='natural gas' SPEC_ID(1)='METHANE', VOLUME_FRACTION(1)=92.2 SPEC_ID(2)='ETHYLENE', VOLUME_FRACTION(2)= 3.3 SPEC_ID(3)='NITROGEN', VOLUME_FRACTION(3)= 3.9 SPEC_ID(4)='CARBON DIOXIDE',VOLUME_FRACTION(4)= 0.6/ &REAC FUEL='natural gas' SOOT_YIELD=0.01 / Fires, however, often involve fuels that do not just consist of C, H, N, and O. For example, chlorine is commonly found in building and household materials, and because of its propensity to form the acid gas HCl, you may want to account for it in the basic reaction scheme. Suppose the predominant fuel in the fire is polyvinyl chloride (PVC). Regardless of its detailed polymeric structure, it can be regarded as C2 H3 Cl for the purpose of modeling. Assuming that all of the Cl in the fuel is converted into HCl, you can derive a single-step reaction mechanism using appropriate soot and CO yields for the specified fuel. In this example, the SFPE Handbook [29] is used to find soot and CO yields for PVC; 0.172 and 0.063, respectively. For a given species, α, its stoichiometric coefficient, να , can be found from its yield, yα , and its molecular weight, Wα , according to the formula: WF να = yα (12.6) Wα Since it is assumed that all of the Cl is converted to HCL, the remainder of the stoichiometric coefficients come from an atom balance. An equation can now be written to include the appropriate numerical values for the stoichiometric coefficients. 1 (C2 H3 Cl) +1 (1.53 O2 + 1.53 (3.76) N2 ) −→ | {z } | {z } Fuel Air 1 (HCl + H2 O + 0.14 CO + 0.96 CO2 + 0.90 C + 1.53 (3.76) N2 ) (12.7) | {z } Products The choice of fuel in this example, PVC, is not defined in Table 11.1, therefore its properties must be defined on a SPEC line. In this example, we use the species’ chemical formula. The example will also use the lumped species formulation to minimize the number of scalar transport equations that need to be solved. Therefore, each species that does not have an explicit transport equation is a LUMPED_COMPONENT_ONLY. &SPEC ID = 'PVC', FORMULA = 'C2H3Cl' / &SPEC ID = 'OXYGEN', LUMPED_COMPONENT_ONLY = .TRUE. / 144 &SPEC &SPEC &SPEC &SPEC &SPEC &SPEC ID ID ID ID ID ID = = = = = = 'NITROGEN', 'HYDROGEN CHLORIDE', 'WATER VAPOR', 'CARBON MONOXIDE', 'CARBON DIOXIDE', 'SOOT',FORMULA='C', LUMPED_COMPONENT_ONLY LUMPED_COMPONENT_ONLY LUMPED_COMPONENT_ONLY LUMPED_COMPONENT_ONLY LUMPED_COMPONENT_ONLY LUMPED_COMPONENT_ONLY = = = = = = .TRUE. .TRUE. .TRUE. .TRUE. .TRUE. .TRUE. / / / / / / For the oxidizer and products, which are both composed of multiple primitive species, SPEC lines are needed to define the composition of the lumped species. You can define the SPEC using either the MASS_FRACTIONS of the component gases or the VOLUME_FRACTIONS. If Eq. (12.7) is properly balanced, you can directly use the stoichiometric coefficients of the primitive species to define the lumped species. &SPEC ID='AIR', BACKGROUND=.TRUE. SPEC_ID(1)='OXYGEN', VOLUME_FRACTION(1)=1.53, SPEC_ID(2)='NITROGEN', VOLUME_FRACTION(2)=5.76 / &SPEC ID='PRODUCTS', SPEC_ID(1)='HYDROGEN CHLORIDE', SPEC_ID(2)='WATER VAPOR', SPEC_ID(3)='CARBON MONOXIDE', SPEC_ID(4)='CARBON DIOXIDE', SPEC_ID(5)='SOOT', SPEC_ID(6)='NITROGEN', VOLUME_FRACTION(1)=1.0, VOLUME_FRACTION(2)=1.0, VOLUME_FRACTION(3)=0.14, VOLUME_FRACTION(4)=0.96, VOLUME_FRACTION(5)=0.90, VOLUME_FRACTION(6)=5.76 / To set the initial concentration of fuel, an INIT line is used: &INIT MASS_FRACTION(1)=0.229, SPEC_ID(1)='PVC' / Since this is not a simple chemistry problem, either the enthalpy of formation of PVC or the heat of combustion of the reaction should be specified. In this case, the heat of combustion for PVC is taken from the SFPE Handbook [29]. &REAC FUEL='PVC', HEAT_OF_COMBUSTION=16400, SPEC_ID_NU='PVC','AIR','PRODUCTS', NU=-1,-1,1, FIXED_MIX_TIME=0.1 / Note that the sign of NU corresponds to whether that species is consumed (-) or produced (+). Figure 12.1 displays the mass fractions of the product species for the sample case PVC_Combustion. A fixed turbulent mixing time of 0.1 s is used for this example only because the reactants are initially mixed within a chamber with no imposed flow. Normally, this parameter is not necessary. 12.2.2 Multiple Chemical Reactions There may be times when the your design/model fire is best described by more than one fuel or by more than a single-step chemical reaction. For this example consider two simultaneous, mixing-controlled reactions of polyurethane and wood. Both of these fuels are complex molecules not included in Table 11.1, so they must be defined on the SPEC line. Polyurethane is defined by the chemical formula C25 H42 O6 N2 and wood is defined by CH1.7 O0.74 N0.002 . Consider a combustion reaction for polyurethane with a soot yield of ys = 145 FDS0−86−g80cff4e 0.3 Species Mass Fraction (PVC) Mass Fraction 0.25 Expected CO2 Expected CO Expected H2O Expected Soot Expected HCl FDS CO2 FDS CO FDS H2O FDS Soot FDS HCl 0.2 0.15 0.1 0.05 0 0 0.5 1 Time (s) 1.5 2 Figure 12.1: Product species mass fractions for model PVC example. 0.131 and a CO yield of yCO = 0.01 [29] is: 1 (C25 H42 O6 N2 ) +27.23436 (O2 + 3.76 N2 ) −→ | {z } | {z } Fuel Air 1 (19.79113 CO2 + 0.166587 CO + 20.71987 H2 O + 5.60253 C0.9 H0.1 + 103.40121 N2 ) (12.8) {z } | Products_1 and the reaction for wood with a soot yield of ys = 0.015 and a CO yield of yCO = 0.004 [29] is: 1 (CH1.7 O0.74 N0.002 ) +1.02121 (O2 + 3.76 N2 ) −→ | {z } | {z } Fuel Air 1 (0.964679 CO2 + 0.003655 CO + 0.848241 H2 O + 0.035184 C0.9 H0.1 + 3.838558 N2 ) (12.9) | {z } Products_2 Similar to example in Section 12.2.1, we will use the lumped species approach to minimize the number of species that FDS needs to transport. Therefore, the species are defined in the following manner: &SPEC &SPEC &SPEC &SPEC &SPEC &SPEC &SPEC &SPEC ID ID ID ID ID ID ID ID = = = = = = = = 'POLYURETHANE', FORMULA = 'C25H42O6N2' / 'WOOD', FORMULA = 'CH1.7O0.74N0.002' / 'OXYGEN', LUMPED_COMPONENT_ONLY 'NITROGEN', LUMPED_COMPONENT_ONLY 'WATER VAPOR', LUMPED_COMPONENT_ONLY 'CARBON MONOXIDE', LUMPED_COMPONENT_ONLY 'CARBON DIOXIDE', LUMPED_COMPONENT_ONLY 'SOOT', LUMPED_COMPONENT_ONLY = = = = = = .TRUE. .TRUE. .TRUE. .TRUE. .TRUE. .TRUE. / / / / / / Examination of Eq. (12.8) and Eq. (12.9) shows that, while Products_1 and Products_2 are composed of the same species, the species do not exist in the same proportion. As a result, we must construct two separate product lumped species. &SPEC ID = 'AIR', SPEC_ID(1) = 'OXYGEN', VOLUME_FRACTION(1)=1, SPEC_ID(2) = 'NITROGEN', VOLUME_FRACTION(2)=3.76, BACKGROUND=.TRUE. / &SPEC ID = 'PRODUCTS_1', 146 SPEC_ID(1) SPEC_ID(2) SPEC_ID(3) SPEC_ID(4) SPEC_ID(5) = = = = = 'CARBON DIOXIDE', 'CARBON MONOXIDE', 'WATER VAPOR', 'SOOT', 'NITROGEN', VOLUME_FRACTION(1) VOLUME_FRACTION(2) VOLUME_FRACTION(3) VOLUME_FRACTION(4) VOLUME_FRACTION(5) = = = = = 19.79113, 0.166587, 20.71987, 5.60253, 103.40121 / &SPEC ID = 'PRODUCTS_2', SPEC_ID(1) = 'CARBON DIOXIDE', SPEC_ID(2) = 'CARBON MONOXIDE', SPEC_ID(3) = 'WATER VAPOR', SPEC_ID(4) = 'SOOT', SPEC_ID(5) = 'NITROGEN', VOLUME_FRACTION(1) VOLUME_FRACTION(2) VOLUME_FRACTION(3) VOLUME_FRACTION(4) VOLUME_FRACTION(5) = = = = = 0.964679, 0.003655, 0.848241, 0.035184, 3.838558 / Once we have constructed the lumped species, we can define the REAC lines. &REAC ID = 'plastic', FUEL = 'POLYURETHANE', HEAT_OF_COMBUSTION=26200, SPEC_ID_NU = 'POLYURETHANE','AIR','PRODUCTS_1' NU=-1,-27.23436,1 / &REAC ID = 'wood' FUEL = 'WOOD', HEAT_OF_COMBUSTION=16400, SPEC_ID_NU = 'WOOD','AIR','PRODUCTS_2' NU=-1,-1.02063,1 / In both of these reactions, the ENTHALPY_OF_FORMATION of the chosen fuels is unknown to FDS, so the HEAT_OF_COMBUSTION is specified for each reaction. Typically, the heat release per unit area (HRRPUA) parameter is used to specify the fire size on the SURF line. The HRRPUA parameter cannot currently be used when there is more than one fuel, so the mass flux of fuel must be specified for each fuel. In this example, we want both fuels to flow out of the same burner and ramp up to a 1200 kW fire after 60 s. First, we create a 1 m2 burner by defining a VENT line: &VENT XB=4.0,5.0,4.0,5.0,0.0,0.0, SURF_ID='FIRE1', COLOR='RED'/ The VENT points to the SURF_ID=’FIRE1’ which we can define using both fuels. The mass flux values for each fuel are determined by the desired heat release, the proportion of the total heat release rate each fuel contributes, the burner area, and the heat of combustion of each fuel. In this case we want a 1200 kW fire where each fuel contributes 50% to the total heat release rate (600 kW). 600 kW 1 = 0.022901 kg/(m2 · s) 1 m2 26200 kJ/kg 600 kW 1 ṁ00wood = = 0.036585 kg/(m2 · s) 2 1 m 16400 kJ/kg ṁ00poly = (12.10) (12.11) &SURF ID='FIRE1', SPEC_ID(1)='POLYURETHANE', MASS_FLUX(1)=0.022901, RAMP_MF(1)='poly' SPEC_ID(2)='WOOD', MASS_FLUX(2)=0.036585, RAMP_MF(2)='wood'/ Note that the SPEC_ID, MASS_FLUX, and RAMP_MF correspond to one another for each fuel. It does not matter which fuel is (1), just that the numbering is consistent. We also want each fuel to follow a ramp such that fire starts at 0 kW at the initial time, reaches 1200 kW at 100 s, and remains at 1200 kW for the remainder of a 600 s simulation. 147 &RAMP &RAMP &RAMP &RAMP ID='poly', ID='poly', ID='poly', ID='poly', T T T T = = = = 0, 50 , 100, 600, F F F F = = = = 0.0 0.5 1.0 1.0 / / / / &RAMP &RAMP &RAMP &RAMP ID='wood', ID='wood', ID='wood', ID='wood', T T T T = = = = 0 , 50 , 100, 600, F F F F = = = = 0.0 0.5 1.0 1.0 / / / / Here, T corresponds to the time and F is the fraction of the mass flux specified on the SURF line. Fig. 12.2 compares the resulting FDS heat release rate (HRR) to the expected HRR from the defined ramp. Note that in this simulation, there is 10 s averaging on the FDS output HRR (&DUMP DT_HRR=10) and the expected results account for this averaging. FDS0−86−g80cff4e 1500 Energy Rate (kW) Energy Budget (two fuels) 1000 500 Expected (HRR) FDS (HRR) 0 0 100 200 300 Time (s) 400 500 600 Figure 12.2: HRR for energy_budget_adiabatic_two_fuels test case. Note that when using multiple chemical reactions, you must set SUPPRESSION=.FALSE. on the MISC line. 12.2.3 Special Topic: Using the EQUATION input parameter When specifying a reaction scheme using all primitive variables (i.e., no lumped species), a convenient shortcut for specifying the reaction is via the input parameter EQUATION, which allows the specification of the reaction in text form. The rules are: • The species must be explicitly tracked. • The species name is its chemical formula as defined on the SPEC line or by the species ID. • The stoichiometry is given before each species and is separated by an asterisk. Real numbers are allowed but exponential notation is not (i.e., 201.1 but not 2.011E2). • The reactants and products are separated by an equals sign. For example, if the reaction defines the complete combustion of methane using primitive species, then the following would be equivalent: 148 &REAC FUEL='METHANE', EQUATION ='METHANE+2*OXYGEN=CARBON DIOXIDE+2*WATER VAPOR' / &REAC FUEL='METHANE', EQUATION ='CH4+2*O2=CO2+2*H2O' / &REAC FUEL='METHANE', EQUATION ='METHANE+2*O2=CO2+2*H2O' / 12.3 Finite Rate Combustion By default, FDS uses a mixing-controlled combustion model, meaning that the reaction rate is infinite and limited only by species concentrations. However, FDS can also employ finite-rate reactions using an Arrhenius model. It is recommended that finite-rate reactions be invoked only when FDS is running in DNS mode (DNS=.TRUE. on the MISC line). You can use the finite-rate reaction model in an LES calculation, but because the temperature in a large scale calculation is smeared out over a mesh cell, some of the reaction parameters may need to be modified to account for the lower cell-averaged temperatures. Consider a single-step reaction mechanism for complete propane combustion: C3 H8 + 5 O2 → 3 CO2 + 4 H2 O (12.12) In this case we explicitly define all primitive species: &SPEC &SPEC &SPEC &SPEC &SPEC ID='NITROGEN', BACKGROUND=.TRUE./ ID='PROPANE' / ID='OXYGEN' / ID='WATER VAPOR' / ID='CARBON DIOXIDE' / The rate expression for the fuel, C3 H8 , is where the rate constant is defined as dCC3 H8 = −k ∏ Cα NS,α dt (12.13) k = A T NT e−Ea /RT (12.14) A is the pre-exponential factor [ ((mol/cm3 )1−n )/s ], where n = ∑ NS,α is the order of the reaction, Ea is the activation energy [ J/mol ], NS,α is an array containing the concentration exponents (default 1), NT is the temperature exponent (default is 0, meaning no temperature dependence). If we use Arrhenius parameters found from experiments or the literature (for this case Westbrook and Dryer [30]), the rate expression for the single-step propane reaction defined by Eq. (12.12) becomes: dCC3 H8 = −8.6 × 1011 T 0 e−125520/RT CC3 H8 0.1 CO2 1.65 dt and the resulting REAC line is: &REAC ID = 'R1' FUEL = 'PROPANE' A = 8.6e11 E = 125520 SPEC_ID_NU = 'PROPANE','OXYGEN','CARBON DIOXIDE','WATER VAPOR' NU = -1,-5,3,4 SPEC_ID_N_S = 'PROPANE','OXYGEN' N_S = 0.1,1.65 149 (12.15) The array SPEC_ID_NU contains the list of tracked species participating in the reaction as a product or reactant. Note that a primitive species with LUMPED_COMPONENT_ONLY=.TRUE. cannot be used in a reaction since it is not tracked separately. The array SPEC_ID_N_S contains the list of species with the exponents, N_S, in Eq. (12.13). This array must contain only primitive species, i.e., no species mixtures. The array of stoichiometric coefficients, NU, can be taken directly from Eq. (12.12). If we extend Eq. (12.12) to include reactions from Westbrook and Dryer [30] that account for carbon monoxide production, we obtain: C3 H8 + 3.5 O2 → 3 CO + 4 H2 O (12.16) CO + 0.5 O2 CO2 In this case we will combine fast chemistry with finite-rate chemistry to create a mixed reaction mechanism. As before, we use primitive species rather than mixtures. A REAC line is needed for each reaction including the reverse reaction. The propane oxidation reaction is a fast chemistry while the reversible carbon monoxide reaction is finite-rate. The Arrhenius parameters are modified from Andersen et al. [31]: &REAC ID = 'R1' FUEL = 'PROPANE' SPEC_ID_NU = 'PROPANE','OXYGEN','CARBON MONOXIDE','WATER VAPOR' NU = -1,-3.5,3,4 CRITICAL_FLAME_TEMPERATURE = 730./ &REAC ID = 'R2' FUEL = 'CARBON MONOXIDE' A = 1.5e9 E = 41840 SPEC_ID_NU = 'CARBON MONOXIDE','OXYGEN','CARBON DIOXIDE' NU = -1,-0.5,1 SPEC_ID_N_S = 'OXYGEN','CARBON MONOXIDE','WATER VAPOR' N_S = 0.25,1,0.5/ &REAC ID = 'R3' FUEL = 'CARBON DIOXIDE' A = 6.16e13 E = 328026 SPEC_ID_NU = 'CARBON DIOXIDE','OXYGEN','CARBON MONOXIDE' NU = -1,0.5,1 SPEC_ID_N_S = 'OXYGEN','CARBON DIOXIDE','WATER VAPOR' N_S = -0.25,1,0.5 N_T = -0.97/ For REAC ID=’R2’, the reaction rate depends on the water vapor concentration, as indicated by its assignment of a value of N_S. However, it does not explicitly participate in the reaction because it is not assigned a stoichiometric coefficient, NU. Reactions of this sort are often written in textbooks as H O 2 a X + b Y −→ Xa Yb (12.17) However, for the purposes of inputting into FDS, since H2 O is neither a product nor a reactant it would not be specified in the chemical reaction using either EQUATION or NU. It would instead be given a rate exponent using N_S to indicate that its presence is required. Some reactions require the presence of any third body to stabilize the reaction rather than just a specific species. Reactions of this type are often write in textbooks as a X + b Y + M → Xa Yb + M 150 (12.18) This type of reaction can be done by adding THIRD_BODY=.TRUE. to the REAC line for the reaction. The species M should not otherwise be defined. As discussed previously (Section 12.1.2), energy release is calculated using the net change of species in a time step along with the enthalpy of formation of each species. This approach makes specifying an endothermic reaction, such as the reversible CO2 reaction (R3), no different than typical FDS exothermic combustion reactions. 12.4 Special Topic: Chemical Time Integration A diagnostic tool for the time integration of chemical reactions is the number of subiterations the ODE solver takes for a given FDS timestep. For infinitely fast chemistry the number of CHEMICAL SUBITERATIONS should always be 1. For finite rate chemical reactions the number can vary between 1 and the minimum between MAX_CHEMISTRY_ITERATIONS and the number needed to satisfy the error tolerance of the ODE solver. If the maximum value is reached, the user can increase the number of iterations or decrease reduce the error tolerate constraints of the integrator. 12.5 Special Topic: Aerosol Deposition It is possible within FDS to model the deposition of smoke and aerosols onto solid surfaces. The aerosol deposition model is invoked by defining a species with the parameter AEROSOL=.TRUE. on the SPEC line along with the parameters DENSITY_SOLID, CONDUCTIVITY_SOLID, and MEAN_DIAMETER. By default, with AEROSOL=.TRUE., FDS will compute all of the aerosol deposition mechanisms discussed in the Technical Reference Guide [1]. For diagnostic purposes, each deposition mechanism can be selectively disabled by using the logical parameters GRAVITATIONAL_DEPOSITION, THERMOPHORETIC_DEPOSITION, TURBULENT_DEPOSITION, and GRAVITATIONAL_SETTLING on the MISC line. GRAVITATIONAL_SETTLING controls the downward movement of aerosols in the gas phase, whereas GRAVITATIONAL_DEPOSITION controls the deposition of aerosols onto upward-facing surfaces due to gravity. The deposition velocity at the wall can be output using QUANTITY=’DEPOSITION VELOCITY’ for a wall cell with DEVC or BNDF. 12.5.1 Example Case: Soot Deposition from a Propane Flame The propane_flame_deposition example shows how to define a reaction that invokes the aerosol deposition model in FDS. The fuel is propane with a specified soot yield of 0.05. Note that this is a fabricated soot yield that is used only for demonstration and verification purposes. The reaction is given by: 1 (C3 H8 ) + 4.81308 (O2 + 3.7619 N2 ) −→ | {z } | {z } Fuel Air 1 (18.10631 N2 + 2.81813 CO2 + 3.98990 H2 O) + 0.20201 C0.9 H0.1 (12.19) | {z } | {z } Products Soot Note that the stoichiometric coefficient for soot ensures that the mass of soot produced is 0.05 times the mass of fuel consumed. This example uses the lumped species formulation to minimize the number of scalar transport equations that need to be solved. Note that for soot to deposit it must be explicitly tracked by defining AEROSOL=.TRUE. on the SPEC line. &SPEC ID = 'PROPANE' / &SPEC ID = 'OXYGEN', &SPEC ID = 'NITROGEN', LUMPED_COMPONENT_ONLY = .TRUE. / LUMPED_COMPONENT_ONLY = .TRUE. / 151 &SPEC ID = 'WATER VAPOR', &SPEC ID = 'CARBON DIOXIDE', &SPEC ID = 'SOOT', LUMPED_COMPONENT_ONLY = .TRUE. / LUMPED_COMPONENT_ONLY = .TRUE. / AEROSOL = .TRUE. / If Eq. (12.19) is properly balanced, you can directly use the stoichiometric coefficients of the primitive species to define the lumped species: &SPEC ID = 'AIR', SPEC_ID ='NITROGEN','OXYGEN', VOLUME_FRACTION = 3.7619,1., BACKGROUND = .TRUE. / &SPEC ID = 'PRODUCTS', SPEC_ID ='NITROGEN','CARBON DIOXIDE','WATER VAPOR', VOLUME_FRACTION = 18.10631,2.81813,3.98990 / The heat of combustion for propane is found in the SFPE Handbook [29]. &REAC FUEL = 'PROPANE', HEAT_OF_COMBUSTION=44715., SPEC_ID_NU = 'PROPANE','AIR','PRODUCTS','SOOT', NU=-1.,-4.81308,1,0.20208/ Note: The sign of NU corresponds to whether that species is consumed (-) or produced (+). Figure 12.3 shows the soot surface deposition on the wall. This boundary quantity is given by the input below. &BNDF QUANTITY='SURFACE DEPOSITION', SPEC_ID='SOOT' / Figure 12.3: Wall soot deposition for the propane_flame_deposition test case. 152 Chapter 13 Radiation For most FDS simulations, thermal radiation transport is computed by default and you need not set any parameters to make this happen. However, there are situations where it is important to be aware of issues related to the radiative transport solver. 13.1 Basic Radiation Parameters: The RADI Namelist Group RADI is the namelist group that contains all of the parameters related to the radiation solver. There can be only one RADI line in the input file. It is possible to turn off the radiation transport solver (saving roughly 20 % in CPU time) by adding the statement RADIATION=.FALSE. to the RADI line. If burning is taking place and radiation is turned off, then the total heat release rate is reduced by the RADIATIVE_FRACTION, which is an input on the REAC line. This radiated energy completely disappears from the calculation. For fire scenarios it is not recommended that you turn off the radiation transport. This feature is used mainly for diagnostic purposes or when the changes in temperature are relatively small. 13.1.1 Radiative Fraction The most important radiation parameter is the fraction of energy released from the fire as thermal radiation, commonly referred to as the radiative fraction, symbolically denoted χr . It is a function of both the flame temperature and chemical composition, neither of which are reliably calculated in a large scale fire calculation because the flame sheet is not well-resolved on a relatively coarse numerical grid. In calculations in which the mesh cells are on the order of a centimeter or larger, the temperature near the flame surface cannot be relied upon when computing the source term in the radiation transport equation, especially because of the T 4 dependence. As a practical alternative, the parameter RADIATIVE_FRACTION on the REAC line allows you to specify explicitly the fraction of the total combustion energy that is released in the form of thermal radiation. Some of that energy may be reabsorbed elsewhere, yielding a net radiative loss from the fire or compartment that is less than the RADIATIVE_FRACTION, depending mainly on the size of the fire and the soot loading. If it is desired to use the radiation transport equation as is, then RADIATIVE_FRACTION ought to be set to zero, and the source term in the radiative transport equation is then based solely on the gas temperature and the chemical composition. By default, the RADIATIVE_FRACTION is based on the reaction’s FUEL for an LES calculation (see 13.1), and zero for DNS. Multi-fuel radiative fraction If multiple fuels are present, e.g., one fuel for cardboard and one fuel for polystyrene combustion, then the radiant fraction will be computed locally as reaction-weighted value, sim153 Table 13.1: Default Radiative Fraction based on species Species ACETONE ACETYLENE BENZENE BUTANE DODECANE ETHANE ETHANOL ETHYLENE HYDROGEN ISOPROPANOL METHANE METHANOL N-DECANE N-HEPTANE N-HEXANE N-OCTANE PROPANE PROPYLENE TOLUENE All other species χr 0.30 0.45 0.45 0.35 0.40 0.25 0.20 0.35 0.10 0.30 0.20 0.20 0.40 0.40 0.40 0.40 0.30 0.35 0.45 0.35 ilar to the approach described by Gupta et al. [32]. For example, if the two fuels are 20% and 40% radiative fraction with, respectively, 80% and 20% of the heat in a grid cell, χr would be 0.8 × 0.2 + 0.2 × 0.4 = 0.24. Thus, χr will vary in space and time. The value of the multi-fuel radiative fraction can be output using the output QUANTITY of CHI_R. Time variation of radiative fraction Even for a single fuel species the global flame radiative fraction may depend on other parameters of the problem like global equivalent ratio. If a time variation of the radiative fraction is necessary, it may be added through a ramp function, RAMP_CHI_R, on the REAC line. &REAC &RAMP &RAMP &RAMP 13.1.2 ..., ID = ID = ID = RADIATIVE_FRACTION 'chi_r', T = 0.0, 'chi_r', T = 5.0, 'chi_r', T = 60.0, = F F F 0.23, RAMP_CHI_R = 'chi_r'/ = 1.0/ = 1.0/ = 0.1/ Spatial and Temporal Resolution of the Radiation Transport Equation There are several ways to improve the spatial and temporal accuracy of the Finite Volume Method in solving the radiation transport equation (RTE), but these will increase the computation time. You can increase the number of angles from the default 100 with the integer parameter NUMBER_RADIATION_ANGLES. The frequency of calls to the radiation solver can be changed from every 3 time steps with an integer called 154 TIME_STEP_INCREMENT. The increment over which the angles are updated can be reduced from 5 with the integer called ANGLE_INCREMENT. If TIME_STEP_INCREMENT and ANGLE_INCREMENT are both set to 1, the radiation field is completely updated in a single time step, but the cost of the calculation increases significantly. By default, the radiation transport equation is fully updated every 15 time steps. If you are using multiple meshes, the radiation solver cannot transfer energy from mesh to mesh within a single time step. If you notice an obvious delay in the propagation of radiative intensity from one mesh to another, you can increase the number of times the radiative intensity is updated within a single time step using RADIATION_ITERATIONS, which is 1 by default. The radiation solver is called before the start of the calculation to establish the radiation field in the event that you specify something to have a non-ambient temperature initially. By default, the radiation and wall boundary routines are iterated 10 times to establish thermal equilibrium. To change the number of iterations, set NUMBER_INITIAL_ITERATIONS on the RADI line. 13.2 Radiative Absorption and Scattering By default FDS employs a gray gas model for the radiation absorption coefficient, a function of gas composition and temperature, which are tabulated in a look-up table using the routines found in RadCal. You can output the absorption coefficient using the output QUANTITY ’ABSORPTION COEFFICIENT’. 13.2.1 RadCal Considerations There are several considerations with regard to RadCal: Path Length Because RadCal computes effective absorption coefficients over a range of wavelengths, it requires a userspecified PATH_LENGTH (m). Its default value is five times the width of a single grid cell. Fuel Species The original version of RadCal included only absorption data for methane, which was used as a surrogate for any fuel. However, more fuel species have been added to RadCal. The current list of fuels includes: METHANE, ETHYLENE, ETHANE, PROPANE, N-HEPTANE, METHANOL, TOLUENE, PROPYLENE, and MMA. These species are in addition to the RadCal species of: CARBON DIOXIDE, CARBON MONOXIDE, WATER VAPOR, and SOOT. 13.2.2 Radiative Absorption and Scattering by Particles The absorption and scattering of thermal radiation by Lagrangian particles is included in the radiation transport equation. The radiative properties can be given by specifying the components of the material refractive index on the corresponding PART line, using keywords REAL_REFRACTIVE_INDEX and COMPLEX_REFRACTIVE_INDEX. Alternatively, wavelength dependent values of these two quantities can be tabulated in a TABLE and called using the RADIATIVE_PROPERTY_TABLE. More details can be found in Section 14.3.2. The radiative properties of the water and fuel particles (droplets) are determined automatically. For fuel, the properties of heptane are assumed. The heptane values can be overriden by specifying them on the PART line. 155 Other parameters affecting the computations of particle-radiation interaction are listed here. RADTMP is the assumed radiative source temperature. It is used in the spectral weighting during the computation of the mean scattering and absorption cross sections. The default is 900 ◦ C. NMIEANG is the number of angles in the numerical integration of the Mie-phase function. Increasing NMIEANG improves the accuracy of the radiative properties of water droplets. The cost of the better accuracy is seen in the initialization phase, not during the actual simulation. The default value for NMIEANG is 15. For each class of particles, the Mie coefficients are calculated for a wide range of droplet diameters to ensure that the all possible runtime situations can be covered. To speed up the initialization phase, the range of diameters can be limited by parameters MIE_MINIMUM_DIAMETER and MIE_MAXIMUM_DIAMETER. Also, the size of the Mie coefficient tables can be specified using MIE_NDG parameter. The radiation properties of most common gases involved in combustion processes (water vapor, carbon dioxide, carbon monoxide, fuel) and soot particles are automatically taken into account if the simulation involves combustion. In simulations with no combustion nor radiating species, it is possible to use a constant absorption coefficient by specifying KAPPA0 on the RADI line. 13.2.3 Wide Band Model The radiation solver has two modes of operation – a gray gas model (default) and a wide band model [1]. If the optional six band model is desired, set WIDE_BAND_MODEL=.TRUE. It is recommended that this option only be used when the fuel is relatively non-sooting because it adds significantly to the cost of the calculation. Read the FDS Technical Reference Guide [19] for more details. Note also that when WIDE_BAND_MODEL=.TRUE., the output QUANTITY ’ABSORPTION COEFFICIENT’ becomes practically useless, because it then corresponds to one individual band of the spectrum. It is also possible to set your own band limits for the wide band model by specifying BAND_LIMITS on the RADI line. The limits should be given in ascending order, in units of microns (µm). The maximum number of bands is 9, in which case you would specify 10 real numbers separated by commas. 156 Chapter 14 Particles and Droplets Lagrangian particles can be used to represent a wide variety of objects that are too small to resolve on the numerical grid. FDS considers three major classes of Lagrangian particles: massless tracers, liquid droplets, and everything else. The parameters describing particles are found on the PART line. 14.1 Basics Properties of different types of Lagrangian particles are designated via the PART namelist group. Once a particular type of particle has been described using a PART line, then the name of that particle type is invoked elsewhere in the input file via the parameter PART_ID. There are no reserved PART_IDs – all must be defined. For example, an input file may have several PART lines that include the properties of different types of Lagrangian particles: &PART ID='my smoke',... / &PART ID='my water',... / Particles are introduced into the calculation in several different ways: they may be introduced via a sprinkler or nozzle (liquid droplets are usually introduced this way), they may be introduced at a blowing vent or burning surface (mass tracer particles or particles representing embers are usually introduced this way), and they may be introduced randomly or at fixed points within a designated volume (solid particles that represent subgrid-scale objects are usually introduced this way). Details are found below. The way to describe particles depends on the type. If you simply want massless tracers, specify MASSLESS=.TRUE. on the PART line. If you specify a SPEC_ID, then FDS automatically assumes that you want relatively small, thermally-thin evaporating liquid droplets. For any other type of particle, such as particles that represent subgrid-scale objects, like office clutter or vegetation, you add a SURF_ID to the PART line. All of these different types of particles are described below. 14.2 Massless Particles The simplest use of Lagrangian particles is for visualization, in which case the particles are considered massless tracers. In this case, the particles are defined via the line &PART ID='tracers', MASSLESS=.TRUE., ... / Note that if the particles are MASSLESS, it is not appropriate to color them according to any particular property. Particles are not colored by gas phase quantities, but rather by properties of the particle itself. For 157 example, ’PARTICLE TEMPERATURE’ for a non-massless particle refers to the temperature of the particle itself rather than the local gas temperature. Also note that if MASSLESS=.TRUE., the SAMPLING_FACTOR (Section 16.9) is set to 1 unless you say otherwise, which would be pointless since MASSLESS particles are for visualization only. Turbulent Dispersion Massless tracer particles may also be useful in modeling dispersion of a tracer gas that does not affect the mean flow field (passive scalar). The number density of the particles then may be translated into a local mass concentration. See how to output number concentration in Sec. 16.10.7. To properly account for subgrid-scale (unresolved) turbulent motions, add the parameter TURBULENT_DISPERSION=.TRUE. to the PART line. The particles will then undergo a random walk based on the subgrid diffusivity. For further information, see the random_walk test cases in the WUI directory of the verification suite. 14.3 Liquid Droplets To define an evaporating liquid droplet, you must explicitly specify the gaseous species via the SPEC namelist group (see Section 11), and then designate the appropriate SPEC_ID on the PART line. If the droplets are defined with SPEC_ID=’WATER VAPOR’, then the particles will be assigned the thermo-physical properties of water, the radiation absorption properties of water, and will be colored blue in Smokeview. 14.3.1 Thermal Properties The following parameters should be specified to control the evaporation. The INITIAL_TEMPERATURE of liquid droplet; assumed ambient, TMPA (◦ C) is specified on PART input. If the fluid given by the SPEC_ID is included in Table 11.1, then no further inputs are required. Otherwise, you must provide all of the following properties of the liquid on the SPEC input: DENSITY_LIQUID The density of the liquid or solid droplet/particle (kg/m3 ). SPECIFIC_HEAT_LIQUID Specific heat of liquid or solid droplet/particle (kJ/(kg · K)). RAMP_CP_L Ramp of temperature vs. specific heat for the solid droplet/particle. VAPORIZATION_TEMPERATURE Boiling temperature of liquid droplet (◦ C). MELTING_TEMPERATURE Melting (solidification) temperature of liquid droplet (◦ C). HEAT_OF_VAPORIZATION Latent heat of vaporization of liquid droplet (kJ/kg). ENTHALPY_OF_FORMATION The heat of formation of the gas (kJ/mol). H_V_REFERENCE_TEMPERATURE The temperature corresponding to the provided HEAT_OF_VAPORIZATION (◦ C). 14.3.2 Radiative Properties The radiative properties of water and fuel droplets are determined automatically. For fuel, the properties of heptane are assumed. For other types of particles, the radiative properties can be given by specifying the components of the material refractive index on the corresponding PART line, using keywords 158 REAL_REFRACTIVE_INDEX and COMPLEX_REFRACTIVE_INDEX. Alternatively, wavelength dependent values of these two quantities can be specified using a spectral property TABLE and specifying the ID of that table is RADIATIVE_PROPERTY_TABLE property on the PART line. Each row of a spectral property table contains three real numbers: wavelength (µm), real and complex components of the refractive index. The real part of the refractive index should be a positive number. If it is greater than 10.0, the particles are treated as perfectly reflecting spheres. The complex part should be a non-negative number. Values less than 10−6 are treated as non-absorbing. Below is an example of the use of spectral property table, listing the properties at wavelengths 1, 5 and 10 µm. &PART &TABL &TABL &TABL ID='particles',..., RADIATIVE_PROPERTY_TABLE='table' / ID='table', TABLE_DATA= 1.0,1.33,0.0001 / ID='table', TABLE_DATA= 5.0,1.33,0.002 / ID='table', TABLE_DATA=10.0,1.33,0.001 / For calculating the absorption of thermal radiation by particles, FDS uses a running average of particle temperature and density. The default averaging factor, RUN_AVG_FAC, is set to 0.5. 14.3.3 Size Distribution The size distribution of liquid droplets is specified using a cumulative volume fraction (CVF)1 indicated by the character string DISTRIBUTION on the PART line. The default is ’ROSIN-RAMMLER-LOGNORMAL’: Z D [ln(D0 /Dv,0.5 )]2 1 1 exp − dD0 (D ≤ Dv,0.5 ) 0 √2π 2 σD 2σ 0 F(D) = (14.1) γ 1 − exp −0.693 D (D > D ) Dv,0.5 v,0.5 Alternatively, you can specify ’LOGNORMAL’ or ’ROSIN-RAMMLER’ alone rather than the combination of the two. Figure 14.1 displays the possible size distributions. Notice that the ’LOGNORMAL’ and ’ROSIN-RAMMLER’ distributions have undesirable attributes at opposite tails, which is why the combination of the two is commonly used. Figure 14.1 also shows a comparison between the prescribed distribution and the actual realized distribution of droplet sizes. The dashed lines show the measured droplet size distributions, while the solid lines show the prescribed sampling distributions. The sampled distributions are measured with the PDPA_HISTOGRAM function. Sample size of 10000 droplets was used. The median volumetric diameter, Dv,0.5 , is specified via the parameter DIAMETER (µm) on the PART line. You must specify the DIAMETER in cases where the droplets evaporate (in which case you also need to specify a SPEC_ID to indicate the gas species generated by the evaporating droplets). The width of the lognormal distribution, σ , is specified with SIGMA_D on the PART line. The width of the Rosin-Rammler distribution, γ, is specified with√GAMMA_D (default 2.4). Note that in the combined distribution, the parameter, σ , is calculated σ = 2/( 2π (ln 2) γ) = 1.15/γ which ensures that the two functions are smoothly joined at D = Dv,0.5 . You can also add a value for SIGMA_D to the PART line if you want to over-ride this feature. The larger the value of γ, the narrower the droplet size is distributed about the median value. You can specify your own cumulative number fraction (CNF)2 by specifying a CNF_RAMP_ID on the PART line and including a RAMP that gives the CNF: &PART ID='my droplets',..., CNF_RAMP_ID='my CNF' / &RAMP ID='my CNF', T= 0., F=0.000000 / 1 The 2 The CVF indicates the fraction of total mass carried by droplets less than the given diameter. CNF indicates the fraction of total droplets whose diameters are less than the given diameter. 159 FDS0−86−g80cff4e FDS0−86−g80cff4e 1 1 Lognormal Distribution Rosin−Rammler−Lognormal Distribution 0.8 0.8 0.6 0.6 0.4 0.4 CNF (Prescribed) CVF (Prescribed) CNF (Sampled) CVF (Sampled) 0.2 0 0 500 1000 Diameter (µm) 1500 CNF (Prescribed) CVF (Prescribed) CNF (Sampled) CVF (Sampled) 0.2 0 0 2000 500 1000 Diameter (µm) FDS0−86−g80cff4e 2000 FDS0−86−g80cff4e 1 1 Rosin−Rammler Distribution User Defined Distribution 0.8 0.8 0.6 0.6 0.4 0.4 CNF (Prescribed) CVF (Prescribed) CNF (Sampled) CVF (Sampled) 0.2 0 0 1500 500 1000 Diameter (µm) 1500 CNF (Prescribed) CVF (Prescribed) CNF (Sampled) CVF (Sampled) 0.2 0 0 2000 500 1000 Diameter (µm) 1500 2000 Figure 14.1: Droplet size distributions. The first three plots are based on a specified CVF from which the CNF is derived. The fourth plot (lower right) is an example of a specified CNF from which the CVF is derived. CVF:s are plotted with black lines, while CNF:s are plotted with red lines. The solid lines show the prescribed sampling distribution, while the dashed lines show the actual sampled droplet size distribution, measured with the PDPA_HISTOGRAM functionality. &RAMP ID='my CNF', T= 200., F=0.000003 / ... &RAMP ID='my CNF', T=2000., F=1.000000 / Note that the RAMP variable T indicates the diameter and is given in micrometers. The fourth plot in Fig. 14.1 is an example of where the CNF is specified and the CVF is calculated from it. It is essentially the reverse of what is shown in the first plot, where the CVF is specified and the CNF is calculated from it. As droplets are created in the simulation, their diameters are randomly chosen based on the given distribution. You can prevent excessively large droplets from being chosen by specifying a MAXIMUM_DIAMETER, which is assigned an infinitely large value by default. Droplets less than a specified MINIMUM_DIAMETER are assumed to evaporate in a single time step. The default value is 0.005 times the value of DIAMETER. The droplet distribution is divided into a series of bins3 . To avoid very small particle weights, the distribution is clipped at the cumulative fractions of CNF_CUTOFF and (1 - CNF_CUTOFF). Note that CNF_CUTOFF is set on the MISC line. The default value of CNF_CUTOFF is 0.005. 3 By default, the range of particle sizes is divided into six bins, and the sampled particles are divided among these bins. This ensures that a reasonable number of particles are assigned to the entire spectrum of sizes. To change the default number of bins, set N_STRATA on the PART line. 160 To prevent FDS from generating a distribution of droplets altogether, set MONODISPERSE to .TRUE. on the PART line, in which case every droplet will be assigned the same DIAMETER. If you set CHECK_DISTRIBUTION=.TRUE. on the PART line, FDS will write out the cumulative distribution function for that particular particle class in a file called CHID_PART_ID_cdf.csv. If you do this, you might want to avoid spaces in the ID of the PART line. 14.3.4 Secondary Breakup If BREAKUP=.TRUE. is set on the PART line, particles may undergo secondary breakup. In this case you should also specify the SURFACE_TENSION (N/m) of the liquid and the resulting ratio of the Sauter mean diameters, BREAKUP_RATIO. Its default is 3/7. Optionally, specify the distribution parameters BREAKUP_GAMMA_D and BREAKUP_SIGMA_D. 14.3.5 Fuel Droplets If the droplets evaporate into the FUEL identified on the REAC line, they will be colored yellow by default in Smokeview and any resulting fuel vapor will burn according to the combustion model specified on the REAC line. The droplets evaporate into an equivalent amount of fuel vapor such that the resulting heat release rate (assuming complete combustion) is equal to the evaporation rate multiplied by the HEAT_OF_COMBUSTION, also specified on the PART line. Note that the burning rate will be adjusted to account for the difference between the heats of combustion of the droplets and the other fuels in the model. If a spray nozzle is used to generate the fuel droplets, its characteristics are specified in the same way as those for a sprinkler. If the fuel species is present in the liquid properties table as a fuel, then the droplets will be given fuel radiation absorption properties. Note that to limit the computational cost of sprinkler simulations, liquid droplets disappear when they hit the “floor” of the computational domain, regardless of whether it is solid or not. However, this may not be desired when using liquid fuels. To stop FDS from removing liquid droplets from the floor of the domain, add the phrase POROUS_FLOOR=.FALSE. to the MISC line. An altnerate solution is make sure the OBST the fuel is landing on is at least one cell thick. Example Case: spray_burner Controlled fire experiments are often conducted using a spray burner, where a liquid fuel is sprayed out of a nozzle and ignited. In this example (spray_burner.fds), heptane from two nozzles is sprayed downwards into a steel pan. The flow rate is increased linearly so that the fire grows to 2 MW in 20 s, burns steadily for another 20 s, and then ramps down linearly in 20 s. The key input parameters are given here: &REAC FUEL='N-HEPTANE',SOOT_YIELD=0.01,HEAT_OF_COMBUSTION=44500./ &DEVC ID='nozzle_1', XYZ=4.0,-.3,0.5, PROP_ID='nozzle', QUANTITY='TIME', SETPOINT=0. / &DEVC ID='nozzle_2', XYZ=4.0,0.3,0.5, PROP_ID='nozzle', QUANTITY='TIME', SETPOINT=0. / &PART ID='heptane droplets', SPEC_ID='N-HEPTANE', QUANTITIES(1:2)='PARTICLE DIAMETER','PARTICLE TEMPERATURE', DIAMETER=1000., HEAT_OF_COMBUSTION=44500., SAMPLING_FACTOR=1 / &PROP ID='nozzle', CLASS='NOZZLE', PART_ID='heptane droplets', FLOW_RATE=1.97, FLOW_RAMP='fuel', PARTICLE_VELOCITY=10., SPRAY_ANGLE=0.,30. / &RAMP ID='fuel', T= 0.0, F=0.0 / &RAMP ID='fuel', T=20.0, F=1.0 / 161 &RAMP ID='fuel', T=40.0, F=1.0 &RAMP ID='fuel', T=60.0, F=0.0 / / Many of these parameters are self-explanatory. Note that a 2 MW fire is achieved via 2 nozzles flowing heptane at 1.96 L/min each: 2 × 1.97 1 min kg 1 m3 kJ L × × 684 3 × × 44500 = 2000 kW min 60 s m 1000 L kg (14.2) The parameter HEAT_OF_COMBUSTION over-rides that for the overall reaction scheme. Thus, if other droplets or solid objects have different heats of combustion, the effective burning rates are adjusted so that the total heat release rate is that which you expect. However, exercises like this ought to be conducted just to ensure that this is the case. The HRR curve for this example is given in Fig. 14.2. FDS0−86−g80cff4e 2500 Heat Release Rate (kW) Heat Release Rate (spray_burner) 2000 1500 1000 500 Specified (HRR) FDS (HRR) 0 0 10 20 30 Time (s) 40 50 60 Figure 14.2: Heat Release Rate (HRR) of spray burner test. Note also that this feature is subject to mesh dependence. If the mesh cells are too coarse, the evaporating fuel can be diluted to such a degree that it may not burn. Proper resolution depends on the type of fuel and the amount of fuel being ejected from the nozzle. Always test your burner at the resolution of your overall simulation. 162 14.4 Solid Particles Lagrangian particles can represent a wide variety of subgrid-scale objects, from office clutter to vegetation. To create solid, non-liquid particles, you must add a SURF_ID to the PART line. The specified SURF line contains the parameters that describe the thermophysical properties and geometric parameters of the particle. These properties are the same as those you would apply to an OBST or VENT. FDS uses the same solid phase conduction and pyrolysis algorithm for particles as it does for solid walls. If the SURF line that is associated with the particle class calls for it, the particles will heat up due to convection from the surrounding gases and radiation from near and distant sources. The convective heat transfer coefficient takes into account the particle geometry, and the radiative heat flux is based on the integrated intensity. That is, the radiation heat flux is the average over all angles. However, you can specify unique directions for the particle if the source of heating does not surround the particles. More about particle splitting is explained in Section 14.4.3. 14.4.1 Basic Geometry and Boundary Conditions To demonstrate the basic syntax for solid particles, the following input lines create a collection of hot spheres: &PART &SURF &PROP &INIT ID='spheres', SURF_ID='HOT', STATIC=.TRUE., PROP_ID='ball' / ID='HOT', TMP_FRONT=500., RADIUS=0.005, GEOMETRY='SPHERICAL' / ID='ball', SMOKEVIEW_ID='SPHERE', SMOKEVIEW_PARAMETERS(1)='D=0.01' / PART_ID='spheres', XB=0.25,0.75,0.25,0.75,0.25,0.75, N_PARTICLES=10 / The PART line establishes the class of particles. In this case, the presence of a SURF_ID indicates that the particles are solids with the properties given by the SURF line ’HOT’. STATIC is a logical parameter whose default is .FALSE. that indicates if the particles are stationary. The PROP_ID references a PROP (property) line that just tells Smokeview that the particles are to be drawn as spheres of diameter 0.01 m. See Section 15.7.3 for details and options. The INIT line randomly fills the given volume with 10 of these hot spheres. See Section 14.5.3 for details. If the SURF line includes a MATL_ID, the particle mass will be based upon the value(s) of DENSITY of the referenced MATL line(s). If there is to be no heat conduction calculation in depth, do not specify a MATL_ID. Instead, you can specify, for example, the surface temperature, TMP_FRONT (◦ C), heat release rate per unit area, HRRPUA (kW/m2 ), or species MASS_FLUX (kg/(m2 · s)). The GEOMETRY options for solid particles are ’SPHERICAL’, ’CYLINDRICAL’, or ’CARTESIAN’. By default, the GEOMETRY is ’CARTESIAN’, in which case you need to provide the LENGTH and WIDTH of the rectangular plate. It is assumed that the plate is symmetric front and back (note this means you should set BACKING=’INSULATED’ on the SURF line). You need only specify the layers that make up the half-thickness. The array THICKNESS(N) indicates the thickness(es) of each layer of the plate, not the total thickness of the plate itself. If the plate is composed of only one material component, the specified THICKNESS is taken as the half-thickness of the plate. For ’CYLINDRICAL’ or ’SPHERICAL’ particles, specify the INNER_RADIUS and THICKNESS of the individual layers. Alternatively, you can just specify the RADIUS if the cylinder or sphere is solid and has only one material component. The default value of INNER_RADIUS is 0 m, which means that the radius of the cylinder or sphere is the sum of the THICKNESS values. Remember that the layers are to be listed starting at the surface, not the center. For ’CYLINDRICAL’ particles, specify a LENGTH as well. 163 14.4.2 Drag For solid particles, the default drag law (i.e., the drag coefficient correlation as a function of the Reynolds number based on particle diameter) is that of a sphere. To invoke the cylinder drag law, set DRAG_LAW to ’CYLINDER’ on the PART line, and to invoke the screen drag law (see Section 14.4.8), set it to ’SCREEN’. If neither of these options is applicable, you may specify a constant value of the drag coefficient for a particle class (a specific PART_ID) by setting a DRAG_COEFFICIENT on the PART line. The DRAG_COEFFICIENT over-rides the DRAG_LAW. If the local particle density is very high, the drag may be reduced by particle wake effects. The threshold volume fraction for considering three-way coupling effects is controlled by the DENSE_VOLUME_FRACTION parameter. Setting this parameter to 1 will turn off the three-way coupling model. 14.4.3 Splitting Particles If the radiation heat flux is not uniformly distributed about the particle, it may be useful to split the particle into pieces, each with its own orientation. This can be done by using the parameter ORIENTATION to specify more than one outward facing directions for the particle. For example, &PART ..., ORIENTATION(1:3,1)=0,0,-1, ORIENTATION(1:3,2)=0,0,1 / specifies that half the particle is facing downwards and half is facing upwards. FDS now essentially tracks two particles. The radiative flux to the downward facing particle is the integrated average over the southern hemisphere; the flux to the upward facing particle is the average over the northern hemisphere. The particle mass and the surface area are scaled by the number of orientations. The splitting along the coordinate axis is demonstrated for all geometries in Fig. 14.3. The orientation direction does not have to align with the coordinate axes. In fact, the ORIENTATIONs do not have to be symmetric or come in pairs, even though for most applications it makes sense to do it this way. For a Cartesian particle (i.e., a plate), only the orientations that are perpendicular to the plate make physical sense. Keep in mind that the heat conducted within the different facets does not transfer through to the other facets. The heat conduction is still only one dimensional, in the direction normal to the face and towards the center. Note that if only a single ORIENTATION vector is assigned on a PART line, the radiative flux to the particle is calculated as if there is a flat plate normal to the direction of the vector, like a conventional heat flux gauge. That is, the heat flux is not an integrated average over the entire particle but rather the directional heat flux with the given orientation. The reason for this exception to the general rule is that often single particles are used as “targets” to record a heat flux at a given point in the domain with a given orientation. These particles can be thought of as tiny heat flux gauges that do not disturb the flow. 14.4.4 Gas Generating Particles Lagrangian particles can be used to generate gases at a specified rate. The syntax is similar to that used for a solid wall. For example, the following input lines create three particles – one shaped like a rectangular plate, one a cylinder, and one a sphere – that generate argon, sulfur dioxide, and helium, respectively. The particles have no mass; they simply are used to generate the gases at a specified rate. &SPEC ID='ARGON' / &SPEC ID='SULFUR DIOXIDE' / &SPEC ID='HELIUM' / &INIT PART_ID='plate', XYZ=-1.,0.,1.5, N_PARTICLES=1 164 / ORIENTATION(1:3,2)=0,0,1 = ORIENTATION(1:3,1)=0,0,-1 ORIENTATION(:,1)=0,0,1 ORIENTATION(:,4)=1,0,0 ORIENTATION(:,2)=-1,0,0 ORIENTATION(:,3)=0,0,-1 ORIENTATION(:,6)=0,0,1 ORIENTATION(:,2)=1,0,0 ORIENTATION(:,3)=0,-1,0 ORIENTATION(:,1)=-1,0,0 ORIENTATION(:,5)=0,0,-1 Figure 14.3: Examples of a Cartesian (plate), cylindrical, and spherical particle split into multiple parts. 165 &INIT PART_ID='tube', &INIT PART_ID='ball', XYZ= 0.,0.,1.5, N_PARTICLES=1 XYZ= 1.,0.,1.5, N_PARTICLES=1 / / &PART ID='plate', SAMPLING_FACTOR=1, SURF_ID='plate bc', STATIC=.TRUE. / &PART ID='tube', SAMPLING_FACTOR=1, SURF_ID='tube bc', STATIC=.TRUE. / &PART ID='ball', SAMPLING_FACTOR=1, SURF_ID='ball bc', STATIC=.TRUE. / &SURF ID='plate bc', THICKNESS=0.001, LENGTH=0.05, WIDTH=0.05, SPEC_ID(1)='ARGON', MASS_FLUX(1)=0.1, TAU_MF(1)=0.001 / &SURF ID='tube bc', GEOMETRY='CYLINDRICAL', LENGTH=0.05, RADIUS=0.01, SPEC_ID(1)='SULFUR DIOXIDE', MASS_FLUX(1)=0.1, TAU_MF(1)=0.001 / &SURF ID='ball bc', GEOMETRY='SPHERICAL', RADIUS=0.01, SPEC_ID(1)='HELIUM', MASS_FLUX(1)=0.1, TAU_MF(1)=0.001 / In this case, there is no calculation of heat conduction in depth. Only the surface area is important. For the plate, the surface area is twice the length times the width. For the cylinder, the area is twice the radius times π times the length. For the sphere, the area is 4π times the radius squared. Figure 14.4 displays the output of the test case called surf_mass_part_specified.fds, demonstrating that the production rate of the gases is as expected. FDS0−86−g80cff4e 0.01 Mass Production (surf_mass_part_specified) Mass (kg) 0.008 0.006 0.004 Expected (Ar mass) Expected (SO2 mass) Expected (He mass) FDS (Ar mass) FDS (SO2 mass) FDS (He mass) 0.002 0 0 5 10 Time (s) 15 20 Figure 14.4: Gas production from three Lagrangian particles. 14.4.5 Vegetation Lagrangian particles can be used to represent different types of vegetation, like leaves, grass, and so on. The best way to explain how to use this feature is by way of example. Suppose we want to describe a collection of wet pine needles that occupy a certain volume. The following lines have been extracted from the sample file WUI/pine_needles.fds. Note that all of the values have been chosen simply to demonstrate the technique. These values should not be used for a real calculation. &PART ID='pine needles', SAMPLING_FACTOR=1, SURF_ID='wet vegetation', PROP_ID='needle image', STATIC=.TRUE. / &INIT PART_ID='pine needles', XB=0.,1.,0.,1.,0.,1., N_PARTICLES=1000, MASS_PER_VOLUME=1. / &PROP ID='needle image', SMOKEVIEW_ID='TUBE', SMOKEVIEW_PARAMETERS='L=0.1','D=0.0005' / 166 &SURF ID = 'wet vegetation' MATL_ID(1,1:2) = 'PINE','MOISTURE' MATL_MASS_FRACTION(1,1:2) = 0.8,0.2 THICKNESS = 0.00025 LENGTH = 0.1 GEOMETRY = 'CYLINDRICAL' / &MATL ID = 'PINE' DENSITY = 500. CONDUCTIVITY = 0.1 SPECIFIC_HEAT = 1.0 N_REACTIONS = 1 REFERENCE_TEMPERATURE = 300. NU_MATL = 0.2 NU_SPEC = 0.8 SPEC_ID = 'GLUCOSE' HEAT_OF_REACTION = 1000 MATL_ID = 'CHAR' / &MATL ID = 'MOISTURE' DENSITY = 1000. CONDUCTIVITY = 0.1 SPECIFIC_HEAT = 4.184 N_REACTIONS = 1 REFERENCE_TEMPERATURE = 100. NU_SPEC = 1.0 SPEC_ID = 'WATER VAPOR' HEAT_OF_REACTION = 2500. / &MATL ID = 'CHAR' DENSITY = 200. CONDUCTIVITY = 1.0 SPECIFIC_HEAT = 1.6 / In the example, 1 kg of pine needles occupy 1 m3 . The number of particles used to represent the pine needles is somewhat arbitrary. FDS will automatically weight the specified number so that the total mass per volume is 1 kg. The needles are modeled as cylinders that are 0.5 mm in diameter. The THICKNESS on the SURF line refers to the radius of the cylinder in units of m. The needles are all 0.1 m long. The needles contain 20 % (by mass) moisture, and 80 % cellulose. The moisture is set to evaporate at 100 ◦ C to create water vapor and the cellulose pyrolyzes at 300 ◦ C to form fuel gas and char. In the example case, the original 1 kg of vegetation is heated until all of the water and fuel evaporate. The fuel is not allowed to burn by setting the ambient oxygen concentration to 1 %. Figure 14.5 shows the evolution of the fuel, water and char mass. Agreement with the expected values means that mass is conserved. 14.4.6 Firebrands Firebrands are small pieces of burning wood and vegetation that can be lofted into the air and blown by the wind ahead of a wildland fire front. Manzello et al. [33] have developed a variety of experimental apparatus designed to generate firebrands in a laboratory setting. The example input file called dragon_5a in the WUI (Wildland-Urban Interface) folder is a very simple mock-up of one of these experiments. The word “dragon” is based on the nickname of the apparatus; 5a is the figure number in Ref. [33] on which this example case is loosely based. In the experiment, 700 g of small dowels (length 50 mm, diameter 8 mm) made of Ponderosa Pine wood were poured into a small steel chamber equipped with several propane burners. The dowels 167 FDS0−86−g80cff4e 1.2 Mass Balance (pine_needles) 1 Mass (kg) 0.8 Expected (Fuel Gas) Expected (Water Vapor) Expected (Char) FDS (fuel gas mass) FDS (water vapor mass) FDS (solid mass) 0.6 0.4 0.2 0 0 2 4 6 8 10 Time (s) Figure 14.5: Evolution of vegetation mass in the pine_needles test case. were left to burn for roughly a minute subject to a slow induced air flow after which time the air flow was increased and firebrands were propelled horizontally out of a 15 cm duct 2.25 m above the lab floor. It is reported that after several replicate experiments, the average mass of the firebrands collected from pans on the floor was 57 g. The average diameter of the collected dowels was 5.6 mm, and the average length was 13.5 mm. It is not possible to simulate the experiment in FDS exactly as it was performed. The reason is that in the experiment, all 700 g of the wooden dowels were poured into the heating chamber at once. FDS cannot handle such a dense packing of Lagrangian particles. Instead, the simulated dowels are introduced at a rate of 10 per second. FDS also does not have a mechanism to break-up the dowels, reducing their length from 50 mm to 13.5 mm. Thus, the initial cylindrical particles are 13.5 mm and remain that length throughout the simulation. The diameter of the cylindrical particles is reduced, however, from 8 mm to 5.6 mm, which takes the initial density of 440 kg/m3 down to 71 kg/m3 because the mass of the firebrands is assumed to be 8 % of the original. The plot in Fig. 14.6 shows the increasing mass of firebrands thrown to the floor in the simulation after 100 s of particle insertion. The total mass of particles inserted into the apparatus is: π (0.004 m)2 × (0.0135 m) × (440 kg/m3 ) × (10 part/s) × (100 s) ≈ 0.3 kg (14.3) The amount expected on the floor is approximately 0.024 kg. 14.4.7 Porous Media A 3-D array of particles can be used to represent the drag exerted by porous media, as in the following example: &SURF ID='LIGAMENT', MATL_ID='ALUMINUM ALLOY', THICKNESS=7.3E-5, GEOMETRY='CYLINDRICAL', HEAT_TRANSFER_COEFFICIENT=10. / &MATL ID='ALUMINUM ALLOY', DENSITY=2690., CONDUCTIVITY=218., SPECIFIC_HEAT=0.9 / &PART ID='FOAM', DRAG_LAW='POROUS MEDIA', SURF_ID='LIGAMENT', POROUS_VOLUME_FRACTION=0.12, STATIC=.TRUE., DRAG_COEFFICIENT=0.1,0.1,0.1, PERMEABILITY=1.0E-7,1.E-7,1.E-7 / &INIT XB=1.010,1.095,0.0,0.5,0.0,0.5, N_PARTICLES_PER_CELL=1, CELL_CENTERED=.TRUE., PART_ID='FOAM' / 168 FDS0−86−g80cff4e 0.04 0.035 Lofting Brands (dragon_5a) Mass (kg) 0.03 0.025 0.02 0.015 0.01 0.005 0 0 Exact FDS 50 100 150 Time (s) 200 250 300 Figure 14.6: Mass generation of firebrands in the dragon_5a test case. These lines are a model of aluminum foam. The basic geometry of the foam ligaments is defined with the SURF line. It is assumed that the ligaments are made of an aluminum alloy whose properties are given on the MATL line. The radius of the assumed cylindrical ligament is indicated by the THICKNESS. Note that the LENGTH of the cylinder which is normally required on the SURF line is computed automatically so that the volume fraction of the grid cell occupied by the foam, specified by POROUS_VOLUME_FRACTION on the PART line, is achieved. In a sense, the foam is modeled by a long cylinder chopped up into small pieces and represented by a single particle in each grid cell. The PART line provides information about the particles. The DRAG_LAW indicates a special empirical model for the porous media. This model states that the pressure drop through the media is given by µ Y 2 ∆p = δ u+ρ √ u (14.4) K K where δ is the thickness of the foam block in the flow direction, µ is the viscosity of the gas, u is the velocity component in the flow direction, ρ is the density of the gas, K is the PERMEABILITY in units of m2 , and Y is a dimensionless inertial term that you specify using the parameter DRAG_COEFFICIENT. When using the porous media model the PERMEABILITY and DRAG_COEFFICIENT must be specified for all three directions. The INIT line designates the volume occupied by the porous media using the sextuplet XB. A single particle is inserted into the center of each cell occupied by the foam by specifying the parameters N_PARTICLES_PER_CELL=1 and CELL_CENTERED=.TRUE.. A sample calculation involving porous media is contained in the folder Sprinklers_and_Sprays. The input file is called porous_media.fds. 14.4.8 Screens A 2-D array of particles can be used to represent the drag exerted by a window screen, as in the following example: &INIT N_PARTICLES_PER_CELL=1, CELL_CENTERED=.TRUE., PART_ID='SCREEN', XB=1.01,1.02,0.0,1.0,0.0,1.0/ &PART ID='SCREEN', DRAG_LAW='SCREEN', FREE_AREA_FRACTION=0.4, STATIC=.TRUE., SURF_ID='SCREEN', ORIENTATION(1:3,1)=1,0,0 / &SURF ID='SCREEN', THICKNESS=0.00015, GEOMETRY='CYLINDRICAL', 169 MATL_ID='ALUMINUM' / &MATL ID='ALUMINUM', DENSITY=2700., CONDUCTIVITY=200., SPECIFIC_HEAT=0.9 / The INIT line designates the plane of the screen using the sextuplet XB. A single particle is inserted into each cell by specifying the parameter N_PARTICLES_PER_CELL=1. The particles are be defined with a SURF_ID containing the material properties of the screen. A special drag law for screens is specified via the DRAG_LAW. ORIENTATION is the direction normal to the screen, and FREE_AREA_FRACTION is the fraction of the screen’s surface area that is open. In the example, an aluminum screen with a 40 % free area and an 0.0003 m wire diameter is placed normal to the x-axis. Note that the LENGTH parameter on the SURF line will be computed automatically so the fraction of the grid cell flow area occupied by the screen is equal to 1 - FREE_AREA_FRACTION. The pressure drop across the screen is given by µ Y ∆p = l u + ρ √ u2 (14.5) K K where l is the screen thickness (equal to the wire diameter), µ is the viscosity of the gas, u is the velocity normal to the screen, ρ is the density of the gas, and Y and K are empirical constants given by K = 3.44 × 10−9 FREE_AREA_FRACTION1.6 m2 Y 14.4.9 2.13 = 0.043 FREE_AREA_FRACTION (14.6) (14.7) Electrical Cable Failure Petra Andersson and Patrick Van Hees of the Swedish National Testing and Research Institute (SP) have proposed that the thermally-induced electrical failure (THIEF) of a cable can be predicted via a simple onedimensional heat transfer calculation, under the assumption that the cable can be treated as a homogeneous cylinder [34]. Their results for PVC cables were encouraging and suggested that the simplification of the analysis is reasonable and that it should extend to other types of cables. The assumptions underlying the THIEF model are as follows: 1. The heat penetration into a cable of circular cross section is largely in the radial direction. This greatly simplifies the analysis, and it is also conservative because it is assumed that the cable is completely surrounded by the heat source. 2. The cable is homogeneous in composition. In reality, a cable is constructed of several different types of polymeric materials, cellulosic fillers, and a conducting metal, most often copper. 3. The thermal properties – conductivity, specific heat, and density – of the assumed homogeneous cable are independent of temperature. In reality, both the thermal conductivity and specific heat of polymers are temperature-dependent, but this information is very difficult to obtain from manufacturers. 4. It is assumed that no decomposition reactions occur within the cable during its heating, and ignition and burning are not considered in the model. In fact, thermoplastic cables melt, thermosets form a char layer, and both off-gas volatiles up to and beyond the point of electrical failure. 5. Electrical failure occurs when the temperature just inside the cable jacket reaches an experimentally determined value. Obviously, there are considerable assumptions inherent in the Andersson and Van Hees THIEF model, but their results for various polyvinyl chloride (PVC) cables suggested that it may be sufficient for engineering analyses of a wider variety of cables. The U.S. Nuclear Regulatory Commission sponsored a study of cable 170 failure known as CAROLFIRE [35]. The primary project objective of CAROLFIRE was to characterize the various modes of electrical failure (e.g., hot shorts, shorts to ground) within bundles of power, control and instrument cables. A secondary objective of the project was to develop a simple model to predict thermally-induced electrical failure when a given interior region of the cable reaches an empirically determined threshold temperature. The measurements used for these purposes are described in Volume II of the CAROLFIRE test report. Volume III describes the modeling. The THIEF model can only predict the temperature profile within the cable as a function of time, given a time-dependent exposing temperature or heat flux. The model does not predict at what temperature the cable fails electrically. This information is gathered from experiment. The CAROLFIRE experimental program included bench-scale, single cable experiments in which temperature measurements were made on the surface of, and at various points within, cables subjected to a uniform heat flux. These experiments provided the link between internal cable temperature and electrical failure. The model can only predict the interior temperature and infer electrical failure when a given temperature is reached. It is presumed that the temperature of the centermost point in the cable is not necessarily the indicator of electrical failure. This analysis method uses the temperature just inside the cable jacket rather than the centermost temperature, as that is where electrical shorts in a multi-conductor cable are most likely to occur first. To use the THIEF model in FDS, add lines similar to the following to the input file: &MATL ID='plastic', DENSITY=2535., CONDUCTIVITY=0.2, SPECIFIC_HEAT=1.5 / &SURF ID='cylinder', THICKNESS=0.00815, LENGTH=0.1, MATL_ID='plastic', GEOMETRY='CYLINDRICAL' / &PART ID='Cable Segment', SURF_ID='cylinder', ORIENTATION(1:3,1)=0.,0.,1., STATIC=.TRUE. / &INIT ID='Cable', XB=0.01,0.01,0.,0.,0.,0., N_PARTICLES=1, PART_ID='Cable Segment' / &DEVC ID='Cable Temp', INIT_ID='Cable', QUANTITY='INSIDE WALL TEMPERATURE', DEPTH=0.0015 / The THIEF model assumes that the cable plastic material has a thermal conductivity of 0.2 W/(m · K) and a specific heat of 1.5 kJ/(kg · K). If you change these values, you are no longer using the THIEF model. The density is the mass per unit length of the cable divided by its cross sectional area. The THICKNESS is the radius of the cylindrical cable in units of m. The LENGTH, in m, is needed by FDS because it assumes that the cable is a cylindrical segment of a certain length. It has no impact on the simulation, and its value it typically the size of a grid cell. The ORIENTATION tells FDS the direction of the prevailing radiative source. The second argument indicates that there can be more than one ORIENTATION. STATIC=.TRUE. prevents the cable from moving. The INIT line is used to position the cable within the computational domain. The DEVC line records the cables inner temperature, in this case 1.5 mm below the surface. This is typically the jacket thickness. 14.5 Particle Insertion There are three ways of introducing droplets or particles into a simulation. The first way is to define a sprinkler or nozzle using a PROP line that includes a PART_ID that specifies the particle or droplet parameters. The second way is to add a PART_ID to a SURF line, in which case particles or droplets will be ejected from that surface. Note that this only works if the surface has a normal velocity pointing into the flow domain. The third way to introduce particles or droplets is via an INIT line that defines a volume within the computational domain in which the particles/droplets are to be introduced initially and/or periodically in time. It is not unusual to include hundreds of thousands of particles in a simulation. Visualizing all of the particles in Smokeview can sometimes be impractical due to memory limitations. To limit the amount of 171 particles, you can make use of the following parameters on the PART line: SAMPLING_FACTOR Sampling factor for the output file CHID.prt5. This parameter can be used to reduce the size of the particle output file used to animate the simulation. The default value is 1 for MASSLESS particles, meaning that every particle or droplet will be shown in Smokeview. The default is 10 for all other types of particles. MASSLESS particles are discussed in Section 14.2. AGE Number of seconds the particle or droplet exists, after which time it is removed from the calcula- tion. This is a useful parameter to use when trying to reduce the number of droplets or particles in a simulation. 14.5.1 Particles Introduced at a Solid Surface If the particles have mass and are introduced from a solid surface, specify PARTICLE_MASS_FLUX on the SURF line. The number of particles inserted at each solid cell every DT_INSERT seconds is specified by NPPC (Number of Particles Per Cell) on the SURF line defining the solid surface. The default value of DT_INSERT is 0.01 s and NPPC is 1. As an example, the following set of input lines: &PART ID='particles', ... / &SURF ID='SLOT', PART_ID='particles', VEL=-5., PARTICLE_MASS_FLUX=0.1 / &OBST XB=-0.2,0.2,-0.2,0.2,4.0,4.4, SURF_IDS='INERT','SLOT','INERT' / creates an obstruction that ejects particles out of its sides at a rate of 0.1 kg/(m2 · s) and a velocity of 5 m/s (the minus sign indicates the particles are ejected from the surface). FDS will adjust the mass flux if the obstruction or vent dimensions are changed to conform to the numerical grid. The IDs have no meaning other than as identifiers. The surface on which particles are specified must have a non-zero normal velocity directed into the computational domain. This happens automatically if the surface is burning, but must be specified if it is not. There is a simple input file called particle_flux.fds that demonstrates how the above input lines can produce a stream of particles from a block. The total mass flux from the block is the product of the PARTICLE_MASS_FLUX times the total area of the sides of the block, 0.4 m × 0.4 m × 4. The expected accumulated mass of particles on the ground after 10 s is expected to be 0.64 kg, as shown in Fig. 14.7. FDS0−86−g80cff4e 0.8 0.7 Particle Mass (particle_flux) Mass (kg) 0.6 0.5 0.4 0.3 0.2 0.1 0 0 Expected (mass) FDS (mass) 5 10 Time (s) 15 20 Figure 14.7: Simple test case to demonstrate mass conservation of particles ejected from an obstruction. 172 Note also that you can independently control particles that emanate from a solid surface. For example, a device might control the activation of a fan, but you can over-ride the device and control the particles separately. To do this, specify either a device or controller via a DEVC_ID or CTRL_ID on the PART line that defines the particles. For more information on devices and controls, see Sections 15.4 and 15.5. 14.5.2 Particles or Droplets Introduced at a Sprinkler or Nozzle A sprinkler or nozzle is added to the simulation using a PROP line to describe the features of the device and a DEVC line to position and orient the device within the computational domain. PARTICLES_PER_SECOND is the number of droplets inserted every second per active sprinkler or nozzle (Default 5000). It is listed on the PROP line that includes other properties of the sprinkler or nozzle. Note that this parameter only affects sprinklers and nozzles. Changing this parameter does not change the flow rate, but rather the number of droplets used to represent the flow. Note that PARTICLES_PER_SECOND can be a very important parameter. In some simulations, it is a good idea to increase this number so that the liquid mass is distributed more uniformly over the droplets. If this parameter is too small, it can lead to a non-physical evaporation pattern, sometimes even to the point of causing a numerical instability. If you encounter a numerical instability shortly after the activation of a sprinkler or nozzle, consider increasing PARTICLES_PER_SECOND to produce a smoother evaporation pattern that is more realistic. Keep in mind that for a real sprinkler or nozzle, there are many more droplets created per second than the number that can be simulated. Note that by default the parameter PARTICLE_CFL is set to .FALSE. (see Section 6.4.10). Thus, particles inserted with a velocity faster than the local fluid entrainment velocity may traverse more than one cell. If the particles represent a fuel spray or sprinkler droplets that are to be collected in a pan, it may be necessary to set PARTICLE_CFL=.TRUE. on MISC to precisely account for particle mass. For example, the particles may represent liquid fuel being sprayed into a pan burner made from a zero thickness obstruction. In this case, if the particle position overshoots the cell face representing the boundary of the pan then either spurious burning of the particle will occur underneath the pan or, if the particle does not burn, the heat release rate will be diminished. 14.5.3 Particles or Droplets Introduced within a Volume Sometimes it is convenient to introduce Lagrangian particles within a particular region of the domain. To do this, use an INIT line which contains the PART_ID for the type of particle to be inserted. Particles specified via an INIT line can represent a number of different kinds of subgrid-scale objects. The particles can be massless tracers or they can be solid or liquid particles with mass. If not massless, specify MASS_PER_VOLUME in units of kg/m3 . Do not confuse this parameter with DENSITY, explained in the next section. For example, water has a DENSITY of 1000 kg/m3 , whereas a liter of water broken up into droplets and spread over a cubic meter has a MASS_PER_VOLUME of 1 kg/m3 . The number of Lagrangian particles inserted is controlled by the parameter N_PARTICLES. Randomly Distributed Particles within a Specified Volume The parameter N_PARTICLES on the INIT line indicates the number of particles to insert within a specified region of the domain. This region can take on a number of shapes, depending on the parameter SHAPE. By default, the region is a rectangular solid designated with the real sextuplet XB. The format for XB is the same as that used on the OBST line. Alternatively, you can specify SHAPE=’CONE’, in which case the particles will be randomly distributed within a vertical cone. This is primarily used for representing trees. The dimensions of the cone are specified via the parameters RADIUS, HEIGHT, and base position XYZ. The 173 latter is a triplet of real numbers designating the point at the center of the base of the cone. Examples of typical INIT lines are: &INIT PART_ID='droplets', XB=..., N_PARTICLES=..., MASS_PER_VOLUME=... / &INIT PART_ID='leaves', XYZ=..., RADIUS=..., HEIGHT=..., SHAPE='CONE', N_PARTICLES=..., MASS_PER_VOLUME=... / Note that the volume of the specified region is calculated according to the SHAPE dimensions, regardless of whether there are solid obstructions within this region. Note also that in most applications, the number of particles, N_PARTICLES, is somewhat arbitrary but should be chosen to provide at least a few particles per grid cell. FDS will then automatically assign a weighting factor to each particle to ensure that the MASS_PER_VOLUME is achieved. In some applications, on the other hand, it may be important to specify the number of particles. For example, if using particles to model the burning of electrical cables, you may want to specify how many cables are actually burning. If the volume specified by the sextuplet XB crosses mesh boundaries, be aware that N_PARTICLES refers to the entire volume, not just the volume within a particular mesh. FDS will automatically compute the necessary number of particles to assign to each mesh. Specifying a Fixed Number of Particles per Grid Cell There are special applications where you might want to specify N_PARTICLES_PER_CELL to indicate the number of particles within each grid cell of a specified region. When using N_PARTICLES_PER_CELL, the particles will be randomly placed within each cell. If you set CELL_CENTERED=.TRUE., the particles will be placed at the center of each cell. Specifying a Weight Factor for Particles Use PARTICLE_WEIGHT_FACTOR to specify how many actual particles each of the computational particles represent. This can be used in conjunction with N_PARTICLES_PER_CELL to reduce the computational cost when a large number of identical particles would be placed in the same grid cell. Single Particle Insertion If you introduce only a single particle, which is often a handy way of creating a target, you may use the real triplet XYZ rather than XB to designate the particle’s position. You can give this single particle an initial velocity using the real triplet UVW. You can also add DX, DY, and/or DZ to create a line of particles that are offset from XYZ by these increments in units of meters. For example, &INIT PART_ID='target', XYZ=1.2,3.4,5.6, N_PARTICLES=10, DX=0.1 / creates a line of 10 particles starting at the point (1.2,3.4,5.6) separated by 0.1 m. This is handy for creating arrays of devices, like heat flux gauges. See Section 16.10.5 for more details. In special cases, you might want a single liquid droplet to be inserted at a particular point with a particular velocity every DT_INSERT s following the activation of a particular device, as follows: &INIT N_PARTICLES=1, XYZ=..., UVW=..., DIAMETER=200., DT_INSERT=0.05, PART_ID='drops', DEVC_ID='nozzle' / Note that the DIAMETER (µm) on the INIT line is only valid for liquid droplets. It over-rides the DIAMETER on the PART line labelled ’drops’. A simple test case that demonstrates this functionality is called 174 bucket_test_3, in which water droplets are launched in different directions from a common point. Their size, velocity, insertion frequency, and mass flux are varied, and a check is made that water mass is conserved (see Fig. 14.8). FDS0−86−g80cff4e 0.01 Accumulated Mass (bucket_test_3) Mass (kg) 0.008 0.006 0.004 0.002 Ideal FDS 0 0 2 4 6 8 10 Time (s) Figure 14.8: Accumulated water collected at the floor in the bucket_test_3 case. Periodic Insertion of Particles within a Specified Volume If you want to introduce particles within a given region periodically in time and not just initially, set DT_INSERT on the INIT line to a positive value indicating the time increment (s) for insertion. The parameter N_PARTICLES now indicates the number of droplets/particles inserted every DT_INSERT seconds. If the droplets/particles have mass, use MASS_PER_TIME (kg/s) instead of MASS_PER_VOLUME to indicate how much mass is to be introduced per second. If you want to delay the insertion of droplets, you can use either a DEVC_ID or a CTRL_ID on the INIT line to name the controlling device. See Section 15.4 for more information on controlling devices. 14.6 Special Topic: Suppression by Water Modeling fire suppression by water has three principal components: transporting the water droplets through the air, tracking the water along the solid surface, and predicting the reduction of the burning rate. This section addresses the latter two. 14.6.1 Velocity on Solid Surfaces When a droplet strikes a solid surface4 , it sticks and is reassigned a new speed and direction. If the surface is horizontal, the direction is randomly chosen. If vertical, the direction is downwards. The rate at which the droplets move over the horizontal and vertical surfaces is difficult to quantify. The parameters HORIZONTAL_VELOCITY and VERTICAL_VELOCITY on the PART line allow you to control the rate at which droplets move horizontally or vertically (downward). The defaults are 0.2 m/s and 0.5 m/s, respectively. 4 If you do not want droplets to accumulate on solid surfaces, set ALLOW_SURFACE_PARTICLES=.FALSE. on the MISC line. It is normally .TRUE. 175 There are some applications, like the suppression of racked storage commodities, where it is useful to allow water droplets to move horizontally along the underside of a solid object. It is difficult to model this phenomenon precisely because it involves surface tension, surface porosity and absorption, and complicated geometry. However, a way to capture some of the effect is to set ALLOW_UNDERSIDE_PARTICLES=.TRUE. on the MISC line. It is normally false. Also, note that when droplets hit obstructions, the vertical direction is assumed to coincide with the z axis, regardless of any change to the gravity vector, GVEC. A useful sample case to demonstrate various features of droplet motion on solid obstructions is the test case called cascade.fds. Figure 14.9 shows a stream of water droplets impinging on the top of a box followed by the cascading of water droplets over the top edge. Figure 14.9: Smokeview rendering of the cascade test case. 14.6.2 Reduction of the Burning Rate Water reduces the fuel pyrolysis rate by cooling the fuel surface and also changing the chemical reactions that liberate fuel gases from the solid. If the solid or liquid fuel has been given reaction parameters via the MATL line, there is no need to set any additional suppression parameters. It is assumed that water impinging on the fuel surface takes energy away from the pyrolysis process and thereby reduces the burning rate of the fuel. If the surface has been assigned a HRRPUA (Heat Release Rate Per Unit Area), a parameter needs to be specified that governs the suppression of the fire by water because this type of simulated fire essentially acts like a gas burner whose flow rate is explicitly specified. An empirical way to account for fire suppression by water is to characterize the reduction of the pyrolysis rate in terms of an exponential function. The local mass loss rate of the fuel is expressed in the form ṁ00f (t) = ṁ00f,0 (t) e− R k(t) dt (14.8) Here ṁ00f,0 (t) is the user-specified burning rate per unit area when no water is applied and k is a function of the local water mass per unit area, m00w , expressed in units of kg/m2 . k(t) = E_COEFFICIENT m00w (t) 1/s (14.9) The parameter E_COEFFICIENT must be obtained experimentally, and it is expressed in units of m2 /(kg · s). Usually, this type of suppression algorithm is invoked when the fuel is complicated, like a cartoned commodity. The example case e_coefficient demonstrates the use of this parameter. A sprinkler is placed 176 over a burner defined with an E_COEFFICIENT as shown below. The sprinkler is set to operate at 5 s. Figure 14.10 shows the heat release rate and burning rate. Note that expected value is computed using the sprinkler flow rate and the FDS results are delayed slightly by the time it takes for droplets to reach and accumulate on the burning surface. &SURF ID='FUEL', HRRPUA=100., E_COEFFICIENT=4. / −3 FDS0−86−g80cff4e 120 2.5 Heat Release Rate Burning Rate 2 Burning Rate (kg/s) Heat Release Rate (kW) 100 80 Expected FDS 60 40 1.5 Expected FDS 1 0.5 20 0 0 FDS0−86−g80cff4e x 10 5 10 Time (s) 15 0 0 20 5 10 Time (s) Figure 14.10: Output of the e_coefficient test case. 177 15 20 178 Chapter 15 Devices and Control Logic Sprinklers, smoke detectors, heat flux gauges, and thermocouples may seem to be completely unrelated, but from the point of view of FDS, they are simply devices that operate in specific ways depending on the properties assigned to them. They can be used to record some quantity of the simulated environment, like a thermocouple, or they can represent a mathematical model of a complex sensor, like a smoke detector, and in some cases they can trigger events to happen, like a timer. All devices, in the broadest sense of the word, are designated via the namelist group DEVC. In addition, advanced functionality and properties are accommodated via additional namelist groups called CTRL (Control) and PROP (Properties). 15.1 Device Location and Orientation: The DEVC Namelist Group (Table 17.5) Regardless of the specific properties, each device needs to be sited either at a point within the computational domain, or over a span of the domain, like a beam smoke detector. For example, a sprinkler is sited within the domain with a line like: &DEVC XYZ=3.0,5.6,2.3, PROP_ID='Acme Sprinkler 123', ID='Spk_39' / The physical coordinates of the device are given by a triplet of real numbers, XYZ. FDS uses these coordinates to determine in which gas or wall cell the device is located. The properties of the device are contained on the PROP line designated by PROP_ID, which will be explained below for each of the special devices included in FDS. The character string ID is merely a descriptor to identify the device in the output files, and if any action is tied to its activation. Not all devices need to be associated with a particular set of properties via the PROP_ID. For example, pointwise output quantities are specified with a single DEVC line, like &DEVC ID='TC-23', XYZ=3.0,5.6,2.3, QUANTITY='TEMPERATURE' / which tells FDS to record the temperature at the given point as a function of time. The ID is a label in the output file whose name is CHID_devc.csv. Note that FDS outputs the data stored for that cell without performing any interpolation with surrounding cells. Some devices have a particular orientation. The parameter IOR (Index of Orientation) is required for any device that is placed on the surface of a solid. The values ±1 or ±2 or ±3 indicate the direction that the device “points.” For example, IOR=-1 means that the device is mounted on a wall that faces in the 179 negative x direction. ORIENTATION is used for devices that are not on a surface and require a directional specification, like a sprinkler. ORIENTATION is specified with a triplet of real number values that indicate the components of the direction vector. The default value of ORIENTATION is (0,0,-1). For example, a default downward-directed sprinkler spray can be redirected in other direction. If you were to specify &DEVC XYZ=3.0,5.6,2.3, PROP_ID='...', ID='...', ORIENTATION=0.707,0.707,0.0 / the sprinkler would point in the direction halfway between the positive x and y directions. For other devices, the ORIENTATION would only change the way the device is drawn by Smokeview. The delivered density to the floor from a sprinkler depends upon where the sprinkler arms are located. Rather than redefining the spray pattern for every possible direction that the sprinkler can be attached to the pipe, the DEVC can be given the parameter ROTATION. The default ROTATION is 0 degrees, which for a downwards pointing sprinkler is the positive x-axis. Positive ROTATION will rotate the 0 degree point towards the positive y-axis. 15.2 Device Output Each device has a QUANTITY associated with it. The time history of each DEVC quantity is output to a comma-delimited ASCII file called CHID_devc.csv (see Section 20.3 for output file format). This file can be imported into most spread sheet software packages. Most spreadsheet programs limit the number of columns to some number (for example the 2003 version Microsoft Excel had a 256 column limit). As a default, FDS places no limit on the amount of columns in a comma-separated value (.csv) file. If your spreadsheet application allows fewer columns than the number of DEVC or CTRL in your input file then set COLUMN_DUMP_LIMIT equal to .TRUE. on the DUMP line. Use DEVC_COLUMN_LIMIT and CTRL_COLUMN_LIMIT to indicate the limit of columns in the device and control output files. Their default values are 254. By default, the DEVC output is written to a file every DT_DEVC seconds. This time increment is specified on the DUMP line. Also, by default, a time-averaged value is written out for each quantity of interest. To prevent FDS from time-averaging the DEVC output, add TIME_AVERAGED=.FALSE. to the DEVC line. A useful option for the DEVC line is to add RELATIVE=.TRUE., which will indicate that only the change in the initial value of the QUANTITY is to be output. This can be useful for verification and validation studies. You can change the values of the output QUANTITY by multiplying by CONVERSION_FACTOR and changing the character string UNITS. You can also change the scaling of the coordinates by applying a COORD_FACTOR. This is handy for plotting nondimensional distance, for example. If you do not want the DEVC QUANTITY to be included in the output file, set OUTPUT=.FALSE. on the DEVC line. Sometimes, devices are just used as clocks or control devices. In these cases, you might want to prevent its output from cluttering the output file. If the DEVC QUANTITY=’TIME’, then OUTPUT is set to .FALSE. automatically. All devices must have a specified QUANTITY. Some special devices (Section 15.3) have their QUANTITY specified on a PROP line. A QUANTITY specified on a PROP line associated with a DEVC line will override a QUANTITY specified on the DEVC line. 180 15.3 Special Device Properties: The PROP Namelist Group (Table 17.20) Many devices are fairly easy to describe, like a point measurement, with only a few parameters which can be included on the DEVC line. However, for more complicated devices, it is inconvenient to list all of the properties on each and every DEVC line. For example, a simulation might include hundreds of sprinklers, but it is tedious to list the properties of the sprinkler each time the sprinkler is sited. For these devices, use a separate namelist group called PROP to store the relevant parameters. Each PROP line is identified by a unique ID, and invoked by a DEVC line by the string PROP_ID. The best way to describe the PROP group is to list the various special devices and their properties. 15.3.1 Sprinklers Here is a very simple example of a sprinkler: &PROP ID='K-11', QUANTITY='SPRINKLER LINK TEMPERATURE', RTI=148., C_FACTOR=0.7, ACTIVATION_TEMPERATURE=74., OFFSET=0.10,PART_ID='water drops', FLOW_RATE=189.3, PARTICLE_VELOCITY=10., SPRAY_ANGLE=30.,80. / &DEVC ID='Spr_60', XYZ=22.88,19.76,7.46, PROP_ID='K-11' / A sprinkler, known as ’Spr_60’, is located at a point in space given by XYZ. It is a ’K-11’ type sprinkler, whose properties are given on the PROP line. Note that the various names (IDs) mean nothing to FDS, except as a means of associating one thing with another, so try to use IDs that are meaningful. The parameter QUANTITY=’SPRINKLER LINK TEMPERATURE’ does have a specific meaning to FDS, directing it to compute the activation of the device using the standard RTI (Response Time Index [36]) algorithm. Properties associated with sprinklers included in the PROP group are: RTI Response Time Index in units of (m·s)1/2 . (Default 100.) C_FACTOR Conduction Factor in units of (m/s)1/2 . (Default 0.) ACTIVATION_TEMPERATURE in units of ◦ C. (Default 74 ◦ C) INITIAL_TEMPERATURE of the link in units of ◦ C. (Default TMPA) FLOW_RATE or MASS_FLOW_RATE in units of L/min or kg/s. An alternative is to provide the K_FACTOR 1 in units of L/(min · bar 2 ) and the OPERATING_PRESSURE, the gauge pressure at the sprinkler, in units √ of bar. The flow rate is then given by K p. Note that 1 bar is equivalent to 14.5 psi, 1 gpm is equiv1 1 alent to 3.785 L/min, 1 gpm/psi 2 is equivalent to 14.41 L/min/bar 2 . If MASS_FLOW_RATE is given then PARTICLE_VELOCITY must also be defined. Note that FLOW_RATE is only appropriate for liquid droplets; solid particles should use MASS_FLOW_RATE OFFSET Radius (m) of a sphere surrounding the sprinkler where the water droplets are initially placed in the simulation. It is assumed that beyond the OFFSET the droplets have completely broken up and are transported independently of each other. (Default 0.05 m) PARTICLE_VELOCITY Initial droplet velocity. (Default 0 m/s) ORIFICE_DIAMETER Diameter of the nozzle orifice in m (default 0 m). This input provides an alternative way to set droplet velocity by giving values for FLOW_RATE and ORIFICE_DIAMETER, in which case the droplet velocity is computed by dividing the flow rate by the orifice area. Use this method if you do not have any information about droplet velocity. However, quite often you must fine-tune the PARTICLE_VELOCITY in order to reproduce a particular spray profile. The ORIFICE_DIAMETER is not used if either PARTICLE_VELOCITY or SPRAY_PATTERN_TABLE is specified. 181 SPRAY_ANGLE A pair of angles (in degrees) through which the droplets are sprayed. The angles out- line a conical spray pattern relative to the south pole of the sphere centered at the sprinkler with radius OFFSET. For example, SPRAY_ANGLE=30.,80. directs the water droplets to leave the sprinkler through a band between 60◦ and 10◦ south latitude, assuming the orientation of the sprinkler is (0,0,-1), the default. Elliptical spray patterns can be defined by giving a pair of spray angles. For example, SPRAY_ANGLE(1,1:2)=0.,60. and SPRAY_ANGLE(2,1:2)=0.,30., defines a spray pattern with 60 degree angle in the direction of x axis and a 30 degree angle in the direction of y axis. SPRAY_PATTERN_SHAPE determines how the droplets are distributed within the specified SPRAY_ANGLE. Choices are ’UNIFORM’ for uniform distribution and ’GAUSSIAN’. The default distribution is ’GAUSSIAN’. The parameter SPRAY_PATTERN_MU controls the latitude of the maximum density of droplets for the ’GAUSSIAN’ distribution. The width of the distribution is controlled by the parameter SPRAY_PATTERN_BETA. SPRAY_PATTERN_TABLE Name of a set of TABL lines containing the description of the spray pattern. PART_ID The name of the PART line containing properties of the droplets. See Chapter 14 for additional details. PRESSURE_RAMP The name of the RAMP lines specifying the dependence of pipe pressure on the number of active sprinklers and nozzles. Be aware that sprinklers can produce many droplets. To limit the computational cost, liquid droplets disappear when they hit the “floor” of the computational domain, regardless of whether it is solid or not. This feature mimics the presence of floor drains. To stop FDS from removing liquid droplets from the floor of the domain, add the phrase POROUS_FLOOR=.FALSE. to the MISC line. Be aware, however, that droplets that land on the floor continue to move horizontally in randomly selected directions; bouncing off obstructions, and consuming CPU time. Note also that solid particles do not disappear from the floor the domain like liquid droplets. For more information about sprinklers, their activation and spray dynamics, read the FDS Technical Reference Guide [1]. Special Topic: Specifying Complex Spray Patterns As an example of the more advanced sprinkler options, a sprinkler with an elliptical spray pattern and uniform mass flux distribution within the spray angle is given by: &PROP ID='K-11', QUANTITY='SPRINKLER LINK TEMPERATURE', RTI=148., C_FACTOR=0.7, ACTIVATION_TEMPERATURE=74., OFFSET=0.10,PART_ID='water drops', FLOW_RATE=189.3, PARTICLE_VELOCITY=10., SPRAY_ANGLE(1:2,1)=0.,60.,SPRAY_ANGLE(1:2,2) = 0.,30., SPRAY_PATTERN_SHAPE='UNIFORM' / &DEVC ID='Spr_60', XYZ=22.88,19.76,7.46, PROP_ID='K-11' / For full-cone sprays, the parameter SPRAY_PATTERN_MU is set to zero by default. For hollow-cone sprays it is set to the average of SPRAY_ANGLE(1:2,1), the spray angle in x direction. The following example uses SPRAY_PATTERN_MU to define a spray that is somewhere between full-cone and hollow-cone spray: &PROP ID='K-11', QUANTITY='SPRINKLER LINK TEMPERATURE', RTI=148., C_FACTOR=0.7, ACTIVATION_TEMPERATURE=74., OFFSET=0.10,PART_ID='water drops', FLOW_RATE=189.3, PARTICLE_VELOCITY=10., SPRAY_ANGLE=0.,30. ,SPRAY_PATTERN_MU=15./ &DEVC ID='Spr_60', XYZ=22.88,19.76,7.46, PROP_ID='K-11' / 182 If a more complex spray pattern is desired than one characterized by a SPRAY_ANGLE, then a SPRAY_PATTERN_TABLE can be specified using the TABL namelist group. Specify the total flow using FLOW_RATE on the PROP line, the name of the spray pattern using SPRAY_PATTERN_TABLE and then one or more TABL lines of the form: &TABL ID='table_id', TABLE_DATA=LAT1,LAT2,LON1,LON2,VELO,FRAC / where each TABL line for a given ’table_id’ provides information about the spherical distribution of the spray pattern for a specified solid angle. LAT1 and LAT2 are the bounds of the solid angle measured in degrees from the south pole (0 is the south pole and 90 is the equator, 180 is the north pole). Note that this is not the conventional way of specifying a latitude, but rather a convenient system based on the fact that a typical sprinkler sprays water downwards, which is why 0 degrees is assigned to the “south pole,” or the −z direction. The parameters LON1 and LON2 are the bounds of the solid angle (also in degrees), where 0 (or 360) is aligned with the −x axis and 90 is aligned with the −y axis. VELO is the velocity (m/s) of the droplets at their point of insertion. FRAC the fraction of the total flow rate of liquid that should emerge from that particular solid angle. In the test case called bucket_test_2, the spray pattern is defined as two jets, each with a velocity of 5 m/s and a total flow rate of 60 L/min. The sprinkler is set to operate for only 5 s. The first jet contains 0.2 of the total flow, the second, 0.8 of the total. The jets are centered at points 30◦ below the “equator,” and are separated by 180◦ . &PROP ID='K-11', QUANTITY='SPRINKLER LINK TEMPERATURE', OFFSET=0.10, PART_ID='water_drops', FLOW_RATE=60., SPRAY_PATTERN_TABLE='TABLE1', SMOKEVIEW_ID='sprinkler_upright', PARTICLE_VELOCITY=10. / &TABL ID='TABLE1',TABLE_DATA=30,31,0,1,5,0.2/ &TABL ID='TABLE1',TABLE_DATA=30,31,179,180,5,0.8/ Note that each set of TABL lines must have a unique ID. Also note that the TABL lines can be specified in any order. Figure 15.1 verifies that the sprinkler releases 5 kg of water (1 kg/s for 5 s). FDS0−86−g80cff4e 6 Accumulated Mass (bucket_test_2) 5 Mass (kg) 4 3 2 1 Ideal FDS 0 0 2 4 6 8 10 Time (s) Figure 15.1: Accumulated water collected at the floor in the bucket_test_2 case. 183 Special Topic: Varying Pipe Pressure In real sprinkler systems, the pipe pressure is affected by the number of actuated sprinklers. Typically, the pressure is higher than the design value when the first sprinkler activates, and decreases as more and more sprinklers are activated. The pipe pressure has an effect on flow rate, droplet velocity and droplet size distribution. In FDS, the varying pipe pressure can be specified on a PROP line using PRESSURE_RAMP. On each RAMP line, the number of open sprinklers or nozzles is associated with certain pipe pressure (bar). For example: &PROP ID='My nozzle' OFFSET=0.1 PART_ID='water drops' FLOW_RATE=0.9 OPERATING_PRESSURE = 10.0 PARTICLE_VELOCITY=15.0 SPRAY_ANGLE=0.0,80.0 PRESSURE_RAMP = 'PR1' / &RAMP ID = 'PR1' T = 1, F = 16. / &RAMP ID = 'PR1' T = 2, F = 10. / &RAMP ID = 'PR1' T = 3, F = 8. / These lines would indicate that the pressure is 16 bar when the first sprinkler activates, 10 bar when two sprinklers are active, and 8 bar when three or more sprinklers are active. When counting the number of active sprinklers, FDS accounts for all active sprinklers or nozzles with a given PART_ID. When pressure ramps are used, both FLOW_RATE and PARTICLE_VELOCITY are dependent on the OPERATING_PRESSURE. Specify either the FLOW_RATE, or the K_FACTOR and OPERATING_PRESSURE. In √ the latter case, the flow rate is given by K p and the droplet velocity by using the liquid density and the ORIFICE_DIAMETER. If spray pattern table is used, the droplet velocity is determined separately for each √ line of the table by applying K p and the ORIFICE_DIAMETER. The median diameter of the particle size distribution is scaled as dm (p) = dm (po )(po /p)1/3 , where po is the OPERATING_PRESSURE and dm (po ) is specified by parameter DIAMETER on the corresponding PART line. For some simulations there may be groups of independent sprinklers or nozzles. For example one might have one set of nozzles for a fuel spray and a second set for water spray. In this case the flow of water would not be impacted by how many fuel spray nozzles are open. To have the PRESSURE_RAMP only count a subset of sprinklers or nozzles, the keyword PIPE_INDEX can be used on the DEVC line. For example: &DEVC &DEVC &DEVC &DEVC ID='Spr_1', ID='Spr_2', ID='Fuel_1', ID='Fuel_2', XYZ=2.00,2.00,8.00, XYZ=1.00,1.00,8.00, XYZ=2.00,2.00,1.00, XYZ=1.00,1.00,1.00, PROP_ID='My nozzle', PROP_ID='My nozzle', PROP_ID='Fuel Spray', PROP_ID='Fuel Spray', PIPE_INDEX=1 PIPE_INDEX=1 PIPE_INDEX=2 PIPE_INDEX=2 / / / / These lines indicate that the fuel spray nozzles are a separate pipe network from the water sprinklers. With these inputs, a PRESSURE_RAMP for the water sprinklers would not count any active fuel spray nozzles. See the example case flow_rate_2 in the Verification Guide for further details on the use of PIPE_INDEX. 15.3.2 Nozzles Nozzles are very much like sprinklers, only they do not activate based on the standard RTI (Response Time Index) model. To simulate a nozzle that activates at a given time, specify a QUANTITY and SETPOINT directly on the DEVC line. The following lines: 184 &DEVC XYZ=23.91,21.28,0.50, PROP_ID='nozzle', ORIENTATION=0,0,1, QUANTITY='TIME', SETPOINT=0., ID='noz_1' / &DEVC XYZ=26.91,21.28,0.50, PROP_ID='nozzle', ORIENTATION=0,0,1, QUANTITY='TIME', SETPOINT=5., ID='noz_2' / &PROP ID='nozzle', PART_ID='heptane drops', FLOW_RATE=2.132, FLOW_TAU=-50., PARTICLE_VELOCITY=5., SPRAY_ANGLE=0.,45. / designate two nozzles of the same type, one which activates at time zero, the other at 5 s. Note that nozzles must have a designated PROP_ID, and the PROP line must have a designated PART_ID to describe the liquid droplets. Example Case: Setting the Flow Rate of a Nozzle This example demonstrates the use of pressure ramps and control logic for opening and closing nozzles. It also serves as a verification test for the water flow rate. There are four nozzles that open at designated times: 0 s, 15 s, 30 s and 45 s. At time 60 s, all the nozzles are closed. The number of open nozzles is measured using a device with quantity ’OPEN NOZZLES’. A comparison of the FDS result and the exact, intended values is shown in Fig. 15.2. Note that ’OPEN NOZZLES’ counts only nozzles belonging to the specified PIPE_INDEX. The pressure ramp has been designed to deliver a total flow rate of 10 L/min at all values of open nozzles. Mathematically this means that √ nK p = 10 L/min ⇒ p= 10 L/min nK 2 (15.1) where n is the number of open nozzles. The corresponding nozzle and pressure ramp definitions are &PROP ID='Head', OFFSET=0.10, PART_ID='water drops', K_FACTOR=2.5, OPERATING_PRESSURE=1., PRESSURE_RAMP='PR', PARTICLE_VELOCITY=2., SPRAY_ANGLE= 0.,60. / &RAMP &RAMP &RAMP &RAMP ID='PR', ID='PR', ID='PR', ID='PR', T= T= T= T= 1., 2., 3., 4., F=16. / F=4. / F=1.778 / F=1. / The water is tracked using a device measuring the accumulated mass per unit area, integrated over the total floor area. The total mass of water should increase from zero to 10 kg in 60 s. A comparison of the FDS prediction and this analytical result is shown in Fig. 15.2. The slight delay of the FDS result is caused by the time it takes from the droplets to fall down on the floor. 15.3.3 Special Topic: Specified Entrainment (Velocity Patch) The details of the sprinkler head geometry and spray atomization are practically impossible to resolve in a fire calculation. As a result, the local gas phase entrainment by the sprinkler is difficult to predict. As an alternative, it is possible to specify the local gas velocity in the vicinity of the sprinkler nozzle. The PROP line may be used to specify a polynomial function for a specific velocity component and this function may be “patched” into the flow field using a device. This device is given the quantity ’VELOCITY PATCH’ and is initially inactive. The velocity patch must be activated with a separate control device, as discussed in Section 15.4. You specify the local region for the velocity patch using XB for the device. The polynomial is defined as a second-order Taylor expansion about the point XYZ (the default value of XYZ is the center of XB). FDS then uses an immersed boundary method to force the local velocity component to satisfy the 185 FDS0−86−g80cff4e FDS0−86−g80cff4e 5 12 Open Nozzles (flow_rate) Accumulated Water (flow_rate) 10 8 3 Mass (kg) Open Nozzles 4 2 6 4 1 2 Analytical (Water) FDS (Nozzles) 0 0 10 20 30 40 Time (s) Analytical (Water) FDS (Water) 50 60 0 0 70 10 20 30 40 Time (s) 50 60 70 Figure 15.2: Output of the flow_rate test case. polynomial. The polynomial is specified by the coefficients P0, PX(1:3), and PXX(1:3,1:3), which represent, respectively, the value of the kth velocity component, the first derivatives, and the second derivatives at point XYZ. Note that the first derivatives are represented by a three component array and the second derivatives are represented by a symmetric 3 × 3 array—only the upper triangular part needs to be specified. The polynomial is given by (note that summation of repeated indices is implied): 2 ri r j ∂ uk ∂ uk uk (r) = (uk )0 + ri + (15.2) | {z } ∂ xi 0 2 ∂ xi ∂ x j 0 {z } | | {z } P0 PX(1:3) PXX(1:3,1:3) The vector r is the position of the velocity storage location relative to the point XYZ. The specific velocity component is specified on PROP by the integer VELOCITY_COMPONENT. Below we provide an example set of PROP and DEVC lines to specify a parabolic profile for the vertical component of velocity. &PROP ID='p1', VELOCITY_COMPONENT=3, P0=-1,PXX(1,1)=5,PXX(2,2)=5 / &DEVC XB=-.1,.1,-.1,.1,.9,.95, QUANTITY='VELOCITY PATCH',PROP_ID='p1', DEVC_ID='t1'/ &DEVC ID='t1', XYZ=0,0,.9, QUANTITY='TIME', SETPOINT=10/ In this example, a velocity patch is activated at 10 s in the simulation. Any w components of velocity with staggered storage locations within the box XB=-.1,.1,-.1,.1,.9,.95 will be driven toward the value specified by the polynomial profile ’p1’. You must ensure that the device box encompasses the staggered storage locations (see the theory manual [1] for a discussion on the face-centered velocity storage locations). 15.3.4 Heat Detectors QUANTITY=’LINK TEMPERATURE’ defines a heat detector, which uses essentially the same activation al- gorithm as a sprinkler, without the water spray. &DEVC ID='HD_66', PROP_ID='Acme Heat', XYZ=2.3,4.6,3.4 / &PROP ID='Acme Heat', QUANTITY='LINK TEMPERATURE', RTI=132., ACTIVATION_TEMPERATURE=74. / √ Like a sprinkler, RTI is the Response Time Index in units of m · s. ACTIVATION_TEMPERATURE is the link activation temperature in degrees C (Default 74 ◦ C). INITIAL_TEMPERATURE is the initial temperature of the link in units of ◦ C (Default TMPA). 186 15.3.5 Smoke Detectors A smoke detector is defined in the input file with an entry similar to: &DEVC ID='SD_29', PROP_ID='Acme Smoke Detector', XYZ=2.3,4.6,3.4 / &PROP ID='Acme Smoke Detector', QUANTITY='CHAMBER OBSCURATION', LENGTH=1.8, ACTIVATION_OBSCURATION=3.24 / for the single parameter Heskestad model. Note that a PROP line is mandatory for a smoke detector, in which case the DEVC QUANTITY can be specified on the PROP line. For the four parameter Cleary model, use a PROP line like: &PROP ID='Acme Smoke Detector I2', QUANTITY='CHAMBER OBSCURATION', ALPHA_E=1.8, BETA_E=-1.1, ALPHA_C=1.0, BETA_C=-0.8, ACTIVATION_OBSCURATION=3.24 / where the two characteristic filling or “lag” times are of the form: δte = αe uβe δtc = αc uβc ; (15.3) The default detector parameters are for the Heskestad model with a characteristic LENGTH of 1.8 m. For the Cleary model, the ALPHAs and BETAs must all be listed explicitly. Suggested constants for unidentified ionization and photoelectric detectors presented in Table 15.1. ACTIVATION_OBSCURATION is the threshold value in units of %/m. The threshold can be set according to the setting commonly provided by the manufacturer. The default setting1 is 3.24 %/m (1 %/ft). Table 15.1: Suggested values for smoke detector model [37]. See Ref. [28] for others. Detector Cleary Ionization I1 Cleary Ionization I2 Cleary Photoelectric P1 Cleary Photoelectric P2 Heskestad Ionization αe 2.5 1.8 1.8 1.8 — βe -0.7 -1.1 -1.0 -0.8 — αc , L 0.8 1.0 1.0 0.8 1.8 βc -0.9 -0.8 -0.8 -0.8 — Defining Smoke By default, FDS assumes that the smoke from a fire is generated in direct proportion to the heat release rate. A value of SOOT_YIELD=0.01 on the REAC line means that the smoke generation rate is 0.01 of the fuel burning rate. The “smoke” in this case is not explicitly tracked by FDS, but rather is assumed to be a function of the combustion products lumped species. 1 Note that the conversion of obscuration from units of %/ft to %/m is given by: " # O[%/ft] 3.28 O[%/m] = 1 − 1 − × 100 100 187 (15.4) Suppose, however, that you want to define your own “smoke” and that you want to specify its production rate independently of the HRR (or even in lieu of an actual fire, like a smoldering source). You might also want to define a mass extinction coefficient for the smoke and an assumed molecular weight (as it will be tracked like a gas species). Finally, you also want to visualize the smoke using the SMOKE3D feature in Smokeview. Use the following lines: &SPEC ID='MY SMOKE', MW=29., MASS_EXTINCTION_COEFFICIENT=8700. / &SURF ID='SMOLDER', TMP_FRONT=1000., MASS_FLUX(1)=0.0001, SPEC_ID='MY SMOKE', COLOR='RED' / &VENT XB=0.6,1.0,0.3,0.7,0.0,0.0, SURF_ID='SMOLDER' / &PROP ID='Acme Smoke', QUANTITY='CHAMBER OBSCURATION', SPEC_ID='MY SMOKE' / &DEVC XYZ=1.00,0.50,0.95, PROP_ID='Acme Smoke', ID='smoke_1' / &DUMP SMOKE3D_QUANTITY='MASS FRACTION', SMOKE3D_SPEC_ID='MY SMOKE' / The same smoke detector model is used that was described above. Only now, the mass fraction of your species ’MY SMOKE’ is used in the algorithm, rather than that associated with the lumped species. Note that your species will not participate in the radiation calculation. It will merely serve as a surrogate for smoke. Note also that if you specify explicitly a smoke surrogate, you should set SOOT_YIELD=0 on the REAC line to prevent FDS from including smoke as a component of the combustion product lumped species. 15.3.6 Beam Detection Systems A beam detector can be defined by specifying the endpoints, (x1,y1,z1) and (x2,y2,z2), of the beam and the total percent obscuration at which the detector activates. The two endpoints must lie in the same mesh. FDS determines which mesh cells lie along the linear path defined by the two endpoints. The beam detector response is evaluated as !! Obscuration = N 1 − exp −Km ∑ ρs,i ∆xi × 100 % (15.5) i=1 where i is a mesh cell along the path of the beam, ρs,i is the soot density of the mesh cell, ∆xi is the distance within the mesh cell that is traversed by the beam, and Km is the mass extinction coefficient. The line in the input file has the form: &DEVC XB=x1,x2,y1,y2,z1,z2, QUANTITY='PATH OBSCURATION', ID='beam1', SETPOINT=33.0 / A similar QUANTITY is ’TRANSMISSION’ which is given by the following expression: ! L0 N Transmission = exp −Km ∑ ρs,i ∆xi × 100 %/m L i=1 (15.6) Note that the transmission is given in units of %/m rather than % like obscuration. L is the total path length of the beam, and L0 is the reference dimension of 1 m. Since a single linear path cannot span more than one mesh, having a beam detector that crosses multiple meshes will require post processing. Break the beam detector path into multiple DEVC lines, one for each mesh that the beam crosses. The total obscuration is given by " # N O = 1 − ∏ (1 − Oi /100) × 100 % i=1 188 (15.7) where Oi is the FDS output for the beam detector of the ith path (note that the bracketed term contains a product rather than a sum). Example Case: A Beam Detector A 10 m by 10 m by 4 m compartment is filled with smoke from burning propane, represented as 0.006 kg/kg of the lumped species variable, PRODUCTS. The soot yield is specified as 0.01 kg/kg, resulting in a uniform soot density of 71.9 mg/m3 . Using the default mass extinction coefficient of 8700 m2 /kg, the optical depth is calculated to be 0.626 1/m. The compartment has a series of obstructions located at increasing distance from the front in increments of 1 m. The correlation for the output quantity VISIBILITY, Eq. (16.6), produces a visibility distance of 4.8 m. When viewing the smoke levels with Smokeview, you should just barely see the fifth obstacle which is at a distance of 5 m from the front of the compartment. If this is the case, Smokeview is properly displaying the obscuration of the smoke. Three beam detectors are also placed in the compartment. These all have a path length of 10 m, but are at different orientations within the compartment. Using the optical depth of 0.626 1/m and the path length of 10 m, the expected total obscuration is 99.81 %, which is the result computed by FDS for each of the three detectors. Figure 15.3: Output of the beam_detector test case. 15.3.7 Aspiration Detection Systems An aspiration detection system groups together a series of smoke measurement devices. An aspiration system consists of a sampling pipe network that draws air from a series of locations to a central point where an obscuration measurement is made. To define such a system in FDS, you must provide the sampling locations, sampling flow rates, the transport time from each sampling location, and if an alarm output is desired, the overall obscuration “setpoint.” One or more DEVC inputs are used to specify details of the sampling locations, and one additional input is used to specify the central detector: &DEVC XYZ=..., QUANTITY='DENSITY', SPEC_ID='SOOT', ID='soot1', DEVC_ID='asp1', FLOWRATE=0.1, DELAY=20 / &DEVC XYZ=..., QUANTITY='DENSITY', SPEC_ID='SOOT', ID='soot2', DEVC_ID='asp1', FLOWRATE=0.2, DELAY=10 / ... &DEVC XYZ=..., QUANTITY='DENSITY', SPEC_ID='SOOT', ID='sootN', DEVC_ID='asp1', FLOWRATE=0.3, DELAY=30 / &DEVC XYZ=..., QUANTITY='ASPIRATION', ID='asp1', BYPASS_FLOWRATE=0.4, SETPOINT=0.02 / 189 where the DEVC_ID is used at each sampling point to reference the central detector, FLOWRATE is the gas flow rate in kg/s, DELAY is the transport time (in seconds) from the sampling location to the central detector, BYPASS_FLOWRATE is the flow rate in kg/s of any air drawn into the system from outside the computational domain (accounts for portions of the sampling network lying outside the domain defined by the MESH inputs), and SETPOINT is the alarm threshold obscuration in units of %/m. The output of the aspiration system is computed as ∑Ni=1 ρs,i (t − td,i ) ṁi Obscuration = 1 − exp −Km × 100 %/m (15.8) ∑Ni=1 ṁi where ṁi is the mass FLOWRATE at sampling location i, ρs,i (t −td,i ) is the soot density at sampling location i, td,i s prior (DELAY) to the current time t, and Km is the MASS_EXTINCTION_COEFFICIENT associated with visible light. Example Case: aspiration_detector A cubical compartment, 2 m on a side has a three sampling location aspiration system. The three locations have equal flow rates of 0.3 kg/s, and transport times of 50, 100, and 150 s, respectively. No bypass flow rate is specified for the aspiration detector. Combustion products are forced into the bottom of the compartment at a rate of 1 kg/s. The SOOT_YIELD=0.001. Mass is removed from the top of the compartment at a rate of 1 kg/s. The aspiration detector shows an increasing obscuration over time. There is a delay of slightly over 50 s in the initial increase which results from the 50 s transport time for the first sampling location plus a short period of time to transport the combustion products to the sampling location. The detector response has three plateaus that result from the delay times of the sampling locations. The sampling points are colocated, so each plateau represents an additional one third of the soot being transported to the detector. The soot density at the sampling point is 7.1 × 10−5 kg/m3 . Using this value the plateaus are computed as 18 %, 33.2 %, and 45.7 %, as seen in Fig. 15.4. FDS0−86−g80cff4e 100 Obscuration (aspiration_detector) Obscuration (%/m) 80 60 40 20 Ideal Value FDS (asp1) 0 0 50 100 Time (s) 150 200 Figure 15.4: Output of aspiration_detector test case. 190 15.4 Basic Control Logic Devices can be used to control various actions, like creating and removing obstructions, or activating and deactivating fans and vents. Every device has an associated QUANTITY, whether it is included directly on the DEVC line or indirectly on the optional PROP line. Using the DEVC parameter SETPOINT, you can trigger an action to occur when the QUANTITY value passes above, or below, the given SETPOINT. The following parameters dictate how a device will control something: SETPOINT The value of the device at which its state changes. For a detection type of device (e.g., heat or smoke) this value is taken from the device’s PROP inputs and need not be specified on the DEVC line. TRIP_DIRECTION A positive integer means the device will change from its INITIAL_STATE when the value of the device is greater than the SETPOINT and be equal to the INITIAL_STATE when the value is less than the SETPOINT. A negative integer has the opposite behavior. The device will change from its INITIAL_STATE when the value of the device is less than the SETPOINT and be equal to the INITIAL_STATE when the value is greater than the SETPOINT. The default value is +1. LATCH If this logical value is set to .TRUE. the device will only change state once. The default value is .TRUE.. INITIAL_STATE This logical value is the initial state of the device. The default value is .FALSE. For example, if an obstruction associated with the device is to disappear, set INITIAL_STATE=.TRUE. If you desire to control FDS using more complex logic than can be provided by the use of a single device and its setpoint, control functions can be specified using the CTRL input. See Section 15.5 for more on CTRL functions. The simplest example of a device is just a timer: &DEVC XYZ=1.2,3.4,5.6, ID='my clock', QUANTITY='TIME', SETPOINT=30. / Anything associated with the device via the parameter, DEVC_ID=’my clock’, will change its state at 30 s. For example, if the text were added to an OBST line, that obstruction would change from its INITIAL_STATE of .FALSE. to .TRUE. after 30 s. In other words, it would be created at 30 s instead of at the start of the simulation. This is a simple way to open a door or window. When using a DEVC output to control FDS, the instantaneous value of the DEVC is used. For some QUANTITY types, such as TEMPERATURE, this output can be very noisy. To prevent a spurious spike from causing a state change of the DEVC you can specify the parameter SMOOTHING_FACTOR. This is a parameter that can vary between 0 and 1. It performs an exponential smoothing of the DEVC output as follows: x̄n = x̄n−1 SMOOTHING_FACTOR + xn (1 − SMOOTHING_FACTOR) (15.9) where n is the time step, x is the instantaneous device output and x̄ is the smoothed output. The SMOOTHING_FACTOR defaults to 0 which means no smoothing is performed. Note that SMOOTHING_FACTOR only changes the value passed to control functions; it has no effect on the value of the DEVC written to the CHID_devc.csv file. 15.4.1 Creating and Removing Obstructions In many fire scenarios, the opening or closing of a door or window can lead to dramatic changes in the course of the fire. Sometimes these actions are taken intentionally, sometimes as a result of the fire. Within the framework of an FDS calculation, these actions are represented by the creation or removal of solid obstacles, or the opening or closing of exterior vents. 191 Remove or create a solid obstruction by assigning the character string DEVC_ID to indicate the name of a DEVC ID on the OBST line that is to be created or removed. This will direct FDS to remove or create the obstruction when the device changes state to .FALSE. or .TRUE., respectively. For example, the lines &OBST XB=..., DEVC_ID='det2' / &DEVC XYZ=..., ID='det2', INITIAL_STATE=.TRUE. / will cause the given obstruction to be removed when the specified DEVC changes state. Creation or removal at a predetermined time can be performed using a DEVC that has TIME as its measured quantity. For example, the following instructions will cause the specified HOLEs and OBSTstructions to appear/disappear at the various designated times. These lines are part of the simple test case called create_remove.fds. &OBST &HOLE &HOLE &OBST &OBST &OBST &HOLE &HOLE XB=0.3,0.4,0.1,0.9,0.1,0.9, XB=0.2,0.4,0.2,0.3,0.2,0.3, XB=0.2,0.4,0.7,0.8,0.7,0.8, XB=0.7,0.8,0.2,0.3,0.2,0.3, XB=0.7,0.8,0.6,0.7,0.6,0.7, XB=0.5,1.0,0.0,1.0,0.0,0.1, XB=0.7,0.8,0.7,0.8,0.0,0.1, XB=0.7,0.8,0.2,0.3,0.0,0.1, &DEVC &DEVC &DEVC &DEVC &DEVC &DEVC &DEVC XYZ=..., XYZ=..., XYZ=..., XYZ=..., XYZ=..., XYZ=..., XYZ=..., ID='timer1', ID='timer2', ID='timer3', ID='timer4', ID='timer5', ID='timer6', ID='timer7', COLOR='PURPLE' / COLOR='RED', COLOR='GREEN', COLOR='BLUE', COLOR='PINK', COLOR='YELLOW', COLOR='BLACK', COLOR='GRAY 50', SETPOINT=1., SETPOINT=2., SETPOINT=3., SETPOINT=4., SETPOINT=5., SETPOINT=6., SETPOINT=6., DEVC_ID='timer1' DEVC_ID='timer2' DEVC_ID='timer3' DEVC_ID='timer4' DEVC_ID='timer5' DEVC_ID='timer6' DEVC_ID='timer7' QUANTITY='TIME', QUANTITY='TIME', QUANTITY='TIME', QUANTITY='TIME', QUANTITY='TIME', QUANTITY='TIME', QUANTITY='TIME', / / / / / / / INITIAL_STATE=.FALSE. INITIAL_STATE=.TRUE. INITIAL_STATE=.FALSE. INITIAL_STATE=.TRUE. INITIAL_STATE=.FALSE. INITIAL_STATE=.TRUE. INITIAL_STATE=.FALSE. / / / / / / / At the start of the simulation, the purple obstruction is present with a red block embedded in it. This red block is actually a HOLE whose initial state is .FALSE., i.e., the hole is filled. Also at the start of the simulation, there is a pink obstruction that is visible. At 1 s the red block disappears. At 2 s the empty hole in the purple obstruction is filled with a green block. This hole was initially true, i.e. empty. The blue obstruction appears at 3 s because its initial state is false, meaning that it does not exist initially. The pink obstruction disappears at 4 s because its initial state is true and this state changes at 4 s. At 5 s a yellow obstruction appears with one empty hole and one embedded gray block. At 6 s the gray block disappears because it is a hole that was initially false and therefore was filled with the gray block when its parent obstruction (yellow) was created. Also at 6 s the hole originally present in the yellow obstruction is filled with a black block because it was a hole that was initially empty and then filled when its DEVC changed state. You should always try a simple example first before embarking on a complicated creation/removal scheme for obstructions and holes. To learn how to create and remove obstructions multiple times, see Section 15.5.5 for information about the custom control feature. 15.4.2 Activating and Deactivating Vents When a device or control function is applied to a VENT, the purpose is to either activate or deactivate any time ramp associated with the VENT via its DEVC_ID. For example, to control a fan, do the following: &SURF ID='FAN', VOLUME_FLOW=5. / &VENT XB=..., SURF_ID='FAN', DEVC_ID='det2' / 192 &DEVC ID='det2', XYZ=..., QUANTITY='TIME', SETPOINT=30., INITIAL_STATE=.FALSE. / Note that at 30 s, the “state” of the ’FAN’ changes from .FALSE. to .TRUE., or more simply, the ’FAN’ turns on. Since there is no explicit time function associated with the ’FAN’, the default 1 s ramp-up will begin at 30 s instead of at 0 s. If INITIAL_STATE=.TRUE., then the fan should turn off at 30 s. Essentially, “activation” of a VENT causes all associated time functions to be delayed until the device SETPOINT is reached. “Deactivation” of a VENT turns off all time functions. Usually this means that the parameters on the SURF line are all nullified, so it is a good idea to check the functionality with a simple example. A ’MIRROR’ or ’OPEN’ VENT should not be activated or deactivated. You can, however, place an obstruction in front of an ’OPEN’ VENT and then create it or remove it to model the closing or opening of a door or window. 15.5 Advanced Control Functions: The CTRL Namelist Group There are many systems whose functionality cannot be described by a simple device with a single “setpoint.” Consider for example, a typical HVAC system. It is controlled by a thermostat that is given a temperature setpoint. The system turns on when the temperature goes below the setpoint by some amount and then turns off when the temperature rises above that same setpoint by some amount. This behavior cannot be defined by merely specifying a single setpoint. You must also define the range or “deadband” around the setpoint, and whether an increasing or decreasing temperature activates the system. For the HVAC example, crossing the lower edge of the deadband activates heating; crossing the upper edge activates cooling. These more complicated behaviors can be modeled in FDS using CTRLs. The following parameters dictate how a control function will behave: ID A name for the control function that is unique over all control functions. FUNCTION_TYPE The type of control function. The possible types are shown in Table 15.2. INPUT_ID A list of DEVC or CTRL IDs that are the inputs to the control function. Up to forty inputs can be specified. If a DEVC or CTRL is being used as an INPUT_ID for a control function, then it must have a unique ID over both devices and control functions. Additionally, a control function cannot be used as an input for itself. SETPOINT The value of the control function at which its state changes. This is only appropriate for func- tions that return numerical values. TRIP_DIRECTION A positive integer means the control function will change state when its value increases past the setpoint and a negative integer means the control function will change state when its value decreases past the setpoint. The default value is +1. LATCH If this logical value is set to .TRUE. the control function will only change state once. The default value is .TRUE.. INITIAL_STATE This logical value is the initial state of the control function. The default value is .FALSE. For example, if an obstruction associated with the control function is to disappear, set INITIAL_STATE to .TRUE. For any object for which a DEVC_ID can be specified (such as OBST or VENT), a CTRL_ID can be specified instead. 193 Table 15.2: Control function types. FUNCTION_TYPE ANY ALL ONLY AT_LEAST TIME_DELAY CUSTOM DEADBAND KILL RESTART SUM SUBTRACT MULTIPLY DIVIDE POWER EXP LOG COS SIN ACOS ASIN PID Purpose Changes state if any INPUTs are .TRUE. Changes state if all INPUTs are .TRUE. Changes state if and only if N INPUTs are .TRUE. Changes state if at least N INPUTs are .TRUE. Changes state DELAY s after INPUT becomes .TRUE. Changes state based on evaluating a RAMP of the function’s input Behaves like a thermostat Terminates code execution if its sole INPUT is .TRUE. Dumps restart files if its sole INPUT is .TRUE. Sums the outputs of the INPUTs Subtracts the second INPUT from the first Multiplies the outputs of the INPUTs Divides the first INPUT by the second The first INPUT to the power of the second The exponential of the INPUT The natural logarithm of the INPUT The cosine of the INPUT The sine of the INPUT The arccosine of the INPUT The arcsine of the INPUT A Proportional-Integral-Derivative control function If you want to design a system of controls and devices that involves multiple changes of state, include the attribute LATCH=.FALSE. on the relevant DEVC or CTRL input lines. By default, devices and controls may only change state once, like a sprinkler activating or smoke detector alarming. LATCH is .TRUE. by default for both devices and controls. If you want a DEVC to operate based on the logical state of a CTRL, set QUANTITY equal to ’CONTROL’ and set the CTRL_ID on the DEVC input to the ID of the control function. The output value of numerical control function is defined by a DEVC line with QUANTITY set equal to ’CONTROL VALUE’ and CTRL_ID set equal to the ID of the control function. You can then use SETPOINT to have the DEVC operate a particular output value of the control function. 15.5.1 Control Functions: ANY, ALL, ONLY, and AT_LEAST Suppose you want an obstruction to be removed (a door is opened, for example) after any of four smoke detectors in a room has activated. Use input lines of the form: &OBST XB=..., SURF_ID='...', CTRL_ID='SD' / &DEVC &DEVC &DEVC &DEVC XYZ=1,1,3, XYZ=1,4,3, XYZ=4,1,3, XYZ=4,4,3, PROP_ID='Acme PROP_ID='Acme PROP_ID='Acme PROP_ID='Acme Smoker', Smoker', Smoker', Smoker', ID='SD_1' ID='SD_2' ID='SD_3' ID='SD_4' 194 / / / / &CTRL ID='SD', FUNCTION_TYPE='ANY', INPUT_ID='SD_1','SD_2','SD_3','SD_4', INITIAL_STATE=.TRUE. / The INITIAL_STATE of the control function SD is .TRUE., meaning that the obstruction exists initially. The “change of state” means that the obstruction is removed when any smoke detector alarms. By default, the INITIAL_STATE of the control function SD is .FALSE., meaning that the obstruction does not exist initially. Suppose that now you want the obstruction to be created (a door is closed, for example) after all four smoke detectors in a room have activated. Use a control line of the form: &CTRL ID='SD', FUNCTION_TYPE='ALL', INPUT_ID='SD_1','SD_2','SD_3','SD_4' / The control functions AT_LEAST and ONLY are generalizations of ANY and ALL. For example, &CTRL ID='SD', FUNCTION_TYPE='AT_LEAST', N=3, INPUT_ID='SD_1','SD_2','SD_3','SD_4' / changes the state from .FALSE. to .TRUE. when at least 3 detectors activate. Note that in this example, and the example below, the parameter N is used to specify the number of activated devices required for the conditions of the control function to be satisfied. The control function, &CTRL ID='SD', FUNCTION_TYPE='ONLY', N=3, INPUT_ID='SD_1','SD_2','SD_3','SD_4' / changes the state from .FALSE. to .TRUE. when 3, and only 3, detectors activate. 15.5.2 Control Function: TIME_DELAY The TIME_DELAY control function starts a timer of length DELAY when its input changes state. When the timer expires, the TIME_DELAY control function will change state. Note, that the timer starts at each change in state of the input; therefore, if the input changes state a second time before the first timer ends, the timer will get reset. This function enables FDS to model time delays between when a device activates and when some other action occurs, like in a dry pipe sprinkler system. &DEVC XYZ=2,2,3, PROP_ID='Acme Sprinkler_link', QUANTITY='LINK TEMPERATURE', ID='Spk_29_link' / &DEVC XYZ=2,2,3, PROP_ID='Acme Sprinkler', QUANTITY='CONTROL', ID='Spk_29', CTRL_ID='dry pipe' / &CTRL ID='dry pipe', FUNCTION_TYPE='TIME_DELAY', INPUT_ID='Spk_29_link', DELAY=30. / This relationship between a sprinkler and its pipes means that the sprinkler spray is controlled (in this case delayed) by the ’dry pipe’, which adds 30 s to the activation time of Spk_29, measured by Spk_29_link, before water can flow out of the head. 15.5.3 Control Function: DEADBAND This control function behaves like an HVAC thermostat. It can operate in one of two modes analogous to heating or cooling. The function is provided with an INPUT_ID which is the DEVC whose value is used by the function, an upper and lower SETPOINT, and the mode of operation by ON_BOUND. If ON_BOUND=’LOWER’, the function changes state from its INITIAL_STATE when the value of the INPUT_ID drops below the lower value in SETPOINT and reverts when it increases past the upper value, i.e., like a heating system. The reverse will occur if ON_BOUND=’UPPER’, i.e., a cooling system. For an HVAC system, the following lines of input would set up a simple thermostat: 195 &SURF &VENT &DEVC &CTRL ID='FAN', TMP_FRONT=40., VOLUME_FLOW=-1. / XB=-0.3,0.3,-0.3,0.3,0.0,0.0, SURF_ID='FAN', CTRL_ID='thermostat' / ID='TC', XYZ=2.4,5.7,3.6, QUANTITY='TEMPERATURE' / ID='thermostat', FUNCTION_TYPE='DEADBAND', INPUT_ID='TC', ON_BOUND='LOWER', SETPOINT=23.,27., LATCH=.FALSE./ Here, we want to control the VENT that simulates the FAN, which blows hot air into the room. A DEVC called TC is positioned in the room to measure the TEMPERATURE. The thermostat uses a SETPOINT to turn on the FAN when the temperature falls below 23 ◦ C (ON_BOUND=’LOWER’) and it turns off when the temperature rises above 27 ◦ C. Note that a deadband controller needs to have LATCH set to .FALSE. 15.5.4 Control Function: RESTART and KILL There are times when you might only want to run a simulation until some goal is reached, or you might want to create some baseline condition and then run multiple permutations of that baseline. For example, you might want to run a series of simulations where different mitigation strategies are tested once a detector alarms. Using the RESTART control function, you can cause a restart file to be created once a desired condition is met. The simulation can continue and the restart files can be copied to have the job identifying string, CHID, of the various permutations (providing of course that the usual restrictions on the use of restart files are followed). For example, the lines &DEVC ID='temp', QUANTITY='TEMPERATURE', SETPOINT=1000., XYZ=4.5,6.7,3.6 / &DEVC ID='velo', QUANTITY='VELOCITY', SETPOINT=10., XYZ=4.5,6.7,3.6 / &CTRL ID='kill', FUNCTION_TYPE='KILL', INPUT_ID='temp' / &CTRL ID='restart', FUNCTION_TYPE='RESTART', INPUT_ID='velo' / will kill the job and output restart files when the temperature at the given point rises above 1000 ◦ C; or just force restart files to be output when the velocity at a given point exceeds 10 m/s. 15.5.5 Control Function: CUSTOM For most of the control function types, the logical (true/false) output of the devices and control functions and the time they last changed state are taken as inputs. A CUSTOM function uses the numerical output of a DEVC along with a RAMP to determine the output of the function. When the RAMP output for the DEVC value is negative, the CTRL will have the value of its INITIAL_STATE. When the RAMP output for the DEVC value is positive, the CTRL will have the opposite value of its INITIAL_STATE. In the case below, the CUSTOM control function uses the numerical output of a timer device as its input. The function returns true (the default value for INITIAL_STATE is .FALSE.) when the F parameter in the ramp specified with RAMP_ID is a positive value and false when the RAMP F value is negative. In this case, the control would start false and would switch to true when the timer reaches 60 s. It would then stay in a true state until the timer reaches 120 s and would then change back to false. Note that when using control functions the IDs assigned to both the CTRL and the DEVC inputs must be unique across both sets of inputs, i.e., you cannot use the same ID for both a control function and a device. You can make a fan operate on a fixed cycle by using a CUSTOM control function based on time: &SURF &VENT &DEVC &CTRL ID='FAN', TMP_FRONT=40., VOLUME_FLOW=-1. / XB=-0.3,0.3,-0.3,0.3,0.0,0.0, SURF_ID='FAN', CTRL_ID='cycling timer' / ID='TIMER', XYZ=2.4,5.7,3.6, QUANTITY='TIME' / ID='cycling timer', FUNCTION_TYPE='CUSTOM', INPUT_ID='TIMER', RAMP_ID='cycle' / 196 &RAMP &RAMP &RAMP &RAMP ID='cycle', ID='cycle', ID='cycle', ID='cycle', T= 59, T= 61, T=119, T=121, F=-1 F= 1 F= 1 F=-1 / / / / In the above example the fan will be off initially, turn on at 60 s and then turn off at 120 s. You can make an obstruction appear and disappear multiple times by using the following lines &OBST &DEVC &CTRL &RAMP &RAMP &RAMP &RAMP &RAMP XB=..., SURF_ID='whatever', CTRL_ID='cycling timer' / ID='TIMER', XYZ=..., QUANTITY='TIME' / ID='cycling timer', FUNCTION_TYPE='CUSTOM', INPUT_ID='TIMER', RAMP_ID='cycle' / ID='cycle', T= 0, F=-1 / ID='cycle', T= 59, F=-1 / ID='cycle', T= 61, F= 1 / ID='cycle', T=119, F= 1 / ID='cycle', T=121, F=-1 / The above will have the obstacle initially removed, then added at 60 s, and removed again at 120 s. Experiment with these combinations using a simple case before trying a case to make sure that FDS indeed is doing what is intended. 15.5.6 Control Function: Math Operations The control functions that perform simple math operations (SUM, SUBTRACT, MULTIPLY, DIVIDE, and POWER) can have a constant value specified as one of their inputs. This is done by specifying one of the INPUT_IDs as ’CONSTANT’ and providing the value using the input CONSTANT. For example, the inputs below represent a control function whose state changes when the square of the velocity exceeds 10 (see Section 15.4 for an explanation of TRIP_DIRECTION). &DEVC ID='SPEED SENSOR', XYZ=..., QUANTITY='VELOCITY' / &CTRL ID='multiplier', FUNCTION_TYPE='POWER', INPUT_ID='SPEED SENSOR','CONSTANT', CONSTANT=2., SETPOINT=10., TRIP_DIRECTION=1 / 15.5.7 Control Function: PID Control Function A PID (Proportional Integral Derivative) control function is a commonly used feedback controller for controlling electrical and mechanical systems. The function computes an error between a process variable and a desired setpoint. The goal of the PID function is to minimize the error. A PID control function is computed as Z t de(t) u(t) = Kp e(t) + Ki e(t) dt + Kd (15.10) dt 0 where Kp , Ki , and Kd are respectively the PROPORTIONAL_GAIN, the INTEGRAL_GAIN, and the DIFFERENTIAL_GAIN; e(t) is the error given by subtracting the TARGET_VALUE from the input; and u(t) is the output. 15.5.8 Combining Control Functions: A Pre-Action Sprinkler System For a pre-action sprinkler system, the normally dry sprinkler pipes are flooded when a detection event occurs. For this example, the detection event is when two of four smoke detectors alarm. It takes 30 s to flood the piping network. The nozzle is a DEVC named ’NOZZLE 1’ controlled by the CTRL named 197 ’nozzle trigger’. The nozzle activates when both detection and the time delay have occurred. Note that the DEVC is specified with QUANTITY=’CONTROL’. &DEVC &DEVC &DEVC &DEVC &DEVC XYZ=1,1,3, XYZ=1,4,3, XYZ=4,1,3, XYZ=4,4,3, XYZ=2,2,3, ID='NOZZLE PROP_ID='Acme Smoker', ID='SD_1' / PROP_ID='Acme Smoker', ID='SD_2' / PROP_ID='Acme Smoker', ID='SD_3' / PROP_ID='Acme Smoker', ID='SD_4' / PROP_ID='Acme Nozzle', QUANTITY='CONTROL', 1', CTRL_ID='nozzle trigger' / &CTRL ID='nozzle trigger', FUNCTION_TYPE='ALL', INPUT_ID='smokey','delay' / &CTRL ID='delay', FUNCTION_TYPE='TIME_DELAY', INPUT_ID='smokey', DELAY=30. / &CTRL ID='smokey', FUNCTION_TYPE='AT_LEAST', N=2, INPUT_ID='SD_1','SD_2','SD_3','SD_4' / Example Case: control_test_2 The control_test_2 example demonstrates the use of the mathematical and PID control functions. Two compartments are defined with the left hand compartment initialized to 20 ◦ C and the right hand compartment to 10 ◦ C. Control functions are defined to: • Add the temperatures in the two compartments • Subtract the right hand compartment temperature from the left hand compartment temperature • Multiply the left hand temperature by 0.5 • Divide the left hand temperature by the right hand temperature • Take the square root of the right hand temperature • Use the time as input to a PID function with a target value of 5 and Kp =-0.5, Ki =0.001, and Kd =1 &CTRL ID='Add',FUNCTION_TYPE='SUM',INPUT_ID='LHS Temp','RHS Temp'/ &CTRL ID='Subtract',FUNCTION_TYPE='SUBTRACT',INPUT_ID='RHS Temp','LHS Temp'/ &CTRL ID='Multiply',FUNCTION_TYPE='MULTIPLY',INPUT_ID='LHS Temp','CONSTANT',CONSTANT=0.5/ &CTRL ID='Divide',FUNCTION_TYPE='DIVIDE',INPUT_ID='LHS Temp','RHS Temp'/ &CTRL ID='Power',FUNCTION_TYPE='POWER',INPUT_ID='RHS Temp','CONSTANT',CONSTANT=0.5/ &CTRL ID='PID',FUNCTION_TYPE='PID',INPUT_ID='Time',TARGET_VALUE=5., PROPORTIONAL_GAIN=-0.5,INTEGRAL_GAIN=0.001,DIFFERENTIAL_GAIN=1./ Results are shown in Fig. 15.5. 15.5.9 Combining Control Functions: A Dry Pipe Sprinkler System For a dry-pipe sprinkler system, the normally dry sprinkler pipes are pressurized with gas. When a link activates in a sprinkler head, the pressure drop allows water to flow into the pipe network. For this example it takes 30 s to flood the piping network once a sprinkler link has activated. The sequence of events required for operation is first ANY of the links must activate which starts the 30 s TIME_DELAY. Once the 30 s delay has occurred, each nozzle with an active link, the ALL control functions, will then flow water. 198 FDS0−86−g80cff4e 40 Control Function Output Control Function Outputs (control_test_2) 30 Add Multiply Subtract Divide Power PID FDS Add FDS Multiply FDS Subtract FDS Divide FDS Power FDS PID 20 10 0 −10 0 2 4 6 8 10 Time (s) Figure 15.5: Results of the control_test_2 case. &DEVC XYZ=2,2,3, PROP_ID='Acme Sprinkler Link', ID='LINK 1' / &DEVC XYZ=2,3,3, PROP_ID='Acme Sprinkler Link', ID='LINK 2' / &PROP ID='Acme Sprinkler Link', QUANTITY='LINK TEMPERATURE', ACTIVATION_TEMPERATURE=74., RTI=30./ &DEVC XYZ=2,2,3, ID='NOZZLE &DEVC XYZ=2,3,3, ID='NOZZLE &CTRL &CTRL &CTRL &CTRL PROP_ID='Acme Nozzle', QUANTITY='CONTROL', 1', CTRL_ID='nozzle 1 trigger' / PROP_ID='Acme Nozzle', QUANTITY='CONTROL', 2', CTRL_ID='nozzle 2 trigger' / ID='check links', FUNCTION_TYPE='ANY', INPUT_ID='LINK 1','LINK 2'/ ID='delay', FUNCTION_TYPE='TIME_DELAY', INPUT_ID='check links', DELAY=30. / ID='nozzle 1 trigger', FUNCTION_TYPE='ALL', INPUT_ID='delay','LINK 1'/ ID='nozzle 2 trigger', FUNCTION_TYPE='ALL', INPUT_ID='delay','LINK 2'/ 15.5.10 Example Case: activate_vents The simple test case called activate_vents demonstrates the several of the control functions. Figure 15.6 shows seven multiply-colored vents that activate at different times, depending on the particular timing or control function. Figure 15.6: Output of the activate_vents test case at 5, 10, and 15 s. 199 15.6 Controlling a RAMP 15.6.1 Changing the Independent variable For any user-defined RAMP, the normal independent variable, for example time for RAMP_V, can be replaced by the output of a DEVC. This is done by specifying the input DEVC_ID on one of the RAMP input lines. When this is done, the current output of the DEVC is used as the independent variable for the RAMP. A CTRL_ID can also be specified as long as the control function outputs a numerical value (i.e., is a mathematical function (Section 15.5.6) or a PID function (Section 15.5.7). In the following example a blower is ramped from 0 % flow at 20 ◦ C, to 50 % flow when the temperature exceeds 100 ◦ C, and to 100 % flow when the temperature exceeds 200 ◦ C. This is similar functionality to the CUSTOM control function, but it allows for variable response rather than just on or off. &SURF &DEVC &RAMP &RAMP &RAMP 15.6.2 ID='BLOWER', VEL=-2, RAMP_V='BLOWER RAMP' / XYZ=2,3,3, QUANTITY='TEMPERATURE', ID='TEMP DEVC' / ID='BLOWER RAMP', T= 20,F=0.0, DEVC_ID='TEMP DEVC' / ID='BLOWER RAMP', T=100,F=0.5 / ID='BLOWER RAMP', T=200,F=1.0 / Freezing the Output Value, Example Case: hrr_freeze There are occasions where you may want the value of a RAMP to stop updating. For example, if you are simulating a growing fire in a room with sprinklers, you may wish to stop the fire from growing when a sprinkler over the fire activates. This type of action can be accomplished by changing the input of the RAMP to a DEVC (see the previous section) and then giving that DEVC either a NO_UPDATE_DEVC_ID or a NO_UPDATE_CTRL_ID. When the specified controller changes its state to .TRUE. it will cause the DEVC to stop updating its value. Since the DEVC is being used as the independent variable to a RAMP, the RAMP will have its output remain the same. This is shown in the example below. A fire is given a linear RAMP from 0 to 1000 kW/m2 over 50 s. Rather than using the simulation time, the RAMP uses a DEVC for the time. The timer is set to freeze when another DEVC measuring time reaches 200 ◦ C. Figure 15.7 shows the result of these inputs in the test case hrr_freeze where it can be seen that the pyrolysis rate stops increasing once the gas temperature reaches 200 ◦ C. &SURF &RAMP &RAMP &DEVC ID='FIRE', HRRPUA=1000., RAMP_Q='FRAMP', COLOR='ORANGE'/ ID='FRAMP', T= 0, F=0, DEVC_ID='FREEZE TIME'/ ID='FRAMP', T=50, F=1/ XYZ=..., QUANTITY='TEMPERATURE', SETPOINT=200., INITIAL_STATE=.FALSE., ID='TEMP'/ &DEVC XYZ=..., QUANTITY='TIME', NO_UPDATE_DEVC_ID='TEMP', ID='FREEZE TIME'/ It should be noted that devices are updated sequentially in the order that they are listed in the input file and that devices in different meshes do not share values until the end of a time step. This means that if the device being frozen is on a different mesh or is listed before the device that freezes it, it will not be frozen until the next time step. 200 −3 FDS0−86−g80cff4e 300 1 Plume Temperature Burning Rate 0.8 Burning Rate (kg/s) Temperature (°C) 250 200 150 100 0.6 0.4 0.2 50 0 0 FDS0−86−g80cff4e x 10 5 10 Time (s) 15 0 0 20 5 10 Time (s) 15 20 Figure 15.7: Temperature (left) and burning rate (right) outputs of the hrr_freeze test case. 201 15.7 Visualizing FDS Devices in Smokeview This section provides an overview of various objects that can be drawn by Smokeview and how to customize their appearance. Further technical details may be found in the Smokeview User’s Guide [2]. 15.7.1 Devices that Indicate Activation Devices like sprinklers and smoke detectors can be drawn in one of two ways so as to indicate activation. When FDS determines that a device has activated it places a message in the .smv file indicating the object number, the activation time and the state (0 for inactive or 1 for active). Smokeview then draws the corresponding object. See Tables 15.3 and 15.4 for images. The character string, SMOKEVIEW_ID, on the PROP line associates an FDS device with a Smokeview object. For example, the following lines instruct Smokeview to draw the device in the shape of a ’target’: &PROP ID='my target', SMOKEVIEW_ID='target' / &DEVC XYZ=0.5,0.8,0.6, QUANTITY='TEMPERATURE', PROP_ID='my target' / Table 15.3: Single frame static objects SMOKEVIEW_ID Image sensor target 202 Table 15.4: Dual frame static objects SMOKEVIEW_ID Image inactive active heat_detector nozzle smoke_detector sprinkler_upright 203 Table 15.4: Dual frame static objects (continued) Image SMOKEVIEW_ID inactive active sprinkler_pendent 15.7.2 Devices with Variable Properties The appearance of Smokeview objects may be modified using data specified with the array of character strings called SMOKEVIEW_PARAMETERS on the PROP line. For example, the input lines &PROP ID='ballprops', SMOKEVIEW_ID='ball', SMOKEVIEW_PARAMETERS(1:6)='R=255','G=0','B=0','DX=0.5','DY=0.25','DZ=0.1' / &DEVC XYZ=0.5,0.8,1.5, QUANTITY='TEMPERATURE', PROP_ID='ballprops' / create an ellipsoid colored red with x, y, and z axis diameters of 0.5 m and 0.25 m and 0.1 m, respectively. Note that these parameters are enclosed within single quotes because they are character strings passed to Smokeview. Table 15.5 lists objects with variable properties. Note that the tsphere object uses a texture map or image to alter its appearance. The texture map is specified by placing the characters t% before the texture file name, for example, t%texturefile.jpg. Table 15.5: Dynamic Smokeview objects SMOKEVIEW_ID Image SMOKEVIEW_PARAMETERS SMOKEVIEW_PARAMETERS(1:6)= ’R=128’,’G=192’,’B=255’, ’DX=0.5’,’DY=.75’,’DZ=1.0’ ball R, G, B - color components (0 to 255) DX, DY, DZ - amount ball is stretched along x, y, z axis (m) cone SMOKEVIEW_PARAMETERS(1:5)= ’R=128’,’G=255’,’B=192’, ’D=0.4’,’H=0.6’ R, G, B - color components (0 to 255) D, H - diameter and height (m) 204 Table 15.5: Dynamic Smokeview objects (continued) SMOKEVIEW_ID Image SMOKEVIEW_PARAMETERS SMOKEVIEW_PARAMETERS(1:11)= ’HUB_R=0’,’HUB_G=0’,’HUB_B=0’, ’HUB_D=0.1’,’HUB_L=0.12’, ’BLADE_R=128’,’BLADE_G=64’, ’BLADE_B=32’,’BLADE_ANGLE=60.0’, ’BLADE_D=0.5’,’BLADE_H=0.09’ fan HUB_R, HUB_G, HUB_B - color components of fan hub (0 to 255) HUB_D, HUB_L - diameter and length of fan hub (m) BLADE_R, BLADE_G, BLADE_B - color components of fan blades (0 to 255) BLADE_ANGLE, BLADE_D, BLADE_H - angle, diameter and height of a fan blade SMOKEVIEW_PARAMETERS(1:9)= ’R=255’,’G=255’,’B=255’, ’AX0=0.0’,’ELEV0=90.0’, ’ROT0=0.0’,’ROTATION_RATE=10.0’, ’D=1.0’, ’tfile="t%sphere_cover_04.png"’ tsphere R, G, B - color components (0 to 255) AX0, ELEV0, ROT0 - initial azimuth, elevation and rotation angle (deg) ROTATION_RATE - rotation rate about z axis (deg/s) D - diameter (m) tfile - name of texture map file 205 Table 15.5: Dynamic Smokeview objects (continued) SMOKEVIEW_ID Image SMOKEVIEW_PARAMETERS SMOKEVIEW_PARAMETERS(1:6)= ’R=192’,’G=192’,’B=128’, ’W=0.5’,’H=1.0’, ’ROT=90.0’ vent inactive vent R, G, B - color components (0 to 255) W, H - width and height (m) ROT - rotation angle (deg) active vent 15.7.3 Objects that Represent Lagrangian Particles Lagrangian particles, like water droplets or small solid particles, are represented in Smokeview as tiny points. However, it is possible to draw Lagrangian particles in other ways, such as those depicted in Table 15.6. For example, the following lines define particles that represent segments of electrical cables that are 10 cm long with a diameter of 1.24 cm: &PART ID='cables', QUANTITIES(1)='PARTICLE TEMPERATURE', ..., PROP_ID='cable image' / &PROP ID='cable image', SMOKEVIEW_ID='tube', SMOKEVIEW_PARAMETERS='L=0.1','D=0.0124' / By default, the cables are colored black, but you can specify your own default color using the parameters R, G, and B. In addition, you can color the particles according to the listed QUANTITIES on the PART line. Menus in Smokeview allow you to toggle between the various color options. You can control the orientation of the ’tube’ objects using a parameter such as ’RANDXY=1’ that causes the cylinders to be drawn randomly in the x − y plane. Objects with the parameters U-VEL, V-VEL, and W-VEL stretch according to the respective velocity components associated with the moving particles. Table 15.6: Dynamic Smokeview objects for Lagrangian particles SMOKEVIEW_ID Image SMOKEVIEW_PARAMETERS SMOKEVIEW_PARAMETERS(1:6)= ’R=192’,’G=255’,’B=128’, ’DX=0.25’,’DY=.5’,’DZ=0.125’ box R, G, B - color components (0 to 255) DX, DY, DZ - amount box is stretched along axes 206 Table 15.6: Dynamic Smokeview objects for Lagrangian particles (continued) SMOKEVIEW_ID Image SMOKEVIEW_PARAMETERS SMOKEVIEW_PARAMETERS(1:6)= ’R=255’,’G=0’,’B=0’, ’D=0.2’,’L=0.6’,’RANDXY=1’ tube R, G, B - color components (0 to 255) D, L - diameter and length (m) RANDXY - randomly orient in x-y plane RANDXZ - randomly orient in x-z plane RANDYZ - randomly orient in y-z plane RANDXYZ - random orientation DIRX, DIRY, DIRZ - orient along axis SMOKEVIEW_PARAMETERS(1:9)= ’R=192’, ’G=64’, ’B=32’ ’U-VEL=1.’, ’V-VEL=1.’, ’W-VEL=1.’ ’VELMIN=0.01’, ’VELMAX=0.2’, ’D=1.0’ velegg R, G, B - color components (0 to 255) U-VEL, V-VEL, W-VEL - velocity components (m/s) VELMIN, VELMAX - minimum and maximum velocity D - diameter of egg at maximum velocity (m) SMOKEVIEW_PARAMETERS(1:9)= ’R=0’, ’G=0’, ’B=0’ ’U-VEL=1.’, ’V-VEL=1.’, ’W-VEL=1.’ ’VELMIN=0.01’, ’VELMAX=0.2’, ’D=0.1’ veltube R, G, B - color components (0 to 255) U-VEL, V-VEL, W-VEL - velocity components (m/s) VELMIN, VELMAX - minimum and maximum velocity D - diameter of tube at VELMAX (m) 207 208 Chapter 16 Output FDS has various types of output files that store computed data. Some of the files are in binary format and intended to be read and rendered by Smokeview. Some of the files are just comma-delimited text files. It is important to remember that you must explicitly declare in the input file most of the FDS output data. A considerable amount of the input file is usually devoted to this. To visualize the flow patterns better, save planar slices of data, either in the gas or solid phases, by using the SLCF (SLiCe File) or BNDF (BouNDary File) namelist group. Both of these output formats permit you to animate these quantities in time. For static pictures of the flow field, use the Plot3D output, a format that is used by many CFD programs as a simple way to store specified quantities over the entire mesh at one instant in time. Finally, tracer particles can be injected into the flow field from vents or obstacles, and then viewed in Smokeview. Use the PART namelist group to control the injection rate, sampling rate and other parameters associated with particles. 16.1 Output Control Parameters: The DUMP Namelist Group The namelist group DUMP contains parameters (Table 17.6) that control the rate at which output files are written, and various other global parameters associated with output files. Its parameters include: NFRAMES Number of output dumps per calculation. The default is 1000. Device data, slice data, parti- cle data, isosurface data, 3D smoke data, boundary data, solid phase profile data, and control function data are saved every (T_END-T_BEGIN)/NFRAMES seconds unless otherwise specified using DT_DEVC, DT_SLCF, DT_PART, DT_ISOF, DT_BNDF, DT_PROF, or DT_CTRL. Note that DT_SLCF controls Smoke3D output. DT_HRR controls the output of heat release rate and associated quantities. MASS_FILE If .TRUE., produce an output file listing the total masses of all gas species as a function of time. It is .FALSE. by default because the calculation of all gas species in all mesh cells is timeconsuming. The parameter DT_MASS controls the frequency of output. MAXIMUM_PARTICLES Maximum number of Lagrangian particles that can be included on any mesh at any given time. (Default 1000000) SMOKE3D If .FALSE., do not produce an animation of the smoke and fire. It is .TRUE. by default. DT_PL3D The time between Plot3D file output. Note that versions of FDS before 6 output Plot3D files by default. Now, you must specify the interval of output using this parameter. Its default value is 1000000 s, meaning that there is no Plot3D output unless specified. 209 FLUSH_FILE_BUFFERS FDS purges the output file buffers periodically and forces the data to be written out into the respective output files. It does this to make it easier to view the case in Smokeview while it is running. It has been noticed on Windows machines that occasionally a runtime error occurs because of file access problems related to the buffer flushing. If this happens, set this parameter to .FALSE., but be aware that it may not be possible to look at output in Smokeview until after the calculation is finished. You may also set DT_FLUSH to control the frequency of the file flushing. Its default value is the duration of the simulation divided by NFRAMES. STATUS_FILES If .TRUE., produces an output file CHID.notready which is deleted, if the simulation is completed successfully. This file can be used as an error indicator. It is .FALSE. by default. 16.2 Device Output: The DEVC Namelist Group Every device DEVC contains a QUANTITY that it monitors. Usually this QUANTITY is written out to a commadelimited spreadsheet file with the suffix _devc.csv. The quantities are listed in Table 16.3. There are two types of DEVC output. The first is a time history of the given QUANTITY over the course of the simulation. The second is a time-averaged profile consisting of a linear array of point devices. Each is explained below. 16.2.1 Single Point Output If you just want to record the time history of the temperature at a given point, add the line: &DEVC XYZ=6.7,2.9,2.1, QUANTITY='TEMPERATURE', ID='T-1' / and a column will be added to the output file CHID_devc.csv under the label ’T-1’. In this case, the ID has no other role than as a column label in the output file. FDS reports the value of the QUANTITY in the cell where the point XYZ is located. Devices on Solid Surfaces When prescribing a solid phase quantity, be sure to position the device at a solid surface. It is not always obvious where the solid surface is since the mesh does not always align with the input obstruction locations. To help locate the appropriate surface, the parameter IOR must be included when designating a solid phase quantity, except when using the STATISTICS feature described in Section 16.10.10 in which case the output quantity is not associated with just a single point on the surface. If the orientation of the solid surface is in the positive x direction, set IOR=1. If it is in the negative x direction, set IOR=-1, and so for the y and z directions. For example, the line &DEVC XYZ=0.7,0.9,2.1, QUANTITY='WALL TEMPERATURE', IOR=-2, ID='...' / designates the surface temperature of a wall facing the negative y direction. There are still instances where FDS cannot determine which solid surface is being designated, in which case an error message appears in the diagnostic output file. Re-position the device and try again. It is best to position the device, via the real triplet XYZ, such that the device location is either at or within a cell width outside of the solid surface. The search algorithm in FDS will look for the nearest solid surface in the direction opposite to that indicated by IOR. 210 Integrated Quantities In addition to point measurements, the DEVC group can be used to report integrated quantities (See Table 16.3). For example, you may want to know the mass flow out of a door or window. To report this, add the line &DEVC XB=0.3,0.5,2.1,2.5,3.0,3.0, QUANTITY='MASS FLOW', ID='whatever' / Note that in this case, a plane is specified rather than a point. The sextuplet XB is used for this purpose. Notice when a flow is desired, two of the six coordinates need to be the same. Another QUANTITY, HRR, can be used to compute the total heat release rate within a subset of the domain. In this case, the sextuplet XB ought to define a volume rather than a plane. Specification of the plane or volume over which the integration is to take place can only be done using XB – avoid planes or volumes that cross multiple mesh boundaries. FDS has to decide which mesh to use in the integration, and it chooses the finest mesh overlapping the centroid of the designated plane or volume. 16.2.2 Linear Array of Point Devices You can use a single DEVC line to specify a linear array of devices. By adding the parameter POINTS and using the sextuple coordinate array XB, you can direct FDS to create a line of devices from (x1 , y1 , z1 ) to (x2 , y2 , z2 ). There are two options. Steady-State Profile Sometimes it is convenient to calculate a steady-state profile. For example, the vertical velocity profile along the centerline of a doorway can be recorded with the following line of input: &DEVC XB=X1,X2,Y1,Y2,Z1,Z2, QUANTITY='U-VELOCITY', ID='vel', POINTS=20 / In a file called CHID_line.csv, there will be between 1 and 4 columns of data associated with this single DEVC line. If X1 is different than X2, there will be a column of x coordinates associated with the linear array of points. The same holds for the y and z coordinates. The last column contains the 20 temperature points averaged over the last DT_DEVC_LINE of the simulation—DT_DEVC_LINE is set on the DUMP line. It is half the total simulation time by default. This is a convenient way to output a time-averaged linear profile of a quantity, like an array of thermocouples. Note that the statistics output to the _line.csv file start being averaged at T=T_END-DT_DEVC_LINE. Prior to this point in the simulation, the raw values are output. This prevents initial transients from biasing the stat values and forces the “line” file output at the end of the simulation to be equivalent to manually processing a point DEVC time history over the last DT_DEVC_LINE of the simulation. A single “line” file can hold more than a single line of data. By default, the coordinate columns are labeled using the ID of the DEVC appended with either -x, -y, or -z. To change these labels, use X_ID, Y_ID, and/or Z_ID. To suppress the coordinate columns altogether, add HIDE_COORDINATES=.TRUE. to the DEVC line. This is convenient if you have multiple arrays of data that p use the same coordinates. If you want the data plotted as a function of the distance from the origin, r = x2 + y2 + z2 , provide the label R_ID. Time-Varying Profile If you do not want a steady-state profile, but rather you just want to specify an array of evenly spaced devices, you can use a similar input line, except with the additional attribute TIME_HISTORY. 211 &DEVC XB=X1,X2,Y1,Y2,Z1,Z2, QUANTITY='U-VELOCITY', ID='vel', POINTS=20, TIME_HISTORY=.TRUE. / This directs FDS to just add 20 devices to the on-going list, saving you from having to write 20 DEVC lines. The ID for each device will be ’vel-01’, ’vel-02’, etc. Single-Point Statistics Mean By default, a line of devices records a mean or time-averaged profile of a particular quantity, φ= ∑ni=1 φi n (16.1) where n is the number of uniform output samples. Min, Max For line devices it is also possible to record the min or the max values of the output quantity for each point on the line. Set STATISTICS=’TIME MIN’ or STATISTICS=’TIME MAX’. FDS will output the values over the last DT_DEVC_LINE of the simulation. 16.2.3 Quantities at Certain Depth To record the temperature inside the surface, you can use a device as follows: &DEVC XYZ=..., QUANTITY='INSIDE WALL TEMPERATURE', DEPTH=0.005, ID='Temp_1', IOR=3 / The parameter DEPTH (m) indicates the distance inside the solid surface. If DEPTH is positive the distance is measured from the front surface. If negative, it is measured from the back surface. Note that if the wall thickness is decreasing over time due to the solid phase reactions, and the distance is measured from the current front surface, the measurement point will be moving towards the back side of the solid. Eventually, the measurement point may emerge from the solid, in which case it starts to show ambient temperature. Measuring the distance from the back surface can then be better suited for the purpose. To record the material component’s density with time, use the output quantity ’SOLID DENSITY’ in the following way: &DEVC ID='...', XYZ=..., IOR=3, QUANTITY='SOLID DENSITY', MATL_ID='wood', DEPTH=0.001 / This produces a time history of the density of the material referred to as ’wood’ on a MATL line. The density is recorded 1 mm beneath the surface which is oriented in the positive z direction. Note that if ’wood’ is part of a mixture, the density represents the mass of ’wood’ per unit volume of the mixture. To record the solid conductivity, use QUANTITY=’SOLID CONDUCTIVITY’. To record the solid specific heat, use QUANTITY=’SOLID SPECIFIC HEAT’. These quantities do not need the MATL_ID keyword. Note that these quantities are allowed only as a DEVC, not a BNDF, output. 16.2.4 Back Surface Temperature If you just want to know the temperature of the back surface of the “wall,” then use &DEVC XYZ=..., QUANTITY='BACK WALL TEMPERATURE', ID='Temp_b', IOR=3 / 212 Note that this quantity is only meaningful if the front or exposed surface of the “wall” has the attribute BACKING=’EXPOSED’ on the SURF line that defines it. The coordinates, XYZ, and orientation, IOR, refer to the front surface. To check that the heat conduction calculation is being done properly, you can add the additional line &DEVC XYZ=..., QUANTITY='WALL TEMPERATURE', ID='Temp_f', IOR=-3 / where now XYZ and IOR refer to the coordinates and orientation of the back side of the wall. These two wall temperatures ought to be the same. Remember that the “wall” in this case can only be at most one mesh cell thick, and its THICKNESS need not be the same as the mesh cell width. Rather, the THICKNESS ought to be the actual thickness of the “wall” through which FDS performs a 1-D heat conduction calculation. 16.3 Profiles of Quantities: The PROF Namelist Group FDS uses a fine one-dimensional mesh at each boundary cell to compute heat transfer within a solid. Use the PROF output to record the properties of the solid over the entire thickness. The parameters (Table 17.19) to specify a given PROFile are similar to those used to specify a surface quantity in the DEVC group. XYZ designates the triplet of coordinates, QUANTITY is the physical quantity to monitor, IOR the orientation, and ID an identifying character string. Here is an example of how you would use this feature to get a time history of temperature profiles within a given solid obstruction: &PROF XYZ=..., QUANTITY='TEMPERATURE', ID='T-1', IOR=3 / Other possible quantities are the total density of the wall (QUANTITY = ’DENSITY’) or densities of solid material components (QUANTITY = ’[MATL_ID]’), where MATL_ID is the name of the material. Each PROF line creates a separate file. The format of the file produced by each PROF line includes the node coordinates and specified quantity every DT_PROF s. However, if you specify FORMAT_INDEX=2 on the PROF line, the resulting file will contain columns containing only the final set of node coordinates and quantity values. This is handy for displaying a steady-state temperature profile. 16.4 Animated Planar Slices: The SLCF Namelist Group The SLCF (“slice file”) namelist group parameters (Table 17.24) allows you to record various gas phase quantities at more than a single point. A “slice” refers to a subset of the whole domain. It can be a line, plane, or volume, depending on the values of XB. The sextuplet XB indicates the boundaries of the “slice” plane. XB is prescribed as in the OBST or VENT groups, with the possibility that 0, 2, or 4 out of the 6 values be the same to indicate a volume, plane or line, respectively. A handy trick is to specify, for example, PBY=5.3 instead of XB if it is desired that the entire plane y = 5.3 slicing through the domain be saved. PBX and PBZ control planes perpendicular to the x and z axes, respectively. By default, 1-D and 2-D slice files are saved NFRAMES times per simulation. You can control the frequency of output with DT_SLCF on the DUMP line. If the “slice” is a 3-D volume, then its output frequency is controlled by the parameter DT_SL3D. By default, FDS sets DT_SL3D=(T_END-T_BEGIN)/5. You may specify a different value of DT_SL3D on DUMP. Note that 3-D slice files can become extremely large if DT_SL3D is small. Animated vectors can be created in Smokeview if a given SLCF line has the attribute VECTOR=.TRUE. If two SLCF entries are in the same plane, then only one of the lines needs to have VECTOR=.TRUE. Otherwise, a redundant set of velocity component slices will be created. 213 Normally, FDS averages slice file data at cell corners. For example, gas temperatures are computed at cell centers, but they are linearly interpolated to cell corners and output to a file that is read by Smokeview. To prevent this from happening, set CELL_CENTERED=.TRUE. This forces FDS to output the actual cellcentered data with no averaging. Note that this feature is mainly useful for diagnostics because it enables you to visualize the values that FDS actually computes. Note also that this feature should only be used for scalar quantities that are computed at cell centers, like temperatures, mass fractions, etc. Slice file information is recorded in files (See Section 20.7) labeled CHID_n.sf, where n is the index of the slice file. A short Fortran program fds2ascii.f90 produces a text file from a line, plane or volume of data. See Section 16.11 for more details. By default, Smokeview will blank slice file data inside obstructions. However, this is expensive to load at startup in Smokeview for large cases. If you wish Smokeview not to store this blanking array, set IBLANK_SMV=.FALSE. on MISC. Another option is to run Smokeview from the command line and to add -noblank as an option. 16.5 Animated Boundary Quantities: The BNDF Namelist Group The BNDF (“boundary file”) namelist group parameters allows you to record surface quantities at all solid obstructions. As with the SLCF group, each quantity is prescribed with a separate BNDF line, and the output files are of the form CHID_n.bf. No physical coordinates need be specified, however, just QUANTITY. See Table 16.3. For certain output quantities, additional parameters need to be specified via the PROP namelist group. In such cases, add the character string, PROP_ID, to the BNDF line to tell FDS where to find the necessary extra information. Note that BNDF files (Section 20.9) can become very large, so be careful in prescribing the time interval, DT_BNDF on the DUMP line. One way to reduce the size of the output file is to turn off the drawing of boundary information on desired obstructions. On any given OBST line, if the string BNDF_OBST=.FALSE. is included, the obstruction is not colored. To turn off all boundary drawing, set BNDF_DEFAULT=.FALSE. on the MISC line. Then individual obstructions can be turned back on with BNDF_OBST=.TRUE. on the appropriate OBST line. Individual faces of a given obstruction can be controlled via BNDF_FACE(IOR), where IOR is the index of orientation (+1 for the positive x direction, -1 for negative, and so on). Normally, FDS averages boundary file data at cell corners. For example, surface temperatures are computed at the center of each surface cell, but they are linearly interpolated to cell corners and output to a file that is read by Smokeview. To prevent this from happening, set CELL_CENTERED=.TRUE. on the BNDF line. This forces FDS to output the actual cell-centered data with no averaging. Note that this feature is mainly useful for diagnostics. Sometimes it is useful to render the QUANTITY integrated over time. For example, a heat flux in units of kW/m2 can be integrated in time producing the total energy absorbed by the surface in units of kJ/m2 . To do this, set STATISTICS equal to ’TIME INTEGRAL’ on the BNDF line. Note that there are no other options for STATISTICS on a BNDF line. 16.6 Animated Isosurfaces: The ISOF Namelist Group The ISOF (“ISOsurface File”) namelist group creates three-dimensional animated contours of gas phase scalar quantities. For example, a 300 ◦ C temperature isosurface is a 3-D surface on which the gas temperature is 300 ◦ C. Three different values of the temperature can be saved via the line: &ISOF QUANTITY='TEMPERATURE', VALUE(1)=50., VALUE(2)=200., VALUE(3)=500. / 214 where the values are in ◦ C. Note that the isosurface output files CHID_n.iso can become very large, so experiment with different sampling rates (DT_ISOF on the DUMP line). Any gas phase quantity can be animated via iso-surfaces, but use caution. To render an iso-surface, the desired quantity must be computed in every mesh cell at every output time step. For quantities like ’TEMPERATURE’, this is not a problem, as FDS computes it and saves it anyway. However, species volume fractions demand substantial amounts of time to compute at each mesh cell. Remember to include the SPEC_ID corresponding to the given QUANTITY if necessary. 16.7 Plot3D Static Data Dumps Data stored in Plot3D [38] files use a format developed by NASA that is used by many CFD programs for representing simulation results. See Section 20.8 for a description of the file structure. Plot3D data is visualized in three ways: as 2-D contours, vector plots and iso-surfaces. Vector plots may be viewed if one or more of the u, v and w velocity components are stored in the Plot3D file. The vector length and direction show the direction and relative speed of the fluid flow. The vector colors show a scalar fluid quantity such as temperature. Five quantities are written out to a file at one instant in time. The default specification is: &DUMP ..., PLOT3D_QUANTITY(1:5)='TEMPERATURE', 'U-VELOCITY','V-VELOCITY','W-VELOCITY','HRRPUV' / It’s best to leave the velocity components as is, because Smokeview uses them to draw velocity vectors. If any of the specified quantities require the additional specification of a particular species, use PLOT3D_SPEC_ID(n) to provide the SPEC_ID for PLOT3D_QUANTITY(n). Plot3D data are stored in files with extension .q . There is an optional file that can be output with coordinate information if another visualization package is being used to render the files. If you write WRITE_XYZ=.TRUE. on the DUMP line, a file with suffix .xyz is written out. Smokeview does not require this file because the coordinate information can be obtained elsewhere. Past versions of FDS (1-5) output Plot3D files by default. Now, you must specify the time interval between dumps using DT_PL3D on the DUMP line. 16.8 SMOKE3D: Realistic Smoke and Fire When you do a fire simulation, FDS automatically creates two output files that are rendered by Smokeview as realistic looking smoke and fire. By default, the output quantities are the ’MASS FRACTION’ of ’SOOT’ and ’HRRPUV’ (Heat Release Rate Per Unit Volume). You have the option of rendering any other species mass fraction instead of ’SOOT’, so long as the MASS_EXTINCTION_COEFFICIENT on the SPEC line is appropriate in describing the attenuation of visible light by the specified gas species. An alternative to SOOT mass fraction can be specified via SMOKE3D_QUANTITY on the DUMP line. If the specified quantity requires the additional specification of a particular species, use SMOKE3D_SPEC_ID to provide the SPEC_ID. Here is an example of how to change the smoke species. Normally, you do not need to do this as the “smoke” is an assumed part of the default combustion model when a non-zero SOOT_YIELD is defined on the REAC line. &SPEC ID='MY SMOKE', MW=29., MASS_EXTINCTION_COEFFICIENT=8700. / &DUMP SMOKE3D_QUANTITY='MASS FRACTION', SMOKE3D_SPEC_ID='MY SMOKE' / The MASS_EXTINCTION_COEFFICIENT is passed to Smokeview to be used for visualization. 215 16.9 Particle Output Quantities This section discusses output options for Lagrangian particles. 16.9.1 Liquid Droplets that are Attached to Solid Surfaces Liquid droplets (as opposed to solid particles) “stick” to solid surfaces unless directed otherwise. There are various quantities that describe these populations. For example, ’MPUA’ is the Mass Per Unit Area of the droplets defined by PART_ID. Likewise, ’AMPUA’ is the Accumulated Mass Per Unit Area. Both of these are given in units of kg/m2 . Think of these outputs as measures of the instantaneous mass density per unit area, and the accumulated total, respectively. These quantities are not identical measures. The quantity ’AMPUA’ is analogous to a “bucket test,” where the droplets are collected in buckets and the total mass determined at the end of a given time period. In this case each grid cell on the floor is considered its own bucket. Each droplet is counted only once when it reaches the floor1 . MPUA counts a droplet whenever it is on any solid surface, including the walls. If the droplet moves from one solid wall cell to another, it will be counted again. The cooling of a solid surface by droplets of a given type is given by ’CPUA’, the Cooling Per Unit Area in units of kW/m2 . Since a typical sprinkler simulation only tracks a small fraction of the droplets emitted from a sprinkler, both MPUA and CPUA also perform an exponential smoothing. This avoids having spotted distributions on surfaces due to the infrequent arrival of droplets that likely have a high weighting factor. Each of the output quantities mentioned above has a variant in which the quantity is summed by species rather than particle type. For example, the quantity ’AMPUA_Z’ along with a specified SPEC_ID rather than a PART_ID will sum the given output quantity over all particle classes with the given SPEC_ID. As an example of how to use these kinds of output quantities, the test case bucket_test_1 describes a single sprinkler mounted 10 cm below a 5 m ceiling. Water flows for 30 s at a constant rate of 180 L/min (ramped up and down in 1 s). The simulation continues for another 10 s to allow water drops time to reach the floor. The total mass of water discharged is 180 L kg 1 min ×1 × × 30 s = 90 kg min L 60 s (16.2) In the simulation, the quantity ’AMPUA’ with STATISTICS=’SURFACE INTEGRAL’ is specified on the DEVC line. This results in FDS summing ’AMPUA’ over each grid cell in the volume defined by XB, in this case the entire floor, analogous to if there were an single bucket present that was the same size as the area specified with XB. Summing the values of ’AMPUA’ over the entire floor yields a total of 90 kg (Fig. 16.1). Note that there really is no need to time-average the results. The quantity is inherently accumulating. 16.9.2 Solid Particles on Solid Surfaces If you want to monitor the accumulation of solid particles that have fallen on a solid surface, use a device as follows: &DEVC ID=..., XB=..., QUANTITY='MPUV', PART_ID='rods', STATISTICS='VOLUME INTEGRAL' / The volume over which to integrate, XB, should be at least one grid cell thick above the surface. 1 Be aware of the fact that the default behavior for liquid droplets hitting the “floor,” that is, the plane z = ZMIN, is to disappear (POROUS_FLOOR=.TRUE. on the MISC line). In this case, ’MPUA’ will be zero, but ’AMPUA’ will not. FDS stores the droplet mass just before removing the droplet from the simulation for the purpose of saving CPU time. 216 FDS0−86−g80cff4e 100 Accumulated Mass (bucket_test_1) Mass (kg) 80 60 40 20 Ideal FDS 0 0 10 20 Time (s) 30 40 Figure 16.1: Accumulated water collected at the floor in the bucket_test_1 case. 16.9.3 Droplet and Particle Densities and Fluxes in the Gas Phase Away from solid surfaces, ’MPUV’ is the Mass Per Unit Volume of particles or droplets of type (PART_ID) in units of kg/m3 . ’MPUV_Z’ provides the same information integrated over all droplets of a single species, (SPEC_ID). The quantities ’PARTICLE FLUX X’, ’PARTICLE FLUX Y’, and ’PARTICLE FLUX Z’ produce slice and Plot3D colored contours of the mass flux of particles in the x, y, and z directions, respectively, in units of kg/m2 /s. You can also apply these quantities to a device. For example, in the case called bucket_test_4.fds, the input line &DEVC XB=..., ID='flux', QUANTITY='PARTICLE FLUX Z', STATISTICS='AREA INTEGRAL' / records the integrated mass flux of all particles passing through the given horizontal plane. Figure 16.2 presents the results of this simple test case in which water spraying at a rate of 0.0005 kg/s for 55 s passes through a measurement plane and onto the floor. The total water accumulated is 0.0275 kg. −4 2 FDS0−86−g80cff4e x 10 FDS0−86−g80cff4e 0.03 Particle Mass Flux (bucket_test_4) Accumulated Mass (bucket_test_4) 0.025 0.02 Mass (kg) Mass Flux (kg/s) 0 −2 −4 0.015 0.01 −6 0.005 Ideal FDS −8 0 10 20 30 Time (s) 40 50 Ideal FDS 60 0 0 10 20 30 Time (s) 40 50 60 Figure 16.2: Mass flux and accumulated water collected at the floor in the bucket_test_4 case. 217 16.9.4 Coloring Particles and Droplets in Smokeview The parameter QUANTITIES on the PART line is an array of character strings indicating which scalar quantities should be used to color particles and droplets in Smokeview. The choices are ’PARTICLE TEMPERATURE’ (◦ C) ’PARTICLE DIAMETER’ (µm) ’PARTICLE VELOCITY’ (m/s) ’PARTICLE MASS’ (kg) ’PARTICLE AGE’ (s) By default, if no QUANTITIES are specified and none are selected in Smokeview, then Smokeview will display particles with a single color. To select this color specify either RGB or COLOR. By default, water droplets are colored blue. All others are colored black. For solid particles with a specified SURF_ID, you may specify any of the solid phase output quantities listed in Table 16.3. If the specified quantity is associated with a species, use the parameter QUANTITIES_SPEC_ID(N) to specify the species. Here N refers to the order of the specified output quantities on the PART line. 16.9.5 Detailed Properties of Solid Particles You may output properties of a single solid particle using a DEVC (device) line. For example, the lines: &INIT ID='my particle', PART_ID='...', XB=..., N_PARTICLES=1 / &DEVC ID='...', INIT_ID='my particle', QUANTITY='WALL TEMPERATURE' / output the surface temperature of a single particle that has been introduced into the simulation via an INIT line. If the particle is split (by means of multiple ORIENTATION vectors on the PART line), you can specify which of the particle fragments you want using the parameter ORIENTATION_NUMBER on the DEVC line. 16.10 Special Output Quantities This section lists a variety of output quantities that are useful for studying thermally-driven flows, combustion, pyrolysis, and so forth. Note that some of the output quantities can be produced in a variety of ways. 16.10.1 Heat Release Rate Quantities associated with the overall energy budget are reported in the comma delimited file CHID_hrr.csv. This file is automatically generated; the only input parameter associated with it is DT_HRR on the DUMP line. The columns in this file record the time history of the integrals of the terms in the enthalpy transport equation. The columns are defined as follows: Z Z Z Z ∂ ρhs dV = ṁb hs,b − ρuhs · dS + q̇b,w + k∇T · dS + ∑ hs,α ρDα ∇Yα · dS |∂t {z } | {z } | {z } |α {z } Q_ENTH Q_CONV Z Q_COND Z Q_DIFF dp + q̇b,r − q̇00r · dS + q̇000 dV + dV + (−q̇b,c − q̇b,r − q̇b,w ) {z } | {z } | {z } | dt {z } | Q_RADI Z HRR Q_PRES (16.3) Q_PART An additional column, Q_TOTAL, includes the sum of the terms on the right hand side of the equation. Ideally, this sum should equal the term on the left, Q_ENTH. All terms are reported in units of kW. Note that 218 the terms that make up Q_PART are summed over the Lagrangian particles. They represent the heat absorbed by the particles via convection, radiation, and conduction from the wall. The other columns in the file contain the total burning rate of fuel, in units of kg/s, and the zone pressures. Note that the reported value of the burning rate is not adjusted to account for the possibility that each individual material might have a different heat of combustion. For this reason, it is not always the case that the reported total burning rate multiplied by the gas phase heat of combustion is equal to the reported heat release rate. Note that the volume integrations in Eq. (16.3) are performed over the entire domain. The differential, dV , is the product of the local grid cell dimensions, dx dy dz. For the special case of two-dimensional cylindrical coordinates, dV = r dr dθ dz, where r = x, dr = dx, and dθ = dy. As a test of the energy balance, a sample case called Pressure_Solver/hallways.fds simulates a fire near the end of five connected hallways. The other end of the hallways is open. As is seen in Fig. 16.3, the quantities Q_ENTH and Q_TOTAL are very closely matched, indicating that the sources of energy loss and gain are properly added and subtracted from the energy equation. As expected, the net energy gain/loss eventually goes to zero as the compartment reaches a quasi-steady state. Rate of Energy Change (kW) FDS0−86−g80cff4e 150 Energy Budget (hallways) 100 Q_ENTH Q_TOTAL 50 0 0 10 20 30 Time (s) 40 50 60 Figure 16.3: The energy budget for the hallways test case. 16.10.2 Visibility and Obscuration If you are performing a fire calculation using the simple chemistry approach, the smoke is tracked along with all other major products of combustion. The most useful quantity for assessing visibility in a space is the light extinction coefficient, K [39]. The intensity of monochromatic light passing a distance L through smoke is attenuated according to I/I0 = e−KL (16.4) The light extinction coefficient, K, is a product of the density of smoke particulate, ρYS , and a mass specific extinction coefficient that is fuel dependent K = Km ρ YS (16.5) Devices that output a % obscuration such as a DEVC with a QUANTITY of ’ASPIRATION’, ’CHAMBER OBSCURATION’ (smoke detector), or ’PATH OBSCURATION’ (beam detector) are discussed respectively in Section 15.3.7, Section 15.3.5, and Section 15.3.6. Estimates of visibility through smoke can be made by using the equation S = C/K 219 (16.6) where C is a non-dimensional constant characteristic of the type of object being viewed through the smoke, i.e., C = 8 for a light-emitting sign and C = 3 for a light-reflecting sign [39]. Since K varies from point to point in the domain, the visibility S does as well. Three parameters control smoke production and visibility. The first is the SOOT_YIELD on the REAC line, defined as the fraction of fuel mass that is converted to soot if the simple chemistry approach is being used. The second parameter, MASS_EXTINCTION_COEFFICIENT, is the Km in Eq. (16.5). It is defined on one or more of the SPEC lines2 for the various light absorbing gas species. Its default value is 8700 m2 /kg, a value suggested for flaming combustion of wood and plastics3 . The third parameter, VISIBILITY_FACTOR on the MISC line, is the constant C in Eq. (16.6). It is 3 by default. The gas phase output quantity ’EXTINCTION COEFFICIENT’ is K. A similar quantity is the ’OPTICAL DENSITY’, D = K/2.3, the result of using log10 in the definition 1 I D ≡ − log10 = K log10 e (16.7) L I0 The visibility S is output via the QUANTITY called ’VISIBILITY’. Note that, by default, the visibility is associated with the smoke that is implicitly defined by the simple chemistry model. However, this quantity can also be associated with an explicitly defined species via the inclusion of a SPEC_ID. In other words, you can specify the output quantity ’VISIBILITY’ along with a SPEC_ID. This does not require that you do a simple chemistry calculation; only that you have specified the given species via a separate SPEC line. You can specify a unique MASS_EXTINCTION_COEFFICIENT on the SPEC line as well. Note that FDS cannot report a visibility of infinity, but rather reports a MAXIMUM_VISIBILITY that you can control via the MISC line. The default is 30 m. 16.10.3 Layer Height and the Average Upper and Lower Layer Temperatures Fire protection engineers often need to estimate the location of the interface between the hot, smoke-laden upper layer and the cooler lower layer in a burning compartment. Relatively simple fire models, often referred to as two-zone models, compute this quantity directly, along with the average temperature of the upper and lower layers. In a computational fluid dynamics (CFD) model like FDS, there are not two distinct zones, but rather a continuous profile of temperature. Nevertheless, there are methods that have been developed to estimate layer height and average temperatures from a continuous vertical profile of temperature. One such method [41] is as follows: Consider a continuous function T (z) defining temperature T as a function of height above the floor z, where z = 0 is the floor and z = H is the ceiling. Define Tu as the upper layer temperature, Tl as the lower layer temperature, and zint as the interface height. Compute the quantities: Z H (H − zint ) Tu + zint Tl = 1 1 (H − zint ) + zint = Tu Tl Solve for zint : zint = 0 Z H 0 T (z) dz = I1 1 dz = I2 T (z) Tl (I1 I2 − H 2 ) I1 + I2 Tl2 − 2 Tl H 2 When (16.8) using the simple chemistry combustion model, you can change the default mass extinction coefficient by adding a line to the input file of the form: &SPEC ID=’SOOT’, MASS_EXTINCTION_COEFFICIENT=..., LUMPED_COMPONENT_ONLY=.TRUE. / 3 For most flaming fuels, a suggested value for K is 8700 m2 /kg ± 1100 m2 /kg at a wavelength of 633 nm [40] m 220 Let Tl be the temperature in the lowest mesh cell and, using Simpson’s Rule, perform the numerical integration of I1 and I2 . Tu is defined as the average upper layer temperature via (H − zint ) Tu = Z H zint T (z) dz (16.9) Further discussion of similar procedures can be found in Ref. [42]. The quantities ’LAYER HEIGHT’, ’UPPER TEMPERATURE’ and ’LOWER TEMPERATURE’ can be designated via DEVC lines in the input file. For example, the line: &DEVC XB=2.0,2.0,3.0,3.0,0.0,3.0, QUANTITY='LAYER HEIGHT', ID='whatever' / produces a time history of the smoke layer height at x = 2 and y = 3 between z = 0 and z = 3. If multiple meshes are being used, the vertical path cannot cross mesh boundaries. 16.10.4 Thermocouples The output quantity THERMOCOUPLE is the temperature of a modeled thermocouple. The thermocouple temperature lags the true gas temperature by an amount determined mainly by its bead size. It is found by solving the following equation for the thermocouple temperature, TTC [43] ρTC cTC dTTC 4 = εTC (U/4 − σ TTC ) + h(Tg − TTC ) = 0 dt (16.10) where εTC is the emissivity of the thermocouple, U is the integrated radiative intensity, Tg is the true gas temperature, and h is the heat transfer coefficient to a small sphere, h = k Nu/DTC . The bead DIAMETER, EMISSIVITY, DENSITY, and SPECIFIC_HEAT are given on the associated PROP line. To over-ride the calculated value of the heat transfer coefficient, set HEAT_TRANSFER_COEFFICIENT on the PROP line (W/(m · K)). The default value for the bead diameter is 0.001 m. The default emissivity is 0.85. The default values for the bead density and specific heat are that of nickel; 8908 kg/m3 and 0.44 kJ/kg/K, respectively. See the discussion on heat transfer to a water droplet in the Technical Reference Guide for details of the convective heat transfer to a small sphere. 16.10.5 Heat Flux There are various output quantities related to heat flux. First, consider the net heat flux to a solid surface: q̇00net = εs q̇00inc,rad − σ Ts4 + h (Tgas − Ts ) (16.11) where q̇00inc,rad is the incident radiative heat flux, εs is the surface emissivity, h is the convective heat transfer coefficient, Ts is the surface temperature, and Tgas is the gas temperature in the vicinity of the surface. If you want to output the net heat flux, use the QUANTITY called ’NET HEAT FLUX’. The individual components, the net convective and radiative fluxes, are ’CONVECTIVE HEAT FLUX’ and ’RADIATIVE HEAT FLUX’, respectively. If you just want to output q̇00inc,rad , use ’INCIDENT HEAT FLUX’. If you want to compare the heat flux predicted by FDS to a measurement made with a heat flux gauge, use ’GAUGE HEAT FLUX’: 4 q̇00gauge = εgauge q̇00inc,rad − σ Tgauge + h (Tgas − Tgauge ) (16.12) If the heat flux gauge used in an experiment has a temperature other than ambient or an emissivity other than 1, use GAUGE_TEMPERATURE (Tgauge ) and GAUGE_EMISSIVITY (εgauge ) on the PROP line associated with the device: 221 &DEVC ID='hf', XYZ=..., IOR=-2, QUANTITY='GAUGE HEAT FLUX', PROP_ID='hf props' / &PROP ID='hf props', GAUGE_TEMPERATURE=80., GAUGE_EMISSIVITY=0.9 / The heat transfer coefficient, h, is that of the solid surface to which the device is attached. Similar to a heat flux gauge, a ’RADIOMETER’ measures only the radiative heat flux: 4 q̇00radiometer = εgauge q̇00inc,rad − σ Tgauge (16.13) The GAUGE_TEMPERATURE (Tgauge ) and GAUGE_EMISSIVITY (εgauge ) can be set on the PROP line associated with the device if their values are different than ambient and 1, respectively. All of the above heat flux output quantities are defined at a solid surface. To record the radiative heat flux away from a solid surface, add a device with the following format: &DEVC ID='flux', QUANTITY='RADIATIVE HEAT FLUX GAS', XYZ=..., ORIENTATION=-1,0,0 / Note that XB and STATISTICS are not appropriate for this quantity. This single line of input is a special shortcut4 for the following lines of input that make use of a Lagrangian particle as a surrogate target: &DEVC &INIT &PART &SURF ID='flux', INIT_ID='f1', QUANTITY='RADIATIVE HEAT FLUX' / ID='f1', XYZ=..., N_PARTICLES=1, PART_ID='rad gauge' / ID='rad gauge', STATIC=.TRUE., ORIENTATION(1:3,1)=-1,0,0, SURF_ID='target' / ID='target', RADIUS=0.001, GEOMETRY='SPHERICAL', EMISSIVITY=1. / Note that the DEVC line does not contain device coordinates, but rather a reference to the INIT line that positions the single surrogate particle at the point XYZ. The INIT line references the PART line, which provides information about the particle, in particular the orientation of the heat flux gauge. The reference to the SURF line is mainly for consistency – FDS needs to know something about the particle’s geometry even though it is really just a “target.” The functionality of surrogate particles can be extended to model an array of devices. Instead of one heat flux gauge, we can create a line of them: &DEVC ID='flux', INIT_ID='f1', POINTS=34, QUANTITY='RADIATIVE HEAT FLUX', X_ID='x' / &INIT ID='f1', XYZ=..., N_PARTICLES=34, DX=0.05, PART_ID='rad gauge' / For more information about specifying arrays of devices via the parameter POINTS, see Section 16.2.2. Note also the parameter DX on the INIT line that creates a line of particles starting at the point XYZ and repeating every 0.05 m. 16.10.6 Adiabatic Surface Temperature FDS includes a calculation of the adiabatic surface temperature (AST), a quantity that is representative of the heat flux to a solid surface. Following the idea proposed by Ulf Wickström [44], TAST is the surface temperature for which the net heat flux (Eq. (16.11) is zero. The following equation can be solved using an analytical solution given by Malendowski [45]: 4 ε q̇00inc,rad − σ TAST + h (Tgas − TAST ) = 0 (16.14) where, q̇00inc,rad is the incident radiative heat flux onto the surface, ε is the surface emissivity, h is the convective heat transfer coefficient, and Tgas is the surrounding gas temperature. 4 This feature maintains backward compatibility with FDS 5. 222 The usefulness of the AST is that it represents an effective exposure temperature that can be passed on to a more detailed model of the solid object. It provides the gas phase thermal boundary condition in a single quantity, and it is not affected by the uncertainty associated with the solid phase heat conduction model within FDS. Obviously, the objective in passing information to a more detailed model is to get a better prediction of the solid temperature (and ultimately its mechanical response) than FDS can provide. To reinforce this notion, you can output the adiabatic surface temperature even when there is no actual solid surface using the following lines: &DEVC ID='AST', XYZ=..., QUANTITY='ADIABATIC SURFACE TEMPERATURE GAS', ORIENTATION=0.707,0.0,0.707, PROP_ID='props' / &PROP ID='props', EMISSIVITY=0.9, HEAT_TRANSFER_COEFFICIENT=10. / This output indicates the maximum achievable solid surface temperature at the given location XYZ that is not actually in the vicinity of a solid surface, facing in any direction as indicated by the ORIENTATION vector. Note that you must set the EMISSIVITY and HEAT_TRANSFER_COEFFICIENT (W/(m2 ·K)) on the PROP line because there is no actual solid surface from which to infer these values. Example: AST vs. Surface Temperature The test case called adiabatic_surface_temperature.fds in the Radiation folder simulates a 0.1 mm steel plate being heated by a thermal plume. The plate is perfectly insulated (BACKING=’INSULATED’ on the SURF line) and its steady-state temperature should be equivalent to its adiabatic surface temperature or AST. The left plot of Fig. 16.10.6 shows the plate temperature rising towards the AST, much as an actual plate thermometer would. The right plot simply shows the AST calculated using the QUANTITY ’ADIABATIC SURFACE TEMPERATURE’ applied via a DEVC at the plate surface, and the AST calculated using the QUANTITY ’ADIABATIC SURFACE TEMPERATURE GAS’ applied via a DEVC positioned just in front of the plate. The idea of the latter device would be to record the AST even if the plate were not actually represented in the simulation. For these two recordings of the AST to be identical, the plate SURFace conditions and the AST-Gas PROPerties must both include the same explicitly-defined HEAT_TRANSFER_COEFFICIENT and EMISSIVITY. FDS0−121−g02067f4−dirty FDS0−121−g02067f4−dirty 500 500 Plate Temp vs AST (adiabatic_surface_temperature) Gas vs Solid AST (adiabatic_surface_temperature) 400 Temperature (°C) Temperature (°C) 400 300 200 100 300 200 100 Plate (temp) AST 0 0 20 40 60 Time (s) 80 100 120 AST AST−Gas 0 0 20 40 60 Time (s) 80 100 120 Figure 16.4: (Left) Surface temperature vs. adiabatic surface temperature of an insulated plate. (Right) AST recorded with a device positioned on the plate surface (AST) and one just off the surface (AST-Gas). 223 16.10.7 Special Topic: Detailed Spray Properties Detailed experimental measurements of sprays using Phase Doppler Particle Analysis (PDPA) provide information on the droplet size distribution, speed and concentration. A special device type is defined via a DEVC line to simulate the PDPA measurement. The actual quantity to measure, and the details of the measurement are defined using an associated PROP line. By default, the PDPA device output at time t is computed as a time integral F(t) = 1 min(t,te ) − ts Z min(t,te ) ts f (t) dt (16.15) but instantaneous values can be obtained by setting PDPA_INTEGRATE equal to .FALSE. on the corresponding PROP line, in which case F(t) = f (t) (16.16) The function f (t) has two forms: f1 (t) = ∑i ni Dm i φ ∑i ni Dni 1 m−n ; f2 (t) = ∑i ni φ V (16.17) where ni is the number of real particles represented by the single simulated particle, Di is the particle diameter, and φ is the quantity to be measured. In each case, the summation goes over all the particles within a sphere with radius PDPA_RADIUS and centered at the location given by the device XYZ. The first form f1 (t) is used for the computation of various mean diameters, with associated properties defined using the following keywords on the PROP line: PDPA_M m, exponent m of diameter. PDPA_N n, exponent n of diameter. In case m = n, the exponent 1/(m − n) is removed from the formula. The second form ( f2 (t)) is used for the computation of mass and energy related variables that do not include the diameter weighting. The concentrations are based on the sampling volume, V , defined by PDPA_RADIUS. The quantity used for x can be chosen with the keyword QUANTITY. A summary of the available PDPA quantities is shown in Table 16.1. It is also possible to output histograms of PDPA output quantities. When PDPA_HISTOGRAM is set to .TRUE., normalized histogram bin counts are output to a comma-separated value (.csv) file from all devices associated with this PROP line. The number of bins and the limits of the histogram are controlled by parameters on the PROP line. The value used in creating the histogram is Dm i φ . Note that when making a histogram of diameters, the limits must be given in meters, not microns. Values falling outside the histogram limits are included in the counts of the first and last bins. Cumulative distributions can be output by setting PDPA_HISTOGRAM_CUMULATIVE=.TRUE. on the PROP line. To output unnormalized counts or cumulative distribution, set PDPA_NORMALIZE to .FALSE.. Note, however, that the counts correspond to the super droplets/particles, not the numerical ones. Due to the stratified sampling technique used (see Section 14.3.3), the counts are not necessarily integers. The histogram output file contains of two-columns for each device. The first column gives the bin centers, and the second column gives the normalized bin count. The bin counts are normalized so that the total area of the histogram is 1. Here the histogram consists of equal width bars centered at the given bin centers and heights equal to the corresponding bin count. Volume and area based distributions can be output by setting the PDPA_N parameter on the PROP line to 3 and 2 respectively. Notice that this works differently from the mean diameter computation where the weighting is based on the PDPA_M parameter. The properties of the PDPA device are defined using the following keywords on the PROP line: 224 Table 16.1: Output quantities available for PDPA. QUANTITY ’DIAMETER’ (default) ’ENTHALPY’ ’PARTICLE FLUX X’ ’PARTICLE FLUX Y’ ’PARTICLE FLUX Z’ ’U-VELOCITY’ ’V-VELOCITY’ ’W-VELOCITY’ ’VELOCITY’ ’TEMPERATURE’ ’MASS CONCENTRATION’ ’NUMBER CONCENTRATION’ ∗ φ 1 (4/3)ρi ri3 (c p,i (Ti )Ti − c p,i (Tm )Tm ) (4/3)ρi ri3 ui (4/3)ρi ri3 vi (4/3)ρi ri3 wi ui vi wi 1 (u2i + v2i + w2i ) 2 Ti (4/3)ρri3 1 f f1 f2 f2 f2 f2 f1 f1 f1 f1 f1 f2 f2 Unit µm kJ/m3 kg/m2 s kg/(m2 · s) kg/(m2 · s) m/s m/s m/s m/s ◦C kg/m3 Tm is the melting temperature of the associated species. PART_ID Name of the particle group to limit the computation to. Do not specify to account for all particles. PDPA_START ts , starting time of time integration in seconds. PDPA output is always a running average over time. As the spray simulation may contain some initial transient phase, it may be useful to specify the starting time of data collection. PDPA_END te , ending time of time integration in seconds. PDPA_INTEGRATE A logical parameter for choosing between time integrated or instantaneous values. .TRUE. by default. PDPA_RADIUS Radius (m) of the sphere, centered at the device location, inside which the particles are monitored. PDPA_NORMALIZE Can be set .FALSE. to force V = 1 in the formula for f2 (t), or to unnormalize the histogram output. QUANTITY Specified on PROP line for choosing the variable φ . PDPA_HISTOGRAM_NBINS Number of bins used for the histogram. The following example is used to measure the Sauter mean diameter, D32 , of the particle type ’water drops’, starting from time 5 s. &PROP ID='pdpa_d32' PART_ID='water drops' PDPA_M=3 PDPA_N=2 PDPA_RADIUS=0.01 PDPA_START=5. / &DEVC XYZ=0.0,0.0,1.0, QUANTITY='PDPA', PROP_ID='pdpa_d32' / The following example is used to write out a histogram of droplet size using 20 equally sized bins between 0 and 2000 µm. 225 &PROP ID='pdpa_d' PART_ID='water drops' QUANTITY="DIAMETER" PDPA_RADIUS=0.01 PDPA_START=0.0 PDPA_M=1 PDPA_HISTOGRAM=.TRUE. PDPA_HISTOGRAM_NBINS=20 PDPA_HISTOGRAM_LIMITS=0,2000E-6 / &DEVC XYZ=0.0,0.0,1.0, QUANTITY='PDPA', PROP_ID='pdpa_d' / 16.10.8 Useful Solid Phase Outputs In addition to the profile (PROF) output, there are various additional quantities that are useful for monitoring reacting surfaces. For example, ’WALL THICKNESS’ gives the overall thickness of the solid surface element. ’SURFACE DENSITY’ gives the overall mass per unit area for the solid surface element, computed as an integral of material density over wall thickness. Both quantities are available both as DEVC and BNDF. Thermogravimetric Analysis (TGA) Output Thermogravimetric Analysis or TGA is a bench-scale measurement in which a very small solid material sample is heated up at a constant rate. The results of a TGA measurement are presented in the form of a normalized mass and normalized mass loss rate. Analogous quantities can be output from FDS: . ’NORMALIZED MASS’ = ∑ m00α (t) ∑ m00α (0) = ∑ ṁ00α (t) ∑ m00α (0) ’NORMALIZED MASS LOSS RATE’ α α . α α (dimensionless) (1/s) Micro-Combustion Calorimetry (MCC) Output Micro-Combustion Calorimetry or MCC is similar to TGA, except the vaporized gas is burned. The result is a normalized heat release rate: . ’NORMALIZED HEAT RELEASE RATE’ = ṁ00f (t) ∆H ∑ m00α (0) (W/g) α Note that ṁ00F is the mass flux of fuel and ∆H is the heat of combustion. Differential Scanning Calorimetry (DSC) Output Differential Scanning Calorimetry or DSC is a measurement of the rate of heat absorption by a small material sample under constant heating. The result is a normalized heat absorption rate: . ’NORMALIZED HEATING RATE’ = q̇00c (t) ∑ m00α (0) α (W/g) Note that it is assumed that the sample is heated purely by convection, in which case q̇00c is the convective heat flux. 226 16.10.9 Fractional Effective Dose (FED) and Fractional Irritant Concentration (FIC) The Fractional Effective Dose index (FED), developed by Purser [46], is a commonly used measure of human incapacitation due to the combustion gases. The FED value is calculated as (16.18) FEDtot = (FEDCO + FEDCN + FEDNOx + FLDirr ) × HVCO2 + FEDO2 The fraction of an incapacitating dose of CO is calculated as FEDCO = Z t 0 2.764 × 10−5 (CCO (t))1.036 dt (16.19) where t is time in minutes and CCO is the CO concentration (ppm). The fraction of an incapacitating dose of CN is calculated as Z t 1 CCN (t) FEDCN = exp − 0.0045 dt (16.20) 220 43 0 where t is time in minutes and CCN is the concentration (ppm) of HCN corrected for the protective effect of NO2 . CCN is calculated as CCN = CHCN −CNO2 −CNO (16.21) The fraction of an incapacitating dose of NOx is calculated as FEDNOx = Z t CNOx (t) 0 1500 dt (16.22) where t is time in minutes and CNOx is the sum of NO and NO2 concentrations (ppm). The Fractional Lethal Dose (FLD) of irritants is calculated as Z t CHBr (t) CHCl (t) CHF (t) CSO2 (t) CNO2 (t) CC3 H4 O (t) CCH2 O (t) FLDirr = + + + + + + dt FFLD,HCl FFLD,HBr FFLD,HF FFLD,SO2 FFLD,NO2 FFLD,C3 H4 O FFLD,CH2 O 0 (16.23) where t is time in minutes, the nominators are the instantaneous concentrations (ppm) of each irritant and the denominators the exposure doses of respective irritants predicted to be lethal to half the population. The lethal exposure doses [46] are given in Table 16.2. To include the effect of an irritant gas not listed in the table, you should specify FFLD in ppm×min using the FLD_LETHAL_DOSE property of the corresponding SPEC line. Table 16.2: Coefficients used for the computation of irritant effects of gases. FFLD (ppm × min) FFIC (ppm) HCl 114000 900 HBr 114000 900 HF 87000 900 SO2 12000 120 NO2 1900 350 C3 H4 O 4500 20 CH2 O 22500 30 The fraction of an incapacitating dose of low O2 hypoxia is calculated as FEDO2 = Z t 0 dt exp [8.13 − 0.54 (20.9 −CO2 (t))] (16.24) where t is time in minutes and CO2 is the O2 concentration (volume percent). The hyperventilation factor induced by carbon dioxide is calculated as HVCO2 = exp(0.1903CCO2 (t) + 2.0004) 7.1 227 (16.25) where t is time in minutes and CCO2 is the CO2 concentration (percent). The Fractional Irritant Concentration (FIC), also developed by Purser [46], represents the toxic effect which depends upon the immediate concentrations of irritants. The overall irritant concentration FIC is calculated as FICirr = CHCl (t) CHBr (t) CHF (t) CSO2 (t) CNO2 (t) CC3 H4 O (t) CCH2 O (t) + + + + + + FFIC,HCl FFIC,HBr FFIC,HF FFIC,SO2 FFIC,NO2 FFIC,C3 H4 O FFIC,CH2 O (16.26) where the nominators are the instantaneous concentrations of each irritant and the denominators the concentrations of respective irritants expected to cause incapacitation in half the population. The incapacitating concentrations [46] are given in Table 16.2. To include the irritant effect of a gas not listed in the table, you should specify FFIC in ppm using the FIC_CONCENTRATION property on the corresponding SPEC line. Note that the spatial integration features (Section 16.10.10) cannot be used with FED output because FED makes use of the ’TIME INTEGRAL’ statistic (Section 16.10.11). For the same reason, FED output is only available as a point measurement. 16.10.10 Spatially-Integrated Outputs A useful feature of a device (DEVC) is to specify an output quantity along with a desired statistic. For example, &DEVC XB=..., QUANTITY='TEMPERATURE', ID='maxT', STATISTICS='MAX' / causes FDS to write out the maximum gas phase temperature over the volume bounded by XB. Note that it does not compute the maximum over the entire computational domain, just the specified volume, and this volume must lie within a single mesh. Other STATISTICS are discussed below. Note that some are appropriate for gas phase output quantities, some for solid phase, and some for both. For solid phase output quantities, like heat fluxes and surface temperatures, the specification of a SURF_ID along with the appropriate statistic limits the calculation to only those surfaces. You can further limit the search by using the sextuplet of coordinates XB to force FDS to only compute statistics for surface cells within the given volume. Be careful to account for the fact that the solid surface might shift to conform to the underlying numerical grid. Also, be careful not to specify a volume that extends beyond a single mesh. Note that you do not (and should not) specify an orientation via the parameter IOR when using a spatial statistic. IOR is only needed to find a specific point on the solid surface. Use the STATISTICS feature with caution because it demands that FDS evaluate the given QUANTITY in all gas or solid phase cells. Minimum or Maximum Value For a given gas phase scalar output quantity defined at the center of each grid cell, φi jk , STATISTICS=’MIN’ or STATISTICS=’MAX’ computes the minimum or maximum value, respectively min φi jk i jk ; max φi jk i jk (16.27) over the cells that are included in the specified volume bounded by XB. Note that this statistic is only appropriate for gas phase quantities. Note also that you must specify a volume to sum over via the coordinate parameters, XB, all of which must be contained within the same mesh. 228 Average Value For a given gas phase scalar output quantity defined at the center of each grid cell, φi jk , STATISTICS=’MEAN’ computes the average value, 1 φi jk (16.28) N ∑ i jk over the cells that are included in the specified volume bounded by XB. Note that this statistic is only appropriate for gas phase quantities. Note also that you must specify a volume to sum over via the coordinate parameters, XB, all of which must be contained within the same mesh. Volume-Weighted Mean For a given gas phase output quantity, φ (x, y, z), STATISTICS=’VOLUME MEAN’ produces the discrete analog of Z 1 φ (x, y, z) dx dy dz (16.29) V which is very similar to ’MEAN’, but it weights the values according to the relative size of the mesh cell. Note that this statistic is only appropriate for gas phase quantities. Note also that you must specify a volume to sum over via the coordinate parameters, XB, all of which must be contained within the same mesh. Mass-Weighted Mean For a given gas phase output quantity, φ (x, y, z), STATISTICS=’MASS MEAN’ produces the discrete analog of R ρ(x, y, z) φ (x, y, z) dx dy dz R (16.30) ρ dx dy dz which is similar to ’VOLUME MEAN’, but it weights the values according to the relative mass of the mesh cell. Note that this statistic is only appropriate for gas phase quantities. Note also that you must specify a volume to sum over via the coordinate parameters, XB, all of which must be contained within the same mesh. Volume Integral For a given gas phase output quantity, φ (x, y, z), STATISTICS=’VOLUME INTEGRAL’ produces the discrete analog of Z φ (x, y, z) dx dy dz (16.31) Note that this statistic is only appropriate for gas phase quantities, in particular those whose units involve m−3 . For example, heat release rate per unit volume is an appropriate output quantity. Note also that you must specify a volume to sum over via the coordinate parameters, XB, all of which must be contained within the same mesh. Mass Integral For a given gas phase output quantity, φ (x, y, z), STATISTICS=’MASS INTEGRAL’ produces the discrete analog of Z ρ(x, y, z) φ (x, y, z) dx dy dz (16.32) Note that this statistic is only appropriate for gas phase quantities. Note also that you must specify a volume to sum over via the coordinate parameters, XB, all of which must be contained within the same mesh. 229 Area Integral For a given gas phase output quantity, φ (x, y, z), STATISTICS=’AREA INTEGRAL’ produces the discrete analog of Z φ (x, y, z) dA (16.33) where dA depends on the coordinates you specify for XB. Note that this statistic is only appropriate for gas phase quantities, in particular those whose units involve m−2 . For example, the quantity ’MASS FLUX X’ along with SPEC_ID=’my gas’ is an appropriate output quantity if you want to know the mass flux of the gas species that you have named ’my gas’ through an area normal to the x direction. Note also that you must specify an area to sum over via the coordinate parameters, XB, all of which must be contained within the same mesh. Surface Integral For a given solid phase output quantity, φ , STATISTICS=’SURFACE INTEGRAL’ produces the discrete analog of Z φ dA (16.34) Note that this statistic is only appropriate for solid phase quantities, in particular those whose units involve m−2 . For example, the various heat and mass fluxes are appropriate output quantities. Limiting the Integration The input parameter QUANTITY_RANGE can be used to limit the region of integration for STATISTICS types of ’AREA INTEGRAL’, ’VOLUME INTEGRAL’, ’MASS INTEGRAL’, and ’SURFACE INTEGRAL’. If QUANTITY_RANGE is set, the integration will only be performed if the value of the QUANTITY lies within the QUANTITY_RANGE where QUANTITY_RANGE(1) is the lower bound and QUANTITY_RANGE(2) is the upper bound. For example: &DEVC XB=..., QUANTITY='MASS FRACTION', SPEC_ID='METHANE', STATISTICS='MASS INTEGRAL', QUANTITY_RANGE=0.03,0.15/ would output the total mass of methane in the volume XB where the mass fraction was between 0.03 and 0.15. A set of additional STATISTICS are available for use with QUANTITY_RANGE: ’AREA’, ’MASS’, ’VOLUME’, and ’SURFACE AREA’. These STATISTICS output the area, mass, volume, or surface area (for a solid phase quantity) where the QUANTITY lies within the QUANTITY_RANGE. For example: &DEVC XB=..., QUANTITY='NET HEAT FLUX', STATISTICS='SURFACE AREA', QUANTITY_RANGE(1)=10./ would output the total surface area in the volume XB where the net heat flux exceeds 10 kW/m2 . Volume, Mass, and Heat Flow The net flow of mass and energy into or out of a planar surface can be a useful diagnostic. There are several outputs that address these. All are prescribed via the device (DEVC) namelist group only. For example: &DEVC XB=0.3,0.5,2.1,2.5,3.0,3.0, QUANTITY='MASS FLOW', ID='whatever' / 230 outputs the net integrated mass flux through the given planar area, oriented in the positive z direction, in this case. The three flows ’VOLUME FLOW’, ’MASS FLOW’, and ’HEAT FLOW’ are defined: V̇ = Z u · dS (16.35) ṁ = Z ρu · dS (16.36) q̇ = Z ρc p (T − T∞ ) u · dS (16.37) The addition of a + or - to the QUANTITY names yields the integral of the flow in the positive or negative direction only. In other words, if you want to know the mass flow out of a compartment, use ’MASS FLOW +’ or ’MASS FLOW -’, depending on the orientation of the door. Volume, Mass, and Heat Flow at a Wall The quantities ’VOLUME FLOW’, ’MASS FLOW’, and ’HEAT FLOW’ cannot be applied at a solid boundary. Instead, use the wall equivalents ’VOLUME FLOW WALL’, ’MASS FLOW WALL’, and ’HEAT FLOW WALL’. These quantities require an IOR for the surface (positive points into the domain). If you want the mass flow of a particular gas species, include the SPEC_ID of that gas species. Note that, in order to be a more useful diagnostic for global energy balances, the heat flow at the wall is defined using the sensible enthalpy of the mixture (composition taken at the wall) and is not relative to ambient conditions: Z q̇w = 16.10.11 ρhs (T ) u · dS (16.38) Temporally-Integrated Outputs In addition to the spatial statistics, a time integral of an DEVC output can be computed by specifying STATISTICS = ’TIME INTEGRAL’ on the DEVC line. This produces a discrete analog of Z t t0 φ (τ) dτ (16.39) Note that the spatial and time integrals can not be used simultaneously. 16.10.12 Statistical Outputs Statistical quantities can also be generated for either point or line devices. Note that these are quantities only have significant meaning if the flow is steady-state. These quantities are estimated using a logarithmic averaging process. The weighting coefficient for the averaging is determined by the desired time interval for the device. For a point device this time interval is starts at STATISTICS_START on the DEVC line and continues until T_END. For a line device is is determined by DT_DEVC_LINE on the DUMP line. Root Mean Square If you set STATISTICS=’RMS’ on the DEVC line, the output will be an unbiased estimate of the root mean square value: s φrms = ∑ni=1 (φi − φ )2 n−1 231 (16.40) Covariance If u = U −U and v = V −V are the deviations for two random variables, U and V , then an unbiased estimate of the covariance is given by ∑n (Ui −U)(Vi −V ) (16.41) u v = i=1 n−1 To output this statistic you must add a QUANTITY2 to the device line and set STATISTICS=’COV’. The following lines would create a line of devices and a single point device equivalent to the first point in the line: &DUMP DT_DEVC_LINE=20/ &DEVC XB=X1,X2,Y1,Y2,Z1,Z2, QUANTITY='CELL W', QUANTITY2='TEMPERATURE', STATISTICS='COV', ID='wT-cov_line', POINTS=20/ &DEVC XYZ=X1,Y1,Z1, QUANTITY='CELL W', QUANTITY2='TEMPERATURE', STATISTICS='COV', ID='wT-cov_point', STATISTICS_DT=20/ Correlation Coefficient Setting STATISTICS=’CORRCOEF’ outputs the correlation coefficient given by ρuv = uv urms vrms (16.42) Here again you must add a QUANTITY2 to the device line. 16.10.13 Wind and the Pressure Coefficient In the field of wind engineering, a commonly used quantity is known as the PRESSURE_COEFFICIENT: Cp = p − p∞ 1 2 2 ρ∞U (16.43) p∞ is the ambient, or “free stream” pressure, and ρ∞ is the ambient density. The parameter U is the freestream wind speed, given as CHARACTERISTIC_VELOCITY on the PROP line &BNDF QUANTITY='PRESSURE COEFFICIENT', PROP_ID='U' / &DEVC ID='pressure tap', XYZ=..., IOR=2, QUANTITY='PRESSURE COEFFICIENT', PROP_ID='U' / &PROP ID='U', CHARACTERISTIC_VELOCITY=3.4 / Thus, you can either paint values of Cp at all surface points, or create a single time history of Cp using a single device at a single point. 16.10.14 Near-wall Grid Resolution Large-eddy simulations of boundary layer flows fall into two general categories: LES with near-wall resolution and LES with near-wall modeling (wall functions). FDS employs the latter. The wall models used in FDS are the Werner-Wengle wall model [47] for smooth walls and a rough wall log law for rough walls [11]. For the wall models to function properly, the grid resolution near the wall should fall within a certain range of y+ , the nondimensional distance from the wall expressed in viscous units. To check this, you may add a boundary file output as follows: 232 &BNDF QUANTITY='YPLUS' / The value of y+ reported is half (since the velocity lives at the cell face center) the wall-normal cell dimension (δ n) divided by the local viscous length scale, δν [11]: y+ = 1 δn ; 2 δν δν = µ/ρ ; uτ uτ = p τw /ρ , (16.44) where τw = µ ∂ |u|/∂ n is the viscous stress evaluated at the wall (τw is computed by the wall function, |u| is taken as an estimate of the streamwise velocity component near the wall); the quantity uτ is the friction velocity. The friction velocity may also be output in a boundary file or via a device attached to a wall. For example: &DEVC XYZ=1,0,0, QUANTITY='FRICTION VELOCITY', IOR=3, ID='u_tau' / Wall functions for LES are still under development, but as a general guideline it is recommended that the first grid cell fall within the log layer: a value y+ = 30 would be considered highly resolved, the upper limit of the log region for statistically stationary boundary layers depends on the Reynolds number, and there are no hard rules for transient flows. Beyond y+ = 1000 the first grid cell is likely to fall in the wake region of the boundary layer and may produce unreliable results. A reasonable target for practical engineering LES is y+ = O(100). 16.10.15 Dry Volume and Mass Fractions During actual experiments, species such as CO and CO2 are typically measured “dry”; that is, the water vapor is removed from the gas sample prior to analysis. For easier comparison of FDS predictions with measured data, you can specify the logical parameter DRY on a DEVC line that reports the ’MASS FRACTION’ or ’VOLUME FRACTION’ of a species. For example, the first line reports the actual volume fraction of CO, and the second line reports the output of a gas analyzer in a typical experiment. &DEVC ID='wet CO', XYZ=..., QUANTITY='VOLUME FRACTION', SPEC_ID='CARBON MONOXIDE'/ &DEVC ID='dry CO', XYZ=..., QUANTITY='VOLUME FRACTION', SPEC_ID='CARBON MONOXIDE', DRY=.TRUE. / 16.10.16 Aerosol and Soot Concentration Currently there are three different device options for outputting aerosol concentration (e.g., soot concentration) from FDS. It is important to recognize what each device is outputting so that the proper selection can be made. &DEVC ID='MF_SOOT', XYZ=..., QUANTITY='MASS FRACTION', SPEC_ID='SOOT'/ &DEVC ID='VF_SOOT', XYZ=..., QUANTITY='VOLUME FRACTION', SPEC_ID='SOOT'/ &DEVC ID='SOOT_VF', XYZ=..., QUANTITY='AEROSOL VOLUME FRACTION', SPEC_ID='SOOT'/ Specifying a DEVC with a ’MASS FRACTION’ and a SPEC_ID of SOOT will output the mass fraction of soot in the gas phase. The quantity ’VOLUME FRACTION’ and a SPEC_ID of SOOT will output the volume fraction of soot in the gas phase treating the soot as if it were an ideal gas. The quantity ’AEROSOL VOLUME FRACTION’ and a SPEC_ID of SOOT will output the volume fraction of soot as if it were a solid particle in the computational cell based on the following equation, 233 fv = ρYa /ρa (16.45) where ρ is the local density, Ya is the local mass fraction of the aerosol, and ρa is density of the aerosol defined using the SPEC input DENSITY_SOLID, which defaults to 1800 (kg/m3 ) for soot. 16.10.17 Gas Velocity The gas velocity components, u, v, and w, can be output as slice (SLCF), point device (DEVC), isosurface (ISOF), or Plot3D quantities using the names ’U-VELOCITY’, ’V-VELOCITY’, and ’W-VELOCITY’. The total velocity is specified as just ’VELOCITY’. Normally, the velocity is always positive, but you can use the parameter VELO_INDEX with a value of 1, 2 or 3 to indicate that the velocity ought to have the same sign as u, v, or w, respectively. This is handy if you are comparing velocity predictions with measurements. For Plot3D files, add PLOT3D_VELO_INDEX(N)=... to the DUMP line, where N refers to the Plot3D quantity 1, 2, 3, 4 or 5. 16.10.18 Enthalpy There are several outputs that report the enthalpy of the gas mixture. First, the ’SPECIFIC ENTHALPY’ and the ’SPECIFIC SENSIBLE ENTHALPY’ are defined: h(T ) = ∆h◦f + Z T Tref c p (T 0 ) dT 0 ; hs (T ) = Z T Tref c p (T 0 ) dT 0 (16.46) Both have units of kJ/kg. The quantities ’ENTHALPY’ and ’SENSIBLE ENTHALPY’ are ρh and ρhs , respectively, in units of kJ/m3 . 16.10.19 Computer Performance There are several useful DEVC QUANTITY’s that can help monitor the performance of your computer: ’ACTUATED SPRINKLERS’ Number of activated sprinklers. ’CFL MAX’ The maximum value of the CFL (Courant-Friedrichs-Lewy) number, the primary constraint on the time step, for the mesh in which the device is located. By default, the time step is chosen so that the CFL number remains within the range of 0.8 to 1.0. If you want to see the CFL number in each grid cell, use a slice (SLCF) file with QUANTITY=’CFL’ and CELL_CENTERED=.TRUE.. ’CPU TIME’ Elapsed CPU time since the start of the simulation, in seconds. ’ITERATION’ Number of time steps completed at the given time of the simulation. ’NUMBER OF PARTICLES’ Number of Lagrangian particles for the MESH in which the DEVC is located. ’TIME STEP’ Duration of a simulation time step, δt, in seconds. ’VN MAX’ The maximum value of the VN (Von Neumann) number, a secondary constraint on the time step, for the mesh in which the device is located. By default, the time step is chosen so that the VN number remains below 1. If you want to see the VN number in each grid cell, use a slice (SLCF) file with QUANTITY=’VN’ and CELL_CENTERED=.TRUE.. ’WALL CLOCK TIME’ Elapsed wall clock time since the start of the simulation, in seconds. 234 ’WALL CLOCK TIME ITERATIONS’ Elapsed wall clock time since the start of the time stepping loop, in seconds. In addition, the following flags can be useful in monitoring the performance of an MPI calculation. They are typically used for debugging. VELOCITY_ERROR_FILE If set to .TRUE. on the DUMP line, this parameter will cause FDS to create a file with a time history of the maximum error associated with the normal component of velocity at solid or interpolated boundaries. MPI_TIMEOUT The amount of time, in seconds, to wait for messages sent via MPI (Message Passing Interface) before timing out. This parameter is set on the MISC line. The default value is 10 s. This parameter is only useful in forcing a job with deadlocked messages to finish and print out information about the lost message. It is very unlikely to solve a deadlock problem. 16.10.20 Output File Precision There are several different output files that have the format of a comma-separated value (.csv) file. These files consist of real numbers in columns separated by commas. By default, the real numbers are formatted -1.2345678E+123 To change the precision of the numbers, use SIG_FIGS on the DUMP line to indicate the number of significant figures in the mantissa (default is 8). Use SIG_FIGS_EXP to change the number of digits in the exponent (default is 3). Keep in mind that the precision of real numbers used internally in an FDS calculation is approximately 12, equivalent to 8 byte or double precision following conventional Fortran rules. 16.10.21 A Posteriori Mesh Quality Metrics The quality of a particular simulation is most directly tied to grid resolution. Three output quantities are discussed here for measuring errors in the velocity and scalar fields. It should be noted that the link between these metrics and true simulation quality is still in the research phase. In other words, a good quality score is not sufficient to assure a good simulation (at the present time). Measure of Turbulence Resolution A scalar quantity referred to as the measure of turbulence resolution [48] is defined locally as ksgs M(x) = hTKEi + ksgs (16.47) where the resolved turbulent kinetic energy per unit mass is TKE = 12 (ũi − hũi i)(ũi − hũi i) TKE must be output via DEVC for each velocity component and its mean: &DEVC &DEVC &DEVC &DEVC &DEVC &DEVC ..., ..., ..., ..., ..., ..., QUANTITY='U-VELOCITY' / QUANTITY='U-VELOCITY', STATISTICS='MEAN' / QUANTITY='V-VELOCITY' / QUANTITY='V-VELOCITY', STATISTICS='MEAN' / QUANTITY='W-VELOCITY' / QUANTITY='V-VELOCITY', STATISTICS='MEAN' / 235 (16.48) You must post-process these outputs to obtain TKE. The subgrid kinetic energy is estimated from Deardorff’s eddy viscosity model ksgs ≈ 12 (ũi − ũˆi )(ũi − ũˆi ) = (µt /(ρCν ∆))2 (16.49) Here, ũi is the resolved LES velocity and ũˆi is test filtered at a scale 2∆ where ∆ is the LES filter width (in FDS, ∆ = δ x). In the bulk flow region, the model for the SGS fluctuations is taken from scale similarity [49]. Cross-term energy is ignored. Keep in the mind, however, that the test filter operation is ill-defined near walls, and so FDS employs constant Smagorinsky in these cells to compute the eddy viscosity. To output an estimate of the subgrid kinetic energy per unit mass use &DEVC ..., QUANTITY='SUBGRID KINETIC ENERGY' / The concept behind the measure of turbulence resolution is illustrated in Figure 16.5. Notice that on the left the difference between the grid signal and the test signal is very small. On the right, the grid signal is highly turbulent and the corresponding test signal is much smoother. We infer then that the flow is underresolved. 1 1 grid signal, ū ˆ test signal, ū 0.5 signal signal 0.5 0 −0.5 −1 0 grid signal, ū ˆ test signal, ū 0 −0.5 1 2 3 space 4 5 −1 0 6 1 2 3 space 4 5 6 Figure 16.5: (Left) Resolved signal, M is small. (Right) Unresolved signal, M is close to unity. For the canonical case of isotropic turbulence Pope actually defines LES such that M < 0.2. That is, LES requires resolution of 80% of the kinetic energy in the flow field (because this puts the grid Nyquist limit within the inertial subrange). The question remains as to whether this critical value is sufficient or necessary for a given engineering problem. As shown in Ref. [50], maintaining mean values of M near 0.2 indeed provides satisfactory results (simulation results within experimental error bounds) for mean velocities and species concentrations in nonreacting, buoyant plumes. Wavelet Error Measure A resolution metric that we call the wavelet error measure or WEM may be output using, for example, &SLCF PBY=0, QUANTITY='WAVELET ERROR', QUANTITY2='HRRPUV' / We begin by providing background on the simple Haar wavelet [51]. For a thorough and more sophisticated review of wavelet methods, the reader is referred to Schneider and Vasilyev [52]. 236 Suppose the scalar function f (r) is sampled at discrete points r j , separated by a distance h, giving values s j . Defining the unit step function over the interval [r1 , r2 ] by 1 if r1 ≤ r < r2 ϕ[r1 ,r2 ] = (16.50) 0 otherwise the simplest possible reconstruction of the signal is the step function approximation f (r) ≈ ∑ s j ϕ[r j ,r j +h] (r) (16.51) j By “viewing” the signal at a coarser resolution, say 2h, an identical reconstruction of the function f over the interval [r j , r j + 2h] may be obtained from f[r j ,r j +2h] (r) = s j + s j+1 s j − s j+1 ϕ[r j ,r j +2h] (r) + ψ (r) 2 2 } [r j ,r j +2h] | {z } | {z a (16.52) c where a is as the average coefficient and c is as the wavelet coefficient. The Haar mother wavelet (Figure 16.6) is identified as 1 if r1 ≤ r < 21 (r1 + r2 ) ψ[r1 ,r2 ] (r) = (16.53) −1 if 12 (r1 + r2 ) ≤ r < r2 ψ[0,1] 1 0 −1 −1 0 1 2 Figure 16.6: Haar mother wavelet on the interval [0,1]. The decomposition of the signal shown in Eq. (16.52) may be repeated at ever coarser resolutions. The result is a wavelet transform. The procedure is entirely analogous to the Fourier transform, but with compact support. If we look at a 1D signal with 2m points, the repeated application of (16.52) results in an m × m matrix of averages a with components ai j and an m × m wavelet coefficient matrix c with components ci j . Each row i of a may be reconstructed from the i + 1 row of a and c. Because of this and because small values of the wavelet coefficient matrix may be discarded, dramatic compression of the signal may be obtained. Here we are interested in using the wavelet analysis to say something about the local level of error due to grid resolution. Very simply, we ask what can be discerned from a sample of four data points along a line. Roughly speaking we might expect to see one of the four scenarios depicted in Figure 16.7. Within each plot window we also show the results of a Haar wavelet transform for that signal. Looking first at the two 237 top plots, on the left we have a step function and on the right we have a straight line. Intuitively, we expect large error for the step function and small error for the line. The following error measure achieves this goal: |c11 + c12 | − |c21 | (16.54) WEM(x,t) = max x,y,z |a21 | Note that we have arbitrarily scaled the measure so that a step function leads to WEM of unity. In practice the transform is performed in all coordinate directions and the max value is reported. The scalar value may be output to Smokeview at the desired time interval. 0.8 0.6 1 a = 0.00 1.00 0.50 0.00 normalized signal normalized signal 1 c = 0.00 0.00 -0.50 0.00 0.4 0.2 0 1 2 3 0.8 0.6 0.2 spatial index normalized signal normalized signal 3 4 1 a = 0.50 0.50 0.50 0.00 0.8 0.6 0.4 0 1 2 spatial index 1 0.2 c = -0.17 -0.17 -0.33 0.00 0.4 0 1 4 a = 0.17 0.83 0.50 0.00 c = -0.50 -0.50 0.00 0.00 2 3 0.8 0.6 0.4 0.2 0 1 4 spatial index a = 0.50 0.50 0.50 0.00 c = -0.50 0.50 0.00 0.00 2 3 4 spatial index Figure 16.7: Averages and coefficients for local Haar wavelet transforms on four typical signals. Looking now at the two plots on the bottom of Figure 16.7, the signal on the left, which may indicate spurious oscillations or unresolved turbulent motion, leads to WEM = 2. Our measure therefore views this situation as the worst case in a sense. The signal to the lower right is indicative of an extremum, which actually is easily resolved by most centered spatial schemes and results again in WEM = 0. In [50], the time average of WEM was reported for LES of a nonreacting buoyant plume at three grid resolutions. From this study, the best advice currently is to maintain average values of WEM less than 0.5. Local Cell Reynolds Number Additionally, we provide an estimate of the local cell Reynolds number given by the ratio of the cell size (LES filter width, ∆) to an estimate of the local Kolmogorov scale, η (see [11]). For a DNS, ∆/η should be 238 less than or equal to one. The Kolmogorov scale is computed from its definition: η≡ (µ/ρ)3 ε 1/4 (16.55) where µ is the molecular dynamic viscosity, ρ is the density, and ε is the kinetic energy dissipation rate, which requires modeling. In FDS, we assume the dissipation rate is locally equivalent to the production of subgrid-scale kinetic energy. This implies ε = (µt /ρ)|S̃|2 (16.56) where µt is the turbulent viscosity and |S̃| is the filtered strain invariant (see FDS Tech Guide). &SLCF PBY=0, QUANTITY='CELL REYNOLDS NUMBER' / 16.10.22 Extinction In combustion, knowing if/when/where chemical reactions have been extinguished is important. The EXTINCTION parameter tells the user whether or not combustion has been prevented by FDS’ extinction routine. By default, EXTINCTION = 1, which means that the FDS extinction routine has not prevented combustion. An EXTINCTION value of 0 means that the routine has prevented combustion. The criteria for an EXTINCTION value of 0 is the presence of fuel and oxidizer without any energy release. An EXTINCTION value of -1 means that there is either no fuel or no oxidizer present. 16.11 Extracting Numbers from the Output Data Files Often it is desired to present results of calculations in some form other than those offered by Smokeview. In this case, there is a short Fortran 90 program called fds2ascii.f90, with a PC compiled version called fds2ascii.exe. To run the program, just type: fds2ascii at the command prompt. You will be asked a series of questions about which type of output file to process, what time interval to time average the data, and so forth. A single file is produced with the name CHID_fds2ascii.csv. A typical command line session looks like this: >> fds2ascii Enter Job ID string (CHID): bucket_test_1 What type of file to parse? PL3D file? Enter 1 SLCF file? Enter 2 BNDF file? Enter 3 3 Enter Sampling Factor for Data? (1 for all data, 2 for every other point, etc.) 1 Limit the domain size? (y or n) y Enter min/max x, y and z -5 5 -5 5 0 1 239 1 MESH 1, WALL TEMPERATURE Enter starting and ending time for averaging (s) 35 36 Enter orientation: (plus or minus 1, 2 or 3) 3 Enter number of variables 1 Enter boundary file index for variable 1 1 Enter output file name: bucket_test_1_fds2ascii.csv Writing to file... bucket_test_1_fds2ascii.csv These commands tell fds2ascii that you want to convert (binary) boundary file data into a text file. You want to sample every data point within the specified volume, you want only those surfaces that point upwards (+3 orientation), you only want 1 variable (only one is listed anyway and its index is 1 – that is just the number used to list the available files). The data will be time-averaged, and it will be output to a file listed at the end of the session. 240 16.12 Summary of Frequently-Used Output Quantities Table 16.3, spread over the following pages, summarizes the various Output Quantities. The column “File Type” lists the allowed output files for the quantities. “B” is for Boundary (BNDF), “D” is for Device (DEVC), “I” is for Iso-surface (ISOF), “P” is for Plot3D, “PA” for PArticle (PART), “S” is for Slice (SLCF). Be careful when specifying complicated quantities for Iso-surface or Plot3D files, as it requires computation in every gas phase cell. For those output quantities that require a species name via SPEC_ID, the species implicitly defined when using the simple chemistry combustion model are ’OXYGEN’, ’NITROGEN’, ’WATER VAPOR’, and ’CARBON DIOXIDE’. If CO_YIELD and/or SOOT_YIELD are specified on the REAC line, then ’CARBON MONOXIDE’ and ’SOOT’ are included as output species. The fuel species can be output via the FUEL specified on the REAC line. As an example of how to use the species names, suppose you want to calculate the integrated mass flux of carbon monoxide through a horizontal plane, like the total amount entrained in a fire plume. Use a “device” as follows &DEVC ID='CO_flow', XB=-5,5,-5,5,2,2, QUANTITY='MASS FLUX Z', SPEC_ID='CARBON MONOXIDE', STATISTICS='AREA INTEGRAL' / Here, the ID is just a label in the output file. When an output quantity is related to a particular gas species or particle type, you must specify the appropriate SPEC_ID or PART_ID on the same input line. Also note that the use of underscores in output quantity names has been eliminated – just remember that all output quantity names ought to be in single quotes. 241 Table 16.3: Summary of frequently used output quantities. QUANTITY ABSORPTION COEFFICIENT ACTUATED SPRINKLERS ADIABATIC SURFACE TEMPERATURE AEROSOL VOLUME FRACTION∗ AMPUA∗∗ AMPUA_Z∗ ASPIRATION BACKGROUND PRESSURE BACK WALL TEMPERATURE BURNING RATE CHAMBER OBSCURATION CHI_R CONDUCTIVITY CONTROL CONTROL VALUE CONVECTIVE HEAT FLUX CPUA∗∗ CPUA_Z∗ CPU TIME DENSITY DEPOSITION VELOCITY DIVERGENCE ENTHALPY EXTINCTION COEFFICIENT FED FIC FRICTION VELOCITY GAUGE HEAT FLUX HEAT FLOW HEAT FLOW WALL NET HEAT FLUX HRR HRRPUA HRRPUV INCIDENT HEAT FLUX INSIDE WALL TEMPERATURE ITERATION LAYER HEIGHT LINK TEMPERATURE LOWER TEMPERATURE MASS FLOW MASS FLOW WALL MASS FLUX∗ Symbol Section 13.2 Section 16.10.19 Section 16.10.6 Section 16.10.16 Section 16.9 Section 16.9 Section 15.3.7 Background pressure, p Section 16.2.4 Mass loss rate of fuel Section 15.3.5 Section 13.1.1 Thermal conductivity Section 15.5 Section 15.5 Section 16.10.5 Section 16.9 Section 16.9 Section 16.10.19 ρ or ρYα with SPEC_ID Section 12.5 ∇·u Section 11.1.2 Section 16.10.2 Section 16.10.9 Section 16.10.9 Section 16.10.14 Section 16.10.5 Section 16.10.10 Section 16.10.10 Section 16.10.5 R 000 q̇ dV 00 q̇ q̇000 Section 16.10.5 Section 16.2.3 Section 16.10.19 Section 16.10.3 Section 15.3.4 Section 16.10.3 Section 16.10.10 Section 16.10.10 Mass flux at solid surface 242 Units 1/m ◦C mol/mol kg/m2 kg/m2 %/m Pa ◦C kg/(m2 · s) %/m W/(m · K) kW/m2 kW/m2 kW/m2 s kg/m3 m/s 1/s kJ/m3 1/m m/s kW/m2 kW kW kW/m2 kW kW/m2 kW/m3 kW/m2 ◦C m ◦C ◦C kg/s kg/s kg/(m2 · s) File Type D,I,P,S D B,D D,I,P,S B,D B,D D D,I,P,S B,D B,D D D,I,S D,I,P,S D D B,D B,D B,D D D,I,P,S B,D D,I,P,S D,I,P,S D,I,P,S D D,S B,D B,D D D B,D D D D,I,P,S B,D D D D D D D D B,D Table 16.3: Summary of frequently used output quantities (continued). QUANTITY X∗ MASS FLUX MASS FLUX Y∗ MASS FLUX Z∗ MASS FRACTION∗ MIXTURE FRACTION MPUA∗∗ MPUA_Z∗ MPUV∗∗ MPUV_Z∗ NORMAL VELOCITY NUMBER OF PARTICLES OPEN NOZZLES OPTICAL DENSITY PATH OBSCURATION PARTICLE AGE PARTICLE DIAMETER PARTICLE FLUX X∗∗ PARTICLE FLUX Y∗∗ PARTICLE FLUX Z∗∗ PARTICLE MASS PARTICLE TEMPERATURE PARTICLE VELOCITY PRESSURE PRESSURE COEFFICIENT PRESSURE ZONE RADIATIVE HEAT FLUX RADIATIVE HEAT FLUX GAS RADIOMETER RELATIVE HUMIDITY SENSIBLE ENTHALPY SOLID CONDUCTIVITY SOLID DENSITY SOLID SPECIFIC HEAT SPECIFIC ENTHALPY SPECIFIC HEAT SPECIFIC SENSIBLE ENTHALPY SPRINKLER LINK TEMPERATURE SURFACE DENSITY SURFACE DEPOSITION∗ TEMPERATURE THERMOCOUPLE TIME TIME STEP Symbol ρuYα ρvYα ρwYα Yα Z Section 16.9 Section 16.9 Section 16.9 Section 16.9 Wall normal velocity Section 16.10.19 Section 16.10.19 Section 16.10.2 Section 15.3.6 Section 16.9 Section 16.9 Section 16.9 Section 16.9 Section 16.9 Section 16.9 Section 16.9 Section 16.9 Perturbation pressure, p̃ − p∞ Section 16.10.13 Section 9.3 Section 16.10.5 Section 16.10.5 Section 16.10.5 Section 12.1.1 Section 16.10.18 Section 16.2.3 Section 16.2.3 Section 16.2.3 Section 16.10.18 cp Section 16.10.18 Section 15.3.1 Section 16.10.8 Section 12.5 Section 16.10.4 Section 16.10.4 Section 15.1 Section 16.10.19 243 Units kg/(m2 · s) kg/(m2 · s) kg/(m2 · s) kg/kg kg/kg kg/m2 kg/m2 kg/m3 kg/m3 m/s 1/m % s µm kg/(m2 · s) kg/(m2 · s) kg/(m2 · s) kg ◦C m/s Pa kW/m2 kW/m2 kW/m2 % kJ/m3 W/(m · K) kg/m3 kJ/(kg · K) kJ/kg kJ/(kg · K) kJ/kg ◦C kg/m2 kg/m2 ◦C ◦C s s File Type D,I,P,S D,I,P,S D,I,P,S D,I,P,S D,I,P,S B,D B,D D,P,S D,P,S D,B D D D,I,P,S D PA PA P,S P,S P,S PA PA PA D,I,P,S B,D D,S B,D D B,D D,I,P,S D,I,P,S D D D D,I,P,S D,I,P,S D,I,P,S D B,D B,D D,I,P,S D D D Table 16.3: Summary of frequently used output quantities (continued). QUANTITY TRANSMISSION U-VELOCITY V-VELOCITY W-VELOCITY UPPER TEMPERATURE VELOCITY∗∗∗ VISCOSITY VISIBILITY VOLUME FLOW VOLUME FLOW WALL VOLUME FRACTION∗∗∗∗ WALL CLOCK TIME WALL CLOCK TIME ITERATIONS WALL TEMPERATURE WALL THICKNESS ∗ ∗∗ ∗∗∗ ∗∗∗∗ Symbol Section 15.3.6 Gas velocity component, u Gas velocity component, v Gas velocity component, w Section 16.10.3 Gas velocity µ Section 16.10.2 Section 16.10.10 Section 16.10.10 Xα Section 16.10.19 Section 16.10.19 Surface temperature Section 16.10.8 Units %/m m/s m/s m/s ◦C m/s kg/(m · s) m m3 /s m3 /s mol/mol s s ◦C m File Type D D,I,P,S D,I,P,S D,I,P,S D D,I,P,S D,I,P,S D,I,P,S D D D,I,P,S D D B,D B,D Quantity requires the specification of a gas species using SPEC_ID. Quantity requires the specification of a particle name using PART_ID. Add VELO_INDEX=1 to the input line if you want to multiply the velocity by the sign of u. Use the indices 2 and 3 for v and w, respectively. Quantity requires the specification of a gas species using SPEC_ID. Do not use for MIXTURE FRACTION. 244 16.13 Summary of Infrequently-Used Output Quantities Table 16.4 below lists some less often used output quantities. These are mainly used for diagnostic purposes. Explanations for most can be found in Volume 1 of the FDS Technical Reference Guide [1]. Table 16.4: Summary of infrequently used output quantities. QUANTITY ADA∗∗ ADA_Z∗ ADD∗∗ ADD_Z∗ ADT∗∗ ADT_Z∗ C_SMAG CABLE TEMPERATURE CELL INDEX I CELL INDEX J CELL INDEX K CELL REYNOLDS NUMBER CELL U CELL V CELL W CFL CFL MAX CHEMICAL SUBITERATIONS DIFFUSIVITY∗ EMISSIVITY EXTINCTION F_X, F_Y, F_Z GAS DENSITY GAS TEMPERATURE H HEAT TRANSFER COEFFICIENT HRRPUL INTEGRATED INTENSITY KINETIC ENERGY KOLMOGOROV LENGTH SCALE MACH NUMBER MASS FLUX WALL CELL MAXIMUM VELOCITY ERROR MIXING TIME NORMALIZED HEATING RATE NORMALIZED HEAT RELEASE RATE NORMALIZED MASS NORMALIZED MASS LOSS RATE∗ PARTICLE PHASE Symbol Average Droplet (cross sectional) Area Average Droplet (cross sectional) Area Average Droplet Diameter Average Droplet Diameter Average Droplet Temperature Average Droplet Temperature Smagorinsky coefficient Inner temperature of cable Mesh cell index in x Mesh cell index in y Mesh cell index in z Section 16.10.21 (ui, j,k + ui−1, j,k )/2 (vi, j,k + vi, j−1,k )/2 (wi, j,k + wi, j,k−1 )/2 Section 16.10.19 Section 16.10.19 Section 12.4 Species diffusivity Surface emissivity (usually constant) Section 16.10.22 Momentum terms, Fx , Fy , Fz Gas Density near wall Gas Temperature near wall H = |u|2 /2 + p̃/ρ Convective heat transfer R 000 q̇ dx dy R U = I ds (u2 + v2 + w2 )/2 Section p 16.10.21 |u|/ (R/W )T γ ρun at a wall cell face Section 6.6 Combustion mixing time Section 16.10.8 Section 16.10.8 Section 16.10.8 Section 16.10.8 Orientation of droplet 245 Units m2 /m3 m2 /m3 µm µm ◦C ◦C ◦C m/s m/s m/s m2 /s m/s2 kg/m3 ◦C (m/s)2 W/(m2 · K) kW/m kW/m2 (m/s)2 m kg/(m2 · s) m/s s W/g W/g 1/s File Type D,I,P,S D,I,P,S D,I,P,S D,I,P,S D,I,P,S D,I,P,S D,I,P,S D D,S D,S D,S D,I,P,S D,I,S D,I,S D,I,S D,I,P,S D D,S D,I,S B,D D,S D,I,P,S B,D B,D D,I,P,S B,D D D,I,P,S D,I,P,S D,I,P,S S,D B,D D D,I,P,S B,D B,D B,D B,D PA Table 16.4: Summary of infrequently used output quantities (continued). QUANTITY PARTICLE RADIATION LOSS PDPA PRESSURE ITERATIONS QABS∗∗ QABS_Z∗ QSCA∗∗ QSCA_Z∗ RADIATION LOSS STRAIN RATE STRAIN RATE X STRAIN RATE Y STRAIN RATE Z SUBGRID KINETIC ENERGY SUM LUMPED MASS FRACTIONS SUM PRIMITIVE MASS FRACTIONS VELOCITY ERROR VN VN MAX VORTICITY X VORTICITY Y VORTICITY Z WALL VISCOSITY WAVELET ERROR∗∗∗ YPLUS ∗ ∗∗ ∗∗∗ Symbol ∇ · q00r due to Lagrangian particles Droplet diagnostics No. pressure iterations Absorption efficiency of droplets Absorption efficiency of droplets Scattering efficiency of droplets Scattering efficiency of droplets ∇ · q00r 2(Si j Si j − 1/3(∇ · u)2 ) ∂ w/∂ y + ∂ v/∂ z ∂ u/∂ z + ∂ w/∂ x ∂ v/∂ x + ∂ u/∂ y Section 16.10.21 ∑i Zi (should be 1) ∑α Yα (should be 1) Section 6.6 Section 16.10.19 Section 16.10.19 ∂ w/∂ y − ∂ v/∂ z ∂ u/∂ z − ∂ w/∂ x ∂ v/∂ x − ∂ u/∂ y Near-wall viscosity, µw Section 16.10.21 Section 16.10.14 Quantity requires the specification of a gas species using SPEC_ID. Quantity requires the specification of a particle name using PART_ID. Quantity requires specification of an additional scalar using QUANTITY2. 246 Units kW/m3 kW/m3 1/s 1/s 1/s 1/s m2 /s2 1/s 1/s 1/s kg/(m · s) File Type D,I,P,S D D D,I,P,S D,I,P,S D,I,P,S D,I,P,S D,I,P,S D,I,P,S D,I,P,S D,I,P,S D,I,P,S D,S D,S D,S B D,I,P,S D D,I,P,S D,I,P,S D,I,P,S B,D S B,D 16.14 Summary of HVAC Output Quantities Table 16.5 summarizes the various Output Quantities for HVAC systems. Quantities for a duct require the specification of a DUCT_ID, and quantities for a node require the specification of a NODE_ID. Mass and volume fraction outputs also require the specification of a SPEC_ID. Table 16.5: Summary of HVAC output quantities. QUANTITY AIRCOIL HEAT EXCHANGE DUCT DENSITY DUCT MASS FLOW DUCT MASS FRACTION DUCT TEMPERATURE DUCT VELOCITY DUCT VOLUME FLOW DUCT VOLUME FRACTION FAN PRESSURE FILTER LOADING FILTER LOSS NODE DENSITY NODE MASS FRACTION NODE PRESSURE NODE PRESSURE DIFFERENCE NODE TEMPERATURE NODE VOLUME FRACTION Symbol Heat exchange rate for an aircoil Density of the flow in a duct Mass flow in a duct Mass fraction of a species in a duct Temperature of the flow in a duct Velocity of a duct Volumetric flow in a duct Volume fraction of a species in a duct Pressure output of a fan in a duct Loading of a species in a filter Flow loss through a filter Density of the flow through a node Mass fraction of a species in a node Pressure of a node Pressure difference between two nodes Temperature of the flow though a node Volume fraction of a species in a node 247 Units kW kg/m3 kg/s kg/kg ◦C m/s m3 /s mol/mol Pa kg kg/m3 kg/kg Pa Pa ◦C mol/mol 248 Chapter 17 Alphabetical List of Input Parameters This appendix lists all of the input parameters for FDS in separate tables grouped by namelist, these tables are in alphabetical order along with the parameters within them. This is intended to be used as a quick reference and does not replace reading the detailed description of the parameters in the main body of this guide. See Table 5.1 for a cross-reference of relevant sections and the tables in this appendix. The reason for this statement is that many of the listed parameters are mutually exclusive – specifying more than one can cause the program to either fail or run in an unpredictable manner. Also, some of the parameters trigger the code to work in a certain mode when specified. For example, specifying the thermal conductivity of a solid surface triggers the code to assume the material to be thermally-thick, mandating that other properties be specified as well. Simply prescribing as many properties as possible from a handbook is bad practice. Only prescribe those parameters which are necessary to describe the desired scenario. Note that you may use the character string FYI on any namelist line to make a note or comment. 249 17.1 BNDF (Boundary File Parameters) Table 17.1: For more information see Section 16.5. BNDF (Boundary File Parameters) CELL_CENTERED PART_ID PROP_ID QUANTITY SPEC_ID STATISTICS Logical Character Character Character Character Character Section 16.5 Section 16.12 Section 16.5 Section 16.12 Section 16.12 Section 16.5 .FALSE. 17.2 CLIP (Clipping Parameters) Table 17.2: For more information see Section 6.7. CLIP (Specified Upper and Lower Limits) Real Real Real Real MAXIMUM_DENSITY MAXIMUM_TEMPERATURE MINIMUM_DENSITY MINIMUM_TEMPERATURE kg/m3 ◦C kg/m3 ◦C Section 6.7 Section 6.7 Section 6.7 Section 6.7 17.3 CSVF (Comma Separated Velocity Files) Table 17.3: For more information see Section 6.4.5. CSVF (Comma Delimited Output Files) UVWFILE Character Section 6.4.5 17.4 CTRL (Control Function Parameters) Table 17.4: For more information see Section 15.5. CTRL (Control Function Parameters) CONSTANT DELAY DIFFERENTIAL_GAIN EVACUATION FUNCTION_TYPE ID Real Real Real Logical Character Character Section 15.5.6 Section 15.5.9 Section 15.5.7 Reference [53] Section 15.4 Section 15.5 250 s 0. 0. .FALSE. Table 17.4: Continued CTRL (Control Function Parameters) INITIAL_STATE INPUT_ID INTEGRAL_GAIN LATCH N ON_BOUND PROPORTIONAL_GAIN RAMP_ID SETPOINT(2) TARGET_VALUE TRIP_DIRECTION Logical Char. Array Real Logical Integer Character Real Character Real Real Integer Section 15.4 Section 15.5 Section 15.5.7 Section 15.4 Section 15.5 Section 15.5.3 Section 15.5.7 Section 15.5.5 Section 15.4 Section 15.5.7 Section 15.4 .FALSE. 0. .TRUE. 1 LOWER 0. 0. 1 17.5 DEVC (Device Parameters) Table 17.5: For more information see Section 15.1. DEVC (Device Parameters) BYPASS_FLOWRATE CONVERSION_FACTOR COORD_FACTOR CTRL_ID DELAY DEPTH DEVC_ID DRY DUCT_ID EVACUATION FLOWRATE HIDE_COORDINATES ID INITIAL_STATE INIT_ID IOR LATCH MATL_ID NODE_ID NO_UPDATE_DEVC_ID NO_UPDATE_CTRL_ID ORIENTATION ORIENTATION_NUMBER OUTPUT PART_ID Real Real Real Character Real Real Character Logical Character Logical Real Logical Character Logical Character Integer Logical Character Character(2) Character Character Real Triplet Integer Logical Character Section 15.3.7 Section 15.2 Section 15.2 Section 15.6.1 Section 15.3.7 Section 16.10.8 Sections 15.3.7 and 15.6.1 Section 16.10.15 Section 9.2 Reference [53] Section 15.3.7 Section 16.2.2 Section 15.1 Section 15.4 Section 14.4 Section 15.1 Section 15.4 Section 16.10.8 Section 9.2 Section 15.6.2 Section 15.6.2 Section 15.1 Section 16.9 Section 15.2 Section 16.12 251 kg/s 0 1 1 s m 0 0 .FALSE. .FALSE. kg/s 0 .FALSE. .FALSE. .TRUE. 0,0,-1 1 .TRUE. Table 17.5: Continued DEVC (Device Parameters) PIPE_INDEX POINTS PROP_ID QUANTITY QUANTITY2 QUANTITY_RANGE RELATIVE R_ID ROTATION SETPOINT STATISTICS STATISTICS_START SMOOTHING_FACTOR SPEC_ID SURF_ID TIME_AVERAGED TIME_HISTORY TRIP_DIRECTION UNITS VELO_INDEX XB(6) XYZ(3) X_ID Y_ID Z_ID Integer Integer Character Character Character Real(2) Logical Character Real Real Character Real Real Character Character Logical Logical Integer Character Integer Real Sextuplet Real Triplet Character Character Character Section 15.3.1 Section 16.2.2 Section 15.1 Section 15.1 Section 16.2.2 Section 16.10.10 Section 15.2 Section 16.2.2 Section 15.1 Section 15.4 Section 16.10.10 Section 16.10.12 Section 15.4 Section 16.12 Section 16.10.10 Section 15.2 Section 16.2.2 Section 15.4 Section 15.2 Section 16.10.17 Section 16.10.10 Section 15.1 Section 16.2.2 Section 16.2.2 Section 16.2.2 1 1 -1.E50,1.E50 .FALSE. deg. 0 s T_BEGIN 0 .TRUE. 1 0 m m ID-x ID-y ID-z 17.6 DUMP (Output Parameters) Table 17.6: For more information see Section 16.1. DUMP (Output Parameters) CLIP_RESTART_FILES COLUMN_DUMP_LIMIT CTRL_COLUMN_LIMIT DEVC_COLUMN_LIMIT DT_BNDF DT_CPU DT_CTRL DT_DEVC DT_DEVC_LINE DT_FLUSH DT_HRR Logical Logical Integer Integer Real Real Real Real Real Real Real Section 6.4.4 Section 15.2 Section 15.2 Section 15.2 Section 16.1 Section 20.5 Section 16.1 Section 16.1 Section 16.2.2 Section 16.1 Section 16.1 252 .TRUE. .FALSE. s s s s s s s 254 254 2 ∆t /NFRAMES 1000000 ∆t /NFRAMES ∆t /NFRAMES ∆t /2 ∆t /NFRAMES ∆t /NFRAMES Table 17.6: Continued DUMP (Output Parameters) DT_ISOF DT_MASS DT_PART DT_PL3D DT_PROF DT_RESTART DT_SL3D DT_SLCF EB_PART_FILE FLUSH_FILE_BUFFERS MASS_FILE MAXIMUM_PARTICLES NFRAMES PLOT3D_PART_ID(5) PLOT3D_QUANTITY(5) PLOT3D_SPEC_ID(5) PLOT3D_VELO_INDEX RENDER_FILE SIG_FIGS SIG_FIGS_EXP SMOKE3D SMOKE3D_QUANTITY SMOKE3D_SPEC_ID STATUS_FILES SUPPRESS_DIAGNOSTICS UVW_TIMER VELOCITY_ERROR_FILE WRITE_XYZ Real Real Real Real Real Real Real Real Logical Logical Logical Integer Integer Char. Quint Char. Quint Char. Quint Int. Quint Character Integer Integer Logical Character Character Logical Logical Real Vector (10) Logical Logical Section 16.1 Section 16.1 Section 16.1 Section 16.1 Section 16.1 Section 16.1 Section 16.1 Section 16.1 Section 16.1 Section 16.1 Section 16.1 Section 16.1 Section 16.1 Section 16.7 Section 16.7 Section 16.7 Section 16.10.17 Reference [2] Section 16.10.20 Section 16.10.20 Section 16.8 Section 16.8 Section 16.8 Section 16.1 Section 3.3 Section 6.4.5 Section 16.10.19 Section 16.7 s s s s s s s s ∆t /NFRAMES ∆t /NFRAMES ∆t /NFRAMES 1.E10 ∆t /NFRAMES 1000000. ∆t /5 ∆t /NFRAMES .FALSE. .TRUE. .FALSE. 1000000 1000 0 8 3 .TRUE. .FALSE. .FALSE. s .FALSE. .FALSE. ∆t=T_END-T_BEGIN 17.7 HEAD (Header Parameters) Table 17.7: For more information see Section 6.1. HEAD (Header Parameters) CHID TITLE Character Character Section 6.1 Section 16.7 253 ’output’ 17.8 HOLE (Obstruction Cutout Parameters) Table 17.8: For more information see Section 7.2.6. HOLE (Obstruction Cutout Parameters) COLOR CTRL_ID DEVC_ID EVACUATION ID MESH_ID MULT_ID RGB(3) TRANSPARENCY XB(6) Character Character Character Logical Character Character Character Integer Triplet Real Real Sextuplet Section 7.4 Section 7.2.6 Section 7.2.6 Reference [53] Identifier for input line Reference [53] Section 7.5 Section 7.4 Section 7.2.6 Section 7.5 m 17.9 HVAC (HVAC System Definition) Table 17.9: For more information see Section 9.2. HVAC (HVAC System Definition) AIRCOIL_ID AMBIENT AREA CLEAN_LOSS COOLANT_MASS_FLOW COOLANT_SPECIFIC_HEAT COOLANT_TEMPERATURE CTRL_ID DAMPER DEVC_ID DIAMETER DUCT_ID EFFICIENCY FAN_ID FILTER_ID FIXED_Q ID LEAK_ENTHALPY LENGTH LOADING LOADING_MULTIPLIER LOSS MASS_FLOW Character Logical Real Real Real Real Real Character Logical Character Real Character Array Real Array Character Character Real Character Logical Real Real Array Real Array Real Array Real Section 9.2.1 Section 9.2.3 Section 9.2.1 Section 9.2.5 Section 9.2.6 Section 9.2.6 Section 9.2.6 Sections 9.2.1, 9.2.4, 9.2.5 Sections 9.2.1, 9.2.2 Sections 9.2.1, 9.2.4, 9.2.5 Section 9.2.1 Section 9.2.3 Sections 9.2.5, 9.2.6 Section 9.2.1 Section 9.2.3 Section 9.2.6 Section 9.2 Section 9.3.2 Section 9.2.1 Section 9.2.5 Section 9.2.5 Sections 9.2.1 – 9.2.5 Section 9.2.1 254 .FALSE. m2 kg/s kJ/(kg · K) ◦C .FALSE. m 1.0 kW .FALSE. m kg 1/kg kg/s 0.0 1.0 0.0 Table 17.9: Continued HVAC (HVAC System Definition) Real Real Character Doublet Real Character Character Logical Real Character Array Real Real Real Character Character Character Real Real Triplet MAX_FLOW MAX_PRESSURE NODE_ID PERIMETER RAMP_ID RAMP_LOSS REVERSE ROUGHNESS SPEC_ID TAU_AC TAU_FAN TAU_VF TYPE_ID VENT_ID VENT2_ID VOLUME_FLOW XYZ Section 9.2.4 Section 9.2.4 Section 9.2.1 Section 9.2.1 Sections 9.2.1, 9.2.5, 9.2.4 Sections 9.2.1, 9.2.2 Section 9.2.1 Section 9.2.1 Section 9.2.5 Section 9.2.6 Section 9.2.4 Section 9.2.1 Section 9.2 Section 9.2.3 Section 9.3.2 Section 9.2.1, 9.2.4 Section 9.2.3 m3 /s Pa m .FALSE. m 0.0 s s s 1.0 1.0 1.0 m3 /s m 0.0 17.10 INIT (Initial Conditions) Table 17.10: For more information see Section 6.5. INIT (Initial Conditions) CELL_CENTERED CTRL_ID DENSITY DEVC_ID DIAMETER DT_INSERT DX DY DZ HEIGHT HRRPUV ID MASS_FRACTION(N) MASS_PER_TIME MASS_PER_VOLUME MULT_ID N_PARTICLES N_PARTICLES_PER_CELL PART_ID Logical Character Real Character Real Real Real Real Real Real Real Character Real Array Real Real Character Integer Integer Character Section 14.5.3 Section 14.5.3 Section 6.5 Section 14.5.3 Section 14.5.3 Section 14.5.3 Section 14.5.3 Section 14.5.3 Section 14.5.3 Section 14.5.3 Section 6.5 Section 14.4 Section 6.5 Section 14.5.3 Section 14.5.3 Section 7.5 Section 14.5.3 Section 14.5.3 Section 14.5.3 255 .FALSE. kg/m3 Ambient µm s m m m m kW/m3 0. 0. 0. kg/kg kg/s kg/m3 Ambient 1 0 0 Table 17.10: Continued INIT (Initial Conditions) PARTICLE_WEIGHT_FACTOR RADIUS SHAPE SPEC_ID(N) TEMPERATURE UVW(3) XB(6) XYZ(3) Real Real Character Character Array Real Real Triplet Real Sextuplet Real Triplet Section 14.5.3 Section 14.5.3 Section 14.5.3 Section 6.5 Section 6.5 Section 14.5.3 Section 6.5 Section 14.5.3 1. m ’BLOCK’ °C m/s m m TMPA 0. 17.11 ISOF (Isosurface Parameters) Table 17.11: For more information see Section 16.6. ISOF (Isosurface Parameters) Character Integer Character Real Array Integer QUANTITY REDUCE_TRIANGLES SPEC_ID VALUE(I) VELO_INDEX Section 16.6 Reference [2] Section 16.6 Section 16.6 Section 16.10.17 1 0 17.12 MATL (Material Properties) Table 17.12: For more information see Section 8.3. MATL (Material Properties) A(:) ABSORPTION_COEFFICIENT ALLOW_SHRINKING ALLOW_SWELLING BOILING_TEMPERATURE CONDUCTIVITY CONDUCTIVITY_RAMP DENSITY E(:) EMISSIVITY GAS_DIFFUSION_DEPTH(:) HEATING_RATE(:) HEAT_OF_COMBUSTION(:) HEAT_OF_REACTION(:) ID Real array Real Logical Logical Real Real Character Real Real array Real Real array Real array Real array Real array Character Section 8.5 Section 8.3.2 Section 8.5.3 Section 8.5.3 Section 8.5.7 Section 8.3.2 Section 8.3.2 Section 8.3.2 Section 8.5 Section 8.3.2 Section 8.5 Section 8.5 Section 8.5 Section 8.5 Section 8.1 256 1/s 1/m 50000. .TRUE. .TRUE. ◦C W/(m · K) kg/m3 kJ/kmol m ◦ C/min kJ/kg kJ/kg 5000. 0. 0. 0.9 0.001 5. 0. Table 17.12: Continued MATL (Material Properties) MATL_ID(:,:) NU_MATL(:,:) NU_SPEC(:,:) N_REACTIONS N_O2(:) N_S(:) N_T(:) PCR(:) PYROLYSIS_RANGE(:) REFERENCE_RATE(:) REFERENCE_TEMPERATURE(:) SPECIFIC_HEAT SPECIFIC_HEAT_RAMP SPEC_ID(:,:) THRESHOLD_SIGN(:) THRESHOLD_TEMPERATURE(:) Character Real array Real array Integer Real array Real array Real array Logical array Real array Real array Real array Real Character Character Real array Real array Section 8.5 Section 8.5 Section 8.5 Section 8.5 Section 8.5 Section 8.5 Section 8.5 Section 8.5 Section 8.5 Section 8.5 Section 8.5 Section 8.3.2 Section 8.3.2 Section 8.5 Section 8.5 Section 8.5 kg/kg kg/kg 0. 0. 0 0. 1. 0. .FALSE. ◦C 80. 1/s ◦C kJ/(kg · K) 0. ◦C 1.0 -273.15 17.13 MESH (Mesh Parameters) Table 17.13: For more information see Section 6.3. MESH (Mesh Parameters) COLOR CYLINDRICAL EVACUATION EVAC_HUMANS EVAC_Z_OFFSET ID IJK LEVEL MPI_PROCESS N_THREADS MULT_ID RGB XB(6) Character Logical Logical Logical Real Character Integer Triplet Integer Integer Integer Character Integer Triplet Real Sextuplet Section 6.3.3 Section 6.3.2 Reference [53] Reference [53] Reference [53] Reference [53] Section 6.3.1 For future use Section 6.3.3 Section 6.3.3 Section 7.5 Section 6.3.3 Section 6.3.1 17.14 MISC (Miscellaneous Parameters) 257 ’BLACK’ .FALSE. .FALSE. .FALSE. m 1 10,10,10 0 m 0,0,0 0,1,0,1,0,1 Table 17.14: For more information see Section 6.4. MISC (Miscellaneous Parameters) ALLOW_SURFACE_PARTICLES ALLOW_UNDERSIDE_PARTICLES ASSUMED_GAS_TEMPERATURE ASSUMED_GAS_TEMPERATURE_RAMP BAROCLINIC BNDF_DEFAULT CNF_CUTOFF CFL_MAX CFL_MIN CFL_VELOCITY_NORM CHECK_HT CHECK_VN CLIP_MASS_FRACTION C_DEARDORFF C_SMAGORINSKY C_VREMAN CONSTANT_SPECIFIC_HEAT_RATIO DNS DRIFT_FLUX DT_HVAC DT_MEAN_FORCING EVACUATION_DRILL EVACUATION_MC_MODE EVAC_PRESSURE_ITERATIONS EVAC_TIME_ITERATIONS FLUX_LIMITER FORCE_VECTOR(3) FREEZE_VELOCITY GAMMA GRAVITATIONAL_DEPOSITION GRAVITATIONAL_SETTLING GROUND_LEVEL GVEC H_F_REFERENCE_TEMPERATURE HUMIDITY IBLANK_SMV INITIAL_UNMIXED_FRACTION LAPSE_RATE MAX_CHEMISTRY_ITERATIONS MAX_LEAK_PATHS MAXIMUM_VISIBILITY MEAN_FORCING(3) MPI_TIMEOUT Logical Logical Real Character Logical Logical Real Real Real Integer Logical Logical Logical Real Real Real Logical Logical Logical Real Real Logical Logical Integer Integer Integer Real Logical Real Logical Logical Real Real triplet Real Real Logical Real Real Integer Integer Real Logical Real 258 Section 14.6.1 Section 14.6.1 Section 8.6 Section 8.6 Section 6.4.8 Section 16.5 Section 14.3.3 Section 6.4.10 Section 6.4.10 Section 6.4.10 Section 6.4.10 Section 6.4.10 Section 6.7 Section 6.4.9 Section 6.4.9 Section 6.4.9 Section 11.1.2 Section 6.4.1 Section 12.5 Section 9.2 Section 6.4.2 Reference [53] Reference [53] Reference [53] Reference [53] Section 6.4.11 Section 6.4.3 Section 6.4.6 Section 11.1.2 Section 12.5 Section 12.5 Section 9.6 Section 6.4.7 Section 16.10.18 Section 11.1.1 Section 16.4 Section 12.1.3 Section 9.6 Section 12.4 Section 9.3.2 Section 16.10.2 Section 6.4.2 Section 16.10.19 .TRUE. .FALSE. ◦C .TRUE. .TRUE. 0.005 1.0 0.8 0 (LES), 1 (DNS) .FALSE. .FALSE. .FALSE. 0.1 0.20 0.07 .FALSE. .FALSE. .TRUE. s s 1.E10 .FALSE. .FALSE. 50 50 2 0. .FALSE. 1.4 .TRUE. .TRUE. m m/s2 °C % 0. 0,0,-9.81 25. 40. .TRUE. °C/m m 1.0 0 1000 200 30 .FALSE. s 10. Table 17.14: Continued MISC (Miscellaneous Parameters) NOISE NOISE_VELOCITY NO_EVACUATION OVERWRITE PARTICLE_CFL PARTICLE_CFL_MAX POROUS_FLOOR PR PROJECTION P_INF RAMP_GX RAMP_GY RAMP_GZ RESTART RESTART_CHID RICHARDSON_ERROR_TOLERANCE RUN_AVG_FAC SC SHARED_FILE_SYSTEM SMOKE_ALBEDO SOLID_PHASE_ONLY STRATIFICATION SUPPRESSION TEXTURE_ORIGIN(3) THERMOPHORETIC_DEPOSITION THICKEN_OBSTRUCTIONS TMPA TURBULENCE_MODEL TURBULENT_DEPOSITION U0,V0,W0 VERBOSE VISIBILITY_FACTOR VN_MAX VN_MIN Y_CO2_INFTY Y_O2_INFTY Logical Real Logical Logical Logical Real Logical Real Logical Real Character Character Character Logical Character Real Real Real Logical Real Logical Logical Logical Real Triplet Logical Logical Real Character Logical Reals Logical Real Real Real Real Real Section 6.4.1 Section 6.4.1 Reference [53] Section 6.4.1 Section 6.4.10 Section 6.4.10 Section 15.3.1 Section 6.4.9 Formal projection Section 6.4.1 Section 6.4.7 Section 6.4.7 Section 6.4.7 Section 6.4.4 Section 6.4.4 Section 12 Section 14.3.2 Section 6.4.9 Section 6.3.3 Reference [2] Section 8.6 Section 9.6 Section 12.1.4 Section 7.4.2 Section 12.5 Section 7.2.1 Section 6.4.1 Section 6.4.9 Section 12.5 Section 6.4.1 Section 6.3.3 Section 16.10.2 Section 6.4.10 Section 6.4.10 Section 12.1.1 Section 12.1.1 17.15 MULT (Multiplier Function Parameters) 259 .TRUE. m/s 0.005 .FALSE. .TRUE. .FALSE. 1.0 .TRUE. 0.5 .FALSE. Pa 101325 .FALSE. CHID 1.0 E-3 0.5 0.5 .TRUE. 0.3 .FALSE. .TRUE. .TRUE. m (0.,0.,0.) .TRUE. .FALSE. °C 20. ’DEARDORFF’ .TRUE. m/s 0. kg/kg kg/kg 3 0.5 0.4 0.000595 0.232378 Table 17.15: For more information see Section 7.5. MULT (Multiplier Function Parameters) DX DXB DX0 DY DY0 DZ DZ0 ID I_LOWER I_UPPER J_LOWER J_UPPER K_LOWER K_UPPER N_LOWER N_UPPER Real Real Sextuplet Real Real Real Real Real Character Integer Integer Integer Integer Integer Integer Integer Integer Spacing in the x direction Spacing for all 6 coordinates Translation in the x direction Spacing in the y direction Translation in the y direction Spacing in the z direction Translation in the z direction Identification tag Lower array bound, x direction Upper array bound, x direction Lower array bound, y direction Upper array bound, y direction Lower array bound, z direction Upper array bound, z direction Lower sequence bound Upper sequence bound m m m m m m m 0. 0. 0. 0. 0. 0. 0. 0 0 0 0 0 0 0 0 17.16 OBST (Obstruction Parameters) Table 17.16: For more information see Section 7.2. OBST (Obstruction Parameters) ALLOW_VENT BNDF_FACE(-3:3) BNDF_OBST BULK_DENSITY COLOR CTRL_ID DEVC_ID EVACUATION ID MESH_ID MULT_ID OUTLINE OVERLAY PERMIT_HOLE PROP_ID REMOVABLE RGB(3) SURF_ID SURF_ID6(6) SURF_IDS(3) Logical Logical Array Logical Real Character Character Character Logical Character Character Character Logical Logical Logical Character Logical Integer Triplet Character Character Sextuplet Character Triplet 260 Section 7.2.1 Section 16.5 Section 16.5 Section 8.5.8 Section 7.2.1 Section 15.4.2 Section 15.4.2 Reference [53] Section 7.2.1 Reference [53] Section 7.5 Section 7.2.1 Section 7.2.1 Section 7.2.6 Reference [2] Section 7.2.6 Section 7.2.1 Section 7.2.1 Section 7.2.1 Section 7.2.1 .TRUE. .TRUE. .TRUE. kg/m3 .FALSE. .FALSE. .TRUE. .TRUE. .TRUE. Table 17.16: Continued OBST (Obstruction Parameters) TEXTURE_ORIGIN(3) THICKEN TRANSPARENCY XB(6) Real Triplet Logical Real Real Sextuplet Section 7.4.2 Section 7.2.1 Section 7.2.1 Section 7.2.1 m (0.,0.,0.) .FALSE. 1 m 17.17 PART (Lagrangian Particles/Droplets) Table 17.17: For more information see Chapter 14. PART (Lagrangian Particles/Droplets) AGE BREAKUP BREAKUP_CNF_RAMP_ID BREAKUP_DISTRIBUTION BREAKUP_GAMMA_D BREAKUP_RATIO BREAKUP_SIGMA_D CHECK_DISTRIBUTION CNF_RAMP_ID COLOR COMPLEX_REFRACTIVE_INDEX CTRL_ID DENSE_VOLUME_FRACTION DEVC_ID DIAMETER DISTRIBUTION DRAG_COEFFICIENT(3) DRAG_LAW FREE_AREA_FRACTION GAMMA_D HEAT_OF_COMBUSTION HORIZONTAL_VELOCITY ID INITIAL_TEMPERATURE MASSLESS MAXIMUM_DIAMETER MINIMUM_DIAMETER MONODISPERSE N_STRATA ORIENTATION(1:3,:) PERMEABILITY(3) PROP_ID Real Logical Character Character Real Real Real Logical Character Character Real Character Real Character Real Character Real Array Character Real Real Real Real Character Real Logical Real Real Logical Integer Real Array Real Array Character 261 Section 16.9 Section 14.3.4 Section 14.3.4 Section 14.3.4 Section 14.3.4 Section 14.3.4 Section 14.3.4 Section 14.3.3 Section 14.3.3 Section 16.9 Section 14.3.2 Section 14.5.1 Section 14.4.2 Section 14.5.1 Section 14.3.3 Section 14.3.3 Section 14.4.2 Section 14.4.2 Section 14.4.8 Section 14.3.3 Section 14.3.5 Section 14.6.1 Section 14.1 Section 14.3.1 Section 14.2 Section 14.3.3 Section 14.3.3 Section 14.3.3 Section 14.3.3 Section 14.4 Section 14.4.7 Section 14.1 s 1 × 105 .FALSE. ’ROSIN...’ 2.4 3/7 .FALSE. ’BLACK’ 0.01 1 × 10−5 µm ’ROSIN...’ ’SPHERE’ 2.4 kJ/kg m/s 0.2 ◦C TMPA .FALSE. µm µm Infinite 20. .FALSE. 7 Table 17.17: Continued PART (Lagrangian Particles/Droplets) QUANTITIES(10) QUANTITIES_SPEC_ID(10) RADIATIVE_PROPERTY_TABLE REAL_REFRACTIVE_INDEX RGB(3) SAMPLING_FACTOR SECOND_ORDER_PARTICLE_TRANSPORT SIGMA_D SPEC_ID STATIC SURFACE_TENSION SURF_ID TURBULENT_DISPERSION VERTICAL_VELOCITY Character Character Real Real Integers Integer Logical Real Character Logical Real Character Logical Real Section 16.9 Section 16.9 Section 14.3.2 Section 14.3.2 Section 16.9 Section 16.9 Section 6.4.10 Section 14.3.3 Section 14.3.1 Section 14.4 Section 14.3.4 Section 14.4 Section 14.2 Section 14.6.1 1.33 1 .FALSE. .FALSE. 7.28 × 104 N/m .FALSE. m/s 0.5 17.18 PRES (Pressure Solver Parameters) Table 17.18: For more information see Section 6.6. PRES (Pressure Solver Parameters) CHECK_POISSON FISHPAK_BC(3) MAX_PRESSURE_ITERATIONS PRESSURE_RELAX_TIME PRESSURE_TOLERANCE RELAXATION_FACTOR SUSPEND_PRESSURE_ITERATIONS VELOCITY_TOLERANCE Logical Integer Integer Real Real Real Logical Real Section 6.6 Section 6.6 Section 6.6 Section 6.6 Section 6.6 Section 6.6 Section 6.6 Section 6.6 .FALSE. s m/s2 10 1. 1. 1. .TRUE. m/s 17.19 PROF (Wall Profile Parameters) Table 17.19: For more information see Section 16.3. PROF (Wall Profile Parameters) ID FORMAT_INDEX IOR QUANTITY XYZ Character Integer Real Character Real Triplet Section 16.3 Section 16.3 Section 16.3 Section 16.3 Section 16.3 262 1 m 17.20 PROP (Device Properties) Table 17.20: For more information see Section 15.3. PROP (Device Properties) ACTIVATION_OBSCURATION ACTIVATION_TEMPERATURE ALPHA_C ALPHA_E BETA_C BETA_E CHARACTERISTIC_VELOCITY C_FACTOR DENSITY DIAMETER EMISSIVITY FLOW_RAMP FLOW_RATE FLOW_TAU GAUGE_EMISSIVITY GAUGE_TEMPERATURE HEAT_TRANSFER_COEFFICIENT ID INITIAL_TEMPERATURE K_FACTOR LENGTH MASS_FLOW_RATE OFFSET OPERATING_PRESSURE ORIFICE_DIAMETER P0,PX(3),PXX(3,3) PARTICLES_PER_SECOND PARTICLE_VELOCITY PART_ID PDPA_END PDPA_HISTOGRAM PDPA_HISTOGRAM_CUMULATIVE PDPA_HISTOGRAM_LIMITS PDPA_HISTOGRAM_NBINS PDPA_INTEGRATE PDPA_M PDPA_N PDPA_NORMALIZE PDPA_RADIUS PDPA_START PRESSURE_RAMP Real Real Real Real Real Real Real Real Real Real Real Character Real Real Real Real Real Character Real Real Real Real Real Real Real Real Integer Real Character Real Logical Logical Real Array Integer Logical Integer Integer Logical Real Real Character Section 15.3.5 Section 15.3.1 Section 15.3.5 Section 15.3.5 Section 15.3.5 Section 15.3.5 Section 16.10.13 Section 15.3.1 Section 16.10.4 Section 16.10.4 Section 16.10.4 Section 15.3.1 Section 15.3.1 Section 15.3.1 Section 16.10.5 Section 16.10.5 Section 16.10.4 Section 15.3 Section 15.3.1 Section 15.3.1 Section 15.3.5 Section 15.3.1 Section 15.3.1 Section 15.3.1 Section 15.3.1 Section 15.3.3 Section 15.3.1 Section 15.3.1 Section 15.3.1 Section 16.10.7 Section 16.10.7 Section 16.10.7 Section 16.10.7 Section 16.10.7 Section 16.10.7 Section 16.10.7 Section 16.10.7 Section 16.10.7 Section 16.10.7 Section 16.10.7 Section 15.3.1 263 %/m ◦C 3.24 74. 1.8 0. 1. 1. 1. 0. 8908. 0.001 0.85 m/s (m/s)1/2 kg/m3 m L/min 0. 0.9 ◦C TMPA W/(m2 · K) ◦C TMPA 1 2 L/(min · bar ) m kg/s m bar m m/s 1. 1.8 m/s 0.05 1. 0. 0. 5000 0. s T_END .FALSE. .FALSE. 10 .TRUE. 0 0 .TRUE. m s 0. 0. Table 17.20: Continued PROP (Device Properties) QUANTITY RTI SMOKEVIEW_ID SMOKEVIEW_PARAMETERS SPEC_ID SPECIFIC_HEAT SPRAY_ANGLE(2,2) SPRAY_PATTERN_BETA SPRAY_PATTERN_MU SPRAY_PATTERN_SHAPE SPRAY_PATTERN_TABLE VELOCITY_COMPONENT Character Real Char. Array Char. Array Character Real Real Integer Integer Character Character Integer Section 15.3.1 Section 15.3.1 Section 15.7.1 Section 15.7.2 Section 15.3.5 Section 16.10.4 Section 15.3.1 Section 15.3.1 Section 15.3.1 Section 15.3.1 Section 15.3.1 Section 15.3.3 √ m·s 100. kJ/(kg · K) deg. deg. deg. 0.44 60.,75. 5 0 ’GAUSSIAN’ 17.21 RADI (Radiation Parameters) Table 17.21: For more information see Section 13.1. RADI (Radiation Parameters) ANGLE_INCREMENT BAND_LIMITS INITIAL_RADIATION_ITERATIONS KAPPA0 MIE_MINIMUM_DIAMETER MIE_MAXIMUM_DIAMETER MIE_NDG NMIEANG NUMBER_RADIATION_ANGLES PATH_LENGTH RADIATION RADIATION_ITERATIONS RADTMP TIME_STEP_INCREMENT WIDE_BAND_MODEL Integer Real Array Integer Real Real Real Integer Integer Integer Real Logical Integer Real Integer Logical Section 13.1.2 Section 13.2.3 Section 13.1.2 Section 13.2.2 Section 13.2.2 Section 13.2.2 Section 13.2.2 Section 13.2.2 Section 13.1.2 Section 13.2.3 Section 13.1 Section 13.1.2 Section 13.2.2 Section 13.1.2 Section 13.2.3 17.22 RAMP (Ramp Function Parameters) Table 17.22: For more information see Chapter 10. RAMP (Ramp Function Parameters) CTRL_ID Character 264 Section 15.6.1 µm 1/m µm µm 5 3 0 0.5 1.5×D 50 15 100 m .TRUE. ◦C 1 900 3 .FALSE. Table 17.22: Continued RAMP (Ramp Function Parameters) DEVC_ID F ID NUMBER_INTERPOLATION_POINTS T X Character Real Character Integer Real Real Section 15.6.1 Chapter 10 Chapter 10 Chapter 10 Chapter 10 Section 6.4.7 5000 s (or m ◦ C) 17.23 REAC (Reaction Parameters) Table 17.23: For more information see Chapter 12. REAC (Reaction Parameters) A AUTO_IGNITION_TEMPERATURE C CHECK_ATOM_BALANCE CO_YIELD CRITICAL_FLAME_TEMPERATURE E EPUMO2 EQUATION FORMULA FUEL FUEL_RADCAL_ID H HEAT_OF_COMBUSTION ID IDEAL N NU(:) N_S(:) N_T O RADIATIVE_FRACTION RAMP_CHI_R REAC_ATOM_ERROR REAC_MASS_ERROR SOOT_H_FRACTION SOOT_YIELD SPEC_ID_N_S(:) SPEC_ID_NU(:) THIRD_BODY Real Real Real Logical Real Real Real Real Character Character Character Character Real Real Character Logical Real Real Array Real Array Real Real Real Character Real Real Real Real Char. Array Char. Array Logical 265 Section 12.3 Section 12.1.4 Section 12.1.1 Section 12.2 Section 12.1.1 Section 12.1.4 Section 12.3 Section 12.1.2 Section 12.2.3 Section 12.1.1 Section 12.1.1 Section 12.1.1 Section 12.1.1 Section 12.1.2 Section 12.1.1 Section 12.1.1 Section 12.1.1 Section 12.3 Section 12.3 Section 12.3 Section 12.1.1 Section 13.1.1 Section 13.1.1 Section 12.2 Section 12.2 Section 12.1.1 Section 12.1.1 Section 12.3 Section 12.3 Section 12.3 ◦C 0 .TRUE. kg/kg ◦C kJ/kmol kJ/kg 0 1327 13100 0 kJ/kg .FALSE. 0 0 atoms kg/kg kg/kg 1.E-5 1.E-4 0.1 0.0 .FALSE. 17.24 SLCF (Slice File Parameters) Table 17.24: For more information see Section 16.4. SLCF (Slice File Parameters) CELL_CENTERED EVACUATION MAXIMUM_VALUE MESH_NUMBER MINIMUM_VALUE PART_ID PBX, PBY, PBZ QUANTITY QUANTITY2 SPEC_ID VECTOR VELO_INDEX XB(6) Logical Logical Real Integer Real Character Real Character Character Character Logical Integer Real Sextuplet Section 16.4 Reference [53] Reference [2] Section 16.4 Reference [2] Section 16.12 Section 16.4 Section 16.12 Section 16.12 Section 16.12 Section 16.4 Section 16.10.17 Section 16.4 .FALSE. .FALSE. m .FALSE. 0 m 17.25 SPEC (Species Parameters) Table 17.25: For more information see Section 11. SPEC (Species Parameters) AEROSOL ALIAS BACKGROUND CONDUCTIVITY CONDUCTIVITY_SOLID DENSITY_LIQUID DENSITY_SOLID DIFFUSIVITY ENTHALPY_OF_FORMATION EPSILONKLJ FIC_CONCENTRATION FLD_LETHAL_DOSE FORMULA HEAT_OF_VAPORIZATION H_V_REFERENCE_TEMPERATURE ID LUMPED_COMPONENT_ONLY MASS_EXTINCTION_COEFFICIENT MASS_FRACTION(:) MASS_FRACTION_0 Logical Character Logical Real Real Real Real Real Real Real Real Real Character Real Real Character Logical Real Real Array Real 266 Section 12.5 Section 11.1.3 Section 11 Section 11.1.2 Section 12.5 Section 14.3.1 Section 12.5 Section 11.1.2 Section 14.3.1 Section 11.1.2 Section 16.10.9 Section 16.10.9 Section 11.1.2 Section 14.3.1 Section 14.3.1 Section 11.1.1 Section 11.2 Section 15.3.5 Section 11.2 Section 11.1.1 .FALSE. .FALSE. W/(m · K) W/(m · K) kg/m3 kg/m3 m2 /s kJ/mol ppm ppm×min 0.26 1800. 0 0. 0. kJ/kg ◦C .FALSE. 0 0 0 Table 17.25: Continued SPEC (Species Parameters) MEAN_DIAMETER MELTING_TEMPERATURE MW PR_GAS PRIMITIVE RADCAL_ID RAMP_CP RAMP_CP_L RAMP_D RAMP_G_F RAMP_K RAMP_MU REFERENCE_ENTHALPY REFERENCE_TEMPERATURE SIGMALJ SPEC_ID(:) SPECIFIC_HEAT SPECIFIC_HEAT_LIQUID VAPORIZATION_TEMPERATURE VISCOSITY VOLUME_FRACTION(:) Real Real Real Real Logical Character Character Character Character Character Character Character Real Real Real Character Array Real Real Real Real Real Array Section 12.5 Section 14.3.1 Section 11.1.2 Section 11.1.2 Section 11.1.2 Section 11.1.3 Section 11.1.2 Section 14.3.1 Section 11.1.2 Section 11.1.2 Section 11.1.2 Section 11.1.2 Section 11.1.2 Section 11.1.2 Section 11.1.2 Section 11.2 Section 11.1.2 Section 14.3.1 Section 14.3.1 Section 11.1.2 Section 11.2 m ◦C g/mol 1.E-6 29. PR kJ/kg ◦C 25. 0 kJ/(kg · K) kJ/(kg · K) ◦C kg/(m · s) 17.26 SURF (Surface Properties) Table 17.26: For more information see Section 7.1. SURF (Surface Properties) ADIABATIC BACKING BURN_AWAY CELL_SIZE_FACTOR C_FORCED_CONSTANT C_FORCED_PR_EXP C_FORCED_RE C_FORCED_RE_EXP C_HORIZONTAL C_VERTICAL COLOR CONVECTION_LENGTH_SCALE CONVECTIVE_HEAT_FLUX CONVERT_VOLUME_TO_MASS DEFAULT Logical Character Logical Real Real Real Real Real Real Real Character Real Real Logical Logical Section 8.2.3 Section 8.3.3 Section 8.5.8 Section 8.3.8 Section 8.2.2 Section 8.2.2 Section 8.2.2 Section 8.2.2 Section 8.2.2 Section 8.2.2 Section 7.4 Section 8.2.2 Section 8.2.2 Section 9.1.6 Section 7.1 267 .FALSE. ’EXPOSED’ .FALSE. 1.0 0.0 0.0 0.0 0.0 1.52 1.31 m kW/m2 1. .FALSE. .FALSE. Table 17.26: Continued SURF (Surface Properties) DT_INSERT EMISSIVITY EMISSIVITY_BACK EVAC_DEFAULT EXTERNAL_FLUX E_COEFFICIENT FREE_SLIP GEOMETRY HEAT_OF_VAPORIZATION HEAT_TRANSFER_COEFFICIENT HEAT_TRANSFER_MODEL HRRPUA ID IGNITION_TEMPERATURE INNER_RADIUS INTERNAL_HEAT_SOURCE LAYER_DIVIDE LEAK_PATH LENGTH MASS_FLUX(:) MASS_FLUX_TOTAL MASS_FLUX_VAR MASS_FRACTION(:) MASS_TRANSFER_COEFFICIENT MATL_ID(NL,NC) MATL_MASS_FRACTION(NL,NC) MINIMUM_LAYER_THICKNESS MLRPUA N_LAYER_CELLS_MAX NET_HEAT_FLUX NO_SLIP NPPC PARTICLE_MASS_FLUX PART_ID PLE PROFILE RADIUS RAMP_EF RAMP_MF(:) RAMP_PART RAMP_Q RAMP_T RAMP_T_I Real Real Real Logical Real Real Logical Character Real Real Character Real Character Real Real Real Array Real Int. Pair Real Real Array Real Real Real Array Real Char. Array Real Array Real Real Integer Array Real Logical Integer Real Character Real Character Real Character Character Character Character Character Character 268 Section 14.5.1 Section 8.2.2 Section 8.3.3 Reference [53] Section 8.6 Section 14.6 Section 9.1.7 Section 8.3.7 Section 8.4.3 Section 8.2.2 Section 8.2.2 Section 8.4.1 Section 7.1 Section 8.4.3 Section 14.4.1 Section 8.3.6 Section 8.3.5 Section 9.3.2 Section 14.4.1 Section 9.1.6 Section 9.1.2 Section 9.1.9 Section 9.1.6 Section 8.5.7 Section 8.5 Section 8.5 Section 8.3.8 Section 8.4.1 Section 8.3.8 Section 8.2.2 Section 9.1.7 Section 14.5.1 Section 14.5.1 Section 14.5.1 Section 9.6 Section 9.5 Section 14.4.1 Section 10.1 Section 10.1 Section 10.1 Section 10.1 Section 10.1 Section 8.3.4 s 0.01 0.9 .FALSE. kW/m2 m2 /(kg · s) .FALSE. ’CARTESIAN’ kJ/kg W/(m2 · K) kW/m2 ◦C 5000. m kW/m3 N_LAYERS/2 m kg/(m2 · s) kg/(m2 · s) m/s m kg/(m2 · s) kW/m2 1.E-6 1000 .FALSE. kg/(m2 · s) 1 0.3 m Table 17.26: Continued SURF (Surface Properties) RAMP_V RAMP_V_X RAMP_V_Y RAMP_V_Z RGB(3) ROUGHNESS SPEC_ID SPREAD_RATE STRETCH_FACTOR(:) TAU_EF TAU_MF(:) TAU_PART TAU_Q TAU_T TAU_V TEXTURE_HEIGHT TEXTURE_MAP TEXTURE_WIDTH TGA_ANALYSIS TGA_FINAL_TEMPERATURE TGA_HEATING_RATE THICKNESS(NL) TMP_BACK TMP_FRONT TMP_INNER(:) TRANSPARENCY VEL VEL_BULK VEL_GRAD VEL_T VOLUME_FLOW WIDTH XYZ(3) Z0 Character Character Character Character Int. Triplet Real Character Real Real Real Real Real Real Real Real Real Character Real Logical Real Real Real Array Real Real Real Array Real Real Real Real Real Pair Real Real Real Triplet Real Section 10.1 Section 10.3 Section 10.3 Section 10.3 Section 7.4 Section 9.1.7 Section 9.1.6 Section 8.4.2 Section 8.3.8 Section 10.1 Section 10.1 Section 10.1 Section 10.1 Section 10.1 Section 10.1 Section 7.4.2 Section 7.4.2 Section 7.4.2 Section 8.6.2 Section 8.6.2 Section 8.6.2 Section 8.1 Section 8.3.4 Section 8.2.1 Section 8.3.4 Section 7.4 Section 9.1 Section 9.5 Section 9.1.5 Section 9.1.4 Section 9.1 Section 14.4.1 Section 8.4.2 Section 9.6 255,204,102 m m/s s s s s s s m 2. 1. 1. 1. 1. 1. 1. 1. m 1. .FALSE. ◦C ◦ C/min m ◦C ◦C ◦C m/s m/s 1/s m/s m3 /s m m m 17.27 TABL (Table Parameters) Table 17.27: For more information see Section 15.3.1. TABL (Table Parameters) ID TABLE_DATA(9) Character Real Array Section 15.3.1 Section 15.3.1 269 0. 800. 5. 20. 20. 20. 1. 10. 17.28 TIME (Time Parameters) Table 17.28: For more information see Section 6.2. TIME (Time Parameters) DT EVAC_DT_FLOWFIELD EVAC_DT_STEADY_STATE LIMITING_DT_RATIO LOCK_TIME_STEP RESTRICT_TIME_STEP T_BEGIN T_END TIME_SHRINK_FACTOR WALL_INCREMENT Real Real Real Real Logical Logical Real Real Real Integer Section 6.2.2 Reference [53] Reference [53] Section 4.2 Section 6.2.2 Section 6.2.2 Section 6.2.1 Section 6.2.1 Section 6.2.3 Section 8.3.8 s s s 0.01 0.05 0.0001 .FALSE. .TRUE. s s 0. 1. 1. 2 17.29 TRNX, TRNY, TRNZ (MESH Transformations) Table 17.29: For more information see Section 6.3.5. TRNX, TRNY, TRNZ (MESH Transformations) CC IDERIV MESH_NUMBER PC Real Integer Integer Real Section 6.3.5 Section 6.3.5 Section 6.3.5 Section 6.3.5 m 17.30 VENT (Vent Parameters) Table 17.30: For more information see Section 7.3. VENT (Vent Parameters) COLOR CTRL_ID DEVC_ID DYNAMIC_PRESSURE EVACUATION ID IOR L_EDDY L_EDDY_IJ(3,3) MB MESH_ID Character Character Character Real Logical Character Integer Real Real Array Character Character Section 7.4 Section 15.4.2 Section 15.4.2 Section 9.4 Reference [53] Section 7.3.1 Section 7.3.4 Section 9.1.8 Section 9.1.8 Section 7.3.1 Reference [53] 270 Pa 0. .FALSE. m m 0. 0. Table 17.30: Continued VENT (Vent Parameters) MULT_ID N_EDDY OUTLINE PBX, PBY, PBZ PRESSURE_RAMP REYNOLDS_STRESS(3,3) RGB(3) SPREAD_RATE SURF_ID TEXTURE_ORIGIN(3) TMP_EXTERIOR TMP_EXTERIOR_RAMP TRANSPARENCY UVW(3) VEL_RMS XB(6) XYZ(3) Character Integer Logical Real Character Real Array Integer Triplet Real Character Real Triplet Real Character Real Real Triplet Real Real Sextuplet Real Triplet Section 7.5 Section 9.1.8 Section 7.3.1 Section 7.3.1 Section 9.4 Section 9.1.8 Section 7.4 Section 8.4.2 Section 7.3.1 Section 7.4.2 Section 7.3.2 Section 7.3.2 Section 7.4 Section 9.2.7 Section 9.1.8 Section 7.3.1 Section 8.4.2 0 .FALSE. m2 /s2 0. m/s 0.05 ’INERT’ m ◦C (0.,0.,0.) 1.0 m/s m/s m m 0. 17.31 ZONE (Pressure Zone Parameters) Table 17.31: For more information see Section 9.3. ZONE (Pressure Zone Parameters) ID LEAK_AREA(N) LEAK_PRESSURE_EXPONENT(N) LEAK_REFERENCE_PRESSURE(N) PERIODIC XB(6) Character Real Real Real Logical Real Sextuplet 271 Section 9.3.1 Section 9.3.2 Section 9.3.2 Section 9.3.2 Section 9.3.1 Section 9.3.1 m2 Pa 0 0.5 4 .FALSE. m 272 Part III FDS and Smokeview Development Tools 273 Chapter 18 The FDS/Smokeview Repository For those interested in obtaining the FDS and Smokeview source codes, either for development work or simply to compile on a particular platform, it is strongly suggested that you download onto your computer the entire FDS/Smokeview “Repository.” All project documents are maintained using the online utility GitHub, a free service that supports software development for open source applications. GitHub uses Git version control software. Under this system, a centralized repository containing all project files resides on a GitHub server. Anyone can obtain a copy of the repository or retrieve a specific revision of the repository. However, only the FDS and Smokeview developers can commit changes directly to the repository. Others must submit a “pull request.” Detailed instructions for checking out the FDS repository can be found at https://github.com/firemodels/fds-smv. The repository contains the following files: 1. FDS and Smokeview source code files 2. FDS and Smokeview documentation 3. Input files for software testing (Examples), verification testing, and validation testing 4. Experimental data files used for validation testing 5. Scripts and post-processing utilities used for software testing 6. Web pages and Wiki Pages The Wiki Pages are particularly useful in describing the details of how you go about working with the repository assets. 275 276 Chapter 19 Compiling FDS If a compiled version of FDS exists for the machine on which the calculation is to be run and no changes have been made to the original source code, there is no need to re-compile the code. For example, the file fds.exe is the compiled program for a Windows-based PC; thus PC users do not need a Fortran compiler and do not need to compile the source code. For machines for which an executable has not been compiled, you must compile the code. A Fortran 2008 compliant compiler is required. 19.1 FDS Source Code Table 19.1 lists the files that make up the FDS source code. Files with the “.f90” suffix contain free-form Fortran 90 instructions conforming to the ANSI and ISO standards (2008 edition). A makefile is available in the FDS-SMV Repository that contains platform specific options for compilation. Note the following: • The source code consists entirely of Fortran statements organized into about 35 files. Some compilers have a standard optimization level, plus various degrees of “aggressive” optimization. Be cautious in using the highest levels of optimization. • For the non-MPI version of FDS, compile with mpis.f90. • The MPI version of FDS uses mpip.f90 instead of mpis.f90, plus additional MPI libraries need to be installed. More details on MPI can be found at the FDS-SMV website, along with links to the necessary organizations who have developed free MPI libraries. 277 Table 19.1: FDS source code files File Name cons.f90 ctrl.f90 data.f90 devc.f90 divg.f90 dump.f90 evac.f90 fire.f90 func.f90 geom.f90 gsmv.f90 hvac.f90 ieva.f90 init.f90 irad.f90 main.f90 mass.f90 mesh.f90 mpip.f90 mpis.f90 part.f90 pois.f90 prec.f90 pres.f90 radi.f90 read.f90 samr.f90 scrc.f90 smvv.f90 soot.f90 turb.f90 type.f90 vege.f90 velo.f90 wall.f90 Description Global arrays and constants Definitions and routines for control functions Data for output quantities and thermophysical properties Derived type definitions and constants for devices Compute the flow divergence Output data dumps into files Egress computations (future capability) Combustion routines Global functions and subroutines Routines supporting complex, unstructured geometry (under development) Routines supporting complex, unstructured geometry (under development) Heating, Ventilation, and Air Conditioning Support routines for evac.f90 Initialize variables and Poisson solver Functions needed for radiation solver, including RadCal Main program Mass equation(s) and thermal boundary conditions Arrays and constants associated with each mesh MPI “include” statement for MPI compilation “Dummy” Fortran/MPI bindings for non-MPI compilation Lagrangian particle transport and sprinkler activation Poisson (pressure) solver Specification of numerical precision Spatial discretization of pressure (Poisson) equation Radiation solver Read input parameters Simplified Adaptive Mesh Refinement (under development) Alternative pressure solver (under development) Routines for computing and outputting 3D smoke and isosurfaces Soot agglomeration and aerosol deposition Turbulence models and manufactured solutions Derived type definitions Experimental vegetation model Momentum equations Wall boundary conditions 278 Chapter 20 Output File Formats The output from the code consists of the file CHID.out, plus various data files that are described below. Most of these output files are written out by the subroutines within dump.f90, and can easily be modified to accommodate various plotting packages. 20.1 Diagnostic Output The file CHID.out contains diagnostic output, including an accounting of various important quantities, including CPU usage. Typically, diagnostic information is printed out every 100 time steps as follows: Time Step 137431 December 27, 2015 00:29:49 Step Size: 0.563E-01 s, Total Time: 1800.04 s Pressure Iterations: 1 Maximum Velocity Error: 0.30E-01 on Mesh 1 at ( 0 44 3) --------------------------------------------------------------Max CFL number: 0.94E+00 at ( 1, 46, 1) Max divergence: 0.13E+00 at ( 66, 12, 1) Min divergence: -0.20E+00 at ( 66, 13, 1) Max VN number: 0.51E+00 at ( 1, 25, 18) No. of Lagrangian Particles: 27 Radiation Loss to Boundaries: 13.830 kW The Time Step indicates the total number of iterations. The date and time indicate the current wall clock time. The STEP SIZE indicates the size of the numerical time step. The Total Time indicates the total simulation time calculated up to that point. The Pressure Iterations are the number of iterations of the pressure solver for the corrector (second) half of the time step. The pressure solver iterations are designed to minimize the error in the normal component of velocity at solid walls or the interface of two meshes. The Maximum Velocity Error indicates this error and in which grid cell it occurs. Max/Min divergence is the max/min value of the function ∇ · u and is used as a diagnostic when the flow is incompressible (i.e., no heating); Max CFL number is the maximum value of the CFL number, the primary time step constraint; Max VN number is the maximum value of the Von Neumann number, the secondary time step constraint. The No. of Lagrangian Particles refers to the number of particles in the current mesh. The Radiation Loss to Boundaries is the amount of energy that is being radiated to the boundaries. As compartments heat up, the energy lost to the boundaries can grow to be an appreciable fraction of the Total Heat Release Rate. Following the completion of a successful run, a summary of the CPU usage per subroutine is listed in the file called CHID_cpu.csv (Section 20.5). This is useful in determining where most of the computational 279 effort is being placed. 20.2 Heat Release Rate and Related Quantities The heat release rate of the fire, plus other global energy-related quantities, are automatically written into a text file called CHID_hrr.csv. The format of the file is as follows s Time T(1) T(2) , , , , kW HRR VAL(1,1) VAL(1,2) . . , , , , kW Q_RADI VAL(2,1) VAL(2,2) , , , , ... ... ... ... , , , , kg/s BURN_RATE VAL(8,1) VAL(8,2) , , , , Pa ZONE_01 VAL(9,1) VAL(9,2) , , , , Pa ZONE_02 VAL(10,1) VAL(10,1) , , , , ... ... ... ... Details of the integrated energy quantities can be found in Section 16.10.1. BURN_RATE is the total mass loss rate of fuel, and ZONE_01, etc., are the background pressures of the various pressure ZONEs. Note that the reported BURN_RATE is not adjusted to account for the possibility that each individual material might have a different heat of combustion. It is the actual burning rate of the fuel as predicted by FDS or specified by you. The background pressure is discussed in Section 9.3. 20.3 Device Output Data Data associated with particular devices (link temperatures, smoke obscuration, thermocouples, etc.) specified in the input file under the namelist group DEVC is output in comma delimited format in a file called CHID_devc.csv. The format of the file is as follows: s Time T(1) T(2) , , , , UNITS(1) ID(1) VAL(1,1) VAL(1,2) , , , , UNITS(2) ID(2) VAL(2,1) VAL(2,2) . . , , , , ... ... ... ... , , , , UNITS(N_DEVC) ID(N_DEVC) VAL(N_DEVC,1) VAL(N_DEVC,2) where N_DEVC is the number of devices, ID(I) is the user-defined ID of the Ith device, UNITS(I) the units, T(J) the time of the Jth dump, and VAL(I,J) the value at the Ith device at the Jth time. The files can be imported into Microsoft Excel or almost any other spread sheet program. If the number of columns exceeds 256, the file will automatically be split into smaller files. 20.4 Control Output Data Data associated with particular control functions specified in the input file under the namelist group CTRL is output in comma delimited format in a file called CHID_ctrl.csv. The format of the file is as follows: s , status , status , ... , status Time , ID(1) , ID(2) , ... , ID(N_CTRL) T(1) , -001 , 001 , ... , -001 . . 280 where N_CTRL is the number of controllers, ID(I) is the user-defined ID of the Ith control function, and plus or minus 1’s represent the state -1 = .FALSE. and +1 = .TRUE. of the Ith control function at the particular time. The files can be imported into Microsoft Excel or almost any other spread sheet program. If the number of columns exceeds 256, the file will automatically be split into smaller files. 20.5 CPU Usage Data The file called CHID_cpu.csv records the amount of CPU time for each of the MPI processes. Rank,MAIN,DIVG, ... , Total T_USED (s) 0, 2.052E+00, 1.058E+01, ... , 5.143+01 1, 2.432E+00, 1.062E+01, ... , 5.123+01 . . where Rank is the number of the MPI process (starting at 0), MAIN, DIVG, and so on, are major routines, and ’Total T_USED (s)’ is the total CPU time consumed by that particular MPI process. Typically, the total time is similar. The time spent in MAIN is essentially overhead – time spent not working on the calculation. If you want to know if your work load is balanced, take a look at the time spent in MAIN. It should be similar for all MPI processes. If one of the MPI processes has a noticeably smaller value for MAIN, then that process is working on the core routines while the other processes sit idle in MAIN. The CHID_cpu.csv file is printed out at the end of the simulation. To force it to be printed out periodically during the simulation, set DT_CPU on the DUMP line. Its default value is very large, meaning that by default, the CPU information is only printed at the end of the simulation. 20.6 Gas Mass Data The total mass of the various gas species at any instant in time is reported in the comma delimited file CHID_mass.csv. The file consists of several columns, the first column containing the time in seconds, the second contains the total mass of all the gas species in the computational domain in units of kg, the next lines contain the total mass of the individual species. You must specifically ask that this file be generated, as it can potentially cost a fair amount of CPU time to generate. Set MASS_FILE=.TRUE. on the DUMP line to create this output file. 20.7 Slice Files The slice files defined under the namelist group SLCF are named CHID_n.sf (n=01,02...), and are written out unformatted, unless otherwise directed. These files are written out from dump.f90 with the following lines: WRITE(LUSF) WRITE(LUSF) WRITE(LUSF) WRITE(LUSF) WRITE(LUSF) WRITE(LUSF) . . WRITE(LUSF) QUANTITY SHORT_NAME UNITS I1,I2,J1,J2,K1,K2 TIME (((QQ(I,J,K),I=I1,I2),J=J1,J2),K=K1,K2) TIME 281 WRITE(LUSF) (((QQ(I,J,K),I=I1,I2),J=J1,J2),K=K1,K2) QUANTITY, SHORT_NAME and UNITS are character strings of length 30. The sextuplet (I1,I2,J1,J2,K1,K2) denotes the bounding mesh cell nodes. The sextuplet indices correspond to mesh cell nodes, or corners, thus the entire mesh would be represented by the sextuplet (0,IBAR,0,JBAR,0,KBAR). There is a short Fortran 90 program provided, called fds2ascii.f90, that can convert slice files into text files that can be read into a variety of graphics packages. The program combines multiple slice files corresponding to the same “slice” of the computational domain, time-averages the data, and writes the values into one file, consisting of a line of numbers for each node. Each line contains the physical coordinates of the node, and the time-averaged quantities corresponding to that node. In particular, the graphics package Tecplot reads this file and produces contour, streamline and/or vector plots. See Section 16.11 for more details about the program fds2ascii. 20.8 Plot3D Data Quantities over the entire mesh can be output in a format used by the graphics package Plot3D. The Plot3D data sets are single precision (32 bit reals), whole and unformatted. Note that there is blanking, that is, blocked out data points are not plotted. If the statement WRITE_XYZ=.TRUE. is included on the DUMP line, then the mesh data is written out to a file called CHID.xyz WRITE(LU13) IBAR+1,JBAR+1,KBAR+1 WRITE(LU13) (((X(I),I=0,IBAR),J=0,JBAR),K=0,KBAR), & (((Y(J),I=0,IBAR),J=0,JBAR),K=0,KBAR), & (((Z(K),I=0,IBAR),J=0,JBAR),K=0,KBAR), & (((IBLK(I,J,K),I=0,IBAR),J=0,JBAR),K=0,KBAR) where X, Y and Z are the coordinates of the cell corners, and IBLK is an indicator of whether or not the cell is blocked. If the point (X,Y,Z) is completely embedded within a solid region, then IBLK is 0. Otherwise, IBLK is 1. Normally, the mesh file is not dumped. The flow variables are written to a file called CHID_****_**.q, where the stars indicate a time at which the data is output. The file is written with the lines WRITE(LU14) IBAR+1,JBAR+1,KBAR+1 WRITE(LU14) ZERO,ZERO,ZERO,ZERO WRITE(LU14) ((((QQ(I,J,K,N),I=0,IBAR),J=0,JBAR),K=0,KBAR),N=1,5) The five channels N=1,5 are by default the temperature (◦ C), the u, v and w components of the velocity (m/s), and the heat release rate per unit volume (kW/m3 ). Alternate variables can be specified with the input parameter PLOT3D_QUANTITY(1:5) on the DUMP line. Note that the data is interpolated at cell corners, thus the dimensions of the Plot3D data sets are one larger than the dimensions of the computational mesh. Smokeview can display the Plot3D data. In addition, the Plot3D data sets can be read into some other graphics programs that accept the data format. This particular format is very convenient, and recognized by a number of graphics packages. 20.9 Boundary Files The boundary files defined under the namelist group BNDF are named CHID_n.bf (n=0001,0002...), and are written out unformatted. These files are written out from dump.f90 with the following lines: 282 WRITE(LUBF) QUANTITY WRITE(LUBF) SHORT_NAME WRITE(LUBF) UNITS WRITE(LUBF) NPATCH WRITE(LUBF) I1,I2,J1,J2,K1,K2,IOR,NB,NM WRITE(LUBF) I1,I2,J1,J2,K1,K2,IOR,NB,NM . . WRITE(LUBF) TIME WRITE(LUBF) (((QQ(I,J,K),I=11,I2),J=J1,J2),K=K1,K2) WRITE(LUBF) (((QQ(I,J,K),I=11,I2),J=J1,J2),K=K1,K2) . . WRITE(LUBF) TIME WRITE(LUBF) (((QQ(I,J,K),I=11,I2),J=J1,J2),K=K1,K2) WRITE(LUBF) (((QQ(I,J,K),I=11,I2),J=J1,J2),K=K1,K2) . . QUANTITY, SHORT_NAME and UNITS are character strings of length 30. NPATCH is the number of planes (or “patches”) that make up the solid boundaries plus the external walls. The sextuplet (I1,I2,J1,J2,K1,K2) defines the cell nodes of each patch. IOR is an integer indicating the orientation of the patch (±1, ±2, ±3). You do not prescribe these. NB is the number of the boundary (zero for external walls) and NM is the number of the mesh. Note that the data is planar, thus one pair of cell nodes is the same. Presently, Smokeview is the only program available to view the boundary files. 20.10 Particle Data Coordinates and specified quantities related to tracer particles, sprinkler droplets, and other Lagrangian particles are written to a FORTRAN unformatted (binary) file called CHID.prt5. Note that the format of this file has changed from previous versions (4 and below). The file consists of some header material, followed by particle data output every DT_PART seconds. The time increment DT_PART is specified on the DUMP line. It is T_END/NFRAMES by default. The header materials is written by the following FORTRAN code in the file called dump.f90. WRITE(LUPF) ONE_INTEGER ! Integer 1 to check Endian-ness WRITE(LUPF) NINT(VERSION*100.) ! FDS version number WRITE(LUPF) N_PART ! Number of PARTicle classes DO N=1,N_PART PC => PARTICLE_CLASS(N) WRITE(LUPF) PC%N_QUANTITIES,ZERO_INTEGER ! ZERO_INTEGER is a place holder DO NN=1,PC%N_QUANTITIES WRITE(LUPF) CDATA(PC%QUANTITIES_INDEX(NN)) ! 30 character output quantity WRITE(LUPF) UDATA(PC%QUANTITIES_INDEX(NN)) ! 30 character output units ENDDO ENDDO Note that the initial printout of the number 1 is used by Smokeview to determine the Endian-ness of the file. The Endian-ness has to do with the particular way real numbers are written into a binary file. The version number is used to distinguish new versus old file formats. The parameter N_PART is not the number of particles, but rather the number of particle classes corresponding to the PART namelist groups in the input file. Every DT_PART seconds the coordinates of the particles and droplets are output as 4 byte reals: WRITE(LUPF) REAL(T,FB) DO N=1,N_PART WRITE(LUPF) NPLIM ! Write out the time T as a 4 byte real ! Number of particles in the PART class 283 WRITE(LUPF) (XP(I),I=1,NPLIM),(YP(I),I=1,NPLIM),(ZP(I),I=1,NPLIM) WRITE(LUPF) (TA(I),I=1,NPLIM) ! Integer "tag" for each particle IF (PC%N_QUANTITIES > 0) WRITE(LUPF) ((QP(I,NN),I=1,NPLIM),NN=1,PC%N_QUANTITIES) ENDDO The particle “tag” is used by Smokeview to keep track of individual particles and droplets for the purpose of drawing streamlines. It is also useful when parsing the file. The quantity data, QP(I,NN), is used by Smokeview to color the particles and droplets. Note that it is now possible with the new format to color the particles and droplets with several different quantities. 20.11 Profile Files The profile files defined under the namelist group PROF are named CHID_prof_nn.csv (nn=01,02...), and are written out formatted. These files are written out from dump.f90 with the following line: WRITE(LU_PROF) T,NWP+1,(X_S(I),I=0,NWP),(Q(I),I=0,NWP) After the time T, the number of node points is given and then the node coordinates. These are written out at every time step because the wall thickness and the local solid phase mesh may change over time due to the solid phase reactions. Array Q contains the values of the output quantity, which may be wall temperature, density or component density. 20.12 3-D Smoke Files 3-D smoke files contain alpha values used by Smokeview to draw semi-transparent planes representing smoke and fire. FDS outputs 3-D smoke data at fixed time intervals. A pseudo-code representation of the 3-D smoke file is given by: WRITE(LU_SMOKE3D) ONE,VERSION,0,NX-1,0,NY-1,0,NZ-1 . . WRITE(LU_SMOKE3D_SIZE,*)TIME,NCHARS_IN,NCHARS_OUT WRITE(LU_SMOKE3D)TIME WRITE(LU_SMOKE3D)NCHARS_IN,NCHARS_OUT IF (NCHARS_OUT > 0)WRITE(LU_SMOKE3D)(BUFFER_OUT(I),I=1,NCHARS_OUT) The first ONE is an endian flag. Smokeview uses this number to determine whether the computer creating the 3-D smoke file and the computer viewing the 3-D smoke file use the same or different byte swap (endian) conventions for storing floating point numbers. The opacity data is converted from 4 byte floating point numbers to one byte integers then compressed using run-length encoding (RLE). The compressed data is contained in BUFFER_OUT. Run-length encoding is a compression scheme where repeated “runs” of data are replaced with a number (number of repeats), and the value repeated. Four or more consecutive identical characters are represented by #nc where # is a special character denoting the beginning of a repeated sequence, n is the number of repeats and c is the character repeated. n can be up to 254 (255 is used to represent the special character). Characters not repeated four or more times are listed as is. For example, the character string aaaaaabbbbbcc is encoded as #6a#5bcc. 284 20.13 Geometry, Isosurface Files Both immersed geometric surfaces (generalized obstructions) and FDS generated isosurfaces are stored using a file format described in this section. Iso-surface files are used to store one or more surfaces where the specified QUANTITY is a specified value. FDS outputs iso-surface data at fixed time intervals. These surfaces are defined in terms of vertices and triangles. A vertex consists of an (x, y, z) coordinate. A triangle consists of 3 connected vertices. The file format allows one to specify objects that change with time. Static geometry is defined once and displayed by Smokeview unchanged at each time step. Dynamic geometry is defined at each time step either in terms of nodes and faces or in terms of a translation and two rotations (azimuthal and elevation) of dynamic geometry defined in the first time step. These files are written out from dump.f90 using lines equivalent to the following: ! header WRITE(LU_GEOM) ONE WRITE(LU_GEOM) VERSION WRITE(LU_GEOM) N_FLOATS IF (N_FLOATS>0) WRITE(LU_GEOM) (FLOAT_HEADER(I),I=1,N_FLOATS) WRITE(LU_GEOM) N_INTS IF (N_INTS>0) WRITE(LU_GEOM) (INT_HEADER(I),I=1,N_INTS) ! static geometry - geometry specified once and appearing at all time steps WRITE(LU_GEOM) N_VERT_S, N_FACE_S IF (N_VERT_S>0) WRITE(LU_GEOM) (Xvert_S(I),Yvert_S(I),Zvert_S(I),I=1,N_VERT_S) IF (N_FACE_S>0) WRITE(LU_GEOM) (FACE1_S(I),FACE2_S(I),FACE3_S(I),I=1,N_FACE_S) IF (N_FACE_S>0) WRITE(LU_GEOM) (SURF_S(I),I=1,N_FACE_S) ! dynamic geometry - geometry specified and appearing for each time step WRITE(LU_GEOM) STIME, GEOM_TYPE IF (GEOM_TYPE.EQ.0) THEN WRITE(LU_GEOM) N_VERT_D, N_FACE_D IF (N_VERT_D>0) WRITE(LU_GEOM) (Xvert_D(I),Yvert_D(I),Zvert_D(I),I=1,N_VERT_D) IF (N_FACE_D>0) WRITE(LU_GEOM) (FACE1_D(I),FACE2_D(I),FACE3_D(I),I=1,N_FACE_D) IF (N_FACE_D>0) WRITE(LU_GEOM) (SURF_D(I),I=1,N_FACE_D) ELSE IF (GEOM_TYPE.EQ.1) THEN ! rotation and translation parameters used to transform geometry from first ! dynamic time step WRITE(LU_GEOM) Xtran, Ytran, Ztran, Xrot0, Yrot0, Zrot0, rot_az, rot_elev ENDIF . . • ONE has the value 1. Smokeview uses this number to determine whether the computer creating the geometry file and the computer viewing the geometry file use the same or different byte swap (endian) conventions for storing floating point numbers. • VERSION currently has value 0 and indicates the version number of this file format. • N_FLOATS, N_INTS The number of floating point and integer data items stored at the beginning of the file. • FLOAT_HEADER, INT_HEADER Floating point and integer data stored at the beginning of the file. 285 • STIME is the FDS simulation time. • N_VERT_S, N_FACE_S, N_VERT_D, N_FACE_D are the number of static and dynamic vertices and faces. • Xvert_S, Yvert_S, Zvert_S, Xvert_D, Yvert_D, Zvert_D are the static and dynamic vertex coordinates. • FACE1_S, FACE2_S, FACE3_S,FACE1_D, FACE2_D, FACE3_D are the static and dynamic vertex indices for each face (triangle). The indices are numbered relative to how vertices were written out earlier. • SURF_S, SURF_D are the static and dynamic SURF indices for each face (triangle). • GEOM_TYPE is flag indicating how dynamic geometry is represented. If GEOM_TYPE is 0 then time dependent geometry is written out in terms of nodes and faces using the same format as the static geometry. If GEOM_TYPE is 1 then time dependent geometry is written out in terms of a translation and two rotations. These transformations are applied to the dynamic geometry defined at the first time step. • Xtran, Ytran, Ztran is the translation applied to the initial dynamic geometry (If GEOM_TYPE is 1) • Xrot0, Yrot0, Zrot0 is the origin about which rotations occur. • rot_az, rot_elev are the azimuthal and elevation rotation angles (in degrees) applied to the initial dynamic geometry. 20.14 Geometry Data Files The geometry data file contains a description of data values computed by FDS on an immersed geometrical objects. This file is analogous to the boundary file. The data written out to a geometry data file MUST correspond to the geometry written out in the corresponding geometry file. Geometry data files are written out from dump.f90 with the lines equivalent to the following: WRITE(LU_GEOM_DATA) ONE WRITE(LU_GEOM_DATA) VERSION WRITE(LU_GEOM_DATA) STIME WRITE(LU_GEOM_DATA) N_VERT_S_VALS,N_VERT_D_VALS,N_FACE_S_VALS,N_FACE_D_VALS IF (N_VERT_S_VALS>0) WRITE(LU_GEOM_DATA) (ValVertStatic(I), I=1,N_VERT_S_VALS) IF (N_VERT_D_VALS>0) WRITE(LU_GEOM_DATA) (ValVertDynamic(I),I=1,N_VERT_D_VALS) IF (N_FACE_S_VALS>0) WRITE(LU_GEOM_DATA) (ValFaceStatic(I), I=1,N_FACE_S_VALS) IF (N_FACE_D_VALS>0) WRITE(LU_GEOM_DATA) (ValFaceDynamic(I),I=1,N_FACE_D_VALS) . . The data values written out in this file correspond to the geometry written out in the geometry file. • ONE has the value 1. 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