CFD Modelling of Indoor Environment Quality Affected by Gas Stoves

Proceedings of Clima 2007 WellBeing Indoors
CFD Modelling of Indoor Environment Quality Affected by Gas Stoves
László Kajtár and Anita Leitner
Budapest University of Technology and Economics, Hungary
Corresponding author email: kajtar@epgep.bme.hu
SUMMARY
Gaseous emission - CO2, NO, and NOx of gas stove has significant and harmful effect on indoor air quality (IAQ) in residential kitchens. To avoid increasing health risk it is essential to
use mechanical ventilation such as kitchen hood. However, according former laboratory studies, extract air flow generated by typical hoods is not adequate to achieve required IAQ. Thus
there is an increasing need to develop a new kitchen ventilation system and to estimate the
required exhaust ventilation rate.
In order to describe required ventilation of kitchens, emission of gas stoves has been investigated for years at Department of Building Service Engineering, BUTE. Laboratory and field
studies were carried out, afterwards, based on the results, pollutant distribution was described
by CFD simulation (FLOVENT).
In this paper an overview of method and results of CFD modelling is published.
INTRODUCTION
Gas stoves as pollutant sources can be described in accordance the power of burners. However occupants’ behaviour (the way of use) takes significant part as well. If gas stoves are
used in order to reduce required heat load of boiler, burner will run free. In this case pollutant concentration could be increase by 200-300%. From health point if view it should be
taken into consideration that occupants normally stand near to the source during the cooking. Thus air in breathing zone contains higher level of pollutants. Therefore occupants are
primary affected by combustion products. Application of hoods decreased the concentration levels but it has not appeared to be satisfactory in IAQ point of view.
In case of gas stoves Hungarian standards has a main issue providing fresh air to ensure burning process and to defend human health. However indoor air quality has an increasing impact
on design process.
In order to help design, implementation and maintenance to develop more effective kitchen
ventilation system, effect of gas stoves has been studied at laboratory and field studies. Based
on results it is proved that trade hoods (Vex,max =180 – 250 m3/h) can not ensure the required
ventilation from IAQ point of view.
To estimate required exhaust air flow CFD modelling has been made by FLOVENT. Number
of cases has been investigated as regards amount of air removed by kitchen hood. Supply air
has been provided using air terminal devices installed above the windows. Validity of CFD
model has been investigated based on results of former laboratory and filed studies.
Proceedings of Clima 2007 WellBeing Indoors
METHODS
Using CFD modelling it can be predicted air moving in the enclosure. Calculations are based
on fundamental flow and energy equations such as:
•
Navier – Stokes equations
•
Energy conservation
•
Mass conservation
•
Concentration equations
•
k – ε turbulence model
To solve these it is necessary to make modelling assumptions in order to describe the investigated phenomenon in an accurate way. This also helps to decrease number of parameters,
required computer capacity and solution time. Main modelling assumptions are follows:
•
outdoor air does not contain any of pollutants;
•
supply air temperature is equal to outdoor air temperature;
•
invariant outdoor climatic circumstances (air temperature, air velocity, pressure);
•
continuous ventilation by given amount of ventilated air.
Geometrical model has developed according architectural parameters of enclosure investigated at field studies. Gas stove model has been made following proportions of a device with
average technical parameters, therefore commonly used one.
Pollution loads have been estimated by laboratory measurements. Considered operating time:
60 minutes. Initial indoor air temperature ti(τ=0) = 20°C.
Figure 1. Plan of model kitchen (Vh = 29,5 m3)
Table 1. Investigated kitchen ventilation systems
Burner power P=5kW
Air flow extracted by hood
(h =80 cm)
0 m3/h
60 m3/h
180 m3/h
250 m3/h
500 m3/h
Supply air terminal device
Pollution load
1 x 60 m3/h
2 x 90 m3/h
2 x 125 m3/h
4 x 125 m3/h
GCO2= 1,80*10-4 kg/s
GNO2= 2,48*10-8 kg/s
GNO = 7,80*10-8 kg/s
Proceedings of Clima 2007 WellBeing Indoors
Thermal properties of stove have been studied in laboratory measurements as well, by measuring air temperature changing around the working stove at 10 points. At the same time surface temperature at different parts of stove envelope has been measured too.
To describe effect of increasing surface temperature approximate functions have been developed based on measurements’ results.
Burner has been model by a heat source element with same heat exchanger area as the real
one. Pollutant source have been placed above to the “burner”. Three sources have been used
for main gaseous products of burning: carbon-dioxide (CO2), nitrogen-oxide (NO) and nitrogen-dioxide (NO2).
For any sort of contaminant initial concentration is zero (kb(τ=0)=0ppm). Outdoor air has
been taken into consideration as clean air.
Modelling grid is a main issue as regards accuracy of model. It is highly recommended to
avoid any over dimensioning or using too poor grid system. In order to provide a proper solution at lower need of computer capacity, different regions have been worked up in the CFD
model. In regions with high expected concentration, temperature or velocity gradients fine
grid has been applied.
Seven regions have been defined with different grid:
1. Burning area (above top plate of stove)
2. Extract area (beside under plate of hood; extracting zone)
3. Front area (beside front plate of stove; primary occupied zone)
4. Left area (left side of stove; secondary occupied zone)
5. Right area (right side of stove; secondary occupied zone)
6. Envelope area (around the stove, along the surface of envelope)
7. Wall area (along inside surface of walls)
Total grid system of the model are contain 72 528 cells.
To describe time depending parameters time-step(s) has to be estimated. Total transient period (60min) has been divided into following steps: 2min, 5min, 10min, 15min, 20min,
30min, 45min and 60min. Data saved at 10th, 30th and 60th minutes.
Changing of calculated parameters (concentration, temperature and air velocity) has been followed at 10 monitor points placed in front of the stave, at 1.5m high along the centreline of
the burner, in every 10cm.
RESULTS
Results are presented in table form and by visualisation as well, at above mentioned transient
time step. At visualisation investigated areas have been selected according monitor points.
In following Figures results at 30min can be seen, in case of no ventilation and 250 m3/h exhaust air flow.
•
CO2 concentration: Figure 2 a. and b.
•
NO concentration: Figure 3 a. and b.
•
NO2 concentration: Figure 4 a. and b.
Proceedings of Clima 2007 WellBeing Indoors
Figure 3. CO2 concentration (30min) a.) V=0 m3/h
b.) V=250 m3/h
Figure 4. NO concentration (30min) a.) V=0 m3/h
b.) V=250 m3/h
Figure 5. NO2 concentration (30min) a.) V=0 m3/h
b.) V=250 m3/h
Proceedings of Clima 2007 WellBeing Indoors
CONCLUSIONS
-
In kitchens equipped gas stoves it is strongly recommended to install mechanical ventilation system to provide required concentration level of CO2 and NOx.
Using kitchen stoves as heating equipment results significantly decreasing indoor air
quality level.
Kitchen hoods with typical technical parameters (Vex,max =180–250 m3/h) can not provide Hungarian requirements for NOx concentration (200 µg/m3)
As regards CO2 concentration required air flow is given in Table 2.
Table 2. Required exhaust air flow
Working period [min]
15
30
60
Vex [m3/h]
cCO2 < 2 500 ppm
65,0
140,0
160,0
cCO2 < 5 000 ppm
0,0
66,0
92,0
ACKNOWLEDGEMENT
This research has been carried out by a commission of Hungarian Research Fund and Association of Gas Distribution Companies.
REFERENCES
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Coimbra, Portugália. 2004. ROOMVENT 2004, 9th International Conference on Air Distribution in Rooms. Book of Abstracts 126-127p. CD 6p.
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by Operation of HVAC – R Systems, 2004. nov. 21-24. Strbske Pleso, Proceeding 155160 p.
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Beijing, China, 2005. Sept. 4-9. 10 th International Conference on Indoor Air Quality and
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Healthy Buildings 2006 Conferece Lissabon 2006.
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