A Web-based Introduction to Fire Modeling
Comparison of Zone and CFD Models
Zone and CFD modeling are the two most used types of fire models on the market today. In order to show the differences of each model we present here a zone and CFD model of the same fire scenario.
The models are of an average sized college dorm room. IN NO WAY SHOULD THESE RESULTS BE INFERRED AS BEING ACCURATE OR ANALYZE THESE RESULTS FURTHER. Although we have modeled an average sized dorm room, we did it for the purposes of comparison between our zone model (C-FAST) and our CFD model (FDS). Absolutely no work was done to verify our results and to use our data would be unethical and wrong.
The dorm room is 12? X 14? X 8?. It has one window (which remains closed the entire time) and one door (which remains open the entire time). The walls and floor are made of concrete and masonry blocks. The ceiling is made of acoustical tiles. For both types of models there are no extra items in the room; i.e. the only item in the room is what is burning, nothing else.
Below are some movie files of both the zone and CFD models.
If you click on them, you can compare what they show for the same exact scenario.
Note: Unfortunately due to bandwidth restriction on our new design,
we are unable to provide the video files on the web at this time.
These movies are not in real time, for every second that passes two seconds have passed in the movie. This has been done to speed the movie up a little.
Zone models usually do not have any visual post processor to make a movie file. In order to have a good visual tool for C-FAST a program called Tecplot was manipulated to show the interface height of the smoke layer. When you watch the zone movie, the line moving down the graph is representative of the smoke layer height.
Scenario 1:
In this scenario our room fire does not reach flashover. Our fire was a double sized bed with a peak heat release rate of 400kW and a burning duration of 1300 (s) or about 22 minutes.
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What to look for:
As soon as the zone model movie starts, we see that the line (smoke layer) has begun to move downward. Zone models assume all smoke goes straight to the ceiling and smoke is created at the beginning of the fire.
As the movies progress we see that the smoke layer in both models is approximately the same. We also see that the two-layer approximation made by zone models is correct up until the end.
The CFD movie also shows all the different temperatures throughout the enclosure, where as zone models assume each layer is uniform in temperature.
Where zone modeling exceeds CFD modeling is that for this room and scenario it took several minutes for C-FAST to model this fire and then 30 minutes to produce the movie. It took several days for FDS to model the fire and make the movie.
Scenario 2:
This scenario our room reaches flashover (when the smoke layer reaches approximately 600oC or 1112oF). Our fire was a wood framed sofa with a peak heat release rate of 3000kW and a burning duration of 430 (s) or about 7 minutes.
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What was said about the non-flashover holds true for this case as well.
