HRR Decreases

[shofly99]

Dear All, I have a question regarding the HRR in CFAST. I expect that the generated HRR would be equal to the input. However, in my case is different. There are four openings to the outside. I also changed the lower oxygen limit but it does not make any difference. What is your opinion on solving this problem?

Rooms.zip

[mcgratta]

I'm not sure exactly how CFAST works, but in FDS, even if you turn off the empirical extinction model, you might still be in a situation where the oxygen in the room is completely depleted and combustion cannot occur. The lower oxygen limit tells CFAST to stop consuming oxygen when its concentration falls below that value. If the LOL is zero, CFAST will try to consume all available oxygen. If there is no oxygen at all, the combustion should stop.

[shofly99]

In my case now, there is oxygen supplied from the four different openings. My concern is why the generated HRR decreases while there are sufficient supply of oxygen. I ran the same case in FDS, and I got the equal HRR that I wanted. I tried to change the area of fire in CFAST, but it seems that was not the solution.

[drjfloyd]

FDS and CFAST are very different codes. You have a small room with an under ventilated fire connected to a larger room with more ventilation. You are probably pushing the bounds of the types of scenarios CFAST is best at. Also FDS reports the HRR over the whole domain whereas CFAST does compartment by compartment. Are you comparing equivalent outputs?

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On Fri, Aug 23, 2024, 17:57 shofly99 ***@***.***> wrote: In my case now, there is oxygen supplied from the four different openings. My concern is why the generated HRR decreases while there are sufficient supply of oxygen. I ran the same case in FDS, and I got the equal HRR that I wanted. I tried to change the area of fire in CFAST, but it seems that was not the solution. — Reply to this email directly, view it on GitHub <#2132 (reply in thread)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ADBUZ4NUPEWCSROSQ5POBWLZS3TP3AVCNFSM6AAAAABM55EFUCVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTANBSG4ZTKMY> . You are receiving this because you are subscribed to this thread.Message ID: ***@***.***>

[shofly99]

I'm not sure If I compare equivalent outputs. I thought that the HRR from CFAST was for the whole domain as well. My intention now is to compare expected and actual HRR from CFAST. I tried to simulate with the lower HRR (1500 kW) and both the expected and actual are the same. When it comes to higher HRR ( > 2000 kW), the HRR suddenly decreases after reaching the peak.

[preneke]

Can you include your actual case? Also can you see if there is a door jet fire or vent fire in the output.

[preneke]

Sorry, it is early in the morning for me. I missed the file.

[preneke]

All or almost all the fuel is being burned. For clarity, the case I have only has one vent into Comp 6 from Comp 7. The plot shows that the fuel not being burned in Comp 6 is being burned in a vent fire in Comp 27, except for a small amount burning outside from the vents connecting to the outside.

[shofly99]

Thanks for your explanation, but why is all the fuel not being burned in Comp 6?

[drjfloyd]

Do a hand calculation for the maximum sustained ventilation flow through the door opening you defined. How much methane would that amount of oxygen support.

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On Mon, Aug 26, 2024, 17:24 shofly99 ***@***.***> wrote: Thanks for your explanation, but why is all the fuel not being burned in Comp 6? — Reply to this email directly, view it on GitHub <#2132 (reply in thread)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ADBUZ4NRALC22OF2M4JPSYLZTLJ2ZAVCNFSM6AAAAABM55EFUCVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTANBUHAYTQOA> . You are receiving this because you commented.Message ID: ***@***.***>

[shofly99]

Thanks all. It is becoming clear now. I really appreciate your support