CFAST low timestep about cfast HOT 6 CLOSED

Also you can put file named <file_name>.stop with an integer maximum number of steps in the directory where you are running CFAST. Unfortunately it will not write the *._calculations.csv, that is a future fix, but it will give you a time from any of the *.csv files you can set at the stop and rerun the case. Not ideal but a work around.

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Underventilation makes the numerics stiff; however, in your case I think the issue is the extreme nature of this scenario. Compartment 1101 is sealed compartment measuring 10 m x 15 m x 3 m with 2 air changes per hour of supply (but no exhaust to relieve pressure). This is 450 m3 or ~124 kg of O2 at t=0. You have 5 fires in this compartment each with a peak HRR in the 10s of MW. In the first 5 s the total HRR over these fires is 116 MW and within 25 s you will have consumed all the O2 in the compartment. 0.0625 m3/s of supply air is only going to support about 200 kW of fire.

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Thank you for this feedback, I suspected it was the underventilation. This is one of the few scenarios out of the run matrix where additional external ventilation (in the form of wall vents) was not modeled. For this case, is the wall / floor leakage of the compartment not enough to act as a pressure relieving exhaust vent or is the leakage simply overpowered by the extreme nature of these fires? I just wanted to make sure I wasn't missing anything obvious with this input deck.

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They eventually relieve pressure but not before the compartment reaches almost 2 atm where the exit flow is compressible flow.

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Keep in mind in this situation fires in CFAST are kind of gas burners so 116 MW with HoC 44000 kW/kg you are pumping in a little over 2.6 kg/s and climbing. This is a lot more mass than the HVAC system and that is just at 5 s. If you are modeling objects that have to pyrolyze the fuel you may want to adjust the HRR due to a lack of thermal feedback from the flame.

It maybe the case that the slow down is due to the oxygen limited burning. If you reduce the HRR to something sustainable in the compartment it may, and I repeat may, run a bit faster.

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Also you can put file named <file_name>.stop with an integer maximum number of steps in the directory where you are running CFAST. Unfortunately it will not write the *._calculations.csv, that is a future fix, but it will give you a time from any of the *.csv files you can set at the stop and rerun the case. Not ideal but a work around.

Excellent, this stop file is a good enough work around for my use case. Thank you everyone for lending your expertise.

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