This does not sound like a problem where CFAST is the appropriate tool.

I don't know if CFAST would allow a breach. The geometry is fixed from the start.

Others might know better, but I am not sure one can set an initial environment inside that is different than outside. This would start the simulation off with a non-sensical discontinuity at the boundary between zones. I think the CFAST solver would choke on that.

Before I completely give up the idea of using CFAST for my task, I want to ask: can the Nitrogen and Oxygen mass fraction of the initial environment be changed by a user from their look-like-preset values of 77% and 23% respectively (from CFAST Technical Reference Guide (p.10)) ? I understand that I cannot do it from within the GUI itself, but is it possible to make the changes manually in the code? Would all the code's calculations and results be valid still after that or they are all based on that 77/23 ratio?

Thank you in advance for any input on this,
Vera

try cfast posted here.
https://github.com/firemodels/test_bundles/releases/tag/CFAST_TEST
To define initial O2 concentration for all interior rooms use INTERIOR_O2_CONC on the &INIT line - default is 0.23 kg/kg. Likewise, to define O2 concentration for the outside use EXTERIOR_O2_CONC on the &INIT line. Nitrogen concentration is set to 1 - O2 concentration. Concentrations must be between 0.0 and 1.0.

The following defined interior O2 concentrations to 0.1 kg/kg and exterior O2 concentration to 0.2 kg/kg
&INIT INTERIOR_O2_CONC=0.1, EXTERIOR_O2_CON=0.2 /

Hi guys, I need your input to understand the t = 0 cell pressure output. I've changed the initial internal pressure from ambient to 4 in w.g. below that. The output text file captures that in the ambient conditions section, showing Interior Pressure (Pa) as 83811.84, but when I scroll down the file to the Time = 0.0 seconds, Cell Pressure (Pa) is shown as zero. The same zero for the Cell pressure at time 0s is shown in the output excel file (name_compartments.xls, cell F5). Question: Why? Is the Interior Pressure in the ambient conditions section of the input file is not really the cell pressure at time = 0s that the excel file shows? What am I missing?

The model I am trying to setup is just a single compartment with inert environment on the inside and ambient conditions on the outside. There is also one small breach on the wall (set it up as a small wall vent for now). No fire simulation yet. I would like to see the cell P and T profiles and also the changes of masses of N2 and O2 in the cell.

The changes I've done to the source code file are really just that, changing the cell ambient environment to inert and the initial cell pressure is now lower than that on the outside of the cell. The rest is controlled through the input interface (inside T 40C, outside T 20C, humidity 0%)

Change 1: Creating the inert environment.
Source file: input_namelist.f90 line 295
!interior_o2_mass_fraction = default_o2_mass_fraction ! kg/kg
changed to:
interior_o2_mass_fraction = 0.0_eb ! kg/kg

The output text file now shows at time 0 I have 100% N2 and 0% O2 (as expected). Since I have a small breach on the wall, as the simulation starts, the ambient air starts flowing in the compartment and some N2 is leaving it. But if I look at the Time = 60 sec in the output text file, it still shows 100% N2 for the upper layer with some O2 also present. So, the sum would be more than 100%. It shouldn't be, right? I need to check if that is just a simple output precision issue and the 100% shown is just a rounded up number.

Change 2: Initial Cell Pressure. I did not see the one place where I thought it would be defined as an input parameter, so I've made the following changes to the couple of places in the code:
2.1. Source file: input.f90 line 108
!interior_abs_pressure = exterior_abs_pressure*interior_ambient_temperature/exterior_ambient_temperature
changed to:
interior_abs_pressure = 83811.84_eb
2.2. Source file: initialization.f90 lines 317 - 325
t_ref = default_temperature
pressure_ref = default_pressure
!interior_abs_pressure = pressure_ref
changed to:
interior_abs_pressure = 83811.84_eb
pressure_offset = pressure_ref
interior_ambient_temperature = t_ref
tgignt = t_ref + 200.0_eb
exterior_ambient_temperature = interior_ambient_temperature
!exterior_abs_pressure = interior_abs_pressure
changed to:
exterior_abs_pressure = pressure_ref

With these changes, the Cell P output for time 0 is shown as 0.00 in both excel and text output files. Am I missing more places in the source code where the changes should be done? If what shown is the relative pressure, relative to the initial cell conditions (it would explain zeros at time zero), but then the absolute cell pressure in the excel file (column G) should be the initial cell pressure (in my case 83811.84Pa) and not the ambient P from the outside, right? But it shows the outside pressure as absolute cell pressure..

I would appreciate your input on this,

Vera

Regarding my message above.. My confusion is that I do not understand what pressure exactly is shown in text output file and excel output file?

From the User manual (p. 51), the text output file shows (at each time interval) "compartment pressure (relative to ambient)".

The ambient conditions are listed in the text output file as T and P for both the Interior and Exterior. In my case, interior pressure is below exterior (by 4 in w.g.). So, when it says "relative to ambient" does it mean to the initial interior pressure (that would explain why it is zero at time = 0)?

When we look at the excel output file, it looks as it shows the same output for pressure in Column F (Pressure Cell, Pa) as the text output file. But per User Manual (p.54) that pressure is "pressure at compartment floor relative to the outside at the absolute height of the floor." So, it is relative to the exterior pressure. Then, it cannot be equal zero at time = 0 as shown by the excel file as the initial value for interior pressure is different than for exterior.

And then we have Absolute Pressure Cell, Pa Column G in the excel file, that I though was the initial interior pressure, but it shows the exterior pressure. The user Manual does not have a description for this output, I guess it was added later.

Is there anyone who can help me to understand what those pressure outputs are and how they relate to the initial interior pressure? And also tell if there are more places in the source code (additional to the two I've identified in the message above) where manual changes should be made to reflect a scenario with different starting point for interior and exterior pressure?

Thanks in advance for any help with this matter,

Vera

You can set up your cases to work or at least to run. The breach is a vent that opens at a certain time. I don't know what inert atmosphere you can be talking about, I am sorry. However, if you talking about a full rich environment with no oxygen, which is technically not burning but it seems misleading to call it inert. If you want to start a fire in the compartment when the oxygen concentration is sufficient I think you probably have to go to FDS. While I think I could jury-rigged that case it would have too many assumptions and work arounds to trust and as such I wouldn't do that. If you mean you have fuel rich environment at low pressure and the interior temp is sufficiently above the combustion temp and what you are interested in is a vent jet fire, that would be external to your compartment that at least can be done, I would not trust the answers. It is not impossible that CFAST would entrain fuel into the oxygen plume in the compartment at the start of the breach and start combustion inside the compartment. It may even represent the actual results well but it feels like it would be outside our assumptions for vent jet fires and we have no test data to compare the model answer to so there is no way to estimate the accuracy if it even worked. In fact I am not sure it even should. You are trying to answer questions about conditions that likely are outside the conditions that CFAST was even considering when it was created. I would again suggest that you talk to the FDS experts about the possibility of FDS giving you the answers you want.

If you want to see the impact start with your vent being f=0, t=0, f=0, t=1, f=1, t=2. When you start as you did CFAST tries to come up with an initial solution at t = 0 and that means you will not see the response you are looking to observe. I would also set the spreadsheet output at 0.1 s. Otherwise all you will see is a step change from one steady state condition to another. You might want to even use a timestep of 0.01. Run the case for 5 s or less and you will see how CFAST responds, in that case, which is to equilibrate internal conditions to external conditions. Now that I see what you are doing, sorry it took so long, this isn't an unreasonable test but it isn't likely to show you that much.

I would agree with @preneke. This is not a good problem for CFAST. Reignition is going to occur somewhere along the contour of the incomming flow where O2 cocentrations support combustion. That event could happen anywhere in the compartment depending on where there is sufficient tempeature to support ignition. CFAST is going to average the door flow over the entire compartment. CFAST does not predict the shape of the incomming plume or its entrainement.

Even with FDS this is a research problem and not common practice. Predicting reignition is challenging, even for CFD.

Yes, I believe you can get CFAST to do this, including the fire. I am not sure what you accomplish beyond learning how to run certain cases in CFAST. While I don't want to discourage you from learning to run CFAST, I am concerned you are entering a realm of misusing CFAST by falling victim to the idea of if CFAST runs it must be a useful calculation. As with all scientific tools you have to be very careful to stay within the assumptions. If you are concerned with the impact of the fire you trying to study and its impacts on a larger building you can use other tools to get a reasonable estimate of the local fire behavior and then you use that estimate as part of the inputs for an analysis of a larger structure with this fire occurring. There are lots of flexibility that CFAST has to help answer all sorts of questions, but it has its limits and it is important to understand that just because you can get CFAST to run doesn't mean that the results from CFAST should be trusted. If you are jury rigging your way about the limits that CFAST has the results might be very difficult to make sense of. Still if you are very careful of using the results and there relationship to CFASTS assumption I do think you can get a case to run.