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CFAST Frequently Asked Questions

  • Q1: I am trying to download CFAST and keep getting a message that says "The requested URL could not be retrieved."

  • Q2: Is CFAST validated?

  • Q3: Can I run CFAST on platforms other than Windows-based PCs?
  • Q4: Is there a user's group for CFAST?

  • Q5: I have just downloaded CFAST and some of the display is not visible.

  • Q6: When I run the test cases supplied with the model, the answers do not match those shown in the user's guide. When I input numbers into the program, only the integer part is shown.
  • Q7: How is wind handled in CFAST?

  • Q8: How is combustion chemistry handled in CFAST

  • Q9: Can I check the mass balance in my calculation?

  • Q10: I've created my own fire and the model does not run.

 

Q1: I am trying to download CFAST and keep getting a message that says "The requested URL could not be retrieved."

The link does work, but we have had compatibility problems with some international downloads. The NIST ftp server requires all IP addresses to have an associated name. Some internet services providers do not provide a name to go along with the numeric IP address. NIST is working to upgrade the ftp server, but this may not be available soon. You can request a copy of the software by e-mail to cfast@nist.gov. If you don't already have the Microsoft .NET framework on your machine, here's a link to the Microsoft download. It is required for operation of the software.

http://www.microsoft.com/downloads/details.aspx?FamilyID=262d25e3-f589-4842-8157-034d1e7cf3a3&displaylang=en

The only difference between the smaller and larger download from the CFAST web site is the inclusion of the .NET framework.

Q2: Is CFAST validated?

Yes and no. CFAST has been compared to a wide range of experimental results. Details and references for these are available in the CFAST technical reference guide available here. We're working to give better guidance as to where the model can be trusted and where it can't, but this is an on-going exercise. Our advice to users is, as always, to validate the model for the type of fire scenario you are interested in. That means simulate an actual experiment with geometry and fire similar to that you plan on using. If the difference in model and experiment is acceptable by whatever criteria you choose to use, go ahead and use the model. If not, let us know, and we'll see if we can improve that feature or suggest another approach.

Q3: Can I run CFAST on platforms other than Windows-based PCs?

CFAST has been compiled on several UNIX platforms, but versions in the last few years have been developed solely on a Windows platform. The GUI front-end, CEdit, is strictly a Windows application. The CFAST source code is available on the Downloads Page, and you will have to compile the code yourself. You'll need a Fortran90/95 compiler. Check the main download page for more details.

Q4: Is there a user's group for CFAST?

There is a very useful Discussion Group where you can post a question to the community. This is closely monitored by all of the developers and is the best place to get your question answered. Posting here instead of just sending an email to one of the developers, will not only open your question up to all the developers at once, but will allow you to tap into the collective expertise of the entire community.

Q5: I have just downloaded CFAST and some of the display is not visible.

For some users, part of the display is cut off and changing the size of the main program window does not fix the problem. window Typically, the program does not show the right-hand portion. For example; the result window for the example run shows half the column for the "Ambient Target Flux" and does not show the scroll bar on the far right. We see this most often with Dell machines. Typically it is caused by the display settings configured for larger than normal fonts. You can check and correct this by going to the control panel (Start, Control Panel, Display), select the Settings tab and then the Advanced button. The DPI setting on the General tab should be set to "Normal Size (96 DPI)." If it is set to Large or Custom, CFAST will not display properly.

Q6: When I run the test cases supplied with the model, the answers do not match those shown in the user's guide. When I input numbers into the program, only the integer part is shown.

Most likely, the error is related to your selection of an international numeric format. CFAST is designed to use the U.S. English numeric format that uses a period as the separator between the integer and fractional part of a number. Unfortunately, the software does not recognize the comma separator and treats the input 2,9 as two separate numbers. To solve the problem, users have simply set up a new user on the machine and set the regional options to "English (United States)" for that user in the "Regional and Language Options" Control Panel applet.

Q7: How is wind handled in CFAST?

The wind modification is applied only to the vents which lead to the exterior. Pressure interior to a structure is calculated simply as a lapse rate based on the NOAA tables. For the exterior, the nominal pressure is modified by:

This modification is applied to the vents which lead to the exterior ambient. The pressure change calculated above is modified by the wind coefficient for each vent. This coefficient, which can vary from -1.0 to +1.0, nominally from -0.8 to +0.8, determines whether the vent is facing away from or into the wind (into increases the pressure, so the coefficient is positive). The pressure change is multiplied by the vent wind coefficient and added to the external ambient for each vent which is connected to the outside.

Q8: How is combustion chemistry handled in CFAST?

Parameters which are important:

HCR - hydrogen to carbon ratio in the fuel 
O2 - oxygen in the fuel 
OD - soot (assumed carbon) in kg per kg of CO2 produced 
HCN and HCL - detracts from the fuel 
CO - kg of carbon monoxide per kg of carbon dioxide produced 
TUHC is what is left over!

It is important to realize that the model does not check for consistency of the chemistry. The heat of combustion builds in chemical kinetics which we do not calculate apriori. Energy release is calculated from the amount of fuel consumed, which is constrained by the oxygen available at the point of combustion. Given the rate of oxygen consumption, oxygen calorimetry yields the heat release rate. The HRR for this process is 13.7 MJ/kg of oxygen.

The default we use for calibration, that is the default for deriving energy and mass balance, is methane. It has a heat of combustion of approximately 50 MJ/ kg, which includes breaking four hydrogen-carbon bonds, and their concomitant combining with oxygen.

There are several aspects of chemistry which are NOT incorporated, for example we do not worry about OH radicals. Nor do we worry about H2. Another assumption is that the mass of soot is carbon. As far as mass balance is concerned, this is reasonable, but there could be anomalous energy bound in the hydrogen.

The hydrogen-carbon ratio is the ratio of hydrogen in the fuel to carbon in the fuel. We have tested this up to 1/3, which is the ratio for methane. The lower limit is zero, and we have tested it (essentially) to this level, using a (mostly) carbon as a test fuel. We have not done this in a full scale test, so one might say it is unverified. But the answer is as close as we can hope for laboratory scale experiments.

As far as energy balance is concerned, higher values should work as well, though the issue mentioned above might become important. But we have only tested (verified) energy balance through methane.

Since the HCR ratio together with the heat of combustion implicitly includes the mass composition and chemical kinetics, it is important to get these numbers be correct for the fuel of interest. Next on the list of things to examine closely would be assumptions related to oxygen content of the fuel and whether you are looking at vitiation effects. The oxygen content is not too important except that it affects the fuel efficiency and does change the (effective) lower oxygen limit. Finally there are the CO/CO2 ratio, and soot in kg/kg of CO2. Given the mass that can result in carbon compounds, these two then determine where the carbon goes.

Since the branching rations (yields) affect the fuel consumption and species production, it is important to make sure all of the input is consistent, so as to produce consistent output.

Q9: Can I check the mass balance in my calculation?

Consider the following excerpt from a CFAST output.

Time =   1200.0 seconds.
Compartment   Upper     Lower     Inter.    Upper           Upper      Lower     Pressure    Ambient    Floor
              Temp.     Temp.     Height    Vol.            Absorb     Absorb                Target     Target               (K)       (K)       (m)       (m^3)           (m^-1)     (m^-1)      (Pa)      (W/m^2)    (W/m^2) -----------------------------------------------------------------------------------------------------------------    1          875.5     439.1    0.7806      15.    ( 68%)  0.000E+00  0.000E+00  -4.19      3.343E+04  3.998E+04
Fires
Compartment    Fire      Plume     Pyrol     Fire      Flame     Fire in   Fire in   Vent      Convec.   Radiat.
                         Flow      Rate      Size      Height    Upper     Lower     Fire
                         (kg/s)    (kg/s)    (W)       (m)       (W)       (W)       (W)       (W)       (W)
------------------------------------------------------------------------------------------------------------------
                  Main    1.57     5.409E-02 1.047E+06  3.31                                   7.328E+05 3.140E+05
    1                     1.57     5.409E-02 1.047E+06           1.353E-10 1.047E+06 0.000E+00
Flow Through Vents (kg/s)
To             Through            Upper Layer               Lower Layer           Mixing       Mixing
Compartment    Vent           Inflow       Outflow      Inflow       Outflow      To Upper     To Lower
--------------------------------------------------------------------------------------------------------
   1           H Outside #1                               6.890E-03
               H Outside #2                  6.662E-03
               H Outside #3                  1.40         1.34                                   0.170
Outside        H Comp  1 #1                                            6.890E-03
               H Comp  1 #2     6.662E-03
               H Comp  1 #3     1.40                                   1.3

The mass balance is just the sum of the flow in minus the flow out. For any compartment, this is just ("Upper Layer Inflow" + "Lower Layer Inflow" + "Pyrol Rate") – ((Upper Layer Outflow + Lower Layer Outflow) with the inflow and outflow summed for each vent. Note that the mixing flows are ignored since they are just exchanges between layers in the same compartment. For the above example, it’s just

(6.890E-03 + 1.34 + 5.409E-02) – (6.662E-03 + 1.40) = -0.005682 kg/s

or less than 1/2 percent of the magnitude of the inflow.

Q10: I've created my own fire and the model does not run.

The easiest way to create your own fire file is to copy an existing one and change the values as appropriate for your specific fire scenario. To copy a fire file, select "Tools" then "Edit Fire Objects" from the menu. The "Duplicate" button copies any selected existing fire object, creating a new object. Simply change the name of the new object and edit the values as appropriate.

If you look at the CFAST output log on the Environment tab in CEdit, you will typically see and error 205 when the model does not run. This is an error in some early versions of CFAST 6 and was fixed in version 6.0.10. The 205 error indicates that a material property is not defined. This could be in the fire definition, compartment surfaces, or targets. Typically, this is caused by a selection of "Off" in the material definition for a fire which is the default selection when you create a totally new fire object with the "Add" button. You do have to select a material for the object. It is used to calculate heat transfer to the object prior to ignition.