Problem with Critical Flame Temperature (CFT)

[brad_fds6]

Hello everybody,

I want compute the Critical Flame Temperature (CFT) for my fuels, i have seen the Beyler's equation in the user guide, but i don't know what are the good values of Heat capacity of products of combustion that we have to use for compute the Critical Flame Temperature (CFT).

I have used  values given in wikipedia, but when i tried to compute the critical flame temperature for the propane for example, i have not found the value of 730 ° c given in the user guide.

I use Heptane and Dodecane as fuels, i want know the good values of Cp of products of combustion in order to be able to compute theirs Critical Flame Temperature correctly.

Thanks for your help

[Randy McDermott]

Run a setup case, T_END=0, and look at the .out file. All the properties of the tracked species are listed.

Sent from my iPhone

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[Randy McDermott]

Looks like Craig compute that for the one step reaction from C3H8 to CO.

On Thu, Feb 5, 2015 at 8:33 AM, 'brad_fds6' via FDS and Smokeview Discussions <fds...@googlegroups.com> wrote:

Dear Randy,

Thanks for your answer, but could i know Why Beylor  has used a critical temperature of 730 K for a propane fire  while the critical flame temperature  in the output file is equal to 1000 K.

I want know how justify the choice of a critical flame temperature different of the critical flame temperature given in the output file.

For example, i have seen that the critical flame temperature of the heptane and the dodecane is equal to 1600 K in the output file, but  my computations give good results with a critical flame temperature of 1100 K.

How could i justify the choice of a critical flame temperature of 1100 K ?

Thanks for your help

To view this discussion on the web visit https://groups.google.com/d/msgid/fds-smv/0e21f951-10e9-4d2a-aa6e-1aa7cc183d37%40googlegroups.com.

[Craig Weinschenk]

The critical flame temperature was calculated for the reaction specified by the REAC line not for the complete combustion reaction.

[brad_fds6]

Excuse me but i don't know where is the example's Craig ?
Is it in the user guide ?

Thanks for your help

[Craig Weinschenk]

It is the Beyler case you are looking at. I computed the critical flame temperature  -- I just followed the propane reaction as it was defined on the REAC line instead of computing the critical flame temperature for complete combustion.

[brad_fds6]

I don't understand, could you explain me again with more details, please.

Thanks for your comprehension

[brad_fds6]

Have you remarked that when you have put a Critical Flame Temperature of 730, in the output file there was 1003 as value of Critical Flame Temperature. And if we make the compute, we remark 1003-730 = 273.

And i have remarked ,that with  1100 in my input file i had 1373 in my output file.

I want understand if the value that we put in the input file is in °C or in K.

Thanks for your help

[Craig Weinschenk]

The critical flame temperature is input in deg. C -- search in  User Guide on CRITICAL_FLAME_TEMPERATURE. It is displayed as output quantity in K - the units are shown in the output. If this is too confusing we can change the output units of CFT for the next release of the code.

[brad_fds6]

No it was my fault. And excuse me, what are the value of the Cp of the CO, H2O and N2 that you have used to compute the critical flame.

I have the value of the Cp of CO equal to 1042.86 J/Kg.K
the Cp of H2O equat to 1410 J/Kg.K
And the Cp of N2 equal to 730 J/Kg.K

Could you give me the good value of these Heat capacity, Please ?

Thanks for your help

[Craig Weinschenk]

I believe I grabbed the values from the SFPE Handbook at T = 1000 C. I do not have my notes in front of me right now. I did do some sensitivity analysis in calculating CFT with varying Cp from JANAF tables values and had a script to iterate to convergence but didn't see significant variation. 

[brad_fds6]

Do you consider the presence of azote in your reaction to compute the Cp of the mixture of products, and the number of moles of products of combustion per mole of fuel/air mixture.

Thanks for your help