Initial Mass Fraction of air

[Guillem Peris]

Hi everyone,

Doing some complex stoichiometry I realized that the initial mass fraction changes depending on the relative humidity (%) and the temperature (there is more water vapor when the temperature increase). I guess that if the relative humidity is 0%, the air mass fraction will be invariable, but if it's more than 0 it depends of the maximum water vapor in air, which depends on the temperature, and the relative humidity. Is it like that?

And if it is, I would be interested to know what is the exactly correlation/formula which fds uses to calculate the initial air mass/volume concentration. I want to obtain the complex stoichiometry using the air mass fraction by default, and i need it exactly to have a balanced stoichiometry.

Thanks in advance,

Guillem,

[dr_jfloyd]

FDS starts with dry air using the default values Y_O2_INFTY=0.232378 and Y_CO2_INFTY=0.000595.  Water vapor is added to this based on the values of HUMIDITY (default is 40 %) and TMPA (default is 20 C). The humidity is determined from the Clausius-Clapeyron relationship.

An easy way to get the air composition for a particular TMPA and HUMIDITY is just to create a short input file with those values and run it for T_END=0.  You can then look into the 

[Guillem Peris]

I used to do that, start a case with a particular TMPA and humidity and read the output .out. My actual goal is to create an automatic algorythm to calculate the initial air mass fraction to later calculate the exact stoichiometry.

I tried to find the values that are used for the Clausius-Calpeyron relationship in the technical reference guide of fds (i.e heat of vaporization) but i didn't find anything. Could you provide me this values? The problem to make an automatic algorythm is that if i don't have the exact values, fds will give me an error in the stoichiometry. Actually if I have this error I deactivate the CHECK_ATOM_BALANCE and change the error tolerance to run the case and later I enter in the .out read the real air mass fraction used by fds, recalculate the stoichiometry and rerun the case.

Thanks,
Guillem,

[Randy McDermott]

See WATER_VAPOR_MASS_FRACTION at Line 2700 of func.f90.

https://github.com/firemodels/fds-smv/blob/development/FDS_Source/func.f90

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[Guillem Peris]

Thanks!

That was exactly i was looking for!

Thanks,

Guillem,