Volume & mass fraction of gases concentrations

[kam]

Hi,
I wanted to know how FDS converses from mass to volume fraction (or
vice versa)
for example, from the output "oxygen mass fraction" in kg/kg to
"oxygen" in mol/mol. Is it by multiplying by the molecular weight of
the mixture and dividing by the molecular weight of the pure gas??

Also is the volume fraction the one should be used to get the gases
percentages to calculate Purser's Fractional Effective Dose?

Thanks in advance.
-kam

[Kevin]

FDS converts volume fraction to mass fraction via

X_i*W_i
Y_i = ---------------
Sum_k (X_k*W_k)

to convert volume fraction to mass fraction, and

Y_i/W_i
X_i = ---------------
Sum_k (Y_k/W_k)

[Bart Van Weyenberge]

Hey Kam,

Did you find out which one to use for concentrations?

I am looking for it too.

Thanks

[Randy McDermott]

For species a, Y_a is the mass fraction, X_a is the mol fraction, and W_a is the molecular weight (kg_a/kmol_a).  Also, W is the molecular weight of the mixture (kg_tot/kmol_tot).

Y_a = X_a*W_a/W

I'll let Craig W. chime in about FED.

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[Craig Weinschenk]

FED concentrations are in parts per million. Please the FDS User Guide Section 16.9.10 for discussion on how we compute FED in FDS and Smokeview Technical Reference Guide Section 3.5 for how it is computed there for visualization.