[lordluan]

Hi There,

I am doing a simulation with gases. In the boundary IN, CFX requires the mass fraction which I added. Looking the results in CFD-post, I see the results as a plot of molar concentration x time (since its transient problem) of a point I placed in the Boundary IN. The values of Molar concentration calculated by the software are higher than the ones calculated using the ideal gas law. So I'd like to know how CFX converts the mass fraction value into molar concentration, since the boundary IN isn't affect by the phenomena I'm simulating.

[ghorrocks]

How does CFX convert from mass fraction to molar concentration? It just divides by molecular weight for each mass fraction, doesn't it?

I have no idea what your post is saying. You have not explained anything about what you are doing.

[lordluan]

Originally Posted by ghorrocks

How does CFX convert from mass fraction to molar concentration? It just divides by molecular weight for each mass fraction, doesn't it?

I have no idea what your post is saying. You have not explained anything about what you are doing.

This procedure is not enough to convert mass fraction (kg/kg) into molar concentration (mol/m³).

[Opaque]

Let us see with equations, with _k for species, and _m for mixture

Mass Fraction_k = Density_k / Density_m

Density_k = Molar Density_k * MolarMass_k

Substituting,

Mass Fraction_k = Molar Density_k * MolarMass_k / Density_m

Then, solving for Molar Density_k (Molar Concentration):

Molar Density_k = Density_m * Mass Fraction_k / MolarMass_k
========================================

Hope the above helps,