[cnqin]

Is it necessary to accurately set the molar mass in multicomponent flow simulations without reactions? Will it have any effect on the mass fraction?

[ghorrocks]

I do not think the molar mass is required when reactions are not present. When no reactions are present you set the mass fraction at the initial condition and boundaries and it convects and diffuses from there - so I cannot see where molar mass comes into it.

If you want to be completely sure, do a test run where you run a model, and then rerun the same model but with the molar masses modified. You should find no difference between the results.

[Martin_Sz]

In multicomponent flow simulations without reactions, the exact value of the molar mass for each component doesn't significantly affect the mass fraction results in ANSYS CFX. Here's why:
* Mass Fraction Independence: CFX primarily relies on mass fractions to track the composition of the mixture. These fractions represent the ratio of a specific component's mass to the total mixture mass. As the molar mass only scales the mass units, it cancels out when calculating mass fractions.
* Solver Focus: The solver concentrates on conserving mass for each component based on their mass fractions. The molar mass doesn't directly influence the mass conservation equations.
However, while the mass fractions themselves might not be sensitive to exact molar mass values, it's still good practice to provide them for the following reasons:
* Accuracy in Derived Quantities: Some post-processing calculations or derived quantities within CFX might depend on the molar mass (e.g., mixture density calculations). Having accurate molar mass values ensures these derived quantities are also accurate.
* Code Consistency: Specifying molar masses maintains consistency within your simulation setup and facilitates data interpretation, especially if you plan to couple the CFX results with other tools that utilize molar mass information.

More at link below
https://howtooansys.blogspot.com/202...onent-cfx.html

[Opaque]

If you are modeling a mixture of ideal gases, each gas a different gas constant, R_gas = R_universal / Molar Mass.

Then, when mixing to obtain the mixture density, the molar mass value propagates through the mixing.

If mixing gases of very different molar mass, say He and H2O, what do you think will be the influence of molar mass.

Hope the above helps.