mass fraction in multicomponent flow model (urgent!!!)
 

I am modelling a pipe flow
I set initial condition: co2 in 0 and air in 1
And boundary condition at inlet: co2 in 0.2 and air in 0.8
However, how can I set mass fractions of co2 and air in the outlet?
Many thanks

in steady state outlet will be the same as inlet
 

If you have a steady state case, then I think the outlet will be the same fractions as inlet.

How can you know the mass fraction at the outlet?
Shouldn't this rather be one of the results of your simulation?

Even if the mass fraction should change from inlet to outlet for example due to reactions, I highly doubt that the simulation would be well posed with a fixed mass fraction at both inlet and outlet.

u have to set the outlet mass fraction just in case u have backflow.
it doesnt matter if u have backflow or not. u have to set it.

u set it like u did it at the inlet. pressure outlet/physics values.

if i set inlet with air and co2
are air and co2 mixed homogeneously?

Yes. As long as you don't specify a position dependent field function, it has to be mixed since there is no information about any separation of air and co2.

How to process a position dependent field function in star ccm?
If I run with this case without specifying any seperation, are they always mixed evenly in a steady flow state?
Thanks

Have a look into the user guide and search for "field function programming reference". You will find all needed information there.

And yes, when your multi-component fluid enters the domain in a mixed state and there is no separation, it will stay mixed. That's some simple logic.
Just keep in mind, separation can occur due to gravity, centrifugal forces etc.

I just wonder how to defy a mass fraction, I am a beginner, I just know how to define volume fraction from tutorial. Thanks a lot!