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-   -   mass fraction in multicomponent flow model (urgent!!!) (https://www.cfd-online.com/Forums/star-ccm/104345-mass-fraction-multicomponent-flow-model-urgent.html)

 itsme_kit July 7, 2012 14:03

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

 siara817 July 9, 2012 07:40

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.

 flotus1 July 9, 2012 07:56

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.

 sebastianh July 9, 2012 08:36

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.

 itsme_kit July 9, 2012 09:01

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

 abdul099 July 14, 2012 03:47

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.

 itsme_kit July 14, 2012 06:04

Quote:
 Originally Posted by abdul099 (Post 371406) 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

 abdul099 July 16, 2012 17:26

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.

 sharon19910319 October 2, 2013 20:30

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!

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