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

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 July 7, 2012, 14:03 mass fraction in multicomponent flow model (urgent!!!) #1 Senior Member   Ke Wu Join Date: Jan 2012 Posts: 179 Rep Power: 7 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

 July 9, 2012, 07:40 in steady state outlet will be the same as inlet #2 Senior Member     siamak rahimi ardkapan Join Date: Jul 2010 Location: Copenhagen, Denmark Posts: 218 Rep Power: 11 If you have a steady state case, then I think the outlet will be the same fractions as inlet.

 July 9, 2012, 07:56 #3 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,534 Rep Power: 25 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.

 July 9, 2012, 08:36 #4 New Member   Sebastian H. Join Date: Apr 2011 Posts: 7 Rep Power: 8 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.

 July 9, 2012, 09:01 #5 Senior Member   Ke Wu Join Date: Jan 2012 Posts: 179 Rep Power: 7 if i set inlet with air and co2 are air and co2 mixed homogeneously?

 July 14, 2012, 03:47 #6 Senior Member   Join Date: Oct 2009 Location: Germany Posts: 637 Rep Power: 14 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. __________________ We do three types of jobs here: GOOD, FAST AND CHEAP You may choose any two!

July 14, 2012, 06:04
#7
Senior Member

Ke Wu
Join Date: Jan 2012
Posts: 179
Rep Power: 7
Quote:
 Originally Posted by abdul099 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

 July 16, 2012, 17:26 #8 Senior Member   Join Date: Oct 2009 Location: Germany Posts: 637 Rep Power: 14 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. __________________ We do three types of jobs here: GOOD, FAST AND CHEAP You may choose any two!

 October 2, 2013, 20:30 #9 New Member   Dezhuang Ye Join Date: Oct 2013 Posts: 2 Rep Power: 0 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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