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

itsme_kit · July 7, 2012, 14:03

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

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

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!

July 7, 2012, 14:03	mass fraction in multicomponent flow model (urgent!!!)	#1
itsme_kit Senior Member Join Date: Jan 2012 Posts: 197 Rep Power: 14	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
siara817 Senior Member siamak rahimi ardkapan Join Date: Jul 2010 Location: Copenhagen, Denmark Posts: 220 Rep Power: 17	If you have a steady state case, then I think the outlet will be the same fractions as inlet.

July 14, 2012, 03:47		#6
abdul099 Senior Member Join Date: Oct 2009 Location: Germany Posts: 636 Rep Power: 21	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 16, 2012, 17:26		#8
abdul099 Senior Member Join Date: Oct 2009 Location: Germany Posts: 636 Rep Power: 21	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!

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
How to model a fan fixing the mass flow rate?	maddalena	OpenFOAM Running, Solving & CFD	55	January 18, 2016 06:52
particle, parcel and mass flow rate balance	flybird	FLUENT	0	May 24, 2007 10:44
volume fraction, Mass fraction and Bulk Mass flow	Dr.jones	CFX	1	January 25, 2006 04:32
Mass Flow Inlet	Pravir Kumar Rai	FLUENT	0	February 19, 2003 14:03
Mass Flow Inlet	Pravir Kumar Rai	FLUENT	0	February 17, 2003 19:23

July 9, 2012, 07:56		#3
flotus1 Super Moderator Alex Join Date: Jun 2012 Location: Germany Posts: 3,399 Rep Power: 46	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
sebastianh New Member Sebastian H. Join Date: Apr 2011 Posts: 7 Rep Power: 15	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
itsme_kit Senior Member Join Date: Jan 2012 Posts: 197 Rep Power: 14	if i set inlet with air and co2 are air and co2 mixed homogeneously?

October 2, 2013, 20:30		#9
sharon19910319 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!