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-   -   Questions about Mass Sources (http://www.cfd-online.com/Forums/cfx/88685-questions-about-mass-sources.html)

jiguozhao May 23, 2011 18:19

Questions about Mass Sources
 
Dear all:
I would like to set a Fluid Mass Flux source on a boundary. After setting the quantity of the Mass Flux Source, some other settings are following like component fraction, temperature and velocity. But I know if the source is negative (that means the fluid disappeared as outflow), the CFX software dosen't consider the following settings anymore. what I am wondering is what is the fraction of component disappearing from the boundary. I checked the result, the mass dosen't obey the fraction which I set following the Mass Flux.
Is there any person who can tell me what fraction the disappeared fluid is based on?
Kind regards

Ji Guozhao
g.ji@uq.edu.au

ghorrocks May 23, 2011 18:55

It does not depend on the fraction (mass or volume). It just consumes whatever fluid is next to it - all phases/components.

jiguozhao May 23, 2011 23:34

Quote:

Originally Posted by ghorrocks (Post 308918)
It does not depend on the fraction (mass or volume). It just consumes whatever fluid is next to it - all phases/components.

Thank you!
Do you meand by that it is disappeared by the local fraction next to the boundary on which I set the source?

Kind regards

Ji Guozhao
g.ji@uq.edu.au

ghorrocks May 24, 2011 07:42

Yes, to rephrase what you said - the volume/mass fraction of the fluid leaving the domain in the mass sink is set by the volume/mass fraction in the control volume adjacent to the mass sink.

jiguozhao June 1, 2011 23:52

why my result is 100% decieded by initialization
 
Dear all:

i was trying to simulate a gas mixture(H2/Argon) going through a gas separation module. there is a change in H2 fraction(or Argon fraction) due to the selectivity of the membrane(more H2 is going through the membrane).
but the H2 fraction distribution result is 100% decided by my initial guess in initialization. if i set 0.1 as H2 fraction in initialization, i get 0.9925 at retentate outlet; if i set 0.3 as H2 fraction in initialiazation, i get 0.293072 at retentate; when i set 0.5 initially, i get 0.491635 at retentate; for setting 0.7 initially, i get 0.692509 finally. in all the cases above, the solving can converge to 10e-5. so which one is the really result?

is there anyone can help me?

kind regards

Ji Guozhao
g.ji@uq.edu.au

NCle May 16, 2012 14:48

[QUOTE=ghorrocks;308995]Yes, to rephrase what you said - the volume/mass fraction of the fluid leaving the domain in the mass sink is set by the volume/mass fraction in the control volume adjacent to the mass sink.[/QUOTED]

I want my sink to remove only one type of fluid, not both, but it seems that I am losing the other type of fluid as well, even though set the volume fraction of that fluid in the sink to zero. Does the specifying of volume fraction allow any selective control over what passes through the sink? I am trying to let only one type of fluid pass through the sink (I am trying to model free surface in a vacuum, and the vacuum is air that I want to escape freely).

ghorrocks May 16, 2012 18:54

Can you explain what you are trying to model? Why does removing only one phase help?

NCle May 17, 2012 07:50

I'm modeling a micron-scale imprinting process: A solid stamp material presses down into the free surface of a viscous melted polymer. In reality, a vacuum is drawn in this process, so the free surface is a polymer-vacuum interface. But in CFX, I must prescribe a gas to the vacuum.
The mass of polymer is conserved during the stamping: the same amount of polymer is in the chamber at start and end. So I don't want it to pass through the sink. However, the "air" must disappear from the chamber, because it is really a vacuum. I want to model a vacuum with air that disappears as if it were never there. So I want to make the air the only phase that passes through the sink. I'll try to attach a picture (cant figure it out).

ghorrocks May 17, 2012 08:00

So why not put an outlet away from the polymer so it never gets there? Then you can just suck stuff out and the polymer will never leave.

Are you sure CFX is the appropriate simulation code for this material? It sounds more like an FEA simualtion to me.

NCle May 18, 2012 09:44

Quote:

Originally Posted by ghorrocks (Post 361650)
So why not put an outlet away from the polymer so it never gets there? Then you can just suck stuff out and the polymer will never leave.

Are you sure CFX is the appropriate simulation code for this material? It sounds more like an FEA simualtion to me.

So, I tried the outlet away fropm polymer and sink method, and I get the same problem: gradually disappearing polymer. When I end the simulation, there is about 2 - 5% less polymer liquid than when I started either way; sink or outlet. I just recently went to an ANSYS informational conference and was counseled by one of the specialists there. They said it seems that the leaking liquid is simply from too coarse a timestep, or too coarse a grid or both. I was just told similarly by the online ANSYS support specialist that it was the timestep being too large.

Good point about the FEA. My Re # is so low its practically a solid, but its right on the line; I am trying to validate this model against another that was done as a thick liquid.

In summary, I guess I am not conserving liquid for reasons other than the sink/outlet issue; just needs temporal/grid refinement.

ghorrocks May 19, 2012 07:11

And convergence is another possibility. You might not have converged tight enough.


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