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Old   June 8, 2010, 11:05
Default Multicomponent diffusion modeling
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Gagan
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Hello everybody,

I would like to ask for help regarding a problem I m trying to solve on CFX 12.1. It is related to multicomponent diffusion (air and syngas(Co/h2) ) in a specific fuel injection geometry at high reynolds number (highly turbulent).

I would like to specify air and syngas at different temperatures and analyse their fractions after mixing. The air has to be a temperature approx 800K and syngas is at 300K. The syngas is injected through 5 orifices in two tubes intersecting inside a main tube through which air is passed.

I am facing a problem that after a certain distance from mixing...they start to "demix" and isosurfaces turn from a conical structure into a thin sheet at the outlet which is physically wrong.
However, when I tried to check with say Air and Ch4, the thin sheet is never formed and something physically possible is simulated. Therefore, I concluded that mesh is not the issue.

However, I am still not sure about several things:
A. I am not sure about how diffusion is modeled in CFX.
B. In post cfx I cannot see any temperature field. The temperate for all the components everywhere is 298K even though I changed it in materials tab.
C. How does it define inter-component diffusivity constants at non default temperatures?

Please let me know if you have answers to any of the questions or you need any more information.

Thanks for your help!

Regards,
Gagan
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Old   June 8, 2010, 16:50
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Gagan
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bump!

Well I was able to generate the temperature field.

However, I really wished to know how well the CFX code is when it comes to modeling multicomponent diffusion. All I get is conical isosufaces of all the components after mixing.

One thing I ve found out is how CFX only consider Ficks Law of Diffusion and does not simulate Stefan Maxwell Equations.

Does anybody know how I could simulate for that? Chemkin or something else?
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Old   June 8, 2010, 18:31
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Glenn Horrocks
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Quote:
A. I am not sure about how diffusion is modeled in CFX.
This is described in the documentation.

Quote:
B. In post cfx I cannot see any temperature field. The temperate for all the components everywhere is 298K even though I changed it in materials tab.
Sounds like a setup error, but looks like you have fixed that now.

Quote:
does not simulate Stefan Maxwell Equations.
After a quick search I have not been able to understand what the additional physics the Stefan Maxwell Equations add. Can you explain what the SM Equations are about?
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