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-   -   Mass_Diffusivity: invalid (zero) diffusivity (http://www.cfd-online.com/Forums/fluent/116171-mass_diffusivity-invalid-zero-diffusivity.html)

Parisa_Khiabani April 14, 2013 13:40

Mass_Diffusivity: invalid (zero) diffusivity
 
Hello my friends,
I am simulating a mixture, using full multi-component mixture and I use kinetic theory.
However, I see the following warning that finally leads to stop my simulation:

Warning: Mass_Diffusivity: invalid (zero) diffusivity

Does anybody know how can I get rid of this annoying problem?
I searched a lot and it shouldn't have any relation with the kinetic theory since when I am using a dilute constant value, the temperature limit causes the simulation diverges.
Thanks,

Parisa

A CFD free user April 14, 2013 16:32

Hi
I think you received the error because you forgot to specify a value for mass diffusivity coefficient for any of the defined species. The default values are zero. Check it out, please.

Parisa_Khiabani April 14, 2013 16:44

Thanks for your comment, dear friend.
However, there is no option to define individual mass difffusivity for each species.
When I use kinetic theory for the mixture, there is no need to set a value for each species diffusivity.
Am I right?
Where can I set the diffusivity for each species?
Thanks,
Parisa

A CFD free user April 14, 2013 17:37

Yes, you right, in case of kinetic theory, there's no need for any further settings. I'm sorry I didn't notice to "kinetic theory", I thought you were using "Multicomponent". It's a weird error really! By the way, you can defined the species by using UDS and then define their diffusivity by the options exist in list or by writing your own UDF. I've already done it by writing my own UDF for a mixture of three gases for a fuel cell.

Parisa_Khiabani April 14, 2013 17:49

Quote:

Originally Posted by A CFD free user (Post 420517)
Yes, you right, in case of kinetic theory, there's no need for any further settings. I'm sorry I didn't notice to "kinetic theory", I thought you were using "Multicomponent". It's a weird error really! By the way, you can defined the species by using UDS and then define their diffusivity by the options exist in list or by writing your own UDF. I've already done it by writing my own UDF for a mixture of three gases for a fuel cell.

Yes that's an odd error.
Did you use kinetic theory as well?
By the way, I am using full-multi component but to calculate the matrix of diffusion coefficients, I use kinetic theory.

epc1 April 14, 2013 20:42

Quote:

Originally Posted by Parisa_Khiabani (Post 420478)
Hello my friends,
I am simulating a mixture, using full multi-component mixture and I use kinetic theory.
However, I see the following warning that finally leads to stop my simulation:

Warning: Mass_Diffusivity: invalid (zero) diffusivity

Does anybody know how can I get rid of this annoying problem?
I searched a lot and it shouldn't have any relation with the kinetic theory since when I am using a dilute constant value, the temperature limit causes the simulation diverges.
Thanks,

Parisa

Hello, check the species L-J parameters. Sometimes one of them is zero and that causes the problem

Parisa_Khiabani April 14, 2013 20:54

Quote:

Originally Posted by epc1 (Post 420538)
Hello, check the species L-J parameters. Sometimes one of them is zero and that causes the problem

Thanks for your answer.
No, none of the L-J parameters are zero. I am using valid results.
This is seriously a strange warning.

Parisa_Khiabani April 14, 2013 20:55

Valid data, sorry I said valid results.

A CFD free user April 15, 2013 04:12

Well, respecting to your question, I used Maxwell-Stefan diffusion coefficients of any two species which are dependent on temperature and pressure. They can be calculated according to the empirical correlation based on kinetic theory. But, in my case, I had porous media, so I had to modify it according to percolation theory as well.

Parisa_Khiabani April 15, 2013 10:06

So, you had already the correlations for diffusion coefficients.
But, I don't have them.
By the way, I think there is a serious bug in FLUENT thermodynamics database.

A CFD free user April 15, 2013 14:18

No I didn't correlate anything myself, just used a correlation from literature. A combined formula based on theory and experimental but based on kinetic theory. It's general not only in my particular case.

Parisa_Khiabani April 15, 2013 14:32

Quote:

Originally Posted by A CFD free user (Post 420755)
No I didn't correlate anything myself, just used a correlation from literature. A combined formula based on theory and experimental but based on kinetic theory. It's general not only in my particular case.

Yes, I know what you are saying. That is a general correlation based on L-J parameters.
I am using that one as well. However, I am trying to develop the code by myself to figure out the bug in FLUENT. I am just developing a small and simple code but that is sufficient enough to realize the bug in FLUENT database.


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