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Old   April 14, 2013, 14:40
Default Mass_Diffusivity: invalid (zero) diffusivity
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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
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Old   April 14, 2013, 17:32
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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.
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Old   April 14, 2013, 17:44
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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
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Old   April 14, 2013, 18:37
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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.
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Old   April 14, 2013, 18:49
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Quote:
Originally Posted by A CFD free user View Post
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.
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Old   April 14, 2013, 21:42
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Quote:
Originally Posted by Parisa_Khiabani View Post
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
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Old   April 14, 2013, 21:54
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Quote:
Originally Posted by epc1 View Post
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.
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Old   April 14, 2013, 21:55
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Valid data, sorry I said valid results.
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Old   April 15, 2013, 05:12
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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.
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Old   April 15, 2013, 11:06
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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.
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Old   April 15, 2013, 15:18
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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.
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Old   April 15, 2013, 15:32
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Quote:
Originally Posted by A CFD free user View Post
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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