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UDF for mass difusivity

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Old   March 9, 2006, 09:51
Default UDF for mass difusivity
Jiri Novak
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I am simulating multicomponent flow using SPECIES. I set the temperature dependent diffusivity for mixture under material s .. which causes an initialization error:

warning mass_diffusivity: invalid(zero) diffusivity phase check material properties.

Does anyone know why and why the mass diffusivity is set for mixture instead of pure species?

My simply UDF looks: DEFINE_PROPERTY(diffusivity, cell, thread) { real diff; real temp = C_T(cell, thread); diff = 6.5e-6 * exp (-20000/temp); return diff; }

Thanx Jiri
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Old   March 9, 2006, 10:31
Default Re: UDF for mass difusivity
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For a temperature of 300K your udf will return 7.2e-35. Fluent wont take such small values and simply set is to zero which causes the error. Even for 2000K the return value is 3e-10 which is very small. Are you shure you function is correct? Adding the exp function to 6.5e-6 instead of multiplying seems to produce more reasonable results...

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Old   March 9, 2006, 13:17
Default Re: UDF for mass difusivity
Jiri Novak
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Thank you. I haven't tested this yet. So, I'm gonna to split the UDF into two regions providing realistic values and not loosing the temperature dependent character.

Regards Jiri
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