Learning Forum

[Weiqiang Liu]

Hi all,

I want calculate my own mass diffusion coefficients with UDF. In the UDF, I need to calculate molecular diffusion coefficients of species according to the equations in ansys theory manual such as below:

however, I do not know what is the right unit in this equation. For example, the unit for Mw,i is g/mol , kg/mol or kg/kmol. Or the unit of pressure is Pa or bar? 

Best 

Weiqiang.

[Amine Ben Hadj Ali]

Pressure is in Pascal. If you know now the unit of the diffusion coefficient you get the one of molar mass. Diffusion is generally provided in kg/ms check the references literature of Chapman.

[Weiqiang Liu]

Hi Amine,

I checked the user guide for fluent. I found that in the equation of calculating binary diffusion coefficient, unit of molar mass is kg/kmol. However, in the equation of calculating kundsen diffusion coefficients, unit of molar mass is kg/mol.

Best.

Weiqiang

[Amine Ben Hadj Ali]

So this topic is solved? Which Knudsen are you talking about?

[Weiqiang Liu]

Hi Amine, above is the definition I got from wiki and the unit of molar mass is kg/mol. I read UDF manual of fluent and it tells me all unit is SI. However, somebody told me that the unit of molar mass fluent uses is kg/kmol. I am really confused.

[Amine Ben Hadj Ali]

But you are working with Fluent. I do not understand the Knudsen issue. Molar weight in kg/kmol.

[Weiqiang Liu]

yes, if I do some dimensional analysis, I can verify that the unit of molar mass is kg/mol. My confusion is what is the unit fluent solver passes to UDF. 

Best regards

Weiqiang

[Amine Ben Hadj Ali]

As you cannot believe what already mentioned make a simple test by just saving molecular weight into UDMI or print it for a particular species or all species.

[Weiqiang Liu]

I am trying this way. Thanks very much!