Mass Diffusion via UDF

Ryan

I am trying to implement mass diffusion in a mixture of CH4 and H2 in my model. Because I'm trying to reproduce a model of the same process done by someone in homemade code I am trying to implement the same mass diffusion coefficient calcs they used, namely Chapman-Enskog (sp?). My thoughts for implementing are to use DEFINE_DIFFUSIVITY in a udf. My question is what to do about the argument 'i' or 'index' required by the DEFINE_DIFFUSIVITY function. I know 'i' refers to species, but will I need a case structure or if then statement to evaluate which species the solver contains or something? Thanks in advance for any clearer explanation. The Fluent documentation is surprisingly lacking in the explanation of this 'i' argument.