I defined species mass diffusion coefficient by dilute approximation and then i want to use diffusion coefficient at some UDF. I found there is a macro "C_DIFF_L(c,t,i,j)" used for diffusion coefficient in FLUENT. I know "i" and "j" meaning. However, the diffusion coefficient defined by dilute approximation is based on species "i" diffusion at mixture and not based on that species "i" diffuse in species "j". So how to use this macro to obtain the species diffusion coefficient defined by dilute approximation and how to set "i,j" at this macro? Thanks.

There is a post on the fluent website on how to specify your own diffusivity model. In it you are shown how to calculate j1,j2 for a binary model from the value of i that is passed in the define diffusivity macro. The dilute model is species i diffusing into background species not each individual species. The C_DIFF_L(c,t,i,j) is a binary value and is used to form D[i[m]]. Check out the fluent site and see if that can't give you any ideas.

DH