Perhaps you have a look into DEFINE_DIFFUSIVITY Macro and check if it is helpful for your case.

If I use UDS diffusivity having unit to be kg/m-s in species transport model and add second term in DEFINE_DIFFUSIVITY macro and insert it in defined per uds as shown belownwill justify the addition of additional diffusion term in species conservation equation as,nPlease correct me if I am wrongnKindly let me know your thoughts on it.n