variable diffusivity
 

I am solving gas diffusion equation using fluent 6.1. I have a porous media and fluid domain next to each other. There is no force convection and velocities are mainly due to diffusive forces in the model. I am using mixture template to model species transport. I want to turn off flow equations to get quick results, but doing that will make the momentum sink terms in porous media model go to zero when flow equations are not solved. To still get the resistance to diffusion in the porous medium, I will like to mass diffusion coeff in the porous medium as compared to the fluid medium. How do I do that. How do I make the mixture template use one value of mass diffusivity in one domain and a different value of mass diffusivity in a different domain. The values wont change with time. Do I need a UDF to do this? Please guide. Thanks in advance.