I want to model solid particles as a scalar in the carrier fluid and want to see how this scalar diffuses in the continuous carrier phase by using STAR-CD. I have already modelled it and my question is that how can I specify the diffusivity using user coding ?

if you want to define the diffusivity of a scalar via user coding, you can go to binary properties in StarGUIde (Thermophysical models and properties -> Additional Scalar), set diffusivity to "User" and click on "Define User Diffusivity". If you get Error 248 by doing this, you need to create a ufile directory. For this, apply the command "ufiles" in the directory you started Prostar.