mass diffusion help
 

Hi everyone. I am new to fluent and am trying to look at mass diffusion. I am just starting off with a 2-D box model with no flux bc's. I know that I need to use species transport and am using mass fractions where I initially put a concentration in the center and just want to see how it diffuses. I am not getting the results I would like to see on this basic problem which concerns me. If anyone has and knowledge of this procedure or can point me in a direction that would be great. Thank you. Jill Benware