How to set BC for the mass diffusion?

Pete

HI,

I'm trying to solve a simple mass diffusion problem. Two reservoirs separated by a thin membrane. One reservoir fills with NaOH, the other just distrilled water. And I want to measure the concentration of OH at the other reservor for a certain time. I just want to do it in 2D ( and only x-dir flow). So the configuration would be very simple, a rectangular (is this correct? or does it need to be 3 rectangulars i.e. left reservoir, membrane (solid), and right reservoir???).

Question?

1. What type of the Boundary condition for left and right side of the membrane? 2. I believe I need to input the mass flow rate (or mass flux), but since the OH is in the reservoir and just diffuses by the concentration gradient, velocity should be zero, shouldn't it? 3. is there any example of diffusion problem in Fluent help?

Thank you very much for your help in advance. Pete