Additional Variable BC

It looks like a diffusion, is that your system? You do not provide very much info on your problem, but if it's like a heat convection, where C is temperature, then you can fix both temperature (C) and transfer coeff. (H) at the same surface, can't you?

Can do this with a gradient boundary with a CEL expression. Should be easy.

Thank you so much Glenn :)
Your advices have always helped me and other students. But would you please explain it more for me? or refer me to a tutorial or example? I don't know how to interpret a gradient boundary. Thanks again 
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C is a scaler variable and shows kind of concentration in the simulation, I just don't know how to define the Left hand of equation. 
Recast the equation as dC/dx=H/K(CC0) then you have the gradient of the scalar at the boundary is a simple function. Set the function to a CEL expression and it is done.
Set this for the additional variable boundary condition at the walls. 
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V*CD*dC/dn = k*C, where V, D and k are constants and C is my additional variable. The problem is how to calculate the dC/dn as it is the gradient in the normal direction from the wall. Any suggestions? 
Will the CEL variables normal, with extensions like _Coordinate_Direction help you? See the CFX reference guide, "Quantitative CEL functions in CFX"

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And can I define AV.Gradient X (...etc) in PRE as well? And btw, how do I define the boundary condition at the wall? The units resolve to [kg/(s m^2)]. Transfer coefficient+additional variable or Flux in? 
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