How to set up a differential equation for boundary
Dear All:
It really bothers me. Does anybody know how to set up a differential equation as a boundary condition? It is a mass transfer problem within a control box. The equation governing the fluid is the Laplace Equation: d^2/dx_i^2 CA=0 Three surfaces all have the same and fixed boundary consition: CA=1 at x,y,z > infinite The other three: dCA/dx=0 at x=0 dCA/dy=0 at y=0 dCA/dz=hCA at z=0 I am stucked on the last one. I'd very appreciated it if anyone could help. Ray ^^ 
Re: How to set up a differential equation for boun
U can use symmetry BC for the same. For detailed understanding, check out details of symmetry BC in Fluent user's manual.

Re: How to set up a differential equation for boun
I did not see how the symmetry bc will happen with the last one. If you have a wall which only some species flow trhough you could use a pseudo "reaction" to impose the concentration gradient but I am not sure.
If your problem is a solution of a mixture where an interface exists and is where the mass transfer occurs you could create a UDF and solve the eq. dCa/dz=hCa(bulk)> Ca(at boundary)= hCa dZ + Ca(at the neighbor cell of the boundary cell) This concentration can be transformed to mass fraction whatever and used to specify the mass fraction defining the interface as a wall. 
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