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 ^^

U can use symmetry BC for the same. For detailed understanding, check out details of symmetry BC in Fluent user's manual.

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.