Hi! I'm simulating a 2D annular heat exchanger with two flow zones(with different fluid zones) in axial direction divided by a steel wall.

The heat exchanger is working in counter-flow and I have serious problems with the continuity convergence.

I think that the reason for the problem is that I'm simulating very different fluids (the inner fluid is a non-Newtonian fluid and the external fluid is water) and Fluent tries to solve two different systems in one solver getting a poor solution, specially in the velocity field (when I display velocity contours, I don't get a real solution).

Can anybody help me? I think that a possible solution can be to impose a UDF boundary condition of velocity in the entire external flow zone(water zone) so that fluent only has to solve the non-Newtonian flow, but I'm not sure if it will work. Can anybody give me more ideas? or tell me about his/her experience in similar problems.

Find what fluid is important to you and solve only it with the solid body and define a constant BC on the other side of the solid. It is not the same but is close to your case.....Hope I was clear....write if you find solution....