Natural draft heat exchanger
 

1) I am trying to model and slove the flow and heat transfer in a vertical heat exchger where cold air and hot oil are driven naturally to in an annular involving conjugate heat transfer. I tested that in 2D first and I could not slove the problem if I specified pressure inlet pressure outlet for air and oil sides. So I end up using assumed oil flow rate and solve for air flow. I have used incompressible boussinsq model and included the gravity effect. Does anyone know what is missing?? 2) Now, I turned to the 3D model and finished the grid but I can not get gambit to speicfy the coupled (conjugate) boundary condition similar to what I did in 2D where as I follows the same steps and knowing that gambit does that automatically one you sp[ecify solid and fluid it couple them once you import the mesh in fluent 5 and enable the heat transfer. Does anyone know what is the problem here and how can I solve it?

Thanks, Tamer