heat transfer between two fluids
 

Hi

I'm simulating a heat exchanger, but I'm having trouble with the thermal part. With few interactions my problem presents a satisfactory result (100 for example), but when you put many interactions (1000 for example) it presents a very unsatisfactory outcome for example my fluid cold comes with temperature 300K and the hot fluid with temperature 350k at the end of the simulation my cold fluid has higher than the temperature of the hot fluid and this further increases in temperature. I tried to solve the simplest cases where the solution converges with few interactions but no heat exchange fluids none. I think it might be a wall condition.

Can someone help me.