Periodic boundary conditions in tube banks

Zhihua

Hi, I am applying periodic boundary conditions in the simulation of flow flield and heat transfer in a circular tube bank in Fluent.Uniform temperature of 360 K is applied on cylinder walls and the upstream fluid temperature is 300 K. I found the following problems after running:

1. The average pressure at the inlet is about the same as the average pressure at the outlet. When I checked the pressure in the middle of the plane in x direction (the line that goes across the center circle), I found the average pressure is higher than that at the inlet. This is not reasonable because the pressure should decrease.

2. The average temperature at the outlet is lower than that at the inlet. This is wrong because the fluid should be heated up by the cylinder.

3. At the last 3 columns of meshes at the outlet , the temperature has a sudden decrease to about 68 K. I think there is something wrong with it.

Can anybody help me? Thanks very much!