I model the flow in circular pipe, then I check the conservation of energy. In case of horizontal pipe, I found that the difference of mdot x ( static enthalpy + 0.5 x v^2) between inlet and outlet is zero approximately. In case of vertical pipe with bouyancy effect, however, I found that mdot x (static enthalpy + 0.5 x v^2 + gh) at inlet is very much higher than the value at exit. I set the boundary condition at pipe wall as adiabatic free-slip wall, so no way for energy dissipation. In addition, I found that the magnitude of difference is about 97% of (mdot x g x h). As I know that the hydrostatic pressure is not included in the 'pressure' of CFX-Post, is it possible that 'gxh' is already included in static enthalpy? Thank you very much for your kind consideration.

Atit

I would like to provide more information. I have checked the entropy change in both cases. In case of horizontal pipe, the irreversibility is 2.6 watts. However, it is 8,217 watts for the flow in vertical pipe. As I told you that the boundary condition of the wall is adiabatic free-slip wall, so the isentropic flow is expected. Do anyone have an idea why the flow in vertical pipe is not isentropic?

Atit