Question on heat transfer coefficient!!!

Benny

Dear All,

Now I have a question on heat transfer The problem concerned is simple: there is a straight circular duct, water flows through the duct, and heat flux loads on the duct's wall. What I want to know is heat transfer coefficient. But I found the results of the calculation are not correct at all comparing with results obtained from the formula: h=Nu*conduct/d, where h is heat transfer coefficient, Nu is the Nusselt number, conduct is thermal conductivity and d is the diameter of the duct. In the Fluent help document, I found that Surface Heat Transfer Coef. is defined by h=q/(T_wall-T_ref), where q is the heat flux, T_wall is the wall temperature and T_ref is the reference temperature which means the average temperature of the fluid. The default value of the T_ref is 288.16K. But I think maybe in different conditions the T_ref is different, so I change it to the average temperature of the fluid. My question is: Is that right to do so and could I get the correct heat transfer coefficient by doing so?

Best regards!

Benny