heat transfer coefficient

Jorge Facao

Hello, I am modelling a laminar steady 3D flow in rectangular channel. The duct is heated on a top face and others walls are adiabatic. The mass flow rate is constant. Since Gr/Re2 >> 1, it's a free convection problem. I am interested in heat transfer coefficient. Why total surface heat flux on top face is different from the m.cp.(Tout-Tin)? I have modelled several top face temperatures but the heat transfer coefficient is different than the literature correlations. Note that Nusselt number is less than 3, when in forced convection we expect 3.8. When I decrease the temperature difference between top face and inlet temperature, the heat transfer coefficient tend to 0, instead to reach forced convection coefficient. Regards, Jorge Facao