Combined forced and free convection heat transfer for fully-developed laminar flow in horizontal annuli

The combined forced and free convection flows in a horizontal annulus are studied numerically by examining the similarity condition of fully-developed laminar flows. The finite-difference equations obtained are solved with the quadratic upstream differencing method to stabilize the convection terms with sufficient accuracy. Two cases of the thermal boundary conditions at the wall are considered for the secondary flow; constant heat flux and constant wall temperature, which lead to an appreciable difference in the flow behavior. The numerical results of the mean Nusselt number are compared with the experiment to show a good agreement.