Civil and Environmental Engineering Publications
Mixed convection in a vertical Poiseuille flow: Comparative study of the bulk heating case and the constant wall-heat-flux condition
INTERNATIONAL COMMUNICATIONS IN HEAT AND MASS TRANSFER
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Numerical study of heat transfer in a laminar now between two vertical parallel plates is reported. Two configurations are considered: the first consists of an imposed constant wall-heat-nux boundary conditions; the second corresponds to a bulk energy dissipation by direct electric-conduction (Joule effect) in the fluid. In the two cases, we have kept the same average temperature-difference between the fluid at the entrance and at the exit of the channel. The now rate is also the same in both cases. Comparison is made between the axial evolution of Nusselt number obtained from this computation and the Nusselt number given by a classical correlation, in forced convection regime. It has been found that they are in good agreement. Heat transfer enhancement in the mixed convection regime is analysed. We show particularly that the temperature distribution at the exit section of the channel is more uniform in the bulk heating case than in the constant wall-heat-flux condition. The secondary-now induced by free convection and its contribution to the temperature uniformity is also studied We show the similarity between the two configurations from the point of view of the secondary-flow structure. In both cases we can observe two convective cells whose centres are located closer to the exit from the channel. It is explained that this phenomenon is due to the presence, in the two cases, of the wall temperature increase along the channel in the downstream direction. (C) 1998 Elsevier Science Ltd.