MIXED CONVECTION IN A VERTICAL POISEUILLE FLOW - COMPARATIVE-STUDY OFTHE BULK HEATING CASE AND THE CONSTANT WALL-HEAT-FLUX CONDITION

Numerical study of heat transfer in a laminar now between two vertical parallel plates is reported. Two configurations are considered: the f irst consists of an imposed constant wall-heat-nux boundary conditions ; the second corresponds to a bulk energy dissipation by direct electr ic-conduction (Joule effect) in the fluid. In the two cases, we have k ept the same average temperature-difference between the fluid at the e ntrance 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 Nusse lt number obtained from this computation and the Nusselt number given by a classical correlation, in forced convection regime. It has been f ound that they are in good agreement. Heat transfer enhancement in the mixed convection regime is analysed. We show particularly that the te mperature distribution at the exit section of the channel is more unif orm in the bulk heating case than in the constant wall-heat-flux condi tion. The secondary-now induced by free convection and its contributio n to the temperature uniformity is also studied We show the similarity between the two configurations from the point of view of the secondar y-flow structure. In both cases we can observe two convective cells wh ose centres are located closer to the exit from the channel. It is exp lained that this phenomenon is due to the presence, in the two cases, of the wall temperature increase along the channel in the downstream d irection. (C) 1998 Elsevier Science Ltd.