Combined free and forced convection heat transfer in a rectangular duct

Abstract Heat Transfer by combined free and forced convection in laminar air flow in the entrance region of a horizontal rectangular duct of aspect ratio 5:1 has been studied experimentally. The investigation was made with one or both of the horizontal upper and lower plates heated with a constant heat flux and the vertical walls unheated. Velocity and temperature profiles have revealed that secondary flows created by natural convection have a significant effect on the flow behaviour and the heat transfer process at the lower plate. At the upper plate the hear transfer is little affected by the secondary flows for low heat fluxes but is reduced sharply for high heat fluxes due to the possibility of recirculation at the upper plate. A correlation of the upper plate results show good agreement with results for pure forced convection and a semi-empirical equation is developed for the lower plate heat transfer process.

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