Mixed convection flow and heat transfer in a heated vertical convergent channel

Abstract

Experimental studies of the mixed convection flow and heat transfer in a vertical convergent channel have been carried out. One of the side walls which maintains the vertical position is heated uniformly, and the opposite wall which has a convergence angle of 3° is insulated. The ratio of the height to the width at the inlet of the channel is 15. During the experiments, the Reynolds number ranges from 100 to 4000 and the buoyancy parameter, Gr/ Re 2, ranges from 0.3 to 907. Flow structure inside the channel is visualized with a heated smoke wire. For both assisted and opposed convection, flow reversal occurs only in the upstream of the channel but not in the downstream where rapid acceleration of flow occurs and the transport process is dominated by forced convection. For assisted convection, the reversed flow forms a steady recirculation cell along the insulated wall, and has no effect on the heat transfer. For opposed convection, the reversed flow occurs along the heated wall, and is unstable and sometimes generates a number of vortices and becomes turbulent. This flow can significantly enhance the heat transfer along the heated wall. Temperature fluctuations at different locations are measured and used to indicate oscillations and fluctuations of the reversed flow. The effect of the buoyancy parameter on the reversed flow structure and the Nusselt number is presented and discussed. The Nusselt number results are correlated in terms of relevant nondimensional parameters for both pure forced and mixed convection, respectively. For the purpose of comparison, the Nusselt numbers for parallel-plate channel are also measured and presented.