REVERSED FLOW STRUCTURE AND HEAT-TRANSFER MEASUREMENTS FOR BUOYANCY-ASSISTED CONVECTION IN A HEATED VERTICAL DUCT

Buoyancy-assisted convection flow and heat transfer processes in a heated vertical channel are studied experimentally for situations where the buoyancy parameter Gr/Re2 is relatively large. The channel wall is made of two parallel plates, with one wall heated uniformly and the opposite wall insulated. A uniform airflow is made to enter the channel from the bottom. The reversed flow is visualized, which occurs initially near the channel exit for the case when Gr/Re2 is greater than a threshold value. The cold reversed flow enters the channel from the outside and forms a V-shaped recirculating flow region in the downstream part of the duct. This region gradually propagates upstream as the buoyancy parameter Gr/Re2 increases. The counter flow motion, leading to mixing between the heated buoyant fluid and the V-shaped recirculation, is shown to be highly unstable and characterized by generation of eddies and vortices when the value of Gr/Re2 is large. An increase in Re has the effect of pushing the reversed flow downstream and making the recirculating region wider. Temperature fluctuations are measured to provide insight into the complex phenomena being studied. The penetration depth of the reversed flow is measured and compared with prediction based on a simple model. Local and average Nusselt numbers are also measured and presented.