Observed flow reversals and heat transfer measurements for buoyancy assisted convection in a heated vertical channel

Buoyancy assisted convection flow and heat transfer processes in a heated vertical channel are studied experimentally for situations where the buoyancy parameter Gr/Re is relatively large. The channel wall is made of two parallel plates, with one wall heated uniformly and the opposite wall insulated. A uniform air flow is made to enter the channel from the bottom. The present study shows experimentally for the first time that flow reversal occurs in assisted flow. The reversed flow occurs initially near the channel exit for the case when Gr/Re is greater than 6. 6 × 10⁴. The cold reversed flow enters the channel from the outside and forms a V-shaped re-circulating flow region in the downstream part of the duct. This region gradually propogates upstream as the buoyancy parameter Gr/Re 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/Re. is large. Temperature fluctuations are measured to provide in-sight 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.