Convective instability and buoyancy effects on forced convection heat transfer in horizontal boundary layers heated from below by a constant temperature plate are studied experimentally for flow regimes with Rex = 2.5×104 ∼ 2.2×106 and Grx = 2×108 ∼ 1.5×1012. Heat transfer results obtained by heat flux gages and temperature measurements are studied using convective instability parameters, Grx/Rex1.5 for laminar flow and Grx/Rex2.7 for turbulent flow, with emphasis on flow in the transition regime. Buoyancy effects on heat transfer are also studied by comparison with predictions from the Pohlhausen solution and the von Ka´rma´n analogy. Convective instability data, velocity and temperature profiles, and flow visualization photographs (obtained by the hydrogen bubble method) and the wall temperature field (obtained by liquid crystal visualization) are presented. Flow visualization photographs reveal the spanwise periodic vortical motion, and the fluctuating velocity and temperature fields near the wall.

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