Experiments are performed to study the mixed convection flow and heat transfer in a horizontal rectangular channel heated from a side. The channel is made of two vertical parallel plates with one of the plates heated uniformly and the opposite plate well insulated. The gap between the parallel plates is small and the height to gap ratio of the channel cross section is 6.67. Both flow visualization and the heat transfer along the heated wall are measured. The Reynolds number ranges from 317 to 2000, the buoyancy parameter, $Gr/Re2,$ from 0 to 20000 and Pr of the air flow is 0.7. Flow structure inside the channel is visualized by injecting smoke at the inlet flowing along the heated side wall. The heated buoyant flow accumulates in the upper region of the channel, which grows in size as the buoyancy parameter increases. The accumulated flow is thermally stable and has a slower motion which can reduce the heat transfer enhancement by the buoyancy force. The effect of the Reynolds number and the buoyancy parameter on the heat transfer is presented and discussed. Comparisons of the Nusselt numbers with the case of the vertical channel flow and the prediction similar to the case of a horizontal flow through a heated vertical plate are also made. The normalized Nusselt numbers are found in proportion to the buoyancy parameter, correlations of the heat transfer data in terms of this parameter have been very successful. [S0022-1481(00)01404-3]

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