Studies on Heat Transfer in Buoyancy-Affected Separated Flows

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The existence of a flow separation and a recirculating region has a significant effect on heat transfer characteristics and heat transfer performance in engineering devices, such as compact heat exchangers, electronic equipment, digital computers, and gas turbines. In some cases these separated flow regions are added to the design of heat transfer devices to enhance the heat transfer, but their effectiveness has not been justified or verified experimentally for buoyancy- affected laminar flows. In situations in which the temperature difference is large, and the flow velocity is small, buoyancy force effects become significant. The role the buoyancy force plays in such a complex flow configuration involving flow separation and reattachment has been studied only numerically in a limited scope by the authors recently, and has not been explored experimentally at all. The proposed research, consisting of coordinated experimental and numerical studies involving separated flows in stepped channels and along stepped flat plates oriented in vertical, inclined, and horizontan positions, is aimed at providing quantitative (measured and predicted) results for the flow and heat transfer in the separated and reattached flow regions for these flow configurations. Emphasis will be placed on the experimental study and the flow and heat transfer results will be obtained for both buoyancy assisting and opposing conditions under heating by uniform wall temperature or by uniform surface heat flux. It is expected that, under the buoyancy assisting flow conditions, the reattachment length will decrease with an increase in buoyancy forces and that when the geometry is inclined or horizontal, a three- dimensional unsteady vortex flow could occur. Experimental observations will define the region and conditions where these characteristics develop in this flow geometry. Both backward- facing and forward-facing steps with different step heights will be treated in the proposed research. Results from the proposed research will be correlated to provide the building blocks that are needed in dealing with heat transfer in complex, buoyancy-affected separated flows that are prevalent in many engineering devices.