Basic Research on Heat Transfer Augmentation in Rotating Boundary Layers

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In order to obtain fundamental knowledge for heat transfer augmentation in rotating machinery, the influence of system rotation on the heat transfer characteristics of transitional and turbulent zero-pressure gradient boundary layers have been experimentally investigated. A test plate is installed in a wind tunnel, which is rotatable around the axis parallel to the plate leading edge with constant speed of rotation. Local heat transfer coefficient during rotation is determined by employing a temperature sensitive liquid crystal. The thermo-couples were employed for compensation of conductive heat loss to the solid wall. Effects of the Coriolis force and the centrifugal buoyancy force have been examined by comparing the heat transfer coefficient with different freestream velocity, rotational speed and wall temperature.When the Coriolis force acts toward the wall, heat transfer in a laminar boundary layer is dramatically enhanced. This can be explained by generation of streamwise vortices by Coriolis torque and resulting enhancement of transverse convection of energy. The turbulent heat transfer is slightly enhanced by wall-ward Coriolis force, which may be attributed to the elevated turbulent diffusion through the modifications of turbulent eddies by system rotation.For the opposite direction of rotation, Coriolis force stabilization exhibits no systematic effect on a laminar heat transfer, while it causes the slight decrease in heat transfer coefficient in a turbulent boundary layer. The latter is attributed to the attenuation of turbulent diffusion by Coriolis stabilization.The centrifugal buoyancy enhances the turbulent heat transfer both in pressure and suction side boundary layers. It has been suggested that this effect may be due to the modified convection through the buoyancy term in the Reynolds stress equation.