Basic Research on Heat Transfer Augmentation in Rotating Boundary Layers

为了获得旋转机械强化传热的基础知识，本文对系统旋转对过渡和湍流零压力梯度边界层传热特性的影响进行了实验研究。试验板安装在风洞中，其可绕平行于板前缘的轴线以恒定的转速旋转。采用温度敏感液晶确定旋转过程中的局部传热系数。采用热电偶补偿固体壁面的传导热损失。通过比较不同自由落体速度、旋转速度和壁面温度下的换热系数，考察了科里奥利力和离心浮力对层流边界层换热的影响，结果表明，科里奥利力作用于壁面时，层流边界层的换热得到了显著的强化。这可以通过科里奥利扭矩产生流向涡以及由此产生的能量横向对流的增强来解释。壁向科里奥利力对湍流换热有一定的强化作用，这可能是由于系统旋转对湍流涡的修正作用增强了湍流扩散;而在相反的旋转方向上，科里奥利力的稳定作用对层流换热没有系统的影响，但会使湍流边界层的换热系数略有降低。离心浮力增强了压力侧和吸力侧边界层中的湍流换热。有人认为，这种影响可能是由于通过雷诺应力方程中的浮力项修改对流。

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.