Studies on Heat Transfer in Buoyancy-Affected Separated Flows

在紧凑型换热器、电子设备、数字计算机和燃气轮机等工程设备中，流动分离和回流区的存在对换热特性和换热性能有重要影响。在某些情况下，这些分离的流区被添加到换热装置的设计中以加强换热，但它们对浮力影响的层流的有效性尚未得到证实或实验验证。在温差较大、流速较小的情况下，浮力效应变得明显。浮力在涉及分离和再附着的复杂流型中所起的作用，作者最近只是在有限的范围内进行了数值研究，完全没有实验研究。这项研究包括阶梯流道和沿垂直、倾斜和水平方向的阶梯平板中分离流动的协调实验和数值研究，目的是为这些流动形态下分离和再附流区的流动和换热提供定量(测量和预测)结果。重点进行了实验研究，给出了均匀壁温加热和均匀表面热流加热时，助浮力和逆浮力作用下的流动和换热结果。预计在浮力辅助流动条件下，再附着长度随浮力的增大而减小，当几何形状倾斜或水平时，可能会出现三维非定常涡流。实验观测将确定这些特性在这种流动几何中发展的区域和条件。在拟议的研究中，不同台阶高度的后向台阶和向前台阶都将被处理。拟议的研究结果将相互关联，以提供在处理复杂的、受浮力影响的分离流动中所需的构建块，这些流动在许多工程设备中非常普遍。

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.