Experimental study on heat and momentum transfer for turbulent flow in rectangular channels with a sharp turn

具有180度急转弯的矩形截面蛇形通道常用于紧凑型热交换器和燃气轮机叶片内部冷却通道等热力设备中。在这种180度转弯的通道中，二次流是由绕转弯段流动的离心力引起的，在转弯段内和转弯段后也发生了流动的分离和再附着。因此，通道内的湍流特性变得非常复杂，具有三维结构，并且在传热表面的小距离上，局部换热率有望发生急剧变化。本文利用萘升华技术，研究了带急转弯的矩形截面蛇形通道内湍流的传热特性。在许多可能的实验参数中，主要关注的是以下参数对通道内热（质）传递特性的影响；(i)转弯间隙，（ii）雷诺数，（iii）通道入口入口流动情况，（iv）隔板倾角。测量了紊流条件下各通道壁面的局部传热特性和壁面局部压力的分布。接下来，通过LDV和PIV测量了通道内的流动特性，特别关注了转弯间隙对转弯段内和转弯段后流动结构的影响，这是这类通道中最重要的几何参数。然后，通过比较壁面沿展向中心线的局部传热（质）率分布与壁面附近的平均速度和湍流强度分布，探讨了壁面的传热（质）传机制。

Rectangular cross-sectioned serpentine channels with a sharp 180-degree turn are often used in thermal equipment such as compact heat exchangers and internal cooling passages in gas turbine blade. In such channels with a 180-degree turn, the secondary flow is induced by the centrifugal force in flowing around the turn section, and the separation and reattachment of the flow also occur in and after the turn section. Thus, the turbulent flow characteristics in the channel become very complex with a three-dimensional structure, and the local heat transfer rates are expected to change steeply over small distances on the heat transfer surface.In this study, detailed characteristics of heat transfer for turbulent flow in the rectangular cross-sectioned serpentine channels with a sharp turn have been examined experimentally using the naphthalene sublimation technique. Among many possible experimental parameters, main attention has been directed to the influence of the following for parameters on the heat (mass) transfer characteristics in the channel ; (i) Turn clearance, (ii) Reynolds number, (iii) Flow-inlet condition at the channel entrance, (iv) Inclination angle of the partition wall. The local heat (mass) transfer characteristics on all the walls of the channels and distributions of the local wall pressure have been measured under the condition of turbulent flow.Next, the flow characteristics in the channels have been measured by LDV and PIV, directing a special attention to the influences of the turn clearance, which is the most important geometrical parameter in this type of channel, on the flow structure in and after the turn section. Then, by comparing the distributions of the local heat (mass) transfer rates on the spanwise centerline on the channel walls with those of the mean velocities and turbulence intensities near the walls, the mechanism of heat (mass) transfer from the channel walls have been examined.