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

180度转弯的矩形截面蛇形通道常用于紧凑型换热器和燃气轮机叶片内冷却通道等热工设备中。在180度转弯的流道中，二次流是在转弯段周围的离心力作用下产生的，在转弯段内和转弯段后也会发生分离和再附着现象。因此，通道内的湍流流动特性变得非常复杂，具有三维结构，局部换热速率预计会在换热面上随距离的变化而急剧变化。本研究利用萘升华技术对急转角矩形截面蛇形通道内的湍流换热特性进行了详细的实验研究。在许多可能的实验参数中，主要关注了以下参数对通道内换热(质)特性的影响：(I)转弯间隙，(Ii)雷诺数，(Iii)通道入口处的进流条件，(Iv)隔墙倾角。在湍流条件下，测量了各流道壁面的局部换热(质)特性和局部壁面压力的分布。其次，利用激光测速和PIV对流道内的流动特性进行了测量，重点研究了流道中最重要的几何参数转弯间隙对流道内和流道后流动结构的影响。然后，通过比较通道壁面沿展向中心线上的局部热(质)传递速率与壁面附近的平均速度和湍流强度的分布，研究了通道壁面的热(质)传递机理。

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.