Structure of Turbulent Heat Transfer in a Separated and Reattched Flow

对于许多类型的换热器来说，分离、再连接和再开发区域的湍流换热预测是非常重要的。大量的研究表明，换热系数在再附着流区达到最大值。此外，最近的研究表明，最大换热系数点通常不同于时间平均再附着点，它位于再附着点的上游。然而，在之前的大多数研究中，使用了两种不同的测试表面。分离区和再附着区以及再附着点下游的湍流边界层内的流动极为复杂，其详细的流动结构，特别是涉及到湍流温度波动的特征；还没有澄清。本研究的目的是研究有限厚度钝平板上的二维气流在分离区、再附着区和更发达区域的紊流换热机理。为了准确地阐明传热行为与流动行为之间的关系，在同一板上进行了传热和流动的测量。所得的要点总结如下。换热系数最大值位于7.8H （H为板厚的一半），平均再附点为9.7H，确定为50%逆流速率点。也就是说，最大换热点存在于流动再附着的下游。在换热系数最大时，回流速率仅为15%，壁面附近的温度波动强度和雷诺数应力相对于再附着点较高。在分离泡中，最大温度波动的位置与最大速度波动的位置不同，位于分离剪切层的外侧区域。温度波动的积分时间尺度在分离泡分界附近达到最大值，在温度波动强度最大的点附近达到最小值。另一方面，在再发育流的整个区域内，它没有表现出本质的变化。功率谱和概率密度函数的特征在很大程度上取决于流区分离、再连接和再发展等流特征。在再附着流区，由于分离泡的大规模涡流脱落，功率谱存在一个宽的驼峰。此外，基于偏度和平整度因子的结果，流动的分离和再附着对温度波动统计特征的影响延伸到下游。少

Prediction of turbulent heat transfer in the separated, reattached, and redeveloped regions is very important in relation to many types of heat exchangers. It has been clarified by numerous previous studies that the heat transfer coefficient reaches a maximum in the reattachment flow region.Furthermore, recent works have shown that the point of maximum heat transfer coefficient is, in general, different from the time-mean reattachment point, and it locates slightly upstream of the reattachment one. However, in the most of these previous studies, two different test surfaces were used. The flow in the separated and reattached regions and in the turbulent boundary layer downstream of the reattachment point is extremely complicated, and its detailed flow structure, especially concerning the characteristics of turbulent temperature fluctuation; has not been clarified.The purpose of the present study was to investigate the turbulent heat transfer mechanism in the separated, reattached, and r … More edeveloped regions of two-dimensional air flow over a blunt flat plate with finite thickness. Measurements of heat transfer and flow were conducted using the same plate in order to clarified exactly the correlation between the heat transfer behaviors and the flow ones.The main points obtained are summarized as follows.The maximum heat transfer coefficient locates at 7.8H ( H being half the plate thickness ) and the mean reattachment point is 9.7H, which was determined as a point of 50% reverse flow rate. That is, the maximum heat transfer point exists somewhat downstream of the flow reattachment. At the point of maximum heat transfer coefficient, the reverse flow rate is only 15% and the temperature fluctuating intensity and the Reynolds stress near the wall are relatively higher compared to those at the reattachment point.In the separation bubble, the location of the maximum temperature fluctuations is different from that of maximum velocity fluctuation and is situated in the outer region of the separated shear layer.The integral time scale of the temperature fluctuation attains maximum near the dividing boundary in the separation bubble and minimum near the point of the maximum temperature fluctuation intensity.On the other hand, it does not show essential change in the whole region of the redeveloped flow.The characteristics of the power spectra and probability density function depend strongly upon the flow feature, such as separated, reattached, and redeveloped flow regions. In the reattached flow region, there exists a broad hump of the power spectrum due to the shedding of large-scale eddies from the separation bubble. Furthermore, based on the results of the skewness and the flatness factors, the effect of the separation and reattachment of the flow upon the statistical features of the temperature fluctuation extends far downstream. Less