Structure of Turbulent Heat Transfer in a Separated and Reattched Flow

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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