Research on the Enhancement of Heat/Mass Transfer due to Periodical Impingement of Eddies

本课题的目的是阐明撞击流强化传热传质的机理，并为强化换热技术的发展提供物理模型。在轴对称射流和二维射流中对大尺度湍流对强化换热的影响进行了实验研究，发现冲击射流中的传热传质本质上是非定常的，强化机理受大尺度湍流准周期冲击换热表面的表面更新效应控制。结果表明，不仅到达速度及其湍流强度(速度尺度)是主要参数，特征频率(时间尺度)也是主要参数。通过对到达速度的时间轨迹进行条件采样的统计分析，确定了表面更新效应的特征频率，并在此基础上对在接近射流中插入圆柱形障碍物的增强效应进行了第二阶段的研究。研究发现，在较短的喷嘴-平板距离下，利用圆柱体后的尾迹流可以有效地产生大尺度的湍流，从而强化射流冲击换热并控制其分布。

The objective of this project is to elucidate the mechanism for enhancement of the heat/mass transfer in impinging flows and to obtain a physical model useful for the development of heat transfer augmentation technology. The effect of large-scale turbulent eddies on heat transfer enhancement was experimentally studied in both axisymmetric and two-dimensional jets.It has been found that the heat/mass transfer in impinging jets is unsteady in nature and that the mechanism for enhancement is controlled by the surface renewal effect of large-scale turbulent eddies impinging quasi-periodically on heat transfer surfaces. It has been found that not only the arrival velocity and its turbulence intensity (velocity scale) but also the characteristic frequency (time scale) are predominant parameters. The characteristic frequency for surface renewal effect was determined by a statistical analysis with conditional sampling applied to time-traces of arrival velocity.Based on these results, the second-phase investigation was also made on the enhancement effect of cylindrical obstacles inserted into approaching jet streams. It has been found that it is possible at short nozzle-to-plate distances to enhance the jet impingement heat transfer and to control its distribution by making use of the wake flows behind cylinders which produce effectively large-scale turbulent eddies.