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

本计画的目的在于阐明撞击流强化热质传递的机制，并获得一个有助于发展强化热质传递技术的物理模型。在轴对称和二维射流中，对大尺度湍流涡强化传热的作用进行了实验研究，发现射流中的传热/传质本质上是非定常的，其强化机理是由大尺度湍流涡准周期性地撞击传热表面的表面更新效应所控制的。结果表明，不仅到达速度及其湍流强度（速度尺度），而且特征频率（时间尺度）是主要参数。通过对到达速度时间轨迹的条件抽样统计分析，确定了表面更新效应的特征频率，并在此基础上，对近场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。已经发现，在短的壁面距离下，可以通过利用柱后的尾流来增强射流冲击传热并控制其分布，该尾流有效地产生大尺度湍流涡流。

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.