Jet Impingement Heat Transfer On Target Surface with Grooves

• Main Authors: Lo, Yuan-Hsiang, 羅元祥
• Other Authors: Liu, Yao-Hsien
• Format: Others
• Language: zh-TW
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/uvtbc3

博士 === 國立交通大學 === 機械工程系所 === 105 === ABSTRACT Transient liquid crystal technology is used for measuring the heat transfer Transient liquid crystal technology was used for measuring the heat transfer coefficient in the impingement cooling channel. The target surface was roughened through the creation of rectangular grooves aligned with the jet holes (Inline pattern) or between the jet holes (Staggered pattern). The grooves were designed either parallel (Longitudinal grooves) or orthogonal (Transverse grooves) to the exit flow directions. Jet-to-jet spacing and jet-to-surface spacing (H/d) were 4 and 3, respectively. In this experimental test, the effect of crossflow was investigated for three exit flow directions, each with a jet Reynolds number ranging from 2500 to 7700. Detailed heat transfer distributions from arrays of impinging jets on a half-smooth, half-rough target surface were investigated. Heat transfer was enhanced near the edge of grooves, whereas the heat transfer was degraded inside grooves. For the half-smooth, half-rough surface, the sudden change in surface geometry broke the flow development and caused intensified flow mixing in the impingement flow channel. Compared with fully roughened surfaces, the half-rough surface was more effective for heat transfer, and an enhancement of more than 50% was achieved for the longitudinal grooves. Compared with smooth surface, Downstream grooves was higher for heat transfer, and an enhanceemnt of more than 19.53% achieved for the smooth in the Reynolds number of 5100 for orientation 1. For the 45˚ Angled grooves, effect of crossflow pushed impinging jets away from the target surface and the heat transfer was reduced downstream when the flow exited from downstream. The jet flow impinged on the groove surfaces and the flow was distributed along the Angled grooves. Thus, the moving fluid stream caused asymmetric Nusselt number distribution on the target surface and produced low average Nusselt numbers. To reduce the influence from the crossflow, the tapered longitudinal grooves were designed such that the groove depth varied along the exit flow direction. For the tapered grooves with decreasing groove depth, a heat transfer enhancement of at least 38% and largest 73.7% was attained compared to non-tapered longitudinal grooves for the flow exiting from downstream. The Nusselt number was enhanced for the small depth grooved region since it intensified impinging jet effect in grooves. Compared with the full roughened surface, For the tapered grooves with increasing groove depth is enhancer for average Nusselt number, and an increase of more than maximum 60.8% and then minimum 28.7% was gotten for non-tapered longitudinal grooves. Furthermore contrast with smooth, Backeard Tapered grooves is higher to accomplish 19.97% for heat transfer. Forward tapered grooves is more beneficent heat transfer than backward tapered grooves, the Nusselt number is higher than 8.7%. The highest impingement heat transfer was found near the regions with minor crossflow effect. The flow exiting from both ends achieved the highest heat transfer because of smallest crossflow effect. For the flow exiting from upstream, the lowest Nusselt numbers were obtained. The tapered grooves with the decreasing groove depth along the streamwise direction achieved the highest impingement heat transfer among all the test cases. Keywords: Jet impingement, Transient heat transfer, Transient liquid crystal technology, Nusselt Number, Grooves