噴流衝擊於具溝槽表面之熱傳研究

Abstract

本研究應用暫態液晶熱像法(Transient liquid crystal thermography)在衝擊冷卻通道內具溝槽表面的熱傳量測。實驗操作流量雷諾數為2500、5100、7700。衝擊目標面分為平滑與矩形溝槽，溝槽方向分為0°、45˚、90°，溝槽底面分為平直與錐形(Tapered)溝槽。溝槽涵盖範圍為全溝槽與半溝槽兩類。衝擊氣流分別為對正(Inline)衝擊於溝槽以及交錯(Staggered)衝擊兩種。衝擊孔口板厚度為5 mm，而圓孔直徑5 mm以4×12矩形陣列，孔口間距為2倍孔洞直徑。噴流孔到受衝擊目標面間距為3倍孔洞直徑。噴流孔口間距與噴流孔口到目標衝擊面間距(H/d)為4 and 3。結果指出橫向對正較縱向對正溝槽擁有更好的熱傳分佈。縱向溝槽的橫向流效應較橫向溝槽小，且噴流確實擊於溝槽處。以縱向對正溝槽為研究的主軸，探討三種出口類型：下游出口、雙向出口、上游出口。雷諾數越高，上游氣流向下推擠，衝擊噴流向出口方向偏移，使噴射流無法正交衝擊於目標處，因此上游熱對流高於下游。以45˚溝槽分佈於全測試片的研究，不同於橫向對正溝槽下游處，因橫向流使噴流偏離出溝槽外，引起的氣流向45˚溝槽流動影響通道內流場的對稱，熱傳平均分佈的效益較低。 而半溝槽分為上游溝槽與下游溝槽兩種，以上游溝槽的整體平均紐塞數高於下游溝槽。其原因為上游的溝槽受到噴流衝擊的流速最大，而越處於下游處的噴射流場因橫流而使噴流偏移，因此上游為溝槽能提高熱傳效應。若下游為溝槽的測試片，因氣流推擠效應，噴射氣流偏離正交溝槽位置與流速削弱。以上游溝槽的類型高於下游溝槽。以全光滑測試片為基準，上游半溝槽縱向對正類型的熱傳效果最高，平均紐塞數在出口方向1雷諾數5100下較全光滑表面約高出19.52%。 錐形溝槽(Tapered grooves)分為由深至淺的正錐形(Forward tapered groooves)與由淺至深的逆錐形(Backward tapered grooves)之縱向溝槽。因在縱向溝槽的橫向流效應較小，氣體於溝槽內，配合正錐形漸淺溝槽深度形成上坡流場，減緩了溝槽內流出速度，增強熱對流與衝擊效應之熱傳。正錐形與縱向水平全溝槽比較方向1出口，其熱傳增強最大範圍為73.7%而最低增強幅度也有38%。亦同出口方向在雷諾數5100下比光滑目標面增強約高30.42%。逆錐形熱傳高於縱向水平全溝槽，而最大強化幅度上升約60.78%,最小為增幅則為28.27%。比起全光滑表面在方向1出口雷諾數5100下約高出19.97%的平均紐塞數。在方向1出口中，錐形溝槽類型以正錐形熱傳衝擊對流效應比逆錐形溝槽高約8.7%平均紐賽數。具錐形溝槽的熱傳效益，優於縱向全溝槽與45˚溝槽。出口類型衝擊熱傳效益，以雙出口類型其次為下游出口而上游出口熱傳效益最低。

關鍵字: 噴流衝擊、暫態熱傳、暫態液晶顯影技術、熱傳紐賽數、溝槽

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