薄霧流於具肋條通道內之熱傳研究

Abstract

薄霧流冷卻為一種利用空氣混合微小液滴形成兩相流來進行散熱的技術，相較於傳統的單相強制對流，薄霧流的散熱效果要高上許多。本研究使用水力直徑為40mm的垂直正方形流道進行實驗，由噴嘴混和水與空氣，送入載流空氣，並在流道後段的一個壁面裝置加熱片，改變載流氣體的流量和加熱片表面，以紅外線熱像儀拍攝其溫度的變化和分布，並配合熱電偶的使用，分析薄霧流的熱傳機制，計算實驗的紐賽數。實驗使用的雷諾數為7900、16000及24000，肋條高度對流道水力直徑比為0.05，肋條間距對高度比為5、10、20。實驗結果顯示薄霧冷卻的效果良好，在平滑表面下的紐賽數約為空氣冷卻的4到6倍，在肋條表面下反而有較差的熱傳增益效果，約是空氣冷卻的2.5到3.5倍，顯示因肋條的安裝而增加的紊流強度是不利於液膜附著的，因此在肋條表面下的平均紐賽數皆低於平滑表面的例子。

Heat transfer in a rib-roughened square duct with mist flow was experimentally determined using infrared thermography. The mist flow was generated by introducing dispersed water droplets into the air stream. The heat transfer measurement was performed on a heated surface located inside the duct with a hydraulic diameter of 4cm. Three Reynolds numbers of 7900, 16000 and 24000 were investigated. The rib pitch-to-height ratios are 5, 10 and 20 and the rib height-to-hydraulic diameter ratio is 0.05. The results showed that mist flow cooling achieved higher heat transfer rates compared to the air cooling. The heat transfer enhancement is 4 to 6 times in the smooth surface case. In the ribbed surface case, the increased turbulence intensities by the presence of ribs hindered the attachment of the liquid film. The average Nusselt number of the ribbed surface was lower than that of the smooth surface.