介電液FC-72在水平矩形狹窄流道內之流動沸騰熱傳與氣泡特性之實驗研究

Abstract

本論文以實驗方法探討介電液FC-72於水平矩形狹窄流道內，完全發展流動的狀態下，流經平滑與鰭片加熱面之沸騰熱傳與氣泡特性，並針對特定實驗結果利用計算流體力學軟體加以分析驗證。實驗流道之間隙高度為2mm、寬度10mm，水力直徑為3.33 mm，實驗的操作條件設定在一大氣壓，測試範圍在雷諾數500~2000，次冷度在10℃至30℃。 實驗結果發現，核沸騰的起始主要受到壁面過熱度與流速所影響，且小間隙的狹窄流道會使沸騰延遲現象減小，而次冷度與流速對加熱壁面溫度的影響在部份發展核沸騰區逐漸減低，在完全發展核沸騰區則幾乎失去影響力。臨界熱通率會隨次冷度與流速的增加而升高，且鰭片加熱面達到臨界熱通率前，其承受的壁面溫度較平滑面高，臨界熱通率也會提升。矩形流道的間隙之大小不同會改變沸騰起始的模式，間隙較小之流道其沸騰起始具有逐漸形成之特性，而相對間隙較大之流道其沸騰起始則具有驟然產生之特性。

Experiments were conducted to investigate the fully developed flow boiling heat transfer of dielectric fluid FC-72 in a horizontal small-gap rectangular channel under 1 atmosphere. The heated surfaces were smooth and fin structure type. The channel was constructed with the height of 2 mm and the width of 10 mm, to attain a hydraulic diameter of 3.33 mm. The testing parameters are: Reynolds number in the range of 500 to 2000 and subcooling temperature 10℃ to 30℃ as well. Results showed that flow velocity and the heated surface temperature are the major factors that would influence the onset of nuclear boiling, and the narrowness of the narrow gap would attenuate the boiling hysteresis. As the boiling mode reached partially developed nuclear boiling, subcooling temperature and flow velocity started to lose their influence on the wall temperature and finally disappeared on CHF (critical heat flux). The CHF always increased as the values of subcooling and flow velocity increased. The wall temperature of the fin heated surface before CHF could sustain a higher temperature than that of smooth heated surface, therefore the CHF could also be promoted. Different gap size of rectangular channel would lead to different on set boiling mode, in which boiling in small gap channel could be formed gradually while boiling in large gap channel would break out suddenly.