介電液FC-72混合可溶解氣體在傾斜狹窄矩形空間之池沸騰研究

Abstract

本論文是研究介電液 FC-72 在傾斜狹窄矩形空間內的池沸騰熱傳，探討在加入可溶解氣體後，不同間隙的池沸騰熱傳特性。池沸騰實驗中所使用的加熱面為 10 × 10 mm 的光滑銅片，加熱面的角度為傾斜 45°，間隙大小(S)分別為 3 mm、2 mm、1 mm及 0.5 mm以及開放式(無間隙)，氣體次冷度為 0、10、20 及 30 K，壓力為 1 大氣壓。在不同間隙大小、不同氣體次冷度下，對起始過熱度、核沸騰熱傳以及臨界熱通量的影響，並將結果與加熱面角度 0°及 90°的結果互相比較。實驗結果發現，可溶解氣體含量的增加會使起始沸騰提早發生，熱傳係數明顯提升。而間隙愈小，起始沸騰會提早發生，但在中高熱通量時則會使得熱傳係數降低，臨界熱通量提早發生。在三種加熱面角度結果發現 45°在間隙 2mm以及 3mm有較佳的熱傳性能。實驗過程中並拍攝在不同壁面溫度下的池沸騰現象，觀察各種情況下的氣泡成長過程。最後並建立一經驗公式以預測臨界熱通量，將預測值與實驗值比較，發現間隙愈小二者誤差值愈小，在 ±10% 以內；而間隙愈大二者誤差值愈大，在 ±10% ~ ±20% 之間。

Experiments were conducted to investigate the pool boiling heat transfer characteristics of dielectric fluid FC-72 under 1 atmosphere with dissolved gas in an inclined narrow rectangular space. The heated surface is a smooth, 10 × 10 mm copper surface with inclination angle of 45°. Gap sizes (S) and gassy-subcooled temperatures are 3, 2, 1, 0.5 mm, open(no gap) and 0, 10, 20 and 30K respectively. Results showed that the incipient surface superheat was reduced and the heat transfer coefficients were enhanced with more dissolved gas. The incipient boiling might occur more easily with decreasing gap sizes. But, under medium and high heat flux conditions, the heat transfer coefficients and the critical heat flux were reduced. The heated surface with inclination angle of 45° has the best heat transfer properties in gap sizes of 3 mm and 2 mm. During the experiment processes, we also took photographs of the bubble growth at different wall temperature in order to observe the bubble characteristics in the pool boiling region. A correlation was also proposed to predict the critical heat flux and was compared with measured data in which the correlation deviated only to a small amount in small gap sizes.