具溶解氣體之介電液FC-72在垂直狹窄矩形空間中的池沸騰熱傳分析

Abstract

本論文主要是研究介電液FC-72在垂直狹窄矩形空間中的池沸騰熱傳，探討在加入可溶解氣體後，不同間隙對池沸騰熱傳的影響。池沸騰實驗中所使用的加熱面為10 mm × 10 mm 的光滑銅片，加熱面的角度為垂直(90°)，間隙大小(S)分別為3 mm、2 mm、1 mm及0.5 mm，氣體次冷度為0、10、20及30 K，壓力為1大氣壓。實驗目的為分析在各種操作參數下，從低熱通量(30%CHF以下)、中熱通量(30%~60%CHF)至高熱通量(60%~90%CHF)的池沸騰熱傳，並分析影響其熱傳係數的原因。實驗結果發現，低熱通量時，相同熱通量下的壁面溫度會隨著可溶解氣體含量的增加而降低，主要原因是增加氣體含量可以降低初始沸騰的熱通量；中高熱通量則因液體中所含的氣體被除氣所致，為純液體核沸騰，熱傳係數無明顯的提升。當間隙逐漸減小，在低熱通量時，起始沸騰熱通量會隨著間隙減小而降低，主要是因為間隙愈小，在間隙中的液體愈容易達到沸騰所需的過熱度，沸騰愈容易發生；但在中高熱通量受到間隙減小的影響，蒸氣膜會提早產生並覆蓋在加熱面上，反而降低了熱傳，而臨界熱通量也因而提早發生。在實驗中並拍攝在低熱通量、中熱通量及高熱通量下的池沸騰現象，觀察氣泡成長過程，以及對池沸騰曲線的影響。最後並利用經驗公式預測臨界熱通量，將預測值與實驗值比較，結果發現，間隙愈大誤差值愈小，約±40%左右，而間隙愈小誤差值愈大，約±90%左右。

Experiments were conducted to investigate the pool boiling heat transfer of dielectric fluid FC-72 under 1 atmosphere with dissolved gas in a vertical narrow rectangular space. The heated surface is a smooth, 10 × 10 mm copper surface. Gap sizes (S) and gassy-subcooled are 3, 2, 1, 0.5 mm, and 0, 10, 20 and 30K respectively. Results showed that heat transfer coefficients were enhanced with increased dissolved gas under low heat flux, because the dissolved gas reduced the incipient surface superheat. Under middle and high heat flux, the liquid was degassed with the result that the boiling was pure liquid nucleate pool boiling, and, hereafter, the heat transfer coefficients could not be raised with increasing dissolved gas. The heat transfer coefficients increasing with decreasing gap sizes in low heat flux, because the smaller gap sizes, the greater the liquid superheat in the narrow space, and incipient boiling might occur more easily. On the contrary, under middle and high heat flux, the heat transfer coefficients decreased with decreasing gap sizes because vapor film might cover on heated surface more easily. The critical heat flux (CHF) also decreased with decreasing gap sizes. In the experiment, we also took photographs of the bubble growth from low to high heat flux in order to observe the bubble characteristics in the pool boiling phenomena. A correlation was proposed to predict the critical heat flux and was compared with measured results which showed that the correlation deviated quite obviously in small gap sizes.