狹窄水平雙套管中R-407C和R-410A冷媒流動沸騰熱傳和氣泡特性研究

Abstract

本研究以實驗方式探討R-407C和R-410A冷媒在水平狹窄雙套管中流動沸騰熱傳(含次冷及飽和流動沸騰)及相關氣泡特徵之影響。流道之間隙由1.0至2.0mm。我們探討了冷媒質通量、飽和溫度、入口次冷度、管徑尺寸以及熱通量對熱傳系數及氣泡特徵的影響。氣泡特徵包含氣泡脫離直徑和頻率以及成核密度可由流場觀測得之。在實驗結果中，冷媒質通量從300到700 kg/m2s，飽和溫度從10到15℃，熱通量從0到45 kW/m2 ，入口次冷度從0到6℃。 首先，提出冷媒R-407C和R-410A在飽和流動沸騰之測試結果。從所量測之沸騰曲線發現，沸騰初始之溫度突降現象在飽和流動沸騰中並不明顯。飽和流動沸騰熱傳係數隨尺寸減少而增加。此外，熱通量增加亦會使得熱傳係數明顯增加。然而，冷媒R-407C之質通量及飽和溫度對熱傳係數影響甚小。但是冷媒R-410A之熱傳係數會隨著質通量及飽和溫度增高而變大。從流場觀測之結果顯示氣泡脫離尺寸隨著質通量增加而稍微減小。再者，熱通量的提升對氣泡的提升，合併及脫離有正向影響。飽和溫度的下降會造成氣泡脫離頻率和有效成核位址的降低。除此之外，在高熱通量下，許多氣泡從加熱表面之成核孔產生並趨向合併成大氣泡。 接著，冷媒R-407C和R-410A在次冷流動沸騰之熱傳結果以沸騰曲線及熱傳係數型式表示。在沸騰曲線中可發現沸騰初始之溫度突降現象在次冷流動沸騰中非常明顯。次冷流動沸騰熱傳係數隨間隙尺寸減少而增加但隨次冷度增加而減少。此外，熱通量增加亦會使得熱傳係數明顯增加。然而，冷媒之質通量及飽和溫度對熱傳係數影響不大。次冷流動沸騰之流場觀測顯示氣泡隨冷媒之質通量及次冷度增加而被抑制，然而增加熱通量對氣泡數目、合併及脫離頻率則有正面影響。熱通量的提升對氣泡的提升，合併及脫離有正向影響。再者，飽和溫度降低造成氣泡脫離頻率及成核密度減小。 冷媒R-134a、R-407C和R-410A在相同水平狹窄雙套管中的流動沸騰比較可看出R-410A的熱傳係數比其他兩種冷媒高。R-410A的脫離氣泡直徑是三者中最小的，但脫離氣泡頻率以及成核密度是最高的。 最後，我們把這個實驗中流動沸騰氣泡脫離直徑、產生頻率及成核密度的資料作分析，求出氣泡特微之3個經驗式，並利用此經驗公式推得氣泡流區之熱傳係數。

The heat transfer and associated bubble characteristics in the saturated and subcooled flow boiling of refrigerants R-407C and R-410A in a horizontal narrow annular duct are investigated experimentally. The gap of the duct is fixed at 1.0 mm and 2.0 mm. We will examine the effects of the refrigerant mass flux, saturation temperature and inlet subcooling, gap size of the duct, and the imposed heat flux on the heat transfer coefficients and bubble characteristics. The bubble characteristics are obtained from the flow visualization, which include the mean bubble departure diameter and frequency and mean active nucleation site density. In the experiment, results are obtained for the refrigerant mass flux varied from 300 to 700 kg/m2s, refrigerant saturation temperature from 10 to 15℃, imposed heat flux from 0 to 45 kW/m2 and inlet subcooling from 0 to 6℃. First, the experimental heat transfer data for the saturated flow boiling of R-407C and R-410A in the horizontal narrow annular duct are examined. From the measured boiling curves, the temperature overshoot at ONB are insignificant for the saturated flow boiling of R-407C and R-410A in the duct. The saturated flow boiling heat transfer coefficient increases with a decrease in the gap size of the duct. Besides, raising the imposed heat flux can also cause a significant increase in the boiling heat transfer coefficients. However, the effects of the refrigerant mass flux and saturated temperature for R-407C on the boiling heat transfer coefficient are small. But increases in the refrigerant mass flux and saturated temperature for R-410A result in higher heat transfer coefficients. The results from the flow visualization show that the mean diameter of the bubbles departing from the heating surface decreases slightly at increasing mass flux. Moreover, raising the imposed heat flux produces positive effects on the bubble population, coalescence, departure frequency. Furthermore, a decrease in the saturated temperature results in a reduction in the bubble departure frequency and active nucleation site density. Besides, at a high imposed heat flux many bubbles generated from the cavities in the heating surface tend to merge together to form big bubbles. Then, the experimental heat transfer data for the subcooled flow boiling of R-407C and R-410A in the narrow duct are expressed in terms of boiling curve and boiling heat transfer coefficient. From the boiling curves, the temperature overshoot at ONB are significant for the subcooled flow boiling of R-407C and R-410A in the duct. The subcooled boiling heat transfer coefficient increases with a reduction in the gap size, but decreases with an increase in the inlet liquid subcooling. Besides, raising the imposed heat flux can cause a significant increase in the boiling heat transfer coefficient. However, the effects of the refrigerant mass flux and saturated temperature on the boiling heat transfer coefficient are milder. Visualization of the subcooled flow boiling processes reveals that the bubbles are suppressed by raising the refrigerant mass flux and inlet subcooling. Moreover, raising the imposed heat flux produces positive effects on the bubble population, coalescence, departure frequency. Furthermore, a decrease in the saturated temperature results in a reduction in the bubble departure frequency and active nucleation site density. Comparison of flow boiling of refrigerants R-134a, R-407C and R-410A in the same annlar duct manifests that the heat transfer coefficients of R-410A are much higher than the other two refrigerants. The mean bubble departure diameter of R-410A is smallest but the mean bubble departure frequency and active nucleation site density are highest among three refrigerants. Correlations for the present data for the subcooled and saturated flow boiling heat transfer coefficients of R-407C and R-410A in the narrow annular duct are proposed. Besides, the present data for some quantitative bubble characteristics such as the mean bubble departure diameter, departure frequency and the mean active nucleation site density for R-407C and R-410A are correlated.