R-410A冷媒在狹窄雙套管中蒸發熱傳之實驗研究

Abstract

摘要 本研究以實驗方式探討R-410A冷媒在水平狹窄雙套管中蒸發熱傳及相關流譜之特徵。流道之間隙固定在2.0mm。我們探討了冷媒質通量、飽和溫度、熱通量以及蒸氣乾度對熱傳系數及蒸發流動之影響。在實驗結果中，冷媒質通量從300到500 kg/m2s，飽和溫度從5到15℃，熱通量從5到15 kW/m2和蒸氣乾度由0.05到0.95。 在實驗數據中，可以很清楚的看到蒸發熱傳遞係數上升隨著冷媒蒸氣乾度的上升呈現性關係以及在高G中上升較明顯。此外，蒸發熱傳遞係數上升在增加q時相當的明顯，並且在增加飽和溫度也是可以很容易的看出來。 我們從側面拍攝了R-410A在蒸發流動時的全管照片，發現在低乾度時，氣泡的成核址在加熱表面是非常重要的，並且它是由小氣泡合併到大氣泡再形成彈狀流。在中乾度的時候，在入口處能然有發現部分的氣泡流，並且在下游處形成環型流，在出口處則是為一環型液膜包覆著加熱表面。在高乾度則是為一環型流，並且在高熱通量時，會在管入口處發生擁塞現象。再著，我們使用的高速攝影機拍攝壁面的狀態，在低乾度的時候，在增大熱通量，成核址的密度也明顯的上升了；並且在中乾度的時候，在高熱通量以及低流量的條件之下，管入口處仍然可以發現到成核址的存在；而在高乾度、q=15kW/m2的時候，在液膜非常薄的情況之下，我們能然可以看到成核址。 在比較R-410A以及R-134a冷媒在相同的管徑以及條件之下，R-410A的熱傳系數明顯的比R-134a高的許多，這可以歸因於R-410A有較高的潛熱以及較低的表面張力。 最後，我們將R-410A蒸發熱傳系數的資料作分析，並求出經驗公式推得在此參數間的熱傳係數。

An experiment is carried out in the present study to investigate the characteristics of the evaporation heat transfer for refrigerant R-410A flowing in a horizontal narrow annular duct. The gap of the duct is fixed at 2.0 mm. In the experiment, the effects of the refrigerant mass flux, saturation temperature, and vapor quality and the imposed heat flux on the measured evaporation heat transfer coefficient and the photos of the evaporating flow are explored. For the duct gap of 2 mm, the refrigerant mass flux G is varied from 300 to 500 kg/m2s, imposed heat flux q from 5 to 15 kW/m2, mean vapor quality xm at the middle axial location of the duct from 0.05 to 0.95 and refrigerant saturation temperature Tsat from 5 to 15℃.

The experimental data for δ=2.0 mm clearly show that the evaporation heat transfer coefficient increases almost linearly with the vapor quality of the refrigerant and the increase is more significant at a higher q. Besides, the evaporation heat transfer coefficient also rises substantially at increasing G. moreover, a significant increase in the evaporation heat transfer coefficient results for a rise in Tsat.

The photos of R-410A evaporating flow taken from the duct side over the entire duct for selected cases show that the bubble nucleation on the heating surface is found to be important at low vapor quality. Besides, at low vapor quality merging of small to from big bubbles and merging of bug bubbles into bubble slugs take place. At intermediate quality only in the entry portion of the duct the bubble nucleation dominates. Slightly downstream merge of big bubbles to form a few bubble slugs appears in the top portion of the annular duct. Near the exit of the duct the evaporation of liquid film flow dominates. For high vapor quality bubble nucleation and strong moving waves can still be seen in the small portion near the duct inlet at a high imposed heat flux, although the liquid film on the heating surface is very thin. Moreover the high-speed photos of the flow for selected small regions in the duct further indicate that at low vapor quality bubble nucleation density on the heating surface increases with the imposed heat flux. At the intermediate vapor quality some bubble nucleation on the heating surface can also be observed in the middle section of the duct at a high q and lower mass flux. And at the high vapor quality bubble nucleation can still be seen at q=15kW/m2 although the liquid film covering the heating surface is very thin.

Comparison of the evaporation heat transfer of refrigerants R-410A and R-134a in the same annular duct manifests that the heat transfer coefficients of R-410A are much higher than R-134a. Besides the annular flow prevails over a larger portion of the duct for the R-134a evaporation for the same set the experimental parameters.