R-134a冷煤在狹窄雙套管中蒸發熱傳之實驗研究

Abstract

本論文是針對R-134a冷媒在狹窄雙套管中的蒸發熱傳和可看見的蒸發流動特性之實驗研究。管子的間隙固定在1.0、2.0和5.0mm。在實驗中，探討了管徑尺寸、冷媒質通量、飽和溫度、熱通量以及蒸氣乾度對熱傳遞係數及蒸發流動特性之影響。在實驗參數的範圍上，在間隙5.0mm中，冷媒質量通量G從100到300kg/m2s，熱通量q從5到15kW/m2，蒸氣乾度由0.05到0.95以及冷媒飽和溫度Tsat從5到15℃。對於間隙2.0和1.0mm，在系統迴路的穩定性考量上，我們設定G的變化分別為300到500以及500到700kg/m2s，其他實驗參數範圍相同於間隙5.0mm。 首先是間隙1.0和2.0mm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近似線性的關係以及在高G中上升較明顯。此外，蒸發熱傳遞係數上升明顯的在增加的q中。並且，蒸發熱傳遞係數顯著上升對於上升的飽和溫度，但是在較窄的管中低熱通量和高質量通量上影響較不明顯。再者，除了在低蒸氣乾度下熱傳遞係數上升明顯隨著上升的冷媒質量通量。在較小的間隙中造成很明顯熱傳遞係數的上升。在較寬間隙5.0mm中與小間隙中有相似的趨勢除了在加熱表面上液膜的乾化。當液膜的乾化現象產生隨著蒸氣乾度的上升蒸發熱傳遞係數則會降低。 除了前述熱傳的數據以外，R-134a蒸發流動的相片由全管的側面以及在管中間小區域所拍攝都在此可以看到。在間隙1.0和2.0mm，低蒸氣乾度下加熱表面氣泡成核很重要。此外，在間隙1.0mm低蒸氣乾度下小氣泡合併成大氣泡再由大氣泡合併成彈狀氣泡較為明顯以及氣泡被驅散在大的液袋區裡。中乾度下，氣泡的成核在加熱表面上仍然可以看見以及在管中主要為蒸氣流過內管的液膜所帶走熱所主導流譜為環形雙向流，在液氣介面上有不規則的波動存在。更高的蒸氣乾度下以環形雙向流主導，環形雙向流主導普遍在高熱通量、低冷媒質量通量和高飽和溫度。在間隙5.0mm中重力的影響變得非常大，蒸氣流在管中上半部，液體流在管子下半部因此以分層雙向流所主導。此外，在低冷媒質量通量和高蒸氣乾度加熱表面上液膜乾化的發生在管中下游的區域，並且在高蒸氣乾度、高熱通量以及低冷媒飽和溫度下會使得乾化往上游移動。 最後，我們將R-134a蒸發熱傳遞係數在所有間隙中的實驗資料做分析，並求出經驗公式。

An experiment is carried out in the present study to investigate the evaporation heat transfer and associated evaporating flow characteristics for refrigerant R-134a flowing in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0, 2.0 and 5.0 mm. In the experiment, the effects of the duct gap, refrigerant mass flux and saturation temperature, imposed heat flux and vapor quality of the refrigerant on the measured evaporation heat transfer coefficient hr and the evaporating flow characteristics will be examined in detail. For the duct gap of 5.0 mm, the refrigerant mass flux G is varied from 100 to 300 kg/m2s, imposed heat flux q from 5 to 15 kW/m2, vapor quality xm from 0.05 to 0.95 and refrigerant saturation temperature Tsat from 5 to 15℃. While for the gap of 2.0 and 1.0 mm, G is respectively varied from 300 to 500 and from 500 to 700 kg/m2s with the other parameters varied in the same ranges as those for δ=5.0 mm. The experimental data for δ=1.0 and 2.0 mm clearly show that evaporation heat transfer coefficient increases almost linearly with the vapor quality of the refrigerant and the increase is more significant at a higher G. Besides, the evaporation heat transfer coefficients also rise substantially at increasing q. Moreover, a significant increase in the evaporation heat transfer coefficients results for a rise in Tsat, but effects are less pronounced in the narrower duct at a low imposed heat flux and a high refrigerant mass flux. Furthermore, the evaporation heat transfer coefficients increase substantially with the refrigerant mass flux except at low vapor quality. We also note that reducing the duct gap causes a significant increase in hr. For the duct with the wider gap of 5.0 mm the effects of the experimental parameters on hr resemble that in the small duct gap except the liquid film covering the heating surface becomes dryout at some downstream locations at low mass flux and high vapor quality. This liquid film dryout results in a reduction of hr with xm. In addition to the heat transfer data presented above, the photos of R-134a evaporating flow are taken from the duct side over the entire duct and over a small region around the middle axial location. In the flow visualization for the small ducts with δ=1.0 and 2.0 mm the bubble nucleation on the heating surface is found to be important at low vapor quality. Besides, at low vapor quality merging of small bubbles to form big bubbles and merging of big bubbles into bubble slugs take place, which is more pronounced at the smaller duct gap for δ=1.0 mm. Moreover, bubbles dispersed in a large liquid slug appear in the duct. At the intermediate vapor quality some bubble nucleation on the heating surface still exists and the flow in the duct is dominated by the vapor flow over thin liquid film around the inner pipe, an annular two-phase flow pattern. Irregular waves appear at the vapor-liquid interface. At the very high vapor quality bubble nucleation can still be seen at high imposed heat flux although the liquid film covering the heating surface is relatively thin. At this high quality the duct is also dominated by the annular two-phase flow. The annular two-phase flow prevails in a larger portion of the duct at higher imposed flux, lower refrigerant mass flux, and higher refrigerant saturated temperature. In the duct with the wider gap of 5.0 mm the effects of the gravity on the evaporating flow are larger, resulting a stratified two-phase flow with the vapor and liquid flows respectively dominated in the upper and lower parts of the duct. Besides, at low refrigerant mass flux and high vapor quality dryout of the liquid film on the heating surface occurs at some downstream locations. These dryout locations move upstream at higher vapor quality, higher imposed heat flux and lower refrigerant saturated temperature. Finally, the empirical correlations for the present measured heat transfer coefficient for the R-134a evaporation in the annular ducts were provided.