R-134a和R-407C冷媒在水平多排小圓管內蒸發熱傳及壓降特性之實驗研究

Abstract

本論文是分別針對R-134a和R-407C兩種冷媒在0.83 mm和2.0 mm兩種內徑的28根水平小圓管內蒸發熱傳及壓降特性之實驗研究。實驗的目的是探討改變冷媒的飽和溫度、冷媒的質通量、測試段的熱通量及測試段的進口蒸氣乾度對R-134a和R-407C兩種冷媒在此兩種小管熱交換器的影響。在實驗參數的範圍上，冷媒的飽和溫度Tsat被設定從5到15℃，測試段熱通量q從5到15 kW/m2，進口蒸氣乾度xin大約從0.2到0.8。在冷媒的質通量G方面，由於管徑從2.0 mm縮小到0.83 mm，因此在系統迴路的穩定性考量上，我們設定G在2.0-mm小管是從200到400 kg/m2s，在0.83-mm小管是從800到1500 kg/m2s。 經由實驗的結果可知，R-134a和R-407C兩種冷媒在0.83-mm和2.0-mm兩種管徑中，我們發現除了在較低的質通量和較高的熱通量，蒸發熱傳係數與摩擦壓降係數皆會隨著進口乾度的增加而明顯地上升。另外，在熱通量的影響方面，熱傳係數也會隨著熱量的增加而上升。但是相較摩擦壓降係數的影響則是很微弱的。而在飽和溫度的影響上，蒸發熱傳係數會隨著系統的飽和溫度上升而增加；但是對於摩擦壓降係數則會隨著飽和溫度的上昇而減少。此外，R-134a和R-407C的蒸發熱傳與摩擦壓降均隨著冷媒質通量的增加而增加。值得一提的是，R-134a冷媒在0.83-mm小管中，若供給較低的冷媒質通量和較高的熱通量，蒸發熱傳係數可能會隨著乾度的增加而下降。這是由於冷媒在小管內流動時，產生部分乾化的現象，因此影響蒸發熱傳係數的下降。 在相同的實驗條件與管徑的比較下，我們發現R-407C冷媒其整體的蒸發熱傳係數會比R-134a冷媒來得高。然而，在摩擦壓降係數方面，則較R-134a冷媒低。 最後，我們將蒸發熱傳係數與摩擦壓降係數在0.83-mm和2.0-mm小管中的實驗資料作分析，並求出經驗公式。

An experiment is carried out in the present study to investigate the characteristics of the evaporation heat transfer and frictional pressure drop for refrigerants R-134a and R-407C flowing in horizontal small tubes having the same inside diameter of 0.83 mm or 2.0 mm. In the experiment for the 2.0-mm small tubes, the refrigerant mass flux G is varied from 200 to 400 kg/m2s, imposed heat flux q from 5 to 15 kW/m2, inlet vapor quality xin from 0.2 to 0.8 and refrigerant saturation temperature Tsat from 5 to 15℃. While for the 0.83-mm small tubes, G is varied from 800 to 1500 kg/m2s with the other parameters varied in the same ranges as those for Di=2.0 mm. In the study the effects of the refrigerant vapor quality, mass flux, saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficients and frictional pressure drops are examined in detail. For R-134a and R-407C in the 2.0-mm or 0.83-mm small tubes, the experimental data clearly show that both the R-134a and R-407C evaporation heat transfer coefficients and frictional pressure drops increase almost linearly and significantly with the vapor quality of the refrigerant, except at low mass flux and high heat flux. Besides, the evaporation heat transfer coefficients also increase substantially with the rise in the imposed heat flux. But the effect of q on the frictional pressure drop is rather weak. Moreover, a significant increase in the evaporation heat transfer coefficients results for a rise in Tsat, but an opposite trend is noted for the frictional pressure drop. Furthermore, both the R-134a and R-407C evaporation heat transfer coefficients and frictional pressure drops increase substantially with the refrigerant mass flux. At low R-134a mass flux and high imposed heat flux the evaporation heat transfer coefficient in the small tubes (Di=0.83 mm) may decline at increasing vapor quality when the quality is high. This decline of hr at rising x is attributed to the partial dryout of the refrigerant flow in the small tubes at these conditions. We also note that under the same xin, Tsat, G, q and Di, refrigerant R-407C has a higher evaporation heat transfer coefficient and a lower frictional pressure drop when compared with that for R-134a. Finally, the empirical correlations for the R-134a and R-407C evaporation heat transfer coefficient and friction factor in 0.83-mm and 2.0-mm small tubes are proposed.