R-407C冷媒於水平雙套鰭管中流動沸騰熱傳及氣泡特性之實 驗研究

Abstract

R-407C冷媒於水平雙套鰭管中流動沸騰熱傳及氣泡特性之實驗研究 學生：施駿達 教授：林清發 教授 國立交通大學 機械工程研究所 摘要 在這篇論文中針對R-407C冷媒於水平雙套鰭管中流動沸騰熱傳之實驗作了詳細的研究，並將水平雙套鰭管與水平雙套平滑管的熱傳特性作比較。此外，也藉由流場觀測研究氣泡於水平雙套鰭管中的特性。實驗參數的範圍，冷媒質通量G從150到250 kg/m2s，測試段熱通量q從5到45 kW/m2，平均蒸氣乾度Xm從0.2到0.8，入口過冷度DTsub為0到7 ℃，系統壓力設定在567，667，776和900 kPa (Tsat =0, 5℃, 10℃ and 15℃)。 在次冷態流動沸騰熱傳結果中，首先比較在加熱面三個不同軸向位置，分別是位於上游、中游及下游的局部沸騰曲線。其結果指出在鰭管下游的位置要達到沸騰起始 (ONB) 所需的壁面過熱度較低。在水平雙套鰭管中，沸騰磁滯現象及沸騰起始所產生的壁溫驟降(temperature undershoot)都不明顯。隨入口過冷度降低，其熱傳係數將增加。由流場觀測結果顯示在較高冷媒質通量下，較高冷媒速度將氣泡從加熱面上掃除，造成較小的氣泡脫離半徑。較高入口過冷度下，較低的冷媒液體溫度將抑制氣泡於加熱面上成長; 但隨熱通量增加，氣泡成長、合併及產生頻率也隨著增加。 在飽和態R-407C冷媒於水平雙套鰭管中之沸騰熱傳實驗研究中，由局部的沸騰曲線觀察無磁滯現象產生，且沸騰起始所需壁面過熱度也很小。熱傳係數隨質通量、熱通量的增加而增加。水平雙套鰭管能比水平平滑管增加熱傳係數30至40%。 而在水平雙套鰭管中R-407C冷媒的蒸發熱傳實驗研究中，蒸發熱傳係數將隨熱通量和冷媒質通量的增加而明顯增加，然而冷媒飽和溫度及平均蒸氣乾度對於熱傳係數的影響則幾乎可以忽略。 最後，我們把這個實驗中流動沸騰熱傳係數以及氣泡脫離半徑和產生頻率的資料作分析，求出經驗公式。

Flow Boiling Heat Transfer and Associated Bubble Characteristics of R407C in a Horizontal Annular Finned Duct Student: Chun-Ta Shih Advisor: Prof. Tsing-Fa Lin Institute of Mechanical Engineering National Chiao Tung University ABSTRACT The subcooled and saturated flow boiling and evaporation heat transfer of R-407C in a horizontal annular duct with integral low fins on the outside surface of the heated inner pipe are experimentally investigated in this study. The heat transfer data for the finned duct are compared with those for the corresponding smooth duct. Besides, the associated bubble characteristics in the finned duct are also examined by inspecting the flow photos taken from the flow visualization. Experiments are carried out for the mass flux G varied from 150 to 250 kg/m2s, imposed heat flux q from 5 to 45 kW/m2, mean vapor quality from 0.2 to 0.8, and liquid inlet subcooling △Tsub from 0℃ to 7℃ for the system pressure set at 567 kPa, 667kPa, 776 kPa and 900 kPa corresponding respectively to the bubble point temperature of 0℃, 5℃, 10℃ and 15℃ for R-407C. The subcooled flow boiling curves at three different axial locations along the heated surface are examined first. The results indicate that the required wall superheat to achieve ONB is lower at the more downstream location of the heated wall. The boiling hysteresis and the temperature undershoot at the onset of nucleate boiling (ONB) is insignificant in the finned duct. Thus the thermal shock caused by the temperature overshoot can be avoided by adding fins to the heating surface. The results for the subcooled flow boiling heat transfer coefficient indicate that the influence of the refrigerant pressure on the boiling heat transfer coefficient can be ignored. Besides, a higher imposed heat flux is required for the ONB at a higher inlet liquid subcooling. The subcooled boiling heat transfer coefficient is slightly higher for a lower inlet subcooling. The flow photos show that at a higher mass flux, the higher liquid refrigerant speed tends to sweep the bubbles away from the heated wall, resulting in a smaller bubble departure diameter. Moreover, at a higher inlet liquid subcooling the bubbles are less populated and are smaller due to the lower bulk liquid temperature. Furthermore, the imposed heat flux shows large effects on the bubble population, coalescence and generation frequency. Next, the saturated flow boiling heat transfer of R-407C in the horizontal annular finned duct are investigated. The saturated flow boiling curves show that no boiling hysteresis is detected in the experiments and the wall superheat needed for the ONB is small. Besides, the boiling curves are mainly affected by the imposed heat flux and refrigerant mass flux. The boiling heat transfer coefficient increases with the imposed heat flux and mass flux. The heat transfer coefficient for the finned duct is about 30﹪to 40﹪over that for the smooth duct. This enhancement of the boiling heat transfer for the finned duct increases as the imposed heat flux is raised. Finally, the evaporation heat transfer of R-407C in the horizontal annular finned duct is investigated. It is found that the evaporation heat transfer coefficient increases with the imposed heat flux and refrigerant mass flux. However, the variations of the evaporation heat transfer coefficient with the mean vapor quality is small for the smooth and finned ducts except at higher mass flux and imposed heat flux