冷媒流量和熱通量振盪引起之暫態新冷媒和FC-72流動沸騰研究

Abstract

由燃燒化石燃料來產生能量所排放之 CO2 氣體已造成嚴重的全球氣候暖化，如何提 升各類工程系統的能源使用效率已廣受世界各國重視。近年來，採用變頻(而不是定頻 的ON/OFF)壓縮機來驅動冷凍與空調系統去因應變動的熱負荷已發現能大幅提昇這些 系統的能源使用效率。在這些變頻冷凍空調系統的refrigeration cycle 中，冷媒流量和熱 負荷都是隨時間在變化。這些隨時間變化的冷媒流量和熱負荷如何影響refrigeration cycle 中的詳細流動沸騰與冷凝特性，到目前為止少有探討。除外，在未來超高元件密 度的電子裝備冷.設計上，採用相變化熱傳方法仍為主要的考慮。在這些電子裝備中， 由於電阻所產生的熱量也是隨時間在變動，因此也須隨時間調整冷.流體流量來適應裝 備的冷.負荷，與此相關的暫態雙相流熱傳特徵也尚未有深入的研究。 在本三年期研究計畫中(民國96 年8 月至99 年7 月)，我們將建立二套實驗環路來 詳細探討隨時間做週期性變化的冷媒流量和熱通量如何影響管流中的流動沸騰熱傳及 氣泡特徵。主要將針對HFCs 冷媒(R-134a，R-407C 及R-410A)在雙套管中與FC-72 在 矩形管中的飽和及次冷態流動沸騰進行實驗。除外，也將對這三種HFC 冷媒在雙套管 中的蒸發熱傳進行量測。雙套管的間隙大小對沸騰和蒸發熱傳的影響將深入探討。同 時，我們也將對冷媒流量和熱通量之振幅和週期對暫態流動沸騰的影響作詳細研究。

The improvement of the energy utilization efficiency in various engineering systems is receiving ever increasing attention worldwide in view of its importance in reducing the release of CO2(g), a global weather warming gas, during the burning of fossil fuel in producing the needed energy. Recently, the use of variable frequency, instead of ON/OFF, compressors in various air condition and refrigeration systems to meet the temporally changing thermal loads has been found to significantly augment the energy efficiencies of these systems. It is important to note that in these systems the refrigerant flow rate varies with time and the systems are subjected to changing thermal loads. How this time dependent refrigerant flow rate and heat flux affect the characteristics of boiling and condensation processes in the refrigeration cycles employed in these air-conditioning and refrigeration systems remains largely unexplored. Besides, in cooling future ultra-high component density electronic devices, methods based on the phase change heat transfer are often considered. Moreover, the power dissipated in these devices is also time dependent and hence the cooling load. The associated time dependent two-phase flow and heat transfer processes in electronics cooling are also poorly understood. In this three-year research project (August 2007 to July 2010), two experimental loops will be established to investigate how the imposed time periodic refrigerant flow rate and heat flux affect the basic channel flow boiling heat transfer and associated bubble characteristics for refrigerants R-314a, R-407C and R-410A and for dielectric coolant FC-72. Experiments will be conducted for the saturated and subcooled boiling of R-134a, R-407C and R-410A in an annular duct and FC-72 in a rectangular duct. The evaporation heat transfer data of R-134a, R-407C, and R-410A flow in an annular duct will be measured. The effects of the gap size in the annular duct on the transient flow boiling and heat transfer characteristics will be examined in detail. Moreover, the transient flow boiling characteristics affected by the amplitudes and periods of the flow rate and heat flux oscillations will be explored.