擴散吸收式冷凍循環(diffusion absorption refrigeration, DAR)是利用熱能作為驅動力的熱力循環，不需要額外增加高耗電的壓縮機、膨脹閥或其它作動組件。DAR系統具有很寬的熱源選擇性，可以來自太陽熱能、地熱、燃料直接燃燒或製程廢熱等，是一種可以替代電能的冷凍空調裝置。本研究探討DAR系統中的重要組件之ㄧ，蒸發器。蒸發器之作動原理是在高濃度製冷劑中通入輔助氣體，使其分壓下降進而造成沸騰或蒸發起始溫度下降，而可以導致有節流閥降壓的作動下達成蒸發致冷的功效。
　　本研究設計加熱面積分別為50 mm × 30 mm及50 mm × 18 mm的迷你單、雙流道，並且在流道底部鑽直徑0.7 mm的氣體進口孔，使得乙醇流體可以將氦氣帶入流體進行氣液混合。本研究之工作流體採用濃度99.8%的乙醇與輔助氣體氦氣。在迷你單流道系統中分別通入液體流量12 ml/min及氦氣流量30與80 ml/min，及迷你雙流道系統中分別通入液體流量10、25與40 ml/min及氦氣流量100、200及300 ml/min，探討此等實驗條件下的對流沸騰熱傳現象。
　　實驗結果顯示，在給定的壁溫下，熱通率隨著乙醇及氦氣流量的增加而上升。在壁過熱度為-10℃時會有較佳的熱傳增益比例，並且在低乙醇流量(10 ml/min)及高氦氣流量(300 ml/min)時，熱傳增益比例最高可達204%。在進入沸騰區域後，其熱傳增益比例將大幅下降。此外，從流譜觀察結果得知，長型彈狀氣泡間推擠碰撞所產生的擾動增加流體間的對流，並且強化沸騰熱傳，此型態為本實驗主要的熱傳機制。在大液體流量(25、40 ml/min)時，熱傳增益比例隨著乾度的增加而大幅度提升，然而在液體流量小(10 ml/min)時，熱傳增益比例隨著乾度增加的幅度較小。

A Diffusion absorption refrigeration (DAR) system is a kind of thermal cycle employing thermal power as the driving force. It works without an external high power consuming compressor, expanded valve and any other working assembling. In the present study, we investigate one of the most important parts of a DAR system, i.e. the gas-assisted evaporator. The working principle of such an evaporator is to reduce the partial pressure of the working fluid by adding a non-condensable gas into the high concentration coolant. Consequently, the working fluid may evaporate at a lower temperature. Under such a situation, the heat transfer in the system may also be affected.
In this study, we have designed test sections with single minichannel or dual minichannels with an import hole located near the inlet bottom of channel for the gas-added. The gasoline imported hole makes helium mix with ethanol fluid by a T-junction. The working fluid is 99.8% ethanol and helium. The flow rate for ethanol is 12 ml/min and is 30/80 ml/min for the helium in the single minichannel system. On the other hand, it is 10, 25 and 40 ml/min for ethanol flow and 100, 200 and 300 ml/min for the helium flow in the dual minichannels. The heat transfer and boiling two-phase flow phenomenon under the conditions above-mentioned are thoroughly investigated.
The results of the study reveal that the heat flux rises while the flow rate of ethanol or helium or both increase. The best heat transfer enhancement, comparing to that without assisted gas is demonstrated at a wall superheat around -10℃. Moreover, under the condition of a low ethanol flow rate of 10 ml/min and a high helium flow rate of 300 ml/min, the enhancement reach the maximum value of 204% in the present study. The enhancement drop obviously in region with positive wall superheat. Experiment observations indicate that the disturbance made by the collision between two neighboring long slug bubbles significantly increases convection and possible ethanol evaporation at the gas-liquid interface, the major heat transfer enhancing mechanisms and boosts the enhancement. The study also reveals that the heat transfer enhancement is linearly proportional to the gas quality with a higher slope for the cases with high ethanol flow rates of 25 and 40 ml/min than that with a low ethanol flow rate of 10 ml/min.

[1] 經濟部能源局, 能源產業技術白皮書, 2014, p. 329.
[2] 經濟部能源局, 經濟部能源局年報, 2012, p. 77.
[3] J. Rodríguez-Muñoz, J. Belman-Flores,“Review of diffusion–absorption refrigeration technologies”, Renewable and Sustainable Energy Reviews, 30, 2014, 145-153.
[4] W. Chen, M. Twu, C. Pan,“Gas–liquid two-phase flow in micro-channels”, International Journal of Multiphase Flow, 28(7), 2002, 1235-1247.
[5] D.A.P. M.K. Akbar, S.M. Ghiaasiaan,“On gas-liquid two-phase flow regimes in microchannels”, International Journal of Multiphase Flow, 29, 2003, 855-865.
[6] S. Saisorn, S. Wongwises,“Flow pattern, void fraction and pressure drop of two-phase air–water flow in a horizontal circular micro-channel”, Experimental Thermal and Fluid Science, 32(3) , 2008, 748-760.
[7] J. Yue, L. Luo, Y. Gonthier, G. Chen, Q. Yuan,“An experimental investigation of gas–liquid two-phase flow in single microchannel contactors”, Chemical Engineering Science, 63(16), 2008, 4189-4202.
[8] K. Triplett, S. Ghiaasiaan, S. Abdel-Khalik, D. Sadowski,“Gas–liquid two-phase flow in microchannels Part I: two-phase flow patterns”, International Journal of Multiphase Flow, 25(3), 1999, 377-394.
[9] K. Triplett, S. Ghiaasiaan, S. Abdel-Khalik, A. LeMouel, B. McCord,“Gas–liquid two-phase flow in microchannels: part II: void fraction and pressure drop”, International Journal of Multiphase Flow, 25(3), 1999, 395-410.
[10] G. Hetsroni, A. Mosyak, E. Pogrebnyak, Z. Segal,“Heat transfer of gas–liquid mixture in micro-channel heat sink”, International Journal of Heat and Mass Transfer, 52(17), 2009, 3963-3971.
[11] Y. Zhao, G. Chen, C. Ye, Q. Yuan,“Gas–liquid two-phase flow in microchannel at elevated pressure”, Chemical engineering science, 87, 2013, 122-132.
[12] P.A. Walsh, E.J. Walsh, Y.S. Muzychka,“Heat transfer model for gas–liquid slug flows under constant flux”, International Journal of Heat and Mass Transfer, 53(15), 2010, 3193-3201.
[13] A.R. Betz, D. Attinger,“Can segmented flow enhance heat transfer in microchannel heat sinks? ”, International Journal of Heat and Mass Transfer, 53(19), 2010, 3683-3691.
[14] Y. Lim, S. Yu, N.-T. Nguyen,“Flow visualization and heat transfer characteristics of gas–liquid two-phase flow in microtube under constant heat flux at wall”, International Journal of Heat and Mass Transfer, 56(1), 2013, 350-359.
[15] S. Saisorn, P. Kuaseng, S. Wongwises,“Heat transfer characteristics of gas–liquid flow in horizontal rectangular micro-channels”, Experimental Thermal and Fluid Science, 55, 2014, 54-61.
[16] K. Choo, S.J. Kim,“Heat transfer and fluid flow characteristics of nonboiling two-phase flow in microchannels”, Journal of Heat Transfer, 133(10), 2011, 102901.
[17] D. Oliver, S. Wright,“Pressure drop and heat transfer in gas-liquid slug flow in horizontal tubes”, British Chemical Engineering, 9(9), 1964, 590-596.
[18] R. Grimes, C. King, E. Walsh, Film thickness for two phase flow in a microchannel, in: ASME 2006 International Mechanical Engineering Congress and Exposition, American Society of Mechanical Engineers, 2006, 207-211.
[19] J.A. Howard, P.A. Walsh, E.J. Walsh,“Prandtl and capillary effects on heat transfer performance within laminar liquid–gas slug flows”, International Journal of Heat and Mass Transfer, 54(21), 2011, 4752-4761.
[20] Y. Han, N. Shikazono,“Measurement of the liquid film thickness in micro tube slug flow”, International Journal of Heat and Fluid Flow, 30(5), 2009, 842-853.
[21] F. Houshmand, Y. Peles,“Heat transfer enhancement with liquid–gas flow in microchannels and the effect of thermal boundary layer”, International Journal of Heat and Mass Transfer, 70, 2014, 725-733.
[22] Y. Lee, P. Lee, S. Chou,“Enhanced thermal transport in microchannel using oblique fins”, Journal of Heat Transfer, 134(10), 2012, 101901.