本研究探討雙迷你流道氣體輔助蒸發器的熱傳性能並量化、研究其在負壁過熱度下的蒸發。此一蒸發器之作動原理是以濃度99.8%的乙醇為工作流體，在其中通入輔助氣體─氦氣，使其分壓下降進而造成蒸發的起始溫度下降，而可以導致蒸發致冷的功效。本研究設計加熱面積為50 mm × 18 mm的迷你雙流道，並且在進口區流道底部鑽直徑0.7 mm的氣體進口孔，使得乙醇流體可以將氦氣帶入流道形成氣液雙相流。本研究之乙醇與氦氣之表像速度分別為 、 與 及 、 及 ，以探討此等實驗條件下的對流蒸發與沸騰熱傳現象。為確認負壁過熱度的蒸發現象，本研究在流道出口設計一氣液分離器及蒸汽冷凝裝置，以量化流道內的蒸發及蒸發率。實驗結果顯示，在給定固定的乙醇表像速度與壁過熱度之條件下，熱通率會隨著氦氣流量上升而上升。其最大熱通率皆發生在本研究的最高壁過熱度約10 °C (受限加熱源耐溫)與氦氣表像速度 時。如:在乙醇表像速度 、 、 ，其最大移熱能力分別為192 kW/m2、258 kW/m2、321 kW/m2。乙醇在加入輔助氣體後，在壁過熱度小於0 °C時，有很大的熱傳增益比例，最大值發生在壁過熱度-10 °C至0 °C之間及乙醇表像速度 與氦氣表像速度 時，最大增益效果為275%。研究結果說明了輔助氣體的確可以降低乙醇的分壓，使其在更低的溫度即有蒸發現象發生。乙醇在固定壁過熱度之下，其蒸氣比例隨著氦氣表像速度升高而升高。進階的蒸發效率分析中，即使在負壁過熱度的條件下，從管壁擴散進來的熱量的確有顯著的比例被用來進行蒸發。其中，乙醇表像速度 與氦氣表像速度 時，蒸氣效率在壁過熱度小於0 oC的區域，最高可達63%。實驗中，另以高速攝影機輔助拍攝雙相流動型態，在雙相流動型態的觀察中，發現乙醇在加入輔助氣體後，且在沒有加熱的情況下，依氦氣流量之不同，會呈現不同長度的彈狀流型態。加熱之後，彈狀氣泡的長度明顯變長，氣液之間的相互作用亦變為更加劇烈。在流動型態上，也佐證了熱傳增益的原因。

The present study investigates the heat transfer characteristic of a gas-assisted evaporator and quantifies the contribution of evaporation under a condition with negative wall superheat. The working principle of a gas-assisted evaporator is to reduce the partial pressure of the working fluid by adding an assistant gas into the coolant. The working fluid may then evaporate at a temperature significantly lower than its normal boiling point with the presence of an assistant gas. Moreover, the presence of the gas phase may also agitate the liquid flow and further enhance heat transfer. Therefore, the heat transfer in the system may be significantly enhanced. In this study, we design test sections with dual mini-channels with an inlet hole located at the channel bottom near the inlet region for the gas-added. The gasoline imported hole makes helium mix with the ethanol flow by a T-junction. The superficial velocity is , and for the ethanol flow and , and for the helium flow in the present study. An innovative gas-liquid separator and condensing unit is designed at the outlet region to confirm and quantity the ethanol evaporated. 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 helium increases under a given ethanol flow rate. Due to the limitation of the power supply, the maximum heat flux take place when the wall superheat is about 10 °C and the helium superficial velocity is . For example, the maximum heat flux is 192 kW/m2, 258 kW/m2, 321 kW/m2 with ethanol superficial velocity , and , respectively. The better heat transfer enhancement is demonstrated at wall superheat below 0 °C after the helium is added. Under the condition of a low ethanol superficial velocity of and a high helium superficial velocity of and wall superheat between -10 °C to 0 °C, the enhancement reaches the maximum value of 275% in the present study. The results confirm the gas-assisted evaporation at a temperature significantly below the normal boiling point of the working fluid. At a constant wall superheat, the quality of vapor increases with increasing the helium superficial velocity. The measurement of ethanol vapor condensate indicates that the contribution of evaporation heat flux may account for up to 63% of wall heat flux at wall superheat under 0 °C. In the experiment, a high-speed video camera is used to photography the flow pattern. The flow pattern shows that the slug bubble will emerge as different length with the different helium superficial velocity without heating. The slug bubble obviously become longer and the interaction between gas and liquid is significant. The flow pattern variation provide another proof for the reason in heat transfer enhancement.

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