本論文之研究目的在於提昇以矽基底漸擴微流道做為蒸發測試段的自然循環迴路的移熱能力。根據本實驗團隊的研究成果指出，流道採取漸擴設計可以有效抑制環路內雙相沸騰時所產生的不穩定性。本研究分別採用了兩種不同設計的微流道，chip 1及chip 2，並首先以95%乙醇為工作流體進行自然循環實驗，蒸發測試段尺寸皆為10.5mm×10.5mm。chip 1的流道進出口尺寸分別為250μm及350μm，蝕刻深度為200μm，流道總數為18條。此種流道設計無法有效增加散熱面積，實驗結果發現其最高移熱能力僅有約70W左右。故再設計chip 2，其流道進出口尺寸分別為150μm及250μm，蝕刻深度為350μm，流道總數為30條。實驗結果發現改良後的流道設計可以將環路流速及質量流率提昇約2.33倍，因此可以有效的提昇環路的移熱能力至120W。
根據高速攝影機所拍攝到蒸發測試段及升流段的流譜，可發現本微型自然循環迴路在低功率時，會發生與大型自然循環迴路相同之閃化不穩定性。本環路的不穩定性在提高輸入功率使質量流率增加後逐漸被抑制，直到環路發生乾化，環路才又處於不穩定的狀態。以不同微流道進行實驗，環路的蒸發段壁臨界熱通率分別為529kW/m^2(chip 1)及481 kW/m^2(chip 2)。
為了進一步提昇環路的移熱能力，本研究改採用0.1莫耳分率的乙醇水溶液進行自然循環實驗。實驗結果顯示在輸入功率為90W時，升流段會發生逆流的現象，因此造成環路不穩定區擴大，故判斷此種工作流體可能不適用於低壓運轉的自然循環迴路。

In this study, the enhancement of heat transfer capacity of a two-phase natural circulation loop with divergent microchannels is explored. The previous research in our labortory shows that the divergent microchannels can significantly stablize the two-phase in the microchannel.
We use two different design, chip1 and chip 2, of microchannel as the evaporator to conduct the natural circulation loop experiments using 95% ethanol. The dimension of the microchannel chip is 10mm×10mm. There are 18 parallel microchannels in chip 1 with width diverging from 250μm to 350μm, with uniform depth of 200μm. This kind of design cannot increase the heat removal area effectively, the experimental results show that highest heat removal capacity is only about 70W. We therefore, re-design the microchannel evaporator as width diverging from 150μm to 250μm with depth of 350μm, and the amount of microchannels is increased to 30 for chip 2. The results reveal that the flow velocity and the mass flow rate of the loop are 2.33 times higher than the previous one, so the heat transfer capacity are advanced effectively with the highest heat removal capability of 110W.
By observing the flow patterns in the evaporator and riser with a high speed video camera, we can reveal the flashing instability phenomenon same as observed in the normal scaled natural circulation loop. The instability of the loop can be suppressed while the input power is increased, after the loop reach the dryout condition, the loop will become unstable again. The experimental results shows the highest wall heat flux with different microchannel chip is 529kW/m^2 (chip 1)and 481 kW/m^2(chip 2).
To make further improvement of heat transfer capacity, we change the working fluid as ethanol-water mixtures as ethanol mole fraction of 0.1. The results show that the two-phase flow in the loop is quite unstable. Moreover, when the input power is 90W, the counter flow appears in the riser. The ethanol-water mixtures may not be suitable for the natural circulation loop at low pressure.

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