近年來，隨著伺服器及電腦工作站的效能快速提升，氣體及液體冷卻系統在未來已無法滿足高端電子產品散熱的應用，兩相蒸發冷卻將成為未來高功率電子設備冷卻之最佳解決方案。然而在應用上，兩相蒸發冷卻在蒸發器中產生氣泡回流及流動不穩定之現象，會使得液體無法補充至流道而造成局部乾涸，影響整體熱傳性能。因此如何解決微流道蒸發器內之汽泡回流成為兩相蒸發冷卻系統是否可實用化的關鍵。本研究透過漸擴流道及微多孔塗層兩種熱傳增強方式，降低氣泡回流現象並提高熱傳性能，製作適用於兩相冷卻系統所需之高性能微流道蒸發器。蒸發器流道型式包含直線雙通、漸擴雙通及直線單通，使用工作流體HFC-245fa並分別對每種蒸發器進行微多孔塗層與否，觀察其前後熱傳性能差異比較。實驗結果顯示在沒有微多孔塗層下之蒸發器以雙通漸擴流道性能表現最佳，其與直線單通蒸發器相比熱傳性能增加了34.2%。從可視化觀察結果顯示漸擴流道在高瓦數時能有效抑制氣泡回流發生，延緩乾涸現象在微流道中發生。當微多孔層塗佈於蒸發器時，因微多孔層內氣泡成核密度增加，使得大量氣泡產生於加熱表面上並帶走大量的熱。在三種微多孔塗層蒸發器中，以單通直線之熱傳系性能表現最好，特別是在塗層厚度為52 m的時候。在微多孔塗佈厚度為98 m時，其熱傳性能增加41% ~ 90%，然而在微多孔塗佈52 m時，其熱傳性能增加65% ~ 148%，且微多孔塗佈對蒸發器壓降影響可以忽略。此兩種熱傳增強方式皆可適用於兩相蒸發冷卻系統，若欲使用表面熱傳增強，則可選擇將微多孔層塗佈於單通直線流道上。若單純使用鰭片製作微流道蒸發器，則可以考慮雙通漸擴流道。;This study explored the flow boiling heat transfer enhancement using refrigerant HFC-245fa in microchannel heat exchangers. Various heat transfer enhancement techniques were discussed and applied to different flow configurations, including 2-pass diverging microchannel, 2-pass straight microchannel, and 1-pass straight microchannel. The results showed that the 2-pass diverging microchannel heat exchanger without coating exhibited the highest heat transfer performance, displaying a 20% increase in heat transfer compared to other configurations. Visualization techniques were employed to validate these test results. Upon applying a porous coating to the heat exchangers, straight microchannel in both 1-pass and 2-pass configurations showed superior heat transfer performance. A 52 μm coating led to enhancements ranging from 65% to 148% compared to a smooth surface, while a 98 μm coating resulted in enhancements of 41% to 90% across different flow rates. Importantly, porous coating thickness had no significant impact on flow boiling pressure drops. For two-phase cooling systems, it is advisable to use 2-pass diverging microchannel without coating or add a 52 μm porous coating to 1-pass straight microchannel. Porous coating emerges as a highly promising technique for enhancing flow boiling heat transfer in two-phase microchannel heat exchanger.
