為了解決二相蒸發冷卻應用於高熱通量電子元件散熱時流動不穩定問題，本研究提出了以漸擴雙通型式的流道來改善微流道蒸發器之性能。分別設計了直線雙通、漸擴雙通與直線單通流道之二相蒸發熱交換器，並比較各型式之性能差異。三種流道型式的蒸發器總熱傳面積相同，直線單通與直線雙通型式的流道間距與高度同樣為1 mm、3 mm，漸擴雙通型式的流道間距則是從入口處0.6 mm逐步漸擴至出口處1.4 mm，漸擴角度為0.449o，流道高度同樣為3 mm。 以雙通型式流道來改善蒸發器性能會增加熱交換器壓降，原因是流過熱交換器的流量相同時，單通型式的流道質量通量較小、流道長度較短，因此熱交換器壓降較小。而直線與漸擴雙通流道的壓降在流道乾涸發生之前並無明顯差異。 直線單通流道在高熱通量時，流道被汽體佔據，液體無法順利補充進流道，導致局部乾涸現象發生使其有較低的熱傳性能。雙通流道相鄰流道之間的流體會相互冷卻，減緩出口處的汽泡成長，讓流體維持在汽泡流狀態，使熱傳性能提升。直線與漸擴雙通流道在乾涸發生前的熱傳性能相同，但漸擴雙通流道有利於汽泡脫離的特性可以延後局部乾涸發生，使其在三種流道型式的蒸發器中有最高的熱傳性能。 以漸擴雙通流道來改善直線單通流道蒸發熱交換器性能，可以提升27至35 %熱傳係數，並在流量273 ml/min時使最小熱阻從0.035降低至0.026 K/W，且最大熱傳係數下的底部熱通量相當，但同時也增加了126%的壓降。 ;This study develops a two-pass diverging microchannel heat exchanger to improve the two-phase flow instability in a microchannel heat exchanger used for the thermal management of high-power electronics. We totally designed three types of two-phase evaporative heat exchangers, including single-pass straight microchannel, two-pass straight microchannel, and two-pass diverging microchannel, and compared the performance of each type. Three types of microchannel have the same heat transfer area. Both single-pass and two-pass straight microchannel fin spacing and fin height are 1 mm and 3 mm, while the two-pass diverging microchannel fin spacing increase from 0.6 mm at the entrance to 1.4 mm at the exit with 0.449o diverging angle, and the height also 3 mm. The experiment results show that a two-pass microchannel would increase the pressure drop of the heat exchanger at the same flow rate, because of the shorter channel length and lower mass flux of the single-pass microchannel. There is no significant pressure drop difference between two-pass straight and two-pass diverging microchannels. Under the condition of high heat flux, the single-pass straight microchannel would be blocked by the large bubble causing the channel surface partial dry-out and decreasing the heat transfer performance. For the same condition in the two-pass microchannel. Due to the heat transfer between neighboring channels suppressing the bubble growth near the exit of the channel, the flow pattern can maintain bubble flow and increase the heat transfer performance. There is no obvious heat transfer performance difference between two-pass straight and two-pass diverging microchannel before partial dry-out happened. Owing to a more stable two-phase flow in the diverging microchannel, the two-pass diverging microchannel has the best heat transfer performance of the three types of microchannels. In addition, using two-pass diverging microchannel to improve the single-pass straight microchannel has 27 % to 35 % increment of heat transfer coefficient, decreasing the minimum thermal resistance from 0.035 to 0.026 K/W, while with 126% increment of pressure drop.
