冷媒R-245fa於直線型與漸擴型微流道中流動沸騰熱傳性能比較;Comparison of flow boiling heat transfer performance of refrigerant R-245fa in straight and diverging microchannel

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题名:	冷媒R-245fa於直線型與漸擴型微流道中流動沸騰熱傳性能比較;Comparison of flow boiling heat transfer performance of refrigerant R-245fa in straight and diverging microchannel
作者:	林哲頡;Lin, Che-Chieh
贡献者:	機械工程學系
关键词:	兩相蒸發熱交換器;直線型流道;漸擴型流道;可視化觀察;Two-phase heat exchanger;Straight channel;Diverging channel;visual observation
日期:	2021-09-10
上传时间:	2021-12-07 13:39:41 (UTC+8)
出版者:	國立中央大學
摘要:	本研究設計直線流道型、漸擴型之兩相蒸發熱交換器，比較兩種型式在50、100、200 ml/min下熱傳性能，並透過可視化觀察分析流動及熱傳、壓降之關係。直線型的流道間距與高度為 7.6 mm、2 mm，而漸擴型的流道則是從入口處逐步增加流道間距從3 mm 逐步漸擴至出口間距 14 mm 而高度同樣為 2 mm。漸擴型流道在低流量下，會發生入口處汽泡阻塞的現象，而阻塞的汽泡無法有較的被推動進入流道內部使得液體難以補充至流道表面發生局部乾涸使其有較低的熱傳係數。而漸擴型式熱交換器在中高流量下有較高的熱傳係數、熱傳性能最好。其原因為流道內之流動情形於低乾度時以汽泡流與柱狀流為主，流道內汽泡順著流動方向離開流道無回流阻塞的現象發生，使表面仍有液體補充。而高乾度的情況下同樣的因汽泡較容易脫離，使其不易發生大面積乾涸。漸擴型流道與直線型流道兩者之間的壓降並無明顯的差異，其原因為雖然漸擴型流道逐步增加其流道的間距，使得下游處的摩擦壓降降低，但其入口處間距與直線型流道相比也小了約2.5倍，因此於上游處也會產生較高的壓降，使得兩種型式熱交換器在壓降上的比較並無明顯的差異。漸擴型流道在 200 ml/min =時具有最低熱阻為 0.057 oC/W，因其與直線型流道相比，汽泡較能順利地脫離流道並無法生阻塞回流等現象，使得熱交換器表面較不易發生局部乾涸等現象。使得其有較好的熱傳性能。 ;In this study, a two-phase evaporative heat exchanger with a straight microchannel and a diverging microchannel was designed. The heat transfer performance of the two types was compared, and the relationship between flow and heat transfer was analyzed by visual observation. The straight fin spacing and height are 7.6 mm and 2 mm, while the gradually diverging fin spacing is gradually increased from 3 mm at the entrance to 14mm at the exit and the height is also 2 mm. Under the condition of low flow rate, the phenomenon of bubble blocking at the inlet will occur, and the blocked bubble cannot be pushed into the inside of the channel, which makes it difficult for liquid to be added to the surface of the flow channel and causes local dry-out and low heat transfer coefficient. The heat transfer coefficient and heat transfer performance of the gradually diverging type heat exchanger are higher at medium and high flow rate. The reason is that the flow in the passage is mainly bubble flow and columnar flow. The bubble in the passage leaves the passage along the flow direction without backflow blocking, so that there is still liquid supplement on the surface. There is no significant difference in the pressure drop between the gradually diverging microchannel and the straight microchannel. The reason is that although the gradually expanding flow channel gradually increases the distance between its flow channels, resulting in a decrease in the friction pressure drop at the downstream, the inlet distance is also 2.5 times smaller than that of the straight type, so a high pressure drop will also be generated at the upstream. There is no obvious difference in pressure drop between the two types of heat exchanger. The lowest thermal resistance of the diverging type is 0.057 oC/W at 200 mL /min, because compared with straight type, the bubble can escape from the flow channel more smoothly and can not generate blocking back flow and other phenomena, making the surface of the heat exchanger less likely to occur local drying and other phenomena. So it has better heat transfer performance.
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