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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/1812


    Title: 冷媒R-134a與R-404A在熱傳增強管上之池沸騰觀察與熱傳性能分析;Visualization and Heat Transfer Performance of Pool Boiling of Refrigerants R-134a and R-404A on Porous and Structured Tubes
    Authors: 范智峰;Chih-Feng Fan
    Contributors: 機械工程研究所
    Keywords: 池沸騰;熱傳增強;池沸騰觀察;氣泡;pool boiling;heat transfer enhancement;visualization;bubble
    Date: 2004-07-02
    Issue Date: 2009-09-21 11:32:19 (UTC+8)
    Publisher: 國立中央大學圖書館
    Abstract: 本研究藉由觀察平滑管和熱傳增強管氣泡產生情形,與量測核沸騰熱傳係數,來了解影響不同熱傳增強管的熱傳機制與熱傳性能的關係。觀察結果顯示,結構表面熱傳增強管在低熱通量時,沸騰機制為溢流模式(flooded mode),小氣泡從管表面下通道中所產生,而大部分的小氣泡會沿著內部通道結合成大氣泡,而後移動到管子上半部從活化的孔洞離開。在高熱通量時,沸騰機制變為吸入-沸騰模式(suction-evaporation mode),液態薄膜從管內通道中蒸發,並從結構表面的孔洞離開,此時並不會有小氣泡從孔洞通過的情形。 多孔表面熱傳增強管由於擁有非常多且不同尺寸的孔洞,因此在低熱通量時就會產生很高密度的氣泡量,然而多孔管的氣泡密度相對表面孔洞的比率還是比結構管的比率來的低,此表示多孔管大部分的孔洞都沒有活化還有很大的改善空間。 量測熱傳性能的結果顯示,在不同熱通量時的熱傳性能都能以氣泡觀察實驗所發現的結果加以解釋,本實驗亦證實Chen等人 [2002]的建議想法,他們是認為表面含有孔洞的增強管或是有表面下通道相連孔洞的管子,這些增強管在通道內的主要的熱傳方式是藉由薄液膜的蒸發來傳遞熱量。而對於管表面開口率較大(big openings)的增強管,其主要的熱傳方式是藉由顯熱熱傳來傳遞熱量,尤其是在高熱通量的時候。 This is experimental work that intend to provide a further understanding of the physical phenomenon for refrigerants R-134a and R-404A boiled on structured and porous tubes. This thesis provides a detail visualization results for bubble formation characteristics and provides a measurement of heat transfer performance on smooth and enhanced tubes. The results show that the relation of heat transfer performance to the heat flux can be well explained by the bubble dynamics as observed in part I of this study. The experimental observation results support the existence of flooded and suction-evaporation mode on structured tubes. At the low heat flux conditions, the boiling mechanism is in flooded mode. Bubbles generated from the sub-tunnel surface of structured tubes that is similar to those generated from a smooth tube surface. Most of the bubbles joined together inside the tunnel to become large bubbles, moved up to the upper part of the sub-tunnel and departed from the surface through the so-called active pores. The other part of the bubbles passing through the pores directly and are the small bubbles. At high heat flux conditions, the boiling mechanism transferred to suction-evaporation mode. Thin liquid film was evaporated in the tunnel and bubbles were ejected from the structured pores. There are no small bubbles passing through the pores during this situation. For porous tube, because of its large amount of various sizes of pores, it possessed a very high bubble density even though at low heat flux condition. However, the ratio of bubble densities to the number of pores for the porous tube is lower than that of structured tubes B and TX. This means that most of the pores on porous tube were not active and needs to be further improved. The research also support the suggestion by Chen et al. [2002] that for tubes with surface pores or with surface pores connected by sub-surface tunnel, the high performance of structured surfaces is largely brought about by the evaporation inside the tunnels. For surface with big openings, the majority of heat load is transported by the sensible heat especially at high flux.
    Appears in Collections:[機械工程研究所] 博碩士論文

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