Mathematical Model of a Loop Heat Pipe with Phase-Change Heat Transfer in a Wick Structure
|Keywords:||迴路式熱管;孔徑分佈曲線;單孔徑毛細結構;雙孔徑毛細結構;相變化熱傳;Loop heat pipe;pore size distribution;monoporous wick;biporous wick;phase-change heat transfer||Issue Date:||2008||Abstract:||
迴路式熱管(Loop Heat Pipe, LHP)數學模型的建立可以幫助設計以及性能上的預測。然而，早期文獻僅針對個別元件進行分析及模擬，甚至忽略考慮毛細結構的相變化熱傳，並且將毛細結構視為單一平均孔徑，這些假設會使模型的適用範圍與應用空間大幅減小。因此，本文將毛細結構的相變化熱傳以及孔徑分佈的概念皆納入考量，建立一穩態模型，能夠廣泛的預測單孔徑與雙孔徑毛細結構於迴路式熱管中，不同輸入熱量下之補償室與蒸發器壁面溫度。經實驗與預測結果比較，最大誤差不超過23%。 在模型的預測中，利用毛細結構臨界孔徑的方法，來討論毛細結構的孔徑分佈對性能的影響。分析結果顯示，孔徑分佈的不同會影響毛細結構內蒸氣累積的情況，而蒸氣薄膜的發生是影響熱傳性能的主要原因，藉由蒸氣薄膜熱阻與系統總熱阻的比值，可做為判斷毛細結構性能的指標，以及性能提升的依據。其中，單孔徑毛細結構因孔徑分佈較窄，蒸氣排除不易，產生之蒸汽薄膜的熱阻會隨輸入瓦數的增加而升高，當輸入熱量達500W時，蒸汽薄膜熱阻高達0.16℃/W，占系統總熱阻(0.26℃/W)60%。 由於雙孔徑毛細結構具有能排除蒸氣的大孔，受到蒸氣薄膜影響的程度較小，藉由模型分析大孔之數量與尺寸對系統性能的影響，可知在大孔尺寸較小、數量較多的情況下，具有較佳的熱傳性能，其中最佳之雙孔徑毛細結構，蒸氣薄膜的熱阻減少至0.002℃/W，占系統熱阻(0.1℃/W)的2%，顯示毛細結構之性能已達上限，說明利用雙孔徑毛細結構可有效降低蒸汽薄膜熱阻，提升迴路式熱管熱傳性能。
A mathematical model for Loop heat pipes (LHPs) can provide a straightforward method of design analysis and performance improvement. However, most of mathematical models were developed for the specific component, either for a wick or a compensation chamber. These models ignored the phase-change heat transfer or the pore size distribution of a wick structure. It will restrict the range of application and prediction of the model. An improved 1-D steady state model was developed in this study. The phase-change heat transfer and the pore size distribution of a wick structure were also taken into account. The evaporator surface temperature was calculated as a function of the heat load. Both of the monoporous wick and biporous wick can also be predicted, the comparison between the predicted results and experimental data are within 23%. The effects of pore size distributions in the wick’s performances were studied by this model. Results of this study showed the different pore size distributions will influence the vapor blanket extent of the wicks, which can be estimated by the thermal resistance. This thermal resistance dominates the heat transfer performance of the wick and thus can be considered as a standard for the wick’s heat transfer capacity. Because the narrow pore size distribution of the monoporous wick would accumulate gradually to form the vapor blanket, it brings the higher thermal resistance with increasing heat flux. As the heat load increased to 500W, the thermal resistance of the vapor blanket would reach to 0.16℃/W, and 60% of the total thermal resistance(0.26℃/W). The large pores in the biporous wick play the role as the path way for vapor to escape, and thus the performance is affected less by the vapor blanket. The size and amount of larger pores in the biporous wicks was analyzed to investigate the heat transfer capacity of the LHP by this model. The Results indicate that the large pores with reducing size and increasing amount have better performance. The optimized biporous wick can obviously reduce the thermal resistance of vapor blanket to 0.002℃/W, only 2% of the total thermal resistance(0.1℃/W), on the other hand, the biporous wick can not only effectively eliminate the thermal resistance of vapor blanket but also improve the heat transfer capacity of the LHP.
|Appears in Collections:||機械工程學系|
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