雙分佈毛細結構於迴路式熱管熱傳增強之實驗與理論研究

Experimental and Theoretical Study on Heat Transfer Enhancement for a Loop Heat Pipe with Bidisperse Wicks

林芳州(Fang-Chou Lin)

指導教授 ： 陳瑤明

國立臺灣大學/工學院/機械工程學研究所/博士(2012年)

https://doi.org/10.6342/NTU.2012.02362

摘要

迴路式熱管為一種高熱傳量、低熱阻、長熱傳距離的被動二相熱傳裝置。本論文為改善以往蒸發器的單孔徑毛細結構在高熱通量時，內部孔洞容易受蒸氣佔據，導致熱傳性能受到限制的缺陷。本研究混合鎳粉與聚丙烯/石蠟/硬脂酸之三元黏結劑構成孔洞球，孔洞球經堆疊後透過燒結方式製作雙分佈毛細結構。將其使毛細結構內部液氣分離的特點，提昇迴路式熱管蒸發器熱傳性能。透過實驗與理論方式分析孔徑分佈對於熱傳性能的影響。
在理論方面，涵括迴路式熱管穩態模型與毛細結構熱傳壓降模型，並考量孔徑分佈對相變化熱傳的影響。預測結果與實驗數據驗證比較，模型的平均絕對誤差百分比在15%之內。理論特點在於本研究所發展毛細結構內部平均邊界孔洞半徑隨熱負載變化的概念，能夠量化評估單、雙分佈毛細結構受蒸氣層累積的影響。對系統熱阻及蒸氣層熱阻的結果分析發現，發現單孔徑毛細結構在低熱負載時，毛細結構二相區孔洞已被蒸氣佔據；而雙分佈毛細結構在高熱負載時，孔洞仍維持良好的液氣分離狀態。
在實驗方面，改變不同孔洞球粒徑範圍藉以調整雙分佈毛細結構內部大/小孔徑比例，探討孔徑比對於熱傳性能影響。孔洞球平均粒徑尺寸對於迴路熱管蒸發器有顯著影響。最佳的孔洞球粒徑參數為20-62μm，相較於其他孔洞球參數的雙分佈毛細結構的孔徑分佈，此參數可得到最小的大/小孔洞比值（13：1）。毛細結構參數測量結果發現，當滲透度與孔隙度差異不大時，雙分佈毛細結構的內部孔徑比為影響熱傳性能的重要指標。熱傳測試結果顯示，最佳雙分佈毛細結構其蒸發器熱傳係數最高達23.3 kW/m2K，在相同熱負載400W下，為單孔徑毛細結構蒸發器熱傳係數10kW/m2K的230%；雙分佈毛細結構最低系統熱阻與最高容許熱負載分別為0.127 oC /W與575Ｗ，各為單孔徑毛細結構的79%與143%。
此外，本研究為改善雙分佈毛細結構在更高熱負載下，由蒸氣熱洩漏造成的性能衰減。以最佳雙分佈毛細結構參數（粒徑20-62 μm）為主毛細結構，單孔徑為次毛細結構製作雙層毛細結構，其製造方法為首次嘗試。其最高蒸發器熱傳係數可達35.6kW/m2K，為單孔徑毛細結構的350%。雙層毛結構最大容許熱負載則可達到750W，約為一般單孔徑毛細結構的190%；最低系統總熱阻上，雙層毛細結構可達0.081oC/W，為單孔徑毛細結構最低熱阻的50%（0.161 oC /W）。
總結本文成果，雙分佈毛細結構及雙層毛細結構有效地提高熱傳量並降低熱阻，大幅提昇迴路式熱管熱傳性能，對高功率元件散熱有高度應用潛力。

關鍵字

迴路式熱管 ； 雙分佈毛細結構 ； 孔徑分佈 ； 電子元件冷卻

並列摘要

Loop heat pipes（LHPs）are passive heat transfer devices that are high heat removal with low thermal resistance in long transporting distance. At high heat fluxes, the vapor blanket developing in the conventional monoporous wick in LHP reduces heat transfer performance. In the present study, bidisperse wicks are developed and utilized to improve the above-mentioned defect. Clusters are formed by mixing nickel powder with the three-component binder consisted of polypropylene, paraffin wax and stearic acid. The Bidiserpse wicks are made by sintering the packed clusters. The impact of pore-size distributions in bidisperse wicks on heat transfer performance is investigated by experimental as well as mathematical analysis.
An improved LHP steady-state model was developed. The effect of pore size distribution on evaporative heat transfer was taken into account. The results showed that the comparison between the predicted results and experimental data were within 15% of the mean absolute percentage error (MAPE). The feature of this model is that it quantitatively evaluates the impact of vapor blanket on performance by the concept of average boundary pore radius in monoporous as well as in bidisperse wick. Moreover, the results of the model calculation also showed that bidisperse wicks were affected less by vapor blanket than the monoporous wick and would improve the heat transfer capacity of a LHP.
In experimental results, the pore size distributions of bidisperse wicks can be adjusted by different cluster size. Experimental data indicates that the cluster size has significant effect on performance. Bidisperse wick with smaller cluster size possesses small pore ratio and better heat transfer performance. The cluster size and pore ratio in the best bidisperse wick are 20-62 μm and 13:1, respectively. It is worthy to mention that the pore size ratio in bidisperse wick is important to heat transfer performance when the permeability and porosity are similar with other bidisperse wicks. The evaporator heat transfer coefficient of bidisperse wick reaches the maximum value of 23.3kW/m2K, which is approximately 230% of the monoporous wick at heat load of 400W. On the other hand, the maximum allowable heat load and minimum thermal resistance of bidisperse wick are 575W and 0.127oC/W, respectively. Compared with monoporous wick, bidisperse wick enhances the maximum heat load for 140% and reduces thermal resistance for 80%.
Besides, in order to improve the defect of the bidisperse wick caused by vapor heat leakage at the higher heat loads, the present study builds bi-layer wick structure using the bidisperse wick（cluster size：20-62 μm） as the primary layer and the monoporous wick as secondary layer, respectively. The heat transfer test results show that the maximum evaporator heat transfer coefficient is 35.6kW/m2K, which is approximately 350% of the monoporous wick. The maximum allowable heat load and minimum thermal resistance are 190% (750W) and 50% (0.081oC/W) compared with monoporosu wick, respectively.
To summarize this study, bidisperse wick and bi-layer wick structures effectively enhance the heat transfer performance of loop heat pipe. It is highly potential for the high-power thermal management applications.

並列關鍵字

Loop heat pipe ； Bidisperse wick ； Pore size distribution ； Electronics cooling

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