鹽析法高分子毛細結構於迴路式熱管之應用研究

Investigation of the Salt-leaching Polymer Wick Structure Applied to Loop Heat Pipe

張云銘(Yun-Ming Chang)

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(2008年)

https://doi.org/10.6342/NTU.2008.00347

摘要

迴路型熱管(Loop Heat Pipe, LHP)為一種高傳熱量、長傳輸距離的被動二相熱傳裝置，具有相當大的潛力應用在航太科技及電子元件散熱。目前迴路式熱管中的毛細結構大多以燒結金屬粉末的方式來製作，應用於迴路式熱管系統時，因金屬材料熱導係數高易有熱洩漏問題而降低性能；再者，對於經燒結所製得之毛細結構，其毛細結構參數(有效孔徑、孔隙度、滲透度)會受到燒結粉末的粒徑、形狀以及燒結時的溫度、時間等因素所影響，因此不易控制其參數來進行探討。有鑑於此，本文以低熱導係數的聚苯乙烯為材料，利用鹽析法來製作可應用於迴路式熱管之高分子毛細結構，藉由改變氯化鈉粒徑大小及其在高分子材料中的比例來調控有效孔徑與孔隙度。經實際製作並量測毛細結構參數結果顯示，其有效孔徑量測值可落於所欲控制孔徑範圍內，孔隙度可精確控制在平均絕對百分比誤差1.65 %內。綜整所量值之參數值，並以曲面擬合的方式求出高分子毛細結構參數關係式：lnκ=-20.47+lnrc+4.31ε2.5，此式將有助於迴路式熱管的設計以及熱傳性能的預測。
將所製作出不同有效半徑、孔隙度、壁厚參數的高分子毛細結構，置入於迴路式熱管中進行熱傳性能測試。測試結果顯示，在結構強度與製程條件允許下，毛細結構壁厚愈薄、有效半徑愈小以及孔隙度愈高，均有助提升迴路式熱管熱傳性能。在蒸發器表面溫度85 ℃的條件下，高分子毛細結構最佳參數組合(壁厚為1.5 mm、有效半徑為5.9 μm、孔隙度為80 %)之迴路式熱管其熱傳量可達320 W，熱阻0.234 ℃/W。實驗所量測補償室溫度與理論預測之值，其平均絕對百分比誤差為5.65 %。再者，將參數相接近的高分子以及金屬毛細結構，先後置入迴路式熱管中進行熱傳性能測試比較，結果顯示具高分子毛細結構迴路式熱管其熱傳性能與具金屬毛細結構迴路式熱管相接近。熱傳導係數為造成性能差異的主因，低熱導係數之高分子毛細結構雖可降低熱洩漏量，使迴路式熱管在較低之操作溫度下運作，但亦會降低毛細結構外部之蒸發表面熱傳係數，使蒸發表面散熱效率變差。
將低熱導係數的高分子毛細結構應用於熱洩漏較為嚴重的平板型迴路式熱管，製作出主要毛細結構為金屬以及次要毛細結構為高分子的複合式雙層毛細結構。在主要毛細結構厚度同為2 mm的條件下，測試結果顯示平板型迴路式熱管性能的熱傳量可達140 W、熱阻為0.38 ℃/W，相較於金屬毛細結構的熱傳量40 W、熱阻0.69 ℃/W，具低熱導係數高分子的複合式雙層毛細結構可有效降低熱洩漏對系統熱傳性能的影響。
利用鹽析法除了可製作出高孔隙度與高滲透度之高分子毛細結構，使具高分子毛細結構迴路式熱管之熱傳性能與具金屬毛細結構迴路式熱管相同，其在製作特性上還具有成本較低、可調控毛細結構參數、可加工性等優點，因此可將鹽析法高分子毛細結構應用於迴路式熱管，有助於提升迴路式熱管之應用性。

關鍵字

高分子毛細結構 ； 鹽析法 ； 迴路式熱管 ； 熱洩漏 ； 聚苯乙烯

並列摘要

Loop heat pipe (LHP), which is a passive two-phase thermal transport device with high heat transfer capacity and long transport distance, has a great potential for applications of spacecrafts and electronic cooling. At present, the wick structures of the LHP are almost manufactured by sintering of metal powder. However, the use of sintered metal wick with high thermal conductivity may cause heat transmit into the evaporator core easily, which is called the problem of “heat leakage”. Besides, the wick parameters of sintered metal are difficult to control in the sintering process. Therefore, the main purpose of this study are using polystyrene with low thermal conductivity as the material and manufacturing the polymer wick structures by the method of salt-leaching in biotechnology area. By the method of salt leaching, the porosity can be controlled by adjusting the volume ratio between polymer material and sodium chloride, and the pore radius also can be controlled by adjusting the particle size of sodium chloride. Hence, polymer wick structures with different porosity and pore radius are manufactured, and the wick parameters that included pore size, pore radius and permeability are also measured. The results show that the pore size parameter lies in the desired range, and the porosity is able to be controlled within 1.65 % MAPE. Summarizing the permeability of the polymer wicks with different porosity and pore radius, the formulas of polystyrene wick structure between permeability, pore radius and porosity expressed specific as lnκ=-20.47+lnrc+4.31ε2.5 is established. The formulas would help the design of LHP and the prediction of the heat transfer performance of LHP.
The polymer wick structures with various wick parameters are installed into a LHP, and the heat transfer performance of LHP system is conducted and compared with sintered metal wick structures. The results of the test reveal when the smaller the thickness and pore radius are, and the larger the porosity is, the performance of the heat transfer would be much better. Hence, a wick structure with the thickness of 1.5 mm, the pore radius of 5.9 micron, the porosity of 80 % is installed into a LHP system to carry out the performance. The capacity of heat transfer in LHP with polymer wick structure approaches 320 W, the thermal resistant is 0.234 ℃/W under the evaporator temperature of 85 ℃. The comparison of the compensation chamber temperature between the predicted results and experimental data shows a good agreement (within 5.65 %). Furthermore, in compare with metal wick structure at the similar porosity and pore radius, the performance of the LHP with metal wick structures is little better than the LHP with polymer wick structure. The thermal conductivity of wick material is major cause of different performance. Polymer wick structure with low thermal conductivity can reduce the compensation chamber temperature, but also decrease the evaporative heat transfer coefficient at the outside surface of wick structure.
According to the literature, the heat leak problem is more serious in the flat LHP than the cylindrical LHP. This problem also makes the thermal resistance of flat LHP higher. Hence this study tries to solve the difficulties by the use of low thermal conductivity polymer wick structure. Then, a composite double layer wick structure, which is composed of the primary wick structure sintered with metal powder and the secondary wick structure made of the low thermal conductivity polymer is applied to the flat loop heat pipe. The results show that when primary wick thickness of composited double-decked wick structure is 2 mm and the temperature of the evaporators is less than 100 ℃. The maximum heat transfer capacity increases from 40 W to 180 W. In addition, the thermal resistance decreases from 0.69 ℃/W to 0.38 ℃/W.
In short, polymer wick structure, comparing with metal wick structure, has some advantages in the characteristics in its production, such as low manufacturing cost, easily controllable, perfect processing, and so forth. Moreover, these merits would promote its application in the LHP.

並列關鍵字

Polymer Wick Structure ； Salt-leaching Method ； Loop Heat Pipe ； Heat Leakage ； Polystyrene

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