平板型迴路式熱管熱洩漏效應之研究

Abstract

迴路式熱管(Loop Heat Pipe)具有高傳熱量、低熱阻、傳輸距離遠等優點，有相當大的潛力應用在航太科技及電子元件散熱。 一般圓管型迴路式熱管必須要加裝鞍部(saddle)才能應用於散熱平面上，而加裝鞍部不僅增加系統熱阻，而且還會使蒸發器表面溫度分布不均，影響性能，所以直接將蒸發器製作成平板狀將可改善加裝鞍部的缺點。 由文獻得知平板型迴路式熱管與圓管迴路式熱管比起來，有較顯著的熱洩漏(heat leak)問題，這也是造成平板型迴路式熱管，其系統熱阻較高的主要原因。因此本文針對熱洩漏問題提出兩種解決方法：第一，藉由增厚毛細結構的厚度，增加熱阻，降低熱洩漏的影響；第二、採用低熱導係數的高分子材料，製作以金屬燒結為主要毛細結構而高分子材料為次要毛細結構的雙層毛細結構，如此可以利用金屬毛細結構產生較高的毛細力，以及高分子毛細結構低熱傳導性質來防止嚴重的熱洩漏，藉此改善平板迴路式熱管熱洩漏的問題。 實驗證實，本文採取的兩種方法皆能改善熱洩漏對系統的影響。當主要毛細結構厚度為4 mm的雙層毛細結構，在蒸發器表面溫度不超過100℃、熱沉溫度30℃的情況下，有最大熱傳量180瓦，熱阻0.38 K/W，較未改善熱洩漏之前熱傳量40瓦、熱阻0.69 K/W，性能有明顯提升。

Loop heat pipes (LHPs), which are generally and widely used with cylindrical evaporators, have a great potential for applications of spacecrafts and electronic cooling due to the advantages of high transfer capacity, low thermal resistance and long transport distances. However, the cylindrical evaporators of loop heat pipes cannot work on a flat thermo-contact surface without saddle. The saddle not only creates an extra thermal resistance but also makes evaporators less isothermal. To avoid the above disadvantages, we adopt the flat evaporators of loop heat pipes. According to the literature, however, the heat leak problem is more serious in the flat evaporators than the cylindrical evaporators. This problem also makes the thermal resistance of flat evaporators higher. Hence this study tries to solve the difficulties and provides two solutions. The first method is to increase the thermal resistance of the wick by raising the wick thickness. The second method, we design a double-decked wick, which is composed of the primary wick sintered with metal powder and the secondary wick made of the low thermal conductivity polymer. The material wick generates higher capillary forces and the polymer has lower thermal conductivity. Such design can ease the heat leak problem. The results show that both methods can ease heat leak problem. When primary wick thickness of double-decked wick is 4 mm, the temperature of the evaporators is less than 100oC and the sink temperature is at 30oC. The maximum heat capacity increases from 40W to 180W. In addition, the thermal resistance decreases from 0.69 K/W to 0.38 K/W