利用電容法量測均溫板在不同加熱功率與傾角下之空隙率變化

Void fraction measurements in vapor chamber under different power inputs and tilting angles using electrical capacitance method

連于仁(Yu-Jen Lien)

指導教授 ： 孫珍理

國立臺灣大學/工學院/機械工程學研究所/碩士(2021年)

https://doi.org/10.6342/NTU202101798

摘要

本研究使用電容法量測均溫板在非對稱加熱下的空隙率隨時間之變化，利用液態水與水蒸氣介電常數的差異，來推算內部的空隙率。我們針對不同的加熱功率與傾角組合進行空隙率量測，俾利探討均溫板之抗重力能力，結果將有助於未來相變化散熱元件之彈性運用及作為設計改良之參據。
由實驗結果可知，當進行非對稱加熱並搭配自然對流冷卻時，受到均溫板中央位置密集的毛細圓柱排列設計的影響，均溫板的空隙率分布會以中央的外環且遠離熱源的位置有較高的空隙率，而以熱源附近的空隙率較低，顯示在此液體回補良好。隨加熱功率增加，高空隙率區域會向外擴張，在10.7 W及傾角為90°時，局部最高空隙率可達0.96。當傾角固定時，隨加熱功率增加，均溫板的穩態平均空隙率亦上升，顯示內部汽化程度與加熱功率為正相關。由空隙率變化可知，在加熱功率為5.7 W與8.5 W時，均溫板的空隙率在傾角0°至90°間較不受傾角變化的影響，穩態平均空隙率的最大變異量分別僅有3.5%與1.7%；隨加熱功率增加至10.7 W時，均溫板的穩態平均空隙率較易受傾角變化的影響，在傾角小於30°時，穩態平均空隙率介於0.897與0.898，但傾角大於30°時，穩態平均空隙率會增加為0.906與0.908之間，代表在高汽化程度下，高傾角所造成之較大的重力作用會導致均溫板內的液體回補速度變緩，使整體空隙率上升。此外，傾角對於啟動溫度之影響亦取決於加熱功率之大小。在加熱功率為5.7 W與8.5 W時，均溫板的啟動溫度較不易受傾角變化的影響，而當加熱功率增加為10.7 W時，高加熱功率所產生之高汽化程度與高傾角(60°與90°)的重力作用，會導致均溫板的均溫性下降，且液態水無法即時回補，進而提高空隙率與內部壓力，啟動溫度隨之上升。

關鍵字

均溫板 ； 電容法 ； 空隙率量測 ； 非對稱加熱 ； 傾角 ； 啟動溫度

並列摘要

In this study, we utilized the electrical capacitance method to measure the change in void fraction of an off-the-shelf vapor chamber with different orientations under asymmetric heating. Due to difference in dielectric properties between liquid water and steam, we were able to determine the distribution of void fraction from the measurements of electrode pairs on flexible printed circuit boards. The tilting angle varied from 0° (horizontal) to 90° (vertical) in order to investigate the ability of the vapor chamber against gravity effect.
From the results, when the vapor chamber was subject to asymmetric heating at lower left corner and natural convection cooling, the far-side close to the center tended to have higher void fractions due to the highly-packed wick columns at the center. In contrast, the region adjacent to the heat source had lower void fractions, indicating sufficient replenishment of water liquid. When the tilting angle was fixed, the augmentation of the power input increased the average void fraction, leading to the expansion of high-void-fraction region and further deterioration of the vapor chamber.
With the vertical orientation at 10.7 W, void fraction could reach as high as 0.96. At low- to mid-level of power inputs (5.7 W and 8.5 W), on the other hand, both void fraction and start-up temperature were nearly independent of the tilting angle. As the power input increased to 10.7 W, the void fraction became susceptible to the tilting angle. The steady-state void fraction increased from 0.897 for tilting angles less than 30° to 0.909 for tilting angles greater than 60°. Moreover, severe vaporization at 10.7 W and great influence of gravity for tilting angles larger than 60° altogether aggravated the temperature uniformity, impeded the circulation of condensed liquid, elevated the void fraction and internal pressure, leading to higher start-up temperatures. The results of this study could facilitate more robust applications of phase-change devices and provide a mean to improve the design of the vapor chamber in the future.

並列關鍵字

vapor chamber ； electrical capacitance method ； void fraction measurement ； asymmetric heating ； tilting angle ； start-up temperature

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