利用電容法量測熱管與均溫板內之空隙率變化

Abstract

本研究提出藉由電容法量測熱管與均溫板在加熱過程與冷卻過程中之空隙率的變化。在不同熱通量下，搭配兩種散熱風扇，分別量測熱管蒸發端、絕熱端、冷凝端與均溫板上不同位置之電容值，由於水蒸氣與液態水之介電常數不同，即可由電容值量測與校正曲線推算空隙率。此方法為非侵入式之量測，且得知空隙率將有助於評估熱管與均溫板之散熱表現與熱通量極限，在熱管理過程中提供可靠的資訊。 由實驗結果可知，在加熱過程由於內部液態水蒸發導致電容值下降，空隙率上升。本研究發現熱管存在一可容忍之熱通量上限，在此上限內改變熱通量並不會使熱管各位置的空隙率產生大幅度之上升，且其隨冷卻能力增加而提高。使用直徑70 mm風扇與直徑80 mm風扇時可容忍之熱通量上限分別為12.2 W與15.2 W，在加熱過程空隙率最高僅上升0.06，但若超過此熱通量上限即可明顯觀察到空隙率隨熱通量的增加而上升。在均溫板之結果中，低熱通量(8.9 W)的加熱過程各位置空隙率之變異率為9%，這是因為此時汽化率較低，均溫板內空隙率分布較不均勻；在高熱通量(12 W)時，空隙率的變異率降為4%，代表均溫板內部汽化率提高會使空隙率分布較均勻。此外，在熱管冷凝端搭配17.7 W之熱通量與均溫板搭配14.2 W之熱通量時，皆有觀察到關閉熱源後500秒內(2000秒至2500秒)，空隙率大幅下降，甚至低於初始狀態之空隙率，這是由於高熱通量伴隨較高之表面溫度，加強熱管與均溫板表面之強制對流，冷卻量增加使熱管與均溫板內大量蒸氣凝結為液態水，由於先前大量蒸氣凝結使內部壓力小於飽和壓力，需有少部分液態水蒸發以維持壓力平衡，導致在2500秒至3000秒時空隙率微幅上升。

This study proposed the use of the capacitance method to measure the change in void fractions of heat pipe and vapor chamber during heating and cooling processes. Due to the different dielectric constants of water vapor and liquid water, void fraction could be estimated from the capacitance measurement. For heat pipe, metal electrodes cut in-house were laminated with double-sided tape and attached to the evaporation, adiabatic, and condensation sections. For vapor chamber, flexible PCBs were customly made to serve as 20 electrode pairs. From the experimental results, evaporation during the heating process resulted in the decrease in capacitance because vapor had lower dielectric constant than liquid water. In addition, we found that heat pipe had an upper limit of input heating rate, below which void fraction showed little deviation across the different sections. The upper limit increased from 12.2 W to 15.2 W when the 70 mm fan was replaced by an 80 mm fan for cooling. For vapor chamber, the spatial variation of void fraction decreased from 9% to 4% when the input heating rate was increased from 8.9 W to 12 W. This was because higher vaporization rate caused less discrete distribution of working fluid. When the heat source was turned off, void fraction experienced a drastic decrease. Especially after cutting off high heating rate, void fraction in heat pipe and vapor chamber could fall below its initial value. The higher surface temperature produced a stronger external convection effect, which led to rapid condensation inside the device. However, excess condensate resulted in a decrease in pressure. When the pressure was lower than the saturation pressure, evaporation took place and void fraction slightly increased to reach phase equilibrium.