具有楔形通道與鰭片結構之低壓兩相冷板的熱傳性能分析與氣泡流動可視化研究

Abstract

本研究旨在探討具有楔形上蓋的低壓兩相水冷板在搭配不同的鰭片結構設計時熱傳性能的表現。研究中共使用三種鰭片結構：全長型鰭片、短型鰭片、漸疏型鰭片，測試裝置的出口壓力設置於50 kPa，工作流體為去離子水，目標為在110°C的結溫以內盡可能提升熱通量。 測試裝置內的加熱過程可根據其氣泡以及熱傳狀態分成四個階段：第一階段為單相對流，表面尚未有氣泡產生，熱傳係數幾乎為定值；第二階段為局部過冷流沸騰，可觀察到局部成核，測試裝置內呈現氣泡流，熱傳係數開始隨加熱表面溫度上升而提升；第三階段為完全發展流沸騰，可觀察到大量成核點啟動，測試裝置內呈現氣泡流且開始出現氣泡合併，熱傳係數隨加熱表面溫度上升的幅度進一步增加；第四階段為飽和流沸騰，可觀察到大量氣泡合併，測試裝置內呈現塞狀流，此時熱傳係數開始隨加熱表面溫度上升而下降。 使用PEEK下蓋與水冷式冷凝器所獲得的實驗結果顯示，在四個階段中，相同熱通量下，短型鰭片的加熱表面中心溫度皆最低，漸疏型鰭片次之，全長型鰭片最高。在第一與第二階段中，短型鰭片與漸疏型鰭片因為下游的截面積改變，流體擾動增加而在下游形成紊流，因此擁有更好的強制對流熱傳表現。第三階段中，大量增加的氣泡會導致排除不順而產生震盪，而下游壓力震盪的頻率會稍微低於氣泡面積震盪的頻率，這是因為氣泡流至下游時體積增加，使得下游氣泡通過頻率降低。根據氣泡流動分析，氣泡面積震盪有助於氣泡排除，在沒有出現氣泡回流的情況下，氣泡面積震盪皆為向下游推送，氣泡面積震盪幅度越大，其單位深度的氣泡移除率越大。在第三階段中，短型鰭片的氣泡面積震盪幅度最大，漸疏型鰭片次之，全長型鰭片最小，因此短型鰭片的熱傳效果最好。第四階段中，所有鰭片結構的氣泡面積震盪幅度與氣泡移除率開始下降，由於下游空間較大的短型鰭片與漸疏型鰭片具更強的氣泡排除能力，因此在大量氣泡堆積的情況下，熱傳性能變差的幅度也較低。故熱傳性能最佳的組合為使用PEEK下蓋與水冷式冷凝器的短型鰭片，其最大熱通量為6.33 W mm-2，所對應的加熱表面中心溫度為106.3°C，總散熱功率可達4906 W，相較於漸疏型鰭片與全長型鰭片，短型鰭片的熱傳係數分別高出5.4%與21.5%。

This study focuses on the performance of a low-pressure two-phase cold plate with a wedge lid. The aim is to maximize the cooling capacity while maintaining a surface temperature below 110°C. Three fin designs are tested: full fins, short fins and divergent fins. Deionized water is used as the working fluid, and the outlet pressure is fixed at 50 kPa. The boiling process in the 2-phase cold plate can be divided into four regimes. Regime I only has single-phase convection and no bubble is found. The heat transfer coefficient remains constant. Regime II is subcooled flow boiling. Bubble nucleation occurs and bubbly flow is presented. The heat transfer coefficient increases with increasing heat flux. Regime III is fully developed flow boiling. More nucleation sites become active and bubble coalescence is observed. The heat transfer coefficient experiences a rapid increase. Regime IV is saturated flow boiling. Extensive bubble coalescence leads to the formation of slug flow and the heat transfer coefficient decreases with a further increase in heat flux. The results show that the short fins consistently yield the lowest surface temperatures and the highest heat transfer coefficient across all regimes. In regimes I and II, the downstream expansion in the designs of short and divergent fins triggers turbulence, improving convective heat transfer. As bubble incipience results in flow-oscillation, the outlet pressure fluctuates slightly more slowly than the variation of bubble area. This is ascribed to bubble growth as it travels downstream. Because reverse flow is not observed, the amplitude of bubble oscillation is directly linked to vapor removal rate per unit depth. The design of short fins leads to the most vigorous bubble oscillation and the best thermal performance. A maximum heat flux of 6.33 W mm-2 is achieved with a surface temperature of 106.3°C by using the short fins design. The corresponding power is 4906 W.