低壓下間歇多噴嘴噴霧冷卻於不同表面結構之熱傳分析

Abstract

本研究架設一套低壓下間歇噴霧冷卻實驗裝置，旨在探討系統壓力在10 kPa下，固定工作流體之體積流率，散熱表面結構、噴嘴數量、工作週期以及噴灑時間對熱傳性能的影響。表面結構包含光滑表面以及周圍燒結表面 (簡稱燒結表面) 二種，單噴頭時工作週期為40%，噴灑時間由0.6 s增加至2 s；四噴頭噴灑時工作週期為10%，噴灑時間由0.15 s增加至0.5 s，使用的工作流體為純水。 依據實驗結果，於相同噴灑條件下，燒結表面有更低的表面溫度，溫度振幅也較小。而在光滑表面上，溫度隨時間變化較易隨機波動，液滴在光滑表面容易形成不均勻的團塊狀分布，而燒結表面則能達到較穩定的溫度變化，推測是因為毛細結構能將液體拉引至表面外圍，在表面中心處形成薄液膜，可穩定藉由蒸發散熱，故表面溫度較低，冷卻性能較佳。在固定流率下，四噴嘴噴灑可達較低的表面溫度，這是因為四噴嘴噴灑下有較大的液體覆蓋面積，表面上液膜較薄，熱阻較低，因此有較低的表面溫度，此外四噴嘴噴灑時，液體較不會堆積在表面中央，故能更快被排除，溫度受噴灑時間的影響較小。在相同工作週期之下，增加噴灑時間溫度震盪幅度也隨之提升，這是因為間隔時間隨噴灑時間增加而變長所致。同時，噴灑時間增加，也會導致表面液膜厚度增加，從而提高熱阻並導致表面溫度升高。然而，燒結表面的液膜厚度隨噴灑時間增加而變化不大，燒結結構能有效地排除多餘液體，使溫度相對穩定，能有效減少溫度振幅並保持表面溫度的穩定，將加熱表面維持在較低的溫度，在低壓間歇性噴灑時表現的更優異。

This study builds an intermittent spray cooling apparatus to investigate the effects of surface microstructures, nozzle quantity, and spray duration on the heat transfer performance. Two types of surfaces are tested: smooth surface and a surface with smooth center region and sintered peripheral (a.k.a. sintered surface). When single nozzle is used, duty cycle is set to 40%, with a spray duration increased from 0.6 s to 2 s. When four nozzles are implemented, duty cycle is 10%, with a spray duration between 0.15 s and 0.5 s so that is the volumetric flow rate is kept at constant. The working fluid is pure water, and the system pressure is 10 kPa. The experimental results show that, the sintered surface can result in lower surface temperatures and smaller temperature fluctuations. In contrast, temperature variations on the smooth surface are more prone to random fluctuations due to patchy distribution of liquid after the spray. The sintered surface has more stable thermal response, likely because its porous structure helps to draw liquid towards the edge and forms a thin film at the center. This thin liquid film leads to better evaporative cooling, resulting in lower surface temperatures. For a given flow rate, the use of four nozzles results in lower surface temperatures because the larger coverage of spray helps to distribute liquid more evenly. Under the same duty cycle, longer spray durations increase the amplitude of temperature fluctuation. This is attributed to the longer intervals between sprays. Moreover, a longer spray duration injects more liquid onto the surface per spray, subsequently thickeming the liquid layer, increasing the heat resistance and the surface temperature. However, the sintered structure can effectively remove excess liquid, maintaining more stable temperatures even with the increase of the spray duration. The sintered surface exhibits superior cooling performance across various spray conditions, effectively minimizing temperature fluctuations and stabilizing surface temperature.