噴霧冷卻及液膜蒸發於低壓下對不同表面結構之熱傳分析

Exploring the performance of spray cooling and film evaporation on different surfaces at low pressure

吳逸軒(Yi-Hsuan Wu)

指導教授 ： 孫珍理

國立臺灣大學/工學院/機械工程學系/碩士(2023年)

https://doi.org/10.6342/NTU202302977

摘要

本研究設計一噴霧冷卻及液膜蒸發的實驗設備，利用降低壓力則飽和溫度下降的特性，探討噴霧及液滴蒸發在系統壓力為10 kPa下，不同銅塊初始溫度、銅塊表面結構與噴灑時間對熱傳的影響。其中銅塊表面結構包含平滑表面 (smooth) 、一般燒結表面 (sintered flat) 及輻射狀燒結表面 (radial sintered fins) 三種。
我們發現平滑表面的主要散熱發生在噴灑階段，藉由液滴與加熱表面的碰撞及流動帶走熱量；而一般燒結表面及輻射狀燒結表面的主要散熱則在於液膜蒸發階段，噴霧後液體在表面上形成液膜，藉由毛細結構將工作流體往外圍擴散，以維持液膜蒸發來散熱。由於燒結結構內流阻變大，大幅降低工作流體流速，不利於噴灑階段的散熱，反而是能藉由毛細作用讓流體往外圍流動補充，來增加液膜蒸發的效果。實驗結果顯示，在相同噴灑時間下，隨著銅塊初始溫度增加，加熱表面的最大溫降、最大冷卻率及總冷卻量均明顯上升。這是因為工作流體與加熱表面的溫差越大，熱交換越多。而在相同銅塊初始溫度下，隨著噴灑時間增加，加熱表面的最大溫降及總冷卻量亦明顯上升。對於燒結結構，提高噴灑時間使工作流體在中心處堆積，能藉由提升位能來增加液膜流速及流動範圍，以改善其冷卻效果。
實驗中的最大溫降、冷卻率最大值及最大總冷卻量均出現在平滑表面，其最大溫降為29.8℃，而冷卻率最大值可達 1218.4 W，較一般燒結表面及輻射狀燒結表面提升 208.4% 及358.9%。相比於單純的液膜蒸發，噴灑時液滴撞擊表面所帶來的換熱效益更高。

關鍵字

噴霧冷卻 ； 液膜蒸發 ； 低壓 ； 燒結結構 ； 初始溫度

並列摘要

This work studies the spray cooling on various types of surfaces under vacuum pressure. The system pressure is set at 10 kPa, and the influences of the surface in the heat transfer mechanisms are discussed. Three different types of surfaces are tested: smooth surface, sintered flat surface, and radial sintered fins.
The results show that cooling is dominated by the impingement of the spray on the smooth surface, while the evaporation of liquid film controls the heat transfer for the sintered flat surface and radial sintered fins after the liquid film is formed by the spray. For surfaces sintered with microstructures, heat transfer is deteriorated during the spraying stage because the porous layer drastically reduces the fluid velocity. However, fluid replenishment can be facilitated by the capillary force, enhancing the evaporation of the liquid film. As the surface is initially heated to a higher temperature, heat transfer increases due to the larger temperature difference. Longer duration of spray also contributes to better cooling performance. For the surfaces of sintered microstructures, increasing the spray amount leads to liquid accumulation in the central region. The higher potential energy results in the increase in liquid velocity and film coverage, both are beneficial to the evaporation of liquid film.
Comparing to liquid film evaporation, spray impingement is a much more efficient mechanism of heat transfer. Therefore, the maximum cooling rate of 1218.4 W can be found for the smooth surface.

並列關鍵字

spray cooling ； liquid film evaporation ； low pressure ； sintered structures ； initial temperature

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