利用分散式微液滴增強迷你流道熱沉之熱傳研究

Abstract

薄霧流是一種空氣中分散著微小液滴之雙相流，比較於傳統的單相強制對流上，有效提升散熱效果，流場特性相對複雜，液滴與空氣的比例和相互作用，均會影響其熱傳效果。本實驗主要探討三種不同大小流道，固定高度3mm，改變寬度分別為0.5mm、1mm、3mm，所對應的流道數目為14條、11條、5條，並改變壁面溫度為40℃、50℃、60℃，雷諾數為1000至6000。利用高速攝影機觀察薄霧流的流場現象，探討迷你流道尺寸效應對薄霧流流場的影響。實驗結果顯示，薄霧流冷卻效果皆高於空氣冷卻，隨壁面溫度上升，蒸發量增加，導致乾燥區出現，因此熱傳效果下降。在不同尺寸流道中，因雷諾數、壁面溫度會有不同的熱傳增益趨勢，最高可至空氣流冷卻4倍，最少則有1.5倍以上熱傳增益。摩擦係數薄霧流較空氣流大，約是空氣流的2至3倍，因微小液滴會附著在壁面上，導致摩擦係數增加，不過隨著溫度的增加，附著在壁面上液滴會逐漸蒸發而降低壓降，因此摩擦係數會降低。

The mist flow, produced by adding micro droplets in the air stream, can achieve higher forced heat convection than the traditional single phase flow. The heat transfer mechanism was very complicated since the mist flow was influenced by the interaction of air and droplets. Three mini channel heat sinks (width of 0.5mm, 1.0mm, and 3mm) with the same height of 0.5mm were investigated. The corresponding number of flow channels was 14, 11 and 5, and the temperature of the wall was 40℃、50℃、60℃. Reynolds number ranged from 1000 to 6000. The flow phenomenon of the mist flow and the effect on the mini-size channels can be observed using high speed camera. The result showed that the effect of the mist flow is better than that of the air cooling. With the rise of wall temperature, the evaporation rate increased, and produced the dry regions, reducing the heat transfer. Owing to the different sizes of channels, Reynolds number and the temperature of the wall can cause different trends of the heat transfer enhancement. The heat transfer from the mist cooling was 1.5 to 4 times as high as the air cooling. The friction coefficient of the mist flow was higher than that of the air flow and it was about 2 to 3 times higher than air flow. The tiny droplets attached to the wall, causing the increased friction coefficient. However, the droplets on the wall started to evaporate and caused lower friction coefficient as the wall temperature increased.