氣水雙介質薄霧流於V型肋條流道內之熱傳研究

Abstract

薄霧流為空氣中混合微小液滴之兩相流，利用液滴及液膜蒸發之潛熱將大幅提升熱傳效果。本實驗使用截面邊長為40 mm之單面加熱的垂直正方形流道，於加熱面上安置連續及破壞型之夾角45度V型肋條且固定壁面溫度為60度，肋條高度對水力直徑比為0.05，肋條間距對肋條高度比為10、20，雷諾數為7900至24000，調整噴嘴空氣壓力為3.5、2 (kg/cm^2)，所對應之噴嘴水流量為0.4、0.75 LPM，使用紅外線熱像儀量測紐賽數並分析薄霧流之熱傳機制。實驗結果顯示，薄霧流冷卻效果隨著噴嘴水流量提升而增加，其熱傳效果皆高於空氣冷卻，在平滑表面熱傳增益可高達9倍，液滴易附著於V型肋條表面且累積液膜於肋條與兩側壁面之夾角，故熱傳效果較佳，流體再接觸點不易附著形成液膜，故於V型肋條表面有2至4倍的增幅。在空氣流及薄霧流中，摩擦係數為肋條高度對肋條間距比為10之連續V形肋條最大。

Mist flow is generated by mixing the air with the dispersed water droplets. The evaporation latent heat of droplets and liquid film enhance the heat transfer. In this study, the hydraulic diameter of the single-side heated square channel is 40mm. The rib configurations are continuous and broken 45°V-shaped. The rib height-to-hydraulic diameter ratio is 0.05. The rib pitch-to-height ratios are 10 and 20. The Reynolds numbers are 7900, 16000 and 24000. The air pressures of the nozzle are 3.5 and 2 kg/cm^2, and the water flow into the nozzle are 0.4 and 0.75 LPM. The infrared thermography is used to obtain the heat transfer coefficients and study the heat transfer mechanism of the mist flow. The experimental results show that the effect of mist flow cooling increases with the water flow ratios into the nozzle, and the heat transfer effect is higher than the air cooling. Droplets easily attaches to the V-shaped rib surface and leads to higher heat transfer. The accumulation of liquid film near the ribs and in the vicinity of the channel wall observed. The heat transfer enhancement of mist cooling is 9 times for the smooth surface and at least 2 times for the V-shaped ribs surface. In the air and the mist flow, the friction coefficient is the highest for the continuous V-shaped rib with a pitch-to-height ratio of 10.