鰭片通道內之薄霧流熱傳研究

Abstract

薄霧流為空氣主流中混合無數微小液滴形成的多相流，利用微小液滴蒸發帶走的潛熱，大幅度的提高熱傳效果。本實驗使用截面邊長為40mm的垂直正方形流道，在單面加熱鰭片通道固定壁面溫度為60℃，雷諾數為7900~24000，鰭片參數為橫向間距對鰭片直徑比例為2，縱向間距對鰭片直徑比例為3、6，鰭片與壁面間隙與鰭片高度比例為1。使用紅外線熱像儀進行平滑表面、單排、雙排與交錯陣列鰭片的熱傳分析。實驗結果顯示，薄霧冷卻效果皆高於空氣冷卻，隨著雷諾數的改變，鰭片周圍液滴有截然不同的累積形式。在低雷諾數下，液滴受到鰭片交錯陣列影響無法在加熱表面形成均勻液膜導致熱傳效益差；在高雷諾數下，較多液滴能沿著主流方向移動，在表面形成液膜有效提高熱傳效果。鰭片陣列雖能破壞流場邊界層形成渦流使流場混合，且增加熱傳面積卻不利於液膜的附著。在單排及雙排陣列中，由於液滴的累積熱傳增益最高到達十倍。在平滑表面熱傳增益約是七倍而在鰭片通道最少有著兩倍以上熱傳增益。

Mist flow is a two-compound flow produced by mixing the dispersed water droplets with air flow. Mist flow cooling achieved by high heat removal rate by evaporating water droplets to absorb heat. In this study, heat transfer in a single-side heated square channel with hydraulic diameter of 40mm was investigated by infrared thermography. Heat transfer behavior was examined for the cases of smooth surface and finned surface (single row , two rows and staggered arrays) at the Reynold numbers of 7900, 16000, and 24000. The results showed the heat transfer enhancement from mist cooling is 7 times for the smooth surface and at least 2 times for the pin fin surface. In the fin case, heat transfer was dominated by water accumulation and liquid film formation. In a single row and two row arrays, heat transfer enhancement using mist flow was 10 times higher compared to the air flow because of droplets accumulation.