影像法用於薄霧流液滴大小及速度量測之研究

Abstract

薄霧流冷卻技術為一種由在空氣中混合微小液滴以進行散熱的方式，此方法的冷卻效果遠高於傳統的單相強制對流冷卻。然而，液滴大小與速度皆影響熱傳效果。本研究量測薄霧流之液滴大小與速度等特性，透過雷射光束形成的薄面使液滴粒子成像，並用高畫素相機拍攝一連串的影像作為電腦程式分析的依據，最後將影像經過去雜訊、銳利化後，再使用平均灰階值法、模式法、疊代法、歐蘇法等二值化法並且計算出液滴的大小、速度場以及相關的流場數據。實驗中使用長40mm與寬40mm的正方形流道，並且在整流完的後段加入240mm的加熱區域。觀測位置設定在加熱流道的入口端及出口端，量測薄霧流在加熱段前後所產生的液滴大小及流場變化，以作為薄霧流的流場特性分析。本實驗透過空氣及水流量的調整，製造出在30-80μm之間大小不同的液滴。水量越大或空氣流量越小，越能製造出較大的液滴。鼓風機提供的乘載空氣流量越大亦越能乘載較大的液滴進入流道。速度方面，則透過拍攝連續雷射下形成的運動軌跡，計算出液滴的移動速度。本研究量測的結果可用於分析薄霧流散熱之特性。

Cooling by mist flow can achieve higher heat transfer rate than cooling by forced convection of single phase flow. The mist flow is produced by mixing the dispersed water droplets with air stream. The heat transfer is affected by the water droplet size and the mist flow velocity. In this investigation, digital image analysis method was used to obtain the droplet size and velocity. The droplet size and velocity were determined by pixel counting and particle tracking method, respectively. Droplets were illuminated with laser light and images were captured by the camera with microscope lens. Noise reduction and image sharpening were applied before image segmentation. The digital images were segmented using thresholding method, and the threshold algorithm used in this study included mean value method, mode method, iterative method, and Otsu method. The channel used in the investigation had the cross-section of 40mm×40mm and the heated length was 240mm. The images were taken at two observation points at the entrance and exit of heated region. Droplet sizes ranging from 30µm to 80µm were produced by adjusting the water and air flow rate. Larger droplets can be generated by combining higher water flow rate and lower air rate. Measurement of droplet size and velocity in the mist flow can be used to analysis the mist cooling performance.