礦物油在水平矩形管道內的層流熱傳實驗

Abstract

本論文為分析二平行排列之寬高比2：1水平矩形管道內的層流熱傳特性。工作流體為石蠟油70。單一矩形管道的長度為250公分，剖面寬為1公分，剖面高為0.5公分，水力直徑為0.67公分。每一流道體積流率為每分鐘0.12至0.9公升，雷諾數的範圍為12至75, 控制入口溫度恆為21℃，溫度測試範圍為21℃至55℃。實驗研究上板加熱、下板加熱與上下板同時加熱三種不同的熱邊界條件，以求得沿流道軸向方向的局部紐賽數和流道的壓力差降值。矩形流道內加熱面與未加熱面之間因為黏度差異產生軸向的速度變形，另外還有自然對流效應，可達到增強熱傳的效果。在固定加熱量及質量流率下，下板加熱的熱傳係數較上板加熱的熱傳係數佳。在固定加熱量下，隨質量流率增加，上板加熱之完全發展紐賽數增加幅度最明顯。在固定質量流率下，隨加熱量量增加，上板加熱、下板加熱或上下板同時加熱紐賽數增加的幅度皆不明顯。

This is an experimental study of laminar heat transfer in two aspect ratio 2:1 rectangular ducts. The long of this test section is 250 centimeter; width of each channel is 1 centimeter; the height is 0.5 centimeter; and the hydraulic diameter is 0.67 centimeter. Volume flow rate of each channel is 0.12~0.9 liter per minute. The range of Reynolds number is 12 to 75. Set the inlet temperature at constant 21℃. The testing range of temperature is at 21℃ to 55℃. Test fluid adopts mineral oil - Paraffinic base oil 70, whose viscosity varies with temperature greatly. Study is under three different heat flux and three different thermal boundary conditions: upper wall heated, lower wall heated, both walls heated to obtain the local Nusselt number along axial direction of the channel and pressure drop value. When the velocity distortion and nature convection are induced between heated surface and unheated surface in the channel, they could both enhance heat transfer. In a constant heat flux and mass flow rate, the heat transfer coefficient of the lower wall heated is greater than the upper wall heated. Under a constant heat flux, the increasing range of the fully developed Nusselt number of upper wall heated is most considerable while the mass flow rate increases. And under a constant mass flow rate, the increasing ranges of the Nusselt number of three thermal boundary conditions are all unobvious.