以可壓縮流探討三維漸縮煙囪管道熱傳之研究

Abstract

隨著工程發展上的需要，傳統煙囪的漸縮管道設計不再僅限於房屋工廠建築中排除由火所引起的氣體或煙塵，如此藉由其管道截面積的改變，以達到增強自然對流或熱傳效果的應用廣泛，包含熱交換器、火箭或飛機引擎推進室、太陽能集熱器與薄膜散熱表面等。因此本研究首先探討關於一具有高溫壁面的三維漸縮煙囪管道中的可壓縮流，分析管道中流體受到管道漸縮角度變化影響於強制對流及自然對流下的現象。在強制對流的研究中，因主流方向中流場截面積逐漸縮小，使流速加快，故在此時流體中央熱傳效果大幅增加，但壁面與壁面的夾角隨之減小，致流經夾角附近的流體，因壁面摩擦阻抗增加流速降低，熱傳效率隨之劣化，此一現象當漸縮角度增大而更為明顯；而在自然對流中流場角落區域受兩方向熱壁面影響，浮力效應明顯，反而流體速度在角落處較快，速度邊界層亦明顯增加，使管道內角落處有熱傳增益。此外，另延伸探討三維垂直漸擴管道自然對流的研究，在接近對應臨界雷諾數的案例裡可以觀察到四面高溫壁面浮力作用引起的向上流動之流體，漸擴管道角落的渦漩，以及出口邊緣由外界吸入的流體所產生的迴流，在管道中形成多個渦漩並不斷的形成再混合後而造成整體自然對流誘導之流場不穩定現象。

A chimney is very important engineering equipment and indispensably used in high temperature systems of combustion, pollution control, ventilation, etc. First, this study investigates the problem of forced convection flow in a three-dimensional tapered chimney with consideration of the flow compressibility numerically. The result shows that the convergent chimney construction can accelerate flow velocities by changing the area of cross section ,and the superior enhancement of heat transfer rates of the center of the heat wall are achieved. With the decreasing of the angle between the walls, velocities of the fluids sides by wall decrease due to the friction and the heat transfer efficiency decrease. Besides, the results in the situation of natural convection reveal that local Nusselt numbers distributed around the corner region are larger than those distributed around the central region. This is a characteristic of the natural convection that is rather different from that of the forced convection. Furthermore, the natural convection in three-dimensional divergent channel with different Rayleigh numbers from stable to unstable situation are investigated numerically, and the phenomena from laminar to unstable transition and heat transfer rate are observed.