混合空氣對流流經底部加熱水平漸縮扁平管道之浮力驅動渦流結構研究

Abstract

本論文研究藉由側板傾斜的底部加熱漸縮管道逐漸加速主流場速度，討論低雷諾數流場穩定性，實驗主要利用流場可視化及暫態溫度量測方法探討渦流的時空特性，本實驗結果並將與側板未傾斜矩形管道作比較，特別著墨在側板傾斜對縱向渦流(longitudinal roll)、橫向渦流(transverse roll)、混合渦流(mixed vortex roll)的效應，實驗操作參數範圍雷諾數介於2到30之間，雷利數則由2,500到10,000，針對側板5度角漸縮作大範圍的浮慣比(buoyancy-to-inertia ratio)研究。 由本實驗研究結果得知在低的浮慣比下，與矩形管道相比較後發現側板傾斜會導致縱向渦流發生的位置較為延後，渦流也較小，縱向渦流的消除也可以觀察出，但在高浮慣比下，側板傾斜導致的流場穩定性將變的不明顯。在間歇渦流流場(intermittent vortex flow)裡，所出現的微弱橫向渦流將逐漸被壓制，因此發現流場型態由間歇渦流流場轉變成規律縱向渦流流場。在漸縮管道裡的混合渦流流場及橫向渦流流場，則發現橫向渦流被擠壓而嚴重變形，其長度也較短，此外，縱向渦流尺寸也較小，再者，橫向渦流的大小沿著向下游流動而逐漸增加，並且也發現同對橫向渦流，其渦流大小不一的情形。最後，提出在漸縮管道裡縱向渦流發生位置、橫向渦流的尺寸及對流速度在軸向增加的經驗公式，並由流場組織圖解釋在漸縮管道裡，區分不同流場型態的邊界。

An experiment is carried out in the present study to investigate the possible stabilization of the low Reynolds number mixed convective vortex air flow by the main flow acceleration due to the reduction in the aspect ratio of the duct through the inclination of the sidewalls in a bottom heated horizontal flat duct. The detailed spatial and temporal characteristics of the vortex flow in the sidewall converging flat duct are procured by experimental flow visualization and transient temperature measurement. The results from the present experiment are compared with those for the corresponding sidewall nonconverging flat duct, which is designed as the “rectangular flat duct”. Particular attention is paid to examining how the sidewall tilting affects the longitudinal, transverse and mixed vortex flows. Experiments are conducted for the Reynolds number varying from 2 to 30 and Rayleigh number from 2,500 to 10,000 for the inclination angle of 5°, covering a wide range of the buoyancy-to-inertia ratio. The results from the present study indicate that at low buoyancy-to-inertia ratios the sidewall inclination causes a slight delay in the onset of longitudinal vortex rolls and the longitudinal vortex rolls are slightly smaller, when compared with those in the rectangular flat duct. The elimination of the longitudinal vortex rolls in the duct core region is also seen. But at high buoyancy-to-inertia ratios the flow stabilization by the sidewall inclination is insignificant. In the intermittent vortex flows the appearance of the weak transverse roll is suppressed by the converging sidewalls. Thus the vortex flow pattern changes from the intermittent vortex flow to regular longitudinal vortex flow. For mixed vortex flow, the transverse vortex rolls in the duct core are squeezed to a larger degree to become somewhat bent and are shorter in the convergent flat duct. Besides, the longitudinal rolls in the sidewall region are smaller in size. Moreover, the transverse vortex rolls grow gradually in size as they move downstream. In the pure transverse vortex flow the sidewall inclination causes the transverse rolls again to grow in size in the downstream direction. In addition, the counter-rotating rolls in the same roll pair have very different size. Empirical correlations are provided for the onset locations of the longitudinal rolls and the size growth and convection speed increase of the transverse rolls in the axial direction in the convergent duct. A flow regime map is also given to delineate the boundaries separating different vortex flow patterns induced in the convergent duct.