水平矩形管道之圓型底板加熱面藉由漸縮與傾斜管道對於浮力所驅動空氣混合對流之迴流延遲與渦旋流結構穩定之影響研究

Abstract

本論文係以探討在水平矩形管道圓型底板加熱面管道內之空氣混合對流，浮力驅動之流場變化。實驗操作參數範圍雷諾數介於5到50之間，雷利數則由7,200到21,000，針對漸縮與傾斜管道對於浮力所驅動空氣混合對流之迴流延遲與渦旋流結構穩定之影響研究。 第一部份將管道漸縮逐漸加速主流場速度，實驗中將測試段分別漸縮5.7∘與11∘，探討低雷諾數流場穩定性，實驗主要利用流場可視化及溫度量測方法探討渦流的特性，實驗結果並將與矩形管道作比較，針對在漸縮管道對縱向渦流(longitudinal roll)、橫向渦流(transverse roll)、迴流(Return Flow)的效應，實驗結果顯示在低的浮慣比(buoyancy-to-inertia ratio)，與矩形管道相比較後發現漸縮管道會導致縱向渦流發生的位置較為延後，在高浮慣比，由於管道漸縮逐漸加速主流場速度，流場型態由不穩定渦流流場轉變成規律縱向渦流流場，迴流亦有效的延遲產生，並且減弱其強度與結構尺寸也縮小，溫度量測亦指出管道漸縮能有效抑制與消除流場中不規則的震盪。 第二部份，針對空氣於水平矩形管道圓型底板加熱面的傾斜管道中，藉由流場觀測及溫度量測來探討管道傾斜角度對空氣混合對流渦流結構的影響。本實驗的角度範圍介於1∘與2∘之間以探討稍微傾斜傾斜管道中迴流與渦旋流結構的軸向發展過程，在稍微傾斜角的情況下，順向混合對流(aiding mixed convection) 即浮力作用在流動方向，結果指出，由於傾斜時浮力助流的作用，使得流場規律且平順，迴流結構尺寸縮小與強度減弱，迴流和渦旋流結構的起始點較水平延後發生；溫度量測顯示，流場中不規則的溫度震盪亦有效的被壓制與消除，浮力成為穩定流場之因素。 最後，根據漸縮與傾斜管道之實驗所得結果分別提出一個與浮慣比有關用來判別回流存在與否的判別法則亦即迴流發生參數之經驗公式，並由流場組織圖說明在傾斜管道內，不同流場型態之邊界。

In this study an experimental flow visualization combined with temperature measurement are conducted to investigate how the sidewall converging and duct inclination affect the buoyancy induced return flow structure and stabilization of vortex flow in mixed convection of gas in a horizontal rectangular duct. The buoyancy driven secondary flow including the return flow and vortex flow is driven by a heated circular disk embedded in the bottom plate of the duct, simulating that in a horizontal longitudinal MOCVD reactor. Specifically, in the first part of the present study the sidewalls of the duct are inclined toward the duct core so that the gas flow in the duct is accelerated, causing the buoyancy-to-inertia ratio to decrease in the main flow direction. While in the second part of the study the duct is inclined upwards with its exit end above the entry end and the component of the buoyancy force normal to the heated plate is reduced. In the experiment the Reynolds and Rayleigh numbers of the flow at the duct inlet are respectively varied from 5 to 50 and from 7,200 to 21,000. In the first part of the study the duct aspect ratio is reduced from 20 at the inlet to 16 or 12 at the exit. The duct is slightly inclined from the horizontal in the second part. Particular attention is paid to delineating the spatial changes of the return flow structure with the sidewall converging and to explore how the duct inclination possibly suppresses and stabilizes the secondary flow. The results obtained in the study show a substantial delay in the onset of the return flow and the effective suppression of the buoyancy driven unstable longitudinal and transverse vortex flows by the sidewall converging and the duct inclination. Besides, the sidewall converging and the duct inclination can weaken the return flow more effectively at slightly higher Reynolds numbers. An empirical equation is provided to correlate the present data for the onset condition of the return flow in the duct. Some preliminary results from the second part of the study indicate that the slight inclination of the duct at 2° can significantly weaken the return flow. The reduction in the size of the return flow zone and the intensity of the return flow is prominent. Besides, the onsets of longitudinal and transverse vortex rolls are delayed substantially.