底部加熱的水平矩形管道中空氣混合對流的渦旋流結構:改變寬高比之影響

Abstract

本篇論文利用流場可視化及溫度量測方式,研究管道寬高比 (aspect ratio) 對於底部加熱的水平矩形管道內低雷諾數 (Reynolds number) 空氣混合對流在浮力驅動的空間和暫態渦旋流結構上之影響。寬高比的範圍從2到8,雷諾數由1.0到10.0,雷利數 (Rayleigh number) 則由2,000到6,000。流場照片顯示在高的寬高比 (A=12) 的混合渦流流場 (mixed vortex flow) 之管道中心部分的規則橫向渦流 (transverse roll),其在中的寬高比A=8的管道中明顯地被側板旁軸向成長的縱向渦流 (longitudinal roll) 擠壓嚴重彎曲,像U形渦流 (U-roll) 結構。而此U形渦流在管道出口部分更被分離成短的縱向渦流。在低的寬高比A=6,縱向渦流和U形渦流之間互動強烈,使得縱向渦流變得很不規則且U形渦流分裂成細胞渦流 (cell)。而在更低的寬高比A□4.0,已無混合渦流結構存在。 改變寬高比對橫向渦流的影響是輕微的,其包含暫態流場振盪振幅及頻率。本實驗所有寬高比的範圍,對於低雷諾數Re=1.0 & 2.5的流場結構主要為橫向渦流。但是在提高浮力-慣性力比時,管道會出現異常變形的橫向渦流。此變形的橫向渦流是由管道入口一半的一些不規則渦流及管道剩餘部分的規則橫向渦流所組成。其非常不同於在高的寬高比A=12所觀察到變形的橫向渦流是出現在管道出口部分。 本研究發現兩個新的渦流形態。特別的是在低的寬高比A=2中的凸向管道出口且低速往下游移動的彎曲渦流 (bent roll)。此外在極低雷諾數Re=1.0的另一渦流形態為管道入口是不移動的橫向渦流及下游為穩定的縱向渦流且其出現在本實驗各個寬高比的範圍。 根據所得到的數據,我們可以整理出各種的渦流形態流場組織圖。

In this study experimental flow visualization combined with transient temperature measurement are conducted to investigate how the duct aspect ratio affects the buoyancy driven spatial and temporal vortex flow structures in low Reynolds number mixed convection of air in a bottom heated horizontal rectangular duct. Results were obtained for the aspect ratio varied from 2 to 8, Reynolds number from 1.0 to 10.0, and Rayleigh number from 2,000 to 6,000. The flow photos show that at an intermediate aspect ratio for A= 8 the regular transverse rolls prevailed in the duct core in the mixed vortex flow for a high aspect ratio (A=12) are significantly squeezed by the axial growth of the longitudinal rolls near the duct sides to become highly curved, like U-rolls, and may further split into short longitudinal rolls in the exit portion of the duct. At a lower aspect ratio of A=6 the interaction between the longitudinal rolls and U-rolls are so strong that the longitudinal rolls become rather irregular and the U-rolls break into cells. At even lower aspect ratios for A□4.0 no mixed vortex roll pattern is noted. The change in the aspect ratio shows milder influences on the transverse vortex flow, including the temporal flow oscillation amplitude and frequency. For the low Reynolds number at Re=1.0 & 2.5, the flow is dominated by the transverse rolls for all aspect ratios examined here. But an unusual deformed transverse vortex flow pattern is noted in the duct at increasing buoyancy-to-inertia ratio. The deformed vortex flow consists of some irregular rolls in the entry half of the duct and regular transverse rolls in the remaining portion of the duct, which is very different from that observed in the high aspect ratio duct for A=12 in which the deformed transverse rolls first appear in the exit portion of the duct. Two new vortex flow patterns are noted in the present study. Specifically, bent vortex rolls which are convex towards the duct exit and move downstream at a slow speed are induced in a low aspect ratio duct with A=2. Besides, at the lowest Reynolds number tested here for Re=1.0 another vortex flow in the form of stationary transverse rolls in the duct entry and steady longitudinal rolls in the downstream is observed for all aspect ratios examined. Based on the present data, flow regime maps delineating various vortex flow patterns in the duct are provided.