混合空氣對流流經圓形加熱底板的水平矩型管道之迴流結構流場觀測

Abstract

本篇文章是利用流場可視化以及溫度量測，對於混合空氣對流流經圓形加熱底板的水平矩型管道，由浮力造成的迴流結構加以探討。內容敘述了迴流在空間上及溫度場的結構。此外，上游的迴流跟下游渦流結構的關係也在文中提及。更重要的是，迴流結構形成的過程也仔細的在文中敘述。實驗的參數範圍是雷諾數 (Reynolds number) 從5到50，雷利數 (Rayleigh number) 從4000到16000，由這些參數可以分別討論雷諾數和雷利數對迴流結構的影響。實驗結果顯示在低浮力與慣性力比值下，只有縱向渦卷存在管道的出口後半部份，管道中並不存在迴流結構。在中等的浮力與慣性力比值下，微弱的迴流存在於管道入口，靠近上板的地方。當浮力與慣性力比值繼續昇高且Re ＜ 10，上游迴流結構強度變強而且佔據了很大的區域。此外，在靠近管道出口也有一個迴流出現。在銅板上的主要主導流場結構是橫向渦卷。有趣的是，位於管道入口處的迴流結構是呈現半圓的形狀圍繞在銅板旁邊。經由流場可視化的觀察發現由浮力造成的一對旋轉方向相反橫向渦卷是從迴流區的下游尖端分裂而出。實驗中也注意到在流場轉變過程中，迴流先出現在靠近管道側板。另外根據實驗的資料，在文章中也列出了有關於迴流結構之發生條件，大小，中心點位置的修正公式。

An experiment combining flow visualization and temperature measurement is conducted here to investigate the buoyancy induced return flow in mixed convection of air over a heated circular plate embedded in the bottom of a horizontal heat duct. Both the spatial and temporal structures of the return flow are examined in detail. Besides, how the return flow induced in the upstream portion of the duct is related to the vortex flow in the downstream is delineated. Moreover, the formation processes leading to the return flow from the unidirectional forced main flow are explored. In the experiment the Reynolds number of the flow is varied from 5 to 50 and the Rayleigh number from 4, 000 to 16, 000 and the effects of the Reynolds and Rayleigh numbers on the return flow are examined in detail. The results show that at a low buoyancy-to-inertia ratio only steady longitudinal vortex rolls (L-rolls) are induced in the exit half of the duct and no return flow appears in the duct. At a intermediate buoyancy-to-inertia ratio a small flow recirculation exists around the duct inlet near the top wall and the L-rolls become unsteady. When the buoyancy-to-inertia ratio is high and the Reynolds number is small with Re＜10, the upstream return flow is strong and occupies a large region. There is another return flow zone around the exit end of the duct. Over the heated circular plate the flow is dominated by the moving transverse rolls. It is of interest to note that the return flow around the duct inlet at steady or statistically stable state is in the form of a semicircular roll around the upstream edge of the circular plate. We further note that during the transient stage the return flow first appears in the side wall region of the duct. Flow visualization also reveals that the splitting of the downstream tip of the return flow zone in the upstream near the duct inlet and the buoyancy driven, spanwisely extended thermal under the tip generate a pair of counter-rotating transverse rolls in the entry portion of the duct. The rolls are then pushed by the main flow to move slowly downstream and grow slightly in size. In addition, the criterion for the onset of the return flow near the duct inlet, the size and the center position of the return flow, based on the present experimental data, are correlated empirically.