水平管道之圓型底板加熱面插入曲線板塊於管道上游位置對於迴流消除之影響

Abstract

本論文研究結合流體可視化及暫態溫度量測之方法，探討因浮力所驅動之迴流(return flow)渦流結構於圓形底板加熱面之矩形管道中如何被消除藉由插入曲線板於管道上游增加主流場速度。實驗操作參數範圍雷諾數和雷利數分別介於5.0 到 50.0 之間，及7,500 到 19,800，插入曲線板於三個分別不同的位置。其實驗所得結果將與未插入曲線板之水平管道做比較，特別將注意力著重於其迴流結構如何藉由插入曲線板而消除；此外， 插入曲線板之位置對於壓制迴流結構之影響也將做一步明細探討於本研究中。 由實驗研究結果顯示得知當插入曲線板於管道增加主流場速度而導致有效的延遲迴流結構產生，並因此明顯的延遲迴流結構的產生。如此致使流體在單一方向具有較廣泛之雷諾數。在臨界浮慣比下的迴流結構受曲線板作用而稍微的減弱其強度、結構變的較小；此外將插入曲線板移向較接近於加熱面時，其迴流結構之產生受曲線板影響而被延遲現象更加顯著。根據實驗所得的結果，簡略的比較在矩形管道中插入曲線板與沒有曲線板作用，對於臨界迴流結構發生之影響，並提出經驗公式。最後，在高浮慣比下，流體之震盪頻率也有效的受曲線板之作用而被壓制.

An experiment combining flow visualization and temperature measurement is carried out here to investigate in mixed convection of gas how the buoyancy induced return flow driven by a heated disk embedded in the bottom of a horizontal rectangular duct is suppressed by the main flow acceleration due to the insertion of the curved blocks into the upstream section of the duct. In the experiment the Reynolds and Rayleigh numbers of the flow are respectively varied from 5 to 50 and 7,500 to 19,800 with the blocks placed at three different positions. The results with the absence and presence of the blocks in the duct are compared with each other. Particular attention is focused on the elimination of the return flow by the insertion of the blocks. Besides, the suppression of the return flow affected by the locations of the blocks is also examined in detail. The results from the present study indicate that when the blocks are inserted into the duct the main flow acceleration causes a significant suppression of the return flow and hence causes a substantial delayed onset of the return flow. Thus the flow is uni-directional over a wider range of the Reynolds number. At supercritical buoyancy-to-inertia ratios the return flow becomes somewhat weaker in intensity and smaller in the size with the insertion of the blocks. Moreover, the onset of the return flow is delayed more substantially by placing the blocks closer to the heated plate. According to the data from the present experiment, the critical conditions for the onset of the return flow in the duct without and with the presence of the blocks are summarized and empirical correlations are proposed. Finally, the temporal oscillation of the flow at high buoyancy-to-inertia ratios can be significantly suppressed by the block insertion.