底部在高溫的水平管道中上板加熱對空氣混合對流渦流結構的影響

Abstract

結合流場觀測及暫態溫度測量的實驗被實現在此論文中，用以研究在底部加熱的水平扁平管道中藉由上板加熱對管道內混合空氣對流渦流結構（如；橫向渦捲、縱向渦捲、混合渦捲）的穩定及消除影響。實驗上，有系統的變化上板溫度用來檢視流場在時間與空間上結構的變化，並詳細的觀察各種流場條件下（上板溫度、流場雷諾數和雷利數）造成流場結構的變化。實驗所使用的開放式混合對流設備是沿用實驗室以往已架設好的設備，其主要測試斷的寬高比為12。實驗範圍包含了廣大的浮力與慣性力比比值（雷諾數從1到50，雷利數從4000到8000，無因次的上板溫度 從0到1；每1/8為一間隔）。 在研究的結果中清楚的指出，管道中上板加熱確實能有效的穩定及消除流場中橫向渦捲、縱向渦捲、混合渦捲和混亂對流的渦流結構。同時對於穩定因底板加熱所產生的不穩定性，也顯示出它重要的影響。且當上板溫度接近於底板溫度時，流場不穩定大都被抑制，而形成單一方向流動情況。在實驗的過程中，發現到一些不常見的的渦流結構；入口存在有1到2個停留的橫向渦捲，伴隨下游產生有10到12捲縱向渦捲，他們僅出現在較高的浮力與慣性力比比值。最後將實驗資料整理成一張表格在（Table 3.2）。

An experiment combining flow visualization and transient temperature measurement is carried out here to study the possible stabilization and elimination of the buoyancy driven unstable longitudinal, transverse and mixed vortex flow in mixed convection of air in a bottom heated horizontal flat duct by the top plate heating. Systematic variations in the top plate temperature were performed to examine its effects on the spatial and temporal flow structures. How the top plate temperature and the Reynolds and Rayleigh numbers of the flow affect the vortex flow characteristics is investigated in detail. An open loop mixed convection apparatus established earlier was employed in this investigation and the test section is a high aspect ratio (A=12) rectangular duct. Experiment is conducted for the Reynolds number varying from 1 to 50, Rayleigh number from 4,000 to 8,000 and the nondimensional top plate temperature from 0 to 1 at an interval of 1/8, covering a wide range of the buoyancy-to-inertia ratio. The results from this study indicate that the top plate heating substantially stabilizes and eliminates the longitudinal, transverse, mixed longitudinal and transverse, and irregular vortex flows. It shows significant effects of stabilizing the vortex flow induced by the buoyancy resulting from the heated bottom plate of the duct. At the high top plate temperature even the entire irregular vortex flow can be eliminated and the flow is unidirectional in the duct. Obviously the transient flow oscillation in the flow is completely suppressed. An unfamiliar vortex structure consisting of stationary transverse rolls in the duct inlet and longitudinal rolls in the downstream is noted in the present study. They appear only at high buoyancy- to-inertia ratios. Various vortex flow patterns observed in the present study are summarized in Table 3.2.