混合空氣對流在小角度側向傾斜的底部加熱水平扁平管道之渦流特性研究

Abstract

本論文研究藉由側向傾斜原為水平的底部加熱矩形管道並逐漸加大側向傾斜角度，討論低雷諾數流場經由側向傾斜之後的不穩定性，實驗主要利用流場可視化及暫態溫度量測方法探討渦流的時空特性，本實驗結果並將與管道未傾斜矩形管道作比較，特別著墨在管道側向傾斜對縱向渦流(longitudinal roll)、橫向渦流(transverse roll)、混合渦流(mixed vortex roll)的效應，實驗操作參數範圍雷諾數介於2到30之間，雷利數則由3,000到8,000，針對管道側向1、2、5及10度角傾斜作大範圍的浮慣比(buoyancy-to-inertia ratio)研究。 由本實驗研究結果得知在低的浮慣比下，與水平矩形管道相比較後發現管道側向傾斜會導致靠近管道較低側板的縱向渦流發生的位置較為延後、渦流也較大；靠近管道較高側板的縱向渦流發生的位置較為提前、渦流也較小。但在高浮慣比下，管道側向傾斜導致的流場不穩定性將變相當明顯。在間歇渦流流場(intermittent vortex flow)裡，所出現的微弱橫向渦流將逐漸被破壞，因此發現流場型態由間歇渦流流場轉變成更不規律閒歇渦流流場。在側向傾斜管道裡的混合渦流流場及橫向渦流流場，則發現即使只是小角度1、2度橫向渦流將被擠壓而嚴重變形破壞消除。橫向渦流的大小沿著向下游流動而逐漸增加，並且也發現同對橫向渦流，其渦流大小不一的情形。此外，在較高角度下在靠近較高側板的管道入口及靠近較低側板的管道出口各有一明顯的迴流生成。所發現的新渦流結構，在高浮慣比或高傾斜角的條件下將會生成此新渦流結構。最後，提出在管道側向傾斜下管道裡縱向渦流發生位置的經驗公式，並由流場組織圖解釋在側向傾斜管道裡，區分不同流場型態的邊界。

An experiment is conducted in the present study to investigate how the vortex flow patterns associated with the mixed convection of air in a bottom heated horizontal flat duct are affected by the lateral inclination of the duct. The detailed spatial and temporal characteristics of the vortex flow in the inclined flat duct are procured by experimental flow visualization and transient temperature measurement. The results from the experiment are compared with those for the corresponding noninclined flat duct, which is designated as the “horizontal flat duct”. Particular attention is paid to how the lateral duct inclination affects the longitudinal, transverse and mixed vortex flows. Experiment will be conducted for Reynolds number varying from 2 to 30 and Rayleigh number from 3,000 to 8,000 for the inclination angle up to 10°, covering a wide range of buoyancy-to-inertia ratio. The present results indicate that at low buoyancy-to-inertia ratios the lateral duct inclination even at very small angles of 1o and 2o causes the longitudinal vortex flow to become spanwisely asymmetric. Besides, the duct inclination respectively results in the delayed and earlier onset of the longitudinal rolls in the lower and upper halfs of the duct. Moreover, the longitudinal rolls in the lower and upper halfs of the duct are respectively stretched and squeezed to become larger and smaller. Furthermore, the L-rolls in the upper half of the duct are less stable. As the inclined angle is increased to 5o and 10o, the asymmetry and onset of the L-rolls are more severely affected. The snaking motion of the longitudinal rolls in the upper half of the duct is noted. In the intermittent vortex flow the weak transverse roll in the upstream core region of the horizontal duct is destabilized to become somewhat irregular by the lateral duct inclination. The effects of the duct inclination on the L-rolls in the intermittent vortex are similar to that in the longitudinal vortex flow. The intermittent vortex flow also becomes spanwisely asymmetric. Especially for higher inclined angles of □□□= 5° & 10° the L-rolls in the upper half of the duct can become completely distorted. At a small inclined angle for □□≦□□2° and at a low buoyancy for Ra ≦ 4,000 the lateral channel inclination mainly causes the transverse vortex rolls in the upstream core region of the duct to become slant in the mixed longitudinal and transverse vortex flow. They deform significantly when moving downstream. The regular mixed vortex flow can become completely disorder even at a very small angle of 2o and irregular flow oscillation appears in the duct. In the pure transverse vortex flow the transverse rolls in the laterally inclined channel at a small inclined angle for □□≦□□2° deform substantially as they move downstream and the L-rolls form near the lower sidewall. The deformation of the T- and L-rolls are more significant for a higher and a higher □□□Even when the duct is only slightly inclined at 2o the pure moving transverse vortex flow pattern prevailed in the horizontal duct (□ = 0o) can be completely distorted by the duct inclination. Empirical correlations are provided for the onset locations of the longitudinal rolls in the axial direction in the inclined duct. Flow regime maps are also given to delineate the boundaries separating different vortex flow patterns induced in the inclined duct.