Experimental Study on the Mixed Convective Vortex Air Flow Structure Driven by a Heated Circular Plate Embedded in the Bottom of a Horizontal Flat Duct
林 清 發
|關鍵字:||化學汽相沉積;混合對流;縱向渦卷流;橫向渦卷流;回流;chemical vapor deposition(CVD);mixed convection;longitudinal vortex roll;transversal vortex roll;return flow|
|摘要:||本文結合流場觀測與溫度量測，在開迴路混合對流實驗系統設計架構下，對具高寬高比(A=20)的水平矩形管道內底板舖設圓形加熱面由浮力所驅動空氣混合對流之渦旋流流場結構，進行廣泛而又深入的探討；實驗參數操作範圍為雷諾數 4.7 到 99.2，雷利數 3,200 至 31,500。
In this study experimental flow visualization combined with transient temperature measurement are conducted to investigate the structure of the buoyancy driven vortex flow in low-Reynolds-number mixed convection of air through a horizontal flat duct with an isothermally heated circular disk embedded in the bottom plate of the duct for the Reynolds number ranging from 4.7 to 99.2 and Rayleigh number from 3,200 to 31,500. The possible presence of various vortex flow patterns is studied by choosing a high-aspect-ratio rectangular duct (A=20) as the test section and the experiment is performed in an open loop mixed convection apparatus. How the circular geometry of the heated surface affects the vortex flow characteristics is investigated in detail. The results indicate that at low buoyancy-to-inertia ratios with moderate Reynolds numbers the generated vortex flow is in the form of longitudinal rolls. Moreover, the longitudinal vortex rolls (L-rolls) closer to the duct axis are induced at more upstream locations, which are completely opposite to those induced in a duct with a uniformly heated bottom. Besides, the thermals driven by the circular heated surface are not evenly spaced in the spanwise direction and tend to be asymmetric spanwisely. It is of interest to note that at a given Rayleigh number Ra the thermals are unstable at high Reynolds numbers, which suggests the existence of the inertia driven instability. Thus the L-rolls evolved from these thermals are also unstable with the presence of nonperiodic generation and disappearance of L-rolls. But at slightly lower Re the thermals and L-rolls are steady and regular. The vortex flow becomes unstable and irregular for a further reduction in the Reynolds number, that is obviously resulted from the buoyancy driven instability. The simultaneous presence of these two instability mechanisms explains the appearance of the reverse transition in the longitudinal vortex flow. At very low Reynolds numbers the buoyancy-induced secondary flow is characterized by the moving transverse vortex rolls (T-rolls) over the heated plate enclosed by an incomplete circular roll. The regular T-rolls are curved at the early stage of their initiation and deformed to some degree due to the presence of the incomplete circular roll around the edge of the heated plate. But in the exit half of the duct the T-rolls are nearly straight and almost spanwisely symmetric with respect to the vertical central plane. More specifically, the generated transverse rolls get stronger and bigger during the downstream moving but do not travel downstream at a constant speed. The transient temperature measurement reveals that the flow oscillation of the T-rolls driven by the circular heated plate is space dependent for given Ra and Re. Moreover, the frequency of the flow oscillation decays substantially with decreasing Reynolds number. Furthermore, the amplitude and frequency of the temperature oscillation are reduced for a raise of Ra but they are slightly affected by the increase in Ra at a higher buoyancy especially when Ra is beyond 11,600. These characteristics are very different from the transverse vortex rolls induced in a duct with a uniformly heated bottom plate. In the cases with a high buoyancy-to-inertia ratio resulted from a very low Reynolds number, a returning flow zone is formed in the entry portion of the duct and transverse vortex rolls prevail over the horizontal heated circular plate. The return flow is in the form of a semicircular roll around the upstream edge of the circular plate. In addition, a downstream return flow zone is also induced near the exit end of the duct. The upstream return flow zone almost blocks the entire duct inlet. Moreover, the return flow zone grows in size and the zone center migrates slightly towards the upstream at increasing buoyancy-to-inertia ratio. Besides, comprehensive temperature measurements suggest that the return flow maintains its steadiness in spite of the vortex flow induced in the downstream region of the duct. Based on the present data, flow regime maps are given to delineate various vortex flow patterns driven by the circular heated plate. In addition, the boundaries for the appearance of the regular transverse vortex flow patterns were empirically correlated. To provide the quantitative reverse flow characteristics, the present data for the size and center position of the upstream return flow zone at the mid-span of the duct are correlated empirically. Besides, empirical correlations for the onset points of the L-rolls and the criterion for the onset of the return flow are also provided.