标题: | 矩形水平管道内底板铺设圆形加热面由浮力所驱动空气混合对流之涡旋流结构实验研究 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 |
作者: | 杜志龍 Jyh-Long Tuh 林 清 發 Tsing-Fa Lin 機械工程學系 |
关键字: | 化学汽相沉积;混合对流;纵向涡卷流;横向涡卷流;回流;chemical vapor deposition(CVD);mixed convection;longitudinal vortex roll;transversal vortex roll;return flow |
公开日期: | 2002 |
摘要: | 本文结合流场观测与温度量测,在开回路混合对流实验系统设计架构下,对具高宽高比(A=20)的水平矩形管道内底板铺设圆形加热面由浮力所驱动空气混合对流之涡旋流流场结构,进行广泛而又深入的探讨;实验参数操作范围为雷诺数 4.7 到 99.2,雷利数 3,200 至 31,500。 结果显示在中高雷诺数所造成的低浮惯比情况下,管道内会出现纵向涡卷流。因为是圆形加热面的关系,愈靠近管道中央所形成的纵向涡卷流它的起始位置就会愈往上游方向移动。可以看出来这和矩形加热面导引纵向涡卷流起始位置的顺序有着相反的趋势。另外流场观测结果发现圆形加热面上的热突出物在横方向上并非等间距分布,连带使得往下游方向成长的涡流也失去对称性。值得注意的是在给定雷利数的条件下 ,这些热突出物在高雷诺数时会显的相当不稳定,表示有惯性力驱动的不稳定性存在,因此下游涡流会有非周期性涡卷消失与再生的不稳定情况发生。但是降低雷诺数之后逐渐有稳定的纵向涡卷流出现,形成从高雷诺数到低雷诺数也就是在增加浮惯比的情况下纵向涡卷流从不稳定转变成稳定的所谓逆势流场转移;再降低雷诺数则又因为浮力驱动的不稳定性而使得涡卷流再度成为不稳定状态。 在极低雷诺数的时候,管道内会出现一个环绕在加热板前缘呈半圆形的环状涡流;同时在圆形加热板上则是布满移动横向涡卷流。规则移动横向涡卷流只存在特定范围的雷利数内而且在入口侧起始位置的横向涡卷是往上游方向弯曲的。涡流在往下游移动的过程中会拉长变直,但是进入出口侧后因为加热板面积收缩的关系,涡流再度变形。此外规则移动横向涡卷流的分布对管道中央垂直面而言几乎完全对称,而涡卷在通过管道中心的位置时会有较大的长度。特别的是在往下游移动的过程当中涡卷强度增强而且体积也有逐渐增加的趋势,但是所有的涡卷并没有以相同的速率往下游方向移动。另外在给定实验参数条件下,瞬时温度量测记录显示规则移动横向涡卷流温度震荡的情形与侦测位置有很大的关系;特别是温度震荡的频率会随着雷诺数的降低而明显地减少,但是在高雷利数的时候温度震荡的频率与振幅随着雷利数变化的情形并不明显。这些特征和矩形加热面所导引的规则横向涡卷流有很大的不同。 在极低雷诺数所造成的高浮惯比情况下,回流会出现在管道入口侧靠近上板的位置,而在加热板上则是移动横向涡卷流。回流的外型是环绕加热板前缘呈逆时针方向旋转的涡旋流,同时我们在稍微离开加热板下缘处也可以看到另一个不同旋转方向的回流,但是这个回流因为加热板上方移动横向涡卷流通过的影响,无法保持一个稳定的状态。此外,上游回流随着雷利数的增加几乎占据了整个管道入口的位置,涡流中心也会前移。值得注意的是温度量测的纪录显示,回流的存在基本上维持稳定而与下游各种涡流的型式无关。 根据各种不同参数条件下对管道内涡旋流流场的观察,我们整理出流场组织图以区别各种不同的涡旋流型式并且列示发生规则移动横向涡卷流的上下边界关连式;另外为了量化上游回流的特征,我们提出一个与浮惯比有关用来判别回流是否存在的法则,结果显示临界浮惯比会随着雷诺数的增加而降低。其他如上游回流轴向及垂直向涡流体积的大小、回流中心的位置以及纵向涡卷流起始位置、不同涡旋流型式的边界等,我们也分别列出了与实验参数有关的关连式。 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. |
URI: | http://140.113.39.130/cdrfb3/record/nctu/#NT910489052 http://hdl.handle.net/11536/70807 |
显示于类别: | Thesis |