【作者】 余业珍；

【导师】 张靖周；

【作者基本信息】 南京航空航天大学 ， 工程热物理， 2009， 博士

【摘要】 突片是一种结构简单的涡激励器,用于控制或改善射流的流动结构。国内外在利用突片改善射流冲击对流换热以及强化掺混方面已经展开了一系列的研究,但突片激励射流强化传热的机理、不同的突片结构对射流冲击换热和同轴射流强化混合的影响规律,以及突片射流与横流相互作用的流场特征,仍然是一个富于挑战和创新的研究课题。本文围绕这一研究方向,进行了两个方面的研究:(1)在射流冲击冷却方面,对单排、双排突片射流以及非均匀横流作用下冲击的换热特性进行数值计算和实验研究,分析不同结构突片对换热效果的影响,揭示突片激励射流的流场控制和冲击换热强化的物理机制;(2)在射流强化掺混方面,分析主流出口安装小突片后,流向涡的产生和发展规律,揭示突片射流改变射流流动结构的机制,研究不同结构和安装方式突片对同轴射流混合效果的影响规律,并提出最佳的突片结构参数。对突片激励射流强化换热机制的研究表明,射流孔边缘突片的插入起到两方面的功用。一方面能够在突片两侧形成一对反向旋转的流向涡对,流向涡随着射流的流动沿流向发展,与周围流体进行着热量和动量的交换。由于流向涡的卷吸和强化掺混作用,加强了掺混边界层内的湍动,湍流动能的增强有利于提高射流冲击的冷却效果;另一方面使得射流出口的流动面积呈现收敛的几何特征,在亚音速射流范围内,射流孔截面收敛有利于提高射流核心区速度,从而改善射流冲击驻点区附近的对流换热效果。对突片激励射流改善冲击冷却换热的数值计算和实验研究表明:突片数目对冲击冷却换热特性的影响和冲击孔的直径有关,突片相对于主流的倾角以及突片伸入主流中的长度对冲击冷却换热特性都有较大的影响。在双排突片射流冲击冷却中,两排孔之间流向孔间距的变化对冲击靶面的温度分布具有较大的影响。对于有非均匀初始横流存在的进气方式,当横流作用较弱的时候,横流的顺排和叉排进气方式对射流的流动和冲击换热影响都很小;当横流作用增强时,两种进气方式表现出了较大的差别。总的来说,突片激励射流对冲击靶板的冷却效果高于常规射流。在射流混合中,主流经过突片以后,周围流体沿径向向主流方向流动,在突片中心处分成两股反向旋转的流体,使突片两侧形成两个反向旋转的流向涡,随着主流与次流的掺混,流向涡的强度不断减弱,并逐渐扩大与周围流体混合,直至逐渐消失。流向涡的存在使主流和次流的掺混程度加快加强,导致主流核心区的缩短;掺混以后周围流体的湍流动能较大。突片射流的出口截面的温度场形状发生了明显变化,在突片插入的位置,低温区扩张,高温区收缩,温度场更加均匀。对突片射流强化主流和次流掺混的数值计算和实验研究表明:合适的突片数目和合适的堵塞比是获得最佳掺混效果的关键因素,突片顶角和突片长度对混合效果也具有重要的影响。在本文研究的参数范围内,最佳的突片数目为8片,突片相对于喷管轴线安装的最佳角度是30°,最佳的突片顶角是90°,突片伸入主流的最合适长度为0.1倍喷口直径。更多还原

【Abstract】 The tabs are a kind of vortex generators with simple configurations and wide applications in control or improvement jet flow structures. Although a series of investigations have been made on jet impingement heat transfer and mixing in coaxial jets using tabbed vortex generator, there are some problems being of challenge and innovation, such as the physical mechanism of tabbed jet impinging heat transfer, the influence rules on tabbed jet impingement heat transfer and mixing enhancement in coaxial jets by different tabs configuration, and the interactional characteristics of flow fields between tabbed jet flow and cross flows. The present paper aims on two aspects:(1) For the jet impingement cooling, the numerical calculation and experiment were conducted to investigate the performance of flow and heat transfer of tabbed jet impingement heat transfer for three jet impinging modes, such as single row jet impingement, double rows jet impingement and single row jet impingement with initial cross flow. The influences of tab configurations on heat transfer enhancement were analyzed and the physical mechanism of flow field control and heat transfer enhancement by tabbed jet was made known.(2) For the mixing in coaxial jets, the producing and developing rules of stream wise vortices and the flowing structure changing by the jet flows with tabs were made known, and the effects of tab configuration with the different structure and installation manner on mixing process were researched. At the same time, the optimum tab configuration parameters were put forward.The investigation on tabbed jet impinging heat transfer enhancement mechanism indicates as follows. In jet impingement cooling, because of the tabs inserting the jet at the orifice edge, two effects could be obtained. On the one hand, a pair of counter-rotating stream wise vortices appears on the both sides of the tab and developes along jet flow direction. It exchanges quantity of heat and momentum with the surrounding fluid. The stream wise vortices induced by tabs intensify turbulence motion in the boundary layer depending under their rolling and mixing process. The augment of turbulent kinetic engegy is benefit to the enhanced convective heat transfer. On the other hand, the jet flow passage takes on convergence, making for the increase of jet core velocity. It is propitious to improve the convective heat transfer near the stagnant region.The numerical calculation and experiment investigation on the impingement cooling in the jet excited by the tabs to improve heat transfer indicates as follows. The influence of the tabs number on the impingement heat transfer is related to the diameter of the impingement orifice. The tab declining angle and the tab length inserting the jet also have influences on the impingement heat transfer. For the impingement cooling with double-rows tabbed jet flows, the change of the stream wise space between the orifices has an important influence on the target temperature distribution. Under the presence of the initial cross flow, the inlet mode of initial cross flow (inline or staggered related to jet) has little influence on the jet flow and heat transfer when the initial cross flow is weak, but behaves great difference when the initial cross flow becomes dominant. In a word, the tabbed jet impingement heat transfer is more sufficient than the common jet.During the mixing of the coaxial jets, the surrounding fluid flows toward the mainstream along the radial after the mainstream passes through the tabs. Then the mainstream is disparted by two counter-rotating fluids so that a pair of counter-rotating stream wise vortices is shaped on the both sides of the tab. The stream wise vortices are weakening uninterruptedly and enlarging and mixing with the surrounding fluid and then disappearing finally along with the flow of the mainstream and the mixing between the mainstream and the secondary flow. The presence of the stream wise vortices enhances and reinforces the mixing between the mainstream and the secondary flow, shortening the mainstream core length. The turbulence kinetic energy in the surrounding fluid is increased after the mixing. The shape of the temperature field takes on obvious change at the exit section of the tabbed jet flow. The low temperature area expands and the high temperature area shrinks at the inserting position of the tabs. The temperature field is even much more equably.The numerical calculation and experiment investigation on the strengthening mixing tab in coaxial jets indicates as follows. The appropriate number of the tabs and the appropriate jam proportion are the key factors for make the optimal mixing effect. The angle and length of the tabs also have important influence on the mixing effect. In the conditions given in the present paper, the optimal tab parameters are obtained. The tab number is 8, the tab angle related to the jet axes is 30°, the tab tip angle is 90°and the ratio of tab length to nozzle diameter is 0.1.更多还原