【作者】 郭加宏；

【导师】 戴世强；

【作者基本信息】 上海大学 ， 流体力学， 2008， 博士

【摘要】 本文主要采用数值计算和分析的方法,结合实验观测,研究了液滴冲击固体表面液膜的流动现象,以及液膜形状的演化过程;研究了受细小喷淋冷却液滴冲击时,热物体表面液膜的稳定性。初步明确了影响液滴与液膜撞击合并过程的诸因素,阐明了各种因素对受细小喷淋冷却液滴冲击的固体表面液膜稳定性的影响,为喷淋液滴冲击冷却的工程应用提供了一些理论依据。论文包括以下主要结果:(1)采用高速摄影仪,在不同液膜厚度条件下,实验观测了不同冲击速度的水滴和乙醇液滴冲击固体表面液膜后的流动现象,探讨了液滴冲击速度、液体粘性、表面张力、液膜厚度和液滴直径等对液滴撞击液膜后的流动现象的影响。实验结果表明:液体的粘性主要影响液滴撞击液膜后产生的水花的厚度及其稳定性;液体的表面张力主要影响撞击后产生的水花的高度,抑制飞溅的发生;液膜厚度增加时,液滴冲击产生的水花高度略有降低,水花的壁厚增加;飞溅出来的液滴个数与冲击液滴的直径有关。(2)采用VOF方法数值模拟了单个液滴撞击固体表面液膜后的流动过程,探讨了液滴撞击液膜形成水花的流动机理。液滴中液体持续冲击进入液膜,液膜内持续产生径向高速流动的流体,冲击周围液膜中原先静止的液体。在高速流动的流体和静止流体撞击位置,流动速度出现间断,该速度间断的位置不断向液滴冲击区域外扩展。速度间断两侧的流体的撞击作用形成冠状水花,水花形成后,由于继续有流体流入,其高度逐步增加。(3)数值分析了单个液滴冲击固体表面液膜时,液滴冲击的Weber数、Reynolds数以及液膜厚度等因素对液滴撞击固体表面液膜后出现的流动现象以及液膜形状演化的影响,与实验结果进行的比较定性地验证了计算结果的正确性。液滴冲击对液膜状态的影响表现为:液滴冲击的Weber数和Reynolds数较大时,液滴撞击液膜后,液膜状态不稳定;反之,液滴撞击液膜后,液膜状态稳定。减小液滴直径和冲击速度,采用张力和粘性较大的液体,都有利于抑制水花和飞溅,也有利于液膜的稳定。(4)数值分析了两个和三个液滴与固体表面液膜的撞击合并过程,得到的主要结论如下:液滴接连不断地冲击液膜时,上下液滴间距离较小时,在液膜上不易产生水花等流动现象,形成的液膜稳定;液滴间距离较大时,在液膜上易产生水花等流动现象,形成的液膜不稳定。液滴并排地同时冲击液膜时,液滴间距离不同,液膜形态的演化基本相同,变动幅度受液滴间距离的影响。多个液滴同时撞击液膜时,液膜形状随时间的演化,将随着液滴数的增加趋于复杂。(5)理论和数值分析了细小喷淋冷却液滴冲击对热物体表面薄液膜的质量和动量传输、液膜自身蒸发和表面张力等因素对液膜稳定性的影响。细小喷淋冲击冷却时选择气化潜热较大、热传导系数小的液体作为冷却液,可使热物体表面形成的薄液膜较为稳定。选择表面张力较大的液体作为冷却液,有利于在热物体表面形成稳定的薄液膜。恰当的喷淋流量分布,能使热物体表面形成的薄液膜保持稳定,冷却性能最佳。流量过小,液膜在各种因素作用下,会产生破裂。流量过大,虽能形成稳定的液膜,但形成的液膜厚度较大,冷却性能并非最佳。喷淋液滴冲击液膜产生的动压力是导致液膜不稳定的因素之一,因此应控制喷淋液滴冲击液膜前的动量。更多还原

【Abstract】 Through numerical simulation and experimental observation, the influence of the droplet impingement on the evolution of the film at the solid surface is analysed. The stability of the film on the hot solid surface impacted by the small cooling droplets is also studied. The influencing factors in the course of coalition of impinging droplet and film are specified, and the effect of each factor on the stability of the liquid film impinged by the small droplets is also elucidated, which may provide the theoretical foundation for the application of small droplets impingement cooling in engineering.The main results of this dissertation are described as follows:(1)With the high-speed camera, the flow resulted from the impact of a water droplet and an ethanol droplet on the film of the same liquid with different thicknesses, is investigated. Based on the experimental results, the influence of the impact speed, the diameter of the droplet, the viscosity and surface tension of the liquid, and the thickness of the film on the evolution of the film are discussed. The fowllowing conclusions are reached. The viscosity of the fluid affects the thickness and stability of the crown formed after the impact of the droplet on the film. The surface tension appears to affect the height of the crown and retard the splash. With increasing thickness of the film, the crown height will be reduced and the thickness of the crown will be increased. The number of the splashing droplets is related with the diameter of the impacting droplet.(2)Through analyzing numerically the flow in the liquid film with the VOF method, the hydrodunamic mechanism resulting in the crown caused by a droplet impingement on the liquid fim is elucidated. Induced by the droplet impingement, continuous high-speed radial flow in the liquid film is formed in the original ambient static fluid. At the striking position of the high-speed fluid to the static fluid, a velocity discontinuity appears, which propogates outwards in the film. The crown appears as a result of the collision of the hige-speed and static fluid on both side of the discontinuity. The height of the crown becomes greater with liquid flowing upward into the crown.(3)The influence of the thickness of the liquid film, the Weber number and the Reynolds number of an impinging droplet on the evolution of the film is numerically analysed. The numerical results herein are in qualitative agreement with the experimental results. The main results of droplet impact on the film are as follows. When the droplet impacts on the film, if the Weber number and the Reynolds number are large, the liquid will be unstable, whereas, if the Weber number and the Reynolds number are small, the liquid will be stable. However, reducing the diameter and the velocity of the droplet and increasing the surface tension and viscosity of the liquid of the droplet, will be favorable to restrain the formation of the crown and splash and to keep the film stable.(4) The collisions of two and three droplets with the liquid film are numerically analysed. The following conclusions are drawn. When the droplets impact the liquid film one by one continuously, if the vertical spacing between two droplets is small, the crown will not appear easily and the liquid film will be stable. If the vertical spacing between two droplets is large, the crown will appear easily and the liquid film will be unstable. When the droplets parallelly impact the liquid at the same time, the evolution of the liquid film is farely alike with different spacings between two droplets, but the rising amplitude is different. However, the flow pattern will be more complicated if more droplets impact on the liquid film.(5)The influences of the small refrigerant droplet impingement, evaporation, surface tension etc on the stability of the liquid film are numerically analysed. For small droplet impingement cooling, the liquid film on the solid surface will be stable if the refrigerant with larger latent heat and thermal conductivity is used. Choosing the refrigerant with larger surface tension will also be favorable for the stability of the liquid film. It is possible to keep stable liquid film on the solid surface with appropriate mass flow of the refrigerant. If the mass flow of the refrigerant is small, the liquid film will rupture, whereas, if the mass flow of the refrigerant is large, the liquid film will become too thick to have good capability of cooling. The dynamic pressure of small droplet impingement is one of the factors causing the liquid film to be unstable. Therefore, the momentun of the droplet should be controlled before it impacts on the liquid film.更多还原