【作者】 徐明；

【导师】 傅新；

【作者基本信息】 浙江大学 ， 机械电子工程， 2013， 博士

【摘要】 空化是流体系统中常出现的现象,当流体内部压强或应力降低到某一临界值后,流体会发生剧烈的气化,游离出大量的气泡,人们将这种现象与沸腾相区别,称之为空化现象。空化发生后,会阻塞流道降低系统效率,造成振动和噪声,甚至使系统发生机械性或者化学性损害,这是液压系统、透平机械、水工结构中应尽力避免的有害现象。随着科技的发展,人类活动范围的不断扩大,涉及空化现象的领域也越来越多,例如：生物医学、核能、航空航天等。空化已成为人类实践活动中必须面对和深刻理解的一个基础性科学问题。空泡临近壁面溃灭时,会在溃灭点位置产生高温、高压以及高速射流。人们利用空泡溃灭时产生的高温、高压加速化学反应的进行,或利用高速射流实现微小粒子的驱动。近壁空泡的力学行为是空泡动力学的重要组成部分,过去多以固体壁面的研究为主。近年来随着空化在生物医学领域应用的进一步深入,近弹性壁面以及管道内的空泡研究也逐步开展起来。本文通过理论分析、数值仿真以及实验观测相结合,对空泡近壁面行为及相关应用进行了系统的研究。利用高速观察的手段,分析了壁面距离、空泡尺寸以及弹性壁面对空泡力学行为的影响,揭示了射流变向的外部临界条件以及壁面与空泡的耦合关系。定量分析了近壁面双空泡的射流行为,揭示空泡间以及空泡与壁面之间的相互作用规律。开展了管道内空泡动力学行为的应用研究,建立了管道内的空泡射流的能量传递模型,并在管道粒子移除实验中进行了验证。论文的主要内容如下：第一章绪论,介绍了单空泡以及多空泡动力学的研究现状,以及空泡在生物医学领域内的最新应用,对高速观测技术在流体领域的应用做了相应的阐述。第二章空泡动力学及数值计算基础,对球形空泡动力学进行了阐述,对用全边界元法对空泡近弹性壁面进行了数值仿真进行了介绍,这可能是模拟瞬态空泡动力学的最有效的仿真方法。建立了基于雷利公式的单空泡动力学模型,对球形空泡的自由溃灭进行了数值模拟,研究了粘性对空泡溃灭的影响；建立受限空间下的单空泡模型,研究受限空间尺寸对空泡溃灭的影响。第三章空泡实验系统,构建了空泡实验系统；采用空泡产生装置对单空泡的自由溃灭进行了实验研究,并与数值仿真进行了对比；开展了双空泡在自由域条件下的验证性实验,与前人的实验结果进行了对比。第四章近壁空泡行为,研究了空泡与生物弹性壁面的相互作用行为,分析了壁面距离对空泡力学行为的影响,给出了空泡距离与弹性壁面的变形量之间的定量关系.研究了空泡与壁面距离丫及空泡间距离γb对空泡射流的影响,给出了射流变向的临界条件.研究了管道弹性对空泡行为的影响,观察空泡溃灭时管道内的伴随空化现象.建立空泡在刚性管内的溃灭的数学模型进行仿真研究,并与实验结果进行了对比。研究了管径对空泡溃灭的影响,给出了管道壁面的变形量与管径的定量的关系。第五章空泡射流及其应用,利用空泡溃灭的微泵效应,实现了管道内的液体输送,利用高速摄像手段,揭示空泡射流作用下液滴形成的动态过程,分析了平板距离对液滴输送速度的影响；利用空泡射流效应,开展了管道端部以及管道内的粒子移除实验,定量分析了空泡与粒子距离对粒子速度的影响,建立了管道内的空泡射流的能量传递模型,揭示了粘度与粒子速度的数学关系,将仿真与实验结果进行了对比.第六章结论与展望,概括了论文的主要研究工作,对论文的创新性进行了总结.更多还原

【Abstract】 Cavitation is a common phenomenon in the fluid system. When the internal pressure or stress in the fluid reduces to a critical value, violent gastification appear in the fluid and a large number bubble can flow. It could be called the caviation phenomenon as distinguished from boiling phenomenon. Caviation was followed flow blockage with the reduction of the system efficiency. Vibration and noise will appear and even cause the mechanical or chemical damage in the system. It should be avoided in the hydraulic system, turbine machinery and hydro-structure. With the development of the technology and expansion of the human activities, the area of cavitation is increasing such as biomedical, nuclear energy and aerospace. Cavitation has become a basic scientific problem in the human practice activities which is needed to face and further understandings.High temperature, high pressure and high speed jet will generate in the collapse point while the cavitation bubble collapse near the boundary. The high temperature and high pressure induced by the cavitation bubble collapse is utilized to speed up chemical reactions or achieve the tiny particle drive. Mechanical behavior of the bubble near the wall is an important component of the bubble dynamics. Much research has focused on the rigid wall. In the recent years, along with the cavitation application in the biomedical area going much further, the research of the bubble near the elastic boundary and in the tube also gradually unfolded.The thesis concentrates on a systematic study on bubble behavior near the wall and related application through the theory analysis, numerical simulation and experimental observation. By means of the high speed observation, the paper analyse wall distance, bubble size and elastic wall that influence the bubble mechanical behavior and reveal the critical condition of external inducing the jet change-of-direction and the coupling relation between wall and bubble. Quantitative analysis of the two bubble jet behavior is carried on and the interation rule between the bubble and wall is revealed. The thesis develops applied research on the bubble dynamic behavior in the tube and establishes the energy transfer model of the cavitation jet in the tube, which is verified in the particle removal experiment. The outline of this thesis is as follows:In chapter1, Introduction, The research situation of single bubble and multiple bubbles cavitation dynamics is introduced. The research progress of the cavitation bubble in the biomedical area and the high speed camera technology in the fluid area is briefly stated.In chapter2, the foundation of cavitation dynamic and numerical calculation, the spherical bubble dynamic is stated. The full boundary element method is introduced which is used to simulate the interaction of the bubble with the elastic boundary and it maybe the most versatile method for transient bubble dynamic simulation. The spherical bubble collapse process is simulated with the Rayleigh model and the effect of liquid viscosity is investigated also. The single bubble model is built under the confined space condition and the influence of the confined space dimension on the bubble collapse is studied.In chapter3, the bubble experimental system, the bubble experimental system is structured. The single bubble is generated by the experimental setup. The bubble collapse in a free field is studied and the result is compared with the numerical result. Two bubbles interaction in the free field is investigated by the experiment which is compared with the previous experimental results.In chapter4, the bubble behavior near the wall, the bubble interaction with the elastic boundary is investigated. The influence on the bubble dynamic behavior of the distance between the bubble and the boundary is analyzed and the quantitative relationship between the bubble distance and the elastic boundary deformation is given. The dimensionless distance between the two bubbles γ and the dimensionless distance between the bubble and the boundary γb are defined. The γ value and γb value influence on the bubble jet behavior is investigated and the critical condition for jet chang-of-direction is present. The effect of the elastic tube on bubble behavior is studied and the accompanied cavitation phenomenon in the tube is observed in the experiment.The theory model of the bubble collapse in the tube is built and the simulation research is conducted. The model prediction results are compared with the experimental results. The effect of the tube diameter on the bubble collapse is considered and the boundary deformation amount with the tube diameter is plotted.In chapter5, the cavitation jet and its application, the micropump effect inducing by the bubble collapse achieve the fluid delivery. The liquid drop formation dynamic process is revealed by the high speed camera and the liquid drop formation velocity is affected by the distance between the plate and the bubble. The particle removal by the bubble-induced jet which contains the particle removal in the tube and in the tube end is investigated by the experiment method. The effect of the distance between the bubble and particle is quantitative analyzed and the energy transfer model of the cavitation jet in the tube is built. The mathematic relation between the particle velocity and liqid viscosity is revealed. The numerical result result is compared with the experimental result.In chapter6, conclusions and prospection, the main research work of this thesis and the innovation of the thesis are summarized.更多还原