【作者】 杜学文；

【导师】 傅新；

【作者基本信息】 浙江大学 ， 机械电子工程， 2008， 博士

【摘要】 本文采用流场仿真、高速观测和频谱测量等方法从流场的微观角度对液压阀口的流道结构、压力梯度、流场流态和空化特性之间的关系进行了系统研究。针对阀口形状复杂、尺寸小、流速快以及流场结构变化剧烈等特点,从各种阀口形式的空化状况分析入手,研究了阀口结构参数、流动参数等因素对空化状态的影响,以及气穴数量、分布等形态要素与系统流量、噪声特性的相关关系。发现气穴的生成和发育受剪切流与涡流两种流态的影响,在高速剪切与低压旋涡并存的情况下,空化更容易受到旋涡的控制。阀口迁移会造成流道内部压力梯度的剧烈变化,进而造成空化初生位置的转移和空化类型的转变。通过多参数寻优的方法得到了空化状态下阀口流量系数的修正公式及流量公式,同时对空化流场进行了可视化观测和频谱分析,发现气泡流的周期性变化频率与空化噪声的高频段基本重合。研究结果为设计高效率、低能耗、低噪声的液压阀和管道系统奠定了基础,具有重要的理论意义和实际应用价值。论文的主要内容如下:第一章讲述了空化研究的发展历史,在此基础上对液体中的核子模式、空化初生及影响因素、空化机理进行了介绍,重点介绍了液压系统中的空化问题及研究现状。高速流场可视化技术在空化研究,特别是流场分析方面有着重要的作用,本章结合近几年的最新进展对该技术进行了详细阐述,最后根据本课题研究的目的和意义,给出了主要研究内容和研究难点。第二章介绍了空化现象的产生机理及空泡动力学的基本理论,静态气泡的生长主要是泡内外压力、温度及表面张力的作用,而运动中的气泡则受到更多因素的影响,除了压强和速度两项主要因素以外,还受到流体粘性、压力梯度以及壁面条件等多种因素干扰,深入了解各项参数对空化过程的作用是空化研究领域中最重要的基础问题之一。在空泡动力学的基础上对空化基本方程以及数值计算的数学模型作了相关介绍。第三章主要对研究过程中的实验手段以及数值计算方法作了介绍,在此基础上阐述了论文的研究思路。流场可视化观测是本课题研究的重要基础,本章对微距模式下的高速摄像技术进行了重点介绍,同时结合频谱测量方案介绍了实验系统的搭建过程。对复杂的节流槽结构进行了简化,并得到了简化结构的过流面积计算公式以及过流面积曲线,最后对流场数值计算的模型、网格划分及边界条件参数的设定情况等进行了说明。第四章运用高速摄像、噪声分析等手段对不同阀口空化流场及流动特性进行了分析,高速摄相机采用微距放大模式进行观测,结合大量观测结果总结出阀口空化的典型特征,以及和阀腔内部压力分布的相关关系,在此基础上研究了阀口结构及流动参数对空化形态、流量与噪声特性的影响。研究发现空化的产生与流动分离受背压的影响最为明显,进口压力对空化形态及噪声影响次之;对于不同阀口,空化初生位置一般位于节流边与孔壁相交的锐缘区附近,随着阀口开度及深度的变化,初生位置会有较大改变。第五章结合大量高速摄像结果,以及流场仿真对阀口内部空化初生情况及初生机理进行了分析,用可视化观测与频谱测量两种方法对空化初生进行了判断,讨论了空化初生的影响因素及预防措施,在此基础上总结出五种典型的空化初生模式,根据每种模式的初生特点给出了空化流场的物理描述,通过对阀口局部压力损失、弯管效应而产生的壁面剥离、沿程压力损失的分析,给出了阀口流动压阻模型,并对旋涡、脉动、流体脱离等因素对空化初生的作用机制进行了分析。第六章主要对空化状态下流量饱和现象进行了研究,分析了有无空化对阀口流量特性的影响,以及空化初生和流量饱和的关系。阀口的结构参数,包括阀口形式、开度与节流槽深度,以及流动参数,包括进出口压力等对阀口的空化状态以及流量饱和特性有着重要的决定作用,本章对此进行了专门讨论。在此基础上通过大量实验对阀口流量系数进行了修正,得到了空化状态下阀口流量的修正公式,空化发生后特别是流量饱和时,理论计算与实验结果完全吻合。第七章结合频谱测量与流场可视化方法对阀口空化噪声的基本特性进行了研究,分析了空化噪声各频率成分的变化同阀口结构、进出口压力以及空化形态的关系,发现并总结出空化状态下阀口高频噪声随结构及流动参数的变化规律,以及气泡流周期性脱落与噪声声级的内在联系,最后根据研究所得的结论,采用二级节流与回流增压等方法,从流量和噪声两个方面对阀口结构进行了优化设计。第八章对本论文的研究内容和成果进行了总结,展望了未来的研究工作。更多还原

【Abstract】 In this paper, the flow simulation, high-speed observation and spectrum measurement methods are adopted to investigate the relationship of valve port structure, pressure gradient, flow pattern and cavitation characteristic from the microscopic point of view. Against the complex shape, small size, high velocity and dramatic change of flow structure, we studied the effect of structure and flow parameters on the state of cavitation, as well as the relations of cavitation distribution, flow discharge and noise characteristics from analyzing the cavitation status under various forms of valve port. The conclusions are that, the generation and development of cavitation are affected by both shear flow and eddy current. When the two states of flow occur simultaneously, cavitation is more susceptible to the vortex. Valve port transportation will cause dramatic change of the pressure gradient inside flow channel, thereby the cavitation inception location and cavitation type will transfer correspondingly. Through multi-parameter optimization method, we get the modified formula of discharge in the state of cavitation. At the same time, through the visualization and spectrum analysis of flow field, we found that the frequency of bubble flow’s cyclical fluctuation is in coincidence with the high frequency of noise. The results are of great theoretical and practical values for the design of high-efficiency, low energy loss and low-noise hydraulic valves and piping system.The main contents of the thesis are as follows:In chapter 1, the nuclear model, cavitation inception and influencing factors, cavitation mechanism are introduced based on the history of the cavitation research. The emphasis is focused on the cavitation problem in the hydraulic system. High-speed visualization technology in the field of cavitation research, in particular the analysis of flow field, plays an important role. In this chapter, the technology is introduced in detail with the latest progress of last few years. In the end, according the purpose and significance of the study, the major contents and the difficulties of the subject are introduced.In chapter 2, the basic theory of cavitation inception mechanism and bubble kinetics are introduced. The growth of the static bubble is the interaction of internal and external pressure, temperature and surface tension. Movement bubbles are subject to more factors, apart from the pressure and speed, other factors such as the fluid viscous, pressure gradient and wall conditions are also of deep interference. Understanding the effect of parameters on cavitation is one of the most important problems in the area of cavitation research. On the basis of bubble kinetics, the fundamental equations and numerical models of cavitation are introduced.In chapter 3, the experiment facility and numerical calculation method are presented. Based on this, the scheme of the study method is summarized. The technology of high speed observation, which is the key basis for the research, combined with the spectrum measurement system is introduced. At the same time, the complex structure of valve port is simplified. At the end of this chapter, the method for numerical simulation such as mesh division and the setting of boundary conditions are explained.In chapter 4, high-speed observation and noise analysis are adopted to investigate the typical characteristics of cavitation flow, and the relations with pressure distribution inside the valve chamber. Based on this, the effect of valve port structure on cavitation patterns, discharge and noise characteristics are revealed. Both the experimental and the numerical results reveal that the cavitation inception and flow separation have close relationship with the back pressure. As the secondary influencing factor, the change of inlet-pressure also affects the characteristics of cavitation and noise. To different valve port, the position of cavitation inception is invariably near the sharp-edged area where the throttling-edge crosses the side-wall of the orifice, and the inception position changes with the change of valve opening and depth.In chapter 5, with large high-speed camera results, as well as simulation of the flow field, the cavitation mechanism within the valve port is analyzed. Cavitation inception is judged by two methods: visual observation and noise spectrum. Based on this, five typical cavitation inception models are summarized. According to the primary characteristics of each model, physical description of flow field is given. Through analyzing the partial pressure loss, elbow effects and pressure loss along the channel, the pressure drag model and coefficient is derived. At last the effect of whirlpools, pulsating and flow separation on cavitation inception is discussed.In chapter 6, flow saturation with and without cavitation, as well as the relation of flow saturation and cavitation inception are studied. The structural parameters, including valve port forms, opening and depth, as well as the flow parameters, including inlet and outlet pressure are key factors which determines the cavitation status and flow saturation inside the valve port, the mechanism is discussed. On this basis, discharge coefficient is amended under the state of cavitation through large experiments; the theoretical calculation consistent well with the experimental results especially after the flow is saturated.In chapter 7, the characteristic of cavitation noise is studied, and the relations of different noise frequency components with valve port structure, inlet-outlet pressure and cavitation forms are discussed. Under the state of cavitation, the regularity of how high-frequency noise changes with structure and flow parameters, and the intrinsic relation between cyclical shedding bubble flow and sound pressure level are summarized. At last, according to the conclusions, two optimized design methods of valve port structure are proposed.In chapter 8, conclusions of the thesis are summarized and the future research proposals are suggested.更多还原