Cavitation Mechanism In The Valve Port And The Effect To Hydraulic System

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.