| With the rapid development of technology,the requirements towards the quality and performance of the hydraulic control parts(servo valve and proportional valve)have become increasingly demanding.So,cavitation,fluid power and flow properties in the development of hydraulic control parts have demonstrated great impact.To tackle this,it is a suitable starting point to study the hydraulic valve cavitation,fluid power and flow coefficient.The research shows that the flow field within the valve is analyzed from a two-dimensional and single-phased flow model based on the fundamental hypothesis of a symmetrical structure of the hydraulic valve.However,the structure of the hydraulic valve is generally not centrosymmetrical due to the influence of the in/out flows through the channel.Therefore,it is not reasonable using the two-dimensional model to simplify the tree-dimensional model.Despite this,the single-phased flow model is favorable to researchers due to its quick calculation time and convenient computational convergence.However,its negligence of the flow density caused with the cavitation phase change reduces the accuracy in simulating cavitation flow field.The dissertation aims to further investigate the cavitation,steady hydro-dynamic force and flow properties of the non-full circumference opening hydraulic slide valve for inflow and outflow models.To achieve this,we examined the hydraulic proportional valve and the large diameter nominal hydraulic slide valve of the throttling groove from the valve sleeve of servo valve.In terms of methodology,three-dimensional two-phased flow numerical simulation and theoretical analysis were adopted.In addition,the pressure and the spool wall pressure distribution were also measured.The results showed that cavitation was more likely to occur at large flow,small valve port opening and small outlet pressure for both inflow and outflow model valves.In addition,the concept of the critical opening degree was raised.It was used to explain that the steady-state fluid power of the inflow model valve gave the valve port an opening trend,when the opening degree reached critical opening degree.Furthermore,a method to measure the wall pressure distribution was proposed,and the fluid power of the steady-state slide valve was calculated through surface integration.It was a common method used in testing the fluid fields within complex valves of small sizes,high pressures and speeds.In the study of the flow coefficient,we found that the differential pressure at a small opening degree had a large impact on the flow coefficient,and the flow coefficient was proportional to the valve port opening degree.Our research delivered a great contribution to the theory and engineering practicability on the design and control of the hydraulic valve.This project was jointly funded by the National Natural Fund of China(51175362)and the Scientific and Technological Research Project of Shanxi Province(20140321008-02).The contents of the dissertation are as follows:(1)The purpose and the meaning of this dissertation were outlined.It also delivered a general introduction on the current studies and development of hydraulic valves at both domestic and international levels.Moreover,we introduced the research progress in hydraulic valves,fluid power and flow properties.Furthermore,the numerical simulation of the flow field data and the application of the flow field measurement technique in the hydraulic technology were briefly discussed.Finally,it summed up the main research content of this dissertation.(2)Started with a theoretical foundation of our research.After that,a three-dimensional model was built using GAMBIT software to study the valve.A grid was then applied to the generated model.The next stage involved in applying the simulation of both single-phased flow model and two-phased flow model to analyze the flow field of the studied slide valve.On the basis of the results,the proposal of the inability of the single-phased flow model to represent the cavitation flow field was raised.This was because the single-phased flow model neglected the influence of cavitation on the flow density and other flow fields.Consequently,the necessity of the use of the two-phased flow model to represent the cavitation flow fields was proposed.(3)Both inflow and outflow models of the hydraulic valve were studied.The initial stage included generating a three-dimensional model for the corresponding opening degree of each valve,applying grids to them.After that,a combination of a two-phased flow model,cavitation model and RNG turbulence model was adopted.This in turn analyzed the flow fields of the valve chambers of both inflow and outflow models of the non-full circumference opening slide valves.Diagrams obtained from the models such as the velocity vector diagram,pressure distribution diagram and gas volume contours were utilized to interpret the data.The results revealed that cavitation occurred in the area of the lowest flow field pressure.Furthermore,cavitation of the inflow model valve occurred near the spool wall close to the entrance position inside the valve chamber.In contrast,the cavitation of the outflow model valve happened along the downstream of the valve port.Under the same conditions,cavitation was more severe in the outflow model valve: the cavitation area was proportional to flow rate,inversely proportional to the opening of the valve port and the outlet pressure.Therefore,a new wall pressure distribution measurement technique was coined.Based on the new technique,the area and the location of the cavitation were interpreted from the lowest measured pressure area of the wall pressure distribution.(4)Analyzed how the fluid power of the steady-state slide valve changed as the flow,valve port opening degree and outlet pressure changed.The main methodology was the use of the wall pressure distribution surface integration.In addition,the measured wall pressure distribution data underwent integration to obtain a fluid power of the slide valve under steady state.The result obtained from integration was then compared to the result from the numerical simulation,with the same trend.Furthermore,the study showed that for both inflow and outflow slide valve models,their steady-state fluid powers increased with the flow rate surge.Another finding was using the concept of critical opening degree to tackle the correlation between the steady-state fluid power of inflow valve model and the opening degree of the valve port.When the valve port opening degree reached critical,the steady-state fluid power shifted from a valve port-closing trend to an opening trend.On the contrary,the steady-state fluid power of the outflow valve model always exhibited a valve port-closing trend,though it was proportional to the degree of valve opening.The study also revealed that when the outlet pressure was small,the steady-state fluid power of the inflow valve model exhibited an increasing trend as the outlet pressure surged.On the other hand,the steady-state fluid power of the outflow valve model showed little effect from the change in the outlet pressure.Another noticeable difference between the inflow and outflow valve models was that the calculated value of the steady-state fluid power equation for the outflow valve model coincided well with the simulated results.However for the inflow valve model,the error between the calculated value and the simulated value was quite large.(5)The flow coefficients of both inflow and outflow valve models were investigated,especially the impact of flow rate,valve port opening degree and outlet pressure on the coefficients.To better understand this,a flow coefficient testing system was designed and the correlation between the flow coefficient and the flow rate,valve port opening degree was tested.The test revealed that parameters such as flow rate,valve port opening degree,inlet and outlet differential pressures were all influential on the flow coefficient.To expand on this,when one parameter was constant,the changes in the other two factors were associated.Therefore,the parameter that was the dominant role in the change determined the changing trend of the flow coefficient.Now,the inflow valve model was compared to the outflow valve model.When the flow rate was constant,the flow coefficient of the inflow valve model was inversely proportional to the opening degree of the valve port.However,for the outflow valve model,when the open degree of the valve port was small,the flow coefficient increased with the surge in the valve port opening degree.This attributed to the cavitation-induced change in differential pressure.Since the cavitation triggered a large change in the differential pressure,it made the differential pressure a dominant role in the change of the flow coefficient.When the opening degree was large,the flow coefficient would then reduce with the increase in the valve port opening degree.If the valve port opening degree was constant and small,and the flow rate range was small,and the flow coefficients for both inflow and outflow valve models were proportional to the flow rate.However,if the flow rate range was large,there was no obvious change in the flow coefficient.It should be noted that within the median section of the large flow rate range,the flow coefficient would decrease with a surge in flow rate.This was because the surge in flow rate delivered an increase in the differential pressure,which made the differential pressure a dominant role,and this in turn reduced the flow coefficient.If the valve port opening degree was large,the flow coefficient reduced with the increase in the flow rate.For both inflow and outflow valve models,when the flow rate was constant,their flow coefficients increased with the surge in the outlet pressure.However,it should be also noted that the flow coefficients for both models did not share the same numerical value.As a result,it was not the most appropriate for the conventional way to choose a fixed value for the flow coefficient.(6)Delivered a general summary of the findings and conclusions from our research.It also addressed how the future research could be carried out to improve on the current study.The findings from this dissertation further improved the current theories of the hydraulic valve fluid mechanics.Furthermore,the dissertation interpreted the properties of the cavitation,steady-state fluid power and flow coefficient of the non-full circumference opening hydraulic slide valve of the throttling groove.It is believed that our research findings make a contribution towards designing and controlling the hydraulic valves in theory and engineering practicability. |
