Study On Clamping Force And Cavitation Characteristics At Pilot Stage Of Two-dimensional Hydraulic Valves

Higher requirements for electro-hydraulic servo valve have been put forward in recent years as the development of aeronautics and astronoutics technology.In order to meet these requirements,the two-dimensional(2D)servo valve,one of high-performance electrohydraulic servo valves,is designed and studied.The valve core has both rotation and sliding movements relative to the axis of the core to achieve pilot control and power amplification.So the servo valve has advantages of simple structure,strong anti-pollution ability,small leakage and good dynamic performance compared with the traditional servo valve because of the special movement of the core,which make the valve widely used in the earthquake simulator and aeronautics and astronautics fields.However,the clamping stagnation happens to the twodimensional servo valve during the process of test and application some time,as well as the self-oscillation phenomenon accompanied with screaming under certain working conditions.In order to reveal hydrodynamic mechanism of these phenomena,it is necessary to reveal the mechanism by means of computational fluid dynamics(CFD),which provides powerful support for relevant theories and experiments.Moreover,the servo spiral mechanism of the pilot stage in the two-dimensional servo valve is very special,which results in different fluid mechanics phenomena in two-dimensional servo valve compared with traditional valve.And other two-dimensional hydraulic valves have been derived on the basis of two-dimensional servo valve,like the two-dimensional digital relief valve,two-dimensional proportional directional valve,two-dimensional cartridge valve,et al.They all have pilot stage in the structure and the core has both rotational and axial movements.Therefore,the basic fluid mechanics problems in the two-dimensional hydraulic valve need to be studied.The main research contents and achievements of this paper are as follows:1.The fluid field of cutting groove within two-dimensional hydraulic valve is analyzed through the numerical simulation to study the relationship between the pressure on the spool shoulder and its influences,which are inlet flow velocity,outlet pressure,the size of the cutting groove and the position of the inlet and outlet ports in the condition of metering-out throttle.And then,the formula of pressure circumferential distribution is established according to the flow equation of the orifice and the Bernoulli equation of the ideal fluid,which is verified through numerical simulation and experiment.It is proposed that the the distribution of the circumferential pressure produced by the Bernoulli Effect on the spool at the cutting groove is not uniform,thus the clamping force exerted on the spool can not be ignored in the case of the smaller depth of the groove and the larger flow rate.2.The flow force mathematical model of the orifice at the pilot stage in two-dimensional servo valve is established by analyzing the particularity structure of the pilot,and the influence factors of flow force at the orifice are discussed through theoretical analysis.It is concluded that the steady flow forces along the axial direction and circumferential direction cause the pilot orifice to close.It is proposed that the radial distribution of flow force is not uniform around the main orifice of the valve core,which causes the radial clamping force and the force increases with the opening of the orifice and the inlet velocity.3.The slit flow between the valve core and the body of the two-dimensional hydraulic valve has the characteristic of two-dimensional flow because the core rotates on startup and slides after rotating.The relationships between the characteristic of two-dimensional slit flow and inlet pressure,the rotation speed of the spool,as well as the clearance of the slit are studied through establishing mathematical model and numerical simulation.By comparison with the results of one dimension slit flow,the two-dimensional slit flow allows smaller clearance than one dimension slit flow under the same situation.The effect factors of leakage and friction of the slit flow at the shoulder of pilot stage and high pressure cavity of two-dimensional servo valve are studied.And the results show that the leakage of the pilot stage is not affected by the inlet pressure,but increases linearly with the inlet flow,while the leakage at the shoulder of high pressure increases linearly with the pressure difference of the entrance and exit,which are verified through experiment.4.The fluid model of the pilot orifice and the special flow channel of the two-dimensional servo valve are established by using the numerical simulation software based on theoretical model of gas-liquid two-phase flow,because the opening of the orifice at pilot stage of twodimensional servo valve is very small and the orifice is formed by two parallel spiral lines in structure.Fluent software is used to simulate gas-liquid phase flow of the fluid model to obtain the process of cavitation formation of the orifice and the gas phase distribution under different inlet velocities,outlet pressures and the openings.According to simulation resutlts,the cavitation phenomenon always appears at the walls of the pilot orifice when the twodimensional servo valve works,and when the sensitive cavity of the valve is in the state of gasliquid phase,it will has bad effect on the stability of the serve valve.5.The experiment test bed of two-dimensional servo valve was set up,and the signals of the valve core's displacement,the sensitive cavity's pressure and the noise were acquired through the high-speed data acquisition system.It turns out that the cavitation is really exist when the two-dimensional servo valve is working according to spectrum analysis of noise signals.Meanwhile,the frequency spectrum analysis method is used to analyze the displacement,noise and pressure signals of the two-dimensional servo valve when the valve is howling because of self-oscillation.It is found that the noise signal not only contains the frequency domain information of the self-oscillation,but also contains the cavitation information.In conclusion,it is the first time to do theoretical,simulation and experimental research on clamping and cavitation phenomena of two-dimensional hydraulic valve,and will continue to study the effect of cavitation on the stability of two-dimensional servo valve,which providing theoretical support for its reliable application in the field of aerospace.