| As a key component of the electro-hydraulic servo control system,electro-hydraulic servo valve affects the control accuracy and response speed of the whole system.Jet pipe servo valve has advantages of strong antipollution ability,and stable performance under low working pressure.However,because of the particularity of the structure in the pilot stage,there exist free turbulent jet,impinging jet,separated flow and flow saturation,which have strong instability and instantaneity.The structural parameters of the pilot stage have great influences on the working performance of the jet pipe servo valve.Therefore,it is of great theoretical significance and application to conduct in-depth research on the flow characteristic,coherent structures and cavitation phenomena in the flow field of the pilot stage.In this paper,the characteristic of flow field,coherent structures,cavitation phenomena and structural optimization of the pilot stage of jet pipe servo valve were studied.The main research contents of this paper are as follows:(1)According to the injection of jet and the structure of receiving region in the jet pipe servo valve,the results of large eddy simulation method and RNG model were compared.It is showed that the overall macroscopic flow and the random pulsation of local small scale vortex could be described more effectively by large eddy simulation.By combining large eddy simulation and multiphase flow,three-dimensional flow field of the pilot stage was simulated.The distribution and morphological evolution law of cavitation phenomena under different working conditions and different nozzle structures were analyzed.Quantitative analyses including mean value,amplitude and peak frequency of local and surface-average cavitation fluctuation with inlet pressure were conduct respectively.Then the effects of transient cavitation phenomenon on the resonant frequency was studied.By studying the effects of the inlet presssure,the outlet pressure,the length of the wedge on the cavitation,effective methods to supress cavitation were put forward.(2)Through the proper orthogonal decomposition method,the mode analyses of the flow field of pilot stage under different inlet pressures and deflection angles of jet pipe were conduct.The mode truncation method was proposed to decompose the flow field into mean flow field,coherent flow field and turbulent flow field.Each modal information was obtained to capture coherent structures in the flow field effectively.By analyzing the velocity field distribution of the vortex at the receiver entrance,it is concluded that the vortex is Lamb-Oseen vortex and exists in the first mode.The effects of boundary conditions on the time-space features of large scale vortex structure were further studied by analyzing the eigenmodes and their coefficient.And the method to improve the stability of the flow field of the pilot stage was put forward.(3)Based on the theory of three-dimensional turbulent jet,the characteristics of the core area and mixing area of the jet under different nozzle shapes were obtained,and the overlap area formulae between the jet and the receiver were established.The mathematical model of the flow field of the pilot stage was established to analyze the pressure and flow characteristics based on the energy conservation and the momentum conservation.A fast multiobjective genetic algorithm,named NASG-?,was utilized to optimize the structure of the flow field of the pilot stage.Taking the recovery pressure and recovery flow rate as the objective function in the NASG-?,the multi-objective optimization was carried out.On this basis,the Pareto optimal solution set was obtained and compared to get the optimal structure of the flow field.(4)Combined with the principle and method of electro-hydraulic servo valve testing,the characteristics of a jet pipe electro-hydraulic servo valve were studied experimentally.Based on the characteristic test bench of electro-hydraulic servo valve and a high-frequency dynamic pressure sensor,the pressure in the receiver was monitored and then analyzed in the time domain and frequency domain to verify the simulation results. |
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