Study Of The Transient Cavitation And Induced Flow Phenomenon In The Pilot Stage Of The Flapper-nozzle Servo-valve

Electro-hydraulic servo valve, which is widely used in many kinds of industrial fields such as aerospace, aviation, chemical industry, and metallurgy machinery, is the core component of the hydraulic control system. Especially the flapper-nozzle two-stage servo-valve is massively used in the hydraulic servo control system due to the advantages of small dimension and high sensitivity. However, according to the special structure and working condition of the pilot stage, the flow field within it has some distinct features such as small scale, high speed and high shearing. In some specific areas flow with cavitation could even occur, which is one of the most substantial factors affecting the performance of the servo-valve. More importantly, the pressure oscillations caused by the collapse of the cloud cavitation could not only disrupt the stability of the flapper but induce the self-excited oscillations of the servo-valve. This research mainly focus on the transient cavitation and other induced flow phenomenon including the pressure fluctuations and transient flow forces on the flaper in the pilot stage. The internal connections among them have been also investigated by comparing the time-frequency parameters, which provides theoretical bases for the research of self-excited oscillations of sevo-valve.The governing equations of the cavitating flow filed in the flapper-nozzle pilot stage are established. The Large Eddy Simulation approach is used in order to calculate the transient vortex and cavitation structure more accurately. Based on the point vapor volume fraction, the surface averaged vapor volume fraction and volume averaged vapor fraction are proposed so as to quantitatively evaluate the cavitation in the nozzle center plane and the whole flow field. Moreover, the analysis method of the transient parameters representing cavitation and flow force has also been developed both in time domain and in frequency domain. The mean value and amplitude of the cavitation, pressure and flow force oscillations are calculated in the time domain. In the frequency domain, the peak frequency and the root-mean-square frequency are calculated.In order to simulate the transient cavitation phenomenon in the flapper-nozzle pilot stage of the servo-valve, the numerical method which is based on the Large Eddy Simulation and mixture two-phase flow model is set up. Both the distribution and evolution characteristics of the transient cavitation have been analyzed at different inlet pressures of the nozzle. The oscillations of the point, surface and volume vapor fractions at different inlet pressures of both the rounded flapper and square flapper are studied in time domain and frequency domain. The changes of the mean value, the amplitude,the peak frequency and the root-mean-square frequency of the cavitation parameters over the inlet pressure are also reported.On the basis of transient cavitation, the pressure fluctuations and transient flow forces on each flapper surface have been also studied numerically. The point, the surface averaged and the volume averaged pressure fluctuations of the flow field are obtained. Also, the resonant frequency of the pilot stage is calculated under the influence of the cavitation. Meanwhile, the transient flow forces on each flapper surface have been investigated, which includes the main flow force and the vortex flow force induced by the impact jet of the nozzle and the eddy cavitating flow respectively. The steady main flow forces are calculated and compared with the results obtained by the empirical method. The oscillations of the vortex flow force are simulated and analyzed in both time and frequency domain. Based on the fluid solid interaction method,the vibrations of the flapper is also analyzed. And the relationship among the transient cavitation, the pressure fluctuations, the transient flow forces and the vibrations of the flapper has been investigated.The unsteady cavitation and the transient flow forces on the flapper in the flapper-nozzle pilot stage are investigated experimentally. The null leakage flow rates of the flapper-nozzle valve are measured at different inlet pressures and the main flow forces of the flapper are evaluated based on a semi-experimental approach. Both the leakage flow rates and main flow forces are compared with the numerical results. Afterwards, the transient cavitation patterns are observed by using the high speed video camera for the rounded flapper and square flapper flow field under different inlet pressures. The corresponding image processing method is also introduced including the pre processing approach and the Structure Similarity Index Measurement(SSIM). The pre processing approach consists of two steps, median filter and gray stretch, which can eliminate the noise and improve the quality of the images. The SSIM method is utilized to investigate the variations of the transient cavitation images and the obtained the information are compared with the numerical results in frequency domain.