Study On Morphological Evolution And Pressure Pulsation Characteristics Of Cavitation In Regulating Valve

The regulating valve is one of the important control element in the hydraulic system,due to its complex structure of its flow channel,when the high-speed flowing medium passes through the throttle port,the flow rate accelerates and the fluid pressure decreases,when the pressure is lower than the saturated vapor pressure,cavitation is easy to occur at the throttle port and downstream in the flow channel.The unsteady process of the regulating valve includes complex phenomena such as the growth,shedding,and collapse of cavitation,and the pressure pulsation induced by the evolution of unsteady cavitation will bring adverse effects such as noise,vibration and so on.Therefore,this thesis studies the unsteady characteristics of cavitation flow in regulating valve,revealing the mechanism of pressure fluctuation induced by the evolution of cavitation morphology,which helps to suppress the adverse effects of cavitation flow in engineering applications.(1)A synchronous acquisition system for cavitating flow field of regulating valve was established,which mainly includes hydraulic experiment system,high-speed fullflow field image acquisition system and pressure signal acquisition system.Based on the synchronous acquisition system of cavitation field of the regulating valve,the evolution of cavitation morphology under the working condition of the regulating valve and the synchronous acquisition of pressure signals at key positions in the flow field are realized,which provides an experimental basis for the evolution of cavitation morphology of unsteady cavitation flow and the synchronous analysis of pressure pulsation at key measurement points.(2)The high-speed full-flow field display system collects the image of cavitation morphology evolution under different cavitation numbers,extracts and analyzes the characteristic values of cavitation morphology evolution process by image processing technology,and the results show that cavitation morphology evolution is roughly divided into three stages under different cavitation numbers,namely cavitation initial stage,transition stage and periodic cloud cavitation stage.By extracting the features of the t-x diagram,the variation law of typical cavity length,laryngeal attachment cavitation shedding rate,and shedding frequency with cavitation number under low cavitation number was obtained.The results show that with the increase of cavitation number,the cavity length of attached cavitation decreases,and the rate of reduction decreases faster and faster with the increase of cavitation number.With the increase of cavitation number,the length of free cavitation cavity decreases,and the rate of decrease is basically consistent with the increase of cavitation number.The development rate of attached vacuoles is generally maintained at 335m/s,and the rate of retraction of adherent vacuoles is generally maintained at 480m/s.Moreover,under different cavitation numbers,the rate of retraction of attached vacuoles has always been greater than the rate of development of adherent vacuoles.In addition,with the increase of cavitation number,the frequency of periodic cloud cavitation shedding gradually increases,and the period of periodic cavitation shedding becomes shorter and shorter.(3)The pressure signal of each measurement point in the flow channel of the regulating valve was measured by the pressure measurement system,and the timedomain frequency domain analysis and time-frequency domain analysis of the pressure signal were carried out,and the time-frequency domain distribution characteristics of the pressure signal in the cavitation flow field under different cavitation numbers were explored.The results show that the frequency domain distribution of pressure signals at upstream measurement points is basically the same under different cavitation numbers,mainly affected by the pulsation cycle of the pump,and the main frequency is 100 Hz.When the cavitation number decreases,the pressure signal of the laryngeal measurement point is relatively irregular and has no obvious main frequency.With the further decrease of cavitation number,the degree of cavitation is further enhanced,the large-scale cavitation group periodically breaks off,collapses develop to the position of the expansion section sensor,and is continuously enhanced by the influence of the cavitation development cycle,and the pressure signal of the measurement point of the expansion section has a peak of about 1100 Hz.Through the time-frequency analysis of the pressure signal,it is found that the pressure pulsation in the regulating valve is a typical non-stationary signal,that is,it is affected by cavitation shedding and also affected by incoming flow pulsation.The time-frequency distribution of laryngeal pressure measurement points is affected by small-scale cavitation clusters,and the distribution is more complicated,and it does not show certain regular changes until a complete attached vacuole is produced.The pressure sensor at the position of the expansion section is mainly affected by the detachment and collapse process of the detached cavitation,and the frequency composition of the pressure signal is relatively single.However,both the larynx and the expansion section are affected by the pressure pulsation of the incoming flow pump and the periodic cavitation of the two periodic development processes,which makes the frequency distribution show different characteristics.(4)Based on the synchronous acquisition system of cavitation flow field of the regulating valve,the synchronous analysis of cavitation morphology evolution image and pressure signal was carried out.The correspondence between the pressure pulsation in the regulating valve and the t-x diagram and the gray value at the measurement point position was explored,and the pressure pulsation law induced by the quasi-periodic development process under low cavitation number was revealed.The results show that the morphological evolution and pressure signal of cavitation are affected by the pulsation period of the upstream flow pump,and the pulsation influence of the pump gradually weakens with the decrease of cavitation number.In addition,the morphological evolution of periodic cloud-like holes and the mechanism of their induced pressure pulsations were studied,indicating that the pressure pulsations generated by periodic cavitation are proportional to the second derivative of the equivalent area of holes during morphological evolution.