| As the key core control element of pneumatic system,pneumatic valve compresses the air,changes the air pressure and flow rate,and then realizes the motion control of the actuator.Therefore,pneumatic valve determines the overall performance of pneumatic system to a certain extent.For this reason,in order to improve the performance of the pneumatic system,the optimization of the pneumatic valve will ultimately be concluded.Therefore,this thesis takes the piezoelectric pneumatic micro-valve with nozzle baffle,which is widely used in the biomedical field,as the research object to explore its internal flow mechanism as well as the source and propagation characteristics of fluid noise,aiming to solve the problem of flow instability and severe vibration of piezoelectric valve.And the roar noise and other problems to provide a certain reference.The main work and research achievements are as follows:(1)Large Eddy Simulation(LES)was used for transient simulation of the internal flow field of the nozzle flapper piezoelectric pneumatic microvalve.The sudden change process and mechanism of energy such as pressure energy and kinetic energy in transient flow field are studied.The main modes of velocity field were extracted to identify the spatio-temporal evolution characteristics.The evolution and development of large scale vortex ring structure formed by supersonic jet is studied.(2)FW-H sound analogy method was adopted to study the noise propagation in the middle and far fields with flow field data as the noise source.The main sources of fluid noise of the nozzle flap-plate piezoelectric pneumatic microvalve,the frequency spectrum characteristics of sudden changes in pressure and velocity,as well as the formation and dissipation position of vortex ring are investigated.Study the directional distribution of noise and trace its cause.(3)Analyze the influence of inlet pressure and valve opening on the flow field and noise characteristics inside the valve.The changes in the core region,wake and vortex ring region of the olive jet were investigated when the inlet pressure and valve opening changed.Study the change of corresponding noise characteristics;Determine the parameter condition with the maximum noise amplitude.The results show that: 1)When the fluid of the nozzle baffle piezoelectric pneumatic microvalve flows through the high pressure cavity,gap valve port and low pressure cavity,the pressure abrupt change inside the gap is the most serious,and the olive supersonic jet core structure is formed in the low pressure cavity,and the wake of the vortex ring is formed in the process of movement.The vortex ring constantly absorbs energy from the main stream and moves towards the exit direction,forming a large-scale vortex structure.Complex vorticity field dominated by strip vortex structure;2)Aerodynamic noise sources include three parts: high-frequency noise generated by pressure pulsation when high-pressure gas passes through micron level slot valve,jet noise generated by supersonic jet structure,and eddy noise generated by main vortex ring structure dissipation;3)The noise pressure level of the monitoring points inside the gap valve port is about 10 d B higher than that in the jet area,and the noise pressure level of the monitoring points in the core jet area is about 20 d B higher than that in the downstream vortex ring formation area.In addition,according to the directionality distribution results,there are noise maxima in three directions,which are most prominent in the direction of 54°.It converges in this direction to form a disordered flow field.Therefore,when designing and optimizing the structure of the piezoelectric pneumatic valve,the low pressure area and supersonic area inside the valve should be avoided reasonably,and the corresponding optimization scheme is proposed to eliminate the internal vortex and achieve the purpose of noise reduction. |
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