| Nowadays,the emission of diesel exhaust has become the most concerned problem.Cavitation phenomenon in diesel nozzle can improve the fuel atomization effect,increase the combustion efficiency,and achieve the effect of energy saving and emission reduction.Therefore,in this thesis,piezoelectric fuel injector was used as the research object,this paper analyzed the influence of jet parameters and geometric parameters on the internal flow of the nozzle and the influence of CH4/DME mixed fuel on the spray characteristics.The specific work contents are as follows:(1)The geometry model of piezoelectric injector was modeled,and the cavitation model of mixed multiphase flow was established.The influence of inlet and outlet pressure,nozzle diameter and length of injection on the internal flow of nozzle were analyzed.The study found that when the outlet pressure was 5MPa,the inlet pressure reached 50 MPa,super cavitation was formed inside the nozzle,the inlet pressure continued to increase,and there was no cavitation falling off,and super cavitation always existed.When the injection pressure was 108 MPa,when the outlet pressure was increased to 30 MPa,the super cavitation in the nozzle became a partial cavitation,which indicates that the increase of outlet pressure can inhibit the flow of nozzle.The mass flow rate,velocity and turbulent kinetic energy at the outlet are increased by increasing the injection pressure or decreasing the outlet back pressure.With the increase of the nozzle diameter,the thickness of the cavity also increased,when the flow reached the super cavitation,the outlet mass flow and turbulent kinetic energy increase with the increase of the nozzle diameter.With the increase of nozzle length,the nozzle outlet velocity and turbulent kinetic energy decreased.With the increase of inlet fillet radius,the cavitation decreased,and the fuel flow velocity and mass flow rate increased,which can restrain the cavitation.(2)The spray atomization field was modeled,and the parameters of cavitation simulation outlet were used as atomization initial conditions to atomization coupling.The effects of inlet and outlet pressure,nozzle diameter,diesel,CH4/DME mixed fuel on spray characteristics were analyzed.It is found that with the increase of inlet pressure,the cavitation was more intense.The disturbance force on the fuel surface is more conducive to the fragmentation of oil droplets.The spray penetration and the spray cone increased,and the SMD decreased.With the increase of outlet pressure,the pressure difference between the inlet and outlet in the nozzle decreased,the fuel flow rate decreased,the cavitation intensity in the nozzle decreased,the turbulent kinetic energy decreased,the spray penetration decreased,the atomization cone decreased,and the SMD of the fuel droplet is larger.In the super cavitation condition,the smaller the diameter of the nozzle,the smaller the bubbles generated by cavitation,and the average turbulent kinetic energy increased,the spray cone increased,the SMD decreased,and the spray penetration became shorter.The CH4/DME(C1D9)mixed fuel with low viscosity and high oxygen content has better air mixing effect,spray cone increased and SMD decreased,good atomization effect,the spray penetration became shorter.(3)Based on the orthogonal test and response surface analysis,the effects of jet parameters and geometric parameters on the fuel flow were studied.It was found that the inlet pressure was an important factor affecting the fuel flow rate.The larger the inlet pressure was,the higher the fuel flow rate was.When the nozzle diameter increased,the nozzle length correspondingly decreased,and the outlet turbulent kinetic energy increased.The nozzle diameter is the main factor affecting the outlet mass flow rate.The nozzle length has the second effect on the outlet flow rate,and the influence of jet parameters on the outlet mass flow rate is relatively smallThe results show that piezoelectric injector can obtain higher mass flow rate and turbulent kinetic energy at the outlet by increasing the pressure difference between the inlet and outlet of the injector,so as to improve the atomization effect of the fuel.In the super cavitation state,the orifice diameter is small and the fuel atomization effect is well.The atomization effect of CH4/DME mixed fuel with low viscosity and high oxygen content is better than that of diesel. |
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