Transient Development Mechanism And Effects On Spray Breakup Of Nozzle Cavitation In GDI Engine

The cavitation effect is one of the three major factors,which are cavitation effect,turbulent disturbance and gas power,affecting fuel atomization process,and its weight increases with the increase of fuel injection pressure.The fuel injection pressure of the new generation of GDI(gasoline direct injection)engine can be up to35 MPa,to further optimize combustion efficiency,transient response characteristics and combustion process controllability of GDI,and 50 MPa ultra-high pressure injection system has been researched.Although the injection pressure is still much lower than that of the diesel engine,but since the saturated vapor pressure of gasoline is higher than that of diesel(Psat|gasoline 80 kPa,Psat|diesel 30kPa),cavitation is more likely to occur.As a result of that,the complex multi-phase flow state induced by nozzle cavitation will appear in GDI nozzle hole and deeply affects the fuel atomization process,which is similar with that in diesel nozzle.In this context,the GDI fuel injection and atomization process is studied starting from the instantaneous cavitation multi-phase flow in the nozzle,and the instantaneous evolution mechanism of cavitation,development,collapse and morphological variation in the nozzle is clarified.Under the premise,the influence of cavitation effect on nozzle flow rate and instantaneous spray characteristics is fully considered,to achieve more accurate and reliable GDI ultra-high pressure fuel injection process control.In this paper,the one-dimensional mathematical model of the critical cavitation number of GDI injector is used as the entry point.On the basis of demonstrating the existence of cavitation multi-phase flow in the GDI injector hole,the research focusing on the two scientific issues of "the instantaneous evolution mechanism of cavitation morphology and its influence mechanism on atomization process" and "the influence mechanism of non-condensable gas on cavitation intensity and active control of cavitation intensity" is made combined with numerical simulation technology and proportional amplification shadow imaging visualization test.The research conclusions can be used to further complete the basic theory related to the cavitation effect in the injector hole and provide an important theoretical basis for the analysis of GDI ultra-high pressure jet flow characteristics,as well as active control of cavitation intensity.In order to prove the existence of the complex multi-phase flow state in the GDI injector hole with cavitation as the cause,and establish the direct connection between the GDI injector hole cavitation and the fuel atomization process at the macro-stable characteristic level,based on the GDI injector,a non-intrusive cavitation test was carried out to establish a one-dimensional cavitation model for calculating the critical cavitation number and the orifice flow coefficient.Then,the common working conditions of the GDI injector are analyzed combining one-dimensional cavitation model,the flow and spray data.The results show that common working conditions of GDI injectors can be completely covered by the cavitation area with the critical cavitation number as the boundary.It is proved from the basic theory that there is cavitation multi-phase flow patterns in GDI injector holes.The effect on the flow rate and spray characteristics of super cavitation flow statement is most significant.In this flow state,the spray cone angle peaks,the flow rate collapses and no longer grows with the increase of the pressure difference.In order to accurately simulate the intensity and instantaneous microscopic morphology of cavitation in the injector hole,on the basis of the multi-dimensional modeling of the cavitation phenomenon involving the main sub-model including multi-phase flow,turbulence and phase change,the effect of multi-phase flow and turbulence calculation method on the accuracy of nozzle cavitation simulation is studied.The results show that the vapor phase can be regarded as the complete follower phase in the multi-dimensional modeling of cavitation in the hole,so that its motion is characterized by the liquid phase.The large eddy simulation(LES)and the Reynolds stress turbulence model(RSM)considering the anisotropy of Reynolds stress can accurately simulate the sheet cavitation and cloud cavitation structure.The calculated gas phase volume fraction in the nozzle hole with LES and RSM is changes periodically(about 1ms),which proves that LES and RSM can be used to analyze the transient evolution mechanism of cavitation in the nozzle.The scale-up shadow imaging visualization test and large eddy simulation are brought together to analyze the recirculation flow induced cavitation.The results can be utilized to explore the inconsistency of the injector spray cone angle when the boundary conditions are constant,reveal the periodic evolution mechanism of sheet cavitation and cloud cavitation and its influence on the atomization process.The results show that the cloud cavitation shedding can be divided into 2 types: the overall shedding of the sheet cavitation and the local shedding of the terminal part of the sheet cavitation.The exact form is determined by the sheet cavitation length and the return jet intensity.The cloud cavitation shedding process will produce a very strong disturbance to flow field.The pressure,velocity and vorticity in the flow field before and after the collapse of cloud cavitation show obvious fluctuations,and the unevenness of the pressure,velocity and vorticity along the lateral distribution in the flow field is also aggravated.The collapse of the cloud cavitation inside and outside of the nozzle will both cause the bump to appear in the spray,and the size of the bump increases with the position of the cloud cavitation near the outlet of the nozzle and the gas phase in cloud cavitation increases.However,the collapse of the cloud cavitation structure is only a sufficient non-essential condition for the spray pump.Velocity tensor second invariant Q numerical simulation and proportional magnification shadow imaging visualization test are brought together to study the vortex flow induced cavitation.The results can be used to explore the cause for skirt spray of the injector under the high pressure and small needle valve lift condition In-hole incentives,to reveal the generation and transient evolution mechanism of vortex flow induced cavitation in the injector hole,and to clarify its influence mechanism on the atomization process.The results show that the liquid phase vortex formed in the nozzle is the direct cause of vortex cavitation.When the watery vortex intensity is large enough,the back pressure gas will reflow under the pseudo-pressure gradient to form the main structure of the vortex cavitation.The non-condensable gas and vapor phase which are expanded at the liquid vortex core is also an important part of vortex cavitation.When the vortex cavitation flow state occurs,the skirt spray appears and the spray cone angle is greatly improved,which is results by the centrifugal force induced by liquid vortex in conjunction with the "change" of the cross section of the outlet(from round to circular)induced by vortex cavitation.The vortex cavitation shape is unstable,and its instantaneous evolution can be subdivided into seven stages.In these seven stages,the back pressure gas is co-aggregated by liquid vortex,while the back pressure gas affect the intensity of liquid vortex,which causes the flow state in the nozzle hole to change periodically.And the spray cone angle is affected by this phenomenon,that means the spray cone is still unsteady under steady conditions,which will be increased with the increase of liquid vortex intensity,and decreased with the increase of the vortex cavitation interference to the liquid phase vortex.The theoretical model of the mixed bubble is proposed,with the inner layer as the vapor phase and the outer layer as the non-condensable gas phase,and the three-phase flow cavitation model considering the influence of non-condensable gas is established through detailed mathematical deduction.Then the three-phase flow cavitation model and scale-up shadow imaging visualization experiments are combined to carry out the study.The results can be used to reveal the mechanism of the influence of non-condensable gas in the liquid phase of the cavitation in the injector hole,and to explore the feasibility of using non-condensable gas to actively control the cavitation intensity.The results show that the non-condensable gas in the molten phase promotes the increase of recirculation zone cavitation strength by increasing the phase transformation rate and the expansion in the low pressure zone in the nozzle hole.When the pressure difference is constant,the flow state in the nozzle hole is significantly affected by the increase of non-condensable gas in the liquid phase,which is the cavitation intensity is strengthened in turn and the primary crushing of the jet is then being promoted.It is shown that the recirculation zone cavitation intensity can be active controlled by filling the liquid phase with non-condensable gas.Throughout the research work,the complex multi-phase flow state induced by cavitation in the GDI injector hole is introduced firstly.Then the multi-dimensional numerical simulation and proportional magnification shadow imaging research is combined to carry out the study.The results can be used to reveal the instantaneous evolution mechanism and influence mechanism on atomization process of microscopic cavitation structure including sheet cavitation.Cloud cavitation and vortex induced cavitation.The significant promotion of cavitation intensity by non-condensable gas,and the active control method of cavitation intensity using non-condensable gas is studied too,whose results can provide a reference for the theoretical analysis and precise control of the atomization process of the ultra-high pressure injection GDI engine.