| Optimizing the quality of fuel injection and atomization is one of the most effective ways to improve the fuel economy and reduce the emission of internal combustion engines.However,due to the high injection pressure and small nozzle geometry,the mechanism of coupling the internal flow and fuel jet breakup in the internal combustion engine is still unclear.Based on the improved Euler hybrid multi-fluid-quasi-VOF model,the coupling characteristics of in-nozzle and near-field spray in the internal combustion engine are studied in the present dissertation.The physical problems of the in-nozzle and near-field spray are investigated from the microscopic process nature.The interaction mechanism of the physical parameters that affect the internal flow of the nozzle and the near field spray is analyzed and determined.The main tasks of the present dissertation are as follows:Primarily,based on the open source OpenFOAM framework,a coupling model between the cavitation sub-model and the Euler hybrid multi-fluid-quasi-VOF model is established in the present dissertation.Based on the classical bubble nucleation theory,the bubble number density in liquid fuel is predicted,and the cavitation sub-model is constructed by combining the bubble number density and the Rayleigh-Plesset bubble growth dynamics equation.Coupling the established cavitation sub-model and multi-fluid model,considering the drag force,virtual mass force,surface tension between phases and the interface compression technology,an improved Euler hybrid multi-fluid-quasi-VOF model is established.The improved Euler hybrid multi-fluid model can not only consider the mass and momentum transfer between phases but also capture the gas-liquid interface.By comparing the simulated cavitation results with the experimental images of the optical nozzle,it is proved that the improved Euler hybrid multi-fluid-quasi-VOF model can accurately predict the cavitation phenomenon in the nozzle.On this basis,the improved hybrid multi-fluid model is used to conduct an overall study of the nozzle internal flow and near-field spray on the cavitating spray C nozzle provided by ECN.The study finds that the parabolic velocity distribution of the fuel in the nozzle can cause the fuel vapor to be separated from the nozzle wall and form a tail at the end of the cavitation area in the developing cavitation.There is a large velocity difference between the liquid fuel and air in the near-field spray area,resulting in a strong drag force between the phases,especially at the mushroom-shaped jet head.The near-field jet breakup is mainly divided into two regions:the mushroom head breakup region caused by the drag force and the main spray region induced by the cavitation,turbulence and drag force.The increase of the back pressure will suppress the formation of cavitation,but can increase the discharge coefficient and increase the jet breakup angle.Secondly,the effect of residual bubbles in the nozzle of the high-pressure internal combustion engine on the internal flow of the nozzle and the near-field spray is studied by using the independently developed and improved hybrid multi-fluid-quasi-VOF model and large eddy simulation method.First,the validity of the model is validated based on the experimental results in the literature.The results show that the transient mixing process of the main spray zone predicted by the improved hybrid multi-fluid model is in good agreement with the experimental high-speed camera imaging results.The study finds that turbulent disturbances,large velocity gradients at the gas-liquid interface,and the velocity difference between fuel and air lead to KH instability.The gas-liquid density gradients and pressure gradients can promote RT instability at the disturbed gas-liquid interface.Residual bubbles can directly enhance primary breakup and turbulent disturbances,especially in the mushroom-shaped jet head region.The larger residual bubbles closer to the nozzle hole inlet promote primary breakup more strongly.Thirdly,based on the improved hybrid multi-fluid-quasi-VOF model,the effects of nozzle geometry and high-frequency fluctuating injection pressure on nozzle internal flow and jet breakup are studied.First,the internal flow and near-field spray characteristics of the geometrically similar cavitating spray C nozzle and non-cavitating spray D nozzle provided by ECN at a constant injection pressure and a sinusoidally varying injection pressure of 100 kHz are compared.Then taking the non-cavitating spray D nozzle as a prototype and keeping the inlet and outlet diameters of the nozzle hole as constant,the nozzle hole diameter at 1/4,1/2,and 3/4 of the nozzle hole length from the nozzle hole inlet are changed as 160 ?m and three different convergent-divergent level nozzles are obtained.The influence of nozzle geometry on nozzle internal flow and near-field spray is studied.The study finds that super-cavitation can greatly enhance the turbulence intensity of the internal flow of the nozzle and the jet surface.The near field spray of the cavitating spray C nozzle is divided into two stages,i.e.the pre-supercavitation stage in which the jet breakup development is relatively slow and the post-supercavitation stage when the jet shatters violently.However,the jet breakup development of the spray D nozzle is much more gradual.Due to unstable cavitation,the effect of fluctuating injection pressure on the jet breakup of spray C nozzle is greater than that of spray D nozzle.The convergent-divergent nozzle can promote the jet breakup,and the greater the convergent-divergent level is,the greater the impact will be on the jet breakup.Finally,a compressible two-fluid-quasi-VOF model is constructed,which takes into account the effect of surface tension.In terms of the physical properties of the working fluid,the density state equation of diesel fuel with temperature and pressure as variables is established based on the experimental data,and the density and viscosity of air are calculated by using the PR state equation and Sutherland equation,respectively.In terms of turbulent vortex structure capture,the interphase turbulent disturbance source term is added to the turbulent kinetic energy equation of the compressible large eddy simulation to consider the interphase perturbation transfer.By comparing the mass flow rate,flow coefficient,spray momentum flux and effective jet velocity under different injection conditions,the results show that the numerical simulation results are in good agreement with the experimental measurements.In the study of cone-shaped nozzle jet breakup,it is found that the jet is divided into an intact liquid column region and a jet breakup region.The disturbance in the intact liquid column region increases along the axial length from the nozzle outlet and is less affected by the injection time.However,the disturbance in the jet breakup region increases dramatically with the increasing injection time and is the main factor for the jet breakup.The surface tension has the dual functions of suppressing atomization of droplets and promoting the breakup of fuel liquid column and ligament.The large velocity gradient at the gas-liquid interface can cause more obvious viscous heat,and the increase in ambient temperature can reduce the air density,resulting in that gas-liquid interaction force is weakened and that the liquid core of jet becomes longer. |
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