Investigations On The Evolution Law Of The Shock Wave Induced By Supersonic Diesel Jet And Its Influence On The Spray Atomization

High diesel injection pressure is regarded as one of the most effective methods to improve the combustion and emission performance of modern diesel engines.The velocity of the diesel jet at the nozzle exit increases with the increased fuel injection pressure.In addition,the application of pre-injection and the low temperature combustion technology produce a low sound speed in the cylinder during the fuel injection process.Therefore,the supersonic fuel jet has become a common phenomenon in modern diesel engines.Shock waves are generated around the spray when the spray is supersonic.The generation and propagation of the shock wave affect the development and atomization of the spray in the cylinder.The evolution law of the shock wave induced by supersonic jet and the effect of the shock wave on spray characteristic are investigated in this paper by means of experiment and numerical simulation.Based on the jet Mach number,the relationship between the shock wave characteristic parameters and the jet Mach number is established.The influence of the shock wave on the spray macroscopic parameters and the mixing characteristics of the oil and gas are quantified in the range of diesel engine operating conditions.At the same time,the effect of the shock wave on the characteristic parameters of spray flow field is studied.The influence of the shock wave on spray macroscopic structure and mixing characteristic of the spray are analyzed from the perspective of mechanism.The following contents have been carried out in the thesis:First,the evolution law of the supersonic diesel spray and shock wave under different diesel injection pressures and ambient pressures are investigated from macroscopic and microscopic perspectives using the Schlieren imaging method and plane laser induced fluorescence method.Based on the jet Mach number,the evolution law of the induced shock wave under different jet Mach number are investigated.The correlation between the shock wave characteristic parameter and jet Mach number is also studied.The results show that the shock wave is divided into detached bow shock wave and attached oblique shock wave.With the increase in the fuel injection pressure,the Mach number of the spray jet increases accordingly,and the shock wave structure is transformed from the bow shock wave to the oblique shock wave.The evolution law of the shock wave characteristic parameters with Mach number of the jet front under different working conditions is analyzed.The empirical correlation of the shock wave angle and the Mach number of the spray front is proposed based on the nonlinear fitting method.The empirical formula can accurately predict the shock angleSecond,the effect of the shock wave on the spray macroscopic structure and mixing characteristic of the spray are investigated in the N2 and SF6 environments.The speed of sound in the SF6 atmosphere is much lower than that in the N2 atmosphere.It is easy to form a shock-wave state in the SF6 environment and a non-shock-wave state in the N2 environment under the same fuel injection pressure and ambient gas density.The spray macroscopic structures under the different gaseous medium conditions are used to analyze the effect of the shock wave on the evolution of the diesel spray.The influence of the shock wave on the spray mixing characteristic is quantified combined with the research on the mass flow rate of the fuel injection.In addition,a combustion chamber structure is designed in this study in order to investigate the shock wave reflection and evaluate the effect of the reflected shock wave on the spray characteristics.The results show that the shock wave promotes the development of the spray.However,the shock wave has an inhibitory effect on the spray development in the radial direction.The spray tip penetration in the SF6 environment is 13.1%higher than that in the N2 environment at the time of 0.5ms after the start of the injection when the fuel injection pressure is 60MPa.However,the differences in the tip penetration decreases when the fuel injection pressure is 140MPa.The spray tip penetration in SF6 is 6.6%higher than that in the N2 gas.By analyzing the air entrainment quantity and the average equivalent ratio,it is found that the shock wave also has a positive effect on the fuel-air mixing in the cylinder.The reflection of the shock waves in the combustion chamber also influences the development and the mixing characteristics of the spray.It is shown that the reflected shock wave has little effect on the development of the spray tip penetration.However,the reflected wave significantly suppresses the development of the spray cone angle.The reflected wave also causes an asymmetric development of the spray interfaceFinally,large eddy simulation combined with the Volume of Fluid(VOF)method is used to further investigate the shock wave characteristic and the effect of the shock wave on the spray characteristics from the perspective of mechanism.The influence of the shock wave on the spray flow field parameters is analyzed,and the interaction between the shock wave and the spray development.Based on the verification of the simulation model,the research scope is extended and the characteristic of the shock wave under ultra-high fuel injection pressure are simulated.The results shown that the structure of the leading edge shock wave has the characteristic of both an oblique shock wave and bow shock wave under high jet Mach number.The shock wave can increase the density,pressure and temperature of the gas.The effect of the shock wave on the flow field parameters increases as the Mach number of the jet increases.The maximum density ratio,pressure ratio and temperature ratio at the fuel injection pressure of 320MPa are 2.46,2.01 and 1.15,respectively.The increase in the temperature behind the shock wave contributes to improving the ignition characteristics of the diesel spray.The pressure disturbance promotes the mixing effect between the spray and the surrounding air.The research of this thesis reveals the shock wave characteristics induced by supersonic fuel jet and the effect of shock wave on spray atomization.It can provide data foundation for the modeling of spray atomization mechanism under new technologies of diesel engines,and can also provide optimization direction for spray atomization in the combustion chamber.The research is of great significance to the optimization design of modern diesel engine.