Development Of A Droplet Breakup Model For Ultra-high Pressure Fuel Sprays And Numerical Study Of Atomization Mechanisms

With the rapid development of China’s economy and society,the energy situation and environmental problems are becoming more and more serious.In particular,the problem of air pollution and its control in recent years has attracted the attention of the whole society.The internal combustion engine,especially the diesel engine,fed with fossil fuels is thought as one of the main sources of fine particulate matter in the atmosphere.The development of high efficiency and low pollution engine has become an important direction of the development of the internal combustion engine industry.High-pressure injection technology is used to be one of the important technologies to improve the combustion efficiency of engine and reduce the formation of particulate matter in cylinder combustion stage.To date,the fuel injection pressure of diesel engine is increasing continually.Ultra-high pressure fuel injection technology will be likely to be applied to future diesel engines,and become one of the main characteristics of advanced diesel engines.Up to now understanding of liquid fuel spray atomization mechanisms under ultra-high injection pressure is still very insufficient.The spray characteristics,atomization mechanisms,methods of experimental/theoretical/numerical simulation,mathematical modeling of ultra-high pressure fuel spray need to be studied and understood.Firstly,prediction performance of KH-RT droplet breakup model,which is widely used in the simulation of high-pressure fuel atomization processes,is analyzed.The research includes the following aspects.(1)Based on the results of spray experiment,the KH-RT breakup model was calibrated.The key constants(B1 and CRT)in the model were calibrated based on the experimental results of macroscopic spray characteristics and droplet size diameter.(2)The prediction performance of the calibrated KH-RT breakup model used in the simulation of ultra-high pressure fuel spray was analyzed.(3)The effects of ambient back pressure and fuel injection pressure on the prediction performance of KH-RT breakup model were studied.Secondly,the KH-RT breakup model was modified to a new breakup model named SS-KH-RT(Supersonic Kelvin-Helmholtz Rayleigh-Taylor)breakup model.The new model takes into account the gas compressibility effect on the supersonic droplets and integrates the Pilch’s model modeling the breakup time of supersonic droplets.And then,the model constant Cb in SS-KH-RT breakup model was calibrated based on the experimental data.The simulation of ultra-high pressure fuel spray was carried out under different back pressure and injection pressure.The results show that simulation error of SS-KH-RT model for ultra-high pressure fuel spray penetration is nearly 50%lower than that of KH-RT model.With the increase of the backpressure,the simulation error of spray penetration based on KH-RT model increases gradually.The spray tip penetration simulated by SS-KH-RT breakup model keeps in good agreement with the experimental results.The adaptability of SS-KH-RT breakup model to variable back pressure is significantly better than that of KH-RT model.Finally,the atomization mechanisms of ultra-high pressure fuel was simulated by SS-KH-RT breakup model.The effects of ultra-high injection pressure on spray evolution,droplet breakup characteristics,spray/air entrainment and transfer characteristics of spray kinetic energy were investigated.Based on the quantitative analysis of the droplet breakup,air entrainment and kinetic energy transfer under ultra-high injection pressure,the special mechanism and law of super-high pressure fuel spray were discussed.The results show that the ability of increasing the spray penetration and decreasing the droplet mean diameter is weakened when the injection pressure exceeds 500 MPa.With the increase of injection pressure,the number of subsonic and supersonic droplets in the spray field increases.The number of subsonic and supersonic droplets at high pressure fuel spray(injection pressure is 200 MPa)is of the order of 1012,which is 1 to 2 orders of magnitude lower than that of ultra-high pressure spray(300 MPa to 700 MPa).When the fuel injection pressure is increased to the ultra-high injection pressure range,the dissipation of the spray kinetic energy increased significantly with the increase of the injection pressure.A novel concept for the characteristics of kinetic energy transfer was provided.The fuel atomization efficiency and liquid-gas kinetic transfer efficiency was defined and calculated.According to the fuel atomization efficiency and liquid-gas kinetic energy transfer efficiency,there existed a critical injection pressure value Pcr,where Pcr is 600 MPa for present conditions.The fuel atomization efficiency and liquid-gas kinetic energy transfer efficiency reached the peak value at the critical injection pressure.As the injection pressure exceeded this critical value,two above efficiencies were simultaneously decreased.