Development Of A Fuel Spray Model And Its Application To Computations Of Combustion And Emissions

The energy crisis and environmental protection have become the two fundamental themes for sustainable human development,diesel engines as the major source of petroleum consumption and emissions to atmosphere have received more attentions.With the development of computer technology and computational fluid dynamics(CFD),numerical simulation has been successfully applied to the study of diesel cylinder working process.In this paper,based on the General transportation equation analysis(GTEA)numerical code,the spray,combustion and emission process of diesel and biodiesel were studied that will better understand the working process of diesel engine.Improve the calculation accuracy of the numerical model was another purpose of this study.The main work of this paper was:(1)A complete fuel spray model which includes fuel injection,droplet movement,droplet tracking,droplet breakup,droplet collision,droplet evaporation,spray/wall interaction,turbulence dispersion sub-model was established for the working process of the diesel engine.The spray model can effectively simulate the fuel spray,evaporation and spray/wall interaction process under different conditions,and better reproduce experimental results.(2)A new droplet breakup model which consider multiple influence factors has been proposed.The model distinguishes between primary breakup and secondary breakup.In modeling primary breakup,the turbulence perturbation was characterized by the turbulence characteristic length and time scales,and the weight coefficient was used when incorporating into the primary breakup model.According to the value of deformation of the droplet,the TAB and KH-RT droplet breakup model were used to calculate the droplet secondary breakup process by the competition mechanism.The study found that the calculation result of the new breakup model was more accurate than the other breakup model.(3)Based on the new droplet breakup model,the fuel spray,evaporation and spray/wall interaction process were studied.The accuracy of different droplet breakup and spray/wall interaction model were compared,then investigate the influence of fuel spray on spray/wall interaction process.The results show that good fuel spray was very helpful for fuel evaporation process,the accurate calculation of fuel spray was very important for study spray/wall interaction process.It is worth note that turbulence inside the nozzle affects themacroscopic morphology of fuel spray and the characteristics of spray/wall interaction process.(4)The detailed chemical reaction kinetics solver(CHEMKIN)and the Shell/CTC Combustion model were integrated into the GTEA code.The problem of excessive calculation of chemical reactions was effective solve by the MPI(Message passing interface)parallel calculation.The GTEA code was used to study the evolution and relationship of the ignition delay and flame lift-off length under different nozzle diameter,injection pressure,ambient density,ambient temperature and oxygen concentration conditions.Based on the new droplet breakup model the effect of turbulence inside the nozzle on the combustion process was quantified.The results show that the turbulence inside the nozzle can decrease the flame lift-off length.(5)The soot model was used to study the soot emission process under different working conditions.The relationship between the lift-off length and the soot emission,and the influence of the fuel spray process on the soot emission were analyzed.Turbulence inside the nozzle can decrease the soot mass.Then,different diesel engines in-cylinder working process were calculated.The results showed the predicted in-cylinder pressure,heat release rate,hydrocarbon(HC),carbon monoxide(CO),soot and nitrogen oxides(NO_x)emissions were in good agreement with the experimental data.(6)The physic properties of biodiesel were added to the GTEA.The spray,combustion and emissions process of biodiesel were investigated,the in-cylinder working process of diesel engine fueled by biodiesel was also studied.In this part,the effect of biodiesel properties on spray process,the characteristics of biodiesel ignition delay and flame lift-off length,and the behavior of biodiesel emissions were studied.It is found that the change of nozzle diameter can decrease biodiesel spray penetration length,ignition delay and lift-off length,but not very helpful for reduce biodiesel NO_x emissions.