Study Of Characteristics Of Fuel Spray Impingement,Film Dynamics And Vaporization

The fuel spray impingement and the wall film characteristics have significant influence on the engine combustion process and engine-out emissions.In this dissertation,serval imporved sub-models of splash,wall film formation,wall film dynamics and vaporization,and film separation were developed on the background of the phenomena related to spray impingement in the internal combustion engines.The main work is as following:(1)A new spray impingement model was developed with special emphasis on the PCCI engine relevant conditions.The new model distinguishes dry wall and wetted wall.The impingement regimes of dry wall include deposition and splash,whereas the regimes for wetted wall consist of stick,rebound,spread,and splash.The regime transition thresholds of splash are determined based on recent experimental observations,which can account for the wide ranges of conditions related to engines.Meanwhile,the characteristics of secondary droplet formed by splash were updated.Comparisons of the predictions from the present model with the experimental measurements and predictions from the previous model were conducted.The results indicate that the numerical predictions from the new model illustrate better agreements with the experimental data than those of the previous model.Finally,the new model coupled with the KIVA-3V code was applied to predict the combustion and emissions characteristics of PCCI engines.The influence of injection timing on the mixture preparation process for the diesel PCCI engine was also investigated.(2)A new splashed mass ratio model was developed based on the recent measurements of spray impingement rather than those of single droplet/wall interaction.It was found that the new model can satisfactorily reproduce the film quantity under various conditions.Then,by coupling the new model with KIVA-3V code,the effects of ambient gas temperature and density,nozzle diameter and injection pressure,impingement distance,and injection mass on the film quantitative characteristics in a constant volume chamber were systematically investigated under PCCI relevant conditions.Finally,dimensionless film quantity correlations under room and high temperature conditions were developed based on dimensionless analysis and power-law scalling approach,respectively.(3)An improved liquid film dynamics,heat transfer and vaporization models based on the Lagrangian method was proposed.By modifying the source term of the impingement momentum,the film dynamics sub-model was improved with consideration of the effect of droplet/film interaction on the film dynamics,as well as the dissipative energy loss during the expansion of the lamella formed by the deformation of the deposited droplet.Taking account of the effect of variable density turbulent flow on the gas/film heat transfer and introducing the Chilton-Colburn analogy for calculation of the film evaporation coefficient,the film heat and mass transfer sub-models were further enhanced.It is found that the improved liquid film model is capable of satisfactorily reproducing the characteristics of film dynamics,heat transfer and vaporization under various engine relevant conditions.(4)Numerical and experimental study of the liquid film separation during the spray/wall interaction at expanding corners were conducted.Firstly,a new film separation and atomization model was developed.To validate the new model and understand the film separation and atomization processes during the spray/wall interaction,the effects of the injection pressure,the impingement distance,and the corner angle on the evolution of the impinging spray and the wall film dynamics after the corner were explored.Furthermore,by coupling the new model with the KIVA-3V code,the performance of the new model on predicting the film separation characteristics were validated against the experimental results,and compared with the predictions from the classical model.It can be concluded that the new proposed model could better reproduce the film separation and atomization characteristics during the spray/wall interaction under various conditions.(5)An enhanced multi-component quasi-dimensional vaporization model for wall film was proposed which consists of three parts,i.e.,the liquid phase,the gas phase,and the vapor-liquid equilibrium.In the liquid phase sub-model,the high-order polynomials are adopted to describe the distributions of the temperature and mass fraction of each species in the film interior.In the gas phase sub-model,on the basis of the Chilton-Colburn analogy with considering the variable Lewis number,the film evaporation coefficient is determined.In the vapor-liquid equilibrium sub-model,under high ambient pressure conditions,the real gas behavior at the film surface is considered.The error distributions of the linear temperation model under various conditions were tested and the effect of the thermal and mass diffusions on the vaporization of the diesel film was also explored.Finally,the linear temperature,quasi-dimensional,and one-dimensional models were integrated into KIVA-3V code for the simulations of film vaporization in the flow over a backward facing step and in a practical diesel engine,and the computational accuracy and efficiency of different models were compared systematically.The results indicate that the improved model while has high computational accuracy and efficiency.