A New Droplet Collision Model And Evaluation Method For Spray Simulation In Internal Combustion Engines

Spray is widely used in industry,agriculture,medical treatment,combustion chamber and other fields.Droplet collision is a fundamental phenomenon in spray process,especially in dense spray.Based on the background of internal combustion engine spray,a new collision model applies to internal combustion engine spray,an evaluation method for droplet collision model and a set of improved criteria for droplet collision outcome classification are established.At the same time,the experimental study on the collision of two jets with severe droplet collision is carried out,and the accuracy of the new established models are verified.First,as part of the Lagrange droplet-Euler fluid(LDEF)method,which is used to simulate the spray process of the engine,an improved hybrid stochastic/trajectory(HST)spray collision model is proposed.The collision frequency of droplets,the generation of collision cell and the selection of collision pairs are optimized.In the HST model,the stochastic method is based on the theoretical framework of the time-counter(TC)direct simulation Monte Carlo(DSMC)method.Meanwhile,to improve the accuracy of the model prediction,the trajectory method is also introduced into the model when the spray parcel is sparse in the collision cells.In the calculation of droplet collision frequency,the factor of "each parcel represents a certain number of real droplets" is fully considered in the new model,however it has not attracted enough attention in the previous research.In addition,to reduce the dependence of the calculation results on the grid,an adaptive collision cell technology is adopted.Moreover,to improve the prediction accuracy of the droplet spatial distribution after collision,a new selection method of the collision pairsthe new is updated.To evaluate the performance of droplet collision model,the experiments of spray impingement under a wide range of conditions were performed and used.Meanwhile,the numerical results of the HST model,the O’Rourke model,the Nordin model and the NTC(no-time-counter)model are compared.The results show that the HST model can better reproduce the macroscopic morphology,collision frequency and Sauter mean diameter(SMD)of droplets for spray.It can be seen that,the HST model is relatively independent of the grid size,time step and the number of parcels.Second,in the spray simulation,the uncertain about the prediction of collision incidence always exists for the droplet collision model based on discrete droplet model(DDM).Therefore,to evaluate the accuracy of the collision incidence in the droplet collision model,the application condition of the Beer’s law in collision model verification is improved by adjusting the spatial distribution of "light-absorbing" parcels,and focusing on the influence of the real droplets number represented by each parcel on the collision rate.Based on the improved method,a theoretical computational efficiency evaluation method for droplet collision model is established,and the computational efficiency of four widely-used droplet collision models is analyzed.In this way,a set of relatively complete theoretical evaluation methods for droplet collision model have been established under the framework of DDM.Three typical droplet collision models,including the O’Rourke model,the Nordin model and the NTC model,as well as the HST model are comprehensively verified by the above methods.The effects of the number and size of parcels,the grid size and the velocity of the reference frame on the collision incidence were investigated.The results show that the HST model can achieve better convergence and accuracy.Moreover,for the HST model and the NTC model,the time of generating adaptive collision cell accounts for the main part of the total calculation time.Finally,the classification criteria of binary droplet collision outcome are improved and updated.The original KIVA-3V program only contains two collision outcomes of droplet coalescence and stretching separation.In this paper,the collision outcome of droplets is extended to the bounce(B),fast coalescence(FC),stretching separation(SS)and reflexive separation(RS).For the bounce collision of droplets,,the kinetic energy causing deformation is not only the kinetic energy of the interaction part of droplets if the viscosity and surface tension are taken into account,but also the kinetic energy of the part and even the whole droplet involved by the interaction.Hence,the bounce criterion of binary droplet collision can be rederived,and the relevant parameters can be fitted according to the existing experimental data.In addition,the widely used SS/FC model proposed by Jiang et al.[J.fluid mech.234,171(1992)]to distinguish stretching separation(SS)from fast coalescence(FC)is modified and improved.Based on the re-derivation of momentum conservation in the process of droplet collision,the correct mathematical expression of tangential velocity along the slip direction is obtained.According to the relevant experimental data,the model constant is expressed as a function of the Ohnesorge number(Oh)and droplet size ratio.In this way,the uncertainty caused by the model constant can be eliminated.Based on the analysis of the existing research(the model of Ashgriz-Poo),the modified model of Gotaas et al.[J.Phys.Fluids,19(10):102106(2007)]is adopted.For the overlapping region of the stretching separation and reflexive separation,a new method to determine the collision results is established by solving the improved SS/C equation and simplifying the solution to obtain the coordinates of curve intersection.The improved models are validated by the existing droplet collision experiments and the spray impingement experiments performed in this paper,and results show that the improved model can achieve much better predictions.