Study On Diesel Spray Characteristics Under Varying Injection Condition And Construction Of Phenomenological Model

With the increasingly stringent emission regulations,achieving higher efficiency requirement has become a major challenge for engine.The mixing process of fuel and air in the cylinder determines the combustion efficiency and pollutant emission.In order to achieve the goal of high efficiency and clean combustion,the mixing and combustion process of fuel and air was controlled by controlling varying injection rate.Therefore,it is very important to study the spray characteristics under varying injection condition.Diesel spray model is an important basic work in the study of diesel spray characteristics and it has become an important tool to analyze and predict the development of diesel spray and improve the mixing characteristics of diesel spray.At present,most spray models are only suitable to calculate the spray characteristic parameters under constant injection rate.Few phenomenological spray models have been developed for varying injection rate and the relevant spray propagation mechanism under varying injection rate is not clear enough.Therefore,the one-dimensional discrete spray model was optimized to make it suitable to calculate spray characteristic parameters under varying injection rate.The results calculated by the optimized one-dimensional discrete spray model were compared with the visual experimental results,one dimensional discrete spray model was proved to be correct for calculating under varying injection rate.Based on the optimized one-dimensional model,the spray characteristics were studied under the condition of constant injection quantity and various injection rate,then the spray characteristic parameters(spray tip penetration,entrainment rate,axial mean velocity and equivalence ratio)under different injection rate were compared and analyzed.During the study of the spray model,it is found that the effective injection velocity is varying with the injection velocity under the varying injection rate in diesel spray.The effective injection velocity was introduced into the calculation of one-dimensional model for the first time.The qualitative analysis between the effective injection velocity at the spray tip and the injection velocity at the nozzle under different varying injection rate were obtained and give the specific reasons for this change.In order to further prove the accuracy of the one-dimensional discrete spray model,the numerical simulation was performed to calculate the development of diesel spray based on three-dimensional CONVERGE software in this paper.By comparing the spray image obtained by CFD simulation and the spray image obtained by visual experiment under different cases,the accuracy of 3D spray CFD simulation was verified further.Then the spray characteristics under the condition of constant injection quantity and various injection rate were studied by 3D simulation software.By comparing the fuel concentration distribution calculated by the threedimensional simulation with the equivalence ratio distribution calculated by the onedimensional model,it is further proved that the one-dimensional discrete spray model is suitable for the calculation of varying injection rate.In the process of model research,it is found that if we can determine the specific functional relationship between the effective injection velocity and the injection velocity,it can lay a foundation for the development of the spray model under varying injection rate.Therefore,the quantitative response relationship between the effective injection velocity and injection velocity was derived based on the jet theory.Then the analytical formula for calculating effective injection velocity was applied to the spray model under the constant injection rate.A phenomenological spray model(zero-dimensional spray model)based on effective injection velocity was constructed and a new spray model under varying injection rate was developed for predicting and calculating the spray tip penetration.Then the results of the zero-dimensional model were compared with the experimental data,the results of the one-dimensional model and the three-dimensional model to further verify the rationality and accuracy of the zerodimensional model in the calculation of varying injection rate so as to provide a certain reference and theoretical value for the further analysis of spray development process and the development of new phenomenological spray model.