Numerical Study On Spray And Combustion Process Based On Eulerian Spray Model And Unsteady-Flamelet/Progress-Variable Combustion Model

With the development of computers,Computational Fluid Dynamics（CFD）numerical methods have become an important tool for engine research and development,especially the optimization of the in-cylinder combustion process in the current research and development of high thermal efficiency and zero carbon emission engines.The development of new combustion modes is inseparable from the numerical simulation analysis of the spray combustion process in engine cylinder,and the key of the the accuracy and reliability spray combustion numerical model is necessary.This paper develops a new?-Y Euler spray model and coupling the Unsteady Flamelet Progress Varaible（UFPV）turbulent combustion model for the compression ignition engines based on the Open FOAM^?platform,which is solved the issue of insufficient predictive ability of the existing spray model in the high temperature and high pressure environment and the difficulty of balancing the calculation accuracy and the calculation cost of the existing turbulent combustion model in the actual engine cylinder.In this paper,the?-Y/UFPV spray combustion model established for n-dodecane and PRF（Primary Reference Fuel,N-Heptane and Iso-Octane）.Analysis of the impact of characteristic accuracy and reliability.The influence of fuel injection operating conditions,environmental operating conditions and fuel attributes on spray combustion characteristics in high temperature and high pressure environments is analyzed in a refined manner based on this coupled model.The main research content and innovative conclusions of this paper are as follows:（1）Aiming at the evaporation process of fuel injection spray in the high temperature and high pressure environment,the diffusion interface method is used to construct the interface between the gas and liquid phases,and the?-Y Euler spray model is established to achieve a more accurate physical description of the dense near-field area of spray.For the fuel spray ignition and combustion process of n-dodecane and PRF,the one-dimensional opposing laminar diffusion flame surface is solved,and the turbulent combustion flame surface database table is built in combination with the probability density function,and it is coupled into the spray combustion three-dimensional CFD numerical code.The coupling of the?-Y Euler spray model and the UFPV turbulent combustion model can realize the high-precision and low-cost three-dimensional numerical calculation of the spray combustion process in the high-temperature and high-pressure environment of the compression ignition engine.（2）Different skeleton chemical reaction mechanisms are applied to the flame surface combustion model.The results of the flame retardation period,the distribution of chemical reaction source terms and the flame structure distribution obtained by the numerical of laminar flame show that the model can reflect all the information of mechanisms.The chemical properties can also finely capture the differences between different skeleton reaction mechanisms.The ignition time,location and duration of low-temperature combustion determine the development of the combustion flame throughout the combustion process.The ignition of low-temperature combustion starts from the oxygen-rich region and shifts to the fuel-rich region;while the high-temperature combustion starts from the fuel-rich area and moves to the stoichiometric ratio.The combustion characteristics are consistent between different skeleton mechanisms.The ignition nucleus under different conditions all appear first in the low mixture fraction region.The ignition delay can be increased by adding low-reactivity fuels（such as iso-octane）or reducing the ambient temperature and oxygen concentration.This method can be used to reduce the maximum combustion temperature;by increasing the ambient temperature and oxygen concentration,the flammability limit of the chemical reaction mechanism and the flame surface model can be effectively extended,that is,a larger scalar dissipation rate can also obtain a stable flame.（3）Based on the?-Y/UFPV spray combustion model,the numerical of the turbulent spray combustion process of n-dodecane fuel in a high temperature and high pressure environment shows that:the ignition delay and lift-off length predicted by the?-Y/UFPV spray combustion model are related to the chemical reaction mechanism;compared with the numercical results of the KH-RT/SAGE spray combustion model used in the current commercial software,the igniton delay and lift-off length predicted by the?-Y/UFPV spray combustion model are closer to the experimental results.It shows that the model has higher accuracy;among the five n-dodecane skeleton mechanisms analyzed in this paper,the Yao mechanism is the most consistent with the experiment in predicting the ignition delay and flame rise length.Different skeleton reaction mechanisms can accurately capture the spray combustion flame structure at the quasi-steady state,and the prediction of nitrogen oxides is not much different,but the prediction of soot is quite different.The?-Y/UFPV spray combustion model can also accurately predict the gas-liquid penetration distance,spray shape,high-temperature ignition delay,lift-off length and quasi-steady state of the two-component PRF characterizing fuel in the high-temperature and high-pressure turbulent spray combustion process The flame structure of the two components is characterized by increasing the proportion of low-reactivity component fuel in the fuel,which will lead to a longer flame retardation period,the position of the lift-off length is farther from the nozzle hole,and lower soot emissions.（4）Based on the?-Y/UFPV spray combustion model,analyze the influence of fuel injection conditions,environmental conditions and fuel attributes on the combustion characteristics of fuel injection spray.Research shows that increasing the ambient temperature and ambient oxygen concentration can extend the flammability limit,The ignition delay and the lift-off length are shortened;increasing the fuel injection pressure can effectively shorten the ignition delay,but the lift-off length will increase.The comparison between numerical simulation and experimental results further shows that the?-Y/UFPV spray combustion model has higher calculation accuracy under relatively low fuel injection pressure,high ambient oxygen concentration and ambient temperature conditions.（5）In the process of predicting the spray combustion characteristics in a high temperature and high pressure environment based on the?-Y/UFPV spray combustion model,the post-processing of the pray combustion characteristic parameters is the key step on analysis results.This paper proposes a method to reconstruct the numerical spray schlieren.This method can capture the transparent phenomenon of the spray head at the time of low-temperature ignition.At the same time,the development of the obtained numerical spray image schlieren profile over time is highly consistent with the experimental image,but The high temperature ignition delay based on the prediction of this method is slightly smaller than the ignition delay determined by the maximum temperature rise rate method.In the post-processing,using the Proj.-OH*can not only accurately capture the lift-off length,but also the spatial distribution of OH*in the experimental results:for n-dodecane,the lift-off length determined based on the OH mass fraction is basically same as the lift-off length obtained by the Proj.-OH*method,and for the PRF,although the numerical calculation results of the two-component fuel with a higher iso-octane content deviate greatly from the experimental results,but the two-dimensional distribution of lift-off length and OH*mass obtained by the Proj.-OH*mass method is consistent with the experimental results.