Simulation Of Spray Combustion Based On Transported Probability Density Function Method At Diesel Engine-like Conditions

Under the background of energy shortage and environmental pollution,the diesel engine will develop in a more clean and efficient way.To develop new combustion technology in the diesel engine,the spray combustion process needs a thorough analysis and the detailed characteristics in the combustion process needs to be captured.At present,considering the average physical information got from the grid scale and the application range of the model,many of the combustion models applied to the research of the IC engines are not accurate enough in coupling with the chemical reaction mechanism.Those lead to a problem that the detailed characteristics of the combustion can't be captured.In recent years,the transported probability density function(transported PDF,or PDF)method has been applied to the engineering analysis.In this paper,the PDF method is applied to liquid spray combustion at diesel engine-like conditions,aiming at further revealing the complex transient process of spray combustion and providing theoretical basis for the effective control of the diesel engine spray combustion process.The PDF and WSR models are carried out to simulate the spray and combustion process,the specific research content is as follows.Firstly,the three-dimension physical models are built up and the non-reacting spray simulation analysis is carried out.The initial ambient temperature and pressure are close to the operating conditions at top dead center of diesel engine.The liquid spray penetration length and mixture fraction are analyzed and compared with experimental data.The results show that the two combustion models can predict the spray structure accurately,and the PDF model has a certain deviation in the prediction of the mixed fraction variance compared to the experimental results.Then,the reacting cases at different environment temperatures and oxygen concentrations are analyzed.The results show that the PDF model can accurately predict the macroscopic physical quantities such as the ignition delay and the flame lift-off length at high temperature and high oxygen concentration.By comparing the distribution of OH concentration using PDF and WSR model,it is found that the PDF model can capture the fluctuation information at the position with the intensity variation of reactions,which can better reflect the position of the flame surface.By comparing the difference of the quasi-steady state flame structure obtained by PDF and WSR model,it is found that the PDF model can more accurately predict flame structure with the reaction zone of high temperature and the high equivalence ratio.In the end,the quasi-steady state and transient flame structures under different conditions are analyzed.Under the quasi-steady state,as the reduction of the ambient oxygen concentration,both the maximum flame temperature and the temperaturedifference between the diffusion reaction zone and premixed reaction zone decreases.The decrease of the ambient temperature can result in the decrease of the equivalence ratio in reaction,especially at the condition of the low oxygen concentration.For transient flame structure,decrease of the initial environmental oxygen concentration can result in the increase of the fuel concentration in reaction,the ignition delay time and the mixing time,this is due to the convection and diffusion process become longer which suppress the flame propagation.And result also shows that equivalence ratio can't be reduced by only decrease the initial ambient oxygen concentration at the initial temperature rising zone.Decrease the initial ambient temperature can not only restrain the temperature rising at high equivalence ratio area,lead to the increase of ignition delay and decrease the premixed equivalence ratio,but also control the formation of the high equivalence ratio premixed reaction zone at quasi-steady combustion flame structure.According to the researches above,the ambient temperature and oxygen concentration are the key factors in controlling the initial temperature rising at the high equivalence ratio zone,and play important roles in clean and efficient combustion in the diesel engine.In this study,it shows that the PDF method can be widely used in the spray combustion simulation of diesel engine.