Experimental And Numerical Simulation Study On Characteristic Of Fuel Droplet And Spray Impact On Oil Film

With the miniaturization of GDI engine and the increase of injection pressure,the spray will impact on the piston or cylinder liner,and the oil dilution caused by spray impact on the cylinder liner will damage the engine.Spray impact on the oil film of cylinder liner causes oil droplets to peel off from the wall,which is considered to be one of the reasons of pre-ignition and super-knock.In this background,this paper carries out fuel spray impact on lubricant oil film experiment under normal atmospheric pressure and temperature,and the single fuel droplet impact lubricant oil film test and simulation,studies the interaction of fuel and lubricant oil,to explain in the oil dilution and pre-ignition formation reasons in engine and provide theoretical basis for improve the spray impingement model.Laser induced fluorescence（LIF）and high-speed photography were used in test,the oil flow characteristics after impingement classified into three categories:accumulation and dilution,central liquid column,and broken splash.The effects of injection pressure,film thickness and film viscosity on the wall collision were qualitatively analyzed,and the conclusions as follow:（1）spray impact on the lubricant oil film will dilute the oil and form a thicker liquid film.（2）improvement of injection pressure will promote oil droplets splash.（2）improvement of the oil viscosity will inhibit the flow and splash of the oil film.（3）When the film thickness increases to50μm,splash oil and central liquid column will formed.LIF and high-speed photography used in test of fuel droplet impact oil film,the test phenomenon can be divided into ring type,stable crown,crown and splash,broken crown and splash.Quantitative analysis the effect of fuel Weber number,viscosity of oilμ,dimensionless oil film thicknessδon dimensionless crown height H^*,and the conclusions as follow:（1）H^*increases as We increases,and broken crown and splash formed under the high We number.（2）The increase ofμwill reduce the maximum height of the crown and reduce the duration time of the crown.（3）As theδincreases,the maximum height of the crown increases and then decreases,reaching the maximum height atδ=0.3;the diameter of the crown rim is greater than the diameter at the crown bottom atδ=0.1,forming a broken crown and splash;the duration of the crown increases with the increase ofδ.（4）The splash threshold formula K=We Oh^-0.4δ^-0.3 and the splashing critical constant K=3548 were fitted.The Volume of Fluid（VOF）model and species transport model were used in simulation calculation of fuel droplet impact lubricant oil film,the velocity and pressure field and the mixing process were quantitative analysis,the conclusion as follows:（1）the formation of crown by dynamic discontinuity point located in mixture area of fuel and oil,lead to the radial velocity translate into axial velocity.（2）A high We number will increase the flow velocity during the impingement and cause a decrease in the mass fraction of fuel in the crown.With the increase ofμ,the flow velocity decreased,and the mass fraction of fuel in the crown increases.At the initial stage of the impact,the velocity field is mainly affected by We number,and at the later stage,the velocity field is mainly affected by theμ.As theδdecreases,the flow velocity increases and the mass fraction of the fuel in the crown increases,there is little difference in the velocity field betweenδ=0.5 andδ=1 during the initial impingement.（3）As the increases of radial velocity gradient in the mixture region of fuel and oil,the axial velocity of the neck jet increases.