Numerical Simulation Of Supercritical Spray Characteristics And Impact On Compression Ignition Engine Performance

With the continuous development of engine technology,the increasingly strict emission regulations,the trend of lightweight and miniaturization of engines,the incylinder pressure and injection pressure are constantly increasing,and the environmental conditions in the engine cylinder at the time of fuel injection are much higher than most hydrocarbon the critical point of fuel thermodynamics.Research on supercritical spray and fuel combustion has also continued to penetrate the field of internal combustion engines.In the supercritical state,the cross section between the fluid and the gas no longer exists,the surface tension of the liquid surface and the latent heat of evaporation are close to zero,the density is close to the liquid,and the transport characteristics are close to the gas.In theory,it can reduce the problem of uneven local mixing in the cylinder,which can effectively optimize the combustion of the engine and reduce the emission of soot particles.Current research focuses on liquid engines in the aerospace field and gas turbine.The main content of this article is about the research on the combustion and emission performance of supercritical fuel injection and compression ignition engine under the background of internal combustion engine.In order to study the spray characteristics of fuel under supercritical conditions,a large eddy simulation turbulence model was used in CONVERGE software to build a spray model.The physical basis of the model was based on the n-heptane transcritical spray experiment conducted by Sandia Lab.The reliability of the model is verified based on the comparison of the fuel spray mass fraction with temperature change curve and the experimental value in the calculation results.The calculation defines transcritical spray as spray A and supercritical spray as spray B.Comparing the mass fraction,it can be seen that the flocculent structure appears at the front of the spray,the jet length of spray B is higher than that of spray A,and the core mass fraction is lower than that of spray B;compared with the temperature distribution,the spray has a significant temperature gradient,and the flocculent structure of spray B appears earlier;Density distribution,the core density of spray B is lower than that of spray A,the density changes faster,and the flocculent structure appears earlier;comparing the axial density of the two,there is a large density gradient between 0-2mm,and the change is more dramatic;the diameter of spray A In the density distribution,the fastest rate of density decline occurs between 0-0.2mm,and the density change between 0-0.5mm is extremely dramatic.It is speculated that gas-liquid phase change occurs in this interval;comparing the turbulent kinetic energy distribution,spray A and spray B The distribution is similar,with an asymmetric structure.Compared with spray A,the turbulent kinetic energy of spray B is significantly higher than that of spray A.And in order to study the influence of fuel on the engine under supercritical conditions,this article uses CONVERGE software to study multiple engine operating conditions under supercritical fuel injection and subcritical fuel injection.In this paper,we have selected a diesel engine model for supercritical / subcritical engine performance simulation based on n-heptane as the fuel for numerical calculations.From the average temperature in the cylinder,average pressure,turbulent kinetic energy,heat release rate,Nitrogen oxides,soot particles,and carbon monoxide were analyzed in several aspects.Studies have shown that the average pressure and temperature in the cylinder under supercritical conditions are higher than those in subcritical conditions.The temperature and pressure peaks in supercritical conditions are shifted forward at the same speed with the increase of the circulating fuel injection.The average turbulent kinetic energy in supercritical conditions is higher than that in subcritical conditions,and the turbulent flow in the cylinder is more intense.The heat release rate and cumulative heat release are also higher in supercritical conditions than subcritical conditions when the circulating fuel injection volume is the same.Compared with subcritical conditions,the quality of NO production under supercritical conditions is higher,the production of soot is reduced,and the production of carbon monoxide does not change much.