Theoretical And Experimental Research On The Evolution Of Soot Particles In Diesel Engine

Soot particlesemitted from diesel engines brought serious harm on human health and atmosphere environment.The harm not only depends on the quality of soot but also largelydepends on the property of soot itself,such as particle size,transport characteristics,and optical properties,which are related to the microscopic structure and size distribution of the particles.The formation process of soot particles from the gas phase to solid phase is extremely a complex physical and chemical process,which has the little knowledge on this process at the current stage.Theoretically calculation andanalysis of soot and soot precursors formation can detailed understanding the growth evolution of the soot particles,can accurately predict the relevant information ofsoot number and size distribution,can be better protect the environmentand improve the atmosphere,and also provide a reliable basis for finding ways to reduce soot emissions and inhibit the soot formation in the combustion process.Firstly,the mathematical model of soot dynamical evolution processwas builtin steady laminar one-dimensional premixed flames,the method of momentsand Monte-Carlo was used to solve this model to obtain the particle size distribution at different height above burner,the effects of hydrogen and oxygenated fuel addition on soot formation are investigated.Results showed that the addition of hydrogen can slow down the formation of soot precursorinthe premixed flames,inhibit particles formation process due to the thermal effect,dilution effect and direct chemically inhibiting effect.Ethanol addition can accelerate the oxidation process of soot particles,then number of large particlesis reduced,the number of small particles is increased,and the peak of particle size distribution pushed forward.Secondly,the paper analyzed the defects of the current soot model that soot quality,quantity and soot formation region can not achieve the simulation accuracy simultaneously,a new eight-steps soot model has been built in this work.Compared to the existing soot models,the eight-step soot model uses a new nucleation species and improves the oxidation of particles to make it more in line with real particles oxidation process.Based on the above model,a complete soot particle dynamic model is built which includes the nucleation,coagulation,surface growth and oxidation.Two fuels,n-heptane and toluene,are used as an alternative fuel mechanism for diesel combustion and PAHs molecule formation mechanism is added so that it can more accurately predict the concentration of PAHs molecules in cylinder.Based on the chemical reaction mechanism described above,the soot model is coupled into Kiva-3v software for a three-dimensional CFD calculation.In-cylinder soot information was obtained in detail,and the constant volume chamber and two different engines are also simulated in order to give an extensive validation and comparison.The results showed that the model has the better reliability and accuracy.Finally,the morphology and size distribution of soot particles of diesel were studied experimentally.The soot size distribution is obtained in different conditions of diesel exhaust using the DMS500 particle analyzer for soot size distribution measurements;the soot formation in different injection timing is also measured.To further investigate the effects of ethanol added to diesel on the soot formation,the particle size distribution under different blend ratios of ethanol was measured,results showed that larger particles are reduced and smaller particles are increased.The collected soot particles from the exhaust pipe are observed in electron microscopy and the images are captured,which can be analyzed to obtain the fractal dimension of particles and other related information.The purpose of this paper is to give a deeper understanding of the physical and chemical processes of soot formation process theoretically and experimentally,the soot particles evolution in one-dimensional laminar premixed flame and three-dimensional turbulent flame are discussed and the soot growth process are researched more comprehensive,which has the great significance for particulating emissions reduction and improving the environment.