Research On The Dynamic Evolution Process Of Soot Particles In Diesel Exhaust Pipe

Diesel engines are widely used in agriculture and industry because of its good economy and power,which also causes that the environmental pollution problems become more and more serious.Diesel emission is one of the main reasons of smog,in addition,soot particles can enter the lungs through the respiratory system and cause disease.Therefore,diesel soot emission has been a research hotspot in the field of combustion.Besides studying the soot particle number concentration,smoke,etc.,transmission electron microscopy is also used to observe the microscopic morphology and structure of the particles.This article took the soot particles in the exhaust pipe as the research object,and the soot particles at different positions before and after the exhaust pipe installed with insulation cotton were collected through the soot particle sampling device.Then the collected soot particle samples were observed by transmission electron microscopy and the particle morphology,structure and size parameters were statistically analyzed by post-processing software.The effects of diesel engine operating conditions,sampling locations and airflow temperature in the exhaust pipe on the morphology,structure,and size of soot particles were studied.Based on the fluid-orbit model,the Lagrange method was used to track the particle trajectory.The trajectory crossover,geometric collision rate and critical velocity were considered to build a dynamic evolution model of particles in a turbulent field.The morphology of soot particles in the exhaust pipe of a diesel engine was simulated and calculated under different operating conditions.The research method combining experiments and simulations was used to study the main factors affecting the morphology,structure and size of soot particles in the exhaust pipe.The paper obtained the following conclusions:(1)At the same engine rotation speed,with the increase of the braking mean effective pressure(BMEP)of the diesel engine,the rotation radius,the projected area of the soot particle,the number of basic carbon particles and the particle diameter of the basic carbon particles develop in a "V" shape.When the BMEP is 0.63 MPa,the value is the smallest.The particle size distribution of basic carbon particles is unimodal,and the range of peaks under each BMEP varies greatly.Soot particles gradually change from simple branches and chains to complex networks and clusters.The agglomeration of soot particles and the fractal dimension of the soot particles increases.At the same BMEP,the morphology of the soot particles does not change much with the increase of the rotation speed.(2)The rotation radius,projected area and number of basic carbon particles at the sampling point b are larger than that at the sampling point a,and the particle diameter of the basic carbon particles at the sampling point b is substantially larger than that at the sampling point a(Except BMEP is 1.13 MPa),this shows that the soot particles still undergo physical growth and change processes such as collision and agglomeration.In addition,the fractal dimension of the soot particles at sampling point b is greater than that at sampling point a.The soot particles are more agglomerated during the movement in the exhaust pipe.(3)After adding insulation cotton,under high load(BMEP is 0.88,1.13 MPa),the average diameter of basic carbon particles and the number of basic carbon particles contained in soot particles are larger than that without insulation cotton,and the average rotation radius of soot particles is reduced under any load.This shows that the rise of the temperature of the airflow in the exhaust pipe promotes the surface growth of basic carbon particles and enhances turbulence,making chain soot particles more easily agglomerated,so the rotation radius of soot particles decreases and the fractal dimension increases.The average fringe separation distance and tortuosity of carbon spacing also decrease.The degree of graphitization is strengthened,especially under high load.(4)The model constructed can well predict the morphology and fractal dimension of the particles after collision.The morphology of the particles is similar to the morphology of soot particles photographed by TEM in the experiment,and both show chain and cluster shapes,and the growth of the particles can be tracked.The increase in temperature has little effect on the collision frequency and particle size distribution of the particles.The particle size is mainly distributed from 60 nm to 80 nm,showing a unimodal distribution.However,the fractal dimension of particles increases with the increase of temperature,which indicates that with the temperature increases,the turbulent energy and the turbulent dissipation rate increase,making the particles more susceptible to distortion and deformation in a strong turbulent environment.In this paper,the morphology,structure,and influencing factors of soot particles in diesel exhaust are studied in depth through a research method combining experimental and simulation.The conclusions can provide theoretical basis and experimental basis for reducing diesel particulate emissions,and have important theoretical value and practical significance for improving environmental quality.