Study On The Collision And Coagulation Process Of Particles In Diesel Engine

Diesel engine exhaust particles are one of the main factors causing atmospheric haze.China's fifth stage of the vehicle emission standards not only limit the mass of particulate matters but also made a strict limit for the number of particulate matters.Diesel engine particles are the product of incomplete combustion of fuels,composed of soot and soluble organic fraction after the chemical nucleation,physical collision and coagulation processes etc..In the process of the formation of diesel engine particles,the particle collision and coagulation process are two important procedures,having significant impacts on the particle size distribution and state characteristics.In this paper,the particle movement in the cylinder of diesel engine is considered,and the process and features of particle collision and coagulation are studied.The collision and coagulation phenomena of the particles were analyzed,and the conditions and evaluation indexes of collision and coagulation were put forward.The collision and coagulation process models were established,and the effects of particle size and initial relative velocity on the collision efficiency,collision frequency and coagulating time were discussed.Main studies and conclusion are as follow:Based on the similarity theory,the collision phenomenon of 186 F diesel engine particles was reproduced by visualization method,and the particle collision process of different piston positions was studied.The collision behavior of particle was analyzed.The results showed that the collision process of the simulated particles was "near-contact-separation or coagulation";In the case of the same number of particles,as the piston downed,the number of particles decreased,and the probability of particle collision declined.The diesel engine bench was established and the particles were collected by using a sampling device.Furthermore,the surface and spatial morphology of the coagulated particle clusters were measured by transmission electron microscopy and atomic force microscopy respectively.Factors affecting the coagulation of particles were analyzed.The results showed that in the coagulated particles,as the size and number of the individual particles increased,the size of the coagulated particle cluster was raised.The research on the variation law of the collision condition and collision process of diesel engine particles was carried out.Based on the regular distribution of particle size,the collision efficiency of diesel engine particles was studied.Based on the force analyses of diesel engine particles,the differential equation of the collision movement of diesel particles was established.The collision process of the equal-size and non-equal-size particles were discussed respectively.The impacts of initial relative velocity,the collision angle and the particle diameter difference on collision compression distance,collision frequency and other indicators were analyzed.The results showed that when the average particle size of diesel particles was 20 nm and the standard deviation was 3nm,the collision efficiency of diesel particles was about 10%.With the increasing of the collision angle,the maximum compression distance increased;The maximum compression distance of the collision increased with the increasing of the initial relative velocity;The larger the particle diameter difference was,the larger the maximum compression distance and more obvious the deformation became;With the particle diameter difference increasing,the particle collision frequency increased significantly.According to the characteristics of physical coagulation process,the model of coagulation rate of diesel particles was established,and the coagulation process was simulated.The influence of particle size,recovery coefficient,coagulation characteristic time on coagulation efficiency,coagulation critical velocity,coagulation time and excess surface area were analyzed.The results showed that when the particle recovery coefficient was 0.026,the coagulation efficiency was about 50.1%,which meant high efficient coagulation.When the coagulation time was the same,the larger the particle size was,the longer the coagulation time would be;When the particle size was in the range of 5nm~12.6nm,the coagulation time was sensitive to the change of the coagulation characteristic time;Under different coagulation characteristic times,with the increasing of the particle residence time in the cylinder,the excess surface area decreased fast at first then slow.The collision and coagulation process of particles have great influence on the scale and morphology.From the studies above,it can be noted that increasing the particle diameter difference can promote the number of collided particles;By improving the particle collision angle,relative velocity or particle size difference,particle coagulation will be facilitated,and the number of particles will be reduced;Smaller particle size will help reduce the particle coagulation time.