| Diesel exhaust particulates are one of the major sources of atmospheric pollutant PM2.5,which has a serious impact on the environment.China's fifth stage of the automobile emission standards impose stringent requirements on diesel engine particle quality and particle number?PN?.The quality and quantity of particulates emitted by diesel engines closely related to the process of particles mutual collision,adsorption and agglomeration.In this dissertation,the particle collision process and agglomeration characteristics of the diesel exhaust process are taken as the research object.By establishing the CFD-DEM coupling model,the parameters change law of particle collision probability in the exhaust process was discussed.The agglomeration characteristic parameters of the particles were measured and analyzed by some test methods,including particle size and number concentration distribution,structure and mechanical parameters.The main research works and conclusion are as follows:Based on the particle size distribution of the 186FA diesel exhaust particle,a typical particle size range of 25 to 50 nm was selected.Hamaker,Kittelson and other theories were used to establish a mathematical model for the interaction force between particles.The variation of van der Waals forces,electrostatic forces,adhesion forces,and adhesion energies among equal and unequal size particles were analyzed.When the particle size increases from25nm to 50nm under the same particle size,the results show that the van der Waals force increases from 1.08×10-15N to 6.43×10-15N,the electrostatic force increases from 1.11×10-17N to 17.89×10-17N,the adhesion force increased from 0.154×10-15N to 0.313×10-15N,and the adhesion energy increased from 0.151×10-15J to 0.309×10-15J;when the particle size ratio was increased from 1 to 2,the van der Waals force increases approximately 3 times,the electrostatic force increases 5.6 times,the adhesion force increases approximately 0.3 times,and the adhesion energy increases by approximately 0.4 times.According to the similarity theory,a CFD-DEM coupling model was established to simulate the particle collision process in the diesel engine air outlet area.The influence of boundary conditions such as exhaust pressure difference,exhaust velocity,and particle size dispersion was analyzed on the particle collision characteristics.The results show that the larger the exhaust pressure difference,the greater the probability of collision between particles and the collision efficiency,the maximum normal overlap distance,and the remaining overlap distance;the higher the exhaust velocity,the more drag and inertia of the particles,etc.Under the action,the speed of motion increases,the collision probability of particles and collision efficiency increase significantly,the maximum normal overlap distance increase,but the remaining overlap distance does not change significantly;with the increase of particle size dispersion,the particle size was from monodisperse to polydisperse changes,the collision probability and collision efficiency between particles increase,the maximum normal overlapping distance,and the remaining overlapping distance become larger.The intensification of collisions between particles plays an important role in increasing the probability of particle agglomeration and reducing the number of exhaust particles.The engine exhaust particle size spectrometer?EEPS?was used to test the particles at the outlet of the 186FA diesel engine air passage.The parameters such as particle size distribution and number concentration on diffierent loads and speeds were analyzed.The results show that under different loads and speeds conditions,the particle size was mainly concentrated in the range of 10100nm,and the particle size distribution was multi-peak;with the increase of diesel engine load,the average particle size of particles increase,and the proportion of nuclear particles decrease.The particles changed to the accumulation state and the particle size dispersion increased.The larger the speed of the diesel engine,the larger the average particle size,the smaller the number of nuclei particles,the more particles in the accumulation state,and the larger particle size dispersion.The increase in the load or speed of the diesel engine lead to the exhaust pressure differential or flow rate increase,and all this will make the particle collision probability and collision force higher.Small-angle X-ray scattering and atomic force microscopy were used to measure the pore structure,pore size,attractive force,and adhesion force of the particles at different loads and speeds.The results show that the qmin and qT parameters of agglomerated particles decrease with the increase of diesel engine load or speeds,and the average cross-sectional diameter,radius of gyration,axial length and fractal dimension of pores decrease.The attractive force Fat,the cohesive force Fad and the adhesion energy Wad,which characterize the stability of the agglomerate particle state,increase.The collision of diesel exhaust particles have a great influence on particle adhesion and agglomeration.Through the study,it can be seen that increasing the exhaust pressure difference,flow velocity and particle size dispersion can exacerbate the particle collision,which have a certain promoting effect on the increase of the density of the agglomerated particle structure,the attraction and adhesion enhancement.The more stable the bond between particles,the more agglomerated particles and the reduced number of exhaust particles. |
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