| Particulate matters with aerodynamic equivalent diameter below2.5μm (PM2.5) are theworst among suspended particles damaging the environment and human health, which havebeen on the list of environmental pollutants to be controlled strictly by many countries. PM2.5are the major component of automobile exhaust emissions, usually formed in the process ofcombustion or mechanical rolling. Studying the diffusion and change properties of this kindof particles has become a difficulty and hotspot in the field of energy and environment. Due tothe high automobile exhaust temperature, it is difficult to directly measure the compositionand diameter of particles in gas emissions at present. Hence, studying the effect of flow fieldcharacteristics on the diffusion law of ultrafine particle pollution among exhaust emissionshas vital significance to the environmental protection and sustainable development.This paper is mainly divided into two parts, one of which is simplifying the automobileexhaust into single inert gas, namely, using tracer gas method for numerical simulation andanalysis; the other is considering the physical or chemical changes including nucleation,condensation/coagulation, deposition of exhaust particulate, and studying the characteristicsof exhaust diffusion in different wake fields with numerical simulation.In the first part of this paper, propane (C3H8) is chosen as the tracer gas. First of all,under the same with speed conditions, study of the influence of different tail shapes on theexhaust gas diffusion without considering the crosswind effect has been carried on, then thesimulation results show that the exhaust gas diffusion is mainly affected by car wake structure,and the exhaust mainly diffuses upward in the section of XY because of the up-wash in thewake; Due to the lateral high-speed airflow, diffusion area doesn’t exceed the body contour;the emission spreads outside the body in the section of XZ due to the tail vortex; Secondly, Inorder to study the influence of different models on the exhaust gas diffusion properties, thecrosswind is set10m/s to simulation. Compared with the situation under no crosswind, great changes are that, the exhaust gas is diffusing from outward to inward of the body, what’s more,the diffusion on horizontal plane occupies a large area. Finally the effect law of differentdriving speed on diffusing is also analyzed with the notchback MIRA model, indicating that afaster speed the car runs, the larger the diffusion area will be, that is more conducive to thediffusion.It’s also worth mentioning that in this paper wind tunnel flow tests and pressure tests areconducted on the notchback MIRA model, then the exhaust gas diffusion characteristics andbody surface pressure coefficient are obtained, which verify the good coherence between theexperiment and numerical simulation.In the second part of this paper, the aerosol model (CTAG model) is adopted to considernucleation, condensation and coagulation, deposition of exhaust particles and a series ofphysical and chemical changes for studying diffusion characteristics. Firstly, diffusionproperties of exhaust particulates caused by different models are analyzed, drawing aconclusion that with certain limits the greater the dilution ratio is, the harder the nucleationwill be, and the turbulent kinetic energy contributes to particle nucleation; Then the dispersioncharacteristics of particulate matter at different speed is studied, and it also shows that theincreasing of the speed enhances the nucleation of particulates and the growth of soot-modeparticles; Finally in this paper, researches on the diffusion characteristics under crosswindcondition are also conducted, the results of which show that the crosswind go against thenucleation of particles. |
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