Numerical Research On Nucleation And Coagulation Of PM In The Exhaust Plume Of A Diesel Vehicle

With good quality of economy and power, diesel is more and more popular in the automobile application. However, diesel emission is the major source of air pollution in cities, especially particulate matter in the diesel exhaust. Current researches for particulate matter emission are mainly based on mass emission of particulate matter. However, the researches on the number concentration and size distribution of particulate matter are very limited. The foreign and domestic researches have showed that a number of physical and chemical processes will occur in the exhaust plume after diesel exhaust emitted in the atmosphere that can alter the number concentration and size distribution, including nucleation, coagulation and condensation. Therefore, the knowledge on number concentration and size distribution of particulate matter in the exhaust plume can be helpful for better understanding the harmfulness of diesel particulate matter, especially fine particulate matter to the environment and human health, which would be a theory basis for developing a efficiently control solution for particulate matter reduction.This thesis made a numerical simulation on forming of particulate matter in the exhaust plume of a diesel vehicle, including two main parts. The first part is fluid simulation on the dispersion of exhaust plume of a diesel vehicle by computational fluid dynamics software package Fluent in order to obtain the ambient temperature, relative humidity and concentration of carbon dioxide fields of exhaust plume. The second one includes the foundation of nucleation model of particulate matter in the exhaust plume based on the theory of binary homogeneous nucleation, the foundation of coagulation and condensation model in the particulate matter growing process based on the theory of molecule dynamics, and numerical calculation of particulate matter forming and growing process in exhaust plume based on the fluid results obtained in the first part as the initial condition in order to obtain the critical nucleus diameter, nucleation rate, number concentration and size distribution of particulate matter in exhaust plume by developing a Fortran code. Meanwhile, the effect of diesel fuel sulfur content, ambient temperature, relative humidity and exhaust dilution ratio on the critical nucleus diameter, nucleation rate, number concentration and size distribution of particulate matter is also analysed.The results shows that the secondary nucleation of particulate matter would increase the number concentration of ultrafine particles, and the coagulation and the condensation would contribute to particle growing and would alter the number concentration and size distribution of particulate matter in the exhaust cooling and dilution process. Diesel fuel sulfur content, ambient temperature, relative humidity and exhaust dilution ratio have a great affection on the secondary nucleation and growing of particulate matter in the atmosphere, not only affect the critical nuclei diameter and the nucleation rate, but also alter the number concentration and size distribution of particulate matter in coagulation and condensation processes. Secondary nucleation of particulate matter in the exhaust plume is mainly affected by ambient temperature and relative humidity around the exhaust pipe and is mainly affected by exhaust dilution ratio near the edge of exhaust plume so that critical nuclei diameter is comparatively bigger and nucleation rate is comparatively low around the exhaust pipe and near the edge of exhaust plume.