| With the quality of economy, power and reliability, diesels have always been a hot issue in the research area. However, its development is being cramped for the pollution caused by its exhaust, especially large amount of particulate matter (PM) in it, which will be the main factor to affect its development in the future. In the past, most researchers focused on the growth of PM in chamber, but overlooked the exhaust drained off into the air. Some foreign investigations have showed that the form of exhaust plume can change remarkably in the air, and then secondary nucleation will take place in that form, which will much affect final formation of PM. Studying the process of PM changing in the air, will help us identify its danger and control it. Therefore, the secondary nucleation of PM is studied in this research.With the effect of the air, different compositions of PM will be condensing, which cause nucleation. After studying the process of nucleation and growth, and some theories on coagulation and deposition both abroad and at home, the author suggests that the growth of PM is the combined effects of gas phase reaction kinetics and solid particle kinetics.In this essay, the author studies the H2O-H2SO4 systems, which are most prone to nucleation. In the research of nucleation, the theory of homogeneous binary nucleation is applied. As for coagulation and condensation, molecular dynamic theory is used. Besides, the implicit governing equations are figured out through the method of finite discrete and semi-implicit.By working on the nucleation of H2O-H2SO4, the author makes out such variables affecting the nucleation rate and its diameter as the amount of the fuel sulfur content, environment temperature, and relative humidity, etc. In addition, by imitating the reaction processes of nucleation, coagulation and deposition, the author studies the function of relative humidity and the amount of sulfur in the growth of PM, obtaining these factors'influence on PM's number density and size distribution. The research also shows the influences of nucleation, coagulation and condensation at different stages in PM's growing process. Nucleation mainly affects the number density and its diameter of initial nucleated PM's; coagulation makes the density smaller and size bigger; while condensation makes the diameter continual grow. All the three reactions are happening simultaneously. Because of the competition between nucleation and coagulation, the number density and size distribution appears like bimodal.At last, there are descriptions on coagulation and deposition of the other submicron particles and nanoparticles such as carbon particle and candle particle, both of which can be described with solid particle kinetics. By comparison, it shows the size distribution of these particles appear like bimodal, too. |
