Research On Nanoparticle-laden Multiphase Via Moment Method

The main objective of the work investigated in nanoparticle-laden multiphase system is to develop a framework for predicting evolution of nanoparticle dynamics in continuum phase, and revealing the interactions between particles and fluid. Here, particles are usually smaller than 1μm and bigger than 1nm in diameter. From fundamental physics, the theory for describing nanoparticle dynamics ranges from simple kinetics theory to continuum theory, and Stokes's law can't be directly used in this problem. In fact, these systems occur commonly in industries and our surroundings, and the conversion from gas to particle and the subsequent particle growth are usually our interests. Generally, nanoparticle dynamics we need to focus on include nucleation or reaction to form nucleus, coagulation or condensation growth, turbulence shear breakage, transport by fluid and et al.Nowadays, most studies in nanoparticle-laden multiphase in the world are limited in highly diluted systems where the interaction between particles can be ignored. For this problem, the most used theory is based on Stokes's, Einstein's and Smoluchowski's works, and the Smoluchowski equation (SE) is the direct and valid description in mathematics. However, the SE equation is a highly non-linear integro-differential equation which is difficult to be solved analytically. In this thesis, the SE equation was firstly converted by moment theory and then closed by Taylor-series expansion technique, and finally a new moment method named TEMOM was proposed. The results showed that this new approach can be used to solve SE equation undergoing Brownian coagulation with sufficient accuracy, but only less computational cost is needed.Based on particle size, there are simple kinetic theory and continuum theory to dispose particle collision problem, respectively. However, it can't be guaranteed all the collision problem involved in the entire size regime can be solved by these two theories. In this thesis, we used the Harmonic mean solution and Otto solution to dispose TEMOM equations in the free molecular regime, Stokes regime and Near-Continuum regime, respectively, and finally obtained the most appropriate method covering all the size regime, TEMOM_Otto method. Based on this find, the TEMOM_Otto method is supposed to be used in the future Brownian coagulation calculations.Nowadays, high nanoparticle emission has been a serious problem, but a great development can't be obtained all the time due to the extraordinary small scale time for particle dynamics evolution and the extraordinary small scale space for particle size. For the first time, this thesis investigated binary homogeneous H₂SO₄-H₂O nucleation and nanoparticle growth in the engine exhaust by using our new developed TEMOM method. This work concentrated on the fundamental effect of fuel sulfur content, environment temperature, relative humidity and upstream turbulence intensity on particle formation and growth, and gave us some guidances for avoiding particulate pollution, establishing emission limit and et al.In the synthesis of nano solid particles, it is the critical technology to control the size and morphology of particles through adjusting the variables of flow and injected precursor concentration. Based on Euler's thought, the two_variable solution in lagrange form disposed by Xiong & Pratsinis was first to be used in QMOM method to predict the synthesis of TiO₂ in diffusion flame, which could help us to further understand the nature of evolution of nanoparticle dynamics in flames, and even gave some advices on how to control nanoparticle properties.In recent years, nanoparticle dynamics in high solid concentration are in the forefront science, in which the classic Smoluchowski mean-field theory can't be further used because the interaction between particles can't be ignored. In this thesis, a new developed overall collision frequency function was obtained by modifying Stokes's drag theory and osmotic pressure theory. This work significantly extended the investigation scope of the classic Smoluchowski theory.In conclusion, the main investigations in this thesis concentrate on how to enhance the abilities of moment method to resolve problems, how to extend the scope of the classic Smoluchowski mean-field theory, and how to deal with the effect of flow on evolution of nanoparticle dynamics in particular circumstances. The achievements of the thesis are expected to help researchers better understand the nature of nanoparticle dynamics in evolved flows, especially for nanoparticle manufacture industries.