Research On The Dynamic Behavior Of Micro/Nano-particles Via Moment Method

The micro/nano-particles refer to particles larger than 1nm and smaller than 100, and micro/nano-particle-laden multiphase system investigates the dynamical evolution of these particles in the continuum phase and the interaction between these particles and the continuum phase. In addition to the transport effect from the fluid phase, micro/nano-particles in such system usually evolve undergoing nucleation or chemical reaction to form nucleus, Brownian coagulation or turbulent coagulation, turbulent shear breakage, condensation/evaporation, deposition and et al., Among all of the above processes, nucleation, coagulation, breakage and deposition are the most common in industrial production and surroundings. The object of the research of micro/nano-particle-laden system is to get all the dynamical information of particles in the above processes.Nowadays, most studies in micro/nano-particle-laden multiphase system focus on the highly diluted system, in which only the effect of fluid on the particles should be considered but neglect the effect of particles on the fluid. The number of particles in such system is too huge to track each particle through Lagrange method, even it is highly diluted, and from the point of engineering application, people pay more attention to the statistical information of particles, such as the detailed size distribution, number concentration, mean diameter, polydispersity and et al. Thus, Euler method is more appropriate. The most widely used Particle General Dynamical Equation (PGDE), which is based on the Smoluchowski mean-field theory, is a highly non-linear integro-differential equation and is difficult to be solved analytically. In this thesis, we will transform PGDE into moment equations via Taylor-expansion moment method (TEMOM) or standard moment method (MOM). All the evolution of particle information as time goes by can be got by solving the moment equation, and the evolution of these information in both time and space can be obtained by solving both the moment equations and the N-S equations simultaneously.Previous studies when referring to particle coagulation usually means Brownian coagulation, but when microparticles are referred in turbulent flow, turbulent shear induced coagulation and breakage dominate. In this thesis, the turbulent shear-induced coagulation and breakage kernels of zero inertial particle are used to derive the responding moment equations, which describe the evolution of microparticles in a planar turbulent jet flow, via TEMOM. These equations, coupled with N-S equations, are solved simultaneously to get all the dynamical information of microparticles in turbulent field.When finite inertial particles are referred, the coagulation kernel is different. It is necessary to consider the effect of turbulent transport and preferential concentration on the coagulation, which can be represented by radial relative velocity and radial distribution function of two parent particles. In this thesis, we conclude the DNS results of previous studies and correct the coagulation kernel of zero inertial particle, to get the coagulation kernel of finite inertial particles. This kernel is further employed in PGDE to get the moment equations of finite particles in a three dimensional turbulent channel flow. Four cases are simulated to research the effect of turbulent transport and preferential concentration on particle evolution.Aerosol particles from the emission of industrial production and generation of these particles are harmful to human health. In nature, they can be removed through the raining process. In this thesis, we express the collision efficiency proposed by Slinn as a polynomial function of particle diameter, and represent the scavenging coefficient as a function of particle diameter, raindrop diameter and terminal velocity of raindrop. The PGDE considering only the wet removal process is transformed into corresponding moment equations via MOM, which are solved to obtain the evolution of aerosol particles.Usually, particle size distribution (PSD) is represented using a log-normal distribution. But lots of experimental results show that PSD should be described as sum of two or more log-normal distributions. In this case, their exists coagulation in one distribution and coagulation between two distributions. In this thesis, we develop the moment equations of such PSD, which is represented as the sum of two log-normal distributions, in the free moleculor regime and (near) continuum regime based on the Muller's work, which describe PSD with one log-normal distribution, via MOM. The harmonic mean method is used to obtain the moment equations in transition regime. These moment equations are used to study the PSD evolution whose initial parameters are in accordance with experimental results.The current research has provided a deep understanding of dynamical evolution of micro/nano-particles undergoing turbulent shear-induced coagulation and breakage, a basis for studying the aerosol removal process, and a guidance when research referring to particle Brown ian coagulation with two or multimodal PSD.