Lattice Boltzmann Two-phase Model For Particle Laden Flow And Simulation Of Particle Capture Process By Fibrous Filter

Due to the "rich in coal" energy structure and rapid development of industry of our country, the inhalable particulate (aerodynamic diameter less than l0μm) gradually becomes the main environmental pollutant. Fibrous filters with the advantages of high particle removal ability and low cost are widely used in air purification in coal-fired power plant, mining/cement industry and other fields. However, the filtration process of suspended particles from the airflow is very complex because it involves various deposition mechanisms of solid particles, as well as the nonsteady evolution of filtration efficiency and pressure drop during the filter clogging. This dissertation established a Lattice Boltzmann (LB) two-phase model for particle laden flow and then used it to simulate the steady and non-steady filtration processes of fibrous filters with various fiber shapes and arrangements. Good understandings are provided in filtration processes of fibrous fibers, which is useful for filter design and arrangement optimization. The main points are as following:(1) The Lattice Boltzmann-Cellular Automation model was improved by using LB method to simulate the flow field and utilizing the Cellular Automation model to describe particle motion.The point-like particles move only at regular grid nodes as fluid particles, and the particle probabilistic position is determined by drag force, gravity and so on. The model is convenient to consider the two-way coupling, random trajectories of particles, dynamic boundary condition and parallel computing. Through simulating the classical experiment of particle laden flow over a backward facing step, it can be concluded that the LB-CA method is better than two-fluids model and achieves the similar precision with the LES-Lagrangian method.(2) The LB-CA model was used to simulate the particle capture process by circular section and non-circular section (elliptical, triangle, rectangle,trilobal and cross) fibers. The results of LB-CA model for filtration process of single circular section fiber agree with the existing theory and experimental results. It was proved that the LB-CA model is suitable for simulating the filtration process with dynamic boundary condition and complicated interphase reaction. With respect to the non-circular section fiber, this paper investigated the effects of fiber arrangements and provided fundamental basis for filter design and optimizing. It was found that, non-circular section fiber has larger capture domain which leads to higher capture efficiency for small particles and such efficiency is in proportion to aspect ratio and does nothing with located angle. When interception or inertial impaction is dominated, the capture efficiency of non-circular section fiber mainly depends on windward area, and the effects of orientation angle and aspect ratio can not be neglected. Optimizing the non-circular section fiber arrangement is nesscery to obtain higher capture efficiency than that of circular section fiber. On the basis of numerical simulation and Levenberg-Marquardt algorithm, numerical fittings were done for virous conditions (for different capture mechanisms, aspect ratios of elliptical fibers, orientation angles and packing density), then some simple empirical formulas were obtained to calculate the pressure drop and capture efficiency for elliptical section fibers. After comparing with the other existing models, these formulas were proved to be usable.(3) The variations of pressure drop and capture efficiency due to different fiber arrangements in multi-fiber filters were studied using LB-CA model. It was found that staggered model performs better than parallel model, especially for the particles with large inertia, althrough its pressure drop is greater. The capture efficiency and pressure drop increase with the space between neighbouring fibers along the streamwise direction, which leads to the decrease of quality factor. Through studying the capture contribution for different fibers, some advices can give to better optimize fiber arrangement. This paper first simulated the unsteady filtration process of multi-fiber filters, the growth and contact of dendritic structure, dynamic evolution of pressure drop, capture efficiency, and capture contribution of each fiber were investigated. It is found that, the "shield effect" of fore fibers is obvious in parallel model; in staggered model, the second row of fibers have the greatest capture contribution because of diversion of fore fibers. (4) Starting from the real structure of fibrous filter and polydisperse particle size distribution, the capture process of real fibrous filter in3D was investigated.Meanwhile, the growth process and structural characteristics of dendrite was discussed in detail. Compared with existing model, LB-CA model is good at considering the effect of fiber arrangement on filtration process. It is found that, when diffusion mechanism is dominated, the porosity of dendritic structure shows unimodal distribution and the average porosity is in inverse proportion to mass loading. However, if polydisperse particles have several dominated capture mechanisms (such as small particles dominated by diffusion, medium particles dominated by interception), the porosity of dendritic structure follows a bimodal distribution and the average porosity has exponent relation to mass loading. During the dust loading process, pressure drop and capture efficiency increase gradually, pressure drop also has exponent relation to mass loading, and meanwhile, capture efficiency reaches steady. However, capture efficiency based on the mass is often higher than that based on particle number, which means fibers are good at capturing large particles than small particles.In conclusion, the LB-CA model offers a useful tool for description of particle capture process by fibrous filters. Not only the steady process of filtration is well simulated through LB-CA model, but also the non-steady process including the growth of dendritic structure formed by deposited particles, evolution of pressure drop and capture efficiency is predicted. By investigating the motion of particles with various properties in the flow field where fibers are located in, the correlation between fiber configuration, particle properties, flow field information and capture performance is explained in detail. The approach can be used to optimize fiber arrangement under different working conditions.