Mesoscale Modeling And CFD Simulation Of Gas-fluidized Bed With Binary Particle Mixture

Gas-solid bubbling fluidized beds have various industrial applications including olefin polymerization,coal/biomass combustion,mineral calcinations etc.In a gas-fluidized bed,particle clusters and bubbles are typical mesoscale structures.In particular,solid particles involved in practical applications usually have a wide size distribution or different densities,showing polydisperse characteristics and making mesoscale mechanisms more complex.Such polydispersity of solid particles leads to segregation phenomena in fluidized beds especially in dense fluidization operated in low velocity,thus greatly influencing the flow hydrodynamics and reaction performance.Understanding the segregation and mixing phenomena is crucial for the proper design and operation of fluidized bed units.Nowadays,computational fluid dynamics(CFD)is emerging as a powerful tool to help understand the complex hydrodynamics of multiphase flow.Among various approaches,the continuum model also called Eulerian approach is the most commonly used method for industrial applications due to its acceptable computational cost.In simulating the gas-solid flows,drag force plays a key role in capturing mesoscale structures.Many researchers pointed out that the drag is the important and dominant force in influencing the segregation and mixing behaviors of binary mixtures at low gas velocities.In the last decade EMMS(Energy Minimization Multiscale method)based drag modeling has been applied successfully to gas-solid fluidization.But most of the previous work is based on monodisperse systems.Further efforts considering the polydispersity is very necessary to truly understand the key features in binary fluidization because there is an evident difference in formation of mesoscale structures between monodisperse and polydisperse flow systems.As a preliminary exploration in this regard,this work aims to develop the EMMS based drag model for binary fluidization at low velocity.The related contents are described as follows.Chapter 1 is a literature review where a brief discussion about challenges in gas-solid fluidization like heterogeneity and typical features associated with polydispersity is presented.Chapter 2 discusses non-equilibrium features of a typical gas-solid fluidized system in terms of solid velocity distribution and how it breaks the local equilibrium assumption in modeling based on numerical experiments of a doubly periodic system and actual fluidized beds,and points out the necessity of mesoscale modeling for heterogeneous gas-solid fluidization.In Chapter 3,as the EMMS/bubbling model for the monodisperse system is considered as the basic framework for further modeling for the fluidized binary particle mixture,a thorough parameter analysis of the model is performed.The investigation of bubble diameter correlations is emphasized due to the importance of bubble characteristics in segregation and mixing phenomena in binary particle system.In Chapter 4,a new model extended from the EMMS/bubbling is established to describe the binary fluidization.Model equations are formulated and a solution scheme for the steady state flow is proposed.The preliminary model validation for the global flow behavior is then performed.In Chapter 5,the new model is used to calculate the drag coefficient for unsteady flow of binary fluidization by introducing the particle and bubble accelerations in force balance equations.The numerical scheme based on some assumptions is proposed and related parameter sensitivity analysis is performed to investigate the reasonability of the assumptions.In Chapter 6,the validation work for the new model by integrating it into the continuum model is presented where two bubbling fluidized beds having different segregations(i.e.,one system where solid components have different sizes and equal densities,the other system where solid components have different sizes and densities)are simulated.At last,conclusions and future work are presented in Chapter 7.