CFD-DEM Coupling Investigation Of Dense Two-phase Flow Mechanisms In Fluidized Beds

Dense gas-solid two-phase flow is frequently encountered in many industrial processes, such as energy, chemical and metallurgy engineering. Deep investigation of this aspect is extremely helpful for exploring the intrinsic transportation mechanism of solid phase and the interaction between these two phases. Most importantly, it can be used for the design and scale-up of the dense fluidizing reactor in the practical operation. In the Eulerian-Lagrangian coupling framework, this thesis developed a parallel numerical platform of computational fluid dynamics combined with discrete element method, which can be used to model the dense gas-solid motion in the fluidizing apparatus of complex structure. In the coupling approach, the gas motion is solved at the computational grid level while the solid motion is resolved at the particle-scale level. Furthermore, the coupling approach has been extended with the convective heat transfer between gas and solid phase, the conductive heat transfer between particles and the erosion of tube surface taking into account. Based on the numerical platform, the bed hydrodynamics in the several kinds of fluidizing apparatus are investigated. Meanwhile, with the advantage of discrete element method, the important details related to solid motion are explored at the particle-scale level. The contents of this thesis can be divided into the following three parts.The first part focuses attention on investigating the bed hydrodynamics, the heat transfer and erosion mechanisms of tubes immersed in the three-dimensional bubbling fluidized bed. Initially, the interaction between the gas-solid motion and bubble phase of the system are studied. Meanwhile, the dispersion and turbulent properties of solid phase are evaluated and the heat transfer property between gas and solid phases is researched. In the following, the strong coupling between the non-uniform distribution of heat transfer coefficient around the tube surface and the local distribution of gas-solid phase is explored in the bubbling fluidizied bed with one tube locating in its central region. Simultaneously, the contribution of different heat transfer mechanisms to the total heat transfer quantity is estimated. Finally, the influences of tube configuration on the preferential bubble path, solid concentration, the dispersion and mixing of solid phase in the system are explored and the quantity adopted to qualitatively predict the erosion distribution of tube bundle is proposed.The contents of second part is related to the transportation mechanism of solid phase in three-dimensional spout-fluid bed and the bed hydrodynamics of double slot-rectangular spouted bed with a partition plate. Initially, the interaction interface of spout-annulus in the spout-fluid bed is explored and the influences of operating parameters and bed geometrical configuration on it are studied. Meanwhile, the circulation behavior of solid phase and its dispersion and resident properties in three different region of the system are investigated. Then, the bed hydrodynamics of gas-solid motion in a three-dimensional lab-scale double slot-rectangular spouted bed are analyzed. Besides, the property of spout boundary and the interaction intensity of spout and annulus are studied. Finally, the influences of plate height on the gas-solid motion of three regions in the system are discussed.The third part explores the typical flow characteristics of gas-solid motion, the dispersion and resident properties of solid phase in the two chambers of three-dimensional internally circulating fluidized bed with a centrally located partition plate. Then, the influence of two-side return of bed material on improving the non-uniformity of gas-solid motion in the riser of three-dimensional full-loop circulating fluidized bed is quantitatively explored. At first, in the internally circulating fluidized bed, the gas-solid flow characteristics combined with the preferential bubble path and the preferential circulating path of solid phase in the system are analyzed. Moreover, the influences of operating parameters (such as fluidizing velocity and particle diameter) and the geometrical configuration (such as the baffle incline angle, the incline angle of right wall and the gap height) on the circulation time, resident and dispersion properties of solid phase are discussed. In the following, the typical flow characteristics (such as non-uniform distribution of gas-solid motion, typical core-annulus structure) of gas-solid phase in the three-dimensional full-loop circulating fluidized bed are studied. At the particle-scale level, the detailed information related to the dispersion, the resident property, the force and rotation of solid phase are investigated. Furthermore, the gas-solid motions in the cyclone, the standpipe and the L-Valve of the apparatus are evaluated. Besides, the effect of two-side return of bed material on improving the gas-solid uniformity in the riser of the circulating fluidized bed is quantitatively studied by comparing the related quantities in the system with one-side return. It is expected that the numerical conclusion can be adopted for the design and scale-up of the dense fluidizing apparatus.