| The gas-solid two-phase flows with different-sized and porous particles occur com-monly in nature and a variety of engineering and technical fields, and such complex flowsare involved in a variety of practical applications, such as atmospheric respirable particulatematter control. Due to the inter-phase and the inner phase interactions between the fluid andthe solid, combined with the complex pore structure of particles, significant challenges willarise when it comes to deep understanding of the flow pattern and internal mechanism ofthese complex fluid-structure interaction problems. Compared with the drawbacks encoun-tered by the traditional numerical methods (for example, the finite difference method, finiteelement method and finite volume method, etc.) in simulating such flows, such as diffi-culties in multi-scale coupling, expensive computational costs, low performance in parallelcomputing and complex treatment of the fluid-structure interactions, the lattice Boltzmannmethod (LBM), an established numerical method in recent decades, has been proven to bevery suitable for studying the gas-solid two-phase flows with unconventional particles dueto its microscopic and mesoscopic nature.Although some results of research on gas-solid flows with unconventional particleshave been achieved using LBM, there are still some basic issues that remain unresolved,such as the performance evaluation of the lattice Boltzmann (LB) models in simulations ofparticulate flows, the appropriate model that describes the motion of pore-structures particlesand the studies of sedimentation of pore-structures particles. In view of the above-mentionedproblems, the aim of this thesis is to improve and develop some models for gas-solid flowswith different-sized and porous particles within the framework of multiple relaxation times(MRT) LBE, and then apply them to conduct detailed studies of such flows. The contents ofthe present thesis can be divided into two parts:One is theory of the LB models:Firstly, the single-relaxation-time model for axisymmetric flows is extended to theMRT LBE model. This model not only maintain the simple form of external force with-out complex gradient, but also can improve the numerical stability.Secondly, an evaluation of performance of the three well-known LB models for par-ticulate flows are conducted in terms of accuracy, stability, efficiency and robustness. Thefindings are helpful for understanding the essence of these LBE models, and helpful forthe choice of proper models. In addition, the sedimentation of128circular particles in anenclosure is simulated to test the power of the MRT model to handle a particle’s group. Thirdly, a MRT-LBE model is proposed for the solvent flow outside and inside a porousmoving particle, which overcome the limitation of sufficiently small fluid velocity amongprevious research works. The present work improves the theory of LBE in the sedimentationof porous particles.The other is application of the LBE for simulating gas-solid flows with unconventionalparticles:Firstly, the proposed MRT axisymmetric LBE model is applied to simulations of theflow past two particles, and three effects, i.e., the gap and the diameter ratio between twoparticles as well as Reynolds number, on both the drag coefficients of two particles and thefluid fields around two particles are studied.Secondly, the drafting, kissing and tumbling (DKT) process of two nonidentical circu-lar particles is numerically studied by using the MRT-LBE model in this thesis. The effectsof initial distance and diameter ratio between two particles on the DKT process under twocases: the larger particle is initially above the smaller one, and the smaller particle is initiallyabove the larger one, are investigated in combination with the results for the sedimentationof two identical particles. The results reveal the transition modes of the DKT phenomenon.Thirdly, in the present thesis, a numerical study of the sedimentation of one and twopermeable particles, including porous particle and composite particle which is composedof a solid core and a surrounding porous shell, is performed using the above MRT-LBEmodel for the suspension flows of a porous particle. For one porous particle, the effects ofporosity and Reynolds number on the settling velocity and the drag coefficient of the particleis studied in detail. Besides these two factors, the thickness effect of the porous layer is alsoanalyzed for one composite particle. For two porous and composites particles, the effectsof porosity and thickness of the porous layer are investigated on the DKT phenomenon.The results show that the strength of hydrodynamic interactions among porous particlesdecreases with increasing their porosity, and that among composite particles lies betweenthe values among porous particles with the same porosity and among solid particles.To sum up, the flow problems with different-sized and porous particles are studiednumerically by the LBM, and the obtained results enhance the understanding of flow patternand internal mechanism of such complex flows. Several valuable efforts are made in thepresent thesis to promote the applications of LBM in gas-solid flows, and these works canserve as a solid basis for further studies. |
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