The Hydrodynamic Behavior And Fluid Structure In Coarse And Fine Particles Fluidized Bed

Fluidized bed is one of the most important reaction equipment with high gas-solid contacting efficiency,and widely used in mineral processing,such as direct reduction of iron ore,magnetizing roasting of low-grade ore,and reduction of pyrolusite at low temperature.In the practical mineral applications,the grinding processing is a necessary pretreatment,which always leads to particles with a wide size distribution(or polydisperse)rather than narrow size distribution(or monodispersed).Meanwhile,the gas-solid flow is a multi-scale complex system with meso-scale structures such as clusters and bubbles.Traditional two-fluid model(TFM),neglecting sub-grid heterogeneous structures,fails to predict the hydrodynamic characteristics,reaction and transport behaviors in fluidized beds.In order to improve the numerical simulation of fluidized beds,it is necessary to take account of the sub-grid heterogeneous structures in the drag model.The multi-scale drag model based on fluidized bed structure-transfer theory enables us to solve the dense-and-dilute two-phase distribution in fluidized beds effectively and quantify the sub-grid drag reduction induced by heterogeneous structures reasonably,and hence has been widely applied in numerical simulations of fluidized beds.In order to take the polydispersity and meso-scale structure into consideration,the drag model based on structure-transfer theory for polydisperse gas-solid flow is proposed in this thesis.Then the structure-based drag model is combined with the continuum model to conduct the CFD simulation for the bi-disperse gas-solid flows.Simulations were conducted to verify the effectiveness of the proposed models.The main points of this thesis are sketched out in the ensuing paragraphs.In the second chapter we studied the effects of correction factors of drag coefficient for polydisperse system on the effective inter-phase drag coefficient of bubbling gas-solid flow within the framework of structure-momentum transfer model,and their consequent effects on CFD results.Although the drag coefficients for individual solid phase were affected much by the choice of correction factors of drag coefficient for polydisperse system,CFD results were in a reasonable agreement with experimental data due to the proper consideration of meso-scale structural effect.The structure-momentum transfer model for bi-disperse bubbling gas-solid flow were constructed in the second chapter.Then the structure-momentum transfer model were combined with the continuum model to conducted simulations for the bi-disperse bubbling gas-solid flow experiments.The results of simulations indicated that new structure based drag model could predict the mixing and segregation of the fine and coarse particles better.Based on structure-transfer theory and homogenous distribution assumption in sub-phases,the structure-momentum transfer model for bi-disperse turbulent gas-sold flow and bi-disperse fast gas-solid flow were established respectively.Two-fluid modeling,integrated with the new drag model,indicated that it could reasonably predict the hydrodynamic characteristics of turbulent fluidized beds and circulating fluidized beds.A new semi-empirical particle-particle drag coefficient was proposed.The"hindrance effect" was taken into consideration by combining the friction stress with conventional particle-particle drag model.Through CFD simulation,the optimal value for the empirical factor is verified.Finally,conclusions and perspectives are given.