| The high-density circulating fluidized bed(HDCFB)is a novel reactor with low energy consumption and high efficiency.The performance of HDCFB is characterized by its excellent characteristics in gas-solids mixing,heat and mass transfer,and chemical reactions,which guarantees its wide applications in industries like chemical,energy,environment,etc.The purpose of this thesis is to develop a simulation approach for the HDCFB,to analyze the gas-solids global and local flow behaviors,to develop a characterization method for clusters,to quantitatively capture the physical characteristics and the spatial distribution of clusters,to track transient motion of particles,to quantitatively capture the backmixing behaviors of particles,and to develop a comprehensive flow-reaction model,and to finally provide theoretical basis for HDCFB reactor design and amplification,and to aid industrial applications for the HDCFB.Considering that the quasi two-dimensional(2.5D)simulation approach can simplify the 3D domain by using wedge area,a simulation method of gas-solids flow for the HDCFB was developed based on the 2.5D simulation approach by adopting the EMMSdrag model in the Eulerian-Eulerian scheme.The comparison between the simulation results and the experimental data under different operating condition indicated that the present gas-solids flow model was accurate in predicting hydrodynamic behaviors in the HDCFB.The gas-solids flow model was then applied to study the global and local gas-solids flow behaviors in the HDCFB.The difference between the LDCFB(low-density CFB)and the HDCFB was analyzed.The image processing method combining with the Matlab toolbox was used to analyze the simulation results obtained with fine mesh,and the physical characteristics and the spatial distribution of clusters were quantitatively analyzed.Results showed that,compared to the LDCFB,the HDCFB tends to result in more distinct S distribution tendency with respect to the axial solids concentration profiles,more non-uniform radial flow for the solids,and longer distance with respect to the feeding zone and accelerating zone.In addition,clusters tend to be present with smaller spherical coefficient and larger inclined angle in both the LDCFB and the HDCFB.More larger clusters tend to form near the wall region,and less smaller clusters in the center region.The spatial distribution of clusters area is wide.The clusters? number in the HDCFB is about twice of that in the LDCFB.Considering that the Computational particle fluid dynamics(CPFD)based on the Euler-Lagrange method can track particles transient motion,the CPFD method combining with the EMMS-matrix drag model was applied to quantitatively predict the particles backmixing behaviors.The effects of operating conditions,type of exit geometry(C-shape,L-shape and T-shape),and size of the exit geometry on the particles flow hydrodynamics and backmixing behaviors in HDCFB were studied.Simulation results demonstrated that particles backmixing mainly occurs in the bottom and near the wall region of the HDCFB.The operating conditions had a greater influence on the gas-solids flow and particles backmixing behaviors than the type and the size of the exit geometry.In order to develop an accurate model for the coupled gas-solids flow and chemical reactions,the ozone decomposition reaction kinetics were incorporated into the aboveestablished gas-solids flow model to finally develop a comprehensive flow-reaction model.A modification factor(?)was introduced to consider the effect of clusters.The suitability of the flow-reaction model was examined by comparing the predicted and experimental results with respect to the ozone decomposition at different operating conditions in the HDCFB.The flow-reaction model was then applied to investigate the effects of chemical reactions with variable volume on the gas-solids flow.Simulation results demonstrated that the variation of gas molar in reaction can have a significant influence on the gas velocity,the solids concentration,and the gas-solids flow nonuniformity.In order to weaken the influential degree of the gas volume variation in chemical reactions and to reach a steady operation,the inert components were introduced into the feed in the simulation,and the effects of expansion ratio on the gas-solids flow hydrodynamics were studied.The simulation results showed that for the reaction of-0.665 and 2.0 in expansion ratio,when the expansion ratio was above-0.25 and below 1.0,respectively,the solids holdup and the radial gas-solids flow non-uniformity were similar,which indicated that gas-solids flow changed slightly for the reaction through adding the inert components. |
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