| Gas-solid two-phase flow is commonly encountered in both industrial manufacture and daily life. This thesis mainly focuses on the dense gas-solid two-phase flow, describing the flow dynamics based on the Eulerian-Lagrange framework and using discrete element method coupled large eddy simulation (DEM-LES) as investigation tool. Based on the proper procedure of numerical validation and parameter sensitivity analysis, the DEM-LES is used to study some basic issues in the conceptually-similar gas-solid fluidization system—internally circulating fluidized bed (ICFB) and circulating fluidized bed (CFB). This thesis is mainly constituted by the following four parts.The first part is the influence of gas-solid properties on the operation of ICFB. Systematic study on the relevant gas-solid properties (particle restitution coefficient, friction coefficient, Young modulus, diameter, density and gas pressure), operational condition (aeration to the chambers) and design parameters (gap height beneth the baffle, incline angle of the baffle and HEC side wall, and so on) are conducted to investigate the solid circulating flux (SCF) and gas bypassing flux (GBF), which are the typical representative of operation state of the facility. The results obtained indicate that the minimum fluidization velocity (Umf) is a key parameter to predict the fluidization state of each chamber and the performance of the bed. Meanwhile, these numerical experiments provide some valuable information for the determination of appropriate operation condition, optimization of design and intensification of relevant process inside the rig.The second part is the mixing mechanisms and the influence of different factors on the mixing of ICFB. Progress of solid mixing inside the ICFB is analyzed. Different stages of mixing and the temporal and spatial range of each mixing mechanism are distinguished. Besides, relevant design, operation and solid parameters’influence on the mixing is quantified. Based on the same aeration, process of solid mixing in the ICFB and the corresponding evenly-aerated bubbling fluidized bed is compared, and the ICFB is found to be more advantage on the solid mixing.The third part is the study of ICFB equipped with immersed tubes. Solid flow dynamics and solid mixing process inside the ICFB with immersed tubes arc quantitatively analyzed. The solid cycle time between the chambers and solid residence time in each of the chambers are comparatively studied for the influence of different number of immersed tubes and the position where they are located. In addition, the circular distribution of tube erosion is extracted and analyzed, and the obtained rule can provide some reference for the wear-proof of such facilities.The last part is the3-D simulation of detailed gas-solid flow dynamics inside a CFB, and the influence of riser cross-section shape on the characteristics of gas-solid flow. A trial to uncover the details of gas-solid flow in the CFB is conducted using numerical method. The distribution of time-averaged solid holdup, pressure, turbulent intensity of solid velocity and gas-solid velocity and its corresponding flux is discussed from whole to local. Emphasis is focused on the comparison of the similarities and differences between the gas-solid flow dynamics of the facilities equipped with square and circular risers. Additional calculation of solid dispersion and its relevant stress and rotation is carried out, and the variation of these parameters under the influence of different fluidization gas velocities is analyzed. Finally, the possibility that whether this information can be used as judgement to the transition of flow pattern is discussed. |
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