| Coal is one of the major fossil energy in the world,and is the foundation of the energy strategy and security of China.However,the environmental pollution caused by coal combustion restricts sustainable development of coal industry and clean coal technology becomes a must in recent years.As a crucial part of clean coal technology,the dry fluidized separation technology provides a favorite way for clean utilization of coal in arid and high altitude regions.Since the particles dynamics within gas-solid fluidized bed is complex and the bed density has typical temporal and spatial multi-scale effects resulted from the motions of bubble phase and emulsion phase,it is difficult to regulate the separation process of coal particles.With the improvement of computer capacity and optimization of relevant models and algorithms,numerical simulation has become an effective tool for studying the interactions of gas and particles.Therefore,in this thesis,the numerical simulation model aimed at the separation process of dry fluidized bed is developed.The multi-scale characteristics including particles motion at micro-scale,bubbles motion at meso-scale and pressure fluctuations at macro-scale are analyzed for exploring the control mechanisms of dry fluidized separation technology.The CFD-DEM method is used to study the segregation and mixing of polydisperse particles by coupling OpenFOAM and in-house DEM code.The accuracy of several polydisperse drag force models for simulating the segregation and mixing process of binary and ternary particles is assessed via against to the experimental data,the most suitable drag model is then selected.Furthermore,the size segregation mechanism of polydisperse particles is investigated and the difference of drag for polydisperse particles is the key factor influencing size segregation.Since there is a great amount of fine medium particles in gas-solid separation fluidized bed,coarse-grained DEM model is used to improve the computational efficiency.Several coarse-grained models available in literature are assessed using experimental data,in terms of axial solid fraction distribution,bed expansion ratio and pressure fluctuation characteristics.The model of EMMS-DPM was proved to be more accurate than others,but the coarse-grained ratio is better kept as less than 5.0.The relationships of pressure fluctuations originating from the particles motion of micro-scale,bubbles motion of meso-scale and bed expansion of macro-scale are finally studied,and the motion of bubbles affected pressure fluctuations largely.By integrating the selected polydisperse drag model,coarse-grained model(EMMS-DPM)and weighted function interpolation algorithm,a multi-scale numerical model of EMMS-DPM-DEM is developed to study the coal particles separation process in fluidized bed.The separation experiments of tracer coal particles are conducted and the axial distribution curve of coal particles are plotted to compare with simulation results,thus validating the accuracy of the proposed model.The dynamic forces of particles pressure gradient force and collisional force of fine medium particles determined the segregation of large coal particles.In chapter seven,the control mechanism of the motion of coal particles are proposed by regulating the density and diameter based on EMMS-DPM-DEM simulation.It was found that the pressure gradient force of fluid and the collisional force from medium particles influenced coal particles segregation by the mechanism of competition in compromising.The effects of scale-up of fluidized bed on coal particles segregation were investigated and increasing the bed size did not influence coal particles segregation when the uniform gas inlet of bed distributor was set.But the segregation degrees decreased a lot when the particles packing height increased.Besides,the back-mixing of fine medium particles on local region and the whole bed have distinct effects on coal particles segregation.The dissertation consists 83 figures,7 tables and 194 references. |
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