Fluidization Characteristics And CPFD Numerical Simulation Of Dense Gas-Solid Fluidized Bed

The air dense medium fluidized bed separation technology is based on Archimedes principle.Material in the bed will be difficult to stratify by density without a uniform density fluidized environment in space,as well as further sorting.The stratified material in the bed will be mixed again under the condition of bed density fluctuating in time domain,which certainly will results in an undesirable separation performance.Considering the air dense medium fluidized bed is used for continuous sorting of materials,it is necessary to study the fluidized state and the spatial and temporal distribution of local bed density with different parameters.Firstly,three factors,initial bed height,dense medium size fraction and fluidization number,were selected to investigate the effects on the distribution of bed density in space and tine domain.The results illustrated a high bed density(1.80-1.88g/cm3)near the side wall,and a lower(1.72-1.82g/cm3)in the intermediate region along the width direction.Correspondingly,there existed a relatively high bed density(1.75-1.84cm3)in the middle section along the bed direction and the bed density in the upper and lower part was close but slightly lower than the middle with 0.01-0.03g/cm3.A higher initial bed height facilitated maintaining the overall bed density in a narrow range.The increase of initial bed height mainly contributed to a more uniform bed density distribution in the horizontal direction,resulting in a reduction in the standard deviation of the overall bed density.The more a position close to the distributor,the more stable local bed density of the position will be in the time domain.Besides,a higher initial bed height made the bed density distribution shift to low density in temporal scope.Then,the effects of dense medium size fraction and fluidization number on the number,diameter and aspect ratio of the air bubbles in the two-dimensional air dense medium fluidized bed were studied by high speed photography and digital image processing.The results showed that the number of bubbles in the top and bottom zone of the bed was significantly higher.With the increase of the dense medium size fraction from 74-125μm to 200-425μm,the number of bubbles in the top zone decreased by 73%.The larger dense medium size fraction(dp=200-425 μm)made the range of bubble diameters in each region much wider,especially in the upper part of the bed.Compared to the middle and upper parts,the bubble diameter distribution in the lower part was more uniform.Finally,the fluidization behaviors in the two dimensional air dense medium fluidized bed was numerically simulated by using the computational particle fluid dynamics(CPFD)model,with the results of which compared with the widely used two-fluid model(TFM).The characteristics of the two models were summarized and analyzed.Combined with the experimental data,the Wen-Yu drag model was selected as the default drag model and the solid volume fraction,time-averaged axial velocity and back-mixing were studied according to the CPFD simulation results.