| Liquid-solid fluidized beds have been widely used in many industrial sections,such as energy,chemical engineering and environmental protection due to their inherent advantages of excellent particles mixing,high process transfer rates.There is a consensus that the liquid-solid two-phase hydrodynamics has a significant effect on the performance of mass and heat transfer and chemical reactions in liquid-solid fluidized beds.The hydrodynamics for homogeneous liquid-solid fluidization are explored by the method of computational fluid dynamic(CFD)combined with experimental validation in this thesis.The Eulerian-Eulerian two-fluid method(TFM)is employed to predict the hydrodynamic characteristics for homogeneous liquid-solid fluidization.Based on the experimental results of this thesis and relevant literatures,the influence of the drag models on the steady-state fluidization characteristics and the interphase force models on the dynamic fluidization characteristics are mainly investigated.Furthermore,the hydrodynamic performance of three kinds of TFMs are compared for the liquid-solid systems.The target of this thesis is to provide theoretical basis and basic data for the design,scale-up and operation optimization of liquid-solid fluidized bed.A laboratory-scale experimental setup of liquid-solid fluidization is designed and constructed for verifying CFD simulations in this thesis.Water and glass beads are used as liquid and solid two phases to investigate the hydrodynamic characteristics in this rectangular fluidized bed.A validation is performed via quantitative comparison of experimental overall solid holdup with that predicted by Richardson-Zaki correlation.The results within an average relative deviation of 5%agree with the predictions of Richardson-Zaki correlation,which validates the reliability of experimental setup and measurement method.The above experiments provide data to support the subsequent numerical simulation of liquid-solid fluidization.Based on the experimental results in this thesis,the hydrodynamics for liquid-solid fluidization under stable state are simulated using TFM with kinetic theory of granular flow(KTGF).The results show that the drag model has a considerable effect on the predicted time-averaged expanded bed height.On the contrary,the solid viscosity and coefficient of restitution derived from KTGF have a little influence on overall solid holdup.However,the drag model,solid viscosity and coefficient of restitution have weak effect on the characteristics of solid motion.The vertical and horizontal components of predicted root mean square(RMS)of solid velocity present opposite trends of "lower in the center and higher near the wall”and "higher in the center and lower near the wall",respectively.Moreover,the hydrodynamics for liquid-solid fluidization in the dynamic processes are predicted using TFM with KTGF,Five drag models,including Wen-Yu,Gidaspow,Syamlal-O’Brien,Dallavalle and TGS,are assessed in the contraction and expansion processes,and furthermore the influence of the classical lift models suggested by Moraga et al.and Legendr and Magnaudet on CFD predictions and the influence mechanism of main inter-phase forces are discussed.It is found that the response time of bed contraction process predicted by Syamlal-O’Brien or TGS drag model is more accurate than those by the others.Moreover,TGS drag model shows the most reasonable prediction in overall solid holdup.For the bed expansion process,TGS drag model gives more reliable prediction in the response time and overall solid holdup than the other models.As a consequence,TGS drag model demonstrates a better hydrodynamic performance in this section.The lift model has little influence on CFD results,and thus it can be ignored in the modelling of inter-phase force for the dynamic characteristics according to the homogeneous liquid-solid fluidized system.Selecting an appropriate drag model to predict the hydrodynamics is crucial to CFD modelling homogenous liquid-solid fluidization.For this purpose,nine drag models are assessed,which cover the particle Reynolds number(Res)from about 20 to 400 using a simplified TFM proposed by Brandani and Zhang.The time-averaged flow characteristics including the radial profiles of solid holdup and vertical solid velocity,expanded bed heights and overall solid holdup are presented and compared with the experimental data from relevant literature and this thesis.CFD results show the performance of drag models is strongly related to Res:BVK drag model predicts the radial profile of solid holdup very well at Res=210;Dallavalle drag model provides relatively accurate predictions for the radial profiles of solid vertical velocity at Res=300 and BVK drag model at Res=390;both TGS and Dallavalle drag models exhibit close agreements with the observations of the expanded bed heights at Res from 116 to 181.On the whole,either BVK or Dallavalle drag model shows superior performance based on the statistical analysis of predictions for time-averaged overall solid holdup.It has been found that the drag models derived from the particle-resolved simulations of flow over fixed spheres is more suitable for homogenous liquid-solid fluidization.Based on the experimental data from relevant publication,the hydrodynamic performance of homogenous liquid-solid fluidization is compared based on inviscid TFM A of Gidaspow,simplified TFM suggested by Brandani and Zhang and the TFM with KTGF.Clearly,the above three TFMs provide reasonable predictions for both the time-average overall solid holdup and the radial profiles of solid vertical velocity.However,the TFM with KTGF overestimates the radial profiles of RMS solid velocity,resulting in the solid motion to be nearly isotropic.Either the inviscid TFM A of Gidaspow or simplified TFM by Brandani and Zhang predicts more reasonably the radial profiles of RMS solid velocity and the level of anisotropy of solid motion,because the modelling of solid stress by these two TFMs are consistent with the characteristics of homogenous liquid-solid system where collisions between particles occur less frequently. |
