Comparative study of two fluid model and dense discrete phase model for simulations of gas-solid hydrodynamics in circulating fluidized beds

Computational fluid dynamics(CFD)has become a valuable tool to study the complex gas-solid hydrodynamics in the circulating fluidized bed(CFB).Based on the two fluid model(TFM)under the Eulerian-Eulerian framework and the dense discrete phase model(DDPM)under the Eulerian-Lagrangian framework,this work conducts the comparative study of the gas-solid hydrodynamics in a CFB riser by these two different models.Results show that DDPM could be used to predict gas-solid hydrodynamics in the circulating fluidized bed,and there are differences between TFM and DDPM,especially in the radial distribution profiles of solid phase.Sensitivity analysis results show that the gas-solid drag model exhibits significant effects on the results for both the two models.The specularity coefficient and the restitution coefficient in the TFM,as well as the reflection coefficient and the parcel number in the DDPM,exhibit less impact on the simulated results.

Numerical analysis and experimental validation of hydrodynamics of a thin bubbling fluidized bed for various particle-size distributions using a three-dimensional dense discrete phase model

A dense discrete phase model combined with the kinetic theory of granular flows was used to study the bubbling characteristics and segregation of poly-dispersed particle mixtures in a thin fluidized bed.Our simulations showed that in using the hybrid Eulerian-Lagrangian method,the common use of one computational cell in the thickness direction of the thin bed does not predict wall friction correctly.Instead,a three-cell discretization of the thickness direction does predict the wall friction well but six cells were needed to prevent overprediction of the bed expansion.The change in specularity factor(SF)of the model not only affected the predictions of the velocity of particles,but also had a considerable impact on their flow pattern.A decrease in SF,which decreases wall friction,showed an over-prediction in the size of bubbles,particle velocities,and void fraction of the bed,and led to a shift in the circulation center toward the bottom of the bed.The segregation of the Geldart B particles was studied in the narrow range from 400 to 600μm with a standard deviation less than 10%of the average diameter.Simulations showed that large particles accumulated close to the distributor at the bottom of the bed and the center of the bed,but small particles moved towards the wall and top surface.The decrease in the mean particle size and spread in shape of the distribution improves mixing by up to 30%at a superficial gas velocity of around 2.5 times the minimum fluidization velocity.Log-normal mixtures with a small proportion of large particles had the most uniform distribution with a thin layer of jetsam forming at the bottom of the bed.Finally,experimental verification of the segregation and mixing of polydisperse particles with narrow size distribution is suggested.

Experimental and numerical investigation of liquid-solid binary fluidized beds： Radioactive particle tracking technique and dense discrete phase model simulations

Liquid-solid binary fluidized beds are widely used in many industries. However, the flow behavior of such beds is not well understood due to the lack of accurate experimental and numerical data. In the current study, the behavior of monodisperse and binary liquid-solid fluidized beds of the same density but dif- ferent sizes is investigated using radioactive particle tracking （RPT） technique and a dense discrete phase model （DDPM）. Experiments and simulations are performed in monodisperse fluidized beds containing two different sizes of glass beads （0.6 and I mm） and a binary fluidized bed of the same particles for vari- ous bed compositions. The results show that both RPT and DDPM can predict the mixing and segregation pattern in liquid-solid binary fluidized beds. The mean velocity predictions of DDPM are in good agree- ment with the experimental findings for both monodisperse and binary fluidized beds. However, the axial root mean square velocity predictions are only reasonable for bigger particles. Particle-particle interac- tions are found to be critical for predicting the flow behavior of solids in liquid-solid binary fluidized beds.