Numerical Study Of Drag Model And Gas-solid Flow Characteristics In Bubbling Fluidized Bed

As the fluidized bed has good characteristics of heat and mass transfer, it is applied in energy, petrochemical and process industries widely. During past few decades, the computational fluid dynamics(CFD) has been being developed to scale up the fluidized bed. How to describe the force exerted on the particles accurately, especially the drag force, is the key to numerical method. This project is concerned with the drag force between gas and particles. A modified drag force model is proposed, because of the drag reduction arising from particle compacting, and coupled with the Euler-Euler model to simulate the characteristics of the gas-solid flow in bubbling fluidized bed. Then, the effect of particle size distribution on gas-solid flow is studied. The following details and results are discussed.To begin with, a new equation of the drag force is derived. The mathematical model has following features: at the range of particle concentration from 0.1 to 0.38, the drag coefficient kgs is lower than Syamlal-O'Brien and Gidaspow models. This coincides with the experiment from literatures.Next, the simulations of gas-solid flow characteristics in bubbling fluidized bed are performed. The results show that the partial pressure drop predicted by modified model do better than Gidaspow and Syamlal-O'Brien models. With the increase of superficial gas velocity, the modified model can be more and more accurate on bed expansion. When superficial gas velocity is 0.36 m/s, the time-averaged axial velocity of particles predicted by modified model show much better than that of Gidaspow model. When gas velocity is 0.46 m/s, the particle concentration, got from the modified model, can be much better than Syamlal-O'Brien model. The equivalent diameters of bubbles are in consistent with the experiment. In terms of flow patterns, the modified model can give the vortex distribution, precisely.Finally, the population balance model(PBM) and the multi-size particle model are studied. We know that the multi-size particles model much overestimate the bed expansion, however, the result of PBM is little higher than the uniform size model. In addition, the equivalent diameter given by PBM is in good agreement with Werther model. With the increasing range of particle size distribution, the peak value of slip velocity between gas and solid shift to low particle concentration.