Hydrodynamic simulation of fluidization by using a modified kinetic theory

For a pseudofluid consisting of a particle assembly, particle stress is tra nsmitted through mutual contact between particles. When the particles are d ensely agglomerated, contacts are usually of long duration and frictional, and this part of the stress is the frictional stress. When the particles ar e sparsely spaced, on the other hand, contacts are temporary and collisiona l, and this part of the stress consists of kinetic and collisional stresses . In many cases the particle contact lies between these two extremes in a g as-solid fluidized bed, and all of these three parts of the stress-kinetic, collisional, and frictional stresses-play important roles in particle-phas e transport. However, the existing kinetic theory for granular flow (KTGF) only involves the kinetic and collisional parts of transport. In this paper , a frictional particle pressure was introduced for correction of KTGF in t he case of highly dense flow, and the solid shear stress was corrected to b e consistent with Einstein's effective viscosity equation for dilute suspen sions. This modified KTGF model may account for the stress over the entire range between two extremes of a densely packed state and a sparsely spaced state. As verification in the dense gas-solid flow, the time-averaged total pressure drop and the particle pressure predicted by this modified KTGF mo del were found to be in agreement with the measurements in a cylindrical fl uidized bed. The inflection point on the particle pressure curve, implying competition among the three transport mechanisms, was also predicted. Moreo ver, instantaneous formation of slugs starting from a homogeneous inflow co ndition was reproduced through simulation and the quantitative comparison o f the slug velocity with empirical correlation was approving. For dilute ga s-solid flow in a circulating fluidized-bed riser, the model predictions ag ree with the time-averaged solid viscosity in order of magnitude. Further m odeling may require a better understanding of the drag force and turbulence .