Hydrodynamic Simulation of Fluidization by Using a Modified Kinetic Theory

Abstract

For a pseudofluid consisting of a particle assembly, particle stress is transmitted through mutual contact between particles. When the particles are densely agglomerated, contacts are usually of long duration and frictional, and this part of the stress is the frictional stress. When the particles are sparsely spaced, on the other hand, contacts are temporary and collisional, 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 gas−solid fluidized bed, and all of these three parts of the stresskinetic, collisional, and frictional stressesplay important roles in particle-phase 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 the case of highly dense flow, and the solid shear stress was corrected to be consistent with Einstein's effective viscosity equation for dilute suspensions. 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 model were found to be in agreement with the measurements in a cylindrical fluidized bed. The inflection point on the particle pressure curve, implying competition among the three transport mechanisms, was also predicted. Moreover, instantaneous formation of slugs starting from a homogeneous inflow condition was reproduced through simulation and the quantitative comparison of the slug velocity with empirical correlation was approving. For dilute gas−solid flow in a circulating fluidized-bed riser, the model predictions agree with the time-averaged solid viscosity in order of magnitude. Further modeling may require a better understanding of the drag force and turbulence.

Simulation of gas–solid two-phase flow by a multi-scale CFD approach—of the EMMS model to the sub-grid level

“…A zero-gradient boundary was used for both phases at the outlet and granular temperature at the wall. The IMF scheme was used to solve the highly coupled model equations whose details can be found elsewhere (Harlow and Amsden, 1975;Wang and Li, 2001). The complete model equations and relevant computational settings were summarized, respectively, in Tables 3 and 4.…”

Two-dimensional coal combustion modeling of CFB

“…This implies that, bottom zone should be modeled in detail as two-phase flow. However, most of the models in the literature do not completely take account of the performance of the bottom zone, consider the bottom zone as well-mixed distributed flow with constant voidage, and use generally lumped formulation [1][2][3][4][5][6][7][8][9][10][11][12][13]. In the present study, bottom zone is modeled as two-phase flow which is subdivided into a solid-free bubble phase and a solid-laden emulsion phase [40,47].…”

“…The modeling has been dealt with in a number of efforts by either 1D, 1.5D, 2D or 3D approaches. The publications in the literature about the CFB modeling may also be subdivided into those that consider the hydrodynamic behavior [1][2][3][4][5][6][7][8][9][10], and the others that investigate the combustion phenomena [11][12][13][14][15][16][17][18][19][20][21].…”

“…Type III: models that employ fundamental equations of fluid dynamics to predict two phase gas/solid flow. They are based on gas and solid phase continuity equations, momentum balances and the appropriate constitutive equations (CFD models) [4][5][6][7][8]. Models [9] have given a comparative analysis of CFD models.…”

A multi-phase theory for the detonation of granular explosives containing an arbitrary number of solid components

“…The modeling of gas-solid flow within fluidized beds has been widely studied in the chemical and petroleum engineering communities because of its relevance to flows in reactors and vertical risers [43,44,10,27,26]. Gas flows in porous media have also been examined [45,20], and reactive dusty gas flows have been investigated because of their potential to detonate in the presence of an ignition source [23,49,40,22].…”