CFD study on solids flow pattern and solids mixing characteristics in bubbling fluidized bed: Effect of fluidization velocity and bed aspect ratio

Abstract

The effect of fluidization velocity and bed aspect ratio on solid flow behavior in bubbling beds was investigated quantitatively and qualitatively. Having used the two-fluid model based on the kinetic theory of granular flow, the set of governing equations was solved by applying finite volume method in two-dimensional Cartesian frame. In the bed with the aspect ratio of 0.8–1.5, Geldart's group B glass beads were fluidized by air at the fluidization number in the range of 1.5–3.5. The predicted solid flow pattern, and the axial velocity of emulsion phase were consistent with the experimental data presented by Laverman et al.. In the studied range, the simulation results demonstrated a strong dependency of solid flow pattern on fluidization velocity, while its dependency on bed aspect ratio was found to be marginal. Aimed at analyzing this solid flow behavior, the predicted solid flow patterns were attributed to the variation of solid mixing characteristics along the bed. Simulation results showed that the local and global mixing intensity increased along the bed from the distributor surface up to the position where two vertically-aligned vortices meet each other. Then, its value remained approximately constant up to the bubble bursting height, where a dramatic surge up to the bed surface occurs. Apart from this, an improvement in the solid mixing characteristics was observed by increasing the fluidization velocity leading to higher solid dispersion and diffusion coefficients, which come along with the top vortices elongation and bottom vortices shrinkage. Regarding the effect of bed aspect ratio, it was observed that the global mixing characteristics increase more significantly in comparison to the local ones for higher bed aspect ratios. Moreover, the simulation results showed that the predicted solid dispersion coefficient are in the range of 2.60×10−4m2/s to 6.18×10−3m2/s which is remarkably larger than the predicted solid diffusivity coefficients in the range of 1.31×10−4m2/s to 4.28×10−4m2/s. The dispersion coefficient changes more profoundly with the variation of superficial gas velocity.