Investigation of mixing behaviors in a spouted bed with different density particles using discrete element method

Abstract

This paper presents the mixing behaviors of binary particle mixtures with equal diameter and different density within a spouted bed by three-dimensional coupled computational fluid dynamics (CFD) and discrete element method (DEM).The particle motion is modeled by the DEM, and the gas motion is modeled by the k−ε two equation turbulent model. The numerical computation is based on a cylindrical spouted bed which the inside diameter, height and conical base are 200mm, 700mm, and 60°, respectively. Binary particle mixtures are composed of spherical particles with equal diameter of 4mm and the heavy-over-light density ratio ranges from 1 to 4. The mixing process, evaluation of mixing quality, particle circulation and distribution of particle concentration along both radial and axial directions are obtained on the basis of simulations. The mixing process is illustrated by the development of solid flow patterns with time. The results show that the process of particle mixing mainly contains three stages: macro-mixing stage, micro-mixing stage and stable mixing stage. The mixing quality is described by Lacy mixing index, and is evaluated by two parameters: mixing degree at the mixing equilibrium phase and the time required reaching the steady value. The effect of sample size act on the mixing degree is also investigated and an optimum size is found. The results show that the mixing quality increases with increasing of gas velocity and decreases with increasing particle density differences of the binary mixture. By comparing the typical trajectories for two tracer particles with different densities, the mixing mechanism is further analyzed. Besides, the mixing rate in radial and axial directions is characterized by the time required for the concentration of the mixtures constituent to maintain a basic equilibrium. It is found that the mixing process along the axial direction is slower than that of the radial direction, and in both directions, mixture with smaller component density difference shows a higher mixing rate and better mixing uniformity.