Experimental Investigation and CFD–DEM Simulation of Solids Mixing in Tapered Fluidized Beds

In this article, the mixing process of binary solid mixtures was investigated in tapered gas–solid fluidized beds through a combination of experimentation and CFD-DEM simulation. Tapered beds exhibit unique dynamic characteristics due to the velocity gradient in the axial direction, which distinguishes them from cylindrical or columnar fluid beds. To explore this, experimental runs were conducted by using a pseudo-3D tapered fluidized bed with an apex angle of 8.13° and a height of 0.9 m. White and red solid particles, with the same density but different sizes and average diameters of 500 and 1000 μm, respectively, were used. A digital image analysis technique was employed to evaluate the mixing index. In addition, the study examines various factors, including the inlet gas velocity, percentage of heavier particles required for minimum fluidization velocity, and mixing degree using the Lacey index. Moreover, the tapered fluidized bed was simulated using the Eulerian–Lagrangian approach, where the solid motion was modeled by the discrete element method and the gas flow was described by the Navier–Stokes equations. The transient simulations yielded a mixing time that agreed with the experimental data. The experimental data clearly indicate that the Umf of the solid mixtures falls between that of the flotsam particles and the jetsam particles. Furthermore, the previously established single-particle correlation was extended to binary solid mixtures in a tapered fluidized bed. The obtained results also revealed that increasing the inlet gas velocity enhances the mixing process, leading to a quicker attainment of the equilibrium mixing index. For example, with a jetsam mass fraction (Xj) of 0.47, the complete mixing times at U0 of 0.52 and 0.67 m/s were 21.2 and 4.41 s, respectively. Additionally, the present work provides valuable data for the numerical modeling of tapered fluidized beds, especially when dealing with mixtures of different particle sizes.