CFD simulation for segregation behavior of a ternary mixture in a bubbling fluidized bed: effect of solid wall boundary condition

The effect of the solid–wall boundary condition on the segregation behavior of a sand ternary mixture differing in size, yet with the same proportion, has been investigated in a gas–solid bubbling fluidized bed. A multi-fluid computational fluid dynamics model incorporating the kinetic theory of granular flow has been used. The mass fraction profiles of different-sized particles along the bed height have been experimentally measured by freeze–sieving method. The simulation results of mass fraction distribution and segregation index have been compared against our experimental data in order to evaluate solid–wall boundary conditions in terms of specularity and particle-wall restitution coefficients. The analysis indicates that the specularity coefficients in the range of 0.5 to 0.9 lead to the satisfactory results, and the best agreement is obtained for ϕ=0.9, which corresponds to partial–slip wall boundary condition, while the particle–wall restitution coefficient has only negligible effect on the results. In addition, maximum segregation index occurs at specularity coefficient of 0.9 at which the segregation pattern may be affected by simultaneous mechanisms of particles circulation and bubbles rising. The effects of superficial gas velocity on the segregation behavior in bubbling regime have also been studied, and a significant reduction in segregation index has been observed with increasing gas velocity from 1.1 Umf to 1.3 Umf.