DEM-CFD simulations of fluidized beds with application in mixing dynamics

Discrete element models of fluidized beds allow the accurate reproduction of many aspects of the flow of the fluid and solid phases on both microscopic and macroscopic scales. In the present paper, we first discuss some of the basics of the DEM-CFD model, along with an illustration of areas where it proved successful. Then, a discrete element computational code is used to analyse in detail the key mechanisms governing the mixing of solids in fluidized beds. Air fluidization of a mixture of glass ballotini and steel shots is simulated, and the steady-state concentration profiles are successfully validated with experimental data. By changing the density of the heavier component, the effect of the gas velocity in combination with the density ratio is investigated in terms of an equilibrium degree of mixing and a characteristic time to reach this condition. The maximum mixing achievable is found to depend strongly on the difference of density. For mixtures with a density ratio close to 3, full mixing is practically impossible due to the presence of a non-mixable region, rich in the heavy component, at the bottom of the bed.