CFD simulations of segregating fluidized bidisperse mixtures of particles differing in size

In this work, we use a multifluid model to investigate numerically the dynamics of segregating fluidized bidisperse mixtures. The model uses the default multifluid equations of commercial computational fluid dynamics (CFD) codes, except for the fluid–particle drag force closure, which Mazzei and Lettieri [1] recently developed and extended to polydisperse systems. The study also comprises some preliminary simulations of collapsing monodisperse beds, focusing on the role of the plastic solid stress. This work tests whether the model is able to correctly predict not only the axial segregation profiles through the bed, but also the minimum fluid velocities at which a) the mixture, being no longer fixed, starts segregating and transient fluidization takes place, and b) the mixture becomes steadily fluidized and fully mixed. To validate the model predictions, we use the experimental findings of Marzocchella et al. [2]. The plastic stress results to play an important role, rendering the simulations more stable and allowing for larger time steps. The model well predicts the stationary axial segregation profiles, and for short computational times estimates correctly the onset of transient fluidization; for longer computational times, however, the system evolves towards a new steady state where, even if the powder is at maximum packing, it partly segregates. The model overestimates the velocity required to fully mix the suspension, probably because the simulated bubbling is not as vigorous as it is experimentally.