CFD simulation of bubbling fluidized bidisperse mixtures: Effect of integration methods and restitution coefficient

In this work we simulated the dynamics of a fluidized bed of a binary solid mixture using the isothermal multi-fluid model of the commercial CFD code Fluent 12. We focused the attention on the role of both restitution coefficient and integration methods on the dynamics of the bed, adopting a mixture of solids assorted in size (with constant density) as case study. We employed two methods of spatial discretization: first-order upwind scheme (FUS) and second-order upwind scheme (SUS). We investigated implicit versus explicit time integration methods as well. The numerical diffusion introduced by the FUS resulted in a low bubble fraction in the bed, in turn reducing solid mixing rates. Simulations carried out adopting the SUS were characterized by a reasonable bubble fraction and associated solid mixing rate. The latter method successfully predicted the transition to fully mixed, uniform fluidization conditions. The value of the restitution coefficient did not significantly affect the results of the simulations, as bubble volume fraction and jetsam concentration profiles did not show any significant change within the interval of restitution coefficient values investigated, with the exception of the nearly ideal value of 0.99.