The influence of binary drag laws on simulations of species segregation in gas-fluidized beds

Abstract

Monodisperse drag laws have been traditionally employed in polydisperse systems using ad hoc assumptions due to the lack of adequate drag laws for polydisperse systems. A key component of both continuum and discrete models used to study segregation in gas fluidized beds is the drag law. In this work, both the ad hoc approach and a new drag treatment developed specifically for polydisperse mixtures using lattice-Boltzmann simulations [M.A. Van der Hoef, R. Beetstra, J.A.M. Kuipers, Lattice-Boltzmann simulations of low-Reynolds-number flow past mono- and bidisperse arrays of spheres: results for the permeability and drag force, J. Fluid Mech. 528 (2005) 233–254] are incorporated into a Multi-Phase Particle-in-Cell (MP-PIC) framework to evaluate their impact on simulations of gas-fluidized, binary mixtures. In particular, several systems composed of Geldart group B particles that differ in size and/or density are considered, with special attention paid to axial species segregation at low fluidization velocities. For a system with size difference only, the Van der Hoef et al. [M.A. Van der Hoef, R. Beetstra, J.A.M. Kuipers, Lattice-Boltzmann simulations of low-Reynolds-number flow past mono- and bidisperse arrays of spheres: results for the permeability and drag force, J. Fluid Mech. 528 (2005) 233–254] drag treatment presents a higher degree of mixing than the ad hoc treatment. For a system with size and density differences, where the small particle is the denser and less massive one, the ad hoc treatment predicts a higher degree of mixing than the Van der Hoef et al. [M.A. Van der Hoef, R. Beetstra, J.A.M. Kuipers, Lattice-Boltzmann simulations of low-Reynolds-number flow past mono- and bidisperse arrays of spheres: results for the permeability and drag force, J. Fluid Mech. 528 (2005) 233–254] treatment. For systems with density differences only or both size and density differences where the large particle is the denser one, both drag treatments generate essentially the same segregation profiles. The relative segregation tendencies predicted by each drag treatment is explained through a qualitative analysis of drag force coefficients of each species in the mixture. The simulation results indicate that the drag law treatment plays a crucial role in the qualitative and quantitative nature of segregation predictions at low gas velocities.