Discrete particle-continuum fluid modelling of gas–solid fluidised beds

Abstract

This paper describes a fluidised bed model developed from the DEM-based Aston granular dynamics code in which the solid–solid interaction rules are based on theoretical contact mechanics thereby enabling particles to be directly specified by material properties such as friction, elasticity, elasto-plasticity and auto-adhesion. For the gas phase which is treated as a continuum, the equations of motion are evaluated with a Navier–Stokes solver originally developed for a two-fluid model. The coupling of the discrete particulate phase and the continuous fluid phase equations is an important attribute of such Eulerian–Lagrangian models and two forms of the coupling terms have been examined: one employing the pressure gradient force (PGF model) and the other a buoyancy force based on the fluid density (FDB model). Uniform fluidisation simulations for a superficial gas velocity of 2.5 m s −1 and bed pressure drop-superficial gas velocity simulations for gas flows from 0.3 to 3.0 m s −1 have been carried out in a pseudo-2D bed of 2400 4 mm -diameter spherical particles using the two model formulations. The two formulations yielded minor differences in qualitative fluidisation behaviour with a gas flow of 2.5 m s −1 . However, there were significant differences in the pressure drop-superficial gas velocity profiles in the fixed bed regime and corresponding significant differences in the prediction of minimum fluidisation velocity. The PGF model showed the best agreement with pressure drop-superficial gas velocity trends and minimum fluidisation velocities predicted by empirical correlations. Two new methods for determining the minimum fluidisation velocity have been introduced and found to give predictions in good agreement with those obtained from pressure drop-superficial gas velocity profiles and empirical correlations.