On the effect of subgrid drag closures

The effect of two subgrid drag closures on the flow of air and Geldart group A particles is presented in this study. A subgrid drag model based on fitting simulation data obtained from finely resolved simulations and a drag model based on the energy minimization approach are both used to solve a gas-solids flow in the riser section of a circulating fluidized bed. The numerical results using a coarse computational grid obtained with these subgrid models are compared with those using a standard drag model as well as experimental data obtained in a pilot-scale riser. Numerical predictions using both subgrid models showed higher solids holdup in the riser indicated by the radial solids density and axial pressure drop profiles in the 2D and 3D system geometries considered in this study. These subgrid models are demonstrated to be both needed and useful as large-scale numerical simulations commonly use coarse computational grids that are unable to resolve the smallest heterogeneous structures observed in the fluidization of small particles.