On the Effect of Sub-Grid Drag Closures

The effect of two sub-grid drag closures on the flow of air and Geldart group-A particles is presented in this study. A sub-grid 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 sub-grid 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 sub-grid models showed higher solids hold-up 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 sub-grid 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 Geldart group-A particles.