On the Subgrid Behavior of Accelerated Riser Flows for a High Stokes Number Particulate

We carried out two-fluid highly resolved subgrid simulations (SGS) to investigate the effective hydrodynamics of solid phases in riser flows. We considered a high Stokes number particulate, solid hold-ups and gas velocities typical of circulating fluidized bed combustors and gasifiers (CFBC-G), namely a 520 mu m size and 2620 kg/m(3) density particulate, average solid fractions from 0.015 to 0.09, and gas velocities from about 3 to 9 m/s. Periodic boundaries were applied in the gravitational direction with the flow driving force provided by an additional gas phase pressure gradient chosen to exceed the gravity acting on the local gas solid mixture. As a consequence, the simulations produced transient accelerated flows, giving rise to instantaneous field predictions and volume averaged effective parameters at growing gas velocities. The results showed significant effects of both gas velocity and solid hold-up over the flow topology, over the effective stresses and pressure of the solid phase, and over the effective drag. A reasonable agreement was found with a few empirical data that were available for effective shear stresses and effective drag.