Accelerating the Convergence of Multiphase Flow Simulations when Employing Non-Uniform Structured Grids

Non-uniform grids inevitably arise in multiphase flow simulation scenarios due to the need to resolve near-wall phenomena and/or large L/D ratios associated with the reactor configuration. This in conjunction with large density ratios of the constituent phases can retard the convergence of the pressure-correction equation that results from applying operator-splitting methods to the incompressible Navier-Stokes equations. Various pre-conditioning strategies to this ill-conditioned pressure-correction matrix are explored in this study for a class of bubbling bed simulations encompassing: different particle densities, bed-heights and dimensions (2D/3D). The right-side Block Jacobi preconditioning option resulted in a 20 - 35% decrease in CPU time that correlated well with a decrease in the number of iterations to reach a specified tolerance.