(149a) Drag Measurements from Particle-Resolved CFD Simulations of Flow Past Random Assemblies of Stationary Ellipsoidal Particles

Buettner, K. E., University of Florida
Sarkar, A., Worldwide Research and Development, Pfizer Inc.
Curtis, J. S., UC Davis
Currently-available literature on fluid-particle drag models for non-spherical shapes are focused on reporting the drag coefficients of individual particles, with very few studies on the drag experienced by an assembly of non-spherical particles. For multiphase-flow applications (e.g., fluidized beds), these single-particle drag coefficients are useful at the very-dilute limit but are generally inaccurate at larger solid fractions. New drag models must be developed at larger solid fractionsâ??as has been done for spherical particles, but not for non-spherical shapes.

In this work, we have performed single-phase computational fluid dynamics (CFD) simulations of fluid flow past random assemblies of stationary ellipsoidal particles. This approach follows the method used in previous studies [Koch and Ladd, J. Fluid Mech. (1997), 349, 31-66; Tenneti et al., Int. J. Multiph. Flow (2011), 77, 1072-1092, to name a few works] to derive drag laws for spherical particles, which is now adapted to ellipsoidal shapes. Our simulations are performed using Gerris, an open-sourced CFD code, which allows automated mesh-refinement of the particle-boundary layer. Using our simulation setup, we probe the influence of varying particle aspect ratio, orientation, solid fraction, and Reynolds number on the fluid-particle drag. We demonstrate that employing a spherical drag model for non-spherical particles misrepresents the actual drag. The particle orientation also plays a key roleâ??the drag is lowest for ellipsoids aligned perfectly with the flow direction and, as expected, and increases non-linearly as the angle between the particlesâ?? axes and the flow increases.

Current simulations of non-spherical particles, typically utilizing CFD-DEM models, apply ad-hoc corrections to spherical drag laws to account for non-sphericity. For example, Zhou et al. [Chem. Eng. Sci. (2011), 66, 6128-6145] calculate fluid-particle drag by substituting the drag coefficient Cd in a spherical drag law with a Cd correlation for non-spherical shapes. We show that such corrections are generally not accurate and, therefore, emphasize the need to develop new drag models specific to beds of non-spherical particles.