Simulation of Bubble and Particle Interactions with Vortical Flows Using a Discrete Element Model

Discrete Element Model (DEM) is used to simulate bubble and particle interactions with vortical flows. In this Euler-Lagrange approach, volume-averaged Navier-Stokes equations are solved using a co-located grid finite volume method. The dispersed phase is assumed spherical and is modeled by tracking the centroids of the particles/bubbles in a Lagrangian frame. In addition to interphase momentum exchange, the volume occupied by the disperse phase is accounted for through fluid void fractions in the momentum and continuity equations. Interactions of the dispersed phase with vortical flows are investigated for both dilute and dense loadings. First, flow generated by the disperse phase motion are simulated to reproduce experimentally observed vortical flow features: (i) the breakup of a viscous blob of particles falling under the influence of gravity and (ii) the transient migration of a buoyant bubble plume. Finally, bubble entrainment and interaction with traveling vortex tube under dilute loadings are simulated in a twodimensional approximation of the experiments by Sridhar and Katz (1999). It is shown that under some conditions, the entrainment of eight small bubbles, less than 1.1 mm in diameter, results in significant levels of vortex distortion, comparable to the experimental observations. We find that the bubble induced distortion is due almost entirely to volumetric displacement effects present in the DEM model. A relative reaction force, defined as the ratio of net bubble to fluid reaction to the local driving force of the vortex, is used to analyze the vortex decay rate. It is shown that the global increases in vortex decay rate are directly proportional to the magnitude of this highly local relative reaction.