Bubble-particle detachment in a turbulent vortex II—Computational methods

Abstract

This is the second in a series of papers concerned with bubble-particle detachment in a turbulent vortex. Part I (Wang et al., 2016a, 2016b) explored, experimentally, the detachment of particles from bubbles due to centrifugal force generated in a rotating vortex in a wall cavity. Detachment was seen to be dependent on interactions between the turbulent vortex and the bubble-particle aggregate. Part II presents calculations of the flow field in the wall cavity, obtained using computational fluid dynamic (CFD) modelling. The probability of particle detachment from a bubble has been predicted using several different turbulence models.The traditional theory assumes that a bubble-particle aggregate is trapped inside an eddy and the attached particle is subjected to a centrifugal force as it rotates along with the eddy. When the centrifugal force on the particle exceeds the capillary force of attachment, the particle breaks away. The centrifugal force is a function of the energy dissipation rate, which requires an advanced turbulence model and high computational resources to achieve reliable predictions from the CFD calculations. To overcome the deficiencies of the current particle detachment model in the simulation of the flotation process, a new detachment model has been developed, relating the centrifugal acceleration to the vorticity in the flow field, where the vorticity calculation does not require the assumption of isotropic flow that is inherent in the dissipation method. Results from the Reynolds Stress Model (RSM) are compared with those from a Large Eddy Simulation (LES). The new vorticity model has advantages over the previous models which used the energy dissipation rate.