A lattice Boltzmann study of the collisions in a particle-bubble system under turbulent flows

Abstract

The Lattice Boltzmann and point particle methods are used to conduct direct numerical simulations of the collisions between particles and bubbles in homogeneous isotropic turbulence. Particle-bubble collision is a complicated process affected by many factors, and the effects of the turbulent flows and the parameters of a particle-bubble system on the collision process are studied. The effects of the preferential concentrations of particles and bubbles in the turbulent flows on the collision process are studied from the perspective of radial distribution functions. To maintain background turbulent flows at targeted turbulent levels, a non-uniform time-dependent large-scale stochastic forcing scheme was implemented within the mesoscopic multiple-relaxation-time LBM approach. A local peak was found in the radial distribution function when the parameters of the particle-bubble system or turbulent levels were changed. Nevertheless, the peak was not found in the collision kernel function as the effect of the preferential concentration was overwhelmed by the effect of the radial relative velocity. It is interesting to find that the distributions of particles and bubbles were affected by the turbulent energy dissipation rate in a way that local accumulations were most apparent when the Stokes number of particles was 1. Nevertheless, the radial distribution functions of particle-bubble pairs remained stable at 1 in the turbulent flows with varied turbulent energy dissipation rates. This means that the distributions of particles and bubbles are not correlated and the effects of individual preferential concentrations of particles and bubbles on the collisions between particles and bubbles are weak. Therefore, the collision kernels of particle-bubble pairs increased with turbulent energy dissipation rate in a similar way as the radial relative velocities between particles and bubbles increased with turbulent energy dissipation rate.