On the effects on inter-particle collisions in turbulent channel flow

The transport of dense particles suspended in a fluid is encountered in many industrial processes and atmospheric flows. Such multiphase flows are complicated by the various ranges of interaction of the particles with the carrier gas. Dense particles also interact with other particles through collisions. The aim of the current work is focused on investigations of particle motions and interactions through numerical simulation of dilute dispersions of solid particles in turbulent gas flow in a channel. The channel provides a dimension of nonuniformity as well as statistical equilibrium.;The gas phase is resolved using large eddy simulation (LES) of the incompressible Navier-Stokes equations. One-way coupling of fluid-particle interactions neglects the influence of the particles upon the gas. Dilute dispersions of small spherical particles are modelled using Lagrangian tracking, and particle motion is governed solely by drag. Predictions of particle transport are obtained for three particle response times in simulations with and without inter-particle collisions. Results show that particle-particle collisions affect the mean properties of the particle populations across the channel. Primary influences of collisions are explained using mean transport equations based on methods of kinetic theory. Measured collision frequencies are compared with relevant kinetic theory models and agree well for the heaviest particle populations simulated. For the intermediate particles, relative error in the collision frequency is greater than 30%. Analysis indicates much of this error is due to the correlation of particle relative velocities. The relative velocity variance is quantified through measurements of the two-point spatial correlations of the particle and fluid velocity fields. Measured spatial correlations of the particle velocity field exhibit a discontinuity at the origin, consistent with a velocity field comprised of distinct random and correlated contributions. Results show that the random component of the particle velocity increases with particle response time, and the fraction of particle motion residing in the random partition is affected by inter-particle collisions. The random collision frequency model is adjusted using the measured fraction of correlated velocity, and the maximum relative error in collision frequency is reduced to 12% for the intermediate particles.