Properties of a bidisperse particle–gas suspension Part 1. Collision time small compared with viscous relaxation time

The properties of a dilute bidisperse particle–gas suspension under low Reynolds number, high Stokes number conditions are studied in the limit τ c τ v using a perturbation analysis in the small parameter v , which is proportional to the ratio of timescales τ c /τ v . Here, τ c is the time between successive collisions of a particle, and t v is the viscous relaxation time. The leading-order distribution functions for the two species are isotropic Gaussian distributions, and are identical to the molecular velocity distributions in a two-component gas at equilibrium. Balance equations are written for the mean and mean-square velocities, using a distribution function that is a small perturbation from the isotropic Gaussian. The collisional terms are calculated by performing an ensemble average over the relative configurations of the colliding particles, and the mean velocity and velocity variances are calculated correct to O ( v 2 ) by solving the balance equations. The difference in the mean velocities of the two species is O ( v ) smaller than the mean velocity of the suspension, and the fluctuating velocity is O ( v ½ ) smaller than the mean velocity.