Particle dispersion in a vertical round sudden-expansion flow

The dispersion of glass beads in an air flow through sudden step expansions in the direction of gravity has been investigated with phase-Doppler anemometry, which provided measurements of the velocity, flux and concentration characteristics. The purpose was to quantify the effect of increasing the air velocity over a factor of 5 for bead diameters of 40 µm and 80 µm and two expansion diameter ratios, 3.33 and 5, and at mass loadings of beads up to 90% of the air mass flow-rate. The results showed that the beads dispersed into the recirculation zone in the lee of the step by interaction with eddies characterized by length and velocity scales of the order of the expansion step height and the downstream area-averaged velocity. Particle dispersion into the recirculation zone was reduced when the bead mean transit time across the recirculation zone was shorter than the bead relaxation time, defined as the time required for a motionless bead suddenly exposed to a constant velocity fluid stream to reach 63% of its surrounding fluid velocity. Also the centrifuging effect, caused by the mean stream line curvature of the recirculation zone, could reduce particle dispersion into the recirculation zone, when its characteristic dimensionless group was less than unity. Beads, because of their mass, left the recirculation zone by sliding down the wall and past the air reattachment point or near the step, giving rise to bimodal probability distributions of velocity.