Particle motion in the stagnation zone of an impinging air jet

Abstract

This study investigated particle behaviour in the stagnation zone of natural and forced round impinging air jets using flow visualization, image analysis, and particle image velocimetry. The jet Reynolds number was 21000, and the nozzle to plate spacing was five diameters. Small mass loadings of glass beads with inertial time constants τp of 1.7 and 7 ms were examined. The Stokes number associated with the mean flow Stm = τpU0/D ranged from 0.6 to 2.4, and the Stokes number associated with vortices in the forced flow St′ = τpf ranged from 0.3 to 1.25 where f is the vortex passage frequency. Particle velocities near the wall deviated strongly from fluid velocities, resulting in rebound and non-Stokesian effects (i.e. significant particle Reynolds numbers Rep). The deceleration associated with rebounding caused long particle residence times in the stagnation zone and significant increases in particle number density above the plate. Rebound height and the height of the region of particle accumulation were well correlated and increased with Stm. Particles associated with lower Stm were accelerated in the radial direction more quickly, not only because of their decreased inertia, but also because of the larger fluid velocties encountered. Shear layer vortices produced spatial variations in particle concentration in the free jet which caused number density near the plate to fluctuate with time. The vortices had little effect on particle motion near the stagnation point, however. Only particles in the vicinity of vortex cores felt the influence of the vortex-induced velocity field. Hence, particle motion in the stagnation zone was most dependent on the mean flow (and thus Stm).