Particle Deposition in Wakes: Inertial and Coulombic Effects on Capture By Single and Multiple Spheres in Line

Abstract

Ten-micrometer dioctylphthalate droplets tagged with sodium fluorescein were collected on six metal spheres, 0.6 cm in diameter and arranged in a line parallel to the axis of flow. The particles carried negative charge in the range 0.9–1.7 × 10⁻⁵ stC while the collectors were charged positively in the range 8.0–23.0 stC. The carrier stream velocities used were 600, 1200, 1500, 1800, and 2400 cm/s and spacings between targets of two through six diameters center-to-center were tested. Collection efficiencies for each target were determined by measuring the number of deposited particles, and the particle concentration in the air stream using fluorometric techniques.

An effective Coulombic attraction parameter K _Eeff , was defined to take into account non-Stokesian drag during particle collection at high velocities. This term was combined with an effective Stokes number to provide the empirical relation, for single spherical targets: η= 1.4[Stk _eff + (2K _Eeff )^1/2]. A model is presented to describe the concentration distribution of particles in the wakes of multiple spherical targets, in-line. This model is based on the lateral turbulent transport of particles into the wake between targets. A correlation between the particle deposition of any trailing target, normalized to that of the leading target, and the modified Graetz number, (n - l)^1/3(Gz)⁻¹, where n is the rank of the target in the series and Gz = 4(z/D)/Pe, enables one to predict deposition on trailing targets.