Local size distributions of particles deposited by inertial impaction on a cylindrical target in dust-laden streams

Abstract

We exploit recent developments on impinging single particle capture laws and rational correlations for inertial impaction on a circular cylinder in high Reynolds number crossflow [Israel and Rosner (1983) Aerosol Sci. Technol. 2, 45–51; Wessel and Righi (1988) Aerosol Sci. Technol. 9, 26–60] to predict the local size distribution of particles deposited by impaction on a cylindrical target when the mainstream particle suspension is “log-normal”. Because of both the aerodynamics of selective impingement, and the nature of the sticking/rebound law, we show that the granular deposit particle size distribution (hereafter abbreviated (PSD) w) is generally quite different from mainstream particle size distribution (PSD) ∞, by so much that (PSD) w generally cannot be characterized accurately by single-mode log-normal distribution parameters. Apart from its relevance in correcting for systematic errors in aerosol sampling from high-speed streams, this local variation of the “granular deposit” PSD along with information on deposit morphology, must be known (in addition to the total mass accumulated per unit area) to predict, say, the loss in convective heat transfer rate associated with the growth of a fouling layer. Three distinct classes of single solid particle capture laws are considered: constant capture fraction (independent of impinging particle velocity and angle of incidence), “on-off” capture behavior expected for impaction on a clean, particle-free, smooth solid surface, and particle capture on a dry, sufficiently thick, granular deposit. Our (PSD) w results are cast in terms of following accessible dimensionless parameters: sensitivity of capture fraction to particle incident velocity and angle, ratio of mainstream velocity to the critical (threshold) velocity for particle rebound (at, say, normal incidence), ratio of mean particle size in the mainstream to the critical size required for impaction on a cylindrical target in crossflow, spread of log-normal mainstream particle size distribution, and the characteristic “slip” Reynolds number for the critical size particle in the mainstream.