Particle deposition on a rotating disk in application to vapor axial deposition (VAD) process

Abstract

Vapor axial deposition (VAD), one of the optical waveguide preform fabrication processes, is performed by deposition of silica particles that are synthesized by combustion of raw chemical materials. In this study, flow field in a VAD process is assumed to be a forced uniform flow perpendicularly impinging on a rotating disk. A set of exact solutions for velocity and temperature distributions are developed by employing a similarity variable obtained from force balance on a control volume near the disk. The solutions depend on the rotating velocity of the disk and the forced flow velocity toward the disk. The solutions are then incorporated into the particle dynamics/transport equation. The particles are approximated to be in a lognormal size distribution. A moment model is used in order to predict distributions of particle number density and size simultaneously. Deposition of the particles on the disk is examined, considering convection, diffusion, thermophoresis, and coagulation with variations of the forced flow velocity and the disk rotating velocity. The deposition rate and the deposition efficiency directly increase as the flow velocity increases, resulting from that the increase of the forced flow velocity causes thinner thermal and diffusion boundary layer thicknesses and, thus, causes the increase of thermophoretic drift and Brownian diffusion of the particles toward the disk. However, the increase of the disk rotating velocity does not result in the direct increase of the deposition rate and the deposition efficiency. Slower flow velocity causes extension of the time scale for coagulation and thus, yields larger mean particle size and its geometric standard deviation at the deposition surface. In the case of coagulation starting farther from the deposition surface, coagulation effects increase, resulting in the increase of the particle size and the decrease of the deposition rate at the surface.