Deposition of polydisperse particles in a Falkner-Skan wedge flow

Abstract

Deposition of silica particles onto a target is utilized for studying the performance of optical fiber fabrication processes. The particles are synthesized from a flame due to chemical reactions, grow by collision and coalescence to become aggregates of approximately 0.2 μm in effective hydrodynamic diameter while they follow gas stream, and are deposited onto the target. Typically, the particles are polydisperse in size and follow a lognormal size distribution. For analysis, Falkner-Skan wedge flow was chosenk as the particle laden flow. Brownian diffusion, thermophoresis, and coagulation of the particles were considered and effects of these phenomena on particle deposition were studied. A moment model was developed in order to predict distribution of particle number density and particle size simultaneously. Particle deposition with various wedge configurations was examined for conditions selected for a typical VAD process. When coagulation was considered, the geometric mean particle size and its geometric standard deviation increased and the particle number density decreased, compared to the case without coagulation. These results proved the fact that coagulation effect expands particle size distribution. The results were discussed with characteristics of thermal and diffusion boundary layers. As the boundary layers grew in thickness, overall temperature and concentration gradient decreased, resulting in decrease of deposition rate and increase of particle residence time in the flow and thus coagulation effect. Effect of wedge configuration on particle deposition efficiency was studied and discussed also with the characteristics of boundary layers.