Numerical modeling of deposition of SiO{sub}2 and GeO{sub}2 in the MCVD process for the manufacture of optical fiber preforms

摘要

A numerical simulation of heat and mass transfer during the multicomponent (SiO{sub}2 and GeO{sub}2) modified chemical vapor deposition (MCVD) process with application to the manufacture of optical fiber preforms is performed. The MCVD process is widely used to manufacture glass preforms for high quality optical fiber fabrication. In this process, a silica tube is rotated on a lathe and is heated by a slowly traversing oxy-hydrogen torch. A mixture of gases, such as SiCl{sub}4, GeCl{sub}4 and O{sub}02, flow into the rotating tube and are heated to high temperatures, when SiO{sub}2 and GeO{sub}2 particles are formed and deposited on the tube wall due to thermophoresis. In general, GeO{sub}2 is used as a dopant for increasing the refractive index of the final optical fiber. In this study, the commercial computational fluid dynamics (CFD) package, FLUENT, based on the finite volume method, is used to solve the governing equations for mass, momentum, energy, and species conservation for gases and transport of SiO{sub}2 and GeO{sub}2 particles. The effects of chemical reactions and temperature-dependent fluid properties as well as the localized heating of the moving torch are included. In the MCVD process, the incorporation of GeO{sub}2 is low due to unfavorable equilibrium of GeCl{sub}4 oxidation reaction in the presence of SiCl{sub}4 at high temperatures. The kinetics of this reversible GeCl{sub}4 oxidation reaction is also taken into account in the current model.