The objective of this study is to understand the dynamic behavior of particles in MCVD. The Modified Chemical Vapor Deposition (MCVD) process is the process for fabricating low-loss optical fibers for telecommunications. This process was invented by MacChesney in 1974. Optical fiber manufacturing processes are composed of fabricating a preform rod, rod sintering, and fiber drawing and coating. The key in the optical fiber fabrications is to make a preform out of glass of extremely high purity.
In general, aerosol reactors to manufacture high purity preform are very complex systems. In these systems, several physicochemical phenomena take place simultaneously. The oxidation of $SiCl_4$ is key reaction in these systems. A homogeneous gas phase reaction occurs above 1100℃ in the reactors. Silica particles nucleate from the reaction products and grow primarily by coagulation in the reaction zone depending on the temperature, diffusion and oxidation of reactants. A suspended particle experiences a net force in the direction of decreasing temperature. This phenomenon is called 'thermophoresis'. The thermophoresis can be the dominant mass transfer mechanism in the deposition of the particle. Therefore, the particles deposit to the preform walls in the deposition zone. The deposited particles are consolidated into clear glass layers as the torch is traversed in the direction of flow.
In this work, a model for MCVD is proposed accounting for aerosol dynamics and deposition efficiency. This analysis includes the general dynamic equation for the aerosol dynamics and the radial velocity of the aerosol is considered. The aerosol size distribution is approximated by a lognormal distribution throughout the reactor. And the evolution of the moments is used to describe the aerosol dynamics for simplicity. The partial differential equations are solved by an explicit finite difference scheme. The evolution of variable profiles (temperature, net particle velocity, concentration of reactants and particles, particle size and polydispersity) along the preform axis is presented. Besides, this study presents the effects of process variables (inlet reactant concentrations, flow rate and burner temperature) on aerosol characteristics and process yields. The radial velocity of particles and the residence time of the gas mixture have important effects on increasing the deposition efficiency. It is also shown that high deposition efficiency in MCVD process can be achieved by lower inlet concentration and slightly high gas flow rate.