Simulation of particle transport and deposition in the modified chemical vapor deposition process

The many complex interactions between system variables means that the experimental understanding of the mechanisms of soot generation, transport and deposition in the modified chemical vapor deposition process used to fabricate silica-based optical fibers is limited. This paper presents a computational fluid dynamics study of the process, in which the particle formation zone and flow field are calculated and silica particles are ‘launched’ at the reaction front and tracked until they either deposit on the substrate tube or exit the system. Variables which can affect this process are tube rotation rate, wall temperature profile, inlet feed composition and gas flow rate. It is shown that buoyancy alone and a combination of buoyancy and tube rotation are ‘symmetry breakers’ in terms of particle generation-deposition and that the most important factors affecting deposition are the gas flow rate and the shape of the wall temperature profile. This sensitivity arises from the competing forces of fluid drag and thermophoresis acting on the particles. It is suggested that altering the wall temperature profile by use of different size burners or changing the substrate tube cooling conditions could achieve significantly different soot deposition layer quality, which would be reflected in the final optical fiber performance.