A three-dimensional model of the neck-down stage of an optical fiber draw furnace was formulated to examine the effects of eccentricity on preform temperature and flow in the furnace. Eccentricity is an unavoidable off-manufacturing condition, which is the deviation between the centers of the cylindrical furnace and the glass preform rod. The Navier-Stokes and energy equations describing the momentum and thermal transfers in the draw furnace were solved simultaneously both in the glass and gas domains using the finite element method. Temperature dependent physical properties were considered. Radiation heat transfer was added into the model as surface-to-surface radiation between the glass surface and furnace wall, using a full enclosure analysis. Radiation inside the glass was approximated by using the Rosseland diffusion approximation.
The circumferential temperature distribution and viscosity change on the preform surface were determined. They increase with the eccentricity ratio. The eccentricity is the distance between the furnace axis and the preform axis. The gas flow is unsymmetrical due to eccentricity. Most of the temperature gradients and viscosity changes were not large within the furnace due to eccentricity. A balanced temperature field in the furnace is due to the high radiation heat transfer. Since the preform necks down within the furnace and becomes thinner in the lower half, the glass becomes more isothermal due to its high radiation coupled glass conductivity. However, although the resulting temperature difference on the glass preform is small, the gradient is high. This temperature gradient becomes important when the exponential temperature dependence of the glass viscosity is considered. The resulting viscosity difference in the neck-down region will cause the glass to flow faster on one side than on the other side resulting in nonuniform consumption of the glass preform. Thus, the desired symmetric root shape of the glass preform may change.