This study represents a three-dimensional numerical analysis of the effects of wall cold-spots on the temperature fields in an optical fiber draw furnace. Radiation heat transfer was added to the energy equation by using the diffusion approximation inside the glass, and a full enclosure analysis in the gas for surface-to-surface radiation between the glass surface and furnace wall was used. The model includes temperature dependent physical properties. Convective heat transfer was treated by solving the flows in the gas and glass, conjugately. The three-dimensional Navier-Stokes equations and the energy equation were solved within the glass preform and gas simultaneously using the finite element method.
Wall cold-spots at different locations and temperatures were examined and their effects on the furnace temperature were determined. These cold spots cause temperature nonuniformities around the preform circumference and delay the heat up of the preform. Asymmetric glass temperatures on the preform caused by cold-spots located in the upper part of the furnace tended to equilibrate before reaching the end of the furnace. The preform diameter at the furnace exit becomes narrow, thus the circumferential temperature distribution on the fiber becomes more uniform and isothermal. However, although the asymmetric temperature differences were small, the temperature gradients and viscosity changes over the preform cross-sections were rather high. Thus, the glass will flow faster through the hot side of the preform. This may result in a nonuniform consumption of the glass preform and may also cause distortion on the desired form of the symmetric neck-down root shape of the preform. Wall cold-spots must be minimized in temperature and size to prevent their effects on the temperature field and final fiber diameter.