| In the optical fiber drawing process, the main objectives have been maintenance of uniformity on final fiber diameter, straight strong fiber structure, small optical loses, and high manufacturing rate. As a result of advantages and technological developments in optical devices, the fiber drawing speed has increased from 3m/s to 20m/s, and the optical loses have been reduced hundreds of times during the last two decades. However, deviations from defined optimum operation conditions due to misalignments and equipment tolerances continue to threaten fiber quality. The neck-down and cooling stages of the furnace fiber drawing process are the two main stages of the manufacturing process in which the dynamic behavior of the fiber and thermal transport define the fiber quality. In this study, deviations in design parameters, off-center glass blanks and wall cold-spots, and cross air-flow cooling of the fiber will be analyzed to examine the effects of these parameters on flow and temperature fields during drawing. These parameters are the sources of temperature nonuniformities on the glass circumference which can cause thermal stresses resulting in a bow in the final fiber, nonuniform consumption of the glass preform, a breakage in continuous drawing and affecting the mechanical and optical properties of final fiber.; Thermal and mass transport in the optical fiber drawing furnace are modeled in three dimensions numerically by using the finite element method. The momentum and energy equations in both glass and gas are solved simultaneously. Temperature dependent physical properties are considered because of strong temperature dependence of properties. A full enclosure analysis is used to calculate surface-to-surface radiation between the glass surface and the furnace wall. Radiation inside the glass is approximated by using the Rosseland diffusion approximation.; The results show that temperature nonuniformity over the fiber circumference is produced by an eccentric glass blank, wall cold-spots, and cross-flow cooling of the fiber. The temperature nonuniformity is large enough to reduce the fiber quality. |
