Optical fibers are typically heated and drawn from silica preforms, which usually consist of two concentric cylinders called the core and the cladding, in a high-temperature furnace. For optical communication purpose, the core always has a higher refractive index than the cladding. In order to investigate the effect of core-cladding structure on the optical fiber drawing, a numerical model has been developed in this work. Axisymmetric flows of a double-layer glass and aiding purge gas in a concentric cylindrical furnace are considered. The thermal and momentum transport in both glass layers and gas are coupled at the interface boundaries. The neck-down profile is generated using an iterative scheme. The zonal method is applied to model the radiation transfer in the glass preform and the gas. Coordinate transformations are used to convert complex domains into cylinders. Stream function, vorticity and energy equations for the core, the cladding and the purge gas are solved by finite different methods using a false transient method coupled with an alternating direction implicit (ADI) method. A second order differencing scheme is used for discretization. The numerical results are validated by comparing with experimental and numerical results available in the literature.

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