A numerical model is developed to study the radiative heat transfer in a furnace for optical fiber drawing with a core-cladding structure in the fiber. The focus is on the effect of the difference in composition and thus the radiation properties in the two regions on radiative transport. The zonal method is applied to calculate the radiative heat transfer within the neck-down region of the preform. The radiative heat transfer between the preform and the furnace is computed by an enclosure analysis. A parallel computational scheme for determining the direct exchange areas is also studied. The radiation model is verified by comparisons with benchmark problems. Numerical results for a pure silica preform, a $GeO2$-doped silica core with a pure silica cladding preform, and a pure silica core with a $B2O3$-doped silica cladding preform are presented. Radiation properties for these are obtained from the literatures and a three-band model is developed to represent the values. It is found that radiative heat flux on the surface of the preform is strongly affected by the differences in the absorption coefficient due to doping. However, changes of about 1% in the refractive index have only a small effect on radiative heat transfer. The basic approach is outlined in order to form the basis for simulating optical fiber drawing processes, which typically involve fibers and preforms with a core and a cladding. Furthermore, the approach can apply to estimate the multi-layer fiber drawing, which is of interest in the fabrication of specialty fibers that have been finding uses in a variety of practical applications. The model can be extended to other similar processes, which involve multiple regions with different radiation properties. The main interest in this study is on the approximate representation of radiation properties and on the modeling of the transport process.

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