By Geraldine Deflandre, Fluent Benelux and Andy Young, Fluent Europe

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Optical fiber drawing is a process that is in wide use today as a result of the growing number of applications that rely on optical fiber technology. It is a challenging process to model accurately because of the complex governing physics inside and outside the fiber material, and because of the changing fiber profile. The fiber begins as molten glass with a steep exponential temperature dependence for the viscosity. During the drawing process, the cross-section can reduce by a factor of more than 10,000, and the draw ratio, or ratio between the velocity through the outlet and inlet sections, can exceed 100,000. The material is subjected to mixed environmental conditions as well: radiative heating and convective cooling at the same time.

The coupled calculation between POLYFLOW and FLUENT; filters are used to export data between the two codes

In a recent project, POLYFLOW and FLUENT were coupled to address fiber drawing simulations in a rigorous manner. The motivation for the work was to develop an industrial-strength methodology that could handle non-axisymmetric and hollow fiber profiles with high draw ratios, and offer a comprehensive heat transfer capability. The iterative calculation uses POLYFLOW to compute the profile shape and interior velocity field, and FLUENT to compute the temperature and velocity fields outside the fiber and the temperature field inside the fiber.

Temperature contours and fiber cross-section for an axisymmetric simulation showing the fiber after the initial POLYFLOW calculation without radiation (top) and after the iterative loop (in which radiation is taken into account ) has reached convergence (bottom)

Each code has particular strengths to meet the specific challenges of this process. POLYFLOW uses the Streamwise remeshing algorithm that relocates the internal nodes as the profile is adjusted after each iteration so that the best element quality is maintained, especially when high draw ratios are encountered. FLUENT offers the discrete ordinates (DO) radiation model that allows for the absorption, transmission, reflection, and refraction of radiation in the presence of the semi-transparent fiber material. The model is capable of correctly capturing the radiative heat transfer to the glass despite the rapid change in glass thickness as the fiber is drawn. Accurate predictions from a simulation such as this can be used by engineers to minimize the heat requirements for the process.

Temperature contours on the fiber surface and pathlines, colored by temperature, in the gas

The solution process begins in POLYFLOW, where an initial fiber shape is computed from the draw velocity. The fiber shape is passed to FLUENT, where heat transfer to (and inside) the fiber is calculated. The new temperature field is then used by POLYFLOW to calculate a revised fiber shape and internal velocity field. The process repeats until the fiber shape and temperature range are within 1% of the previous values. In 2D and 3D test simulations, convergence was achieved after 3 and 4 global iterations, respectively, and predictions of the final fiber shape and temperature profile were in very good agreement with expectations.

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