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By Denis Tschumperlé, Alcatel, Conflans, France
The manufacture of optical fiber requires a series of successive steps.
At Alcatel, the process begins by creating a preform of doped silica using
MCVD (Modified Chemical Vapor Deposition). A silica overclad is then deposited
onto the preform using a plasma torch. This massive silica preform is
then drawn into a 125 micron diameter fiber. For this to occur, the preform
is heated up to the silica softening point in a drawing furnace, and then
drawn into a fiber. After the fiber is cooled, it is passed through UV
furnaces, where it is coated with one or more polymers to give it additional
strength. Throughout the entire process, the control of temperature and
gas composition is crucial to meet quality requirements.
Schematic of the fiber drawing process
FLUENT has been used at Alcatel to simulate several of the steps in the
process. The first simulation performed was of the simple case of a fiber
cooling in air1. After validating these results, engineers were able to
predict the temperature of the fiber as it travels through air at different
temperatures and draw speeds.
FLUENT has also been used to simulate the drawing furnace, where the
temperature pattern, gas flow, and gas composition must be controlled
to avoid any degradation of the internal walls of the furnace or of the
preform. Because very few measurements can be made in this harsh environment,
simulation is essential to better understand the influence of these parameters
on the fiber product quality. The furnace models include fluid flow and
radiative heat transfer. Using the discrete ordinates model, the silica
preform is treated as a semi-transparent medium, and the radiation spectrum
is divided into several bands. The model predictions of temperature on
the furnace centerline have been successfully validated against thermocouple
measurements. In fact, the computed temperatures are believed to be more
representative of reality than the measured values, due to the radiative
effect of the furnace on the metallic thermocouples.
Comparison of computed (blue line) and experimental (red circles) temperature
in the centerline of the drawing furnace
Modifications to the process are continually being evaluated for ways
to improve it, and FLUENT has been used to assess and optimize these new
designs. For example, if the draw speed is increased while the height
of the draw tower (where the heating, drawing, cooling, and coating processes
take place) remains constant, the fiber must be cooled more efficiently.
One method for doing this is to direct cooling jets of gas onto the fiber2.
The position of these jets on the draw tower strongly influences the fiber
temperature, and depending on the jet positions, radiation may or may
not play a role in the process. Experience has shown that the temperature
dependence of the material properties, for both the fiber and cooling
gas, can greatly influence the computed temperatures, so this must be
carefully taken into account. It has also been determined that the turbulent
flow – with very different characteristics near the fiber and out
in the free stream – is best modeled using the two layer zonal treatment.
With the many modeling choices available, Alcatel engineers feel that
FLUENT can help them identify the best scenarios for improving this complex
process.
Pathlines colored by temperature in the cooling device
References
1. D. Tschumperle, M. Nicolardot, “Fiber Cooling Modelization During
Draw Using CFD”, ASME PVP Vol. 424-1, Volume 1, 2001.
2. D. Tschumperle, J.F. Bourhis, S. Dubois, A. Leon, “Study of Cooling
Tubes for Fiber Draw Using CFD”, Proceedings of 50th IWCS, Lake Buena
Vista, Florida, November 12-15, 2001.
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