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CONVECTION AND RADIATION EFFECTS IN HOLLOW, COMPOUND OPTICAL FIBERS

Juan I. Ramos
Escuela de Ingenierias Industriales, Universidad de Malaga, Dr. Ortiz Ramos, s/n 29071 Malaga, Spain

Abstrakt

A coupled model for the study of hollow, compound optical fiber drawing processes that accounts for the heat transfer in the preform and for the motion of the gases surrounding the preform and fiber by means of two-dimensional equations, employs a net radiative model for the radiative heat exchanges amongst the preform, fiber, irises and furnace walls, and uses an asymptotic method to derive one-dimensional equations for the geometry, axial velocity component and temperature along the fiber for small Biot numbers is presented. It is shown that radiative heat exchanges are about three times larger than forced convection effects, free convection is not important, and the fiber's geometry, axial velocity and temperature were in remarkable good agreement with those obtained with the one-dimensional model for hollow, compound fibers and a constant Biot number. It is also shown that, as the thermal radiative losses from the fiber increase, the fiber's dynamic viscosity increases, the fiber exhibits a strong necking phenomenon and the fiber's axial velocity increases rapidly from its value at the die's exit to a constant value downstream and then remains constant. For the boundary conditions considered in this paper, it is shown that the activation energies of the viscosity laws for the inner and outer materials of the hollow, compound fiber do not have very strong effects on the fiber's geometry, axial velocity component and temperature, whereas the fiber's solidification point moves towards the die as the thermal Preclet number is decreased. It is also shown that the pre-exponential factor and activation energy of the viscosity law do not play a key role in determining the fiber's geometry and temperature for the conditions analyzed in this paper.

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