Thermal transport due to material and gas flow in a furnace for drawing an optical fiber

Abstract

The transport processes involved in the neck-down region for optical fiber drawing are numerically investigated. In this manufacturing process, a moving glass rod is heated in a furnace containing an inert gas environment and drawn into a thin optical fiber. The conjugate problem is solved considering both radiation and convection, with focus on the latter. Two different flow configurations, involving inert gas flow in the same as well as in the opposite direction as the moving preform/fiber, are considered in this study. A coordinate transformation is used to change the complicated computational domains in the gas and the fiber to cylindrical ones. The transport in the fiber is coupled with that in the gas through the boundary conditions. The radiative thermal transport is calculated using an enclosure model developed in an earlier study. The numerical results on convective flow and transport are validated by comparing with results available in the literaturefor simpler configurations. The effects of several important parameters such as fiber draw speed, inert gas velocity, furnace dimensions, and gas properties on the flow and temperature distributions are investigated. For the aiding flow case, in which the inert gases flow in the same direction as the fiber, heat transfer to the fiber increases as the gas velocity increases. For opposing flow, a recirculating region appears in the gas, close to the moving fiber surface, causing reduction in heat transfer as compared to the aiding case. The thickness of this recirculating zone decreases with increasing inert gas velocity. Radiation is found to be the dominant mode of heat transfer in the overall heating of the preform/fiber, with nitrogen as the inert gas. However, near the edges of the furnace, radiation heat transfer is relatively small and convection becomes very important. Also, the convective transfer rate is relatively large near the flow entrance because of the large temperature difference between the gas and the fiber. However, away from the entrance, the gas heats up and the temperature difference relative to the fiber decreases, resulting in a smaller convective heat transfer rate. The relevance of the results to various aspects of the fiber-drawing process is discussed.

Numerical modeling and analysis of glass fiber drawing process from large sized silica preform

“…Too high draw tension may cause the fiber breakage, or, if it is too low, it could be the source of undesirable fiber vibration and process instability in the continuous drawing process. The strength of draw tension recommended in fiberoptics industries is known to be on the order of 100 g or 1 N. By considering the force balance between draw tension and viscous and gravitational forces, Yong Ki Kim, Choi, Kwak and Kyoungjin Kim, Journal of Thermal Science and Technology, Vol.12, No.2 (2017) draw tension acting on the drawn glass fiber or preform at a given arbitrary z location can be evaluated as (Choudhury and Jaluria, 1998) …”

“…Paek and Runk (1978) and Paek (1999) established a one-dimensional momentum balance model for the prediction of preform neck-down profile but the effects of purge gas in draw furnace was treated simply by prescribing convective heat transfer coefficient estimated from the experimental data. Also, there have been multi-dimensional computational investigations on glass fiber drawing by considering the convective transport of purge gas around the preform neck-down shapes which profiles were taken from the measurements (Lee and Jaluria, 1997;Choudhury and Jaluria, 1998;Yan and Pitchumani, 2006).…”

“…Specifically, the preform diameter in the previous theoretical and computational studies of glass fiber drawing has been limited to less than 5 cm in 1990s (Imoto et al, 1989;Lee and Jaluria, 1997;Choudhury and Jaluria, 1998) and less than 10 cm in 2000s (Cheng and Jaluria, 2005;Wei et al, 2004;Kim et al, 2012). Therefore, it would be necessary to investigate the size effects of enlarged preform on every aspect of drawing process (such as formation of neck-down profile, draw tension, fiber cooling) as well as the validity of modeling approaches, while the fiberoptics industries are constantly demanding stable process techniques with larger sized preform at higher drawing speed.…”

Optimization of a thermal manufacturing process: Drawing of optical fibers

“…Thus, the equations are coupled because of property variation and viscous dissipation. For the convenient implementation of the finite difference method, the flow domains for the glass and purge gas are transformed into cylindrical ones [15,19]. The boundary conditions for a free surface, arising from the force balance, are employed here at the interface between the glass and the purge gas in order to determine the neck-down profile.…”

Computational Model for Predicting the Location of Glass Solidification in Optic Fiber Drawing

“…They studied a 5cm diameter preform and it was shown that the glass velocity distribution is close to 1D while the glass temperature has a 2D distribution. Other 2D studies include Lee and Jaluria [7], Choudhury and Jaluria [8], Yin and Jaluria [9] and Cheng and Jaluria [10]. The above studies only considered the furnace domain, and have based on either one of the two assumptions; the fiber solidifies within the furnace, or an arbitrary glass diameter is specified at the furnace exit.…”