Investigation of convective heating in a polymer fiber drawing process

Polymer optical fibers are drawn from a preform rod in a manufacturing process that requires heating. A polymer preform in a tubular furnace enclosure receives energy from the furnace wall via natural convection and thermal radiation. Natural convection, in comparison to radiation, contributes a smaller fraction of the total energy required during the transient heating of the preform and also during the fiber drawing. Based on numerical predictions, natural convection and radiation contribute approximately equal heating to the preform when it is initially introduced at room temperature into a preheated furnace. As the preform temperature rises, the fraction of convective heating decreases as a result of weakening of gas‐phase circulation cell(s) between the furnace wall and preform. These findings are supported by the measured temperature histories in the interior of the preforms that, for all of the cases studied, differed by less than 1.3°C from numerical predictions. Although radiation contributes a larger fraction of the total required energy, natural convection can nevertheless have a strong and detrimental effect on the fiber quality. Specifically, convective instabilities caused air temperature oscillations of 0.3 to 2°C with frequencies from 0.01 to 0.30 Hz. Experimental observations show that these gas phase temperature oscillations promote unwanted diameter variations during fiber drawing.