The Effect of Convective Heat Transfer Instabilities on the Diameter of Polymer Optical Fiber

The effect of various natural convection regimes on the diameter of drawn polymer fiber was studied experimentally. Results indicate that as the buoyant potential of air within a cylindrical furnace enclosure is increased the natural convection regime transitions from laminar to oscillatory, and finally, to chaotic flow. The time-dependent heating caused by the oscillatory and chaotic regimes alters the rheology of the elongating polymer preform, causing detrimental time-dependent variations in the fiber diameter. The gas-phase temperature oscillations recorded on opposite sides of the necking perform were not identically in-phase indicating the convective flow is asymmetric in nature. The period of the recorded oscillations was found to be a function of the driving temperature difference, consistent with prior investigations. Furthermore, near the transition to chaotic flow sub-harmonics were observed within the recorded oscillations. When subjected to oscillatory and chaotic natural convection the standard deviation of the fiber diameter variations was up to 2.5 to 10 times greater, respectively, than that measured under laminar heating conditions. This represents a significant instability mechanism, one that has not been investigated within the context of the fiber drawing process to date.