Polymer Optical Fiber Manufacturing - Heating Instabilities





During fiber drawing the furnace wall is typically hotter than the top iris. This temperature difference promotes air circulation as hot air next to the wall rises while the cold air next to the top iris sinks. Researchers investigating natural convection flow in tall annular enclosures have shown that as the intensity of this buoyant circulation is increased the convective flow can become unstable. Steady unicellular flow can transition to oscillatory multicellular flow, through a supercritical Hopf bifurcation. Other phenomena common in the field of chaos theory have been observed, such as period doubling, multiple solutions, intermittency, and quasi-periodic and fully chaotic flow.

Experiments in our lab have confirmed that convective instabilities can occur during fiber drawing. More importantly these variations can cause significant and detrimental variation in the fiber diameter. This is the first time convective instabilities and their impact have been identified in the fiber drawing process.

Time-Variant Heating

The figure below shows the furnace air temperature histories recorded as the temperature of the top iris was decreased. When the top iris is hot the air circulation within the furnace is laminar and the recorded air temperature is constant (as shown in the top temperature history below, a). When the top iris temperature is decreased (thus increasing the buoyant potential) the flow becomes unstable and the recorded air temperature starts to oscillate with a frequency of 0.23 Hz (see the middle plot, b). As the temperature of the top iris is further reduced the flow suddenly becomes chaotic with variations in temperature of up to 3 C (see the bottom plot below, c).



The frequency of the oscillations was found to be a function of the temperature difference between the furnace wall and the top iris. This behavior is consistent with prior investigations.


Effect on Fiber Diameter

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 figure above shows the fiber diameter histories corresponding to the laminar, oscillatory, and chaotic flow regimes discussed above. The standard deviation of the fiber diameter under laminar heating conditions is 0.6 microns. The diameter variation increases significantly for the oscillatory and chaotic heating regimes with standard deviations of 1.5 and 6.9 microns, respectively.

A transparent model of the fiber drawing furnace was constructed that allowed optical access to the convective flow instabilities that had been measured during fiber drawing. In this model a quenched PMMA preform was used to create the curved inner geometry. Seed particles were illuminated with a vertical laser sheet (see figure below left). This allowed a two dimensional vector plot of the natural convection to be created (below right).



Interrogation of the axial velocity measured near the interface between two convective cells (marked with a white cross in the figure located above right) shows that the flow oscillates in a periodic fashion (see below). An animation (1.2Mb avi file) shows the oscillation in the vector field for the time duration shown below.