Polymer Optical Fiber Manufacturing - Preform Heating



Experimental and numerical investigation of the initial preform heating was conducted in order to understand the heat transfer phenomena involved in fiber production. A better understanding of the system will help minimize preform waste and minimize the time required to reach steady-state drawing.



The experimental setup is shown above. A cold one-inch diameter preform was inserted into the furnace and allowed to heat until it reached thermal equilibrium. Thermocouples at locations 1-4 monitored the temperature within the preform as a function of time. Irises at the top and bottom of the furnace prevented the flow of air into or out of the furnace. Thermocouples were located in the irises and flush with the inner surface of the furnace wall.

Numerical Model

The heat transfer from the furnace wall to the air and preform was calculated using the numerical domain shown in red in the figure above. In order to accurately capture the natural convection heat transfer the conjugate problem was solved. Therefore, the full axi-symmetric continuity, momentum, and energy equations were solved for the entire numerical domain. The initial preform temperature was uniform at 20 C and the furnace was preheated. The experimentally measured temperature profile of the furnace wall was used as the thermal boundary condition in the numerical model. The no-slip condition was applied to all bounding surfaces.

The governing equations were solved using the SIMPLER finite volume algorithm. The air was considered incompressible and buoyancy forces were modeled using the Boussinesq approximation. The exchange of radiative heat transfer was solved using the net enclosure method. All surfaces were modeled as gray and diffuse. The air did not participate in the radiative exchange.

The numerical domain was discretized into 434 axial nodes and 32 radial nodes. The fully implicit SIMPLER method used a time-step of 0.5 seconds. The numerical model displayed satisfactory self-consistency when subject to both spatial and temporal refinement. Results obtained with a mesh of twice the nodal density (866x462) gave results within 0.6%. Using a time step of 0.01s gave temperature fields within 1% of those found using a time step of 0.5s.

The plots below show the numerically predicted velocity and temperature fields within the preform and furnace at three different times during the transient heating of a preform. The furnace wall was approximately isothermal with a temperature of 85 C.

After 100 seconds there is strong natural convection due to the large temperature difference between the hot wall and the cold preform. After 2000 seconds natural convection continues strongly with a unicellular flow pattern. As the furnace environment reaches steady state (t=14 hours) the majority of the lower preform is isothermal. Unlike before, the convection is multicellular and weak as the natural convection is now driven by the vertical variation in both the wall and the preform surface temperatures, not the radial temperature difference as before.

Effect of Preform Emissivity

To test the numerical model, experimentally measured preform temperature histories were compared with predicted values for a range of well-defined preform emissivities. Preforms coated with both black paint and aluminum foil were used. The discrepancies between the predicted preform temperatures and the experimental observations never exceeded 1.3 C (see below). These results show that not only does the numerical model accurately predict the total transient heating, but it also successfully predicts the individual contribution of radiative heat transfer over almost the entire range of surface emissivities. This allows us to make an accurate estimation of the relative contributions of thermal radiation and natural convection heat transfer.




Contribution of Thermal Radiation

The above figure shows the predicted rate of radiative and total heat transfer experienced by a half-inch diameter poly(methyl) methacrylate (PMMA) preform exposed to a parabolic axial wall temperature profile.

Natural convection and thermal radiation contribute approximately equal heating to the cold preform when it is initially introduced into the preheated furnace. As the preform temperature rises, the fraction of convective heating decreases due to the weakening of the air circulation cells between the furnace wall and preform. By the end of the initial preform heating radiation typically accounts for 90% of the total heat transfer.

Hence 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.

Preform Temperature Profile

Due to the importance of radiative heat transfer the resulting vertical temperature profile within the preform is coupled to that of the furnace wall. This is because radiative heat transfer is greatest between surfaces in close proximity to each other. That is why the preform temperature profile seen below closely resembles the parabolic temperature profile of the furnace wall.



This has important implications because the initial reduction in the polymer's viscosity and the onset of necking typically occurs at the hottest point in the preform provided there is sufficient weight of polymer below. Therefore the location of initial necking can be controlled by modifying the furnace wall temperature. This can aid in the reduction of preform waste. Finally, the furnace wall temperature profile can be used to generate a preform temperature distribution that most resembles that of steady-state drawing, thereby minimizing initial setup time.