Isothermal flow in an optical fiber coating applicator and die system

The isothermal flow generated by a fast-moving optical fiber in a coating applicator and die was studied numerically. Although it is expected that thermal effects can be important in typical coating processes, the isothermal flow behavior should be thoroughly investigated before including the heat transfer considerations. Thus, isothermal flow is studied in this paper, and key parameters that play a major role in the optical fiber coating process are investigated in detail. The coupled partial differential equations that govern the fluid flow are solved on a transformed nonuniform staggered grid. A finite volume method is employed with a semi implicit method for pressure linked equations (SIMPLE)-based algorithm for the pressure calculations. The fiber speed is varied from 0.1 up to 22 m/s, and various applicator sizes are considered. The diameters at the die entrance and at the die exit are varied separately, since the geometry of the applicator and of the die can play an important role. Both free and solid wall surfaces are taken for the boundary condition at the top surface in the applicator. The shear-free condition can be regarded as direct contact of the coating fluid with the cooling gas at the top surface. Three different viscosities were chosen, based on reference temperatures, taken as the fluid inlet temperature, the fiber inlet temperature, and the average temperature. Relatively low pressure differences, compared to those in the applicator and the die, were found at the dynamic contact point where the fiber first enters the coating applicator. This suggests that air or gases used for cooling the fiber may be able to enter through this location. It was also found that the shear rates in this region are comparatively higher. The temperature-dependent viscosities were found to be of critical importance in the flow. The change in size of the applicator did not significantly affect the mass flow rates at the die exit, whereas the pressure gradients and shear rates near the dynamic contact point were strongly affected. The flow field in the die changed substantially with a change in the exit size. Maximum pressures and shear rates in the die increased drastically when the die exit became smaller. This study also suggests that a more accurate representation of the upper meniscus is needed to simulate flow near the fiber entry into the applicator.