The drawing speeds employed in the manufacturing of optical fibers have been rising in recent years due to growing worldwide demand. However, increasing speeds have placed stringent demands on the manufacturing process, mainly because of large temperature gradients that can generate thermally induced defects and undesirable variations in fiber characteristics. Heat transfer and glass flow that arise in drawing fibers of diameters 100–125 microns from cylindrical silica preforms of diameters 5–10 cm play a critical role in the success of the process and in the maintenance of fiber quality. This paper presents an analytical and numerical study of the optical fiber drawing process for relatively large diameter preforms and draw speeds as high as 20 m/s. The free surface, which defines the neck-down profile, is not assumed but is determined by using a balance of forces. An iterative numerical scheme is employed to obtain the profile under steady conditions. The transport in the glass is calculated to obtain the temperature, velocity and defect distributions. A zone radiation model, developed earlier, is used for calculating radiative transport within the glass. Because of the large reduction in the diameter of the preform/fiber, the velocity level increases dramatically and the geometry becomes complicated. A coordinate transformation is used to convert the computational domains to cylindrical ones. The numerical results are compared with experimental and numerical results in the literature for smaller draw speeds for validation. The effects of high draw speeds and of other physical variables on defects generated in the fiber, on the neck-down profile, and on the feasible domain for the process are determined. [S0022-1481(00)02302-1]
Precise modeling of radiative heat exchange between the furnace and the glass preform is a very important part of the modeling of the fiber drawing process in a high temperature furnace. Most earlier studies on this process have used the optically thick approximation, i.e., the radiative heat exchange is assumed to depend only on the preform surface temperature while the transmission, emission, and absorption within the preform are approximated as a diffusion process. The validity of this approximation in the modeling of the fiber drawing process is dubious since the diameter of the preform undergoes a drastic reduction during the drawing process. The objectives of this research are to use a more accurate approach—the zonal method—to replace the optically thick approximation for computing the radiative heat exchange between the furnace and the preform, and to determine if the optically thick approximation is valid for this process. In applying the zonal method, the preform surface is assumed to be diffuse to both transmission and reflection. An enclosure analysis is performed for the radiative exchange between the furnace and the outer surface of the preform and the zonal method is employed to consider the radiative exchange within the glass preform. The emissivity for the glass preform has been calculated based on the diffuse surface assumption and applied to the computation of radiative heat flux with the optically thick approximation for the purpose of comparison with the present work. The results obtained by the zonal method show that the radiative heat flux is strongly influenced by the radial temperature variation within the preform, while those obtained by the optically thick approximation do not show this effect, as expected. Comparisons of the results obtained by these two approaches reveal that the optically thick approximation predicts the radiative heat flux satisfactorily for a range of axial temperature variations, but only when the radial temperature variation within the preform is small. The diameter change in the neck-down region has almost no effect on the validity of the optically thick approximation.
The thermal transport associated with optical fiber drawing at relatively high drawing speeds, ranging up to around 15 m/s, has been numerically investigated. A conjugate problem involving the glass and the purge gas regions is solved. The transport in the preform/fiber is coupled, through the boundary conditions, with that in the purge gas, which is used to provide an inert environment in the furnace. The zonal method, which models radiative transport between finite zones in a participating medium, has been employed to compute the radiative heat transfer in the glass. The flow of glass due to the drawing process is modeled with a prescribed free-surface neck-down profile. The numerical results are compared with the few that are available in the literature. The effects of important physical variables such as draw speed, purge gas velocity and properties, furnace temperature, and preform diameter on the flow and the thermal field are investigated. It is found that the fiber drawing speed, the furnace temperature, and the preform diameter have significant effects on the temperature field in the preform/fiber, while the effects of the purge gas velocity and properties are relatively minor. The overall heating of the preform/fiber is largely due to radiative transport in the furnace and the changes needed in the furnace temperature distribution in order to heat the glass to its softening point at high speeds are determined.
We introduce a temperature-dependent parameterization in the modified embedded-atom method and combine it with molecular dynamics to simulate the diverse physical properties of the δ and ε phases of elemental plutonium. The aim of this temperature-dependent parameterization is to mimic the different magnitudes of correlation strength of the Pu 5f electrons at different temperatures. Compared to previous temperature independent parameterization, our approach captures the negative thermal expansion and temperature dependence of the bulk moduli in the δ-phase. We trace this improvement to a strong softening of phonons near the zone boundary and an increase of the flike partial density and anharmonic effects induced by the temperature-dependent parameterization upon increasing temperature. Our study suggests it is important to include temperature-dependent parameterization in classical force field methods to simulate complex materials such as Pu.
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