Modeling and optimization of the sagging process for large-size and high-purity silica glass synthesis

Glass sagging is a subsequent process to the CVD process used for large-size and high-purity silica glass synthesis. Physical phenomena taking place in this process are complicated which need an in-depth understanding for better control. In this paper, a comprehensive study is conducted for the sagging process using a level-set and enthalpy-porosity coupled model. With this model, the deforming behavior of glass ingot and evolution of OH uniformly distributed region are well predicted. Then, two performance indices (the effective yield rate and maximum extension radius of OH uniformly distributed region) are proposed based on different applications, and important factors, including geometrical parameters (the ingot initial length, crucible diameter and pedestal height) and operating parameter (the heater power allocation scheme), are explored for their effects on the two indices. The orthogonal test design method is adopted to further determine the collective effects of the four factors. According to the range analysis results, the initial ingot length has the greatest effect, while the crucible diameter has the least effect on the effective yield rate; and for the maximum extension radius, the crucible diameter becomes the major factor, while the pedestal height is the most insensitive factor. The corresponding optimal schemes are proposed for the two indices finally, which are believed to provide useful guidance for improving the sagging process.