Optimization of Production Process of Optical Fiber Preform by Vapor-Phase Axial Deposition Method

将气相轴向沉积(VAD)法制备二氧化硅微粒的过程模拟为三个阶段：(1)氢扩散火焰中的燃烧反应；(2)二氧化硅微粒的生成；(3)二氧化硅微粒在火焰中的运动。在阶段(1)中，使用由21对基元反应组成的化学模型来模拟燃烧反应。在阶段(2)中，通过添加四氯化硅与氧气的反应，讨论了上述燃烧反应与二氧化硅颗粒生成反应的关系。最后，基于流体力学理论，模拟了SiO_2颗粒在燃烧气流中的运动及其在预制件表面的沉积过程，寻找了预制体高效生长的必要条件。结果表明：(1)SiO_2颗粒的燃烧和生成区域完全分离，燃烧反应主要发生在火焰底部。过剩的氧气在燃烧反应区周围流动，接近火焰的中心部分。(2)四氯化硅沿中心流经燃烧区，无反应。在那里，它被燃烧反应加热，最后与氧气反应，生成细小的二氧化硅颗粒。(3)在预制件表面附近，气流速度减小，二氧化硅颗粒的运动对热泳现象的变化非常敏感。因此，必须使温度梯度更大，以提高预制件上的二氧化硅沉积产率。(4)为了获得均匀的沉积，燃烧气体必须将预制件的整个表面进行层状包裹。

The production process of silica fine particles by the vapor-phase axial deposition (VAD) method was simulated considering the whole process as a sequence of the following three stages: (1) combustion reaction in hydrogen diffusion flames; (2) production of silica particles; and (3) movement of silica particles in the flame. In the stage (1), the combustion reaction was simulated using a chemical model composed of twenty-one pairs of elementary reactions. In the stage (2), the relation between the above combustion reaction and the production reaction of silica particles was discussed by adding a reaction of silicon tetrachloride with oxygen. In the last stage, the movement of the silica particles in the flow of the combustion gas and their deposition on the preform surface were simulated on the basis of the fluid dynamics so that the necessary conditions for the efficient growth of the preform were looked for.The following results were obtained: (1) The regions of the combustion and the production of silica particles are completely separated, and the combustion reactions take place mainly at the base of the flame. The oxygen in excess flows around the combustion-reaction region and them approaches the central part of the flame. (2) Silicon tetrachloride flows along the center through the combustion region without reaction. There it is heated by the combustion reactions and finally reacts with oxygen producing fine particles of silica. (3) Near the preform surface, the flow velocity decreases and then the movement of the silica particles becomes very sensible to change with thermophoresis phenomena. The temperature gradient must be, therefore, made larger to raise the silica deposition yield on the preform. (4) The combustion gas must envelope laminarly the preform all around its surface in order to get uniform deposition.