Double Diffusive Natural Convection in Crystal Growth

在纯材料的结晶生长过程中，在凝固前沿的外围会有极少量的杂质积聚。这就产生了浓度梯度和浓度自然对流。冷却温度梯度和浓度梯度诱导双扩散自然对流。这是本研究的主题。一开始，采用两相层作为最简单的双扩散系统。下半层为致密咸水层，上半层为纯水层。两层从一个垂直侧加热，从相反的垂直侧冷却。两层开始加热和冷却后，盐组分不断向上层扩散，盐厚羽沿加热壁向上抬升。由于系统的高刘易斯数，浓稠的浓度边界层能保持较长时间，核心状态长时间为相对纯净的水。这在上层和下层都引起振荡对流。研究发现，温度依赖性使两层界面不断向上移动。传质率的关系式是单一的。它不是由浓度差而是由扩散系数决定的。研究了不同温度和浓度方向相反或相同的浮力系统的振荡对流特性。

In the process of crystal growing of pure materials, very small amount of impurities are accumulated just outside of the solidification front. This induces the concentration gradient an concentration natural convection. The cooling temperature gradient and concentration gradient induce so to speak double diffusive natural convection.This is the theme of the present research. At the beginning, two phase layrs were adopted as a most simple system of double diffusion. The lower half layr is a dense salt water layr and the upper half layr a pure water layr. Both layrs are heated from the one vertical side and cooled from opposing vertical side. After two layrs are strted to be heated and cooled, salt component is continuously diffused to the upper layr and the salt-thick plume is raised upward along the heated wall. Because of the high Lewis number of the system, the thick concentration boundary layr is kept for long and the core regime is relatively pure water for many hours. This induces oscillatory convection both in the upper and lower layrs. The temperature dependence of the properties were found to move the two-layr interface constantly upward. The mass transfer rate was correlated by a single equation. It is determined not by the concentration differece but by the diffusion coefficient. Various systems for the buoyant forces due to temperature and concentration either opposite or the same directions were studied for the oscillatory convection characterstics.