Plume convection and freckle formation in a mushy layer daring solidification for multi-component system

在多组分体系从下往下冷却的单向凝固过程中，糊状层的局部区域会形成若干通道，称为雀斑。雀斑的起源与糊状层中羽流对流的开始有关。本文描述了双星系统凝固过程中产生的羽流结构。在Hele-Shaw电池中，采用透明的氯化铵水溶液进行过共晶生长。采用颗粒跟踪法和染料示踪法分别测量了熔融层内羽流对流速度和糊状层内间隙流体流动速度。确定了每个对流流的几个重要特征。特别是羽流对流是由向上的气流包裹在向下的气流中构成的，即双流结构。向下流动增强了喷出羽流的通道出口附近的凝固，就像火山一样。浆糊层内的间隙流体均匀向下移动，这是由羽流对流引起的。随着水溶液初始温度的升高，由于垂直温度梯度的稳定作用，羽流对流强度变弱。

In the unidirectional solidification cooled from below for multi-component system, several channels are formed in localized regions of the mushy layer, known as freckles. The origin of freckles is related to the onset of plume convection in the mushy layer. The present article describes the structure of plumes generated during solidification of a binary system. A transparent aqueous ammonium chrolide solution is employed for super-eutectic growth in a Hele-Shaw cell. The velocities of plume convection in the melt layer and interstitial fluid flow within the mushy layer are measured by the particle tracking and dye tracing methods, respectively. Several important features are identified for each convective flow. In particular, the plume convection is found to consist of the upward flow enveloped in the downward flow, i.e, double flow structure. The downward flow enhances the solidification in the neighborhood of the exit of the channel emanating the plume, like a volcano. Interstitial fluid within the mushy layer is observed to move downward uniformly, which is induced by the plume convection. As the initial temperature of the aqueous solution increases the strength of plume convection becomes weak due to the stabilizing effect of the vertical temperature gradient.