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

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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.