Flow-solidification interaction: A numerical study on solidification of NH4Cl – 70 wt.%H2O solution in a water-cooled mould with a large sample thickness

Abstract

This study focuses on the flow-solidification interaction during the solidification of NH4Cl – 70.43 wt.%H2O solution in a water-cooled mould with a large sample thickness (100 × 80 × 10 mm3). First part of the paper is to use a volume-average-based dendritic solidification model to reproduce an experiment in which transient flow is monitored in-situ using particle image velocimetry, and the solidification sequence is recorded with a light camera. The striking features of the solidification experiment are that the crystal morphology is dominant in a columnar structure, and four typical flow regimes appear in the bulk solution during cooling/solidification, namely pure thermal convection, unstable turbulent flow caused by double diffusive convection, turbulent-stratified flow, and meandering flow. The results show that there is quantitative agreement between the simulation and experiment results. The focus of this paper (second part) is to study the flow-solidification interaction by correlating the above flow regimes with the solidification quantities in the mushy zone. Owing to the enhanced mass (solute) and energy transport by advection, flow suppresses the growth of the mushy zone, while speeding up the solidification inside the mushy zone, leading to the formation of dense mush and massive columnar dendrite trunks. The crystalline morphology of NH4Cl, as presented by a dendritic envelope and described by morphological parameters, is an important factor that influences the flow; as a feedback, the flow further influences the solidification inside the mushy zone. The numerical model also provides valuable information about the formation of freckle trails.