Hydrodynamical instability of the mushy layer during directional solidification of ammonium chloride solution in a Hele-Shaw cell

Experiments of directional solidification in a Hele-Shaw cell, 240 mm x 245 mm x 1 mm, were carried out to study the effects of convective flows and the evolution of structure in the "mushy layer", consisting of fluid and crystallites. Three different concentrations of ammonium chloride (NH₄Cl) solution, 26%, 27% and 28%, with a constant bottom cooling temperature ranging from 5°C to -20°C were used as an analogue model for metallic alloy systems. The experimental design included techniques using Peltier heat pumps along with PID feedback control for temperature regulation. A shadowgraphic visualization method made possible the simultaneous observation of the evolution of convection in the liquid region and the phenomena of chimney formation within the mushy layer. More than one hundred runs were carried out. Temperature information and simultaneously acquired digital images were recorded automatically. The time evolution of the solidification process was traced by image processing technique from the digital images. Results show that the phenomenon of intermittent plumes frequently occurs. The number of plumes near the liquid-mush interface decreases from a larger number in the beginning to about the same number of high plumes in the liquid region or chimneys in the mush. The number of chimneys is more static than that of plumes due to the relatively low speed of solidification or dissolution. The mush consists of three different layers. A thin layer of generally upward oriented crystal forms rapidly in the beginning, followed by a second layer aggregate gradually covering it. Finally a third layer of dendritic crystals dominates the top of the growing mush. Preliminary experiments in a Hele-Shaw cell inclined at 21 degrees caused the mush structures to develop differently than in the normal geometry. The significant differences include the dendritic crystal form, the lacunae of the mush, the shape of the chimneys, and the porosity changes of the structures. Directions for future research are suggested