FLOW AND MORPHOLOGICAL CONDITIONS ASSOCIATED WITH UNIDIRECTIONAL SOLIDIFICATION OF AQUEOUS AMMONIUM-CHLORIDE

Using a 27 percent aqueous ammonium chloride solution as a transparent analog, shadowgraph and dye injection techniques have been employed t o observe flow and morphological conditions associated with unidirecti onal solidification (UDS) from below. Dendritic crystals, which initia lly form at the cold surface, reject lighter, solute deficient fluid, and the attendant instability is manifested by finger-type double-diff usive convection phenomena. As a two-phase (solid/liquid), or mushy, r egion grows from the bottom surface, vertical channels develop in the mushy region, and solutal plumes that emanate from the channels are ch aracterized primarily by an ascending, oscillatory motion and secondar ily by wisps of fluid, which detach and descend from bends in the plum es. In time, double-diffusive convection layers also form in the melt. From a numerical simulation of the process, it is concluded that the channels originate from perturbations at the liquid interface, which c ause localized remelting and create conditions conducive to developmen t of the channels.