Solidification of an alloy in a cavity cooled through its top surface

Abstract

An experimental study on the phenomenon of solidification of a binary alloy (water and ammonium chloride) in a cavity cooled through its top wall is reported. Three distinct regions exist in the cavity during the solidification process. A region occupied by the solid, a region occupied by the liquid and a mixed phase region (mushy zone) consisting of a complex combination of dendrites and liquid. The mixed phase region is of considerable extent, often larger than that of the solid. The growth of the solid/mush interface and the mush/liquid interface with time as well as the temperature distribution in the cavity are determined in the course of the study for a host of initial concentrations. The temperature field in the mixed phase and the solid regions is conduction dominated. This, despite the fact that the mushy region is permeable and allows for flow within it. Apparently, in the experiments of this study the flow in the mush is not sufficiently strong to render convection the main heat transfer mode in the mush. In the liquid phase a bicellular flow field exists. The flow has an effect on the shape of the mush/liquid interface. This effect, however, is weak and the interface remains predominantly flat especially for higher concentrations. Double diffusive convection takes place in the liquid region because ammonium chloride is constantly released into the liquid as the solidification progresses downwards, after it started in the vicinity of the top wall. Salt fingers are clearly visible penetrating into the liquid region. The thickness of the mushy zone generally increases with time. Increasing the initial concentration of the alloy reduces the thickness of the mixed phase region.