SOLIDIFICATION OF A BINARY MIXTURE IN A RECTANGULAR CAVITY

Abstract

1. ABSTRACT An experimental study of solidification of aqueous sodium chloride solution on a vertical wall of a rectangular cavity is reported. The effects of buoyancy driven thermosolutal convection on the freezing process have been examined. The temperature distribution in the solid, mush and liquid regions has been recorded. Using a hypodermic needle, samples of solution were periodically withdrawn from predetermined locations and with a refractometer, the concentration was determined. The progress of freezing was recorded photographically. Volume fraction at different times has been determined. As the solution was frozen, solute was rejected establishing a concentration gradient. Along with the thermal gradient thermosolutal convective flow was established. In the bulk melt there were two regions, one where convective flow was present (convection region) and below it a region where the liquid was stagnant (stratified region). The two regions were separated by a thin visible zone. In the convection region, there was little variation in concentration and the temperature was vertically and stably stratified. However, near the thin separating zone, the temperature distribution indicated thermal instability (i.e. cold liquid above hot liquid), but the concentration distribution was stably stratified (i.e. dense liquid above light liquid). In the stratified region, the concentration increased significantly from the initial value and this relative increase was inversely proportional to the initial concentration. Also with increase in initial concentration, the rate of freezing was reduced. For experiments with initial concentrations of 5% and 20%, the maximum percent increases in concentration (t = 6 h), are 51.5% and 9.94%, which corresponds to actual concentrations of 7.6% and 21.9%, respectively. The respective total (solid + mush) volume fractions were 0.17 and 0.12 (at t=6 h).