Upward solidification of a binary solution saturated porous medium

Abstract

Upward solidification of a binary solution saturated porous medium was investigated experimentally and theoretically to identify the dominant physical phenomena and to document the experimental results. A solid matrix of the porous medium was a packed bed of glass beads, and the saturating liquid was an aqueous sodium chloride solution on the water-rich side of the eutectic composition. Freezing experiments were performed in a square cross-section enclosure under constant temperature conditions at the cold bottom and hot top walls. Transient temperature profiles and the liquidus and solidus positions were measured for different combinations of bead size, initial temperature and initial salt concentration. Simultaneous measurements of local concentration and temperature at selected locations were also made in a separate series of experiments. An analytical model, based on heat and species conservation principles and relations from the equilibrium phase diagram is suggested, and the predictions are compared with experimental data. The uniform distribution of the predicted average concentration suggests that species transport by diffusion is negligible, and the effect of bead size on the temperature profiles was confined to the region near the cold bottom. The behavior of dimensionless temperature under different initial temperature and concentration conditions could be explained by the relation between the local freezing rate and nonequilibrium undercooling. Also, the ‘over-enriching’ was observed in the mushy region away from the liquidus, but a quantitative analysis was not performed. The importance of incorporating models, which account for the effects of nonequilibrium undercooling and density differences between phases, and the need for experimental diagnostics, which do not disturb the physical phenomena occurring during the solidification of the mixture, are emphasized.