Formation and suppression of channels during upward solidification of a binary mixture

Characteristics of the mushy zone were experimentally investigated in the upward solidification of a binary mixture. The formation of channels and techniques for their suppression were explored. The experiments were carried out in a rectangular test section using aqueous ammonium chloride as the phase-change material at hypo- and hypereutectic concentrations ranging from 15-33 wt.% of salt. The cold-plate temperature was varied in the range of -60 deg C to -14 deg C. Transient temperature profiles and the positions of the liquidus and solidus interfaces were obtained. The mushy-zone characteristics, the nature and distribution of channels, and the associated fluid flows were studied as a function of initial solution concentration and cold-plate temperature. The application of low-amplitude vibration to the test cell was found to reduce the mushy-zone thickness and the number of channels for all concentrations; channeling was almost completely suppressed at the lower concentrations. Channel suppression was influenced by vibration amplitude rather than frequency, with the larger-amplitude vibration being the more effective. The vibration results suggest that the formation and sustenance of channels is influenced more by developments within the mushy zone than by bulk liquid behavior.