Steady-state solidification of aqueous ammonium chloride

Journal article

We report on a series of experiments in which a Hele-Shaw cell containing aqueous solutions of NH4Cl was translated at prescribed rates through a steady temperature gradient. The salt formed the primary solid phase of a mushy layer as the solution solidified, with the salt-depleted residual fluid driving buoyancy-driven convection and the development of chimneys in the mushy layer. Depending on the operating conditions, several morphological transitions occurred. A regime diagram is presented quantifying these transitions as a function of freezing rate and the initial concentration of the solution. In general, for a given concentration, increasing the freezing rate caused the steady-state system to change from a convecting mushy layer with chimneys to a non-convecting mushy layer below a relatively quiescent liquid, and then to a much thinner mushy layer separated from the liquid by a region of active secondary nucleation. At higher initial concentrations the second of these states did not occur. At lower concentrations, but still above the eutectic, the mushy layer disappeared. A simple mathematical model of the system is developed which compares well with the experimental measurements of the intermediate, non-convecting state and serves as a benchmark against which to understand some of the effects of convection. Movies are available with the online version of the paper. © 2008 Cambridge University Press.

Authors: Peppin, S; Huppert, H; Worster, MG

• Publication status: Published
• Journal: JOURNAL OF FLUID MECHANICS
• Volume: 599
• Pages: 465-476
• Publication date: 2008-03-25
• DOI: 10.1017/S0022112008000219
• EISSN: 1469-7645
• ISSN: 0022-1120
• Language: English