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Steady-state solidification of aqueous ammonium chloride
Published online by Cambridge University Press: 06 March 2008
Abstract
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.
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Peppin et al. supplementary material
Movie 1. The time-lapse movie shows the solidification of a 23 wt% solution of aqueous NH4Cl. The pulling speed is initially 2 microns per second (point b in figure 4) and the mushy layer is relatively uniform. Eventually the speed is decreased to 0.5 microns per second (point a in figure 4) and several chimneys form. Plumes of buoyant fluid can be seen emanating from the chimneys. The speed is then increased back to 2 microns per second and the chimneys disappear. The entire movie represents 1 hour and 10 minutes of real time. The width of the Hele-Shaw cell is 12 centimetres.
Peppin et al. supplementary material
Movie 2. In this movie a 25 wt% NH4Cl solution is being translated at 6 microns per second (point c in figure 4). Secondary crystals can be seen in the supercooled melt above the mushy layer. The entire time-lapse movie represents 1 hour and 10 minutes of real time.
Peppin et al. supplementary material
Movie 3. This movie illustrates the disappearance of the mushy layer during the solidification of a 21 wt% solution translated at 1 micron per second (point d in figure 4). Although the initial concentration was above the eutectic concentration of 19.7 wt% the growing solid is the pure composite eutectic and no mushy layer appeared. The entire time-lapse movie represents 1 hour and 40 minutes of real time. The gradations on the ruler at the left are in millimetres.
Peppin et al. supplementary material
Movie 4. This final movie shows a 'breathing mode' which occurred during the solidification of a 23 wt% solution. This system has the same initial concentration as in movie 1, but the pulling speed is intermediate between the speeds used there. Here the pulling speed is 1 micron per second, which is near to the chimney-extinction boundary (figure 4), and the chimneys form and die out and then form again in phase. The entire time-lapse movie represents 12 hours of real time.
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