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Steady-state solidification of aqueous ammonium chloride

Published online by Cambridge University Press:  06 March 2008

S. S. L. PEPPIN ,
HERBERT E. HUPPERT  and
M. GRAE WORSTER
S. S. L. PEPPIN
Affiliation:
Institute of Theoretical Geophysics, Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK
HERBERT E. HUPPERT
Affiliation:
Institute of Theoretical Geophysics, Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK
M. GRAE WORSTER
Affiliation:
Institute of Theoretical Geophysics, Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK
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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.

JFM classification

Convection: Convection in porous media
Type
Papers
Information
Journal of Fluid Mechanics , Volume 599 , 25 March 2008 , pp. 465 - 476
Copyright
Copyright © Cambridge University Press 2008

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References

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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.

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Video 2.7 MB

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.

Download Peppin et al. supplementary material(Video)
Video 1.3 MB

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.

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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.

Download Peppin et al. supplementary material(Video)
Video 1.7 MB