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Experimental study of convection in a mushy layer during directional solidification

  • C. F. Chen (a1)
Abstract

Results of experiments using a number of techniques to study the nature of convection in a mushy layer generated by directional solidification of aqueous ammonium chloride solutions are reported. The techniques include flow visualization using a dye tracing method to study convection within the mushy layer before and after the onset of plume convection, and X-ray tomography to measure the solid fraction of a growing mush. The principal results are as follows. (i) Prior to the onset of chimneys, there is no convective motion in the mush, in spite of the vigorous finger convection at the mush-liquid interface. (ii) When the plume convection is fully developed, the flow of fluid in the mush consists of a nearly uniform downward motion toward the bottom of the tank, horizontal motion along the bottom toward the chimneys, then upward plume motion through the chimneys in the liquid region above the mush. (iii) The solid fraction of a growing mush as determined by X-ray tomography shows a significant decrease toward the bottom of the tank after the chimneys are fully developed. As a result, the concomitant increase in the local permeability can be as much as 50%. Some of the results reported herein confirm theoretical predictions of Worster (1992) and Amberg & Homsey (1993). Others reveal phenomena not observed heretofore.

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Copley, S. M., Giamei, A. F., Johnson, S. M. & Hornbecker, M. F.1970The origin of freckles in unidirectionally solidified castings. Metall. Trans.1, 21932204.

Hellawell, A., Serazin, J. R. & Steube, R. S.1993Channel convection in partially solidified systems. Phil. Trans. Roy. Soc. Lond. A 345, 507544.

McDonald, R. J. & Hunt, J. D.1970Convective fluid motion within the interdendritic liquid of a casting. Metall. Trans.1, 17871788.

Sample, A. K. & Hellawell, A.1984The mechanisms of formation and prevention of channel segregation during alloy solidification. Metall. Trans. A 15, 21632173.

Sarazin, J. R. & Hellawell, A.1988Channel formation in Pb-Sn, Pb-Sb, and Pb-Sn-Sb alloy ingots and comparison with the system NH4Cl-H2O. Metall. Trans. A 19, 18611871.

Tait, S. & Jaupart, C.1992aCompositional convection in a reactive crystalline mush and melt differentiation. J. Geophys. Res.97, 67356756.

Worster, M. G. 1991 Natural convection in a mushy layer. J. Fluid Mech. 167, 481501.

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Journal of Fluid Mechanics
  • ISSN: 0022-1120
  • EISSN: 1469-7645
  • URL: /core/journals/journal-of-fluid-mechanics
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