Studies on transport phenomena during directional solidification of a noneutectic binary solution cooled from the top
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10/31/03
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Studies on Transport Phenomena during Directional Solidification of a Noneutectic Binary Solution Cooled from the Top P. KUMAR, S. CHAKRABORTY, K. SRINIVASAN, and P. DUTTA In this article, we investigate the effects of laminar natural convection on directional solidification of binary fluids with noneutectic compositions when cooled and solidified from the top. The study is performed using aqueous ammonium chloride solution as the model fluid. In the first case, the initial concentration of ammonium chloride is less than the eutectic composition, leading to an aiding of the double-diffusive convection. In this case, solidification leads to the formation of a diffused matrix of dendritic crystals (mushy region) separating the pure solid and liquid regions. The mushy interface is characterized by a waviness, which is caused by a Rayleigh–Benard type of cellular motion in the liquid region. The cellular motions, which are caused by thermal and solutal buoyancy, cease once the thickness of the liquid layer falls below a critical value. The second case leads to a unique situation, in which crystals nucleated at the top wall of the cavity detach and descend through the lighter bulk fluid and, finally, settle at the floor of the cavity. In both the aforementioned cases, the features of convective transport are visualized using a sheet of laser light scattered through neutrally buoyant glass particles seeded in the solution. Numerical simulations are also performed for the first case, and the agreement with experimental results is found to be good.
I. INTRODUCTION
WHEN a homogeneous, stagnant fluid is subjected to a vertical temperature gradient, the resulting instabilities can give rise to Rayleigh–Benard type convective flow patterns, once a critical value of the Rayleigh number is reached. However, if the resulting temperature at a particular location falls below the local liquidus temperature (i.e., the temperature below which the liquid starts crystallizing), it is likely to promote solidification. Also, if the initial melt happens to be a multicomponent system, a mushy layer of dendritic crystals may form during solidification, the interstices of which accommodate the residual solute that is rejected during the process. Thus, in addition to temperature gradients created by externally imposed boundary conditions as well as due to release of latent heat, concentration gradients are also created simultaneously by the preferential rejection of solute across the solid/liquid interface. The rejected solute may be transported by diffusion on a local scale (leading to “microsegregation”) and by convective flow on a larger scale (leading to “macrosegregation”). Depending upon the externally imposed boundary conditions, there can be a wide variety of physical issues pertinent to solidification processes. In the case in which a melt is cooled from below, the fluid layer is always thermally stable, and, hence, a thermal-buoyancy-driven flow does not occur. Additionally, if the system rejects a den
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