Application of a hybrid model of mushy zone to macrosegregation in alloy solidification

Solidification of an aqueous ammonium chloride (NH4Cl-H2O) solution inside a two-dimensional cavity is numerically investigated using a continuum mixt ure mathematical model. The mushy region where solid and liquid phases co-e xist is considered a non-Newtonian fluid below a critical solid fraction, a nd a porous medium thereafter. This critical solid fraction is chosen as th at corresponding to the coherency point, where a solid skeleton begins to f orm. The numerical results show that the solidification of a hypereutectic NH4Cl-H2O solution is mainly characterized by the rejection of solute at th e mushy region and double diffusive convection induced by the opposing solu tal and thermal buoyancy forces. The mathematical model agrees satisfactori ly with the available experimental and numerical data. (C) 2001 Elsevier Sc ience Ltd. All rights reserved.