Theoretical Analysis and Model Experiment on Thermosolutal Convection during Unidirectional Solidification in the Vertical Direction

Abstract

Double diffusive phenomena which causes various types of convection in a fluid phase stems from two coexisting diffusive components with different diffusivities relating to heat or mass. Typical convection regimes due to the double diffusive phenomena are the finger interface and the diffusive interface.
On the basis of a set of governing equations which describes the double diffusive phenomena, thermal Rayleigh number (Ra_T) and solutal Rayleigh number (Ra_C) were extracted as the controlling dimensionless numbers. Various convection regimes were classified on a Ra_T vs. Ra_C diagram. A cold model experiment using NH₄Cl-H₂0 system has been conducted to confirm the relationship on the Ra_T vs. Ra_C diagram. Transitional changes in the finger and the diffusive interfaces were visualized by the shadowgraph method. The convection regimes observed in the experimental system were successfully simulated by numerical computation of the governing equations.
Double diffusive phenomena during unidirectional solidification in the vertical direction was investigated based on both the numerical simulations and experiments for the NH₄Cl-H₂O system and Sn-Pb system. Observations and theoretical predictions suggested that finger type upward flows which selectively grow in liquid phase in the first stage of solidification induce channels in mushy zone and which in turn resulted in freckles in solidified ingot.