Convective instabilities during directional solidification

Abstract

Results of a ground-based experimental program that supports a low-gravity space processing Spacelab experiment are reported. The phenomena which precipitate pluming and thus freckling in a metal alloy are studied in detail using a metal analog (ammonium chloride and water) and the sequential events leading to massive channeling and convection are optically documented. The pluming is shown not to be a random burst of unstable fluid from a preferred channel but, rather, a natural occurrence resulting from a fundamental (Rayleigh-Benard) fluid dynamic instability at the density inversion interface. Rayleigh numbers are calculated for the instability and a critical Rayleigh number is determined.