Mushy Zones Evolution Under Microgravity Conditions

Abstract

High-purity pivalic acid (PVA) dendrites were observed under convection-free conditions on STS-87 as part of the United States Microgravity Payload Mission (USMP4) flown on NASA’s space shuttle Columbia in 1997. Our telemetry video data show that PVA dendrites melt without relative motion with respect to the quiescent melt phase. With a small fixed superheat above the melting point, ∆T ≡ Tm − T∞, designated in the theory by a Stefan number, dendritic fragments melt and shrink steadily. Fragmentation of the dendrites is observed at higher initial supercoolings. Individual fragments follow a squareroot time-dependence as predicted using a quasi-static conduction-limited approach [1]. Agreement between the analytic theory and experiments was found when the melting process occurs under shape-preserving conditions, where needle-like crystal fragments may be approximated as prolate spheroids with a constant C/A ratio. In microgravity experiments where C/A ratio is not constant, because of interactions in the mushy zone, a “sectorizing” approach was employed that divides the melting process into a series of steps, each approximated b ya constant average value of the C/A ratio. Sectorization allows prediction of melting kinetics using quasistatic theory. Theoretical Stefan numbers were calculated for each sector of melting independently using the initial and final axial lengths for that interval.