Diffusion-limited melting in microgravity

Abstract

The Isothermal Dendritic Growth Experiment (IDGE) constituted a series of three NASA-supported microgravity experiments (USMP-2, -3, and -4), all of which flew aboard the space shuttle Columbia. These experimental space flight experiments grew and recorded dendrites in the absence of gravity-induced convective heat transfer, producing benchmark-quality data for testing solidification scaling laws for two substances: 1) succinonitrile, a BCC material, and 2) pivalic acid, an FCC crystal. The IDGE instruments aboard USMP2 and -3 provided in-flight CCD images, and 35-mm films (postflight). USMP-4, for the first time, allowed streaming of near-real-time video data. Using 30 fps video data, it became possible to study both freezing and melting sequences for pivalic acid (PVA) at different supercoolings. The melting study reported here involved image capture and digital processing methods using a custom-designed image processing system. Steady-state crystal growth data from the IDGE, documented elsewhere, indicate that dendritic growth under microgravity conditions is limited by conduction heat transport. We now report on the melting process of a PVA dendritic mushy zone, observed for the first time under convection-free conditions. Conduction-limited melting processes are of importance in orbital melting of materials, meteoritic genesis, mushy-zone evolution, and in fusion weld pools where the length scales for thermal buoyancy are highly restricted. Microgravity video data show that PVA dendrites melt into fragments that shrink at accelerating rates to extinction. The melting paths of individual fragments follow a characteristic time dependence for the diminishing length scales within the mushy zone. The theoretical melting kinetics against which the experimental observations are compared is based on the conduction-limited quasi-static process of melting under shape-preserving conditions. Good agreement between theory and experiment was found for the melting of a needle-shaped prolate spheroidal PVA crystal fragment with an aspect ratio o f C / A = 12. (c)2002 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.