Combustion of metal droplets ignited by flash heating

Abstract

The use of single, intense pulses of light from a capacitor discharge lamp to form and ignite submillimeter droplets of metals in a glass oxidation chamber is described. The droplets were photographed in free fall, or were quenched in liquid argon at various stages of combustion and analyzed conventionally. It has been possible to investigate burning droplets of Zr, Ta, Ti, Mo, W, Pu, Sm, Re, and Al by this technique. The combustion of Zr is emphasized here. The violent fragmentation or explosion of zirconium droplets that occurs a short time after ignition has been shown to be related to the presence of nitrogen and possibly other gas-related impurities in the combustion system. Zirconium droplets that do not explode when burning in pure oxygen may be induced to fragmenteither by dissolving a small amount of nitrogen in the metal before ignition or by adding a few per cent of gaseous nitrogen to the oxygen. A similar effect can be produced by coating the metal thinly with a heavy hydrocarbon wax prior to ignition. In the explosion-free combustion of zirconium droplets, it has been possible to observe spearpointing—the abrupt brightening of a droplet near the end of combustion—by photographic techniques. This has been ascribed to the sudden freezing of the droplet and is probably due to nucleation of the droplet after appreciable supercooling. This phenomenon should provide a calibration point for the determination of burning temperatures of certain metal droplets. Direct microweighing of droplets quenched in liquid argon at various stages of combustion has provided preliminary information on the kinetics of oxygen uptake by molten zirconium. In pure oxygen near atmospherie pressure, a 525-μ droplet reacts with less than 3% of the oxygen molecules that strike its surface during combustion, an indication that mass-transfer processes within the droplet limit the rate of oxidation.