On the mechanism of aluminum ignition in steam explosions

An available theory [Epstein, M., Fauske, H.K., 1994. A crystallization the ory of underwater aluminum ignition. Nucl. Eng. Des. 146, 147-164] of the i gnition of aluminum melt drops under water, which is based on the assumptio n that the aluminum oxide (Al2O3) drop-surface skin first appears in a meta stable molten state, is compared with existing experimental data on the ign ition of aluminum drops behind shock waves in water [Theofanous, T.G., Chen , X., DiPiazza, P., Epstein, M., Fauske, H.K., 1994. Ignition of aluminum d roplets behind shock waves in water, Phys. Fluids 6, 3513-3515]. The predic ted and measured ignition temperature of about 1770 K coincides approximate ly with the spontaneous nucleation temperature of supercooled liquid Al2O3 (1760 K). This suggests that the crystallization of the oxide layer represe nts a strong 'barrier' to aluminum drop ignition under water. Apparently a similar interpretation is applicable to aluminum drop ignition in gaseous o xidizing atmospheres. We conclude from the theory that the low-temperature aluminum ignitions tin the range 1100-1600 K) that have been observed durin g steam explosions are a consequence of the short aluminum drop oxidation t imes in this environment relative to the characteristic time for Al2O3 crys tallization. Several aspects of the aluminum drop/shock interaction experim ents besides ignition are discussed in the paper. Tn particular, the experi ments provide strong evidence that during the course of a vapor explosion m etal fragmentation occurs via a thermal mechanism at low pressure and prece des the development of a high-pressure shock. (C) 2000 Elsevier Science S.A . All rights reserved.