CRITERIA FOR THE DETONATION OF ALUMINUM PARTICLES DISPERSED IN AIR

An experimental investigation was conducted to establish and evaluate the criteria for the detonation of aluminum powders dispersed in air. Most of the experimental effort was conducted in a specially designed and highly instrumented detonation-tube facility. Flake-aluminum powder of 3 to 4 m('2)/g was readily detonated using 2.8g high explosive for initiation. Non-detonating initiators, such as high radiant/thermal squibbs, were not successful in achieving detonation. Results indicated that this flake powder detonates in similar fashion as homogeneous gas-phase systems; i.e., the induction times between the shock and reaction fronts were in the (mu)sec range. The detonation velocities were as high as 1.65 km/sec, comparable to the Chapman-Jouguet velocity of 1.85 km/sec. Detonation pressures were generally about double the C-J value of about 2.5 GPa. Spinning detonations were observed. Atomized-aluminum powder also detonated but with greater difficulty and degraded detonation characteristics. If the aluminum oxide coating of both types of powder was purposely increased by heating to 250 or 330(DEGREES)C, degradation of ease of initiation was observed with associated increased induction times if detonation was achieved at all. Powders of 15 to 25 (mu)m could not be detonated.; In large-scale field tests using 4.54 kg of aluminum powder and explosive dissemination, only the flake powder could be detonated. Explosive charges of 454 g were inadequate for initiation, but 2.27 kg explosive charges did initiate detonation in dispersed flake powder clouds of 6 to 8-m diameter. The average concentration was 0.7 kg/m('3), about half the stoichiometric concentration. The detonation velocity, obtained from high-speed film records, was about 2 km/sec. In the case of the atomized powder, detonation would not propagate although there was an initial incipient blast enhancement due to the extensive overdrive from the initiator charge. Three types of explosive charges were used: (1) high shock/low radiant energy, (2) low shock/high radiant energy, and (3) high shock/high radiant energy. It was observed that (1) caused severe disproportionation in the cloud whereas (3) appeared to minimize induction time for detonation initiation.; It is concluded that the unconfined detonation of aluminum powders dispersed in air is achievable, even in inhomogeneous dispersions, but that very strong shock energy is required. The role of radiation appears to be major since it allows initiation of detonation with less induction time if combined with shock initiation. Heating the particles to the melting point of the aluminum oxide coating is believed to be necessary for initiation. Calculations demonstrated that with particle sizes greater than several (mu)m the induction time becomes large and hence causes decoupling of the shock and reaction fronts.