Unconfined hybrid detonation waves were experimentally investigated in a suspension of aluminum (Al) particles and liquid fuel spray in air. The study reported here included the results from both 30 kg and 200 kg Al-liquid fuel mass. A hydrocarbon liquid fuel was used and its detonability is characterized by a mean detonation cell size of 33 mm in its stoichiometric vapor-air mixture as determined from laboratory detonation tube experiments. Micrometric Al powder saturated with the liquid fuel was contained in a polyethylene cylindrical casing with an explosive cased in a glass tube in the center. Explosion of the central explosive dispersed the Al-liquid fuel to an unconfined cylindrical particle-spray suspension in air to a radius and a height with the suspension touching the ground at a given dispersal time. The cylindrical Al-spray-air cloud showed 130-1100 m3 in volume and 5-10 m in radius for 30-200 kg fuel mass, respectively. Detonation of the cloud was then initiated by an initiation (i.e., a secondary) charge near the center of the cloud and propagated cylindrically outwards. The resulting detonation phenomenon was recorded using high-speed video cameras and Endevco piezo-resistive pressure transducers radially located on a ground-level concrete pad at 1 m intervals. Detonation cell sizes were registered using a 1.2 x 0.9 m2 Al smoke foil installed on the ground.
Pressure histories showed a single shock-front detonation wave with an average propagation velocity of 1700 m/s along the cloud radius. The hybrid detonation exhibited a detonation cell size of about 20 mm in average as recorded on the smoke foil. The smaller hybrid detonation cell size, with respect to the 33 mm cell size of the vapor-air mixture of the baseline liquid fuel, indicated that the Al particles react considerably within the fuel spray reaction zone and therefore enhance the spray detonation. In addition to fine detonation cellular structure, the flame surface of the hybrid detonation wave front, recorded by the high-speed cameras for the experiments of both 30 kg and 200 kg fuel, displayed structures in a scale larger than the detonation cell size by one to two orders of magnitude. These structures indicated the interactions of some macroscopic transverse waves. They were generated by the heat release from the nonuniform local particle concentrations depicted as particle jets inherently formed during the explosive dispersal of Al-liquid particles. This non-uniformity or particle clustering and jetting also manifested itself in the detonation velocity and pressure fluctuation. The particle jetting instability phenomenon was further analyzed and the formation mechanism for the primary particle jetting was discussed from both bulk and grain-scale physics.
