Detonability of RDX Dust in Air/Oxygen Mixtures

A combined experimental and theoretical study was conducted to determine the detonability characteristics of high-explosive RDX dust dispersed in air. Toward this end, a special shock tube was constructed in which the dust was transported through the tube by a gas flow. Detonation of high-pressure gases in the driver served to transmit a strong blast wave into the dust mixture. The resultant wave was monitored by pressure switches, pressure transducers, a photocell, and streak photography. Within the limits of the facility, detonation was realized for dust with the larger of Ihe particles (37 fim) tested in oxygen-enrich ed air (12% O2 + 88% air by volume) and with ammonium perchlorate added to the dust mixture as an additional oxidizer (11% AP + 83% RDX by mass) in air. However, under the same conditions, the finer dust with the smaller particles (2 jim) could not be detonated. A theoretical model was developed to explain the effect of particle size on the detonability of dust/oxidizer mixtures. In this model, an unsteady particle heat-transfer equation with an Arrhenius type of reactive source term was coupled with the flow conservation equations. The thermal effect due to the presence of particles on the flow prior to ignition were calculated for various particle sizes and loading ratios. It was shown that a large concentration of very fine dust mixed with the gaseous oxidizer would cool the gas behind the incident shock wave sufficiently to deter the ignition of the particles. This provides an explanation for why the finer dust failed to detonate.