Explosives dust cloud combustion

Data are presented for the lean limit concentrations, explosion pressures, and rates of pressure rise for air-dispersions of the following explosives dusts: HMX, HNS, RDX, TATB, TNT, tetryl, HNAB, PETN, and picric, acid. While the literature contains extensive studies of their detonability and ignition sensitivity as dense solids or packed poweders, data on their explosibility as dust clouds was limited. These explosive dusts sustain flame propagation when dispersed in air above lean limit concentrations of between 100 g/m 3 and 300 g/m 3 . Their explosibility properties at low to moderate concentrations are comparable to those of plastics or carbonaceous dusts containing equivalent combustible volatile fuel contents. At higher concentrations, most are considerably more hazardous. For intermediate particle sizes, in the 20 to 50 μm range, and at low to moderate concentrations, the −NO 2 oxidizer groups in their chemical structure appear to make little or no contribution to their explosibility, because oxygen is already present in excess in the air. However, at higher concentrations and especially for coarser particles with sizes greater than 100 μm, there is a contribution from the −NO 2 groups, and some of the dusts even exhibit explosibility when dispersed in pure nitrogen. Particle size dependency measurements reveal that the finest dusts (below 20 μm) are less explosible in air than dusts of intermediate size (20–50 μm), a behavior markedly different from that of pure fuel dusts. Eventually, for the coarsest sizes, their explosibility diminishes as with pure fuel dusts. Their combustion mechanisms appear to involve two modes of flame propagation: the normal one observed for pure fuel dusts; and a second mode which requires no air, and which is quenched by mixing dilution and heat losses to the dispersing gas. This second, pure monopropellant mode, involves flame zones attached to the particles. The second mode is favored by higher concentrations and coarse particle sizes. The precise nature of the coupling between the two modes requires further study.