Mechanisms of prompt and delayed ignition and combustion of explosively dispersed aluminum powder

A multidimensional numerical study was performed to explore the ignition and combustion mechanisms of an annular aluminum particle shell surrounding a trinitrotoluene (TNT) charge. The model equations consist of a fully compressible reacting gas coupled to a kinetic-theory-based Eulerian granular multiphase model. The parametric study explored the influence of thickness of the Al particle layer, initial Al packing, and diameter of the Al particles. The computed results show ignition and flame structures that are consistent with both delayed and prompt ignition of explosively dispersed reactive powder. In delayed ignition, the Al particles ignite several milliseconds after the dispersal when the particles interact with the surface of the TNT fireball during the negative phase of the blast. The resulting turbulent Al dust flame propagates from the inside toward the outside of the dispersed Al dust cloud. The results show that smaller-diameter Al particles ignite more quickly and propagate through the dispersed dust cloud more rapidly. Paradoxically, larger amounts of smaller Al particles are unburned after the turbulent flame propagates through the dust cloud in comparison to larger-diameter particles. Prompt ignition occurs when the Al particles ignite in the shock-heated air almost immediately as the particles start to disperse. The flame structure for prompt ignition scenarios is a non-premixed dust flame where the fuel and air are mixed by velocity slip between the gas and particles.