Post-Detonation Afterburning of High Explosives

Abstract: High performance of an explosive compound with respect to afterburning requires sufficient combustible properties of the explosive, and a careful determination of the most appropriate charge positioning to achieve the desired afterburning effect. Understanding the physical processes of post-detonation afterburning and how these are affected by the surroundings, e.g. the Height of Burst (HoB), facilitates the optimal use of the explosive and also helps in designing protection against it. The use of Large Eddy Simulation (LES) for investigating afterburning properties of an explosive charge can be a cost effective approach to identify the most optimal conditions for subsequent full-scale experiments. Of particular interest are Enhanced Blast eXplosives (EBX), to which metal particles, usually aluminium, are added to the explosive compound in order to increase the afterburning energy release by allowing the metal particles and detonation products combust with air. This presents a further modelling challenge since the combustion becomes multi-phased.This thesis presents modelling, simulation and experimental efforts in studying this two-phase post-detonation combustion event at different HoB of 1 kg trinitrotoluene (TNT) and TNT/aluminium charges. The main objectives of this work is to demonstrate the use of LES with finite rate chemistry for these types of applications, to elucidate the physical processes involved in near-ground air blasts, to demonstrate what effects the HoB has on the afterburning, and how aluminium particles affect the combustion. Simulation results, supported by experimental data, show that the main mechanism responsible for the mixing, and therefore afterburning, is the rise of hydrodynamic instabilities, which trigger the build up of a mixing layer. Shock-mixing layer interactions further create more instabilities. Thus, in order to achieve maximum effect of the afterburning during an explosive blast, the existence of a turbulent mixing layer has to be combined with repeated shock propagation through it, by which the duration of the afterburning is maintained. The presence of reacting particles increases the vorticity generated by instabilities since the particles create perturbations in the detonation product cloud, hence disrupting the alignment of the pressure and density gradients. Burning particles intensify mixing even further through volumetric expansion induced by increased heat-release from particle combustion. The mixing intensity in its turn varies with HoB, as the shock propagation pattern is different for all HoB, which means that in order to achieve maximum effect from aluminium inclusion to an explosive, HoB must be considered as a parameter.