In mining, condensed-phase explosives are sensitised on site, by adding inhomogeneities to the body of the explosive for optimum performance. The inhomogeneities are in the form of gas cavities or solid beads. The interaction of a shock wave with these inhomogeneities generates hotspots; localised regions of high pressure and temperature that lead to an earlier ignition than the neat material.
Sponsor
This project is funded by ORICA. 
The challenge
Ignition is a temperature-dirven phenomenon but conventional mathematical models find it challenging to recover realistic temperature fields. In this study, we have overcome this problem.
To study appropriately the shock-induced collapse of voids (or interaction with beads) in explosives, complex equations of state for describing the materials involved in the simulation are needed, large (1000:1) density differences across the material interface boundary, maintaining oscillation-free interfaces (in terms of pressures, velocities, and temperatures), recovering accurate temperature fields and heavy computations for well-resolved, three-dimensional simulations are also required. We use the MiNi16 formulation to model the explosive and air-cavities, an elasto-plastic formulation for the beads and a multiphysics technique when both types of inhomogeneities are present.
The research
We study the evolution of the temperature field during the cavity collapse in liquid nitromethane, in the inert and reactive scenario. The main objective is to understand the origin of localised temperature peaks which play a leading order role at the early stages of ignition. We perform three dimensional numerical simulations of shock-induced single gas-cavity collapse in liquid nitromethane, and two dimensional simulations of the shock interaction of multiple cavities, multiple beads (and a combination of the two) in nitromethane.
Results
Single cavity collapse
We demonstrated that, compared to experiments, the generated hot spots have a more complex topological structure and that additional hot spots arise in regions away from the cavity centreline. The ignition in both the centreline hot spots and the hotspots generated by Mach stems occurs in less than half the ignition time of neat nitromethane.
Cavity and bead combination
We study the effect of a single cavity, a single PMMA bead, a 4x4 matrix of cavities, a 4x4 matrix of PMMA beads and a 4x4 matrix of a combination of two cavities and two PMMA beads, on the NM temperature we conclude that the cavities have a more profound effect on sensitisation compared to PMMA particles. Thus, the cavities can be used to increase the sensitivity drastically, and combinations of PMMA particles to fine-tune it.
Publications
- A hybrid formulation for the numerical simulation of condensed phase explosives, L. Michael, N. Nikiforakis, Journal of Computational Physics, vol. 316, 2016, 193-217, https://doi.org/10.1016/j.jcp.2016.04.017
- The evolution of the temperature field during cavity collapse in liquid nitromethane. Part II: Reactive case, Michael L. and Nikiforakis N. Shock Waves 29(1), 173-191 (2019), https://doi.org/10.1007/s00193-018-0803-7
- The evolution of the temperature field during cavity collapse in liquid nitromethane. Part I: Inert case, Michael L. and Nikiforakis N. Shock Waves 29(1), 153-172 (2019) https://doi.org/10.1007/s00193-018-0802-8
- Control of condensed-phase explosive behaviour by means of voids and solid particles, Michael L and Nikiforakis N. Active Flow and Combustion Control 2018, Notes on Numerical Fluid Mechanics and Multidisciplinary Design, Springer, https://doi.org/10.1007/978-3-319-98177-2_18