Title: The evolution of the temperature field during cavity collapse in liquid nitromethane. Part II: Reactive case,
Abstract: We study effect of cavity collapse in non-ideal explosives as a means of
controlling their sensitivity. The main aim is to understand the origin of
localised temperature peaks (hot spots) that play a leading order role at early
ignition stages. Thus, we perform 2D and 3D numerical simulations of shock
induced single gas-cavity collapse in nitromethane. Ignition is the result of a
complex interplay between fluid dynamics and exothermic chemical reaction. In
part I of this work we focused on the hydrodynamic effects in the collapse
process by switching off the reaction terms in the mathematical model. Here, we
reinstate the reactive terms and study the collapse of the cavity in the
presence of chemical reactions. We use a multi-phase formulation which
overcomes current challenges of cavity collapse modelling in reactive media to
obtain oscillation-free temperature fields across material interfaces to allow
the use of a temperature-based reaction rate law. The mathematical and physical
models are validated against experimental and analytic data. We identify which
of the previously-determined (in part I of this work) high-temperature regions
lead to ignition and comment on their reactive strength and reaction growth
rate. We quantify the sensitisation of nitromethane by the collapse of the
cavity by comparing ignition times of neat and single-cavity material; the
ignition occurs in less than half the ignition time of the neat material. We
compare 2D and 3D simulations to examine the change in topology, temperature
and reactive strength of the hot spots by the third dimension. It is apparent
that belated ignition times can be avoided by the use of 3D simulations. The
effect of the chemical reactions on the topology and strength of the hot spots
in the timescales considered is studied by comparing inert and reactive
simulations and examine maximum temperature fields and their growth rates.