Numerical study of detonation in solid explosives under hydrodynamic and elastic-plastic confinement

Initiation devices used in mining have strict requirements for safety and efficiency. However, the analysis of their operation is encumbered by their complex design which involves multiple explosive charges and inert materials. We use numerical simulations to study detonation in configurations involving complex geometry and multiple materials with the aim of revealing key features of their internal processes and improving their reliability and performance. The mathematical model is based on a two-phase reactive formulation and is extended with porosity and shock desensitization models. It is coupled with appropriate inert material models for fluids and solids to accurately capture their interaction with the detonation wave. We initially consider detonation propagation in annular charges. The model and implementation are validated against experimental data for steady state propagation. Then, the numerical solution is used to obtain a detailed description of the speed of the detonation wave along the annular arc and a new description of the transition phase is proposed. Further, a parametric study is performed in which the dependence of the transition phase and steady state on the dimensions of the annulus is analysed. The rest of the thesis examines detonation in explosive devices used in the initiation of tertiary explosives in mining. First, we consider the response of a detonator in isolation, guided by an underwater explosion test. Following validation, the strength of the blast wave is examined at several distances from the detonator. Results show that the blast wave in the near field is asymmetric and stronger along the axis of the detonator. Further, the near field blast wave varies considerably between detonators of different shell material and thickness while the pulse in the far field is similar. This indicates that the fine differences between detonators cannot be captured by tests that consider the blast wave at a single point in the far field. Lastly, we study the complete configuration used to initiate explosives in mining blastholes which involves a detonator and a booster. The reactive model is extended to account for shock desensitization. The model is validated and a series of simulations of the detonator and booster configuration, with and without desensitization, are performed. These show that the influence of desensitization is significant and can lead to the formation of dead zones in the explosive which have a critical impact on booster performance. Depending on the material of the detonator shell, the initiation of the booster can result in only a small non-reacted region or in an extensive desensitized zone which prevents the detonation of a large portion of the explosive.EPSRC DTP studentship (ref. 1498435

Doctor of Philosophy

dissertationThe detonation of hundreds of explosive devices from either a transportation or storage accident is an extremely dangerous event. Motivation for this work came from a transportation accident where a truck carrying 16,000 kg of seismic boosters overturned, caught fire and detonated. The damage was catastrophic, creating a crater 24 m wide by 10 m deep in the middle of the highway. Our particular interest is understanding the fundamental physical mechanisms by which convective deflagration of cylindrical PBX-9501 devices can transition to a fully-developed detonation in transportation and storage accidents. Predictive computer simulations of large-scale deflagrations and detonations are dependent on the availability of robust reaction models embedded in a computational framework capable of running on massively parallel computer architectures. Our research group has been developing such models in the Uintah Computational Framework, which is capable of scaling up to 512 K cores. The current Deflagration to Detonation Transition (DDT) model merges a combustion model from Ward, Son, and Brewster that captures the effects of pressure and initial temperature on the burn rate, with a criteria model for burning in cracks of damaged explosives from Berghout et al., and a detonation model from Souers describing fully developed detonation. The prior extensive validation against experimental tests was extended to a wide range of temporal and spatial scales. We made changes to the reactant equation of state-enabling predictions of combustions, explosions, and detonations over a range of pressures spanning five orders of magnitude. A resolution dependence was eliminated from the reaction model facilitating large scale simulations to be run at a resolution of 2 mm without loss of fidelity. Adjustments were also made to slow down the flame propagation of conductive and convective deflagration. Large two- and three-dimensional simulations revealed two dominant mechanisms for the initiation of a DDT, inertial confinement and Impact to Detonation Transition. Understanding these mechanisms led to identifying ways to package and store explosive devices that reduced the probability of a detonation. We determined that the arrangement of the explosive cylinders and the number of devices packed in a box greatly affected the propensity to transition to a detonation

A multi-physics methodology for the simulation of reactive flow and elastoplastic structural response

We propose a numerical methodology for the numerical simulation of distinct, interacting physical processes described by a combination of compressible, inert and reactive forms of the Euler equations, multiphase equations and elastoplastic equations. These systems of equations are usually solved by coupling finite element and CFD models. Here we solve them simultaneously, by recasting all the equations in the same, hyperbolic form and solving them on the same grid with the same finite-volume numerical schemes. The proposed compressible, multiphase, hydrodynamic formulation can employ a hierarchy of five reactive and non-reactive flow models, which allows simple to more involved applications to be directly described by the appropriate selection. The communication between the hydrodynamic and elastoplastic systems is facilitated by means of mixed-material Riemann solvers at the boundaries of the systems, which represent physical material boundaries. To this end we derive approximate mixed Riemann solvers for each pair of the above models based on characteristic equations. The components for reactive flow and elastoplastic solid modelling are validated separately before presenting validation for the full, coupled systems. Multi-dimensional use cases demonstrate the suitability of the reactive flow-solid interaction methodology in the context of impact-driven initiation of reactive flow and structural response due to violent reaction in automotive (e.g. car crash) or defence (e.g. explosive reactive armour) applications. Several types of explosives (C4, Detasheet, nitromethane, gaseous fuel) in gaseous, liquid and solid state are considered.This work was supported by Jaguar Land Rover and the UK-EPSRC grant EP/K014188/1 as part of the jointly funded Programme for Simulation Innovation

The 1999 Center for Simulation of Dynamic Response in Materials Annual Technical Report

Introduction: This annual report describes research accomplishments for FY 99 of the Center for Simulation of Dynamic Response of Materials. The Center is constructing a virtual shock physics facility in which the full three dimensional response of a variety of target materials can be computed for a wide range of compressive, ten- sional, and shear loadings, including those produced by detonation of energetic materials. The goals are to facilitate computation of a variety of experiments in which strong shock and detonation waves are made to impinge on targets consisting of various combinations of materials, compute the subsequent dy- namic response of the target materials, and validate these computations against experimental data

The evolution of the temperature field during cavity collapse in liquid nitromethane. Part I: inert case

We study the effect of cavity collapse in non-ideal explosives as a means of controlling their sensitivity. The aim is to understand the origin of localised temperature peaks (hot spots) which play a key role at the early stages of ignition. Thus we perform 2D and 3D numerical simulations of shock induced gas-cavity collapse in nitromethane. Ignition is the result of a complex interplay between fluid dynamics and exothermic chemical reaction. To understand the relative contribution between these two processes we consider in this first part of the work the evolution of the physical system in the absence of chemical reactions. We employ a multi-phase mathematical formulation which accounts for the large density difference across the gas-liquid interface without generating spurious temperature peaks. The mathematical and physical models are validated against experimental, analytic and numerical data. Previous studies identified the impact of the upwind side of the cavity wall to the downwind one as the main reason for the generation of a hot-spot outside of the cavity; this is also observed in this work. However, it is apparent that the topology of the temperature field is more complex than previously thought and additional hot spots locations exist, arising from the generation of Mach stems rather than jet impact. To explain the generation mechanisms and topology of the hot spots we follow the complex wave patterns generated and identify the temperature elevation or reduction generated by each wave. This allows to track each hot spot back to its origins. We show that the highest hot spot temperatures can be more than twice the post-incident shock temperature of the neat material and can thus lead to ignition. By comparing the maximum temperature observed in the domain in 2D and 3D simulations we show that 3D calculations are necessary to avoid belated ignition times in reactive scenarios

The evolution of the temperature field during cavity collapse in liquid nitromethane. Part II: reactive case

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