Bulletin of the American Physical Society
19th Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 60, Number 8
Sunday–Friday, June 14–19, 2015; Tampa, Florida
Session B1: Detonation and Shock-Induced Chemistry I: DDT Chemistry |
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Chair: Harold Sandusky, Naval Surface Warfare Center, Indian Head, Steven Son, Purdue University Room: Grand E |
Monday, June 15, 2015 9:15AM - 9:45AM |
B1.00001: Mesoscale challenge of extending atomistic scale chemistry of initiation reactions to deflagration-to-detonation transition Invited Speaker: Santanu Chaudhuri Predictive simulations connecting chemistry that follow the shock or thermal initiation of energetic materials to subsequent deflagration or detonation events is currently outside the realm of possibilities. Molecular dynamics and first-principles based dynamics have made progress in understanding reactions in picosecond to nanosecond time scale. However, connecting the events that leads to deflagration will require simulations using much larger length and time scale to connect the full reaction network. This constitutes a mesoscale challenge in energetic materials research. Recent advances in addressing this mesoscale chemistry challenge in other domains will be discussed. Development in coarse-grain simulations and accelerating reactive MD simulations faces the challenge of simplifying the chemistry by making assumptions on the mechanism with consequences on the outcome. For example, results from thermal ignition of different phases of RDX shows a complex reaction and deterministic behavior for critical temperature before ignition. First-principles calculations for validation of key pathways observed will be discussed. The kinetics observed is dependent on the hot spot temperature, system size and thermal conductivity. Smaller hot spots in simulations needed higher temperature for ignition of the solid. For cases where ignition is observed, the incubation period is dominated by intermolecular and intramolecular hydrogen transfer reactions. The gradual temperature and pressure increase in the incubation period is accompanied by accumulation of heavier polyradicals. The polyradicals with triazine rings from the RDX molecules intact undergo ring-opening reactions which fuel a series of rapid exothermic chemical reactions. Our ongoing work on connecting mesoscale and continuum scale will be discussed. [Preview Abstract] |
Monday, June 15, 2015 9:45AM - 10:00AM |
B1.00002: Modelling propagation of deflagration waves out of hot spots Yehuda Partom It is widely accepted that shock initiation and detonation of heterogeneous explosives come about by a two-step process known as ignition and growth. In the first step a shock sweeping an explosive cell (control volume) creates hot spots that become ignition sites. In the second step deflagration waves (or burn waves) propagate out of those hot spots and transform the reactant in the cell into reaction products. The macroscopic (or average) reaction rate of the reactant in a cell depends on the speed of those deflagration waves and on the average distance between neighbouring hot spots. Here we simulate the propagation of deflagration waves out of hot spots on the mesoscale in axial symmetry using a 2D hydrocode, to which we add heat conduction and bulk reaction. The propagation speed of the deflagration wave depends on both pressure and temperature, where pressure dependence is dominant at low shock level, and temperature dependence is dominant at a higher shock level. From the simulation we obtain deflagration (or burn) fronts emanating out of the hot spots. For intermediate shock levels the deflagration waves consume the explosive between hot spots. For higher shock levels the deflagration waves strengthen to become detonation waves on the mesoscale. From the simulation results we extract average deflagration wave speeds and show how they depend on reaction rate and on other material parameters. [Preview Abstract] |
Monday, June 15, 2015 10:00AM - 10:15AM |
B1.00003: Geometry effects on detonation in vapor-deposited hexanitroazobenzene (HNAB) Alexander S. Tappan, Ryan R. Wixom, Robert Knepper Physical vapor deposition is a technique that can be used to produce explosive films with controlled geometry and microstructure. Films of the high explosive hexanitroazobenzene (HNAB) were deposited by vacuum thermal evaporation. HNAB deposits in an amorphous state that crystallizes over time into a polycrystalline material with high density and a consistent porosity distribution. In previous work, we have evaluated detonation critical thickness in HNAB films in an effectively infinite slab geometry with insignificant side losses. In this work, the effect of geometry on detonation failure was investigated by performing experiments on films with different thicknesses, while also changing dimensions such that side losses became significant. Films were characterized with surface profilometry and scanning electron microscopy. The results of these experiments will be discussed in the context of small sample geometry, deposited film microstructure, and density. [Preview Abstract] |
Monday, June 15, 2015 10:15AM - 10:30AM |
B1.00004: Multistage reaction pathways in detonating high explosives Ying Li, Rajiv Kalia, Aiichiro Nakano, Priya Vashishta Atomistic mechanisms underlying the reaction time and intermediate reaction products of detonating high explosives far from equilibrium have been elusive. This is because detonation is one of the hardest multiscale physics problems, in which diverse length and time scales play important roles. Here, large spatiotemporal-scale reactive molecular dynamics simulations validated by quantum molecular dynamics simulations reveal a two-stage reaction mechanism during the detonation of cyclotrimethylenetrinitramine crystal. Rapid production of N$_{2}$ and H$_{2}$O within 10 ps is followed by delayed production of CO molecules beyond ns. We found that further decomposition towards the final products is inhibited by the formation of large metastable carbon- and oxygen-rich clusters with fractal geometry. In addition, we found distinct uni-molecular and intermolecular reaction pathways, respectively, for the rapid N$_{2}$ and H$_{2}$O productions. [Preview Abstract] |
Monday, June 15, 2015 10:30AM - 10:45AM |
B1.00005: Exploring a Detonation Nature of Mesoscopic Perturbations and Ejecta Formation from the Mesoscale Probing of the PBX-driven Liners Igor Plaksin, Raafat Guiruis, Luis Rodrigues, Ricardo Mendes, Svyatoslav Plaksin, Eduardo Fernandes, Claudia Ferreira Ejecting debris from free surface of liner is of considerable interest at optimization of explosive devices, in which the PBX-driven liner effects shock compression of gaseous matter. Following factors were historically considered as main drivers of material ejection: granular microstructure of liner material, roughness and surface defects of liner, and shock pressure time history in PBX-driven liner. In contrast to existing models, we are considering the small scale fluctuations of detonation flow as probable dominating factor of surface jetting in the PBX-driven collapsing liners. Obtained experimental evidence is indicative that jetting from the liners is caused by meso-scale perturbations of PBX detonations, which are identified as (1) ejecta of overdriven detonation products through detonation front, (2) ejecta-driven detonation cells, and (3) galloping detonation front motion. Spatially resolved scenarios of each of phenomena (1-3) were obtained in experiments with copper-liners and HMX-based PBXs fabricated on maximum packing density of crystalline constituents. Both the DRZ-induced perturbations translated to a PBX-driven liner and the ejected debris were recorded and quantitatively measured in the mesoscale range with application of the 96-channel optical analyzer MCOA-UC. [Preview Abstract] |
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