Doctor of Philosophy (PhD)
Mechanical and Industrial Engineering
Deflagration-to-detonation transition (DDT) in confined, low-density granular HMX (65%-85% Theoretical Maximum Density, TMD) occurs by a complex mechanism that involves compaction shock interactions within the material. Piston driven DDT experiments indicate that detonation is abruptly triggered by the interaction of a strong burn-supported secondary shock and a piston-supported primary (input) shock, where the nature of the interaction depends on initial packing density and primary shock strength. These interactions influence transition by affecting hot-spot formation within the micro-structure during pore collapse. In this study, meso-scale simulations of hot-spot formation in shock loaded granular HMX are used to guide the development of a new hot-spot based macro-scale ignition and burn (I&B) model. The model is conceptually similar to conventional I&B models but describes ignition in terms of pressure-dependent hot-spot formation rate and describes burn in terms of a dissipation-dependent regression rate that accounts for the onset of hot-spot facilitated burn for sufficiently strong shocks. Inert macro- and averaged meso-scale predictions show good agreement, provided that the averaging area size is suitably selected. The I&B model reasonably predicts features representative of a Type-I DDT mechanism that is typical of particulate beds. The mechanism involves the formation of a solid-plug (i.e., a region having 100% TMD) within the bed that significantly affects reaction provided that the local dissipated work is insufficient to trigger hot-spot facilitated burn. Hence, the solid-plug affects the wave dynamics associated with transition. The model also predicts features characteristic of ignition and burn-controlled transition mechanisms and reasonably predicts time and distance to detonation over a wide range of piston impact speeds (150-600 m/s) and initial packing densities (68%-83% TMD). The shock strength required for transition from ignition to burn-controlled initiation increases with initial packing density, and is estimated to be approximately 0.2, 0.32, and 0.39 GPa for \phi_0= 0.68, 0.77 and 0.83, respectively. Predictions also highlight conditions favorable for the formation of spontaneous combustion waves whose propagation speed is influenced by shallow spatial gradients in solid volume fraction within the plug region.
Rao, Pratap Thamanna, "A Multi-Scale Approach for Modeling Shock Ignition and Burn of Granular HMX" (2017). LSU Doctoral Dissertations. 4174.
Gonthier, Keith A.