Applications of chemical kinetics to detonation: Wave coalescence and homogeneous initiation in single crystal 1,3-propanediol-2,2-bis[(nitrooxy)methyl]-tetranitrate (PETN)

A thermal ignition model is used in parallel with single crystal solid and product fluid Equations of State (EOS) from the literature to calculate the time and distance to homogeneous shock initiation in single crystal PETN. The ignition model is a highly constrained, globalized representation of the temperature and pressure dependent decomposition chemistry of PETN. Initiation in the single crystal proceeds via a homogeneous initiation mechanism where thermal ignition results from a well-defined initial shock state (P,T,V). The transition to steady detonation at an observed location (x*, t*) then follows when a superdetonation wave initiated by this thermal ignition overtakes the input shock wave. In the traditional approach P and V are determined directly from measurements, leaving T relatively unconstrained due to the extremely nonlinear dependence of T on P, V in the EOS. We present an iterative algorithm that captures and clearly separates these phenomena by calculating T as a function of P from the chemical kinetics of the thermal ignition. This allows the determination of the input shock state (P,T,V) without recourse to typical assumptions of energy and momentum conservation across an infinitely thin shock discontinuity or constraints on the entropy change. The calculated states compare favorably with pressures and states of compression observed in experiment but provide a new and interesting set of temperatures associated with each state. We discuss these fully determined initial shock states in (P, V, T) in the context of PETN thermodynamics, specifically the solid/liquid phase boundary at high temperature and pressure.