MACAW equations of state for PETN reactants and products

Continuum reactive burn models for high explosives use separate equations of state (EOS) to describe the unreacted material and the detonation products. Common deficiencies in empirical EOS models include: (1) simplistic representations of specific heat capacities and/or the Grüneisen parameter, (2) regions of thermodynamic instability due to negative bulk moduli, or (3) the lack of a well-defined entropy. These issues are observed in several widely used EOS models, such as the Davis reactants and the linear shock-particle velocity relation for condensed-phase materials, or the Davis products and the Jones-Wilkins-Lee (JWL) EOS for supercritical fluids. Building on the analytic framework of Lozano et al. (J. Appl. Phys. 134, 125102, 2023), we present a two-phase thermodynamic model that addresses these deficiencies. Additional terms are incorporated to more accurately represent supercritical fluids, such as those found in high explosive detonation products. Porosity is also introduced into the mixture model using the small-λ formalism with pressure-temperature closure. Using experimental data and density functional theory calculations, we calibrate separate parameters for the reactants and products of Pentaerythritol tetranitrate (PETN) and compare the model against cylinder tests and overdriven experiments.