Explosive behavoir: Characterizing the freeze-out assumption in post-detonation explosive reactions
Explosive behavoir: Characterizing the freeze-out assumption in post-detonation explosive reactions lead image
As research into explosive reactions continues to advance, increasing focus is placed on the detailed chemical reactions that occur post-detonation. Features of the freeze-out assumption, where the expansion of the blast outpaces chemical reactions, had yet to be explored with modern simulation methods.
Egeln et al. developed a comprehensive modeling framework that couples the detonation of an explosive charge to afterburning processes. Using finite-rate detailed chemical kinetics, the group conducted a series of numerical simulations of post-detonation reaction processes to produce a detailed model consisting of 59 species and 368 reactions for the detonation of a 12 mm-diameter hemispherical pentaerythritol tetranitrate (PETN) charge.
“With this model, we can look at a really complex multi-physics system where you have all these physics coming together for how these extreme flows behave,” said author Ryan Houim. “We examined the freeze-out assumption, but it can let you explore other phenomena.”
After an initial period of near-instantaneous reactions, the blast cools to a point at which blast products expand and reactions slow, leading to a “frozen” chemical state.
The team found that assuming activity coefficients equal to unity produces freeze-out mole fractions that closely align with experiments.
“Sometimes the temperature at which people use chemical equilibrium codes for freeze-out can be a little arbitrary,” said author Anthony Elgen. “This model can provide guidance on what temperature to set.”
The results also showed that smaller charges can accurately predict the freeze-out temperature of larger charges, providing a potential reduction in computation time.
The group next looks to extend this framework to investigate the late-time mixing behavior between the ambient air and the fireball.
Source: “Post-detonation fireball modeling: Validation of freeze-out approximations,” by Anthony A. Egeln Jr., John C. Hewson, Daniel R. Guildenbecher, Ryan T. Marinis, Marc C. Welliver, and Ryan W. Houim, Physics of Fluids (2023). The article can be accessed at https://doi.org/10.1063/5.0153334 .
