Numerical Simulation of Chemical Freeze-Out in Explosive Post-Detonation Flows

A numerical simulation study was performed to examine the post-detonation reaction processes produced by the detonation of a 12 mm-diameter hemispherical PETN explosive charge. The simulations used a finite rate detailed chemical reaction model consisting of 59 species and 368 reactions to capture post-detonation reaction processes. The BKW real-gas equation of state is used for the gas phase to allow for the mixing of reactive species. A recent simplified reactive burn model is used to propagate the detonation through the charge and allow for detailed post-detonation reaction processes. The computed blast, shock structures, and mole fractions of detonation product species agree well with experimental measurements. A comparison of the simulation results to equilibrium calculations indicates that the assumption of a local equilibrium is fairly accurate until the detonation products rapidly cool to temperatures below 1500 to 1800 K by expansion waves. Below this range, the computed results show mole fractions that are nearly chemically frozen within the detonation products for a significant portion of expansion. These results are consistent with the freeze-out approximation used in the blast modeling community.