Modeling of the quenching of blast products from energetic materials by expansion into vacuum

Condensed phase energetic materials include propellants and explosives. Their detonation or burning products generate dense, high pressure states that are often adjacent to regions that are at vacuum or near-vacuum conditions. An important chemical diagnostic experiment is the time of flight mass spectroscopy experiment that initiates an energetic material sample via an impact from a flyer plate, whose products expand into a vacuum. The rapid expansion quenches the reaction in the products so that the products can be differentiated by molecular weight detection as they stream past a detector. Analysis of this experiment requires a gas dynamic simulation of the products of a reacting multi-component gas that flows into a vacuum region. Extreme computational difficulties can arise if flow near the vacuum interface is not carefully and accurately computed. We modify an algorithm proposed by Munz [1], that computed the fluxes appropriate to a gas-vacuum interface for an inert ideal gas, and extend it to a multi-component mixture of reacting chemical components reactions with general, non-ideal equations of state. We illustrate how to incorporate that extension in the context of a complete set of algorithms for a general, cell-based flow solver. A key step is to use the local exact solution for an isentropic expansion fan, for the mixture that connects the computed flow states to the vacuum. Regularity conditions (i.e. the Liu-Smoller conditions) are necessary conditions that must be imposed on the equation of state of the multicomponent fluid in the limit of a vacuum state. We show that the Jones, Wilkins, Lee (JWL) equation of state meets these requirements.