Simplified chemical-kinetic mechanisms for characterizing the structure of the dark zones of double base and nitramine propellants

Abstract

Simplified chemical-kinetic mechanisms were employed to calculate the structure of the dark zone formed over the burning surface of double base and nitramine propellants. It is believed that ignition delays in large-caliber-gun ballistie cycles are related to the chemistry taking place in the dark zone. Skeletal chemical-kinetic mechanisms comprising 22 elementary reactions among 15 species and 23 elementary reactions among 17 species were used to calculate the structure of the dark zone of double base and nitramine propellants, respectively. These skeletal chemical-kinetic mechanisms were previously extracted from a detailed chemical-kinetic mechanism incorporating 190 elementary reactions involving 41 species. Ignition-delay times are calculated using these mechanisms in homogeneous mixtures in which the initial concentrations of various reactants are similar to those found at the beginning of the dark zones of double base and nitramine propellants. These calculations were performed for various initial pressures and temperatures. The ignition-delay times calculated using the skeletal chemical-kinetic mechanisms are found to agree reasonably well with those calculated using the detailed chemical-kinetic mechanism. Reduced chemical-kinetic mechanisms were derived from the skeletal mechanisms by introducing steady-state approximations for a number of species. For double base propellants, a reduced mechanism of three global reactions is obtained. Reduced mechanisms using six and four global reactions are deduced for nitramine propellants. The ignition-delay times and structures of the dark zones calculated using the reduced three-step mechanism for double base propellants and six-step mechanism for nitramine propellants are in agreement with calculations made using the skeletal mechanisms. The ignition-delay times calculated using the four-step reduced mechanism for nitramine propellants are found to be significantly smaller than those calculated using the skeletal mechanism. The reduced chemical-kinetic mechanisms are expected to be useful in interior ballistics calculations.