Combustion Mechanisms and Simplified-Kinetics Modeling of Homogeneous Energetic Solids

The combustion mechanisms of homogeneous energetic solids, such as double-base propellant, are considered in the framework of simplified-kinetics modeling. The framework is the classical approach of quasi-steady gas and condensed-phase reaction zones, homogeneous solid, and one-dimensional heat feedback and flame structure. The primary system performance parameter of interest is the burning rate or regression rate of the solid. The effect of important environmental parameters such as pressure, temperature, and radiative heat flux, as well as intrinsic system parameters such as temperature sensitivity of condensed- and gas-phase reaction zones, are considered. Validation of modeling assumptions is addressed via comparison of theoretical predictions with experimental observations both for steady state and quasi-steady, time-dependent conditions for the common materials NC/NG and HMX. The approach is to seek a balance between theoretical complexity and physical fidelity (i.e., predictive capability) that achieves some sort of optimum between these competing interests. For example, complexity for its own sake, e.g., in detailed chemistry (particularly if it is uncertain), is not included if it cannot be justified by demonstrated improvement in macroscopic system performance simulation capability. Similarly, mathematical simplicity is not retained for its own sake if it entails serious deleterious effects in physical fidelity. The result of this approach is a simplified mathematical (usually analytical) model that has the ability to elucidate some important fundamental mechanisms as well as simulate both steady and unsteady behavior of these complex chemically reacting dynamic systems with surprising fidelity, given the simplicity of the assumptions. The results illustrate the stark contrast that exists between the limiting assumptions of an asymptotically high gas-phase activation energy versus a vanishingly small one. The former approach, which is the more common one, is actually shown to be less accurate in simulating the macroscopic combustion behavior of common homogeneous energetic solids. Where possible, connections are also made between global modeling parameters and fundamental ones, such as decomposition activation energies and bond strengths.