Multidimensional modeling of composite solid propellant combustion

Abstract

A two-dimensional reactive flow model was developed to study AP/HTPB propellant combustion phenomena using CONVERGE CFD. A detailed kinetic mechanism representative of AP and HTPB combustion was thermodynamically coupled to the solid propellant. The solid phase was represented by a physical model and coupled to the reactive flow as boundary conditions. This coupled system produces two-dimensional flow and temperature fields above the burning propellant surface, the flame structure. This flame structure formed above the composite AP based propellant was calculated over a range of pressures and with two characteristic AP particle sizes. The variations in the calculated flame structures and their trends with varying pressure and particle size conform to expectations from theorized AP combustion properties.

This dissertation describes studies which characterize composite ammonium perchlorate (AP) based propellants and develop numerical models for the combustion of these propellants. Modeling the combustion of composite propellants requires that many physical phenomena be captured and represented. The developed simulations were designed taking advantage of various streams of information, such as: microstructure, chemical kinetics, and geometry. The dependence of the AP-based propellant burn rate and flame structure on pressure, ingredient mixture fractions, and particle size was discussed alongside simulated results depicting the behavior using a variety of models.

Microscopy data of the propellant was processed and resulting material characteristics were introduced to the models. Image processing and stereological techniques were used to characterize the heterogeneity of composite propellant and inform a predictive burn rate model. Composite propellant samples made up of ammonium perchlorate (AP), hydroxyl-terminated polybutadiene (HTPB), and aluminum (Al) were faced with an ion mill and imaged with a scanning electron microscope (SEM) and x-ray tomography (micro-CT). Properties of both the bulk and individual components of the composite propellant were determined from a variety of image processing tools.

An algebraic model, based on the improved Beckstead-Derr-Price model developed by Cohen and Strand, was used to predict the steady-state burning of the aluminized composite propellant. In the presented model the presence of aluminum particles within the propellant was introduced. The thermal effects of aluminum particles are accounted for at the solid-gas propellant surface interface and aluminum combustion is considered in the gas phase using a single global reaction. Properties derived from image processing were used directly as model inputs, leading to a sample-specific predictive combustion model. This algebraic model was used to investigate the burn rate of various propellant formulation types, such as monopropellant, mono- and bimodal AP particle size, and aluminized propellants.