Aluminized composite propellant combustion modeling with Heterogeneous Quasi-One dimensional (HeQu1-D) approach

The Heterogeneous Quasi One-dimensional (HeQu1-D) model for AP/HTPB composite propellant combustion is extended to aluminized propellants. Following the serial burning approach of HeQu1-D, a statistical particle path through an aluminized propellant is taken to consist of aluminized binder-matrix coated AP particles of various sizes. Extending the idea of homogenization of fine-AP particles, aluminum particles are assumed to be homogenized with the binder-matrix. Large Al particles (nominal size >= 15 mu m) in the binder-matrix are assumed to get ejected into the gas phase and hence do not contribute to heat feedback to the burning surface. With these modifications, the model is shown to accurately predict the burn rate variation with pressure and initial temperature of propellants using Al of nominal sizes in the range of 15-50 mu m (termed conventional aluminum, CAl). The experimentally observed reduction in propellant burn rate with substitution of AP by CAl is shown to be due to the increase in fuel richness and energy sink effect of melting of Al. Catalytic effects due to addition of burn rate modifiers (Fe2O3 in space shuttle booster propellant, for instance) are also accounted for by modifications to gas phase rate parameters. Increase in burn rate up to 25% with Al particles of a few pm (3-6 mu m) in comparison with non-aluminized propellant is explained by recognizing that there will be non-negligible fraction of sub-micron Al due to the associated particle size distribution. This leads to heat release by sub-micron Al combustion close to the surface and an enhancement in the heat feedback, by a combination of convective and radiative mechanisms. Dramatic increase in burn rate with sub-micron Al (a factor of 4-5 as compared to CAl) is captured by invoking radiation from fine-Al/Al2O3 particles to propellant surface. Agglomeration of sub micron Al particles is invoked to explain the saturation of burn rate enhancement with reduction in Al particle size and the effectiveness of sub-micron Al substitution in smaller fractions (bi-modal Al). Predictions for over fifteen different propellants with varying fractions of fine and coarse aluminum from earlier literature have been presented. Comparisons of the predictions from the model with experimental results from different sources is shown to be excellent-to-good for most cases. The hitherto unknown radiation dominated ablation (r(over dot) > 50 mm/s) of coarse AP particles and the effect of Al particle size on propellant temperature sensitivity are brought out. The MATLAB (R) code based on the model offers opportunity for design of AP-HTPB composite propellants with combination of fine and coarse aluminum with confidence. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved