| With the goal of understanding the behavior of aluminum in solid propellant combustion, the flame structure of isolated aluminum particles burning in a variety of controlled environments has been examined experimentally and computationally. Spherical aluminum particles of 220 mum diameter were generated continuously by mechanical chopping of wire strands and Co2 laser heating, and were released into cold, quiescent environments consisting of pure CO2,H2O, N2O,CO, and mixtures of 21% O2/Ar, 50% O2/Ar and 21% O2/N2. Spatially resolved species and temperature measurements were carried out during a quasi-steady, near spherically symmetric burning period using an in situ two-excitation-line ratiometric planar laser-induced fluorescence (PLIF) technique. The radial distributions of the condensed-phase products formed in the flame, and the composition within quenched particles, were measured using off-line electron probe microanalysis (EPMA). To aid the interpretation of the experimental results, a detailed local equilibrium numerical model of aluminum particle combustion has been developed. The important aspect of the model is that neither thin flame sheet, nor thin condensation sheet assumptions were invoked.; In accord with Glassman's Criterion for the vapor-phase combustion of metals, aluminum particles burned with an intensely luminous, detached flame in all atmospheres studied, with the exception of CO. During quasi-steady particle combustion, AlO was a gas-phase intermediate with non-zero concentrations on the particle surface. The 21% O2/Ar, 21% O2/N2 and N2O combustion systems attained a nearly constant flame temperature over significant radial distances. These systems were thermodynamically expected to reach a ""limit"" temperature because the heat of combustion is sufficient to partially decompose Al2O3. In contrast, the temperature profile for combustion in CO2 did not exhibit a plateau, but had the shape of a classical diffusion flame. Except in the CO atmosphere, spherical particles between 100--200 nm, consisting of stoichiometric Al2O3, were produced in the detached envelope at r/rs > 2. The presence of N in the combustion gases significantly altered the flame structure, and also promoted the formation of large (5--60 mum) residual particles.; Qualitatively, model and experiment were in excellent agreement for combustion in an air-equivalent mixture of O2 and Ar. The model predicted a distributed reaction zone in which the flame temperature was controlled by the decomposing condensed-phase product. The model also predicted the pressure dependence of the mass burning rate found in the literature. Contrary to previous explanations, however, this pressure sensitivity was found to originate from the pressure dependence of the gasification temperature of Al and decomposition temperature of Al2O3 l . Model results obtained for aluminum combustion in pure CO2 agreed less well with the experimental data, and hence indicated the importance of sub-processes such as the nucleation, coagulation, growth and migration of condensed-phase oxides. |
