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A Summary of Aluminum Combustion

The combustion characteristics of aluminum combustion are summarised in an overview of the subject, focusing on the burning time of individual particles. The fundamental concepts that control aluminum combustion are discussed starting with a discussion of the “D n ” law. Combustion data from over ten different sources with almost 400 datum points have been cataloged and correlated. Available models have also been used to evaluate combustion trends with key environmental parameters. The exponent is shown to be less than two, with nominal values of ~1.5 to 1.8 being typical. The effect of oxidizer is pronounced with oxygen being twice as effective as water and about five times more effective than carbon dioxide. The observed effect of pressure and initial temperature is minimal. In the second part of the paper a two-dimensional unsteady state kinetic-diffusion-vaporization controlled numerical model for aluminum particle combustion is presented. The model solves the conservation equations, while accounting for the species generation and destruction with a 15 reaction kinetic mechanism. Two of the major phenomena that differentiate aluminum combustion from hydrocarbon droplet combustion, namely the condensation of the aluminum oxide product and the subsequent deposition of part of the condensed oxide, are accounted for in detail with a sub-model for each phenomenon. The effect of the oxide cap in the distortion of the profiles around the particle has been included in the model. The results obtained from the model, which include two-dimensional species and temperature profiles, are analyzed and compared with experimental data. The combustion process is found to approach a diffusion controlled process for the oxidizers and conditions treated. The flame zone location and thickness is found to vary with oxidizer. The result shows that the exponent of the particle diameter dependence of burning time is not a constant and changes from about 1.2 for larger diameter particles to 1.9 for smaller diameter particles. Due to the deposition of the aluminum oxide on the particle surface, particle velocity oscillates. The effect of pressure is analyzed for a few oxidizers.