(646a) Combustion Dynamics of Mechanically Alloyed Al?Mg Powders in Turbulent Flames

Metal powders are widely used as high-energy density additives to energetic formulations.  Recent efforts focused on development of new metal-based materials, that are expected to ignite more readily and burn faster than pure metal powders, such as Al, used in many solid propellants, explosives, and pyrotechnics.  Full scale tests of energetic systems with multiple new components are expensive and thus predictive models for particle ignition and combustion rates are required in order to establish whether the novel materials are compatible with the present system configurations.  Most current models and descriptions are based on laboratory experiments performed in stationary or laminar combustion configurations and with relatively coarse powders of pure metals.  Experimental data on various new materials, especially including fine powders burning in practically interesting turbulently mixing environments are lacking.  In our recent work, an approach to characterize combustion of fine aluminum powders in turbulently mixing gas flows was discussed.  In this paper, the approach is extended to characterize combustion of fine, mechanically alloyed Al∙Mg powders.  The powders with particle sizes roughly from 1 to 20 µm are prepared by ball-milling elemental aluminum and magnesium in a planetary mill.  The Al/Mg ratios are varied to explore powders with different compositions.  The experiments are aimed to measure burn times for the alloyed particles burning in environments with different levels of turbulence. For each material composition, the powders are injected using a nitrogen flow into an air-acetylene flame.  Experiments are performed with both laminar flame and flames turbulized by auxiliary tangential jets of air.  The air flows with adjustable flow rates are used to achieve different controlled levels of turbulence intensity.  Optical emission of the burning particles was recorded using filtered photomultiplier tubes.  Measured durations of individual particle emission pulses were assumed to represent their burn times; these data were classified into logarithmically spaced time bins and are correlated with the particle size distribution to obtain the burn time as a function of the particle size. It is observed that the effect of particle size on burn rate is stronger for the mechanically alloyed Al∙Mg powders compared to that for pure aluminum.  Experiments will be presented for powders of different compositions and burning in environments with different levels of turbulence.  It is expected that these results will serve to develop a mechanistic model for burn rates of Al∙Mg alloys in environments with varied turbulence intensities.  The experimental methodology is expected to be useful for a broad range of reactive powders.