Combustion Dynamics of Mechanically Alloyed Al∙Mg Powders in Different Oxidizing Environments

Metal powders are widely used as fuel additives due to their high energy densities and their ability to enhance energetic formulations. Although Al is the most common metal fuel additive because of its high heat of combustion, both ignition delays and burn times of Al particles are relatively long. Powder of Mg ignites easier and may burn faster than Al powder with similar particle sizes; however, Mg has a significantly lower heat of combustion. This work is focused on characterizing the combustion dynamics of fine, mechanically alloyed Al∙Mg powders that might optimize the performance combining high energy density with high reactivity. The powders were prepared in a planetary mill using a staged milling approach to produce fine particle sizes. The Al/Mg ratios were varied to explore powders with different compositions. The experiments were aimed to measure burn times and combustion temperatures for the alloyed particles burning in different oxidizers.  In one set of experiments, powders were injected into an air-acetylene flame.  Thus, powders ignited and burned in a mixture of carbon dioxide and water.  Auxiliary tangential air flows around the flame were used to generate different levels of turbulence. In another experiment, powders were injected in a laminar hydrogen/oxygen flame, so that their combustion occurred in water vapor.  Optical emission of burning particles was recorded using filtered photomultiplier tubes.  Emission durations for individual pulses were interpreted as particle burn times.  They were correlated with their respective particle size distributions assuming larger particles burn longer.  It was found that an increase in flame turbulence results in shorter particle burn times.  More interestingly, the burn times became longer for alloys with increased Mg concentrations.  Additional details regarding combustion temperatures and burn rates in different environments will be reported and discussed.