Combustion characteristics of micron-sized aluminum particles in oxygenated environments

Experimental data describing combustion of micron-sized aluminum particles as a function of their size are limited. Often combustion characteristics are derived indirectly, from experiments with aerosolized powder clouds. In a recently developed experiment, micron-sized particles cross two laser beams. When each particle crosses the first, low-power laser, it produces a scattered light pulse proportional to the particle diameter. The second, powerful CO₂ laser beam ignites the particle. The optical emission pulse of the burning particle is correlated with its scattered light pulse, so that the combustion characteristics are directly correlated with the size for each particle. In this work, emission signatures of the ignited Al particles are recorded using an array of filtered photomultipliers to enable optical pyrometry and evaluate the molecular AlO emission. Processing of the generated data for multiple particles is streamlined. Experiments are performed with spherical aluminum powder burning in atmospheric pressure O₂/N₂ gas mixtures with the oxygen concentrations of 10%, 15%, and 21% (air). In air, the AlO emission peaks prior to the maximum in the overall emission intensity, and the latter occur before the maximum of the particle temperature. The temperatures at which particles burn steadily increase with particle size for particles less than 7.4μm. For coarser particles, the flame temperature remains constant at about 3040K. In the gas mixture with 15% O₂, the flame temperatures are observed to increase with particle size for the entire range of particle sizes considered, 2-20μm. At 10% O₂, the flame temperatures are significantly lower, close to 2000K for all particles. The intensity of AlO emission decays at lower oxygen concentrations; however, it remains discernible for all environments. The results of this study are expected to be useful for constructing the Al combustion models relaxing the assumption of the steady state burning.