Experimental investigation of the aluminum combustion in different O2 oxidizing mixtures: Effect of the diluent gases

The existence of different distinct phases during the aluminum combustion process was unambiguously proved and widely discussed in several studies on single burning aluminum particles. Indeed, in the case of a combustion in diffusion vapor phase, the formation of growing aluminum oxide caps on the droplet surface is suspected to drive the transition from a symmetric to an asymmetric regime. Especially based on diverse observations for aluminum particles burning in O-2/N-2, a hypothesis was proposed on the effect of N-2 which probably contributes to the regime transition occurrence. To extend research efforts and get a further understanding of the asymmetric transition development, and, more generally on aluminum combustion, an analysis in O-2/N-2, O-2/Ar and O-2/He oxidizing mixtures is presented. This work is based on an advanced experimental technique allowing to levitate a single particle and visualize its self-sustained combustion with high temporal and spatial resolution. Combustion parameters of ignited micron-sized aluminum particles (30-100 mu m) at atmospheric pressure were accurately determined using optical diagnostics. Results provide a new quantified dataset and allow to compare the relative effect of N-2, Ar and He as diluent gases in the aluminum combustion process. Burning times are found to be similar, regardless of the considered diluent gas, as well as the aluminum consumption rate in symmetric regime. However, estimations on the relative duration of the symmetric phase demonstrate a potential influence of N-2 which promotes the regime transition in comparison with Ar and He for low molar fraction of oxidizer. The ratio between flame and droplet diameters is also modified depending on the inert gas and is determined. Observed trends are discussed and compared to available data to improve the description and quantification of the relative influence of the inert gases.