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

Abstract

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 O2/N2, a hypothesis was proposed on the effect of N2 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 O2/N2, O2/Ar and O2/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 to 100 µm) at atmospheric pressure were accurately determined using optical diagnostics. Results provide a new quantified dataset and allow to compare the relative effect of N2, 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 N2 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.

The role of thermophoresis on aluminum oxide lobe formation

“…We moreover assume that electrophoresis (due to electric charges) is weak, because of intense thermoionic emission at the high temperatures of smoke in the flame. This seems supported by experiments in our electrodynamic levitator [34,35,16] where no deformations of the flame due to the imposed electric field were ever observed.…”

“…We do believe that this "asymmetric" regime is actually just related to the rapid build-up of smoke above the lobe. This symmetric/asymmetric transition was studied recently by the present authors [16] and we found that the nature of gas is of primary importance. The fraction τ of the burning time spent in the symmetric regime is in the range 0.9∼1 (i.e., no asymmetric regime) for O 2 /He and O 2 /Ar mixtures at large dilution ratio (O 2 < 40 %).…”

“…Lower values β ≈ 0.2 were also attested in quench bomb measurements on aluminized solid propellants, with a strong effect of the pressurant [15]. Similarly, experimental observations suggest that the lobe is more massive when nitrogen is used as a diluent compared to argon or helium [16,17,18]. Other experiments show that the rate of oxide accumulation grows for larger aluminum particles and lower oxygen content [19].…”

“…The present study was to some extent motivated by some of our recent experimental observations conducted in our electrostatic levitator [34,35,16]. In this set-up, a single aluminum droplet is levitated by electrostatic forces, ignited by a CO 2 laser beam, and burns in a controlled environment, allowing a large spectrum of burning conditions, such as pressure or oxidizers.…”

“…Most computations performed in this work will focus on an aluminum particle with diameter d Al =70 µm because it is an average diameter of particles studied in our past experiments [34,35,16] and also because it is a typical size of aluminum agglomerates released during solid propellant combustion. We consider a quiescent environment without any imposed flow.…”