Aluminum (Al) fuel particles have been shown to ignite via different mechanisms on the micron versus nanometer particle scale. Micron Al particles react by diffusion of mass through a growing oxide shell. It has been proposed that nano-Al particles react by a melt dispersion mechanism in which the volume change due to melting of Al induces high pressures and causes spallation of the oxide shell and dispersion of liquid Al at high velocity. The particle core-shell structure plays a critical role in promoting the melt dispersion mechanism for nano-Al particles while the presence of a shell acts as a barrier to the diffusion process for micron-Al particles. The goal of this study is to understand the role of the passivation shell and associated reaction mechanism on flame propagation. Specifically, the flame propagation rates of perfluoroalkyl carboxylic acid (C13F27COOH) treated Al particles with and without an alumina shell were measured. Results show that when there is no alumina passivation shell encasing the Al core the melt dispersion mechanism cannot operate and flame propagation rates are reduced by two orders of magnitude. These results imply that the melt dispersion reaction mechanism is responsible for high energy propagation rates observed in these confined tube experiments. All flame propagation experiments were performed in a confined flame tube apparatus which may not be ideal for characterizing materials with significantly different ignition sensitivities. Results indicate that flame speeds measured with this apparatus are dependent on the ignition sensitivity of the mixture. Acid treated Al particles are shown to be roughly five times less ignition sensitive than the Al particles with no acid treatment; however, both particles produce roughly the same heat of combustion in thermal equilibrium measurements.
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The Influence of Al Passivation On the Reaction Mechanism: Flame Propagation Studies
Aluminum (Al) fuel particles have been shown to ignite via different mechanisms on the micron versus nanometer particle scale. Micron Al particles react by diffusion of mass through a growing oxide shell. It has been proposed that nano-Al particles react by a melt dispersion mechanism in which the volume change due to melting of Al induces high pressures and causes spallation of the oxide shell and dispersion of liquid Al at high velocity. The particle core-shell structure plays a critical role in promoting the melt dispersion mechanism for nano-Al particles while the presence of a shell acts as a barrier to the diffusion process for micron-Al particles. The goal of this study is to understand the role of the passivation shell and associated reaction mechanism on flame propagation. Specifically, the flame propagation rates of perfluoroalkyl carboxylic acid (C13F27COOH) treated Al particles with and without an alumina shell were measured. Results show that when there is no alumina passivation shell encasing the Al core the melt dispersion mechanism cannot operate and flame propagation rates are reduced by two orders of magnitude. These results imply that the melt dispersion reaction mechanism is responsible for high energy propagation rates observed in these confined tube experiments. All flame propagation experiments were performed in a confined flame tube apparatus which may not be ideal for characterizing materials with significantly different ignition sensitivities. Results indicate that flame speeds measured with this apparatus are dependent on the ignition sensitivity of the mixture. Acid treated Al particles are shown to be roughly five times less ignition sensitive than the Al particles with no acid treatment; however, both particles produce roughly the same heat of combustion in thermal equilibrium measurements.