Combustion of Nanoaluminum and Water Propellants: Effect of Equivalence Ratio and Safety/Aging Characterization

The deflagration and combustion efficiency of 80nm aluminum/ice (ALICE) mixtures with equivalence ratios of phi=1.0, 0.75, and 0.67 were experimentally investigated. We find that pressure exponent and burning rate vary little between these three mixtures, with the exponent varying only from 0.42 to 0.50 and burning rate at 6.9MPa varying from 2.05 to 2.10cms1. However, reducing the equivalence ratio from 1.0 to 0.67 surprisingly increases combustion efficiency from 70% to 95% with unburned aluminum agglomerates visible in electron microscopy photographs of 70% combustion efficiency (phi=1.0) products. Our findings suggest that nanoaluminum/water combustion is diffusionally limited for all conditions considered. Aging tests on the propellant show that storage at 30 degrees C essentially stops the Al/H2O reaction such that little nanoaluminum degradation occurs after 200 days. Electrostatic discharge (ESD), shock initiation, and impact sensitivity tests indicate that the propellant is insensitive to ignition by these stimuli. Specifically, while neat nanoaluminum powders are highly ESD sensitive (ignition threshold 0.314mJ), nAl/H2O mixtures are insensitive to ESD and have ignition thresholds in excess of 400mJ. Likewise, nAl/H2O mixtures are insensitive to impact ignition, having an ignition threshold in excess of 2.2m. Propellants containing 80nm or larger average particle size aluminum were also found to be insensitive to shock initiation.