Combustion of nanoaluminum at elevated pressure and temperature behind reflected shock waves

This study presents experimental measurements on the combustion of nanoaluminum particles behind reflected shock waves in a shock tube. These experiments were performed at elevated pressures (4–32 atm) and temperatures (1200–2100 K) in the oxidizers oxygen and carbon dioxide, with nitrogen also present. The light emission from the reacting particles was monitored. For all cases, a brief period of intense light emission was observed soon after exposure to the reflected shock conditions. The time scales of this emission event are quantified by the 10–90% integrated emission intensity method to yield a reaction time for this rapid exothermic process. The duration of the emission is found to be 50–500 μs for the conditions tested here. Reaction times in 50% O2 and 50% N2 were shown to decrease significantly with ambient temperature, with Arrhenius-type exponentials fitting reasonably well to the observed experimental data. The reaction times were also dependent on pressure, with the timescales decreasing by 1.6–4 times as the pressure was increased from 8 to 32 atm over the range of temperatures in the experiments. In 50% CO2 and 50% N2, the reaction occurs in two sequential stages, with more of the emission at earlier times under higher-temperature conditions. Particle temperatures were also measured. During the bright emission event, the temperature rises above the ambient and then cools to near the ambient as the emission event ends. The peak temperature of the particle varied with ambient temperature, pressure, and oxidizer, with high ambient temperatures (2000 K), high pressures (32 atm), and high oxygen mole fractions (50%) giving the highest particle temperatures (∼3500 K∼3500 K). Conversely, 50% CO2 atmospheres produced particle temperatures just slightly above the ambient. The spectral output of the light emission was shown to be dominated by broadband emission. At high temperatures and pressures in oxygen, weak emission from the AlO B–XB–X transition was observed.