Spatiotemporal quantification of AlO around a burning micro-sized Al droplet using laser absorption imaging

By combining the quantitative advantage of laser absorption spectroscopy and the superior spatiotemporal resolution of high-speed microscopic imaging, we report, for the first time, the transient aluminum monoxide (AlO) concentration distribution around a burning micro-sized aluminum droplet in water-vapor-rich ambient. Arrangement of two lasers at resonant and non-resonant wavelengths, respectively, eliminates the non-resonant extinction interference from condensed-phase products. The AlO concentrations are found to increase as the combustion proceeds and reach a plateau approximately 10 ms after ignition. The AlO concentration increases from a negligible level near the droplet surface to a peak within the condensed layer, decreases, and spreads beyond it. The maximum molar concentration observed over time reaches approximately 2%. The proposed method enables high spatiotemporally resolved inspection of AlO species, offering a powerful diagnostic tool for further insights into the mechanism of Al combustion process. Novelty and significance statement High spatiotemporally resolved AlO distribution around a burning micro-sized aluminum droplet is obtained quantitatively for the first time using high-speed laser absorption imaging. Two lasers at resonant and non-resonant wavelengths, respectively, eliminate the non-resonant extinction interference from condensed-phase products. The proposed experimental system can readily provide instantaneous AlO concentration data for mechanism and modeling studies of Al combustion.