Quantitative measurement of aluminum atom number density around a burning micron-sized aluminum droplet using spatially resolved laser absorption spectroscopy

In this study, we demonstrate a successful application of laser absorption spectroscopy in an in-situ optical diagnostic system to map the radial distribution of the number density of vapor-phase aluminum (Al) atoms around a burning micron-sized Al droplet. This technique overcomes the challenges associated with the short optical path (at micron scale) and offers high sensitivity to Al atom concentration variations. Results indicate that the number density of Al atoms decreases sharply from ∼1.1 × 10²²/m³ within r/r₀ = 1.2∼1.4 (r₀ is the radius of the Al droplet and r is the distance from the center of the droplet) to ∼4.0 × 10²¹/m³ within r/r₀ = 1.4∼1.6, prior to the formation of the Al₂O₃ condensation layer (r/r₀ = 1.6∼1.9). This largest decline rate in the radial direction indicates the ‘flame front’ location. Additionally, a substantial number of Al atoms, i.e., at the scale of 10¹⁹∼10²¹/m³, are still present beyond the Al₂O₃ condensation layer, and their number densities continue to decrease further outwards gradually. This result agrees with the trend predicted by our detailed numerical simulation. Moreover, it is shown that the produced Al₂O₃ droplets are stable, as no detectable absorption signals from Al atoms can be found from the Al₂O₃ droplet dissociation even at ∼3500 K. In addition, as we used the radial temperature profiles obtained from simulations to correlate the number densities of Al atoms in different ground states, an uncertainty analysis was performed. It was shown that this might introduce a maximum uncertainty of 1.84 % in the total Al atom number density. To the awareness of the authors, this work represents the first quantitative in-situ measurement of the Al atom number density around a burning micron-sized Al droplet.