Visualization of unsteady combustion of single aluminum droplets: coalescence, eruption and fragmentation

Detailed characterization of the unsteady combustion behaviors of individual aluminum (Al) droplets is scarce, due to the experimental challenges in resolving the transient phenomena at the micrometer scale. In this work, we spatiotemporally elucidate the complex interplay, including droplet coalescence, eruption, and fragmentation, of individually burning Al droplets and alumina (Al 2 O 3 ) products, in a hot H₂O/N₂/O₂ flow. Flame incandescence and droplet shadowgraphs are simultaneously imaged using two high-speed cameras, and the surface temperature evolution is tracked in separate measurements via high-speed two-color pyrometry imaging. Initially, the Al droplet burns in a symmetric phase, during which a visible flame sheet fully encapsulates the Al droplet. This is frequently followed by an unsteady asymmetric stage where the droplet locally breaks or entirely fragments. The transition from symmetric to asymmetric combustion is triggered by the coalescence of high-temperature Al 2 O 3 satellite droplets with the parent Al core, forming a Janus droplet with two immiscible components (the Al 2 O 3 cap and the Al core). This coalescence rapidly heats the interfacial surface to temperatures above the Al boiling point, causing a sharp increase in Al vapor pressure and high vaporization rates, i.e., Al vapor ejection. The ejection is driven by a temperature gradient between the Al 2 O 3 cap and the contacted Al surface. The Janus droplet morphology is quantitatively analyzed to derive the interfacial tension between the liquid Al and Al 2 O 3 , enabling predictions of the droplet's cross-sectional geometry at various stages. Furthermore, ex-situ sample analysis shows that the eruption significantly enhances the formation of nanometer Al 2 O 3 particles. These observations advance the understanding of the unsteady behaviors inherent to Al droplet combustion.