Combustion characteristics of sub-millimeter aluminum droplets in high-temperature steam

Combustion processes of single aluminum droplets with sub-millimeter diameters in high-temperature steam atmosphere are experimentally investigated using diffuse backlight illumination imaging. The droplets generated by melting aluminum foil disks are burned in the exhaust gases of two inverse hydrogen–oxygen/nitrogen diffusion flames. High spatiotemporal resolution images reveal that the combustion of an aluminum droplet in high-temperature steam is diffusion-controlled, as evidenced by the formation of an enveloping vapor-phase flame and a condensation layer. A portion of the alumina droplets generated in the condensation layer are convected downstream by the surrounding flow, while others diffuse or collide back onto the parent aluminum droplet surface, forming an alumina cap, above which no condensation layer is observed. In the early combustion phase, the condensation layer diameter increases as the vapor-phase flame develops and subsequently decreases with the growth of the alumina cap, whereas the aluminum droplet diameter remains relatively constant. The maximum diameter ratio between the condensation layer and aluminum droplet increases with the initial droplet size. The measured burning times are longer than those predicted by previous empirical models, which can be attributed to the formation of a large alumina cap and the radiative cooling from the alumina droplet surface.