Flame structure of single aluminum droplets burning in hot steam-dominated flows

In this work, a specially designed experimental setup is employed to study the ignition and combustion of single aluminum droplets in hot steam-dominated flows. The transient burning behaviors of Al droplets of different sizes are characterized by simultaneously visualizing the flame incandescence and droplet shadowgraphs with two high-speed cameras at high magnification. The combustion process can be described in three stages: Al ignition and droplet generation, droplet evaporation and flame development, and steady combustion. During the steady combustion stage, a bright flame sheet, characterized by a narrow layer of dense nano-micron-sized alumina droplets, encapsulates the Al droplet core. The flame sheet composed of alumina droplets is located on a stagnation plane where the radial velocities relative to the droplet core are close to zero. The standoff ratio is around two, and it slightly decreases with the droplet size and increases with the oxygen content in the ambient gas. The thickness of the flame sheet (the alumina particle layer) is analyzed using Abel inversion of the projected profile of the flame incandescence and optical depth, revealing a thickness of about 50 μm for a burning droplet of a 550 μm diameter. Based on the shadowgraph images, the evaporation rate of the Al droplets is determined from the shrinking rate of the droplet projected area. Size-dependent evaporation rates are found to be related to different slip velocities, and the addition of oxygen to the oxidizer can significantly increase the evaporation rate. Finally, a conceptual model of a burning Al droplet in the steady combustion stage is proposed based on the experimental findings. The presented results provide novel datasets that contribute to model development and deepen the understanding of the physical and chemical processes involved in aluminum droplet combustion.