High-speed digital off-axis holography to study atomization and evaporation of burning droplets

Droplet formation and evaporation are key processes of spray combustion, the mechanism of which is yet not fully understood. In this study, we develop a method based on high-speed (25 kHz) digital off-axis holography (DOH) to visualize and measure burning ethanol droplets in the atomization. Ligaments and fragments from a burning drop internally blown to disintegrate are studied after hologram processing. Reconstructed images are nearly free from influences of the flame with the measured droplets showing clear borders. Size and depth location uncertainties reduce to nearly 1/10 of those of commonly used with digital in-line holography. A time-resolved observation shows the dynamic formation of secondary droplets with stages classified as vapor penetration, sheet breakup, and ligament disintegration. Sizes and velocities of fragments are extracted and their distributions are analyzed at 1.12 ms, 3.12 ms, and 13 ms after the initiation of atomization. Results show that new secondary droplets have larger sizes and smaller velocities with time. Lagrangian measurements are performed with 3D droplet tracking algorithm. Evaporation rates of 31 droplets with different positions, sizes, and velocities are obtained and the results are in good accordance with previous studies. The DOH technique is promising to investigate the combustion of individual fuel droplets in a polydisperse droplet cloud as the real spray combustion.