Periodic Partial Extinction Regime in Acoustically Coupled Fuel Droplet Combustion

This experimental study investigates the response of burning liquid fuel droplets exposed to standing acoustic waves, extending prior studies quantifying mean and temporal flame response to moderate acoustic excitation\footnote{Sevilla, et al., Comb. Flame \textbf{161}, pp. 1604-1619, 2014}. This investigation explores alternative fuels exposed to a range of acoustic forcing conditions (frequencies and amplitudes), with a focus on ethanol and JP-8. Three fundamental flame regimes are observed: sustained oscillatory combustion, periodic partial extinction and reignition (PPER), and full extinction. Phase-locked OH* chemiluminescence imaging and local temporal pressure measurements allow quantification of the combustion-acoustic coupling through the local Rayleigh index $G$. As expected, PPER produces negative $G$ values, despite having clear flame oscillations. PPER is observed to occur at low-frequency, high amplitude excitation, where the acoustic time scales are large compared with kinetic/reaction times scales for diffusion-limited combustion processes. These quantitative differences in behavior are determined to depend on localized fluid mechanical strain created by the acoustic excitation as well as reaction kinetics.