Acoustic excitation of an n-heptane droplet: Evaporation, ignition and combustion characteristics

In aerospace propulsion systems, the coupling between droplet combustion and sound waves can be a key mechanism for driving thermoacoustic instability. In this experimental study, we examine the evaporation, ignition and combustion characteristics of an n-heptane droplet subjected to time-periodic acoustic excitation of different amplitudes (12.5 to 37.5 Pa) and frequencies (100 to 500 Hz). On applying acoustic excitation, we find that the ignition delay time of the droplet decreases, while both the evaporation rate and the combustion rate increase. All three effects strengthen as the excitation amplitude increases, while the first and third effects strengthen as the excitation frequency decreases. These trends are corroborated by analysis of time-resolved CH* chemiluminescence images, which reveal that the heat-release-rate oscillations of the burning droplet grow in amplitude either as the excitation amplitude increases or as the excitation frequency decreases. The pronounced effects on droplet ignition and combustion observed during low-frequency acoustic excitation can be attributed to the time scale of chemical reaction being significantly shorter than that of the acoustic excitation. This study contributes to a better understanding of droplet combustion in the presence of time-periodic acoustic oscillations, facilitating the development of aerospace propulsion systems that can potentially be more resistant to thermoacoustic instability.