The influence of acoustic fields on combustion of a single fuel droplet has been investigated using microgravity. The natural convection-free conditions allow the role of sound alone to be established by avoiding the coupling of sound-induced alternating convection and natural convection. Experiments were done with n-decane single droplets of about 1.5 mm in diameter. Sounds of 66.5 to 3353 Hz with sound pressure levels up to 135 dB were applied in a duct, which achieved maximum velocity amplitudes of about 1 m/s.
As a result, a significantly deformed flame and a soot ring instead of soot shell were found in cases of large velocity amplitude. Burning rates increased with increasing velocity amplitude. The other parameters such as sound pressure level or frequency had little influence. Different influences of sound were found for droplets burning at different locations relative to the velocity antinode of the applied sound. All these observed phenomena are explained by the alternating convection due to sound and a unidirectional convection, which is a form of acoustic streaming, driven by the acoustic radiation force. A simple analysis has been made of the streaming mechanism and the increased burning rate. Differences with droplet location are explained by the enhancement in heat and mass transfer due to the two kinds of convection.