Acoustically excited droplet combustion in normal gravity and microgravity
This experimental study focussed on methanol droplet combustion characteristics during exposure to external acoustical perturbations in both normal gravity and microgravity conditions. Focus was placed on examination of excitation conditions in which the droplet was situated at or near: (1) a velocity antinode (pressure node), where the droplet experienced the greatest effects of velocity perturbations, or (2) at a velocity node (pressure antinode), where the droplet was exposed to minimal velocity fluctuations. It was found that acoustic excitation had a significantly greater influence on droplet burning rates and flame structures in microgravity as compared with that in normal gravity. In normal gravity, acoustic excitation of droplets situated near a pressure node produced only very moderate increases in burning rate (about 11–15% higher than for non-acoustically excited, burning droplets). No significant change in burning rate occurred at a pressure antinode in normal gravity. In microgravity, for the same range in sound pressure level, droplet burning rates increased by over 75% and 200% for droplets situated at or near pressure antinode and pressure node locations, respectively. In microgravity, condensed phase combustion processes appeared to be far more extensively influenced and enhanced by both velocity and pressure perturbations introduced via acoustical excitation. Observed flame deformation for droplets situated near pressure nodes or antinodes were consistent with acoustic radiation forces arising from acoustic streaming.