An experimental and theoretical investigation on the effects of acoustics on spray combustion

Spray characteristics in an ethanol-fired, Rijke-tube combustor were studied experimentally by measuring Sauter-mean diameter axial and radial profiles with a phase-Doppler instrument. Predictions of axial Sauter-mean diameter profiles were also obtained by use of a simplified model describing the spray burning in an acoustic field. The effect of flame locations (L/2, L/3, L/4, and L/6) and acoustic modes (first [118 Hz], second [259 Hz], and third [393 Hz]) were studied parametrically at a constant sound pressure level of 150 dB. Both theoretical and experimental results showed that oscillations enhanced the combustion process and resulted in a smaller burnout distance and higher slopes in the Sauter-mean diameter axial profiles when the spray was positioned at locations of high acoustic-velocity fluctuations. Comparisons of these experimental results and model predictions showed very good qualitative agreement. Flame lift also played a significant role delaying the spray ignition at locations of high acoustic-velocity fluctuations. The mean axial droplet velocity component along the axial and radial directions were not affected significantly by the acoustic oscillations. Finally, the theoretical Rayleight integrals showed that the particular spray used here had very little acoustic-driving capability, a result that has been also supported by experimental observations.