The Interaction of Well-Characterized, Longitudinal Acoustic Waves With a Nonreacting Spray

The effect of a variable acoustic field on the evaporation characteristics of a nonreacting ethanol spray has been investigated in a long, vertical tube using a phase Doppler particle analyzer. The parametric effect of the first, second, and third acoustic modes were studied fora constant sound pressure level, using the steady flow condition as a reference. Axial and radial profiles of Sauter mean and arithmetic mean droplet diameters, and time-resolved droplet velocities, were measured and compared for the four experimental conditions while holding the spray location fixed inside the tube. Spectral analysis of the droplet axial velocity component revealed dominant frequencies equal to the frequencies of the acoustic waves in the tube. Average droplet velocities under all four experimental conditions remained the same. Droplet diameters decreased between 13% and 31%, 15% on average, in the presence of the first and third modes, compared to steady values. The second mode had little effect on the droplet diameters because the location of the nozzle corresponded to a velocity node of the stationary acoustic field for the second harmonic. A simple mathematical model describing the evaporation of a droplet in a pulsating flow field yielded results that agreed qualitatively well with the experimental results obtained.