Enhancement of water droplet evaporation rate by application of low frequency acoustic field

Evaporation rates of low-density water aerosols in a cylindrical acoustic resonator were investigated experimentally, motivated by the potential use of water droplet aerosols in two-phase thermoacoustic devices. Measurements of water aerosol evaporation rates were conducted using a novel direct visualization method combining mass balance quantification based on the light scattering from water droplets and two-dimensional PIV measurements with the droplets acting as the flow visualization particles. Water droplet aerosol evaporation was monitored at a location between the resonator pressure and velocity anti-nodes, while being subjected to a low frequency (110 Hz) acoustic standing wave with various acoustic pressure amplitude (APA) values. Application of the acoustic field was found to significantly increase water droplet evaporation rates, exhibiting a linear dependency on the applied APA in the range 600 Pa < APA < 1720 Pa Normalization of the obtained results using a simple dimensional model enabled the identification of the main governing parameters controlling droplet evaporation rates. These were found to be the known Reynolds number based on relative droplet motion, the newly introduced acoustic drive ratio, and droplet-to-air temperature difference over air water vapor concentration ratio. In addition, significant variations in the acoustic intensity, detected during the experiments, were shown to be a useful marker for measurements of droplet evaporation rate.