AN EXPERIMENTAL INVESTIGATION OF THE BEHAVIOR OF DROPLETS IN AXIAL ACOUSTIC FIELDS

This paper describes an experimental investigation of the behavior of water droplets in axial acoustic fields. It was motivated by the incre asing interest in the use of pulsations to improve the performance of energy intensive, industrial processes. The presence of an acoustic fi eld is believed to enhance heat and mass transfer to and from the drop lets, probably because of the relative motion between the droplets and the gas phase. This relative motion is characterized by the ratio of the amplitude of the oscillatory droplet velocity to that of the acous tic velocity (entrainment factor), and by the phase between the drople t and gas phase oscillations. An experimental set-up was developed to investigate the effect of acoustic oscillations on the motion of indiv idual droplets. In these experiments a droplet produced by a piezo-cer amic droplet generator is allowed to fall through a transparent rest s ection in which an acoustic field has been set up using a pair of acou stic drivers. Images of the droplets in the test section acquired at c onsecutive instants using a high speed, intensified imaging system wer e used to determine the rime dependent droplet trajectory and velocity . The acoustic velocity was calculated from measured acoustic pressure distributions. The entrainment factor and the phase difference were t hen determined from these data. The results show how the entrainment f actor decreases and the phase difference increases with increasing dro plet diameter and frequency, indicating that larger diameters and high er frequencies reduce the ''ability'' of the droplets to follow the ga s phase oscillations. The measured data are in excellent agreement wit h the prediction of the Hjelmfelt and Mockros model. Both theoretical predictions and measured data were correlated with the Stokes number, which accounts for the effects of droplet diameter and frequency. It w as also shown that acoustic oscillations decrease the mean terminal ve locity of the droplets.