Abstract
Acoustic levitation method is one of levitation techniques and describes some nonlinear behaviors such as the deformation of droplet interface. The purpose of this paper is to clarify the interaction between the flow behavior generated by the ultrasonic wave and the transport phenomena at the droplet interface. The following results were experimentally obtained: in the case of volatile fluids, a pair of the toroidal vortex was observed in the vicinity of droplet interface. As the saturated vapor pressure of droplet increases, the size of the circular vortex decreases. By measuring the change of surface area and surface temperature of the levitated droplet, the heat transfer coefficient was estimated. In the case of the droplet with the circular vortex, the heat transfer coefficient was larger than that obtained from the Ranz-Marshall correlation. Then, the thickness of thermal-boundary layer was calculated by using the heat transfer coefficient. The range of thickness was from 0.2 mm to 0.6 mm. We estimated the correlation between the flow structure and boundary layer around a droplet by using above mentioned result. By comparing the thickness of boundary layer, we concluded that the Stokes layer was inside the diffusion boundary layer. Thus, physical properties of the surrounding atmosphere were affected by the evaporation behavior. In the case of the droplets with the evaporation, the result suggested that the kinematic viscosity variation of gas phase around the droplet affected the flow structure.
