Application of acoustic levitation for studying convective heat and mass transfer during droplet evaporation

The observation of acoustically levitated droplets offers great potential for studying their evaporation characteristics under well-defined ambient conditions. In contrast to alternatives such as sessile droplets or droplets suspended by a tiny fiber, the key advantage of acoustic levitation is the contact-free nature of the experimental method. However, acoustic streaming effects or droplet deformation by acoustic forces may affect the heat and mass transfer considerably. Consequently, the droplet evaporation process is usually dominated by these intrusive effects of the acoustic field. The main objective of this study is to present an experimental setup, which minimizes the disturbing effects of the levitation technique in order to enable a solid investigation of convective heat and mass transfer during droplet evaporation. To achieve this goal, a high resonance frequency of 100 kHz is chosen and the investigation is limited to substantially smaller droplets compared to other studies in the literature. Moreover, an air flow is injected through a hole pattern in the reflector in order to control the relative velocity between the droplet and the gas phase. Particle Shadow Velocimetry is used to characterize the gas flow field in a stepwise approach, initially without any levitated droplet. Subsequently, the vortex structures around non-evaporating droplets are discussed. Eventually, evaporating water droplets are studied. The applied optical diagnostics offer the distinct advantage that the droplet size can be measured simultaneously with the surrounding gas flow field. Hence, the evaporation rate can be evaluated for different combinations of droplet size and relative velocity to the gas phase. For the entire range of resulting Reynolds numbers, the experimentally obtained evaporation rates were found to agree very well with results of a numerical evaporation model when a well-established correlation is used to account for convective heat and mass transfer.