Quantitative experimental determination of evaporation influencing factors in single droplet levitation

Abstract

Acoustic levitation has been used successfully as a model system for spray polymerization processes and the determination of particle morphology. The experimental extension for the investigation of heat and mass transfer in levitated liquid droplets requires much more effort. In order to enable the levitation method as a universal tool for the tracking of heat and mass transfer, influencing experimental parameters and their effect on relevant characteristics like droplet deformation, temperature and surrounding flow field were analyzed. These parameters were elucidated experimentally to reveal their mutual dependencies. It was shown that the evaporation rate is affected by the aspect ratio of a droplet, which is directly correlated with its wet-bulb temperature. This was primarily explained by varying acoustic flow field patterns based on classical fluid mechanics, gas velocities and acoustic boundary layer thicknesses, which were visualized and quantified by particle image velocimetry. In order to study the wet-bulb temperature, thermocouple measurements on the levitated droplet were performed. It turned out, that the wet-bulb temperature is dependent on the power input and the resulting sound pressure level as it changes the acoustic field around the droplet. Moreover, the temperature measurement revealed the necessity to establish a temperature correction to account for heat conduction effects along the thermocouple shaft into the droplet. The illustrated experimental setup and measurement procedure is supposed to provide guidelines for the experimental determination of heat and mass transfer in evaporating droplets.