Evaporation of an acoustically levitated droplet

The rate of heat and mass transfer to droplets in sprays is a critical issue in the design of many practical spray systems. Applications like fuel injection in internal combustion engines or spray drying stimulate interest in studying the processes related to the evaporation of droplets. Since experiments on real sprays are difficult to conduct and interpret, simpler and idealized configurations are generally preferred. An acoustic levitator is used to observe a droplet during the entire time of its evaporation. The introduction of a droplet into the sound field leads to the formation of two steady toroidal vortices close to the droplet surface. The results illustrate how this 'outer acoustic streaming' affects the heat and mass transfer. A calibration-free, high-resolution laser absorption spectrometer intended for the non-intrusive detection of the water vapour distribution around the droplet has been developed and used for first experiments. A sensitivity of 1500 ppm in a 1mm absorption path could be demonstrated. The present study is also part of larger project aiming to implement highly resolved measurement techniques to observe the heat and mass transfers in the vicinity of the droplet surface or within the droplet.