Evaporation of pure liquid droplets: Comparison of droplet evaporation in an acoustic field versus glass-filament

The rate of heat and mass transfer to droplets in sprays is a critical issue in the design of many industrial 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. In this study an acoustic levitator and the glass filament method are used to observe single droplets during evaporation. The introduction of a droplet into the acoustic field leads to the formation of two steady toroidal vortices close to the droplet surface, known as outer acoustic streaming. The results of this study illustrate how this “outer acoustic streaming” affects the heat and mass transfer. The elimination of these vortices has been achieved through applying a ventilating air flow. Particle Image Velocimetry has been used to characterize the flow field and the interfacial transfer. Furthermore, the resulting evaporation rates have been verified by comparing them with existing model predictions. A comparison of these results to those obtained with the glass-filament method revealed good agreement when the air flow was increased to a limit where the inner acoustic streaming is eliminated, i.e. when forced convection was the primary mechanism in determining the evaporation rate of the liquid droplet. For other air-flow regimes no direct comparisons between results obtained with the two techniques were permissable, as confirmed by the differing Sherwood numbers obtained in the experiments.

► Evaporation of droplets on glass filament compared to those in an acoustic levitator.
► Role of ventilation in an acoustic levitator.
► Influence of acoustic streaming on evaporation of droplets in an acoustic levitator.
► Visualization of acoustic streaming—inner and outer.
► Sherwood number measurement of evaporating drops in an acoustic levitator.