An investigation of droplet evaporation characteristics in an ultrasound environment
This study investigates and quantifies the effect of an imposed ultrasound field on the evaporation of water droplets in, for example, humidifiers used in medical respiratory treatments. The purpose of the ultrasound field is to accelerate the droplet evaporation process. This would have benefits in terms of improved efficiencies, more compact equipment sizes and better process controllability. A preliminary investigation was carried out to identify the most promising mechanisms for the effect of the imposed ultrasound field on the evaporating droplets – this being the enhancement of the normal mass and heat transfer processes involved. From this, theoretical models of normal and ultrasound enhanced droplet evaporation were developed to predict the rates of water evaporation and also changes to the droplet size distribution during evaporation. An experimental investigation was carried out to measure water droplet evaporation rates and changes to the droplet size distribution under normal and ultrasound enhanced conditions. It was found that the ultrasound field improved droplet evaporation rates in all cases tested, even at very low power levels. Improvements varied from 1 – 30%. An increase in the strength of the ultrasound field increased the improvement in evaporation rate. However, air flow above a certain threshold diminished this improvement by disrupting the ultrasound field. Investigation of the changes to the droplet size distribution indicated that at high ultrasound power levels and low air flow rates a significant amount of droplet coalescence occurred which caused the droplet distribution for the remaining droplets to shift to much larger droplet sizes. Results from theoretical models compared well to the experimental results for most experimental conditions. Differences between model and experiment occurred for the very small droplet sizes and where the effect of the ultrasound field caused maximum droplet coalescence and heating of the air and these areas warrant further future investigation. It was concluded that the ultrasound enhancement of water droplet evaporation does occur by enhancing the heat and mass transfer processes involved, that improvements in evaporation rate up to 30% could be achieved and that this could be applied to medical respiratory equipment to improve its operation and efficiency.