Evaporation from freely falling droplets

Improvements in experimental techniques for studying the behavior of freely falling droplets are reported. The Re has a significant effect on the evaporation rate of a droplet. Above a Re of 24, the flow detaches and a viscous, slow region forms over the rear of the droplet, thus enhancing the evaporation rate. The vortex region elongates above a Re of 30 and extends 0.8 diam to the rear at a Re of 100. Adding a nonvolatile fluid to the water (molasses was used in experiments) results in a lowered evaporation rate. The technique gives a better simulation to actual aircraft spraying conditions, where the toroidal motion of the fluid will produce a small 'skin' of nonvolatile fluid around the droplet.