ID 10

The Evaporation Rate of Droplets inside a Spray Cloud

 

Abstract:

The spectrum of spray droplet sizes emitted from agricultural nozzles on a spray boom can be represented by a drop size distribution. The evaporation of these droplets can be represented by a D-squared law, in which the square of the droplet diameter decreases linearly with an increase in time. In this approach time is normalized by an evaporation time scale that depends on the evaporation rate, the ambient wet bulb temperature depression, and the Sherwood number. Current evaporation models treat each released droplet in the distribution in isolation, as if each were released into the ambient background environment of temperature, relative humidity, and wind speed. Recent laboratory tests idealized the effect of the spray cloud by stacking droplets on multiple threads, each thread farther downwind of the thread ahead of it. In this way the evaporation rate could be measured on the most upwind thread, and cloud effects on evaporation could be measured on the most downwind threads. These tests provided sufficient data (tracking the evaporation of 223 droplets) to reach two conclusions: (1) a more appropriate representation of evaporation behavior for droplets inside a spray cloud is one that retains the D-squared rule but is quadratic in time; and (2) as the Reynolds number of the droplet decreases toward zero, the evaporation rate appears to decrease to one-half its value. For data collected on WHO water droplets, the evaporation rate appears to follow a cubic in Reynolds number up to a Reynolds number of five.