Effect of Nozzle Type and Operation on Spray Droplet Size

Droplet size distribution analyses were carried out for several types of aerial spray nozzles in an airstream to simulate operation on an aircraft. A laser-imaging spray droplet spectrometer probe was used to measure the droplets and determine the effects of spray pressure, airspeed, and nozzle orientation (angle of spray discharge relative to the airstream) on droplet size for solid stream, disc-core hollow cone, swirl-type hollow cone, elliptical orifice fan, and deflector fan nozzles. Surfactant in tap water was used to simulate aqueous pesticide spray mixtures. Results indicated that spray pressure, and thus relative velocity (the difference between the airspeed and the velocity of spray exiting a nozzle), in combination with nozzle orientation, was extremely important in droplet size control. Increasing spray pressure for nozzles oriented so that the liquid was emitted parallel with or at a small angle to the airstream reduced relative velocity, increased droplet size, and reduced the percent of spray volume in small droplets that are most susceptible to spray drift. Increasing spray pressure for nozzles operated so that the liquid was emitted at larger angles to the airstream reduced droplet size and increased the percent of spray volume in small droplets. Increasing airspeed from 193 to 241 km/h (120 to 150 mph) resulted in smaller droplet size and a higher percent spray volume in droplets most susceptible to spray drift.