Evaluation of a prototype air-assist agricultural sprayer nozzle

Droplet spectra and lateral spray distributions from a proto type air-assist agricultural sprayer nozzle were observed and evaluated at the United States Department of Agriculture-Agricultural Research Service (USDA-ARS) in Beltsville, Maryland and at The University of Tennessee Department of Agricultural Engineering in Knoxville, Tennessee. Evaluations considered the effects of various air and liquid pressure combinations, different liquid physical properties, and other operational variables on the spray emitted by the nozzle unit. Evaluations of droplet spectra from six nozzle tips and at various positions within a given distribution were also performed.

Data describing the droplet spectra were collected using a helium-neon laser spectrometer. Air pressures of 34, 52, and 69 kPa in combination with liquid pressures of 207, 276, 345, and 414 kPa were used during the evaluation. Tapwater, a hard well water, distilled water, and an oil-in-water solution were used to analyze the effects of various liquid properties on droplet spectra emitted from the nozzle. Changes in the volume-surface (Sauter) mean diameter (SMD), the volume median diameter (VMD), and the 10 and 90 percent intercepts on the cumulative volume curve (DV1 and DV9, respectively) were analyzed for all test conditions.

Data characterizing the lateral spray distributions from the air-assist nozzle were collected using a patternator. The air and liquid pressures noted above were used during the evaluation. Only tapwater and the oil-in-water solution were tested for their effects on the spray pattern emitted by the nozzle. Boom heights were 43, 48, and 53 cm above the patternator surface. Coefficients of variation for the lateral spray distributions produced were used to determine the appropriate nozzle spacing along the boom.

Results from the droplet spectra analyses suggest that droplet size decreases as air pressure is increased for a given liquid pressure; however, droplet size increases as liquid pressure increases for a given air pressure. The latter trend is opposite that which occurs in conventional hydraulic atomizers. These phenomena occurred for all liquids tested. Further, the evaluation indicated that little variation existed among the droplet diameters analyzed for liquid pressures of 276 kPa and 345 kPa in combination with the 52 kPa air pressure setting for the oil-in-water solution. The same trend was observed for tapwater at the same liquid pressures, but in combination with all air pressures.