Droplet Size Distributions from Hollow-Cone Nozzles Coupled with PWM Valves

Abstract

HighlightsVolumetric diameters DV0.1, DV0.5, and DV0.9 for PWM-controlled hollow-cone nozzles were investigated.Droplet sizes were influenced greatly by nozzle size, operating pressure, duty cycle, and slightly by PWM valve type.Volume fractions were reported for droplets smaller than 100 µm, between 100 and 300 µm, and greater than 300 µm.Measured droplet sizes were classified as different categories based on the ASABE standard 572.3.Abstract. Integration of variable-rate disc-core nozzles in orchard sprayers is the optimal solution to achieve precision spray applications of agrochemicals; however, little information is available on the droplet size distribution and classification for these hollow-cone nozzles. Investigations were performed for the comprehensive understanding of spray droplet size spectrum discharged from hollow-cone nozzles which were manipulated with pulse width modulation (PWM) valves to produce variable flow rates. Test variables included five disc-core hollow-cone nozzle sizes (D2-DC25, D2-DC45, D4-DC25, D4-DC45, and D5-DC25), two 10-Hz PWM valve designs, five operating pressures (276, 414, 552, 689, and 827 kPa), and 10 duty cycles (DUCs) ranging from 10% to 100% at 10% intervals. Droplet diameters were measured with a laser imaging particle system. Volumetric diameters (DV0.1, DV0.5, and DV0.9) varied greatly with the nozzle orifice size, operating pressure, and DUC, and varied slightly by PWM valve type. Higher operating pressures and larger nozzles generated droplets with more consistent size distributions across DUCs from 10% to 100%. The differences in relative spans among DUCs increased with the nozzle size increase and the operating pressure decrease. Droplet size classifications from very fine to coarse, based on ASABE 572.3 standard definitions, varied with the nozzle size and operating pressure but were similar for two PWM valve designs at all DUCs. Spray volume fractions also varied with the droplet size range, DUC and pressure. For droplets smaller than 100 µm, the volume fraction remained relatively consistent or slightly decreased as DUC increased, but increased as the pressure increased. In comparison, the fraction increased as both DUC and pressure increased for droplets between 100 and 300 µm and decreased for the portion of droplets greater than 300 µm. As a result, the discovered information would be implemented for the improvement of precision variable-rate spray systems equipped with PWM valves. Keywords: Atomization, Droplet diameter, Orchard sprayer, Precision agriculture, Variable rate.