Characterization of volumetric droplet size distributions from PWM-controlled hollow-cone nozzles designed for variable-rate air-assisted sprayers

Abstract

<b><sc>Abstract.</sc></b> The intermittent flow control of hollow cone nozzles coupled with pulse width modulation (PWM) solenoid valves enables significant reductions in pesticide use without changing orchard sprayer parameters. However, little information is available on droplet size distributions associated with these nozzles. Experiments were conducted to characterize the droplet spectra produced from five TeeJet disc-core nozzles (D2DC25, D2DC45, D4DC25, D4DC45, D5DC25) connected to a PWM valve at a frequency of 10 Hz. Volumetric droplet size distributions (D_V0.1, D_V0.5, D_V0.9) were measured with a laser imaging particle and droplet size measurement system in an air-conditioned room. Variables were the combinations of duty cycles of PWM valves (from 10% to 100% at 10% intervals) and five operating pressures (276, 414, 552, 689, 827 kPa). Optimal heights of nozzles were determined for the measurements of droplet sizes that represented those droplets across hollow-cone spray patterns. Nozzles were moving back and forth from the laser focal area until a minimum of 10,000 droplets were counted. During the measurement, pressures upstream and downstream of the PWM valve, flow rates discharged from nozzles and spray angles were also recorded with sensors and stored in a computer. The test results provided a baseline for the design of variable-rate orchard sprayers equipped with the PWM valves to achieve consistent and accurate spray applications.