Droplet Size Distributions and Flow Rates of Pulse-Width-Modulated Low-Drift Nozzles

Although a pulse-width modulation (PWM) technique controls nozzle flow rate with minimal pressure variation, its effects on droplet size distribution and flow regulation when combined with low-drift nozzle designs are still not well documented. Therefore, the objective of this research was to investigate the effects of PWM on droplet size distribution and flow rate of low-drift nozzles used in pesticide application systems. Experiments were conducted under controlled laboratory conditions to evaluate eight flat-fan nozzles with different designs to increase spray droplet sizes. Each nozzle was coupled with a PWM valve, and tested at duty cycles (DUC) from 20% to 100% in 20% increments, and operating pressures of 276 and 414 kPa. Droplet size distribution was determined using a laser diffraction technique, and nozzle flow rate was evaluated to assess the effects of DUC on spray characteristics. PWM operation showed a strong linear relationship between DUC and flow rate (R2 ≥ 0.99). In addition, measured flow rates showed good agreement with theoretical values at DUCs ≥ 60%, whereas substantial deviations were observed at lower DUCs. The effects of DUC on droplet size characteristics varied by nozzle design, pressure, and the parameter evaluated. Low DUCs tended to increase droplet size heterogeneity and the proportion of drift-prone droplets (<150 µm), although these effects were dependent on nozzle type and operating pressure and were not observed consistently across all nozzles. Overall, excessively low DUCs may compromise flow accuracy and spray quality in PWM systems.