Spray droplet sizes with additives discharged from an air-assisted variable-rate nozzle

Abstract

Understanding droplet size distributions is essential to achieve constant spray quality for real-time variable-rate sprayers that synchronize spray outputs with canopy structures. Droplet sizes were measured for a custom-designed, air-assisted, five-port nozzle coupled with a pulse width modulated (PWM) solenoid valve under various operating conditions. Parameters used for the measurements included five air velocities (0, 10, 30, 50, 70 m/s), three spray solutions (water only, water mixed with a surfactant, water mixed with a drift retardant), five operating pressures (138, 207, 276, 345, 414 kPa), and seven pulse width modulation rates (10, 20, 40, 50, 60, 80, 100%). Droplet sizes did not vary significantly with the modulation rate except at 10%, but the variation of droplet sizes was highly significant with spray solution formulation and operation pressure while slightly significant with air velocity. A dimensionless parameter which was the ratio of liquid viscosity and relative discharge velocity to surface tension had reasonable correlations with DV0.1, DV0.5 and DV0.9 of droplets for the three spray solutions. To minimize droplet size variations, the variable-rate nozzle should be operated at a constant liquid pressure and should avoid 10% or lower modulation rate. Compared to the range of droplet sizes for a given condition, the variation of water and surfactant droplet sizes because of changes in the air velocity was acceptable for the new sprayer development.