Droplet size and spray pattern characteristics of PWM-based continuously variable spray

A Pulse-Width-Modulation-based (PWM-based) continuously variable sprayer was developed using a proportional regulating solenoid valve. Variable flow-rate was obtained by varying the duty cycle of the actuating signal with 24 kHz frequency. Flow-rate regulating ranges of the PWM-based continuously variable spray (i.e. the turndown ratio responding to 100%-40% duty cycle) are 7.14:1, 3.57:1, and 3.70:1 for flat-fan, hollow-cone and solid-cone nozzles, respectively. The purpose of the study was to evaluate the PWM-based continuously variable spray. The method was to quantify the effects of flow-rate control on spray characteristics in terms of droplet size spectra, spray distribution patterns, and spray angle for flat-fan, hollow-cone, solid-cone nozzles. For all nozzles tested, spray distribution concentrated on the center of the spray field with the decrease of flow-rate. But the spray shape is still symmetrical. The sensitivities of the spray angles to flow-rate were 0.83, 0.67, and 0.58 (o)/% respectively for flat-fan, hollow-cone and solid-cone nozzles. Compared with the sensitivities of spray angle for PWM-based intermittent variable spray, they are somewhat larger. As flow-rate was reduced from the maximum (100% flow-rate) to the minimum controllable rate, the observed median diameter of spray droplets decreased by 5.4%, 9.8%, and 9.9 % for flat-fan, hollow-cone and solid-cone nozzles, respectively. This indicates that spray droplet size was affected slightly by flow-rate control.