Droplet size distributions from hollow-cone nozzles coupled with PWM valves

Authors: SALCEDO, RAMON - The Ohio State University; Zhu, Heping; Jeon, Hongyoung; OZKAN, ERDAL - The Ohio State University; WEI, ZHIMING - Shandong Academy Of Agricultural Sciences; GIL, EMILIO - Technical University Of Catalonia; CAMPOS, JAVIER - The Ohio State University; ROMAN, CARLA - The Ohio State University

Submitted to: American Society of Agricultural and Biological Engineers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2022
Publication Date: 8/30/2022
Citation: Salcedo, R., Zhu, H., Jeon, H., Ozkan, E., Wei, Z., Gil, E., Campos, J., Roman, C. 2022. Droplet size distributions from hollow-cone nozzles coupled with PWM valves. American Society of Agricultural and Biological Engineers. 65(4):695-706. https://doi.org/10.13031/ja.15064.
DOI: https://doi.org/10.13031/ja.15064

Interpretive Summary: Target-oriented intelligent sprayers guided with laser sensors have been developed for effective protection of fruit and nursery crops from attacks by pest insects and diseases with minimum impact to the environment. Pulse width modulation (PWM) valves are used in these sprayers to manipulate variable-rate spray applications. However, characteristics of spray droplets discharged from variable-rate hollow-cone nozzles commonly used for these sprayers have not been documented. In this research, the droplet size distributions of PWM valve-controlled hollow-cone nozzles were determined under various operating pressure and duty cycle conditions. Test results revealed that spray droplet sizes from the hollow-cone nozzles with PWM valves were fallen in the classifications of coarse, medium, fine and very fine, which varied with the nozzle orifice size, pressure, PWM valve fabrication and duty cycle. This information provided understanding of how PWM valve manipulations affect volumetric diameters of droplets and other spray quality parameters including relative spans, volume fractions of droplets in various diameter ranges, and droplet size categories defined by the ASABE standard. It will be used to further improve the variable-rate accuracy and drift reduction of new air-assisted intelligent sprayers for specialty crops such as orchards, vineyards and nurseries.

Technical Abstract: Integration of variable-rate hollow-cone 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 nozzles. Investigations were performed for 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 fabrications, 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 slightly with PWM valve fabrication. 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, based on ASABE 572.3 standard definitions, varied with the nozzle size and operating pressure but were similar for two PWM valve fabrications at all DUCs. Spray volume fractions maintained relatively consistent or slightly decreased as DUC increased but increased as the pressure increased for the portion of droplets smaller than 100 µm. In comparison, the spray volume fractions increased as both DUC and pressure increased for the portion of 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 in improvement of precision variable-rate spray systems equipped with PWM valves.