Measurement comparison and fitted distribution equation of droplet size for agricultural nozzles

Abstract: In order to investigate the function expression of droplet size distribution in the spray sheet for agricultural nozzles to improve the pesticide efficacy, some common nozzle types were tested in this study. At present many methods and equipment were used for measuring droplet size. Droplet size is a main parameter influencing the deposition rate and distribution uniformity of pesticide on the target. However, different test results may be caused by different methods or equipment. For selecting a suitable analyzer to test droplet size distribution, three common droplet size analyzers were applied to measure droplet sizes of ST110-03 and ST110-02 nozzles which were standard flat-fan nozzles manufactured by Lechler GmbH. The three analyzers were Particle/Droplet image analysis system, Sympatec HELOS Vario particle size analyzer and Spraytec laser diffraction system and were referred to as PDIA, Sympatec and Spraytec, respectively. Their respective locations were the Institute for Application Techniques in Plant Protection of JKI (Julius Kühn-Institut), Germany, the Centre for Pesticide Application and Safety of the University of Queensland, Australia, and the Centre for Chemicals Application Technology of China Agricultural University, China. For all the sprays in this study, tap water was sprayed at an operating pressure of 0.3 MPa, and each apparatus was operated complying with its corresponding experimental procedure. For each nozzle type, 3 nozzles were tested with three replications. Results showed that absolute results differed between different tests depending on measuring protocol and type of measuring apparatus, but the nozzle classifications were the same, comparing the results with limits of BCPC nozzle classification obtained by PDIA in JKI. Spraytec was more accessible to authors than other analyzers; therefore, it was selected to study the distribution of droplet size. The volume median diameters (VMDs) of air-induction compact nozzles (IDK), standard flat-fan nozzles (ST) and hollow-cone nozzles (TR), with the orifice sizes of 02 and 03 for each type, were measured at different positions in the spray sheet. The nozzles were all produced by Lechler GmbH in Germany. It was found that the VMD distributions were symmetric for all tested nozzles and the axis of symmetry was the centerline of spray sheet. The VMDs of IDK nozzles were significantly larger than other two types'. The coefficient of variation (CV) of droplet sizes, which were tested at different spray heights but the same horizontal position, indicated that the VMDs of IDK nozzles varied with spray height obviously. Meanwhile, at a fixed spray height, the VMD distribution of IDK along the horizontal direction appeared to be W-shaped; the distributions of ST and TR were parabolas. The parabola opening of ST was larger than that of TR. The tested VMD was then fitted with program code using Matlab software based on least square method. In the fitted VMD distribution equation, independent variables were spray height and horizontal position and dependent variable was VMD. The significant relationship between distribution position and VMD was found, the significance threshold α was set at 0.05. Results also showed that the F-statistic calculated from the data of each nozzle was greater than the critical value of the F-distribution for the desired false-rejection probability of 0.05. The coefficient of determination was greater than 0.8 for all fitted equations. All of these pointed that the obtained equations could describe the droplet size distribution correctly and predicate the size at any position in the spray sheet with precision. The fitted function research involved in this paper will provide the valuable basis to study the VMD distribution of overlapped spray sheet for boom sprayer; the study will improve the uniformity of deposition rate and biological efficacy. Meanwhile, the fine droplet zone in the spray sheet is the target of drift control. Therefore, the VMD distribution is also conducive to the development of novel anti-drift sprayer to reduce the risk of pesticide.