Parameter optimization and experiment of air-assisted spraying on pear trees

Abstract: Air-assisted spraying pattern has been widely used in pesticide application area, and air-assisted spraying parameters have a significant influence on droplet distribution. The optimization of spraying parameters is an important way to improve the spray efficiency. However, there are few bases to determine air-assisted spraying parameters for tree canopies of different foliage area volume densities. In this article, cannon sprayer was used, and air velocity from sprayer outlet was adjusted by changing fan speed from 0 to 3 000 r/min with the aid of frequency converter. Firstly, droplet attenuation regulation without tree canopy was researched under different air velocities and spraying distances in the three-dimensional space. The results turned out that air outlet velocities and spraying distances had a significant influence on the droplet penetration flow rate without tree canopy. In order to reduce the influence of droplet gravity and air resistance, the droplet relative penetration rate was introduced into the article, which meant the ratio of droplet flow penetration rate with tree canopy to that without tree canopy in the same three-dimensional position. Secondly, pear trees with different foliage area volume densities in different growth stages were used as test samples, and the regulation of droplet penetration rate, deposition rate and drift rate inside and outside the tree canopy were researched under different air velocities, foliage area volume densities and spraying distances. It revealed that the drift rate increased significantly as the air velocities increased. To some extent, as for the droplet penetration rate and deposition rate, the spraying effect was improved greatly as the air velocities increased at the beginning, and then the influence of air velocity tended to be less. Droplet penetration rate, deposition rate and drift rate all decreased as the foliage area volume density of tree canopies or the spraying distances increased. Therefore, these factors should be considered as a whole in the spraying application to improve the spraying effect. On the basis of this idea, a way to determine the optimum air velocity range at the outlet for tree canopies with different foliage area volume densities was put forward, and the droplet penetration rate, deposition rate and drift rate were regarded as comprehensive evaluation index. After that, the air flow replacement principle and the spraying application criteria for unit area of leaves were used to estimate sprayer traveling speed. The estimated results revealed that there was a relatively big difference in sprayer traveling speed for different foliage area volume densities of tree canopies, which should be considered in the spraying application. On the basis of experimental data analysis, the attenuation models of the droplet deposition rate and penetration flow rate inside the tree canopy were established. In these models, air outlet velocities, foliage area volume densities, and spraying distances were regarded as independent variables, and the droplet deposition rate and penetration rate were regarded as dependent variables. Coefficient of determination, relative error and root mean squared error (RMSE) were used to evaluate the attenuation models. The results showed that coefficients of determination for the models were over 0.90, average relative errors between the estimated and measured values were less than 20%, and RMSEs were relatively low. The models could be used to predict penetration flow rate and deposition rate accurately inside the tree canopy. By further using the model, the range of optimum outlet air velocity with different foliage area volume densities was estimated, and there was a high consistency between the estimated and experimental results, which showed that the attenuation model could be used to estimate the optimum outlet air velocity range. This research provides experimental method and basis for the spraying parameters optimization in practical spraying operation.