Spatial Drift Behavior And Drift Loss Regulation Of Pesticide Droplets Under Conventional Volume Spray

Chemical pesticides,one of the most important methods to prevent and control agricultural pests and diseases,are essential for ensuring crop yield and food security.At present,the consumption of pesticides in China ranks first in the world but the utilization efficiency is relatively low,which is only 40.2% for the three major grain crops in 2020.More than 30% of the pesticides are drifting during spatial transport process,resulting in considerable pesticide waste and ecological damage.This thesis focuses on the pesticide drift mechanisms based on the comprehensive platform of droplet motion.The dynamic behavior and mass transport efficiency of pesticide droplets are studied from the horizontal and longitudinal perspectives,respectively,which is beneficial to clarify the drift mechanism and propose the regulation strategy.The numerical and experimental platforms for spatial motion of pesticide droplets are established.The CFD models coupled with multiple factors are founded,and the accuracy is verified by the experimental results of droplet trajectory,diameter attenuation and velocity evolution.A multi-functional measurement platform is constructed for the real-time detection of the process parameters and the analysis of pesticide drift.Based on the differences of horizontal drift distance,the sensitive droplets are determined.It shows that the droplet diameter and spray height are the main controlling factors that obviously affect pesticide drift.Sensitivity analysis further indicates that the droplets below 150μm are prone to drift.Among them,the droplets below 100 μm tend to totally drift and those between 100 and 150 μm are easily drift affected by the operation parameter and environmental condition,which are defined as sensitive droplets.As the ambient conditions worsen,the diameter of sensitive droplet rapidly reduces below 100 μm due to the sharp increase of evaporation rate,leading to the cross-region drift.Besides,it is found that the spatial velocity of sensitive droplets decreases sharply near the nozzle.Due to their poor ability to maintain the initial kinetic energy,the evolution of spatial velocity depends on the ambient conditions.The spatial dynamic force analysis reveals that the ratio of net gravity to initial drag dominates the spatial velocity evolution and horizontal drift.The longitudinal mass transport presents a typical regional effect,which is manifested in the frequent droplet collision in the dense region near the nozzle and the droplet diffusion in the dispersed region away from the nozzle.The droplets below 150 μm tend to be emerged by the larger droplets during the collision process,reducing the proportion of easily drifting droplets and thus improving the uniformity of droplet spectrum.In addition,narrowing the droplet spectrum could improve the deposition uniformity of droplets onto the target.The critical drift height that obviously determines drift loss is established.The rationality and effectiveness are verified by the wind tunnel experiment,and the critical drift heights of different droplets under various environmental conditions are further determined.It suggests that narrowing the droplet spectrum can promote the mass transport efficiency only within the critical drift height,and otherwise the drift loss will increase.The effects of wheat canopy on the airflow distribution and mass transport efficiency at different growth stages are systematically studied.It is found that the obstruction of canopy makes the airflow velocity present the gradient distribution along the longitudinal direction,which is characterized by "velocity attenuation within canopy","velocity fluctuation near canopy" and "velocity stabilization far from canopy".Further,the effective height of the fluctuation region is nearly independent of the ambient air velocity but mainly determined by the canopy height,and it’s basically equal to the canopy height in the middle and late stages.In the early stage,the environmental parameters played a leading role in the droplet behavior and mass transport efficiency,and the canopy structure and environmental parameters play a synergistic effect in the middle growth stage.While,the canopy structure plays a leading role in the later stage,when the collision behavior disappears and the "upward drift" behavior of sensitive droplets occurs,increasing the spatial drift loss.Based on the above results,the regulation strategy of mixing spray adjuvants to change the physiochemical properties for improving the droplet spectrum and reducing the spatial drift is put forward.It shows that increasing the viscosity can increase the droplet diameter,while reducing the surface tension can narrow droplet spectrum,thus improving the uniformity.Additions of vegetable oil and organosilicon adjuvant increase the droplet diameter of TR80015 and ST110 015,significantly reducing the initial proportion of easily drifting droplets.The drift experiments in the wind tunnel indicate that these two kinds of spray adjuvants reduce about 50% of the drift loss for TR and ST nozzles,compared with pure water.The IDK nozzle produces larger droplets,showing a good anti-drift effect and reducing the drift loss by 90%compared with the TR and ST nozzles.