Research On Multiple Factors Mechanism Of Pesticide Spray Droplet Movement In The Air

The rough use of pesticide in agricultural spray not only leads to extravagance of pesticide,but also the severe environmental pollution and jeopardization to healthy living of organisms.Study on the movement of pesticide spray is regarded as the key step in solving the problem of pesticide excessive use and the comprehension of single droplet movement is the foundation of knowing the law of spray drift.Through numerical simulation method of gas-liquid interface tracking,this thesis constructs the two-dimensional droplet movement model to investigate the movement of droplet in the cross-wind and the mechanism of different influencing factors on the trajectory and deformation during the motion process are revealed.Improvements are made on the set conditions of uniform and stable air velocity fields in previous studies and unstable and non-uniform air flow field are established in this thesis.Multiple factors of flow field and droplet are inspected in order to find suitable ways to control spray drift.Drag force performs as the decisive factor on droplet movement in cross-wind.Larger droplet moving in faster wind exhibits more driftable characteristics and the physicochemical properties of the liquid barely influence droplet trajectory.Previous studies on the movement of spray usually ignored the morphological evolution of droplets by regarding droplet as a mass point;researches on droplet impact also took perfect sphere as starting shape.However,droplet deforms under the effects of inertia force,viscous force and surface tension.The effects of droplet size,wind velocity,viscosity of liquid phase and the surface tension between two phases on droplet deformation are discussed in the present thesis and the mathematical relations of deformation amount and cycle with Re and Oh numbers are posted,which could be used to predict droplet deformation under certain conditions.Droplet movement in air flow where the vertical velocity fluctuates by sine function are investigated.It has been found that the drift distance presents to be longer in the fluctuating air fields than uniform ones;droplets in larger sizes tends to have better drift prevention ability in fluctuating cross-wind;frequencies and amplitudes of sine functions in the inspection range perform no obvious influences on drift.When the initial velocity of droplet increases,drift angle decreases.Drift distance reduces when initial direction of droplet deviates towards upwind but at the same time the high horizontal velocity area on the right hand expands and ant the same time the low horizontal velocity area formed by the trajectory of droplet reduces.Air velocity fields around wheat canopy are calculated basing on the data given in literatures and drift of droplets near canopy in shooting stage and green stage is explored.The inverse proportional relations between droplet initial kinetic energy and drift angle are set up where the droplets with larger initial kinetic energy tend to be more drift preventing and it’s difficult for the droplet to deposit when spray height is higher.