Coordination of distinctive pesticide adjuvants and atomization nozzles on droplet spectrum evolution for spatial drift reduction

Pesticide adjuvants,as crop protection products,have been widely used to reduce drift loss and improve utilization efficiency by regulating droplet spectrum.However,the coordinated regulation mechanisms of adjuvants and nozzles on droplet spectrum remain unclear.Here,we established the relationship between droplet spectrum evolution and liquid atomization by investigating the typical characteristics of droplet diameter distribution near the nozzle.Based on this,the regulation mechanisms of distinctive pesticide adjuvants on droplet spectrum were clarified,and the corresponding drift reduction performances were quantitively evaluated by wind tunnel experiments.It shows that the droplet diameter firstly shifts to the smaller due to the liquid sheet breakup and then prefers to increase caused by droplet interactions.Reducing the surface tension of sprayed liquid facilitates the uniform liquid breakup and increasing the viscosity inhibits the liquid deformation,which prolong the atomization process and effectively improve the droplet spectrum.As a result,the drift losses of flat-fan and hollow cone nozzles are reduced by about 50%after adding organosilicon and vegetable oil adjuvants.By contrast,the air induction nozzle shows a superior anti-drift ability,regardless of distinctive adjuvants.Our findings provide insights into rational adjuvant design and nozzle selection in the field application.

Numerical investigation of droplet pre-dispersion in a monodisperse droplet spray dryer

Monodisperse droplet spray dryers have great advantages in particle formation through spray drying because of their ability to produce uniform sized particles. Experimental analyses of this system have shown that droplets atomized through the piezoceramic nozzle need to be sufficiently well dispersed before entering the drying chamber to achieve sufficiently dried particles. However, the dispersion dynamics cannot be readily observed because of experimental limitations, and key factors influencing the dispersion state currently remain unclear. This study carried out numerical simulations for droplet dispersions in the dispersion chamber, which allow this important process to be visualized. The system- atic and quantitative analyses on the dispersion states provide valuable data for improving the design of the dispersion chamber, and optimizing the spray drying operation.

The effects of wind-driven waves and ocean spray on the drag coefficient and near-surface wind profiles over the ocean

By introducing a wave-induced component and a spray-induced component to the total stress, a mathematical model based on the Ekman theory is proposed to detail the influence of wind-driven waves and ocean spray on the momentum transport in a marine atmosphere boundary layer（MABL）. An analytic solution of the modified Ekman model can be obtained. The effect of the wave-induced stress is evaluated by a wind wave spectrum and a wave growth rate. It is found that the wave-induced stress and spray stress have a small impact compared with the turbulent stress on the drag coefficient and the wind profiles for low-to-medium wind speed. The spray contribution to the surface stress should be much more taken into account than the winddriven waves when the wind speed reaches above 25 m/s through the action of a ＂spray stress＂. As a result, the drag coefficient starts to decrease with increasing wind speed for high wind speed. The effects of the winddriven waves and spray droplets on the near-surface wind profiles are illustrated for different wave ages, which indicates that the production of the spray droplets leads the wind velocity to increase in the MABL. The solutions are also compared with the existed field observational data. Illustrative examples and the comparisons between field observations and the theoretical solutions demonstrate that the spray stress has more significant effect on the marine atmosphere boundary layer in the condition of the high wind speed compared with wave-induced stress.

Effects of Sea-Surface Waves and Ocean Spray on Air-Sea Momentum Fluxes

The effects of sea-surface waves and ocean spray on the marine atmospheric boundary layer （MABL） at different wind speeds and wave ages were investigated. An MABL model was developed that introduces a wave-induced component and spray force to the total surface stress. The theoretical model solution was determined assuming the eddy viscosity coefficient varied linearly with height above the sea surface. The wave-induced component was evaluated using a directional wave spectrum and growth rate. Spray force was described using interactions between ocean-spray droplets and wind-velocity shear. Wind profiles and sea-surface drag coefficients were calculated for low to high wind speeds for wind-generated sea at different wave ages to examine surface-wave and ocean-spray effects on MABL momentum distribution. The theoretical solutions were compared with model solutions neglecting wave-induced stress and/or spray stress. Surface waves strongly affected near-surface wind profiles and sea-surface drag coefficients at low to moderate wind speeds. Drag coefficients and near-surface wind speeds were lower for young than for old waves. At high wind speeds, ocean-spray droplets produced by wind-tearing breaking-wave crests affected the MABL strongly in comparison with surface waves, implying that wave age affects the MABL only negligibly. Low drag coefficients at high wind caused by ocean-spray production increased turbulent stress in the sea-spray generation layer, accelerating near-sea-surface wind. Comparing the analytical drag coefficient values with laboratory measurements and field observations indicated that surface waves and ocean spray significantly affect the MABL at different wind speeds and wave ages.

Rapid velocity reduction and drift potential assessment of off-nozzle pesticide droplets

The droplet velocity and diameter significantly affect both the spatial drift loss and the interfacial deposition behaviors, thus determining the ultimate utilization efficiency during pesticide spraying.Investigating the spatial velocity and diameter evolutions can reveal the mechanism of drift loss and guide to design regulation strategy. Here, we explored the spatial velocity distribution of droplets after leaving the nozzle by particle image velocimetry technology and particle tracking model, considering that the effect of nozzle configuration and the air velocity. It shows that all droplets decelerate rapidly with the velocity attenuation ratio ranging from 50% to 80% within the region of 200 mm below the nozzle.The spatial velocity evolution differences between droplets in crossflow are determined by the competition of vertical drag force and net gravity, and the drag force sharply increases as the droplet diameter decreases, especially for that smaller than 150 μm. Based on the spatial evolution differences of the droplet velocity and diameter, a functional adjuvant was added to the liquid for improving the diameter distribution. And the drift loss was significantly reduced due to the reduction of the proportion of easily drifting droplets.

Single-jet Spray Mixing with a Confined Crossflow

In order to achieve uniform mixing between spray droplets and crossflow, cold-model experiment of a hollow-cone water spray in an air crossflow is investigated via a numerical simulation. The simulation cases are designed by using the orthogonal design method. The Eulerian-Lagrangian formulation is employed for modeling the droplets-crossflow two-phase flow while the realizable k-ε turbulence model is used to describe the turbulence. A new index, mixedness quality, is proposed to assess the overall mixing of the droplets in the crossflow. The simulation results demonstrate that the counter-rotating vortex pair (CVP) imposes a more significant impact on the spatial distribution than on the size distribution of the droplets. Pairs of CVP with smaller scales are preferable for achieving a better mixing. The influencing factors are listed in the following order in terms of the degree of their impact from the greatest to the least: the Sauter diameter of the initial droplets, the mixing tube diameter, the spray angle, the velocity of the inlet crossflow, and the vertical velocity of the initial droplets. A moderate droplet diameter, a smaller tube diameter, a moderate spray angle, a greater crossflow velocity and a moderate vertical velocity of the droplet are favorable for achieving a higher mixedness quality of the jet spray in a confined crossflow.

植保施药机械喷雾雾滴飘移研究进展

中国农药产品80%以上通过喷雾方式施用,药液从喷头到靶标作物过程中产生的随风飘移和蒸发飘移是农药造成人畜健康风险、生态环境破坏的重要因素之一。随着航空施药技术的发展,解决或减少喷雾雾滴飘移的问题成为施药技术研究的重点和热点。基于此,本文分别从喷雾雾滴(尺寸分布、黏度、表面张力、蒸气压、挥发性、密度等)、喷雾模式(喷头类型、喷雾速度和高度、喷施方法)和外界条件(风速、风向、温度、湿度、气流等环境条件和操作人员技术水平等)等方面系统分析了产生雾滴飘移的主要原因;同时详细综述了采用田间试验、室内试验、计算机模拟及新型测试技术测定喷雾雾滴飘移的优点及局限性,针对植保无人飞机施药,提出应通过室内测定、田间试验与计算机模拟相结合的方式开展雾滴飘移研究。在此基础上分析并总结了从改变雾滴的运动方式、理化性质等方面直接控制和从田间布局间接控制的喷雾雾滴飘移风险控制技术。

Analysis of pneumatic atomizer spray profiles

Our objective is to analyze the atomization processes of a pneumatic atomizer by measuring the size and velocity distributions of droplets in a liquid paint spray. The droplet size and velocity distributions have been determined at different axial positions in the spray; a mathematical description of how these quan- tities vary throughout the spray is then proposed. Additionally, the relative number density of droplets and the relative local mass flux are estimated.

Predominance of inhalation route in short-range transmission of respiratory viruses:Investigation based on computational fluid dynamics

During the coronavirus disease 2019 pandemic,short-range virus transmission has been observed to have a higher risk of causing infection than long-range virus transmission.However,the roles played by the inhalation and large droplet routes cannot be distinguished in practice.A recent analytical study revealed the predominance of short-range inhalation over the large droplet spray route as causes of respiratory infections.In the current study,short-range exposure was analyzed via computational fluid dynamics(CFD)simulations using a discrete phase model.Detailed facial membranes,including eyes,nostrils,and a mouth,were considered.In CFD simulations,there is no need for a spherical approximation of the human head for estimating deposition nor the“anisokinetic aerosol sampling”approximation for estimating inhalation in the analytical model.We considered two scenarios(with two spheres[Scenario 1]and two human manikins[Scenario 2]),source-target distances of 0.2 to 2 m,and droplet diameters of 3 to 1,500μm.The overall CFD exposure results agree well with data previously obtained from a simple analytical model.The CFD results confirm the predominance of the short-range inhalation route beyond 0.2 m for expiratory droplets smaller than 50μm during talking and coughing.A critical droplet size of 87.5μm was found to differentiate droplet behaviors.The number of droplets deposited on the target head exceeded those exposed to facial membranes,which implies a risk of exposure through the immediate surface route over a short range.