Research And Optimization On Gas-liquid Two Phase Flow Of The Air-assist Boom Spraying

Air-assist boom spraying is an important technique of plant protection, which improves the efficiency of pesticide application and reduces spray droplets drift. Research and optimization on gas-liquid two-phase flow of the air-assist boom spraying will provide fundamental support for spray system optimization design, and improve the spray quality more economically and rapidly. Based on the large amounts of associated literatures at home and abroad, air-assist boom spraying process are studied by experiments, simulations, optimizations and field compliance tests.An air-assist boom spraying and measuring system was developed with gas-liquid two-phase flow. The system consists of air bag, boom, spray control device, and Phase Doppler Particle Analyzer (PDPA). The system can provide various fan rotating speeds, spray pressures, relative positions and angles between air bag outlets and nozzle. The droplets average size, spray angle and velocity were measured in the test. The experimental results show that, droplets velocity at x direction is not affected by air bag fan rotating speed or spray pressure. As the rotating speed increasing, droplets velocity at y direction is reduced, but increased at z direction. Droplets velocity at y and z directions is increasing with spray pressures. Higher rotating speed, the larger droplets average size, lower droplets number and higher variation coefficient of droplets size distribution. Increasing spray pressure could reduce droplets average size, increase droplets number. At spray pressure 0.5MPa, droplets velocity in z direction and droplets number are significantly affected by fan rotating speed. The droplets velocity in z direction and average size are reduced with increasing the vertical distance between air bag outlets and nozzle. They are also increasing and then decreasing when the horizontal distance between air bag outlets and nozzle is increased. Droplets velocity in x, z direction and droplets average size are reduced as the angle between air bag outlets and nozzle increasing. There is negative x direction droplet velocity value at angle 45° between air bag outlets and nozzle, where the droplets is moving in the opposite direction to wind.The gas-liquid two-phase flow of the air-assist boom spraying is transient simulated by CFD software Ansys Fluent. Discrete Phase Model (DPM), RNG κ-ε turbulence model are used. The simulation results were compared with the measuring data by PDPA. The transient simulation shows that, the downward velocity of the droplets was reduced by larger air resistance. It turns to zero after left far away from nozzle. Then the droplets move with wind and drifting. When droplets are falling in the air, larger droplets at the windward side drop down easily due to much more weight. But small droplets are easily drifting. Air flow with high velocity comes from air bang outlets. Flow direction is decided by the vector sum of air bag flow and wind. The high velocity of air flow could change the trajectory of droplets and reduce drifting.In order to obtain the best air-assist boom spraying operating point, a multi-objective optimization was developed by response surface methodology. Anti-drift coefficient (f) and droplet deposition distribution variation coefficient (CV) are defined as objective value. At single factor condition, the anti-drift coefficient is increased with the fan rotating speed, but reduced with the spray pressure and the vertical distance between air bag outlets and nozzle. As the horizontal distance and angle between air bag outlets and nozzle increasing, the anti-drift coefficient is increasing and then decreasing. The droplet deposition distribution variation coefficient is increased and then decreased when the fan rotating speed, horizontal distance and vertical distance between air bag outlets and nozzle increasing. It is also reduced when spray pressure and angle between air bag outlets and nozzle increasing. The variation trend at multi- factor condition is the same to that in single factor condition. The result of the maximum anti-drift coefficient and minimum droplet deposition distribution variation coefficient after multi-objective optimization is obtained with fan speed 3000r/min, spray pressure 0.3MPa, vertical distance 90mm between air bag outlets, horizontal distance 140mm between air bag outlets, and the angle 23.53°between air bag outlets. At optimum spray condition, anti-drift coefficient is 81.4%, droplet deposition distribution variation coefficient is 11.54%.Researches and optimization on air-assist boom spraying indicate that it is able to improving pesticide deposition, reducing drift, and increasing droplet deposition uniformity in the spray process. The transient simulation and multi-objective optimization on gas-liquid two-phase flow of air-assist boom spraying are established to get functional relation between spraying operation and pesticides application index for agriculture plant protection.