Design and test of circulating air-assisted sprayer for dwarfed orchard

Abstract: Conventional air-assisted spraying declines rapidly to stagnate in general, particularly when the droplets reach the canopy in an orchard. Unfavorable spray effects thus often occur, such as "difficult to penetrate" and "difficult to deposit inside the canopy and the back of leaves". In this study, a new idea was proposed to form multi-source wind disturbance for the direction change of airflow inside the canopy, namely, "from outside to inside, and then from bottom to top". Firstly, the movement tracking of droplet flow under the surrounding airflow was analyzed to determine the structure of a sprayer and the key parameters. The sprayer with a "door" type structure was composed of a surrounding air-assisted system, a spraying and recovery system, a hydraulic drive system, and a crawler chassis. An axial flow fan was placed at the top of the canopy. Specifically, the air inlet of the fan was facing the top of the canopy, whereas, the air outlet of the fan was connected with an air duct to transport the air into eight subsequent outlets. Meanwhile, the negative-pressure suction was generated through the air inlet of the fan, thereby moving the air flow "from bottom to top" in the canopy. Four flumes were arranged below the shields to receive the lost droplets. Two pumps were utilized to transfer the recovered droplets into the auxiliary tank for environmental protection. The size of the gate-type opening was adjusted in a certain range for various planting patterns in an orchard. The key parameters of the surrounding air-assisted system were also optimized using the displacement theory of air volume and jets. The air velocity of the outlet was determined to be 10-20 m/s, while, the wind pressure provided by the axial flow fan cannot be less than 857.8 Pa. Secondly, the stress of the gantry frame was analyzed under the service condition to meet the user needs, where the bending and torsion resistance were verified in the theoretical evaluation. Thirdly, the five-hole probes and ribbon method were selected to field test the distribution of flow direction in a prototype of the sprayer. Meanwhile, the velocity distribution of the airflow field was also measured to verify whether the sprayer can produce the droplets flow from the outside to the inside and from the bottom to the top. It was found that the airflow angle changed significantly inside the canopy, especially in the height of 0.8-1.8 m and the center area of 0.25 m on both sides of the center line of a trunk. There was an obvious increase in airflow velocity under the surrounding air-assisted spraying. Finally, the spraying effects with and without surrounding air-assisted were compared at the fan speed of 1 000 r·min-1, where the coverage rate of the droplet was selected as an evaluation index. The coverage rate of the droplet on the leaf face increased by 42.9%, while that of the leaf back increased by 40.4%, where the overall leaf back increased by 33.7%, compared with traditional air-assisted spraying. It infered that the surrounding airflow significantly improved the droplets deposition coverage in the center of a canopy and leaf back. The findings can provide an insightful design idea for the surrounding air-assisted sprayer to produce the airflow suitable for plant protection in an orchard with dwarfed fruit trees. Follow-up experiments can be performed on the canopies of different sizes and thicknesses to clarify the influence of boundary conditions on the surrounding air-assisted spraying.