Structure Optimization And Deposition Improvement Method Of Centrifugal Nozzle For Aviation Spray In Tea Garden

At present,the planting area of tea gardens in China has reached 47.48 million mu,and most of them are located in remote hilly area.Therefore,it is difficult,inefficient,and costly to apply pesticides manually.With the shortage of labor force in tea gardens and the development of agricultural mechanization,it has become an inevitable trend to apply efficient and flexible plant protection unmanned aerial vehicle(UAV)instead of manual application in tea garden pest control.However,due to the high distance between the aerial spraying nozzle and crop canopy,it is difficult for droplets to deposit effectively,which will lead to problems such as pesticide loss and air pollution.Optimizing the nozzle structure and exploring the matching application parameters are effective means to improve the deposition distribution of the liquid medicine.In this paper,the rotary centrifugal atomizing with uniform atomization and controllable droplet size was taken as the research object.The optimization of droplet deposition characteristics of aerial plant protection for tea garden was carried out from three aspects:nozzle working parameters,atomization disc structure and liquid medicine properties,which provided the theoretical basis and technical reference for the realization of intelligent plant protection operations in tea garden.The main research content are as follows:The main factors affecting the deposition characteristics of centrifugal atomized droplets were obtained.According to the working principle of rotary centrifugal atomizer,the atomization mechanisms of rotary disc centrifugal atomizer and the mutual transformation reason were deeply explored by analyzing the formation process atomizing droplets under centrifugal force,and then the main factors affecting the deposition of centrifugal droplets were obtained as follows:the working parameters of nozzle(flow rate and rotation speed),the disc diameter and the liquid characteristics(viscosity and surface tension).The droplet behavior in the air flow and gravity field was studied according to the theory of fluid mechanics,and the force of a single droplet was analyzed.The velocity change and trajectory equations of droplets with different sizes from throwing to landing were established,which provided the theoretical basis for explaining spray deposition.The spray deposition tests of rotary disc centrifugal atomizer under different working and structure parameters were carried out by building a variable spray test bench,and then the influence rules and weights of nozzle working parameters(spay height,atomizer rotating speed,and nozzle flow rate)and disc structure parameters(diameter,teeth,and cone angle)on droplet deposition were studied.The optimum working parameters were obtained as follows:the spay height was 80 cm,the atomizer rotating speed was 5000 r/min,and the nozzle flow rate was 800 mL/min.The diameter of the improved disc was 65 cm,the number of teeth was 180,and the cone angle was 30°.Spray adjuvants were added to improve the deposition,diffusion,and adhesion of pesticide droplets on the surface of tea leaves.The mechanism of adjuvant molecules changing droplet deposition characteristics was analyzed,and the results showed that the adjuvants can accelerate the deposition and diffusion of bifenthrin droplets by reducing surface tension.The contact angle of droplets on tea leaves at two growth stages decreased with the increasing concentration of adjuvants,and pesticides were easier to wet and spread on rough mature leaves than on the new buds.The extreme wetting phenomenon of Silwet L-77 reduced the binding force between the two-phase molecules,leading to the loss of pesticides,while the droplets with 0.2%6501 had the desirable wettability and adhesion on the surfaces of fresh buds and young leaves.The tea garden spraying test was carried out by UAV equipped with the optimized nozzle to explore the influence of flight speed and flight height on the distribution of droplet deposition.When the flight altitude was 2 m and the flight speed was 4 m/s,or the flight altitude was 2.5 m and the flight speed was 3 m/s,it could satisfy the uniform droplet deposition,wide spraying range,and large coverage rate at the same time.The optimization results based on the spray test bench were verified.The volume median diameters of droplets at each sampling point were reduced by 4～13%,and the relative span values of droplets were significantly reduced.The addition of adjuvants could not only improve the uniformity of droplet deposition,but also increase the deposition size of droplets,thus increasing the coverage rate of droplets.