| In recent years,unmanned aerial vehicles（UAVs）applied to agricultural plant protection have been developed rapidly in China.However,during the spraying operation of plant protection UAVs,the pesticide drift caused by their spraying operations can seriously endanger the environmental safety and the health of surrounding animals and plants.At present,the drift reduction technology of plant protection UAVs is not mature,especially since the research on nozzle atomization characteristics is weak.The influence mechanism of many factors on liquid sheet breakup and atomization is unclear,resulting in the lack of systematic test method guidance for pesticide drift control of UAVs.In view of this situation,this paper carried out mechanism research on the liquid sheet breakup and atomization characteristics of a hydraulic fan nozzle of the plant protection UAVs,established a model for the initial droplet distribution of the fan nozzle,and developed the model into a model for the plant protection drone combined with the rotor wind field of the UAV.At the same time,the relationship between the liquid sheet breakup and atomization characteristics and the droplet drift of the fan nozzle has been explored.Based on wind tunnel tests and field tests,the study of droplet deposition and drift during aerial spraying of UAVs was conducted.The main researches and conclusions are as follows:（1）The effect of the physical properties of the spray liquid on the breakup of the liquid sheet and the atomization characteristics of the fan nozzle was experimentally studied,and the initial droplet distribution model of the fan nozzle was established.The surfactant sodium dodecyl benzene sulfonate（SDS）solutions and typical viscous glycerol solutions with different concentrations were used in this experiment.The breakup mode of liquid sheet of SDS surfactant solution and glycerol solutions gradually changed from rimless mode to wave or ligament mode,and finally fully developed mode with the increase of spray pressure.Higher spray pressure reduced the number of droplets in the range of droplet size greater than 200μm and increased the number of droplets in the range of droplet size less than 200μm.The absolute values of velocity U and velocity V gradually decreased with the increase of glycerol solution concentration.（2）In order to apply the initial droplet distribution model of the fan nozzle to plant protection UAVs,this model was improved,and expanded to develop into the initial droplet distribution model for the plant protection UAVs.The effects of rotor wind field and ambient wind speed on liquid sheet breakup and atomization characteristics of plant protection UAVs were experimentally studied under different rotor speeds and wind tunnel speeds.The rotor speed increased the overall velocity distribution of the droplet flow field.When the rotor speed increased from 0 to 2200 rpm,the absolute value of velocity V increased from 6.95 m/s to 9.26m/s.The median diameter of the droplet volume(DV0.5)at the rotor speed of 2200 rpm was18.70%larger than that without the rotor.The rotor wind field can prevent the droplets from drifting downwind in the crosswind,and the spray pressure promoted the movement of droplets to the ground in the rotor wind field.（3）In order to explore the correlation between the liquid sheet breakup and atomization characteristics and the droplet drift,the atomization characteristics of liquid sheet breakup and the droplet drift distribution of the XR fan shaped nozzle sprayed with different adjuvant solutions were studied.A comparison was made between the XR fan nozzle and the AIXR anti-drift nozzle,confirming that the anti-drift effect of the XR nozzle was better.The results showed that there was a significant strong correlation between the volume percentages smaller than 150μm of V<150(%vol)with XR and AIX nozzles and their drift potentials,and the correlation coefficients were 0.921 and 0.974,respectively.The drift potential of SDS solution sprayed by the XR nozzle（2.14%）was lower than that of aqueous solution sprayed by the AIXR anti-drift nozzle（2.47%）,indicating that selecting appropriate adjuvant for the XR fan nozzle can also achieve good anti-drift effects.（4）In order to study the drift characteristics of the plant protection UAV under the controllable conditions of the wind tunnel,the two-phase flow test platform of UAV and wind tunnel were used to construct the research conditions.Combined with the initial droplet distribution characteristics of the plant protection UAVs,the effect of spray pressure and lateral wind speed on droplet drift of plant protection UAV was analyzed.In the medium and low speed wind environment less than 4 m/s,a smaller spray pressure of 0.20 MPa should be selected,and the total drift rate was 1.05%.In the higher lateral wind environment,a larger spray pressure of0.50 MPa should be selected,and the total drift rate was 6.00%.The amount of droplet drift at the wind speed of 4 m/s was significantly lower than that at 6 m/s collected from the drifting tower at 20 m.The rotor wind field can significantly inhibit droplet drift.To reduce pesticide drift,operating conditions with low wind speeds should be selected based on meeting operational requirements.（5）In order to optimize the operational parameters of plant protection UAVs,the three aviation operation modes designed in this paper were evaluated based on the droplet deposition in the operational area and the degree of drift in the drift area.At the same time,the internal driving forces of droplet deposition and drift in different operational modes affected by the initial distribution of droplets were revealed from the perspective of spray flow and flight speed,respectively.The optimized operation mode parameters were as follows:1 L/mu of spraying volume,4.5 m/s of flight speed and 2.025 L/min of flow rate.The coverage in operation area of this operation mode was the largest,which was 5.54%.The average deposition rate was the largest,which was 38.2%,and the drift distance of droplet was the shortest,which was 8 m.In the analysis of factors affecting the deposition quality of droplets in the operation mode,high flow rate promoted the deposition of droplets in the operation area and leaded to a longer drift distance.Higher flight speed leaded to a longer drift distance in the drift area.In addition,the use of adjuvants helped to deposit droplets in the operation area.The research results showed that the initial droplet distribution model established in this paper can effectively predict the initial droplet distribution during the spraying operation of the plant protection UAVs,and can achieve the goal of reducing the droplet drift of airborne spray by regulating the atomization process of the nozzle of the plant protection UAVs,which can provide a reference for the application of airborne spray drift reduction. |
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