Droplet size distribution of aerial nozzle for plant protection in wind tunnel and flight conditions

Abstract: Aerial plant protection has great development potential in the Chinese modern agriculture because of its advantages such as high work efficiency, low cost, ability of dealing with sudden disaster, and being applicable in complex site conditions of large-area agriculture and forestry pest control. Droplet size is a direct factor to reduce the droplet drift and improve spraying effect. For obtaining the spray particle size distribution of GP-81A nozzle, the droplet size distribution test was carried out under the wind tunnel conditions and flight conditions based on the advantages of the 2 methods. In September 2015, droplet size distribution of GP-81A nozzle was tested with the high speed wind tunnel, which simulated the high speed air flow produced by the fight of the fixed-wing Y5B aircraft at Intelligent Agricultural Equipment Technology Research Center, Beijing Academy of Agricultural and Forestry Sciences, and studied the effect of airflow on droplet size. Six kinds of different apertures with the GP-81A nozzle of from 2# to 7# were selected, and spray pressure was set at 0.25 MPa, which was similar with spray pressure of Y5B aircraft. Wind tunnel flow changed from 0 to 84.9 m/s, and the test of droplet size was performed every 5 Hz, in order to obtain droplet size distribution under different airflow condition and different nozzle diameter. The test results indicated that when the wind speed was less than 33.8 m/s, the droplet size increased with the increase of the airflow speed, and when the wind speed was more than 33.8 m/s, the droplet diameter decreased with the increase of the airflow speed, for large enough airflow could make droplet further atomized. When the airflow speed was 33.8 m/s, the droplet diameter of 7# nozzle was the maximum that was 491.1 μm; when the airflow speed was 84.87 m/s, the droplet diameter of 2# nozzle was the smallest that was 202.1 μm. The relative width range of droplet spectrum was 1.067-2.124, showing that this series of nozzle atomization droplets were overall uniform, and with the increase of wind speed, the droplet spectrum relative width increased and the uniformity of droplet size distribution decreased. In October 2015, at the airport in Xuzhou Agricultural Aviation Station, the droplet size distribution was tested under flight conditions with different nozzle. Test aircraft was Y5B aircraft for agricultural application, which had the flight speed of 180 km/h, the flying height of 6 m, and 50 GP-81A air fan nozzles fixed on both sides of the wing. The test objects were 2#, 4# and 7# nozzles and the spray pressure was 0.2-0.3 MPa. Sampling slices of droplets were placed at sampling points and the statistical calculation of droplet size was carried out by the computer system. The test results showed that the volume medium diameters (VMD) of 2#, 4# and 7# nozzles were 151, 260 and 322 μm, respectively. With the increase of the nozzle aperture, the nozzle flow increased, and the droplet size also increased; the droplet spectrum relative width range was 0.637-1.425, indicating that the droplet spectrum width was narrow, the droplet atomization performance was better, and the droplet distribution was uniform. At the same time, the droplet size and distribution were compared between the wind tunnel conditions and the flight conditions with similar spray pressure. Under the same spray pressure conditions, the droplet size in wind tunnel conditions was slightly higher than that in the flight test conditions, and the main reason was that the distance from the nozzle exit to test position was different. According to the results, the droplet relative spectral width values were close to 1 in wind tunnel and flight conditions, indicated that the droplet distribution was uniform, and the droplet distribution in flight conditions was more uniform. The research results provide a reference for the further optimization of working parameters of aerial nozzle to reduce the droplet drift.