Test on airflow field and spray characteristics for long-range air-blast sprayer

Abstract: When the long-range air-blast sprayer is carrying out spraying, droplets are sent to the distant targets by the high speed air flow from fan. At the same time, the air flow in spatial wind field will blow the targets and shake the target branches, which allows droplets attachment to the back and surface of leaves of target crops as well as the surface and inner of the fruit tree canopy. It has been reported that the deposition and distribution of droplets on the fruit trees are mainly affected by blowing volume of air-blast sprayer fan, which is determined by the wind speed of the fan. Therefore, the characteristics of wind field with air-blast sprayer and the variations of wind speed will directly affect the control effects on pests and diseases in orchards. Scholars from China and other countries have done much research on the air-assisted sprayer in orchards. The range of the sprayer is relatively short, which can be applied to the orchard plant protection in plain areas. In addition, in the research on the long-range air-blast sprayer (also called cannon sprayer), most of them focus on the penetrativity, deposition and distribution of droplets, optimal designs of sprayer and variations of droplet diameter. Based on above, the study in this paper focused on spatial wind field and spray characteristics for long-range air-blast sprayer. As the prototype for testing, the long-range air-blast sprayer is equipped with 10 hollow-cone type nozzles distributed circumferentially, which are evenly distributed on the round outlet edge of spray-duct. The followings are the instruments and equipments used in the test: AZ-8901 anemometer with wind speed measurement range of 0.4-35 m/s, resolution of 0.01 m/s, and accuracy of ±2%; wind speed measurement positioning frame (homemade) which is 3 m high and 2 m wide and its grid size is 11 cm × 11 cm with a wire radius of 1.25 mm; a digital microscope with a magnification from 1 to 300. With the wind speed measurement positioning frame, the tests on the characteristics of spatial wind field for long-range air-blast sprayer and on the spraying width were carried out. The wire of sampling frame can be regarded as the cylinder in the flow field. When measuring, the anemometer was 20 mm away from the sampling frame in the direction of incoming flow. Thus, the influence coefficient of wire on wind speed was calculated, which was 3.9×10-3. Therefore, the interference to the flow field caused by sampling frame can be neglected. At the same time, the wind speed in testing environment had a great influence on the test and the measurement of spatial wind field and spray characteristics for long-range air-blast sprayer. Therefore, in order to ensure the reliability of the test results, repeated measurements of ambient wind speed have been carried out before each test. The test results showed that the vertical average wind speed on the axis of long-range air-blast sprayer decreased with the increase of distance between sampling point and nozzle and there was a negative logarithm relationship between them. This characteristic is in accord with the flow variations of submerged jet. The wind speed at the same sampling point in the axis direction of long-range air-blast sprayer was linearly and positively correlated to the power frequency of the fan with a high correlation coefficient. The horizontal spraying width of long-range air-blast sprayer was bilaterally symmetrical in the axial direction of spray-dust; in contrast, the vertical counterpart in the upper and lower axial direction of spray-dust was asymmetric. Bernoulli effect and Coanda effect resulted from entrainment are the important reasons for the formation of asymmetric vertical spraying width. However, the effects can be eliminated by spraying with an angle of elevation. By measuring the average wind speed at the outlet of air duct, the blowing volume was calculated. The result showed that the blowing volume of long-range air-blast sprayer and the average speed of flow at the outlet were proportional to the frequency of the fan. The sampling card method was adopted to measure the horizontal operation width of the sprayer and the width of the experimental prototype was 22 m. The long-range air-blast sprayer can be applied not only to agricultural plant protection but also to dedusting spray and dust suppressing spray for environmental purification. The findings in the study provide theoretical basis and technical guidance for the production and application of the long-range air-blast sprayer.