Influence of nozzle pore diameter and spraying pressure on size and deposition distribution of droplets from two types of orchard spray machines

Abstract:【Objective】Chemical pesticide plays an irreplaceable role in control of plant diseases, pests and parasitic weeds and guarantees food security. Rational use of pesticide is helpful for improving agri- cultural productivity and crop quality. The pesticide spraying is still the main control method for pest control. Thus, the machine used for plant protection is important in orchards. At present, the machine presents a new situation, which is the motorized spray gun with high-capacity and high-pressure is the main stream and the air-blast spray with low-capacity is auxiliary. Compared with developed countries, the machine used for plant protection is relatively backward in China. However, the basic research is comparable with developed countries in China. The research is not in touch with the practical application. It is necessary to carry out basic and practical research on the machine used for plant protection in orchards.【Methods】In this paper, the volume median diameters (VMD) of two common spray ma- chines in orchards (motorized sprayer with stretcher and plunger pump and conventional axial flow sprayer) under different pressures and pore sizes were studied. The droplet deposition distribution, ground loss rate and effective deposition amount of two machines applied on the apple trees were measured. The relationship of droplet size and pressure of machines and canopy distribution of pesticide in orchards was studied and parameters were optimized. It can provide technical guidance for the design and choice of spraying machinery in orchards.【Results】The research showed that the VMD of droplet decreased with the increase of pressure for motorized sprayer with stretcher and plunger pump at three pore sizes (0.7 mm, 1.0 mm and 1.4 mm). However, the decrease of VMD tended to be stable with the continuous increase of pressure. When the pressure was greater than 2 MPa, the effect of distance and pressure on particle size of the droplets reduced greatly. When the pressures were 1.5-3.5 MPa, the dis- tribution ranges of particle size were from 180 μm to 250 μm for motorized sprayer with stretcher and plunger pump and conventional axial flow sprayer. In addition, the change of droplet size of conventional axial flow sprayer with spraying distance was larger than that of motorized sprayer with stretcher and plunger pump. For motorized sprayer with stretcher and plunger pump, the deposition in the middle of fruit tree canopy was the highest, followed by the lower part of fruit tree canopy, and the deposition in the upper part was lowest. The deposition of fruit tree canopy increased with the increase of pore size for motorized sprayer with stretcher and plunger pump at three pore sizes of 0.7 mm, 1.0 mm and 1.4 mm. With the pore size of 1.4 mm, the depositions in the middle and outer part of fruit tree canopy were the largest (29.48 m·m-2). With the pore size of 0.7 mm, the depositions in the top and inner part of fruit tree canopy were the smallest (5.14 m·m-2). By comparing to the ground loss rate and the effective deposition rate, it could be seen that the ground loss rate of motorized sprayer with stretcher and plunger pump increased with the increase of pore size, while the effective deposition rate decreased with the in- crease of pore size, which had an obviously negative correlation. When the pore size was 0.7 mm, the ground loss rate was the lowest (22.04%) and the effective deposition rate was the highest (45.92%). When the pore size was 1.4 mm, the ground loss rate was the largest (35.21%) and the effective deposi- tion rate was the smallest (35.87%). There was a slight difference in the deposition structure of canopy after spraying between conventional axial flow sprayer and motorized sprayer with stretcher and plung- er pump. The area with the most deposition was the lower part of the canopy, followed by the middle part, and the upper part was least. Compared with motorized sprayer with stretcher and plunger pump, under the condition of small pore size (0.5 mm), the amounts of deposition in and out of the upper cano- py were similar, ranging from 5.49 m·m-2 to 6.66 m·m-2. However, under the condition of other pore siz- es (0.7 mm and 1.0 mm) and canopy area (middle and lower), the amount of deposition of conventional axial flow sprayer was far lower than that of motorized sprayer with stretcher and plunger pump. It could be seen that the ground losses of the conventional axial flow sprayer at three pore sizes were not significantly different, ranging from 16.34% to 17.78%, which were far lower than that of motorized sprayer with stretcher and plunger pump. It could be seen that the effective deposition rate of the conventional axial flow sprayer increased with the decrease of the pore size. When the pore size was 0.5 mm, the effective deposition rate reached the highest by 35.30%, which was equivalent to the effective deposition rate of the motorized sprayer with stretcher and plunger pump at 1.4 mm pore size.【Conclusion】For the motorized sprayer with stretcher and plunger pump, reducing the diameter of nozzle hole could decrease the droplet size, which was helpful to improve the effective deposition rate and reduce the ground loss rate. For the conventional axial flow sprayer, although reducing the droplet size had a certain effect on improving the effective deposition rate, the effect on the ground loss rate was limited. There was difference in the route of liquid loss between two machines. The loss route of the convention- al axial flow sprayer was mainly by means of the amount of droplet floating in the air. However, the loss route of the motorized sprayer with stretcher and plunger pump was mainly caused by the liquid falling from the leaf surface. In order to improve the effective deposition rate of two machines, the au- thor proposed the corresponding improvement measures, such as adding air feeding device on the motorized sprayer with stretcher and plunger pump, or installing infrared detector on the conventional axi- al flow sprayer to realize variable spraying.