Design and experiment of NJS-1 type open-circuit closed wind tunnel for plant protection

Abstract: Drift potential is one of the most important factors affecting spraying quality. Plant protection wind tunnel is an important and preferred tool for quantitative study on drift potential, which is an effective, reliable and feasible research tool for the parameter localization of spray model building. Wind tunnel can provide a well controlled environment for drift measurements under a range of wind speeds and spraying conditions. The NJS-1 type of plant protection wind tunnel was designed based on the drift potential theory, principle of wind tunnel design and atmospheric boundary layer similarity theory, which was composed of inlet section, power section, flexible section, transition section, diffusion section, stability section, contraction section and working section. The total length of the wind tunnel was about 20.0 m; the length of the working section (including five subsections) was about 7.5 m, while the cross section the of the working section was 1.2 m wide and 1.8 m high; the contraction section was 3.0 m long and the contraction ratio was 4; the settling chamber was 2.5 m long and the cross section was 2.4 m wide and 3.6 m high; a hexagon honeycomb of stainless steel and two 18-mesh stainless steel screens were embedded inside the settling chamber; the diffusion section was 2.5 m long, and diffusion ratio was 4 and diffusion angle was 40°; the transition section was 2.8 m long and its main function coupled the power section and the diffusion section. The power section was composed of collector device, impeller, nose cowl, guide vane unit, trail hood and air duct, and was produced by an axial fan which was driven by the 25 kW electric machine. The design of the air supply device and main technical parameters of key parts was based on the theories of fluid mechanics. The air volume of the fan was 21.6 m3/s at the fan speed of 960 r/min, which was determined to adopt 8 impellers and 9 guide glades, the impeller diameter was 1.2 m and the guide glade width was 0.18 m. Wind profiles and flow dynamic characteristics were investigated for the NJS-1 type of plant protection wind tunnel. The objective of this research was to determine the suitability of the wind tunnel for the experiment research and provide the basic information for droplet drift and deposit experiments in this wind tunnel. The experiment was conducted under the condition of the empty wind tunnel. And the experiment strictly followed the flow field specification on high-speed wind tunnel and low-speed wind tunnel (GJB1179-1991). Pitot tube, spectrum analyzer and contractive segment were applied under different wind velocities. Some tests were finished, which referred to the main characteristic parameter testing of field aerodynamics flows in the plant protection wind-tunnel such as air velocity, turbulence intensity, flow stability and flow field uniformity. The results showed that the flow stability coefficient was less than 1.5%, the turbulence was about 1% within air velocity scope from 1.0 to 9.0 m/s, and wind field distribution was more uniform with the increase of the air velocity. This study suggests that the NJS-1 type of plant protection wind tunnel meets the design standard and experiment requirement.