The Trajectory And Deposition Behavior Of Droplets Around Target Affected By Air Flow Field In Greenhouse

For air assisted pesticide spraying in greenhouse, the trajectory and adhesion behavior of droplets are closely related with several factors:the velocity field and pressure field of the airflow, the droplets properties (such as droplet size, initial velocity, etc.), spray angle, as well as parameters of target (such as shape, size, position, etc.).Air-assisted sprayer system could send droplets to distance target and improve the droplet deposit on targets. The system can increase the efficiency and reduce labor intensity during the production process in greenhouse. For this reason, air assistance spraying system is widely used in orchard and greenhouse. But the adhesion behavior of the spray around the target is still unclear at present time, although it will be helpful for system improvement or design. Experiment and CFD (Computational Fluid Dynamics) method are two main means of spray study. Compared to experiment, CFD technique has a wide range of applications in all fields including the domain of droplets spraying as it give advantages of low cost, fast speed and high precision. This article mainly explores the influence which the flow field around target brings to the trajectory and adhesion of droplets. A CFD method was introduced for droplet trajectory simulation in a simple airflow field in which droplets were traced by the discrete phase particle tracking method, on the basis of which explores adhesive conditions of droplets. Experimental results were compared with simulation results to check deposition rate.The designed flow field was a rectangle with the size 1600mmx720mm×1000mm in streamwise (x), normal (y) and transverse (z) directions. The flow model included a rectangle target (a 120mm×120mmx60mm) located 840mm away from the left side of the boundary and 415mm above the ground. The computational domain was discretized by a structured control volume mesh which consisted of 361857 codes. A local mesh refinement method was employed around the target in order to investigate the fluid flow and adhesion of the droplets more accurately. Because the air flow and the droplets pass the target would have a dramatic change. Symmetric method was used in present simulation, so only half of the flow field (1600mx360mmx 1000mm) was computed. Since the length, width and height of the flow field were at least 6 times more than the corresponding sizes of the target, that meant the boundary conditions for the computational domain could only have a little influence on the area around the target. This paper presented the effects of droplet velocity, droplet sizes and injection angles on deposition rate by the CFD simulation and experiments. The results proved that:1) The droplet size was 50μm; 60° (the angle between the horizontal XY plane and the velocity of flow at inlet) was selected as injection angle. After analyzing the trajectories of droplets in the region around target, the maximum moving time of droplets in x and y direction should simultaneously longer than the maximum moving time of droplets in z direction to achieve adhesion on target.2) The droplet sizes were 10μm,30μm,400m,50μm,60μm,70μm,80μm, and 100μm; 15°,30°,45°, 60° and 75° were selected as injection angles. The distributions of droplets deposition rate on target were found out in different calculate conditions. The deposition rate of the droplets would increase with enlarged droplet sizes and raised injection angles. When the droplet sizes were small (d≤40μm), the spray angle had less effect on the deposition rate, but the influence was getting greater when the droplet sizes were greater than 50μm.3) 75°、60°、45°、30°and 15° were selected as injection angle; comparing the trajectories of the droplets (the droplet of 80μm and 40μm). The trajectories of the droplets depend on the pressure distribution of air flow and original locations of the droplets at inlet of computational domain. In the same pressure field, larger droplets could be trapped by the target more easily. There was a high pressure area above the target which could accelerate the air flow to the edge of the target, but the accelerated air flow had little effect on the larger droplets (bigger than 50μm), so such droplets could be trapped easier by the target.4) 15°,45° and 75° were selected as injection angles. There is a region below target where droplets cannot reach while spraying, the length of which is related to spray angles. When the injection angles were small than 45°, the increased injection angle had much effect on length, the length would increase with increased injection angles. When the injection angles were large than 45°, the increased injection angle had little effect on length.5) The droplet size was 50μm; 60° was selected as injection angle; the air velocities were 0.5m/s, 0.75m/s, 1.0m/s,1.25m/s,1.5m/s,1.75m/s,2.0m/s,2.25m/s,2.5m/s,2.75m/s,3.0m/s. The distributions of droplets deposition rate on target were found out in different calculate conditions. The adhesion behavior of droplets was affected by air velocity and pressure at different injection velocities. When the droplet size was 50μm, spray angle was 60°, the larger the spray velocity, the lower the deposition rate.