该文主要探讨在温室环境中用气流辅助方式喷施农药时，施药对象（靶标）周围的流场对雾滴飞行轨迹及雾滴附着行为产生的影响，以期为精准施药技术提供参考。该文基于CFD模拟，用离散相粒子跟踪法模拟流场中的雾滴运动轨迹，在此基础上探寻雾滴附着靶标的条件，并采用试验手段验证模拟结果的可靠性。研究表明，在靶标附近区域，对区域内的雾滴轨迹进行分析后，得出了雾滴在靶标上表面实现附着的时间条件，即雾滴从进口处运行到靶标边缘所需的时间必须比雾滴从进口处运行到靶标上表面等高位置所用时间长，雾滴才能实现附着；喷雾过程中靶标下方存在雾滴不能到达的遮挡区域，靶标对雾滴运动的遮挡长度与喷雾角度有关；气流速度与雾滴粒径是雾滴附着靶标的关键因素，在雾滴粒径为50μm，喷雾角度为60°的情况下，喷雾速度越高，流场中靶标上表面压力越大，喷雾雾滴的沉积率越低。%The trajectory and adhesion behavior of droplets inair-assisted pesticide spraying in greenhouse are closely related with several factors: the velocity field and pressure field of the airflow, the droplets properties (such as droplet size, initial velocity), the spraying angle, as well as the target parameters (such as shape, size and position). This study explored the conditions that influenced the droplets carried by airflow, with a keen emphasis on whether it could touch, instead of going around, and stay on the target. A CFD (computational fluid dynamics) model was introduced for droplet trajectory simulation in an airflow field, in which droplets were traced by discrete phase particle tracking method. A computation region of 1600, 720 and 1000 mm respectively in streamwise, spanwise and normalwise was established. A target with the dimension of 120 mm × 120 mm × 30 mm was placed 400 mm above ground and 840 mm away from the left boundary of the computational domain. To simplify computational complexity, only half of the computational region (1600 mm × 360 mm × 1000 mm) was computed since the whole region was symmetric on both sides of the sprayer in streamwise. The grid number of the actual computation domain was about 0.36 million. A local mesh encryption method was applied around the target in order to increase the resolution of the simulation. The particle diameters involved in this simulation were 10, 30, 40, 50, 60, 70, 80 and 100μm; and the injection angles were adjusted to 90°, 75°, 60°, 45°, 30° and 15° respectively. A discrete phase boundary condition was set which trapped the droplets on the ground (wall) or the target surfaces. The rest of boundaries for discrete phase were set to boundary condition of droplet escaping. The influences of droplet velocity, droplet size and injection angle on deposition rate were evaluated by the CFD simulation and the experiments proved that: the condition of a droplet touching and adhering on target was that the maximum moving time of droplets inx andy direction should be simultaneously longer than the maximum moving time of droplets inz direction in the region around the target. There was a region below the target where droplets could not reach while spraying, and the length of the region was related to spraying angle. The adhesion behavior of droplets was affected by air velocity and droplet size. When the droplet size was 50μm and spraying angle was 60°, the larger the spraying velocity, the lower the deposition rate. The comparison of the deposition rates from simulations and experiments proves that the experiment data agree well with the data from simulation, so the simulation can be taken as reliable and valid measure in droplet deposition evaluation under greenhouse condition.
展开▼
