果园风送喷雾机导流板角度对气流场三维分布的影响

风送喷雾条件下,雾滴是在空气流携带下进入果树冠层的各个部位,所以喷雾机气流场的运动和分布对雾滴的分布和穿透非常重要。为了研究果园风送喷雾机导流板角度变化对外部气流速度场三维空间分布的影响,该文采用ICEM建立几何模型,并进行全结构网格划分,采用k-ε湍流模型和CFX求解器进行数值求解。通过变换上导流板角度(30°、45°、60°、90°)与下导流板角度(0°、10°、20°、30°),来模拟分析风机外部流场在各工况下的空间稳态流场、湍流状态,以及对气流场空间分布的影响。结果表明,下导流板角度由0°增加至30°过程中,由于地面摩擦阻力对气流的影响逐渐减小,同时地面摩擦阻力与两侧空气阻力形成的夹角越来越大,因此单一气流束逐渐分成3条气流束,这样的气流分布优于单一方向气流对果树枝叶的吹动效果,有利于气流携带雾滴进入果树冠层;上下导流板导向气流主要集中在导流板指向区域,因此,导流板的角度设置应根据树冠高度、树干高度来调整。通过设置合理的导流板角度,使得风场分布与果树冠形相吻合,达到仿形喷雾效果。对于行距4 m、树高3.0~3.2 m的果园喷雾,上、下导流板角度均为30°;对于棚架果园,上导流板角度为90°(或卸掉上导流板),下导流板为30°。该研究有利于指导田间喷雾作业、喷雾参数调整,可达到更好的喷雾效果、减少环境污染。

Influence of deflector angles for orchard air-assisted sprayer on 3D airflow distribution

Air-assisted orchard sprayers are characterized by a strong airflow that carries the pesticide droplets to the target canopy and assists the plant parts to move so as to allow the whole tree penetrated with pesticide droplets. The airflow distribution and movement characteristics are key parameters of sprayer for droplet deposition on target canopy. In order to study the 3D (three-dimensional) spatial distribution of airflow field from an air-assisted orchard sprayer, this article used ICEM CFD (Integrated Computer Engineering and Manufacturing code for Computational Fluid Dynamics) to establish a geometric model, whose whole structure was meshed, and the k-ε turbulence model and CFX solver were adopted. The effect of different environmental systems on airflow 3D distribution for air-assisted orchard sprayer was estimated, in which the upper deflector angle was set as 30°, 45°, 60°, and 90°, and the lower deflector angle was set as 0°, 10°, 20° and 30°, respectively. Results show that the 3D spatial distribution model of the airflow from air-assisted orchard sprayer can reflect the airflow distribution directly, and the simulated values of airflow characteristic curve are in good agreement with the measured values. However, in the low airflow velocity region (the velocity below 2 m/s), the simulated velocity values of some sampling points are quite different from measurements. That is caused by the perturbation of the natural wind. In the high airflow velocity region with 1.2 m width, the average relative error between the measured value and the simulated value of wind speed is less than 10%. That can confirm that the simulation results are credible. Meanwhile, airflow guided by deflectors is mainly focused in the area the deflectors pointed to, with no obvious effect on airflow diffusion in the middle area, only increasing the airflow velocity and letting airflow gather near deflector area. So the adjustment of deflector angle should be based on the height of canopy and tree trunk, through setting lower deflector angle to fit the foliage limit of the orchard, and setting upper deflector angle to point at position which is little lower than the topmost of tree canopy in the orchard. For further research, to change airflow velocity of the fan has no significant effect on airflow distribution in the airflow field, and merely increases airflow diffusion region. When the lower deflector angle increases from 0° to 30°, the effect of ground friction resistance on the airflow decreases gradually, and the angle between ground friction resistance and air resistance on both sides increases constantly, so one single air current is divided into 3 air currents gradually. This kind of air distribution on horizontal plane has advantages on blowing branches and leaves and penetrating droplets into fruit tree canopy. The airflow velocity distribution in the vertical direction considerably affects the distribution of spray droplets in canopy. The vertical profile of the airflow velocity should be even along the whole vegetation wall, and matched with the canopy shape curve. But the deflector can only adjust the upper and lower airflow, some deflectors are suggested to be fixed in the middle area for adjusting airflow distribution in vertical direction. This study found that a better effect could be achieved by setting the upper and lower deflectors angle as 30° for spraying in the Chinese general orchard where fruit trees had 3.0-3.2 m height and were planted with 4 m row spacing, and by setting the upper deflector angle of 90° (or removing), and the lower deflector angle of 30° for spraying in trellised orchard. The results can provide the reference for the spraying in the orchard, and the higher spraying efficiency and the reduction of environmental pollution can be achieved through the calibration of sprayer operational parameters.