AGRICULTURAL SPRAYS IN NON-UNIFORM CROSS-FLOW
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The size and velocity characteristics of droplets in agricultural sprays generated from two flat fan nozzles were studied in a wind tunnel using a Phase-Doppler Particle Analyzer. A sheared cross flow pattern, having lower velocities near the wind tunnel floor and increasing velocities with increasing distance towards the wind tunnel roof was used. The nozzle was located in the region of high shear. The study was conducted in four stages. In stage 1, size-velocity characteristics of droplets in spray from a nozzle (Teejet 8003) generated at 275 kPa pressure and subjected to 5 km/h (average) crosswind were characterized. In stage 2, size-velocity \ characteristics of droplets in sprays of 'fine' (from Teejet 8001) and 'medium' (from Teejet 8003) quality, were compared. In stage 3, the effect of variation in crosswind velocity on droplet characteristics was studied by adopting three crosswinds, having a similar velocity distribution pattern, but corresponding to average velocities of 5 km/h (low), 15 km/h (medium) and 25 km/h (high). In stage 4, the effect of variation in nozzle operating pressure on droplet size-velocity characteristics was studied by generating spray fromaTeejet8003 nozzle at pressures of 150,275 and 400 kPa. Size and velocity of the droplets were measured in the central region of the spray at various downwind locations in a 1x 0.3 m grid. Variables of interest included droplet size, droplet horizontal velocity, droplet vertical velocity, and droplet concentration. These variables were used to evaluate the dynamics of the droplets in various size groups. Results indicated that due to reverse flow in the wake region of the sprays, some droplets acquired negative horizontal velocities, (i.e., opposite to the cross flow direction), and deposited close to the .nozzle axis. The reverse flow in the 'fine' spray moved droplets of all sizes that were present backwards while in the 'medium' spray only the smallest droplets « 50 um) traveled back. However, the reverse flow in the 'fine' spray did not persist away from the nozzle. With increased crosswind velocity, the strength of the reverse flow diminished. At high crosswind, the wind completely penetrated the spray and dragged both small and large droplets downstream. The deposit distances for droplets of various sizes in the sprays, studied at various stages of this research, indicated that droplet movement strongly depended on the spray quality from which droplets originated. The presence of larger droplets in the spray increased the possibility of deposit of smaller droplets uear the nozzle axis. Reverse flow was found to be a significant factor that affected the movement of the droplets of different size groups in the sprays. The study of droplet size distributions at downwind locations in the 'medium' spray indicated that droplets <105 um, < 210 urn and < 300 um diameter were separated from the spray by the low, medium and high wind speeds, respectively and could be defined as the 'driftable' droplet sizes for each wind speed for the operating conditions in the study. The driftable droplet sizes for the 'fine' and 'medium' sprays subjected to 5 km/h crosswind velocity were < 78 um and< 105 um, respectively.