Spray drift from hydraulic spray nozzles: the use of a computer simulation model to examine factors influencing drift
1993
The effects on spray drift of nozzle size, angle and operating pressures for boom-mounted hydraulic nozzles operating over a range of meteorological and crop conditions, was investigated using computer simulation. The results showed that the measurements of droplet size, particularly the percentages of spray volume in droplets less than 100 micrometer in diameter, critically influenced spray drift. Measurements made with an optical imaging system gave data which showed insensitivity to some of the parameters studied, in particular, nozzle size. Results using measurements from this instrument indicated greater spray drift from 80 degrees nozzles mounted at 0.5 m above the crop than from 110 degrees nozzles operating over a comparable range of flowrates and positioned 0.35 m above the crop, despite the coarser spray quality produced by the nozzle with the smaller fan angle. Further simulations were carried out using data from a Phase Doppler analyser in which better resolution of size was possible at droplet diameters < 100 micrometer. Using this data, the model indicated an increase in the mass flux of down-wind airborne spray in conditions that encouraged evaporation; this effect was probably over-estimated because of the assumed constant local air moisture content. Spray drift was shown to increase approximately linearly with wind speed: the rate of increase for a given spray sheet angle and operating pressure was a function of nozzle size. The simulation was also used to demonstrate that a measurement of wind speed made at a height of 2 m above a tall crop signifies a greater drift hazard than the same wind speed measured over a short crop at the same height above the ground.
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