R. D. Brazee

Simulation of Drift of Discrete Sizes of Water Droplets from Field Sprayers

The drift distances of water droplets from field sprayers were determined for several variables with a computational fluid dynamics computer program. The simulation variables for drift distances up to 200 m included: droplet size (10 to 2000 mm), wind velocity (0.5 to 10.0 m/s), initial droplet velocity (0 to 50 m/s), discharge height (0.25 to 4.0 m), temperature (10° to 30° C), relative humidity (10 to 100%), and 20% turbulence intensity. Except at low temperature and high relative humidity, all 50-mm-diameter and smaller droplets completely evaporated before depositing 0.5 m below the point of discharge for all simulated conditions. Drift distances increased with increasing wind velocity and discharge height, but decreased with increasing initial downward droplet velocity for 100-mm-diameter and larger water droplets. Changes in ambient temperature and relative humidity had much greater influence on drift distances of water droplets less than 100-mm-diameter than on 200-mm-diameter and larger droplets.

DROPLET SPECTRA AND WIND TUNNEL EVALUATION OF VENTURI AND PRE-ORIFICE NOZZLES

Small- to medium-size droplets are desirable when applying insecticides and fungicides because they provide better penetration into the canopy and better coverage than larger sizes. However, small droplets can drift long distances. Several agricultural nozzle manufacturers have recently introduced so-called “low-drift” nozzles. Although manufacturers of low-drift nozzles claim these nozzles are considerably more effective in reducing spray drift than standard flat-fan nozzles, no independent data are available to support their claim. The objective of this study was to determine the effectiveness of two low-drift nozzles (TurboDrop® and Turbo TeeJet®) in reducing drift. The TurboDrop nozzle design incorporates a venturi air intake port and a pre-orifice chamber while the Turbo TeeJet nozzle design includes only a pre-orifice chamber. Nozzle evaluations were accomplished by measuring droplet sizes with a laser particle sizer and deposition distances of droplets in a wind tunnel (5 m/s). Data from measurements obtained with the low-drift nozzles were compared to data from a standard flat-fan nozzle. The low-drift nozzles produced fewer drift prone droplets and significantly lower downwind airborne deposits than the standard flat-fan nozzle (XR). In general, droplet size measurements taken along the long axis of the spray patterns showed less variation in volume median diameters for the XR and Turbo TeeJet (TT) nozzles than for the TurboDrop (TD) nozzles. The TD nozzles produced lower downwind deposits than TT nozzles operated at similar pressures (276 kPa); however, a larger orifice TT nozzle operated at a lower pressure (176 kPa) produced significantly lower downwind airborne deposits than the TD nozzle operated at 276 kPa. Covering or restricting the venturi air intake port on the TD nozzle increased nozzle output but had little impact on the overall droplet spectrum and downwind drift.

Coverage and Drift Produced by Air Induction and Conventional Hydraulic Nozzles Used for Orchard Applications

A conventional, axial-flow, air-blast orchard sprayer was used to make applications to the outside row of a semi-dwarf apple block. Fluorescent tracer was applied at the same rate using either disc-core nozzle sets or air-induction nozzles fitted with flat-fan tips. The experiment included measuring the percent area of spray coverage on leaves after three variations in spray application method. Each of the variations used a different type of nozzle on the same conventional, axial-fan orchard sprayer. The three nozzle variations were a Spraying Systems D3-25 nozzle set, a Spraying Systems D4-25 nozzle set, and a TurboDrop 02 (TD02) air-induction nozzle set. Canopy spray deposits, downwind sedimentation, and airborne spray losses were also measured following treatment on the inside half of the outside row using D4-25 nozzles or TD02 nozzles. The small droplet spectrum D3-25 nozzle set produced the highest leaf surface coverage on both upperside and underside surfaces at 2.0 and 3.0 m heights in the canopy. The upperside leaf surface coverage produced by the D3-25 nozzle was only somewhat greater than the TD02 nozzle. It was, however, significantly higher than the D4-25 nozzle set at the 3.0 m height. Conversely, the underside leaf surface coverage produced by the D3-25 was significantly greater than the TD02 nozzle set at both 2.0 and 3.0 m heights and not statistically different from the D4-25 nozzle set at the lower sampling height. There were relatively few differences in canopy spray deposits between the D4-25 and TD02 nozzle sets. The TD02 treatment produced the lowest downwind sedimentation deposits on targets 8 to 32 m from the edge of the orchard. The D4-25 produced approximately three times higher deposits up to 9 m above the ground than the TD02 treatment on passive nylon screens located 8 m downwind from the edge of the orchard. The D4-25 treatment produced significantly higher airborne deposits on elevated, high-volume, air sampler filters out to 64 m. At 128 m, sedimentation and airborne deposits were similar for the D4-25 and TD02 treatments.

Computer Simulation of Variables that Influence Spray Drift

A computer program was used to determine the effects of several variables on drift distances of spray droplets. Variables were initial droplet size, velocity and height of discharge, wind velocity, turbulence intensity and relative humidity, and volatility of the liquid. For relative humidity and wind velocity ranges of 20-80% and 0.5-4.0 m/s, all water droplets 50 µm diameter and smaller that were directed downward with initial velocity of 20 m/s either completely evaporated or drifted farther than 6 m before depositing 0.5 m below point of discharge. With 20 m/s initial droplet velocity, 2.0 m/s wind velocity (20% turbulence intensity) and 60% relative humidity, 100 and 200 µm diameter droplets deposited at mean distances from point of discharge of 2.6 and 0.13 m respectively. Drift distances of water droplets as large as 200 µm diameter were influenced by initial droplet velocity and height of discharge. Experimental data verified the accuracy of the computer program in predicting drift distances of water droplets.

A History of Air-Blast Sprayer Development and Future Prospects

The design and operating procedures of air-blast sprayers have been greatly improved over the past 50 years. Early tree and vine spray application equipment used hand-guns that required a large amount of water. Later, sprayers with efficient fans, producing large volumes of air at high velocities, were developed for large fruit and nut trees. Recently, apple growers have planted many dwarf and semi-dwarf trees. In general, it is easier to produce more uniform coverage with less drift when spraying small trees than when spraying large trees. Modern designs such as tower, directed jet, and tunnel sprayers should reduce airborne spray drift and produce more uniform coverage. For optimum effectiveness, sprayer air-jet velocity, volume, and droplet spectra should be matched to the tree size, shape, and density. Besides optimizing delivery parameters, future sprayers will likely be required to handle biological materials with a greater variety of physical properties than the standard chemical materials used now. In addition, these materials will require spray systems that protect the live spray products from damage from heat, mechanical stress, and other factors that may kill the beneficial organism being applied.

Wind Tunnel Evaluation of a Computer Program to Model Spray Drift

A computer program (Fluent“) was evaluated for use in modeling the drift of spray droplets delivered by atomizers. A uniform-size droplet generator and wind tunnel were also used to determine drift distances for various droplet sizes, wind velocities and other operating conditions. Drift distances determined experimentally for droplet sizes ranging from 148 to 424 mm diameter and wind velocities ranging from 0.5 to 6.2 m/s agreed well with distances predicted by the computer program.

Fluorescent Intensity of Dye Solutions under Different pH Conditions

Fluorescent tracers are widely used for assessment of spray quantity in the field due to their relatively high sensitivity, low cost and user safety. However, many concerns have been raised over their measurement accuracy due to questions of stability of fluorescence during tests. Stable analysis of fluorescence is essential to ensure accurate evaluation of pesticide spray application efficiency. The objective of this research was to determine the stability of fluorescent intensity of five tracers dissolved in solutions with various pH conditions in an effort to minimize analytical errors in the measurement of spray deposition and drift. The fluorescent intensity of five fluorescent tracers commonly used for the quantitative assessment of spray deposition and off-target loss was investigated with wash solutions over pH conditions from 6.9-10.4. The tracers selected in the tests were Brilliant Sulfaflavine (BSF), Fluorescein, Pyranine, Tinopal, and Eosin. The fluorescence of Pyranine was the most sensitive to the solution pH conditions, followed by Fluorescein and Tinopal, while BSF and Eosin had a nearly constant fluorescent intensity over the pH range from 6.9-10.4. The fluorescence of Fluorescein increased 1.3 times, Tinopal 1.25 times, and Pyranine 3.0 times as the pH value increased from 6.9- 8.4, but it became nearly constant when pH value was greater than 8.4. However, Pyranine, Fluorescein, and Tinopal showed much stronger fluorescence than BSF and Eosin. A solution containing Fluorescein at pH 8.4 and higher demonstrated 83 times greater fluorescent intensity than the solution containing the same amount of BSF. In conclusion, the fluorescence of tracers should be examined under various pH conditions during the selection of tracers for pesticide spray deposition and drift trials.

Downwind Residues from Spraying a Semi-dwarf Apple Orchard

Airborne spray and ground deposits were measured downwind from a row of semi-dwarf apple trees sprayed with an orchard air sprayer. Airborne spray collectors used were: string, plastic tape, bottles, and high-volume air-sampler filters; collector locations were at 7.5 to 240 m from the tree row, 0.5- to 10-m elevation. Ground deposits were collected on lines perpendicular to the tree row and at distances from 3 to 240 m downwind from the tree row. A mixture of water and a fluorescent tracer was sprayed on the trees at 468 L/ha during a total of 16 passes on four days and climatological data were recorded.

Turbulent Jet Theory Applied to Air Sprayers

ABSTRAC THE air sprayer is represented as a fan jet by means of turbulent jet theory. Equations describing air velocities at any point in the air sprayer jet are given as a function of outlet velocity and sprayer outlet configura-tion. Both lateral and axial air velocity profiles were measured in air jets produced by two orchard air spray-ers. These measured velocities agreed with values calcu-lated from equations developed from ideal turbulent fan jet theory. The equations given can be used to design a sprayer with an optimum combination of outlet width and exit velocity.

Downwind residue from air spraying of a dwarf apple orchard.

ABSTRACT The edge row of dwarf apple trees was sprayed with an air sprayer; fluorescent dye was used to trace spray drift deposits. Tracer deposited on the ground was measured with plastic collectors and airborne spray was captured and measured with string, bottle, and high-volume, air-sampling filter collectors. Microclimatic variables including vertical heat flux were measured. Ground deposit decreased greatly beyond 120 m; about 0.03% of total material sprayed was deposited between 122 and 152 m downwind. Airborne spray between the ground and a 20 m level at 122 m downwind was estimated to be about 3.5%. Ground collectors and an unobstructed array of string collectors located 5.0 m from the sprayer captured about 75% of the total material sprayed.