Characterizing turbulent spray deposition from self-propelled sprayers

Key Result

The uniformity of spray deposits from modern, self-propelled sprayers was lower than predicted. It was found that the “best” configuration of low boom height, slow travel speed and coarser spray was only somewhat more uniform compared to the alternative of higher boom height, faster travel speed and finer spray.

Project Summary

Overview

The uniformity of a spray deposit is fundamental to a successful spray application. Spray dosage is directly related to pesticide performance. The timely, accurate and efficient application of pesticides is an important activity in modern crop production methods. Increased productivity continues to be demanded by the market; however, minimizing the environmental impact of pesticide application due to spray drift remains paramount to sustainable and responsible agricultural activities. Understanding the factors that affect the behavior of spray droplets upon release from a high-clearance sprayer is a critical building block to reducing the drift of pesticide.

Objectives

The objectives of this study were to document the spray deposit distribution of a high-clearance spray boom under a variety of test conditions with a view to identifying means of improving deposition uniformity. Initially, the intent was to test many configurations and sprayers to define the ideal way to minimize deposit variability. After reviewing data from initial trials, it was apparent that the variability of deposits across the boom width was greater than expected and that changes in sprayer configuration rarely translated into different deposit patterns. The emphasis on this project became to try to understand the basic components of the deposit profile with a view to identify sprayer design features that may contribute to the observed variability.

Figure 1. A number of strings arranged in the direction of sprayer travel.

Experimental details

This study by Dr. Tom Wolf was conducted in several parts. First, deposition data from historical drift trials using petri plates were analyzed to identify trends with application method. Then, the effect of boom stability on deposition patternation was tested for a number of travel speeds and boom heights. Next, the effect of spray quality, boom height, and travel speed on boom-wide spray deposits was quantified across and along the direction of travel using a 2 mm diameter monofilament string (Figure 1). This type of collector is internationally recognized as a suitable collector of agricultural sprays, but it does preferentially collect smaller droplets and utilizes a comparatively small sampling volume. As such, it was considered a useful simulation for smaller targets such as cotyledon stage weeds. Lastly, the aerodynamic turbulence caused by a sprayer was evaluated using computational fluid dynamics (CFD).

The study that looked at the effect of boom height and travel speed on deposition was conducted with a Rogator RG1100B equipped with a 36m boom fitted with Greenleaf AirMix 11004 nozzles. Nozzles were spaced at 20” and operated at 40psi, producing a coarse spray quality. Variables were four boom heights (8”, 16”, 32” and 64” above target) and three travel speeds (8, 14 and 20 mph). The study that looked at spray quality, boom height, travel speed and sprayer model was conducted over three years at four locations with four different sprayers and configurations. Trial one in 2017 looked at the impact of two spray qualities (coarse and extremely coarse), travel speeds of 15 and 7.5 mph, boom heights of 25 and 35” on spray deposition uniformity across the entire 36 m boom width of a John Deere R4045 sprayer fitted with 380/105 R50 tires. The second trial, in 2019, was conducted with a John Deere R4045 with a 120’ boom fitted with 380/105 R50 tires. Travel speeds were 18 and 7.3 mph, boom height was 24 and 40” and a medium spray quality was used. The third trial in 2019 as well was conducted with a John Deere 4830 with a 100’ boom. Six sprayer configurations were looked at, consisting of two boom heights of 28 and 45” each at three travel speeds of 8.9, 13.4 and 20 mph. The fourth trial conducted in 2020, was done with a John Deere 4830 with a 100’ boom fitted with 320/90/R50 tires and row dividers on all four wheels. Two treatments were looked at- high booms (35”) and fast travel speeds (18 mph) and low boom (20”) and slow travel speed (9 mph).

Results

Deposition was less uniform across the width of the boom as it was along the direction of travel. Higher wind speeds increased variability, as did a combination of higher booms and faster travel speeds. Finer sprays also tended to deposit less uniformly. Deposit patterns, and the magnitude of the associated variability, were only moderately repeatable, but some similar trends were apparent. The first was the downwind displacement of the edges of the spray swath. The second was the overall lower deposition in the wake of the sprayer wheel tracks. The final common observation was the greater variability of the deposit in the centre of the spray boom, behind the tractor unit, than on either of the left or right spray boom wings.

Computational fluid dynamics studies showed that disturbances in the flow field were increased with travel speed and boom height. In computer modelled data, both upward and lateral components were increased similarly, increasing the potential for spray droplets to be directed off target. Tire width had an impact on flow field disturbances. Not only was greater turbulence observed in the wake of the wider tires, the width of the tire-induced wake was several tire widths more than the width of the tractor unit.

This study demonstrated that the uniformity of spray deposits from modern self-propelled sprayers is lower than predicted in existing lab and field studies. The observed variability, as measured by CV (Coefficient of Variation), exceeded 20% in nearly all cases and reached as high as 50% despite the use of nozzles with little wear, proper boom height, uniform driving speed and smooth, level terrain. It was surprising that the CV of the sprays was not only high, but also stubbornly so. Efforts to reduce the variability using lower booms, lower speeds, and coarser sprays was met with some success. However, even the “best” configuration (low boom, slow travel speed, and coarser spray) was only somewhat more uniform compared to the alternative (high boom, fast speed, and finer sprays). Calmer conditions may, in hindsight have lowered the CVs, but side winds were used as they are recommended for spraying. The alternative, headwinds, create dramatically different aerodynamic environments depending on whether one heads into them or drives with them. Although the overall effect of slower speeds and lower booms were not as large as hoped, they nonetheless represent the single best tool available to applicators at this time.

Conclusions

Researchers wanted to look at the spray deposit distribution of a high clearance spray boom to try to improve deposition uniformity. A variety of variables were tested- boom height, spray quality, travel speed and sprayer model. They found that the uniformity of spray deposits from modern, self-propelled sprayers was lower than predicted in existing lab and field studies. It was found that the “best” configuration of low boom height, slow travel speed and coarser spray was only somewhat more uniform compared to the alternative of higher boom height, faster travel speed and finer spray. The effect of lower boom heights and slower speeds was not as significant as hoped, but it remains the single best tool to farmers and applicators.