BLADE ANGLE EFFECT ON DROPLET SIZE SPECTRUM OF ROTARY ATOMIZERS USED IN BRAZIL

_________________________ 2 Universidade Estadual Paulista, Faculdade de Ciências Agronômicas (UNESP/FCA)/ Botucatu SP, Brazil. 3 AgroEfetiva/ Botucatu SP, Brazil. 4 University of Nebraska-Lincoln, West Central Research and Extension Center, North Platte/ United States of America. Received in: 4-8-2016 Accepted in: 6-21-2016 Eng. Agríc., Jaboticabal, v.36, n.6, p.1118-1125, nov./dez. 2016 BLADE ANGLE EFFECT ON DROPLET SIZE SPECTRUM OF ROTARY ATOMIZERS USED IN BRAZIL


INTRODUCTION
Brazil has the second largest fleet of agricultural aircraft in the world, and rotary atomizer usage for aerial pesticides applications is a very popular practice (BAYER et al., 2011;ANTUNIASSI, 2015).These centrifugal-energy nozzles allow control the droplet size according to its rotational speed, and it is the main factor in influencing the droplet size spectrum (TESKE et al., 2005).
In general, cage or discs rotary atomizers have been used to spray pesticides.The cage type atomizer is constituted of a cylindrical monel metal wire gauze rotating around a fixed hollow spindle (HUANG et al., 2014).The spray solution is released through the hollow spindle and impacts on the cage, resulting in the droplets generation.Thereafter, these droplets also have impact with the air, which also affects the droplets size (TESKE et al., 2005).Cages also can be manufactured with laser designed metal gauze instead of wires.The other type of atomizer is composed of discs with grooves and teeth, by where the liquid is released into the air, also fixed around a hollow spindle (CRAIG et al., 2014).
Blades are used to drive the cage or discs around the spindle.It can be adjusted to select the rotational speed and, consequently, the droplet size.The faster rotation is, the smaller droplets will be produced (TESKE et al., 2005;CRAIG et al., 2014;HUANG et al., 2014).
Droplet size and spray droplet size spectrum are the main factors affecting pesticides efficacy and spray drift.Despite having potential to control insects, diseases and weeds, small droplets are more prone to drift downwind from the application area than larger droplets (ANTUNIASSI et al., 2011;CHECHETTO et al., 2013;GANDOLFO et al., 2014).
The droplet size also affects deposition distance downwind from the sprayed area (CRAIG et al., 2014;FERGUSON et al., 2014).Others parameters that have impact on spray drift are the tank mix, operating pressure, swath width, flight height and weather conditions (HUANG et al., 2012;HILZ & VERMEER, 2013;FERGUSON et al., 2015).
Examples of aerial spray drift prediction models about drift potential and downwind distance are the AgDRIFT and AgDISP (TESKE et al., 2005;HOFFMANN et al., 2013).These models were created using a database of studies conducted mainly in the USA and Australia (Spray Drift Task Force Database).They promote the adoption of practices to help reduce spray drift during the spray applications of pesticides, increasing environmental protection (FRITZ et al., 2012a).Despite this scenario, there is lack of information about droplet size spectrum of rotary atomizers that have been used in Brazil (BAYER et al., 2011).Differences in diseases control or spray drift reported in the literature (ANTUNIASSI et al., 2011) may be a consequence of differences between atomizers types and brands.
According to these concerns, the aim of the present study was to characterize the droplet size spectrum of several models of rotary atomizers used in Brazil according to the blade angle, in a high speed wind tunnel.

MATERIAL AND METHODS
The droplet size spectrum of six rotary atomizers used to spray applications in Brazil was evaluated in a high speed wind tunnel.Five of the atomizers were made by national manufacturers and one of them (ASC-A10) by an international manufacture from the USA.Table 1 gives us details about the treatments abbreviations, atomizer model, blade number, manufacturer, type (disc or cage), as well as the materials used to construct the cages or discs.All atomizers were set up to apply 6 L min -1 .The spray solution was composed of water plus the fungicide Aproach ® Prima (SC, DuPont™), at 20 mL c.p. L -1 (Picoxystrobin, 9 g a.i L -1 + Cyproconazole, 3.6 g a.i.L -1 ).It was selected this pesticide to adequate the physical-chemical properties of the spray solution according to usual spray applications conducted in Brazil, where the use of fungicides with suspension concentrate formulations are common.There were three repetitions per treatment for all evaluated parameters.
The droplet size measurements were conducted in a high speed wind tunnel, based in a methodology described by FRITZ et al. (2012a).The airspeed of 180 km h -1 (112.5 mph) was used for all treatments.It represents the flight speed of the Ipanema airplane, a Brazilian-made aircraft manufactured by Embraer (ANTUNIASSI, 2015).
The study was conducted in the Pesticide Application Technology Laboratory, at the West Central Research and Extension Center of the University of Nebraska-Lincoln in North Platte, Nebraska, USA.The laboratory presented controlled temperature of 20º C (±2º C) and relative humidity of 60-70%.
The facility contains an opened circuit wind tunnel, with an outlet section of 0.3 m x 0.3 m, capable of producing wind speed up to 300 km h -1 .The wind tunnel contains a transparent working section (1.2 m height, 1.2 m width and 6 m length) in which a mounted spray boom section was used as a supporter to the atomizers.An exhaust air scrubber is used to recover the sprayed pesticides.The wind tunnel is also equipped with a flow straightener to produce laminar air flow through the tunnel working section.
A Sympatec HELOS laser diffraction particle-sizing system (Sympatec Inc., Clausthal, Germany) was used to measure the droplet size spectra downwind of the atomizers, and each replication comprised of operating the atomizers for 10 seconds.It was equipped with a 623 nm He-Ne laser and was fitted with a lens (denoted by the manufacturer by R6) able to analyze a range of droplets between 0.5 µm and 1750 µm.The HELOS laser system has an emitter and receiver, which was positioned across from each other and outside of the wind tunnel.
The atomizers were positioned in such way that the center of the spray cloud is crossing the laser beam (TESKE et al., 2001).This configuration was selected because according to these authors, the droplet size changes according to the radial distances from the hub, with the larger droplets going further from the hub and the small droplets keeping closer during the spray applications.The distance from the atomizer to the laser beam was 85 cm, also respecting the parameters proposed by those authors.It was also observed that there were no droplets touching the ceiling, walls or floor of the wind tunnel before crossing the laser.
Three blade angles of 45, 60 and 75 degrees were selected, resulting in different rotational speed (rpm, revolutions per minute).A wide range of other blade angles are available to rotary atomizers, and are usually used from 35 to 80 degrees (HOOPER & SPURGIN, 1995).The blade angles were adjusted using a protractor and the rotational speed was measured using a laser photo tachometer (Cybertech, DT2234A), while spraying the solution (wet rotation).
The parameters of interest were DV10 (µm, 10% of the spray volume consists of droplets with a diameter smaller than the value of DV10); VMD (µm, volume median diameter, 50% of the spray volume consists of droplets with a diameter smaller than the value of VMD); DV90 (µm, 90% of the spray volume consists of droplets with a diameter smaller than the value of DV90); %<100 µm (percentage of volume of droplets having a diameter smaller than 100 µm); RS (relative span, this is the difference in diameter for 90% and 10% of the spray by volume divided by the VMD) (FERGUSON et al., 2014;AL HEIDARY et al., 2014).
Relative span is a dimensionless parameter indicative of the uniformity of the droplet size distribution, whereas smaller values indicate narrower spectrum (HEWITT, 2007), and %<100 µm has a positive correlation with the potential risk of spray drift (OLIVEIRA et al. 2015).
The spray category classification values were based on data generated using the guidelines established in ASAE S572.1 "Spray Nozzle Classification by Droplet size spectrum" (ASAE, 2009;FRITZ et al., 2012b).Data generated in this study were analyzed using SAS Enterprise Guide (SAS, Cary, NC, USA).A modified PROC MIXED command was used with replication set as a random factor in analysis.A Tukey's means separation procedure was used to determine statistical significance with α=0.05.

RESULTS AND DISCUSSION
It was observed that increasing the blade angle of the rotary atomizers resulted in slower rotational speed.Although the atomizers were evaluated at the same angles, there were observed differences in the rotational speed between them.It may be resultant of differences such as number of blades, dimension of cage or discs, and material.These results are in accordance with what was described by TESKE et al. (2005).
It was observed that at 45 degrees (Table 2) the ASC atomizer showed the highest value of VMD, 141 µm, but the value of % < 100 µm, 31%, was statistically similar to the % < 100 µm observed to the CBB atomizer, 32%.At the same angle, the Stol and Microbell atomizers resulted in the lowest values of VMD, 65 µm and 68 µm, respectively, and the highest values of % < 100 µm, 71% and 70%, respectively.The other atomizers showed VMD between 106 µm and 123 µm, and % < 100 between 32% and 47%.Droplets with diameter smaller than 100 µm have low sedimentation velocity, about 25 cm s -1 (CRAIG et al., 2014).The authors inform that these droplets, despite having potential against pests, are more vulnerable to evaporation, off target movement, and loss to the environment.Droplets bigger than 300 µm have high sedimentation velocity, but they are considered too big for the applications of fungicides and insecticides, because of unreasonable coverage on leaves, or are inappropriate to control insects.
With the exception of the CBB atomizer, that showed a relative span of 1.12, all the atomizers had RS above 1.39 at 45 degrees.The RS of the Microspin was the highest, 1.95.HOOPER & SPURGIN (1995) observed values of RS to the AU5000 atomizer at a blade angle of 50 degrees, flow rate of 5 L min -1 , wind speed of 180 km h -1 and rotational speed of 5410 rpm, equal to 1.92.At the same conditions, the authors observed that the VMD had changed in between 78 and 92 µm when the flow rate was increased from 3.5 L min -1 to 5 L min -1 , respectively, but the observed rotational speed decreased from 5510 rpm to 5410 rpm.It was also observed by TESKE et al. (2005) that the flow rate itself did not have a large effect on atomization, while rotational speed and tank mix had a large effect on droplet size.
The lower value of RS observed in the disc atomizer is explained by the process of droplet formation.For discs atomizers it is created by regular ligaments, and there is no impact of the droplet with the cage, resulting in more uniform droplet size, compared to cage atomizers (CRAIG et al., 2014).
The process of droplet formation for cage atomizers is based on the release of droplets through the hollow spindle, that will be broken up by the rotational cage, before having impact with the air (CRAIG et al., 2014).According to the same authors, the disc atomizer has a different mechanism of droplet formation; the liquid is just released through the teeth into the air, in a process known as "ligaments", and does not have impact with the cage.
At 60 degrees (Table 3) the Microspin atomizer showed the highest value of VMD, but not the lowest value of small droplets (%<100 µm).It is explained by the high value of RS, 1.51.The CBB atomizer showed VMD of 174.41 µm, and the lowest values of % < 100 µm and RS, 16.8% at 1.17, respectively.The other atomizers resulted in values of RS above 1.39.At 60 degrees the Stol atomizer showed the lowest value of VMD, 103 µm, and the highest values of % < 100 µm, 48%.The atomizer Microspin produced the highest value of VMD, 198 µm, and the lowest % < 100 µm, 20%.The VMD of the CBB atomizer was 174 µm, and it resulted in the lowest value of % < 100 µm, 17% and RS, 1.17.
The only change of droplet size category from fine to medium was observed at the angle of 75 degrees, to the Microspin atomizer (Table 4).This atomizer produced VMD of 257 µm, differing statistically from the other atomizers.Higher air speeds increase the rotational speed and cause more air shear across the atomizer, which can produce finer droplets, although the relative liquid to air velocity is an important consideration (TESKE, et al., 2005).Once the rotational speed is provided to the pilot by fitting an electronic application monitor on the airplane (HUANG et al., 2014), it can be used as a reference of droplet size to the evaluated atomizers.
This study provides results that help to understand the characteristics of aerial applications conducted in Brazil.These results may be considered as a reference before buying atomizers, and also help to plan strategies to reduce spray drift potential, as well as to increase the efficacy of the applications.

CONCLUSIONS
The blade angle of 75 degrees is recommended to decrease drift potential during spray applications of pesticides in Brazil compared with the blade angles of 45 or 60 degrees.The angle is not the only parameter to be considered, since atomizer model and brand influence the droplet size spectrum.

TABLE 1 .
Treatment abbreviation, atomizer model, blade number, manufacturer, atomizer type and cage or disc materials.

TABLE 2 .
Treatments, atomizer rotational speed (rpm), means of droplet size statistics, percent of droplets smaller than 100 µm, relative span and droplet size spectrum classification, at the blade angle of 45 degrees.The flow rate and wind speed were 6 L min -1 and 180 km h -1 , respectively.Category classification values are based on data generated using the guidelines established in ASAE S572.1.Means with the same letters in the column do not differ by Tukey's test at α = 0.05 level. *

TABLE 3 .
Treatments, atomizer rotational speed (rpm), means of droplet size statistics, percent of droplets smaller than 100 µm, relative span and droplet size spectrum classification, at the blade angle of 60 degrees.The flow rate and wind speed were 6 L min -1 and 180 km h -1 , respectively.Category classification values are based on data generated using the guidelines established in ASAE S572.1.Means with the same letters in the column do not differ by Tukey's test at α = 0.05 level. *

TABLE 4 .
Treatments, atomizer rotational speed (rpm), means of droplet size statistics, percent of droplets smaller than 100 µm, relative span and droplet size spectrum classification, at the blade angle of 75 degrees.The flow rate and wind speed were 6 L min -1 and 180 km h -1 , respectively.