College of Aeronautics Report 8223 August 1982
TECHNISCHE HOGESCHOOL DELFT
LUCHTVAART- EN RUIMTEVAARTTECHNIEK
BIBLIOTHEEK Kluyverweg 1 - DELFT
Design and Development of a Disc-Windmill Atomiser for Aerial Applications
2 9 JULl 1983 by J.J. Spillman
College of Aeronautics Cranfield Institute of Technology
August 1982
Design and Development of a Disc-Windmill Atomiser
for Aerial Applications
by J.J. Spillman
College of Aeronautics Cranfield Institute of Technology
Cranfield, Bedford, UK
ISBN 0 902937 78 2 £7.50
"The views expressed herein are those of the authors alone and do not necessarily represent those of the Institute. "
DESIGN AND DEVELOPMENT OF A DISC-WINDMILL ATOMISER FOR AERIAL APPLICATIONS.
J. Spillman and R. Sanderson
INTRODUCTION
To improve the efficiency of aerial applications of pesticides it is essential to get a greater fraction of the emitted formulation to the true biological target. To do this and at the same time ensure that environmental pollution is a minimum requires a careful choice of droplet size
spectrum as indicated by Himel and Moore (196 7) and Spillman (1979) (1980). The best atomisers currently available commercially for aircraft use produce at best only about 40% of their emission in sizes within 25% of the desired size and in many cases the percentage is less than half this value. A wide range of droplet sizes in an emission makes it impossible to achieve the precision
placement sprays required for example in herbicide applications and in the treatment of oil slicks, whilst a large fraction of the emission is wasted when small droplets are required.
Research at Cranfield sponsored by the British Science and Engineering Research Council has been directed at
designing an atomiser to emit most of its output within 25% of a specified size at flow rates suitable for aerial applications. The design concepts and initial results of this work were first discussed at the Second International Conference on Liquid Atomisation and Spray Systems by
Spillman and Sanderson (1982). This note reports some of the results obtained more recently.
PRINCIPLE OF THE DISC-WINDMILL ATOMISER
In order to obtain uniformity in droplet size
it is essential to accelerate the fluid to form a very thin, uniform sheet and thus to control carefully the break-up
of this sheet into filaments and then into droplets. Whilst direct droplet formation would give less variation in droplet size the flow rates attainable would be unacceptably low. Spinning disc atomisers utilise these principles well,
especially those which use radial grooves to minimise liquid slip during the acceleration phase and peripheral teeth to regulate the atomisation process. However, their maximum flow rates, even in the filament mode, are insufficient for aerial operations. Attempts have been made to gang together groups of discs but their performances have been disappointing
because of the difficulties of getting an even
feed rate to each disc and an even airflow at their peripheries.
The limiting factor on flow rates in the filament mode is the low velocity of the fluid in the filaments because
of the effects of surface tension at the periphery. Attempts to increase this speed by increasing the rotational speed
and hence the centrigugal force qn the fluid simply cause the whole fluid sheet to extend beyond the periphery and then break-up randomly as in a hydraulic nozzle, resulting in the same wide range of droplet sizes. In the disc-windmill
atomiser the airflow approaching the disc at the speed of the aircraft is turned by the disc to give a very high radial speed at its periphery as illustrated in figure 1. The friction of this intense radial flow on the liquid filaments helps draw them off the perimeter of the disc.dramatically increasing their radial velocities.
Experiments have shown that this e ffeet can increase the maximum flow rate of a disc operating in filament mode
by almost an order of magnitude when operating in a 45m/sec. (100m.p.h.) wind. Even at speeds half this value the flow rates are significantly increased.
3
-The airflow can also be used to rotate the disc rather than rely on electrical, pneumatic or hydraulic power, so simplifying the whole atomiser. The disc can be converted into a windmill with 100% solidity by making a series of radial cuts from the perimeter to about half radius and then twisting each of the segments to form a blade. Changing a disc in this manner does not affect its performance as an atomiser but turns it into a very
effective, although inefficient, windmill. Rotational speeds of over 26,000r.p.m. have been measured for a disc-windmill in an airflow of 45m/sec. at zero fluid flow rate; normal operating conditions are between one third and one half of
this rotational speed.
Figure 2 shows schematically the extremely simple form of the disc-windmill atomiser. The bearing is shielded from the formulation and all orifices are at least 1mm. in diameter except for the flow restrictor. Figure 3 shows one of the 50mm. diameter prototype atomisers being tested in a wind
tunnel whilst figure 4 shows one assembled with a standard cut-off valve.
TEST RESULTS
A range of prototype disc-windmill atomisers with different blade settings have been tested in wind speeds between 40 and 100 knots. Spark photography was used to check that the emission from each tooth was as uniform as
possible and that the radial cuts did not change the atomisation from the periphery. No evidence of fluid leaving the trailing edge of the blades was found when radial grooving was
used. Figure 5 shows the filaments and droplets leaving the atomiser frozen by the illuminating flash. The apparent variation in drop size at the top of the picture is
points the smaller satellite droplets are being caught by the f aster movirxg larger droplets. The lighting on the
filaments from the botton blade shows the waves forming
in them prior to break up. The increasing distance between highlights shows how the fluid is being accelerated,
stretching it out towards the break-up points.
Figure 6 shows that the volume median diameter of the emitted spray can be expressed simply as
Volume Median Diameter co ^ 448,000 Q , .
= bo + — ^ — J — (i)
of Emission (microus) y2 Q/
where
Q is the flow rate in litres per minute per disc V is the flight speed in knots and
e is the blade setting at its tip to the plane of rotation measured in degrees.
With only three exceptions in forty-eight results, this expression is accurate to within 10pm. for all the test
results which cover flow rates varying from 0.15 to 1.1 litres per minute, flight speeds from 40 to 100 knots and blade
angles from 8.5° to 22°.
The reason for this linear relationship with the
parameter —r~r is not wholly understood. Arguments relating
VJQT
power input and output as well as dimensional analysis indicate the suitability of the powers of the flow rate, speed and
blade angle terms but to date a satisfactory overall
explanation has not been found. Other parameters such as the number median diameter of the emission and rotational
speed do not collapse to a single line when plotted against the same parameter although there is some suggestion
5
-When the ratio of voliime median diameter to number medium diameter is plotted against the parameter Jf
|-vz eT as in figure 7, it is clear that over the range
0.8xlO~ < — 7 — r < 3.3xlO~ the ratio is less than v^ e?
2.0, indicating a satisfactory narrow size spectrum. The ratios obtained under similar test conditions for hydraulic nozzles and spinning cages have been more than twice this value.
Equation (1) shows that the volume median diameter of the spray can be obtained at a given flow rate and flight speed by choosing the correct blade setting, within the
range 5° to 22°. The lower limit has been suggested because of the sensitivity of the results to blade setting at the small values. The upper limit is the upper value of the test results, and may possibly be higher.
As an excimple consider an aircraft flying at 45m/sec. (lOOmph) and drift spraying a fungicide over fruit trees. If a volume median diameter of 140ym. and a number median diameter of 80pm. is required then the value of ^ i
_4 V2 et would be 1.9x10 and, for a blade setting of
22°, each disc windmill would emit 0.73i/min. With 40 such atomisers and a lane separation of 30 metres an application rate of 3.6£/ha could be achieved. Assuming a leaf area to ground area ratio, counting both leaf sides of 5 : 1 then
2
this corresponds to 27 drops/cm on average, if the drops are assumed to be 80pm. diameter. If a smaller coverage was required the blade setting could be reduced, keeping —^^
6 ^ constant, and hence the drop spectrum constant.
The results shown in figures 6 and 7 are for water
plus a small eunount of surfactcint. Clearly the results will
be different for significantly different formulations. However, tests with butyl dioxitol and water with various additives
have shown very little difference from the basic results. Because there is no orifice smaller than one millimeter in diameter in the device, except for the flow regulator, so blockages, even with wettable powder formulations should be minimal.
It is planned to make sets of disc-atomisers specific to particlar applications. Since they are designed to screw into a standard spray boom it is envisaged that an operator will have a set of booms with suitable disc-windmill
atomisersfor each type of application and will simply wash out and change booms between different kinds of application. Field trials of this system should start shortly.
REFERENCES
Spillman J.J. (1980) "The efficiency of aerial spraying." The Aeronautical Journal. Royal
Aeronautical Society 84 (830) 60-69
Himel C M . Moore A.D.
(1967) Spray Droplet Size in the control of Spruce Budworm, Boll weevil, Bollworm and Cabbage Looper.
J.Econ.Entomology,62(4) 916-918.
Spillman J.J.
Spillman, J.J. Sanderson R.
(1979) 'Optimum Droplet Sizes for Spraying against Flying Insects.'
Agricultural Aviation iJ (164),October. (1982) 'A Disc-Windmill Atomiser for the
Aerial Application of Pesticides.'
Proceedings of the Second International Conference on Liquid Atomisation and Spray Systems.
Fig. 3 Prototype atomiser under test in a windtunnel
Fig. 4 Prototype atomiser with a grooved and serrated disc
FILAMENT AND DROPLET
300 VOLUME 250 MEDIAN 200 DIAMETER 150 MICRONS 100 50
-NOTE:- HALF THE EMITTED VOLUME IS IN SIZES SMALLER THAN THE VOLUME MEDIAN DIAMETER, HALF 15 IN GREATER SIZES.
1 y y Q v3^2 . y QVZ^IO"^ . V^<^y " . „_ „„ i , y . ^ ^ ' • ' ^ y ^ ^ .^^ KEY Q 40 KNOTS 22° ANGLE + 60 KNOTS 22° ANGLE ^ 80 KNOTS 22° ANGLE y 90 KNOTS 22° ANGLE 0 90 KNOTS 15 ° ANGLE V 90 KNOTS 8-5°ANGLE • 100 KNOTS 22° ANGLE 1 1 i O 10 2 0 3 0 4 0 5 0
VARIATION OF VOLUME MEDIAN DIAMETER WITH FLOW RATE, Q i/min,
AIRSPEED. V KNOTS AND BLADE ANGLE. 0°.
2 5
V.M.D N.M.D
2 0
