Abstract:
Handheld electrically driven spinning disc atomisers are
capable of applying pesticides more efficiently than with
hydraulic nozzles, because the narrower droplet spectra they
produce leads to reduced drift and wastage of chemical.
Despite these savings however, farmers are frequently
reluctant to use such methods because of poor reliability,
and high maintenance costs of the electrical drive systems.
This research has therefore examined an alternative drive
system using a jet of the pesticide fluid to power the
atomiser. Experiments have been carried out to increase
understanding of the processes involved, with the aim of
producing an optimum design suitable for mounting to a
knapsack sprayer.
The fluid drive mechanism is comparable to that of a Pelton
Wheel; driving torque arises from a change in momentum of a
fluid jet as it strikes the inside of a cup causing it to
rotate. The fluid is emitted from grooves and teeth to form
ligaments which produce uniform droplets. Peripheral
distribution of fluid is made uniform by a series of slotted
weirs on the inside wall of the cup.
Form and size of various components including the nozzle,
bearing, cup and atomising disc have been investigated. The
requirement for low flowrate necessitates the use of a small
nozzle with filter. Supply pressure available from hand-
pressurised knapsack sprayers is also restricted. Available
input energy is therefore limited, requiring that all the
processes leading up to atomisation are as efficient as
possible.
Rotational speed is a function of jet velocity, mass
flowrate, inner and outer cup radius, and bearing frictional
torque. A simple computer model has been developed to
predict the effects of changing these parameters. Radial
exit velocity of the fluid has been derived from a
consideration of the viscous forces to which thin fluid
layers are subject during centrifugal acceleration upon
rotating surfaces. This has enabled undisturbed ligament
radius to be calculated, and has therefore provided a
satisfactory method for the prediction of dropsize.
Spectral and deposit characteristics are satisfactory for
CDA herbicide application with water based formulations,
although antifoam agent is required for formulations
containing surfactant. Typical average droplet size for an
operating pressure of 3 bar is 250μm, and optimum swath
width is 1.2m for an intended application rate of 30 to 40
litres per hectare.
