  The effect of viscoelasticity on sprays produced from agricultural flat fan
nozzles is investigated experimentally using dilute aqueous solutions of
polyethylene oxide (PEO). Measurements of the droplet size distribution using
laser diffraction reveal that polymer addition to water results in the
formation of overall bigger droplets with a broader size distribution. The
median droplet size $D_{50}$ is found to increase linearly with the extensional
relaxation time of the liquid. The non-dimensional median droplet sizes of
different polymer solutions, sprayed at different operating pressures from
nozzles of different sizes, rescale on a single master curve when plotted
against an empirical function of the Weber and Deborah numbers. Using
high-speed photography of the spraying process, we show that the increase in
droplet size with viscoelasticity can be partly attributed to an increase of
the wavelength of the flapping motion responsible for the sheet breakup. We
also show that droplet size distributions, rescaled by the average drop size,
are well described by a compound gamma distribution with parameters $n$ and $m$
encoding for the ligament corrugation and the width of the ligament size
distribution, respectively. These parameters are found to saturate to values
$n=4$ and $m=4$ at high polymer concentrations.
