Atomization from a liquid sheet is a critical process particularly in the context of its
application in liquid-fuelled propulsion systems. The acoustic field in the combustors
can affect atomization process by modifying the surface waves on liquid sheet and the
droplets leading to time-varying fuel delivery resulting in heat release oscillations to
form a closed-loop feedback cycle triggering combustion instability The objective of
the present work is to investigate the dynamics of primary atomization of a hollow cone
spray from a pressure swirl nozzle in the presence and absence of strong swirling flow
when subjected to axial acoustic excitation. This is inferred by comparing the unexcited
spray with the excited cases with and without swirl flow. The effect of acoustic pressure
and acoustic velocity on the dynamics of primary atomization of the hollow cone spray
in strong swirling flow field is also investigated by comparing the spray positioned at
an acoustic velocity node, antinode, and a mixed point locations in the standing wave
field with and without swirl flow. Results from high-speed back-light imaging acquired
in-sync with dynamic pressure measurements are processed to clarify the unstable
behaviour observed in the spray dynamics; this was achieved by extracting key
parameters such as breakup length, spatial growth rates, phase differences and by
employing Proper Orthogonal Decomposition (POD). A novel method to obtain the
breakup length of a hollow cone spray from the position of maximum wave amplitude
is presented. The breakup length is the smallest for the mixed point. The phase

v

difference between the left and right half-angle fluctuations shows that the flapping
motion of the spray is predominantly observed at the mixed point for different air to
liquid ratios. Another novel approach is adopted to identify the physical mechanisms
corresponding to each POD spatial mode by comparing POD spatial modes obtained
from experiments to those generated artificially.