KEYWORDS: coaxial atomizer; high-speed imaging; breakup length; frequency;
flapping; nozzle geometry; swirling air; volumetric mean diameter;
liquid mass flux.
This research work aims to investigate liquid jet instabilities and spray unsteadiness in

liquid-centered coaxial airblast atomizers, which find wide range of engineering appli-
cations including liquid-propelled rocket engines, coal gasification, chemical and metal

spray, industrial burners, etc. The objectives of the present study are to understand the
influence of annular air swirl and nozzle exit geometry on the liquid jet instabilities,
particularly on the large-scale instabilities which cause strong structural modifications
of the liquid jet, and, furthermore to study its consequence on the downstream spray
fluctuations.
Experiments were conducted using coaxial airblast atomizers for a wide range of
injector operating conditions by varying inlet flow rates of air and liquid (water in the
present case) under ambient atmospheric conditions. In order to examine the effect of
annular air swirl, axial swirlers with different vane angles were used. To investigate
the influence of nozzle exit geometry on the liquid jet breakup process, three variants
of liquid nozzles with different inner diameter and same lip thickness were used in the
coaxial atomizers. Two different techniques, viz. Optical Connectivity (OC) technique
and high-speed shadowgraphic imaging were employed to visualize the liquid jet. The

former was used to characterize jet breakup length, while the latter was utilized to char-
acterize large-scale instabilities as well as primary Kelvin-Helmholtz (KH) instability.

Experiments were also conducted for time-resolved shadowgraphic visualization of lig-
aments and/or droplets at two different axial locations i.e. z = 8Dl and 30Dl (where

Dl
is inner diameter of the liquid nozzle). Proper Orthogonal Decomposition (POD)
analysis of the images was carried out to extract the dominant large-scale spatial modes
present during the jet breakup as well as in the downstream spray.
The results revealed that apart from gas to liquid momentum flux ratio, the mean jet
breakup length also reduces in the presence of annular air swirl. A correlation for liquid

jet breakup length was obtained, which accounts for swirl number in addition to mo-
mentum flux ratio. In contrast to the mean jet breakup length, the fluctuations in the jet

breakup length were found to be proportional to momentum flux ratio and were further
enhanced in the presence of annular air swirl. Three distinct large-scale instabilities,
viz. flapping instability, wavy instability and explosive breakup, were identified which

governs the unsteady jet breakup behaviour. While the flapping instability is the dom-
inant mode of instability for momentum flux ratio upto about 20, explosive breakup

becomes the dominant mode of instability for higher range of momentum flux ratio.
Wavy instability is always a subservient mode of jet instability and disappears for high
range of momentum flux ratio. In the presence of annular air swirl, the hierarchy of the
instabilities is modified and explosive breakup becomes dominant mode of instability
even for lower limit of momentum flux ratio (M > 12). Apart from the cross-sectional
area ratio of fluid passage, lip thickness ratio of the liquid nozzle also plays a vital role
in triggering the interfacial disturbances as it conditions the liquid and air flow prior
to their interaction, thereby influencing the liquid jet instabilities. Though, mean jet
breakup length is weakly influenced by nozzle geometric parameters, the jet breakup
length fluctuations are strongly influenced by the nozzle exit geometry and particularly
vary inversely with lip thickness ratio.
In the far downstream location, where the secondary breakup is almost over, the
volumetric mean diameter of droplets showed a positive correlation with the mean jet
breakup length while the fluctuation in the droplets size decreased with increase in the

jet breakup length fluctuations. The liquid shedding rate and rate of variation of char-
acteristic droplet size measured at z = 8Dl and 30Dl

, respectively, are found to be
correlated to KH instability wave generation rate at the nozzle exit via. momentum
flux ratio and liquid jet Reynolds number, although the influence of former is always

dominant. It is also found that the flapping instability of the liquid jet perpetuates its in-
fluence in the downstream region and leads to preferential accumulation of liquid mass

on either side of the atomizer axis, thereby affecting the droplet size and local liquid
mass flux. These results give evidence that the liquid jet disintegration memory effect
is retained far downstream of the injector exit and causes significant spray fluctuations.