Visualization Studies of an Acoustically Excited Liquid Sheet

Abstract

The interaction between atomizing air and a liquid sheet initiates interfacial instabilities that develop into large amplitude waves, which ultimately promote liquid disintegration. However, to reduce atomizing airflow, the dynamic influence of the surrounding gas is not sufficient to affect the inner core of the liquid. In other words, the characteristic length scales of perturbation promoting break-up are small in comparison with the initial liquid momentum. Therefore, to enhance the disintegration process, the intact liquid-core near the nozzle outlet must be influenced by artificial or external excitations. To accomplish this, the present investigation employed a plane laminar liquid-sheet that was perturbed by acoustic excitations at an optimum frequency through the impinging air-streams.

On the nature of instabilities in externally perturbed liquid sheets

“…It was observed that the interaction of the acoustic field with the sheet generates a sinuous wave riding on it. Flat liquids sheet having large aspect ratio sandwiched between two impinging air streams in the presence of acoustic excitation was studied by Sivadas & Heitor (2002) and Sivadas, Fernandes & Heitor (2003). Their result showed that the breakup of the liquid sheet gets enhanced and there is reduction in the breakup length of the sheet with acoustic excitation.…”

Effect of acoustic velocity on the primary atomization of a hollow cone spray in a swirling flow field

“…Acoustic oscillations affect the spray by forming large-scale vortical structures which entrains droplets, [3,4] alters velocity field [4] and can increase evaporation rates, [4] thereby decreasing droplet size [5,6], resulting in finer atomization [7] and enhancing combustion process [8]. These oscillations improve spray development and patternation by changing liquid disintegration characteristics [9,10]. Studies performed on flat liquid sheets showed that acoustic excitation results in structurally different liquid disintegration characteristics as compared to unexcited flows [10].…”

“…These oscillations improve spray development and patternation by changing liquid disintegration characteristics [9,10]. Studies performed on flat liquid sheets showed that acoustic excitation results in structurally different liquid disintegration characteristics as compared to unexcited flows [10]. In the presence of acoustic oscillations, the interfacial wave development on the liquid sheet reduces surface tension dominated instability [10].…”

“…Studies performed on flat liquid sheets showed that acoustic excitation results in structurally different liquid disintegration characteristics as compared to unexcited flows [10]. In the presence of acoustic oscillations, the interfacial wave development on the liquid sheet reduces surface tension dominated instability [10]. These oscillations augment the growth of sinuous waves responsible for sheet breakup [11], resulting in flag-like flapping [12,13] and synchronizing droplet shedding and axial velocity fluctuations with an excitation frequency [13,14].…”

“…These oscillations augment the growth of sinuous waves responsible for sheet breakup [11], resulting in flag-like flapping [12,13] and synchronizing droplet shedding and axial velocity fluctuations with an excitation frequency [13,14]. The liquid sheet breakup processes are enhanced [10], thereby shortening breakup length [6,9]. Experiments on head-on impinging jets show that the sheet responds only above a certain minimum sound pressure level and this minimum sound pressure level increases with an increase in the excitation frequency [6].…”