Hydrodynamic response of an annular swirling liquid sheet surrounding a forced gas jet

Abstract

Gas–liquid coaxial injectors invariably involve a shear layer between a fast-moving gas stream and a slow-moving liquid stream. This shear layer within the confinement of a recess has a region of absolute instability beyond a critical momentum flux ratio. This causes the spray to exhibit self-pulsation at discrete natural frequencies. We apply sinusoidal acoustic forcing to the gas jet over a range of frequencies and amplitudes to explore the frequency response of the spray. The fluctuating spray width near the injector orifice is measured as a time-resolved tracer characteristic, and its power spectral density is used to determine the spectral response of the spray. The non-pulsating spray that involves a primarily convectively unstable shear layer responds unconditionally to all the forcing frequencies. However, for a self-pulsating spray, when the forcing frequency is far from the natural frequency and both are incommensurate, the spectral response involves both the frequencies, their linear combinations and harmonics representing a state of quasi-periodicity. When the forcing frequency is close enough or the amplitude is high enough, 1 : 1 lock-in is observed where the natural mode is suppressed completely and the spray behaves just like the unforced flow with the peak shifted to the forcing frequency. Combining the experimental observations and application of the van der Pol oscillator model, we could demonstrate the analogous behaviour of this multiphase system with other hydrodynamically self-excited systems with external forcing. The results presented here can prove significant in understanding the dynamics of the atomization process in thermoacoustic coupling and possible control of it.