Transversal oscillation of a planar liquid sheet induced by co-current airflows

Air-blast atomizers are commonly used in gas turbine engines. The atomizer discharges a liquid fuel sheet and co-current air flows into the combustor. However, the detailed atomization characteristics of the atomization process are not fully understood because of their complicated multi-phase and multi-scale characteristics. The liquid sheet injected from the atomizer oscillates longitudinally by Kelvin-Helmholtz (KH) instability, and then oscillates transversally before bag formation and breakup. The characteristics of the longitudinal and transversal oscillations strongly affect the spray characteristics, such as the spray angle and droplet diameter. Hence, in the present study, the oscillation phenomena in the spanwise direction of a planar liquid sheet are investigated in order to clarify the role of the characteristics in the atomization process. We propose a model on the wavelengths of the transversal oscillations based on Rayleigh-Taylor (RT) instability, which is induced by the liquid sheet accelerating due to KH instability and aerodynamic force. High-speed photography of a planar liquid sheet with co-current airflows is carried out to measure the wavelengths of the transversal oscillations. The validity of the model is examined through comparisons between the predicted and measured wavelengths of various gas and liquid velocities and densities, the surface tension, and the thicknesses of the liquid sheet and the lip (between the gas and liquid). From the results obtained, we clarify that the transversal oscillation of an air-blasted liquid sheet is governed by RT instability based on both the acceleration by KH instability and aerodynamic force.