4D Characterization of the swirling gas jet flow field in a coaxial two-fluid atomizer*

The dynamics of the swirling gas jet in the near field of a coaxial two-fluid atomizer play a key role in the development of interfacial instabilities, the liquid break-up process, and spray characteristics. Due to the overwhelming momentum from the gas, and its turbulent state, the instabilities exhibit a wide range of spatial and temporal scales. Moreover, the addition of angular momentum in the gas dramatically influences the spray, including droplet formation and dispersion. This study investigates the near-field velocity field of the gas-only annular jet from the atomizer using 3D-PTV. As the liquid injection cross-sectional area and momentum are both negligible, the gas-only annular accurately represents the spray flow field. Momentum ratios M=5-174 and the swirl ratios between SR=0-1 are studied, with the mean and turbulence velocity statistics measured with high resolution. The near-field wake region grows with increasing swirl ratio, and the azimuthal velocity peaks for a value of the critical swirl number that is a function of the gas Reynolds number. Robust PCA is conducted to extract dominant flow structures, including the axial shear layer and helical vortices. Turbulent intensity depends on the momentum and swirl ratio in response to the instabilities triggered.