Absolute instabilities and dynamics of helical vortices in twin annular swirling jets

Abstract

Modern low-emissions gas turbine combustors commonly employ a twin annular swirling flow configuration that consists of a central annular inner jet and a surrounding annular outer jet. This paper investigates the instability dynamics of helical vortices of such a flow configuration in non-reacting laminar setting with a varying outer jet swirling ratio S. The corresponding base flow features a centerbody wake (CBW), an outer recirculation zone, and a lip recirculation zone at low swirl ratios, whereas at high swirl ratios, the CBW is replaced by a central recirculation zone (CRZ). The azimuthal mode with wavenumber m=1 is found to be absolutely unstable in the CBW region at low swirl ratios (S<0.8), though not large enough to trigger global oscillations. With further increased swirl ratio (S≥0.8), the CBW is suppressed and the CRZ supports a large region of absolute instability for both m=1 and m=2 modes. A three-dimensional nonlinear time stepping performed at S=0.8 confirms that the absolute instability of m=1 mode near the nozzle exit leads to the formation of a single-helix vortex in the near-field. Downstream of the CRZ, the m=1 mode transits to convective instability, whereas the m=2 mode is absolutely unstable. The single-helix vortex is consistently found to disappear in the far-field, where the flow dynamics is dominated by a double-helix vortex counter-winding around the tail of the CRZ.