Hysteresis and Bi-Stability in Transversely Excited Swirling Flows

Abstract

Abstract We investigate the effect of transverse acoustic excitation on non-reacting swirling jets. The work is motivated by the azimuthal instabilities in annular gas turbine combustor which are one of the major challenges in aero-engines. We have designed and fabricated a multi-nozzle linear array combustor to simulate the flow conditions of an annular combustor. The nozzle features a dual co-rotating radial swirler configuration. Two compression drivers placed on either side of the combustion chamber are used to generate acoustic fields in the direction transverse to the flow. Simultaneous 2D PIV and high-frequency pressure measurements are conducted to measure the time-averaged velocity field and the chamber acoustics, respectively. It is observed that once the swirling jet is excited with a transverse acoustic forcing, it instantaneously transitions to a wall-jet state. In wall-jet state, the flow moves radially outwards and remains attached to the walls on either side of the nozzle, and is characterised by a recirculation zone with a strong negative axial velocity. In our experiments we demonstrate that transverse acoustic excitation can lead to a bistable state in swirling flows. We investigated the acoustic response to low-amplitude forcing on the combustion chamber by performing a forced acoustic response analysis using COMSOL. It is observed that acoustic forcing leads to a peak response of the radial and azimuthal velocities at the flare of the swirler, which could induce an axisymmetric deflection of the jet shear layer pushing it into a wall-jet state.