Hysteresis and Bi-Stability in Transversely Excited Swirling Flows

Abstract

Abstract In this work, 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 combustors which are one of the major challenges in aero-engines. Annular combustion chambers have a longer circumferential length in comparison to their length or height. As a result, thermoacoustic instabilities are driven by azimuthal acoustic modes. We have designed and fabricated a multi-nozzle (up to three nozzles) linear array combustor to simulate the flow conditions of an annular combustor. The nozzle features a dual co-rotating radial swirler, with a flow split of 44% and 56% between the primary and secondary swirlers. Two compression drivers (power 400 W each) 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. The time-averaged flow field of the quiescent swirling jet shows a typical low swirl number flow-field that features a narrow recirculation zone surrounded by a high-velocity annular jet. It is observed that once the swirling jet is excited with a transverse acoustic forcing, it instantaneously transitions to a conical type of vortex breakdown (CVB). However, this phenomenon is observed only above a threshold value of perturbation amplitude. In this condition, the flow moves radially outward and remains attached to the walls on either side of the nozzle, and is characterized by a recirculation zone with strong a negative axial velocity. It is a known phenomenon that swirling jets transition to a CVB upon transverse acoustic excitation. However, to the best of our knowledge, this is the first instance where it is shown 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 CVB state. In the case of a multi nozzle array of three identical swirlers, we observed that the external acoustic forcing changes the time averaged base flow of the central nozzle.