Impact of precessing vortex core dynamics on the thermoacoustic instabilities in a swirl-stabilized combustor

Abstract

Global instabilities in swirling flows can significantly alter the flame and flow dynamics of swirl-stabilized flames, such as those in modern gas turbine engines. In this study, we characterize the interaction between the precessing vortex core (PVC), which is the consequence of a global hydrodynamic instability, and thermoacoustic instabilities, which are the result of a coupling between combustor acoustics and the unsteady heat release rate. This study is performed using experimental data obtained from a model gas turbine combustor employing two concentric swirling nozzles of air, separated by a ring of fuel injectors, operating at five bar pressure. The flow split between the two streams is systematically varied to observe the impact of flow structure variation on the system dynamics at both non-reacting and reacting conditions. High-speed stereoscopic particle image velocimetry, OH planar laser-induced fluorescence and acetone planar laser-induced fluorescence are used to obtain information about the velocity fields, flame and fuel flow behaviour, respectively. Spectral proper orthogonal decomposition and a complex network analysis are used to identify and characterize the dominant oscillation mechanisms driving the system. In the non-reacting data, a PVC is present in most cases and the amplitude of the oscillation increases with increasing flow through the centre nozzle. In the reacting data, three dominant modes are seen: two thermoacoustic modes and the PVC. Our results show that in the cases where the frequency of the PVC overlaps with either of the thermoacoustic modes, the thermoacoustic modes are suppressed. The complex network analysis coupled with a weakly nonlinear theoretical analysis suggests the mechanisms by which this coupling and suppression of the thermoacoustic mode occur.

Impact of Hydrogen Addition on the Thermoacoustic Instability and Precessing Vortex Core Dynamics in a CH4/H2/Air Technically Premixed Combustor

“…7b and c St ∼ 0.47 and St ∼ 1.5 were confirmed by examining the spatial amplitude distributions of their SPOD modes. These, results are consistent with a recent study [43] which shows theoretically, that these types of oscillations arise due to non-linear coupling between the helical hydrodynamic VBB precession mode that generates the PVC and the axi-symmetric hydrodynamic response to the forcing imposed on the flow by the thermoacoustic oscillation.…”

“…, generated by non-linear hydrodynamic mode coupling [43]. In the present experiment, the St for this oscillation happens to be very close to St T A and is therefore captured within the wavelet-filtering band around St T A .…”

Analysis of Thermoacoustic Instabilities Using the Helmholtz Method in a Swirled Premixed Combustor

“…This phenomenon involves a combination of flame dynamics, combustor acoustics, burner aerodynamics, etc. [8][9][10]. If the thermoacoustic instabilities for a certain gas turbine combustor can be accurately numerically predicted in the design stage, the development cost and period can be significantly reduced [11].…”