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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.
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A Comparison of Implicit and Explicit Simulations of Vortex Breakdown
Vortex breakdown is an instability which occurs in swirling flows and is seen in both incompressible and compressible flow regimes. The choice of numerical algorithm to com- pute this flow is critical to resolving the relevant physical phenomena. In previous work, we described the procedure of combining a Barely Implicit Correction (BIC) algorithm with a fourth-order Flux-Corrected Transport (FCT) algorithm to eliminate the sound speed limit for explicit calculations of low-speed flows, and showed that BIC-FCT could predict the three major vortex breakdown modes (spiral, bubble and double-helix). Here, we show a direct comparison of the implicit BIC and explicit FCT algorithms by using the same time step within the explicit stability limit. The comparison is conducted on simulations of a three-dimensional, swirling jet flow with vortex breakdown, by examining the flow structures and examining the property fields. Differences are observed in the phase of the downstream spiral and double-helix modes. Based on a pressure wave analysis, we attribute this phase shift to the difference of how the explicit FCT and implicit BIC treat an open outflow boundary.
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- Award ID(s):
- 1839510
- PAR ID:
- 10104190
- Date Published:
- Journal Name:
- AIAA Scitech 2019 Forum
- Page Range / eLocation ID:
- 1144
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.2514/6.2019-1144
