Abstract
This work examines the complex flow field in a bistable turbulent swirl flame, where the flame alternates irregularly between a lifted-off M-shape and an attached V-shape. The flow field consists of various types of intermittent dynamics due to flame-shape switching that occur on different time scales. In order to properly identify, separate and temporally resolve these dynamic components, a novel method of multiresolution proper orthogonal decomposition (MRPOD) is developed by interfacing the maximum overlap discrete wavelet packet transform (MODWPT) of multidimensional data series with the conventional snapshot POD. Specific care is taken to select the wavelet filter, decomposition level and reconstruction bandwidth to achieve variable spectral bandpasses and adequate temporal resolutions. When applied to data series from high-speed three-component velocity field measurements in the bistable swirl flame, MRPOD is capable of isolating frequency components that are usually lumped into a single POD mode, with enhanced spatial/temporal coherence for even weak and highly intermittent dynamics. Owing to the improved spectral purity, a series of previously unknown dynamics is uncovered alongside well-characterised instabilities such as the precessing vortex core (PVC) and thermoacoustic (TA) instabilities. In particular, a non-periodic switch mode is found to couple with the previously identified shift mode only during flame-shape transitions, with pronounced modifications on the backflow and near the inlet of the combustor, a region known to influence the growth rate of PVC. TA oscillations are seen to drive repetitive flame reattachment during M–V transitions that eventually settles into a V-flame. But sustained high TA amplitudes alone do not appear to necessarily presage the onset of such a transition. Additional higher-order harmonics of PVC and evidence of TA modulations on PVC dynamics are uncovered, which also exhibit bi-modal behaviours: while maintaining their characteristic frequencies, these instabilities could attain either a single or double helical structure and are only active during either V- or M-flame periods.
Highlights
Turbulent flows in modern gas turbine combustors often exhibit periodic oscillations due to hydrodynamic and acoustic instabilities as well as non-periodic dynamics such 882 A30-2Z
A method of modal analysis based on maximum overlap discrete wavelet packet transform is proposed with an aim to time–frequency localise intermittent dynamics in turbulent flows
maximum overlap discrete wavelet packet transform (MODWPT) of time series is generalised into a matrix form to facilitate handling of multidimensional data series for efficient computation
Summary
Data-driven modal decomposition methods such as POD (Lumley 1967; Sirovich 1987) and dynamic mode decomposition (DMD) (Rowley et al 2009; Schmid 2010) have been implemented most commonly to extract periodic, coherent structures in turbulent flows (Taira et al 2017). In their most basic forms, their applicability to problems such as the bistable flame is hindered due to their inadequacies at identifying and separating intermittent dynamic components, which may possess similar spatial structures but which occur on a multitude of time scales.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
