Lagrangian Saddle Point Analysis in the Flow-Field of a Bluff-Body Stabilized Combustor

Abstract

Experiments are performed in a partially-premixed bluff-body stabilized turbulent combustor by varying the mean flow velocity. Simultaneous measurements obtained for unsteady pressure, velocity and heat release rate are used to investigate the dynamic regimes of intermittency (10.1 m/s) and thermoacoustic instability (12.3 m/s). Using wavelet analysis, we show that during intermittency, modulation of heat release rate occurring at the acoustic frequency fa by the heat release rate occurring at the hydrodynamic frequency fh results in epochs of heat release rate fluctuations where the heat release is phase locked with the acoustic pressure. We also show that the flame position during intermittency and thermoacoustic instability are essentially dictated by saddle point dynamics in the dump plane and the leading edge of the bluff-body.

Multilayer network analysis to study complex inter-subsystem interactions in a turbulent thermoacoustic system

“…Such a critical region was also identified in previous studies investigating the hydrodynamic subsystem independently. For example, a similar critical was region identified by Premchand et al (2019) corresponding to the dominant acoustic mode using flow decomposition techniques during the occurrence of thermoacoustic instability. In addition, a similar critical region was identified by Krishnan et al (2021) as the hubs of single-layer vorticity networks during the state of thermoacoustic instability.…”

“…Thus, different studies have theorised that there are inter-dependencies between the acoustic modes, coherent structures and heat release rate fluctuations during the state of thermoacoustic instability. Subsequently, the role of acoustic modes on the evolution of coherent structures during the state of thermoacoustic instability has been investigated using different techniques such as phase averaging and proper orthogonal decomposition (Lacarelle et al 2010;Oberleithner et al 2011;Tammisola & Juniper 2016), dynamic mode decomposition (Schmid et al 2011;Premchand et al 2019) and also using numerical models (Matveev & Culick 2003). Moreover, active control techniques have been developed by modifying vortex shedding frequencies and suppressing the coherence of large vortices that aid in exciting thermoacoustic instability (McManus, Poinsot & Candel 1993;Paschereit, Gutmark & Weisenstein 1999).…”