Pattern formation during transition from combustion noise to thermoacoustic instability via intermittency

Abstract

Gas turbine engines are prone to the phenomenon of thermoacoustic instability, which is highly detrimental to their components. Recently, in turbulent combustors, it was observed that the transition to thermoacoustic instability occurs through an intermediate state, known as intermittency, where the system exhibits epochs of ordered behaviour, randomly appearing amidst disordered dynamics. We investigate the onset of intermittency and the ensuing self-organization in the reactive flow field, which, under certain conditions, could result in the transition to thermoacoustic instability. We characterize this transition from a state of disordered and incoherent dynamics to a state of ordered and coherent dynamics as pattern formation in the turbulent combustor, utilizing high-speed flame images representing the distribution of the local heat release rate fluctuations, flow field measurements (two-dimensional particle image velocimetry), unsteady pressure and global heat release rate signals. Separately, through planar Mie scattering images using oil droplets, the collective behaviour of small scale vortices interacting and resulting in the emergence of large scale coherent structures is illustrated. We show the emergence of spatial patterns using statistical tools used to study transitions in other pattern forming systems. In this paper, we propose that the intertwined and highly intricate interactions between the wide spatio-temporal scales in the flame, the flow and the acoustics are through pattern formation.

Multifractal analysis of flame dynamics during transition to thermoacoustic instability in a turbulent combustor

“…In this figure, from the roll-up of the flame, we can discern that small-scale vortical structures exist (marked by purple circles) within the mixing layer. George et al (2018) showed that such small-scale vortical structures cause spatial and temporal incoherence in the acoustic power production.…”

“…The spatiotemporal dynamics of a turbulent combustor is largely influenced by the coupling between the various subsystems such as the hydrodynamic field, the reactive field (flame) and the acoustic field (Candel 1992). George et al (2018) showed that the emergence of order from chaos during thermoacoustic instability is the result of interactions among the patterns formed in various subsystems. The patterns formed are the standing wave pattern in the acoustic field, the large-scale coherent structures in the flow field and the coherent patterns of the flame.…”

On the emergence of large clusters of acoustic power sources at the onset of thermoacoustic instability in a turbulent combustor

“…Firstly, the size and strength of the coherent structure are not as high as in that observed during thermoacoustic instability. George et al (2018) quantitatively measured the size of the coherent structures during the different stages of combustor operation using a vortex detection technique. Also, the strength of the coherent structure, which is reflected in the magnitude of the positive node strength, is lower than that during thermoacoustic instability.…”

Advances by the Marie Curie project TANGO in thermoacoustics

“…The governing differential equations (16) for the sound field in a combustion chamber and 17for the tailored Green's function, have exactly the same structure, and their solutions satisfy the same boundary conditions. These features can be exploited mathematically to derive a governing integral equation for the acoustic field in terms of the tailored Green's function.…”

“…In a further study, George et al. 16 looked in more detail into the spatio-temporal dynamics of a bluff-body stabilised turbulent combustor. They used an armoury of diagnostic equipment to measure the instantaneous local acoustic power production, p'(t)q'(x,y,t), of the flame, and applied the Mie scattering technique to reveal the interaction of vortices.…”