Flame dynamics during intermittency and secondary bifurcation to longitudinal thermoacoustic instability in a swirl-stabilized annular combustor

Roy, Amitesh; Singh, Samarjeet; Nair, Asalatha; Chaudhuri, Swetaprovo; Sujith, R. I.

doi:10.1016/j.proci.2020.08.053

Cited by 28 publications

(11 citation statements)

References 17 publications

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“…Ananthkrishnan et al [13,14] hypothesized the possibility of a secondary bifurcation from an initially stable primary limit cycle to a large amplitude secondary limit cycle solution in thermoacoustic systems having higher-order nonlinearities. Secondary bifurcation was then experimentally confirmed in laminar [15] and, very recently, in turbulent [16][17][18] thermoacoustic systems.…”

Section: Introductionmentioning

confidence: 98%

Abrupt transitions in turbulent thermoacoustic systems

Bhavi,

Pavithran,

Roy

et al. 2022

Preprint

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Abrupt transitions to the state of thermoacoustic instability (TAI) in gas turbine combustors are a significant challenge plaguing the development of next-generation lowemission aircraft and power generation engines. In this paper, we present the observation of abrupt transition in three disparate turbulent thermoacoustic systems: an annular combustor, a swirl-stabilized combustor, and a preheated bluff-body stabilized combustor. Using a low-order stochastic thermoacoustic model, we show that the reported abrupt transitions occur when an initially stable, supercritical limit cycle becomes unstable, leading to a secondary bifurcation to a large amplitude limit cycle solution. The states of combustion noise and intermittency observed in these turbulent combustors are well captured by the additive stochastic noise in the model. Through amplitude reduction, we analyze the underlying potential functions affecting the stability of the observed dynamical states. Finally, we make use of the Fokker-Planck equation, educing the effect of stochastic fluctuations on subcritical and secondary bifurcation. We conclude that a high enough intensity of stochastic fluctuations which transforms a subcritical bifurcation into an intermittency-facilitated continuous transition may have little effect on the abrupt nature of secondary bifurcation. Our findings imply the high likelihood of abrupt transitions in turbulent combustors possessing higher-order nonlinearities where turbulence intensities are disproportionate to the large amplitude limit cycle solution.

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Section: Introductionmentioning

confidence: 98%

Abrupt transitions in turbulent thermoacoustic systems

Bhavi,

Pavithran,

Roy

et al. 2022

Preprint

Self Cite

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“…The assumption is drastic, but is justified on the reason that decoupled oscillators result in oscillations at their own intrinsic frequency leading to stable combustion. Whereas coupled oscillators, with coupling provided through parametric or functional means [31,32,33], result in synchronized behavior. Further, a range of qualitatively similar dynamical states that range from combustion noise to limit cycle oscillations have been reproduced by such models [32,33], thereby providing reliable lower order models for less demanding physical and computational resources.…”

Section: Introductionmentioning

confidence: 99%

“…Whereas coupled oscillators, with coupling provided through parametric or functional means [31,32,33], result in synchronized behavior. Further, a range of qualitatively similar dynamical states that range from combustion noise to limit cycle oscillations have been reproduced by such models [32,33], thereby providing reliable lower order models for less demanding physical and computational resources. A key advantage of adopting this approach is the ability to control thermo-acoustic instabilities by linking the ad-hoc coupling terms to physical processes.…”

Section: Introductionmentioning

confidence: 99%

Understanding dynamical states observed in a thermo-acoustic system from perspective of frequency-phase relationship derived from a probabilistic oscillator model

Ramanan

Ramankutty

Sreed

et al. 2022

Preprint

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We perform lab scale experiments in a swirl combustor by reducing the equivalence ratio for two cases involving slightly different inlet air flow rates. Reduction in equivalence ratio at a constant air flow results in the combustor transiting from stable to unstable combustion. The transition passes through stable, type-2 intermittency and beat oscillations. The beat oscillations are seen to fluctuate at a single frequency, and hence suggestive of a process involving spatio-temporal variations in the phase of the driver (i.e., heat release rate fluctuations). This is understood in the manner of a reduced order model, where the driver is considered to an ensemble of phase oscillators with time delays and a probabilistic distribution of natural frequencies, similar to Kuramoto oscillator. The acoustic field is modeled as a Van-der-pol Duffing system, with natural frequency equal to the duct acoustic mode. The coupling between the oscillators is varied based on the physical premise of Rayleigh criterion. The coupled system is seen to qualitatively and quantitatively match the pressure data obtained from experiments. Insights into various conditions illustrate the role of mean and fluctuating instantaneous frequency amongst the phase oscillators in determining the modeled pressure oscillations. By quantifying the extent of fluctuating frequency coupling among the phase oscillators, it is observed that beat oscillations have high correlation with lower deviation compared to intermittency and stable oscillations.

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“…There are a number of physical mechanisms which generate heat release rate fluctuations such as equivalence ratio fluctuations [5], flame-vortex interactions [6], entropy fluctuations [7], and swirl number fluctuation [8]. Heat release rate fluctuations generated through these mechanisms undergo mutual synchronization with acoustic pressure fluctuations, resulting in the occurrence of thermoacoustic instability [9][10][11]. Consequently, mitigation strategies aim at interrupting the coupling between the acoustic waves and the unsteady heat release rate fluctuations.…”

Section: Introductionmentioning

confidence: 99%

Mean-field synchronization model for open-loop, swirl controlled thermoacoustic system

Singh¹,

Dutta²,

Dhadphale³

et al. 2022

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Open-loop control is known to be an effective strategy for controlling self-excited thermoacoustic oscillations in turbulent combustors. In this study, we investigate the suppression of thermoacoustic instability in a lean premixed, laboratory-scale combustor using experiments and analysis. Starting with a self-excited thermoacoustic instability in the combustor, we find that a progressive increase in the swirler rotation rate transitions the system from thermoacoustic instability to the suppressed state through a state of intermittency. To model such transition while also quantifying the underlying synchronization characteristics, we extend the model of Dutta et al. [Phys. Rev. E 99, 032215 (2019)] by introducing a feedback between the ensemble of mean-field phase oscillators and the basis expansion of the acoustic pressure governing equation. The assumption that coupling strength among the oscillators is a linear combination of acoustic and swirler rotation frequency is justified a posteriori. The link between the model and experimental results is quantitatively established by implementing an optimization algorithm for model parameter estimation. We show that the model replicates the bifurcation characteristics, time series, probability density function (PDF), and power spectral density (PSD) of the various dynamical states observed during the transition to the suppressed state, to excellent accuracy. Specifically, the model captures the change in the PDF of pressure and heat release rate fluctuations from a bimodal distribution during thermoacoustic instability to a unimodal distribution during suppression. Finally, we discuss the global and local flame dynamics and show that the model qualitatively captures various aspects of spatio-temporal synchronization that underlies the transition.

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Flame dynamics during intermittency and secondary bifurcation to longitudinal thermoacoustic instability in a swirl-stabilized annular combustor

Cited by 28 publications

References 17 publications

Abrupt transitions in turbulent thermoacoustic systems

Abrupt transitions in turbulent thermoacoustic systems

Understanding dynamical states observed in a thermo-acoustic system from perspective of frequency-phase relationship derived from a probabilistic oscillator model

Mean-field synchronization model for open-loop, swirl controlled thermoacoustic system

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