Now showing 1 - 10 of 12
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    Physical mechanisms that cause intermittency that presages combustion instability and blowout in a turbulent lifted jet flame combustor
    (01-02-2018)
    Murugesan, Meenatchidevi
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    The physics of intermittent dynamics that presages the onset of impending combustion instability and blowout in a turbulent lifted jet flame combustor is investigated. It is observed that the transition from combustion noise to combustion instability happens via intermittency while varying the relative location of the burner inside a confinement as a bifurcation parameter. For further change in burner location past the condition of combustion instability, the intermittency before the flame blows out is once again observed. The authors show that the coupled interaction of flow, flame dynamics, and combustion chamber acoustics is the physical reason for the occurrence of intermittency prior to combustion instability. In contrast, intermittent dynamics that presages blowout occurs due to the interplay between flame blowout precursor events and the driving of high-amplitude oscillations as the flame propagates towards the burner.
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    Precursors to flutter instability by an intermittency route: A model free approach
    (01-02-2016)
    Venkatramani, J.
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    Nair, Vineeth
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    The aeroelastic response of a NACA 0012 airfoil in the flow regimes prior to flutter is investigated in a wind tunnel. We observe intermittent bursts of periodic oscillations in the pitch and plunge response, that appear in an irregular manner from a background of relatively lower amplitude aperiodic fluctuations. As the flow speed is increased, the intermittent bursts last longer in time until eventually transitioning to a fully developed periodic response, indicating the onset of flutter. The repeating patterns in the measured response are visualized using recurrence plots. We show that statistics of the recurrence states extracted from these plots can be used to develop model-free precursors that forewarn an impending transition to flutter, well before its onset.
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    Flame dynamics during intermittency in a turbulent combustor
    (01-01-2017)
    Unni, Vishnu R.
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    The dynamic characteristics of the intermittency exhibited by turbulent combustors using LPG as fuel before and after the occurrence of thermoacoustic instability were compared. The unsteady pressure fluctuations analysis suggested that the intermittency before and after thermoacoustic instability are of type II. The flame dynamics during either state were also compared through a high speed Mie scattering images acquired simultaneously with unsteady pressure. During both regimes of intermittency the flame switches between an oscillation where the flame oscillates in a periodic manner due to the inherent turbulent fluctuations and an oscillation where the flame exhibits periodic roll-up as a consequence of the periodic vortex shedding at the dump plane. The flame dynamics during intermittency before and after thermoacoustic instability varied. The flame along the inner shear layer was stabilized by the stagnation point flow behind the bluff body during intermittency before the occurrence of thermoacoustic instability while the flame along the inner shear layer was stabilized by the recirculation zone created by the bluff body during intermittency after the occurrence of thermoacoustic instability.
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    Effect of preheating of the reactants on the transition to thermoacoustic instability in a bluff-body stabilized dump combustor
    (01-01-2021)
    Pawar, Samadhan A.
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    Raghunathan, Manikandan
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    Reeja, K. V.
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    Midhun, P. R.
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    In the present study, we investigate the effect of preheating of the reactants on the dynamics of a turbulent bluff-body-stabilized dump combustor during the onset of thermoacoustic instability. As we decrease the equivalence ratio towards a low fuel-lean value, we observe the dynamical transition of the combustor from the state of combustion noise to thermoacoustic instability via intermittency. Such an intermittency route to thermoacoustic instability is preserved for the range of preheat temperatures considered in this study. Further, we notice that an increase in the temperature of the reactants advances the point of the onset of thermoacoustic instability in the system. During the state of combustion noise, high temperature preheating of the reactants shifts the anchoring point of the flame from the shaft to the tip of the bluff-body and suppresses the oscillatory behavior of the global heat release rate in the system. We also observe a change in the multifractal characteristics of the heat release rate fluctuations such that the signal properties change from short-range to long-range correlations. On the other hand, preheating of the reactants to high temperatures during the state of thermoacoustic instability engenders a shift in the frequency of the acoustic field from the third unstable mode to the first unstable mode. Moreover, such preheating expanses the flame surface from one half of the combustor (lower) to both halves and, hence, increases the Pearson's correlation between the local heat release rate fluctuations measured on both halves of the combustor. Thus, we report that the preheating of the reactants significantly alters the dynamical characteristics of a turbulent combustor during the transition to thermoacoustic instability from that observed in the absence of preheating.
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    Intermittency as a Transition State in Combustor Dynamics: An Explanation for Flame Dynamics Near Lean Blowout
    (02-11-2015)
    Nair, Vineeth
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    The dynamic transitions preceding lean blowout were investigated experimentally in a laboratory scale turbulent combustor by systematically varying the flow Reynolds number (Re). Previous studies on combustor dynamics have shown that the onset of large-amplitude, combustion-driven oscillations is, at times, presaged by intermittent bursts of high-amplitude periodic pressure pulsations. These intermittent bursts appear in a near random fashion amidst regions of aperiodic low-amplitude fluctuations, provided the underlying flow-field is turbulent. In the present study, we show that intermittent burst oscillations are also observed in combustors close to the lean blowout limit. We show that such intermittent oscillations emerge through the establishment of homoclinic orbits in the phase space of pressure oscillations. The formation of such orbits points to the complex nature of the interaction between the hydrodynamics and acoustic subsystems, which operate over a range of different time scales. High-speed flame images reveal that the intermittent states observed prior to lean blowout correspond to aperiodic detachment of the flame from the bluff-body lip. These findings are consistent with other reports of possibly intermittent states in the literature.
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    Abrupt transitions in turbulent thermoacoustic systems
    (17-03-2023)
    Bhavi, Ramesh S.
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    Pavithran, Induja
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    Roy, Amitesh
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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 low-emission 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. Consequently, secondary bifurcations would be much more resilient to control, requiring improved control strategies.
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    A reduced-order model for the onset of combustion instability: Physical mechanisms for intermittency and precursors
    (01-01-2015)
    Nair, Vineeth
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    In combustors, the transition from low-amplitude, aperiodic fluctuations termed combustion noise to large-amplitude, periodic oscillations termed combustion instability is presaged by an intermediate regime in flow conditions characterized by bursts of intermittent, high-amplitude, periodic oscillations that appear in a near-random fashion amidst aperiodic fluctuations. In this study, we show that, a reduced-order model from first principles that incorporates the hydrodynamic-acoustic coupling can capture these intermittent burst oscillations and the subsequent flow-acoustic lock-in observed in combustors. The physical mechanism that leads to intermittency in pressure fluctuations is described using the model. The paper concludes by illustrating through the model, ideas that use intermittency in the signal as an early warning signal - a precursor - to an impending combustion instability.
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    Rate dependent transition to thermoacoustic instability via intermittency in a turbulent afterburner
    (01-06-2020)
    Manikandan, S.
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    Experiments were conducted to study the rate dependent transition to thermoacoustic instability in a turbulent afterburner rig. The afterburner rig contains v-gutters as flame stabilizers and simulates elevated inlet gas temperature using a preheater. Under quasi-static increase in flow Reynolds number of the afterburner, screech is not observed; on the contrary, screech onsets when the Reynolds number is increased at a higher rate. Such a phenomenon is known as rate induced tipping or R-tipping. When the Reynolds number is increased at a lower rate, screech appears as a few bursts of high-amplitude periodic oscillations amidst low-amplitude aperiodic oscillations, a state known as intermittency. As the rate of change of Reynolds number is increased, more bursts appear and with further increase in the rate, the proportion of periodic oscillations of the bursts increases in the time series of pressure fluctuations, approaching a state of limit-cycle oscillations. We show for the first time, a rate-dependent transition to thermoacoustic instability in a turbulent afterburner happening through intermittency, with respect to increasing the rate of change of Reynolds number. The onset of burst occurs earlier, i.e. bursts occurring at a lower Reynolds number as the rate of change of Reynolds number is increased. The amplitude of bursts during intermittency is higher than that of the limit cycle oscillations. The rate dependent transition to screech through intermittency is analyzed by studying the variation of probability density function (PDF) of the amplitude of pressure, variation of the amplitude of bursts, variation of the number of bursts of periodic oscillations, and the inlet conditions corresponding to the onset of bursts, with respect to different rates of change of Reynolds number. Rate dependent transition observed in the model afterburner suggests that combustion system of a gas turbine engine should be subjected to different engine throttling rates to define the thermoacoustic stability map.
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    Synchronization-based model for turbulent thermoacoustic systems
    (01-07-2023)
    Weng, Yue
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    Unni, Vishnu R.
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    Saha, Abhishek
    We present a phenomenological reduced-order model to capture the transition to thermoacoustic instability in turbulent combustors. Based on the synchronization framework, the model considers the acoustic field and the unsteady heat release rate from turbulent reactive flow as two nonlinearly coupled sub-systems. To model combustion noise, we use a pair of nonlinearly coupled second-order ODEs to represent the unsteady heat release rate. This simple configuration, while nonlinearly coupled to another oscillator that represents the independent sub-system of acoustics (pressure oscillations) in the combustor, is able to produce chaos. Previous experimental studies have reported a route from low amplitude chaotic oscillation (i.e., combustion noise) to periodic oscillation through intermittency in turbulent combustors. By varying the coupling strength, the model can replicate the route of transition observed and reflect the coupled dynamics arising from the interplay of unsteady heat release rate and pressure oscillations.
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    Phase synchronization and collective interaction of multiple flamelets in a laboratory scale spray combustor
    (01-01-2019)
    Pawar, Samadhan A.
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    In this paper, we investigate the coupled behvior of the acoustic field in the confinement and the unsteady flame dynamics in a laboratory scale spray combustor. We study this interaction during the intermittency route to thermoacoustic instability when the location of the flame is varied inside the combustor. As the flame location is changed, the synchronization properties of the coupled acoustic pressure and heat release rate signals change from desynchronized aperiodicity (combustion noise) to phase synchronized periodicity (thermoacoustic instability) through intermittent phase synchronization (intermittency). We also characterize the collective interaction between the multiple flamelets anchored at the flame holder and the acoustic field in the system, during different dynamical states observed in the combustor operation. When the signals are desynchronized, we notice that the flamelets exhibit a steady combustion without the exhibition of a prominent feedback with the acoustic field. In a state of intermittent phase synchronization, we observe the existence of a short-term coupling between the heat release rate and the acoustic field. We notice that the onset of collective synchronization in the oscillations of multiple flamelets and the acoustic field leads to the simultaneous emergence of periodicity in the global dynamics of the system. This collective periodicity in both the subsystems causes enhancement of oscillations during epochs of amplitude growth in the intermittency signal. On the contrary, the weakening of the coupling induces suppression of periodic oscillations during epochs of amplitude decay in the intermittency signal. During phase synchronization, we notice a sustained synchronized movement of all flamelets with the periodicity of the acoustic cycle in the system.