Intermittency route to thermoacoustic instability in turbulent combustors

2014 ◽  
Vol 756 ◽  
pp. 470-487 ◽  
Author(s):  
Vineeth Nair ◽  
Gireeshkumaran Thampi ◽  
R. I. Sujith
Keyword(s):  
Bluff Body ◽  
Combustion Instability ◽  
Signal Amplitude ◽  
High Amplitude ◽  
Operating Conditions ◽  
Combustion Noise ◽  
Continuous Measure ◽  
Periodic Oscillations ◽  
Thermoacoustic Instability ◽  
Amplitude Fluctuations

AbstractThe dynamic transition from combustion noise to combustion instability was investigated experimentally in two laboratory-scale turbulent combustors (namely, swirl-stabilized and bluff-body-stabilized backward-facing-step combustors) by systematically varying the flow Reynolds number. We observe that the onset of combustion-driven oscillations is always presaged by intermittent bursts of high-amplitude periodic oscillations that appear in a near-random fashion amidst regions of aperiodic low-amplitude fluctuations. These excursions to periodic oscillations last longer in time as operating conditions approach instability and finally the system transitions completely into periodic oscillations. A continuous measure to quantify this bifurcation in dynamics can be obtained by defining an order parameter as the probability of the signal amplitude exceeding a predefined threshold. A hysteresis zone was observed in the bluff-body-stabilized configuration that was absent in the swirl-stabilized configuration. The recurrence properties of the dynamics of intermittent burst oscillations were quantified using recurrence plots and the distribution of the aperiodic phases was examined. From the statistics of these aperiodic phases, robust early-warning signals of an impending combustion instability may be obtained.

Intermittency in the Dynamics of Turbulent Combustors

2014 ◽  
Author(s):  
Vineeth Nair ◽  
R. I. Sujith
Keyword(s):  
Reynolds Number ◽  
High Speed ◽  
Bluff Body ◽  
Combustion Instability ◽  
Homoclinic Orbits ◽  
High Amplitude ◽  
Operating Conditions ◽  
Periodic Oscillations ◽  
Lean Blowout ◽  
Amplitude Fluctuations

The dynamic transitions preceding combustion instability and lean blowout were investigated experimentally in a laboratory scale turbulent combustor by systematically varying the flow Reynolds number. We observe that the onset of combustion-driven oscillations is always presaged by intermittent bursts of high-amplitude periodic oscillations that appear in a near random fashion amidst regions of aperiodic, low-amplitude fluctuations. The onset of high-amplitude, combustion-driven oscillations in turbulent combustors thus corresponds to a transition in dynamics from chaos to limit cycle oscillations through a state characterized as intermittency in dynamical systems theory. These excursions to periodic oscillations become last longer in time as operating conditions approach instability and finally the system transitions completely into periodic oscillations. Such intermittent oscillations emerge through the establishment of homoclinic orbits in the phase space of the global system which is composed of hydrodynamic and acoustic subsystems that operate over different time scales. Such intermittent burst oscillations are also observed in the combustor on increasing the Reynolds number further past conditions of combustion instability towards the lean blowout limit. 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 intermittent oscillations are thus of prognostic value and can be utilized to provide early warning signals to combustion instability as well as lean blowout.

scholarly journals Mathematical Modeling of Unsteady Gas Transmission System Operating Conditions under Insufficient Loading

Energies ◽  
2019 ◽  
Vol 12 (7) ◽  
pp. 1325 ◽  
Author(s):  
Vasyl Zapukhliak ◽  
Lyubomyr Poberezhny ◽  
Pavlo Maruschak ◽  
Volodymyr Grudz ◽  
Roman Stasiuk ◽  
...  
Keyword(s):  
Mathematical Modeling ◽  
Pressure Fluctuations ◽  
High Amplitude ◽  
Transmission System ◽  
Gas Pipeline ◽  
Operating Conditions ◽  
Amplitude Fluctuations ◽  
Pipeline Route ◽  
Main Gas Pipeline ◽  
System Operating

Under insufficient loading of a main gas transmission system, high-amplitude fluctuations of pressure may occur in it. A mathematical model is proposed to estimate the amplitude of pressure fluctuations in a gas pipeline along its length. It has been revealed that the shutdown of compressor stations along the gas pipeline route has a significant impact on the parameters of the unsteady transient operating conditions. The possibility of minimizing oscillation processes by disconnecting compressor stations is substantiated for the “Soyuz” main gas pipeline.

scholarly journals Seeds of phase transition to thermoacoustic instability

2021 ◽  
Author(s):  
Raghunathan Manikandan ◽  
Nitin Babu George ◽  
Vishnu Unni ◽  
R. Sujith ◽  
Jürgen Kurths ◽  
...  
Keyword(s):  
Power Plants ◽  
High Amplitude ◽  
Spatiotemporal Patterns ◽  
Critical Threshold ◽  
Reacting Flow ◽  
Periodic Oscillations ◽  
Thermoacoustic Instability ◽  
Stable Operating ◽  
Operating Regimes ◽  
Early Manifestation

Abstract Tackling the problem of emissions is at the forefront of scientific research today. While industrial engines designed to operate in stable regimes produce emissions, attempts to operate them at "greener" conditions often fail due to thermoacoustic instability. During thermoacoustic instability, hazardous high amplitude periodic oscillations lead to failure of these engines in power plants, aircrafts and rockets. Yet, identifying the onset of thermoacoustic instability remains elusive due to spatial variability and the continuous evolution of spatiotemporal patterns in the reacting flow field. Here, we show experimental evidence of early manifestation of the onset of thermoacoustic instability at certain zones. Our findings allow us to identify a critical threshold that enables us to distinguish stable operating regimes from hazardous operations. This opens new perspectives for predicting the onset of thermoacoustic instability and could be a step forward to "greener" operations. The developed methodology is applicable for other systems exhibiting phase transitions.

Multifractality in combustion noise: predicting an impending combustion instability

2014 ◽  
Vol 747 ◽  
pp. 635-655 ◽  
Author(s):  
Vineeth Nair ◽  
R. I. Sujith
Keyword(s):  
Scale Invariance ◽  
Prognostic Value ◽  
Combustion Instability ◽  
Pressure Fluctuations ◽  
High Amplitude ◽  
Combustion Noise ◽  
Warning Signals ◽  
Stochastic Background ◽  
Turbulent Flow Field ◽  
Low Amplitude

AbstractThe transition in dynamics from low-amplitude, aperiodic, combustion noise to high-amplitude, periodic, combustion instability in confined, combustion environments was studied experimentally in a laboratory-scale combustor with two different flameholding devices in a turbulent flow field. We show that the low-amplitude, irregular pressure fluctuations acquired during stable regimes, termed ‘combustion noise’, display scale invariance and have a multifractal signature that disappears at the onset of combustion instability. Traditional analysis often treats combustion noise and combustion instability as acoustic problems wherein the irregular fluctuations observed in experiments are often considered as a stochastic background to the dynamics. We demonstrate that the irregular fluctuations contain useful information of prognostic value by defining representative measures such as Hurst exponents that can act as early warning signals to impending instability in fielded combustors.

Engineering Precursors to Forewarn the Onset of an Impending Combustion Instability

2014 ◽  
Author(s):  
Vineeth Nair ◽  
Gireeshkumaran Thampi ◽  
R. I. Sujith
Keyword(s):  
Time Scales ◽  
Combustion Instability ◽  
Pressure Fluctuations ◽  
High Amplitude ◽  
Combustion Noise ◽  
Additional Measure ◽  
Statistical Measures ◽  
Random Manner ◽  
Low Amplitude ◽  
Measured Pressure

The present study aims at arming an operator of fielded turbulent combustors with a repertoire of mathematical measures for real-time monitoring to forewarn the onset of impending combustion instability. In turbulent combustors, the route to high-amplitude, periodic, combustion-driven oscillations from conditions of low-amplitude, chaotic, combustion noise happens through an intermediate regime in flow conditions where the measured pressure fluctuations display bursts of intermittent, high-amplitude, periodic oscillations that appear in a near-random manner amidst chaotic fluctuations. This loss of chaos from combustion noise to combustion instability can be quantified to serve as a precursor to impending instability. The recurrence properties of intermittent burst oscillations can be quantified using dynamical systems theory by tracking the distribution of the aperiodic segments in the measured signals. Several statistical measures may be constructed through such recurrence quantification that provide robust early warning signals to an impending instability. Further, the transition to combustion driven oscillations leads to a collapse of the number of relevant time scales involved in the problem. This collapse in time scales can be quantified using generalized Hurst exponents which serve as an additional measure that captures the onset of an impending combustion instability well in advance. The various patent pending measures illustrated in this paper serve as precursors due to the inevitable presence of an intermittent regime of burst oscillations in turbulent combustors.

Onset of thermoacoustic instability in turbulent combustors: an emergence of synchronized periodicity through formation of chimera-like states

2016 ◽  
Vol 811 ◽  
pp. 659-681 ◽  
Author(s):  
Sirshendu Mondal ◽  
Vishnu R. Unni ◽  
R. I. Sujith
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Large Amplitude ◽  
Local Heat ◽  
Combustion Noise ◽  
Periodic Oscillations ◽  
Thermoacoustic Instability ◽  
Chimera State ◽  
Rate Oscillations

Thermoacoustic systems with a turbulent reactive flow, prevalent in the fields of power and propulsion, are highly susceptible to oscillatory instabilities. Recent studies showed that such systems transition from combustion noise to thermoacoustic instability through a dynamical state known as intermittency, where bursts of large-amplitude periodic oscillations appear in a near-random fashion in between regions of low-amplitude aperiodic fluctuations. However, as these analyses were in the temporal domain, this transition remains still unexplored spatiotemporally. Here, we present the spatiotemporal dynamics during the transition from combustion noise to limit cycle oscillations in a turbulent bluff-body stabilized combustor. To that end, we acquire the pressure oscillations and the field of heat release rate oscillations through high-speed chemiluminescence ($CH^{\ast }$) images of the reaction zone. With a view to get an insight into this complex dynamics, we compute the instantaneous phases between acoustic pressure and local heat release rate oscillations. We observe that the aperiodic oscillations during combustion noise are phase asynchronous, while the large-amplitude periodic oscillations seen during thermoacoustic instability are phase synchronous. We find something interesting during intermittency: patches of synchronized periodic oscillations and desynchronized aperiodic oscillations coexist in the reaction zone. In other words, the emergence of order from disorder happens through a dynamical state wherein regions of order and disorder coexist, resembling a chimera state. Generally, mutually coupled chaotic oscillators synchronize but retain their dynamical nature; the same is true for coupled periodic oscillators. In contrast, during intermittency, we find that patches of desynchronized aperiodic oscillations turn into patches of synchronized periodic oscillations and vice versa. Therefore, the dynamics of local heat release rate oscillations change from aperiodic to periodic as they synchronize intermittently. The temporal variations in global synchrony, estimated through the Kuramoto order parameter, echoes the breathing nature of a chimera state.

Effect of Hydrogen on Steady-State and Transient Combustion Instability Characteristics

2020 ◽  
Author(s):  
John Strollo ◽  
Stephen Peluso ◽  
Jacqueline O’Connor
Keyword(s):  
Steady State ◽  
High Speed ◽  
Combustion Instability ◽  
Heat Rate ◽  
Flame Stabilization ◽  
Operating Conditions ◽  
System Stability ◽  
Chamber Pressure ◽  
Thermoacoustic Instability ◽  
Hydrogen Addition

Abstract This paper examines the effects of steady-state and transient hydrogen enrichment on thermoacoustic instability in a model gas turbine combustor. Combustion instability, a feedback loop between flame heat release rate oscillations and combustor acoustics, is characterized in a swirl-stabilized flame operated at a range of hydrogen-natural gas fuel blends and heat rates. Measurements of combustor chamber pressure fluctuations and CH* chemiluminescence imaging are used to characterize instability at a range of operating conditions. Steady-state tests show that both mixture heat rate and hydrogen content affect system stability. At a given heat rate, higher levels of hydrogen result in unstable combustion. As heat rate increases, instability occurs at lower concentrations of hydrogen in the fuel. Transient operation was tested in two directions — instability onset and decay — and two hydrogen-addition times — a short time of 1 millisecond and a longer time of 4 seconds. Results show that instability onset processes, through the transient addition of hydrogen, are highly repeatable regardless of the timescale of hydrogen addition. Certain instability decay processes are less repeatable, resulting in cases that do not fully transition from unstable to stable combustion despite similar changes in hydrogen fuel flow rate. Flame behavior before, during, and after the transient is characterized using high-speed CH* chemiluminescence imaging. Analysis of the high-speed images show changes in flame stabilization and dynamics during the onset and decay processes. The results of this study can have implications for systems that experience variations in fuel composition, particularly in light of growing interest in hydrogen as a renewable fuel.

Thermoacoustic Analysis of Combustion Instability Through a Distributed Flame Response Function

2012 ◽  
Author(s):  
Giovanni Campa ◽  
Sergio Mario Camporeale ◽  
Ezio Cosatto ◽  
Giulio Mori
Keyword(s):  
Time Delay ◽  
Combustion Chamber ◽  
Response Function ◽  
Gas Turbines ◽  
Combustion Instability ◽  
Operating Conditions ◽  
Intensity Coefficient ◽  
Injection Point ◽  
Thermoacoustic Instability ◽  
Flame Response

Modern gas turbines equipped with lean premixed dry low emission combustion systems suffer the problem of thermoacoustic combustion instability. The acoustic characteristics of the combustion chamber and of the burners, as well as the response of the flame to the fluctuations of pressure and equivalence ratio, exert a fundamental influence on the conditions in which the instability may occur. A three dimensional finite element code has been developed in order to solve the Helmholtz equation with a source term that models the heat release fluctuations. The code is able to identify the frequencies at which thermoacoustic instabilities are expected and the growth rate of the pressure oscillations at the onset of instability. The code is able to treat complex geometries such as annular combustion chambers equipped with several burners. The adopted acoustic model is based upon the definition of the Flame Response Function (FRF) to acoustic pressure and velocity fluctuations in the burners. In this paper, data from CFD simulations are used to obtain a distribution of FRF of the κ-τ type as a function of the position within the chamber. The intensity coefficient, κ, is assumed to be proportional to the reaction rate of methane in a two-step mechanism. The time delay τ is estimated on the basis of the trajectories of the fuel particles from the injection point in the burner to the flame front. The paper shows the results obtained from the application of FRF with spatial distributions of both κ and τ. The present paper also shows the comparison between the application of the proposed model for the FRF and the traditional application of the FRF over a concentrated flame in a narrow area at the entrance to the combustion chamber. The distribution of the intensity coefficient and the time delay proves to have an influence, both on the eigenfrequency values and on the growth rates, in several of the examined modes. The proposed method is therefore able to establish a theoretical relation of the characteristics of the flame (depending on the burner geometry and operating conditions) to the onset of the thermoacoustic instability.

Quantification of Variation in Combustion Instability Amplitude in a Multi-Nozzle Can Combustor

2020 ◽  
Author(s):  
Seth Westfall ◽  
Olivia Sekulich ◽  
Wyatt Culler ◽  
Stephen Peluso ◽  
Jacqueline O’Connor
Keyword(s):  
Growth Rates ◽  
Combustion Instability ◽  
Oscillation Amplitude ◽  
Pressure Oscillation ◽  
Operating Conditions ◽  
Permutation Entropy ◽  
State Variables ◽  
Amplitude Variation ◽  
Thermoacoustic Instability ◽  
Fuel Staging

Abstract In this work, we quantify the level of variation in unstable combustion oscillation amplitudes and identify the source of this variation in a multi-nozzle can combustor. At conditions where pollutant emissions are reduced, lean-premixed combustors can undergo thermoacoustic instability when pressure and heat release rate oscillations couple. A commonly used method for suppressing instability is fuel staging, which is a method where fuel is unevenly distributed between nozzles in a multi-nozzle combustor. Our work follows others who have characterized the effect of fuel staging on combustion instability and the mechanisms by which it works during both steady-state and transient operation. One of the outcomes from our previous work was that certain instability operating points display a high level of pressure oscillation amplitude variation. Instead of oscillating at a constant limit-cycle amplitude, the pressure oscillation amplitude varies significantly in time. In this work, we use the concept of permutation entropy to quantify the level of variation in the pressure fluctuation amplitude. We correlate the level of variation with a number of state variables over a range of operating conditions, 291 test cases in all. These state variables include mixture equivalence ratio; transient timescale, amplitude, and direction; hardware temperatures; gas temperatures; and thermoacoustic damping and growth rates. Significant pressure oscillation amplitude variation occurs when the thermoacoustic damping rate is low. The damping and growth rates can be low for a number of reasons, but they are highly correlated with the metal temperature of the centerbody, where the flames are anchored; lower temperatures result in lower damping rates during stable operation and lower growth rates during unstable operation. These results show the importance of the thermal boundary condition on the time-dependent behavior of the thermoacoustic instability.

Thermoacoustic instability as mutual synchronization between the acoustic field of the confinement and turbulent reactive flow

2017 ◽  
Vol 827 ◽  
pp. 664-693 ◽  
Author(s):  
Samadhan A. Pawar ◽  
Akshay Seshadri ◽  
Vishnu R. Unni ◽  
R. I. Sujith
Keyword(s):  
Heat Release ◽  
Limit Cycle ◽  
Acoustic Field ◽  
Heat Release Rate ◽  
Release Rate ◽  
Bluff Body ◽  
Acoustic Pressure ◽  
Mutual Synchronization ◽  
Periodic Oscillations ◽  
Thermoacoustic Instability

Thermoacoustic instability is the result of a positive coupling between the acoustic field in the duct and the heat release rate fluctuations from the flame. Recently, in several turbulent combustors, it has been observed that the onset of thermoacoustic instability is preceded by intermittent oscillations, which consist of bursts of periodic oscillations amidst regions of aperiodic oscillations. Quantitative analysis of the intermittency route to thermoacoustic instability has been performed hitherto using the pressure oscillations alone. We perform experiments on a laboratory-scale bluff-body-stabilized turbulent combustor with a backward-facing step at the inlet to obtain simultaneous data of acoustic pressure and heat release rate fluctuations. With this, we show that the onset of thermoacoustic instability is a phenomenon of mutual synchronization between the acoustic pressure and the heat release rate signals, thus emphasizing the importance of the coupling between these non-identical oscillators. We demonstrate that the stable operation corresponds to desynchronized aperiodic oscillations, which, with an increase in the mean velocity of the flow, transition to synchronized periodic oscillations. In between these states, there exists a state of intermittent phase synchronized oscillations, wherein the two oscillators are synchronized during the periodic epochs and desynchronized during the aperiodic epochs of their oscillations. Furthermore, we discover two different types of limit cycle oscillations in our system. We notice a significant increase in the linear correlation between the acoustic pressure and the heat release rate oscillations during the transition from a lower-amplitude limit cycle to a higher-amplitude limit cycle. Further, we present a phenomenological model that qualitatively captures all of the dynamical states of synchronization observed in the experiment. Our analysis shows that the times at which vortices that are shed from the inlet step reach the bluff body play a dominant role in determining the behaviour of the limit cycle oscillations.

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