Dynamical Characterization of Thermoacoustic Oscillations in a Hydrogen-Enriched Partially Premixed Swirl-Stabilized Methane/Air Combustor

2021 ◽  
Author(s):  
Abhishek Kushwaha ◽  
Praveen Kasthuri ◽  
Samadhan A. Pawar ◽  
R. I. Sujith ◽  
Ianko Chterev ◽  
...  
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Acoustic Pressure ◽  
Thermal Power ◽  
Frequency Locking ◽  
Thermoacoustic Instability ◽  
Period 2 ◽  
Partially Premixed ◽  
Hydrogen Enrichment

Abstract In this study, we systematically analyze the effects of hydrogen enrichment in the well-known PRECCINSTA burner, a partially premixed swirl-stabilized methane/air combustor. Keeping the equivalence ratio and thermal power constant, we vary the hydrogen percentage in the fuel. Successive increments in hydrogen fuel fraction increase the adiabatic flame temperature and also shift the dominant frequencies of acoustic pressure fluctuations to higher values. Under hydrogen enrichment, we observe the emergence of periodicity in the combustor resulting from the interaction between acoustic modes. As a result of the interaction between these modes, the combustor exhibits a variety of dynamical states, including period-1 limit cycle oscillations (LCO), period-2 LCO, chaotic oscillations, and intermittency. The flame and flow behavior is found to be significantly different for each dynamical state. Analyzing the coupled behavior of the acoustic pressure and the heat release rate oscillations during the states of thermoacoustic instability, we report the occurrence of 2:1 frequency-locking during period-2 LCO, where two cycles of acoustic pressure lock with one cycle of the heat release rate. During period-1 LCO, we notice 1:1 frequency-locking, where both acoustic pressure and heat release rate repeat their behavior in every cycle.

Dynamical Characterization of Thermoacoustic Oscillations in a Hydrogen-Enriched Partially Premixed Swirl-Stabilized Methane/Air Combustor

2021 ◽  
Author(s):  
Abhishek Kushwaha ◽  
Praveen Kasthuri ◽  
Samadhan A. Pawar ◽  
R. I. Sujith ◽  
Ianko Chterev ◽  
...  
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Acoustic Pressure ◽  
Thermal Power ◽  
Frequency Locking ◽  
Thermoacoustic Instability ◽  
Period 2 ◽  
Partially Premixed ◽  
Hydrogen Enrichment

Abstract In this study, we systematically analyze the effects of hydrogen enrichment in the well-known PRECCINSTA burner, a partially premixed swirl-stabilized methane/air combustor. Keeping the equivalence ratio and thermal power constant, we vary the hydrogen percentage in the fuel. Successive increments in hydrogen fuel fraction increase the adiabatic flame temperature and also shift the dominant frequencies of acoustic pressure fluctuations to higher values. Under hydrogen enrichment, we observe the emergence of periodicity in the combustor resulting from the interaction between acoustic modes. As a result of the interaction between these modes, the combustor exhibits a variety of dynamical states, including period-1 limit cycle oscillations (LCO), period-2 LCO, chaotic oscillations, and intermittency. The flame and flow behavior is found to be significantly different for each dynamical state. Analyzing the coupled behavior of the acoustic pressure and the heat release rate oscillations during the states of thermoacoustic instability, we report the occurrence of 2:1 frequency-locking during period-2 LCO, where two cycles of acoustic pressure lock with one cycle of the heat release rate. During period-1 LCO, we notice 1:1 frequency-locking, where both acoustic pressure and heat release rate repeat their behavior in every cycle.

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.

scholarly journals Flame Dynamics Intermittency in the Bi-Stable Region Near a Subcritical Hopf Bifurcation

2017 ◽  
Author(s):  
D. Ebi ◽  
A. Denisov ◽  
G. Bonciolini ◽  
E. Boujo ◽  
N. Noiray
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Acoustic Pressure ◽  
Oscillation Amplitude ◽  
Stable Region ◽  
Describing Function ◽  
Acoustic Damping ◽  
Intermittent Behavior ◽  
Flame Describing Function

We report experimental evidence of thermoacoustic bi-stability in a lab-scale turbulent combustor over a well-defined range of fuel-air equivalence ratios. Pressure oscillations are characterized by an intermittent behavior with “bursts”, i.e. sudden jumps between low and high amplitudes occurring at random time instants. The corresponding probability density functions of the acoustic pressure signal show clearly separated maxima when the burner is operated in the bi-stable region. A flame describing function, which links acoustic pressure to heat release rate fluctuations, is estimated at the modal frequency from simultaneously recorded flame chemiluminescence and acoustic pressure. The representation of its statistics is new and particularly informative. It shows that this describing function is characterized, in average, by a nearly constant gain and by a significant drift of the phase as function of the oscillation amplitude. This finding suggests that the bi-stability does not result from an amplitude-dependent balance between flame gain and acoustic damping, but rather from the non-constant phase difference between the acoustic pressure and the coherent fluctuations of heat release rate.

Smart Passive Control of Thermoacoustic Instability in a Bluff-Body Stabilized Combustor: A Lagrangian Analysis of Critical Structures

2020 ◽  
Author(s):  
C. P. Premchand ◽  
Manikandan Raghunathan ◽  
Midhun Raghunath ◽  
K. V. Reeja ◽  
R. I. Sujith ◽  
...  
Keyword(s):  
Wavelet Transform ◽  
Flow Field ◽  
Heat Release ◽  
Shear Layer ◽  
Heat Release Rate ◽  
Release Rate ◽  
Sound Production ◽  
Bluff Body ◽  
Passive Control ◽  
Thermoacoustic Instability

Abstract The tonal sound production during thermoacoustic instability is detrimental to the components of gas turbine and rocket engines. Identifying the root cause and controlling this oscillatory instability would enable manufacturers to save in costs of power outages and maintenance. An optimal method is to identify the structures in the flow-field that are critical to tonal sound production and perform control measures to disrupt those “critical structures”. Passive control experiments were performed by injecting a secondary micro-jet of air onto the identified regions with critical structures in the flow-field of a bluff-body stabilized, dump, turbulent combustor. Simultaneous measurements such as unsteady pressure, velocity, local and global heat release rate fluctuations are acquired in the regime of thermoacoustic instability before and after control action. The tonal sound production in this combustor is accompanied by a periodic flapping of the shear layer present in the region between the dump plane (backward-facing step) and the leading edge of the bluff-body. We obtain the trajectory of Lagrangian saddle points that dictate the flow and flame dynamics in the shear layer during thermoacoustic instability accurately by computing Lagrangian Coherent Structures. Upon injecting a secondary micro-jet with a mass flow rate of only 4% of the primary flow, nearly 90% suppression in the amplitude of pressure fluctuations are observed. The suppression thus results in sound pressure levels comparable to those obtained during stable operation of the combustor. Using Morlet wavelet transform, we see that the coherence in the dominant frequency of pressure and heat release rate oscillations during thermoacoustic instability is affected by secondary injection. The disruption of saddle point trajectories breaks the positive feedback loop between pressure and heat release rate fluctuations resulting in the observed break of coherence. Wavelet transform of global heat release rate shows a redistribution of energy content from the dominant instability frequency (acoustic time scale) to other time scales.

Adjoint Methods for Elimination of Thermoacoustic Oscillations in a Model Annular Combustor via Small Geometry Modifications

2018 ◽  
Author(s):  
José G. Aguilar ◽  
Matthew P. Juniper
Keyword(s):  
Sensitivity Analysis ◽  
Time Delay ◽  
Heat Release ◽  
Combustion Chamber ◽  
Heat Release Rate ◽  
Release Rate ◽  
Gas Turbines ◽  
Acoustic Pressure ◽  
Annular Combustor ◽  
Thermoacoustic Oscillations

In gas turbines, thermoacoustic oscillations grow if moments of high fluctuating heat release rate coincide with moments of high acoustic pressure. The phase between the heat release rate and the acoustic pressure depends strongly on the flame behaviour (specifically the time delay) and on the acoustic period. This makes the growth rate of thermoacoustic oscillations exceedingly sensitive to small changes in the acoustic boundary conditions, geometry changes, and the flame time delay. In this paper, adjoint-based sensitivity analysis is applied to a thermoacoustic network model of an annular combustor. This reveals how each eigenvalue is affected by every parameter of the system. This information is combined with an optimization algorithm in order to stabilize all thermoacoustic modes of the combustor by making only small changes to the geometry. The final configuration has a larger plenum area, a smaller premix duct area and a larger combustion chamber volume. All changes are less than 6% of the original values. The technique is readily scalable to more complex models and geometries and the inclusion of further constraints, such that the combustion chamber itself should not change. This demonstrates why adjoint-based sensitivity analysis and optimization could become an indispensible tool for the design of thermoacoustically-stable combustors.

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

2019 ◽  
Author(s):  
C. P. Premchand ◽  
Nitin B. George ◽  
Manikandan Raghunathan ◽  
Vishnu R. Unni ◽  
R. I. Sujith ◽  
...  
Keyword(s):  
Saddle Point ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Bluff Body ◽  
Leading Edge ◽  
Mean Flow ◽  
Acoustic Frequency ◽  
Thermoacoustic Instability ◽  
Point Analysis

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.

scholarly journals Investigation of flame behavior and dynamics prior to lean blowout in a combustor with varying mixedness of reactants for the early detection of lean blowout

2018 ◽  
Vol 11 ◽  
pp. 175682771881251 ◽  
Author(s):  
Somnath De ◽  
Arijit Bhattacharya ◽  
Sirshendu Mondal ◽  
Achintya Mukhopadhyay ◽  
Swarnendu Sen
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Premixed Flames ◽  
Early Prediction ◽  
Lean Blowout ◽  
Partially Premixed Flames ◽  
Rate Oscillations ◽  
Partially Premixed ◽  
Flame Behavior

Lean blowout is one of the major challenges faced when the gas turbine combustors are operated with lean fuel–air mixture to meet the emission norm. We experimentally study the flame behavior and the dynamics of heat release rate fluctuations during a transition to lean blowout. The study comprising flame visualization and estimating several measures to predict lean blowout for both premixed and partially premixed flames (using fuel ports F1 to F5) in a swirl stabilized dump combustor. To that end, we acquire unsteady heat release rate in terms of CH* chemiluminescence obtained through a photomultiplier tube with a narrow band-pass filter. For evaluating different statistical measures, we use National Instrument Labview software while acquiring the heat release rate oscillations. For premixed and partially premixed flames, such measures and the flame behavior show a different and, in some cases, even opposite trends as lean blowout is approached. However, in both premixed and partially premixed flames, the mean and root mean square values of the heat release rate fluctuation decrease as we decrease the equivalence ratio. Further, we show that the value of mean frequency calculated using Hilbert transform of the heat release rate fluctuations is a good indicator of lean blowout. Apart from the early prediction of lean blowout, different statistics of heat release rate oscillations, such as kurtosis and skewness, are shown to identify only the occurrence of lean blowout for premixed (F1 and F2) and flames with lower level of premixing (F3). They are not useful for the flames with high levels of unmixedness like F4 and F5. On the other side, probability density function is seen useful for both premixed and partially premixed flames. In short, we present the relative importance of different measures stated earlier for the identification and early prediction of lean blowout for both premixed and partially premixed flames.

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.

Lagrangian Analysis of Flame Dynamics in the Flow Field of a Bluff Body-Stabilized Combustor

2019 ◽  
Vol 142 (1) ◽  
Author(s):  
C. P. Premchand ◽  
Nitin B. George ◽  
Manikandan Raghunathan ◽  
Vishnu R. Unni ◽  
R. I. Sujith ◽  
...  
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Bluff Body ◽  
Leading Edge ◽  
Mean Flow ◽  
Lagrangian Analysis ◽  
Acoustic Frequency ◽  
Thermoacoustic Instability ◽  
Flame Dynamics

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 rate 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.

The Effect of Sauter Mean Diameter Fluctuations on the Heat Release Rate in a Lean-Burn Aero-Engine Combustor

2019 ◽  
Author(s):  
Nicholas C. W. Treleaven ◽  
Andrew Garmory ◽  
Gary J. Page
Keyword(s):  
Heat Release ◽  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Heat Release Rate ◽  
Release Rate ◽  
Sauter Mean Diameter ◽  
Lean Burn ◽  
Thermoacoustic Instability ◽  
Mean Diameter

Abstract It has been shown that the fluctuations of pressure caused by a thermoacoustic instability can affect the mass flow rate of air and atomisation of the liquid fuel inside a gas turbine. Tests with premixed flames have confirmed that the fluctuations of the mass flow rate of air can affect the heat release rate through purely aerodynamic phenomenon but little work has been done to test the sensitivity of the heat release rate to changes in the fuel atomisation process. In this study, a lean-burn combustor geometry is supplied with a fuel spray fluctuation of SMD (Sauter mean diameter) of 20% with respect to the mean value and the heat release rate predicted using Large Eddy Simulation (LES) with combustion predicted using a presumed probability density function (PPDF), flamelet generated manifolds (FGM) method. Previous work has shown that at atmospheric conditions the SMD may fluctuate by up to 16% percent and at low frequencies may be reasonably well predicted by using a correlation based on the instantaneous velocity and mass flow rate of air close to the air-blast atomiser. Analysis of the flow fields highlights a complicated spray, flame and wall interaction as being responsible for this observed fluctuation of heat release rate. The heat release rate predicted by the LES shows a 20% fluctuation which implies that even small fluctuations of SMD will significantly contribute to thermoacoustic instabilities.

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