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.

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.

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.

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.

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

2018 ◽  
Vol 849 ◽  
pp. 615-644 ◽  
Author(s):  
Nitin B. George ◽  
Vishnu R. Unni ◽  
Manikandan Raghunathan ◽  
R. I. Sujith
Keyword(s):  
Pattern Formation ◽  
Flow Field ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
High Speed ◽  
Mie Scattering ◽  
Combustion Noise ◽  
Small Scale ◽  
Thermoacoustic Instability

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.

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.

On the Impact of Shear Flow Instabilities on Global Heat Release Rate Fluctuations: Linear Stability Analysis of an Isothermal and a Reacting Swirling Jet

2012 ◽  
Author(s):  
Kilian Oberleithner ◽  
Sebastian Schimek ◽  
Christian Oliver Paschereit
Keyword(s):  
Stability Analysis ◽  
Heat Release ◽  
Shear Layer ◽  
Linear Stability ◽  
Heat Release Rate ◽  
Release Rate ◽  
Linear Stability Analysis ◽  
Amplitude Dependence ◽  
Swirling Jet ◽  
The Impact

The prediction of large-scale flow structures in combustor flows and their impact on the flame dynamics is of great importance to avoid thermoacoustic instabilities in modern gas turbine design. The streamwise growth of these so-called coherent structures depends on the receptivity of the shear layers, which can be predicted numerically by means of linear stability analysis. We demonstrate this approach on an isothermal swirling jet that is dominated by a self-excited helical mode that features a precessing vortex core, showing that this theoretical concept successfully predicts the frequency, the source, and the shape of this mode. The analysis is further applied to a reacting flow with a swirl-stabilized flame, pointing out important connections between the shear layer receptivity and the measured amplitude dependence of the flame transfer function. The theoretical findings suggest that the saturation of the global heat release rate fluctuations observed at moderate forcing amplitudes is caused by vanishing shear layer receptivity.

Effect of a Premixed Pilot Flame on the Velocity-Forced Flame Response in a Lean-Premixed Swirl-Stabilized Combustor

2019 ◽  
Author(s):  
Jihang Li ◽  
Stephen Peluso ◽  
Domenic Santavicca ◽  
James Blust
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Shear Layer ◽  
Heat Release Rate ◽  
Release Rate ◽  
Wall Region ◽  
Rate Fluctuation ◽  
Phase Images ◽  
Flame Response ◽  
Near Wall

Abstract The effect of a fully-premixed pilot flame on the velocity-forced flame response of a fully premixed flame in a single-nozzle lean-premixed swirl combustor operating on natural gas fuel is investigated. Measurements of the flame transfer function show that as the percent pilot is increased there is a decrease in the flame transfer function gain at all frequencies, a decrease in the frequencies at which the gain minima and maxima occurred, and a decrease in the flame transfer function phase at high frequencies. High-speed CH* chemiluminescence flame imaging is used to gain a better understanding of the mechanism(s) whereby the pilot flame affects flame dynamics and thereby the flame transfer function. Time-averaged flame images show that the location of the maximum heat release rate does not change with forcing frequency or percent pilot, although the flame extends further upstream into the inner shear layer with increasing percent pilot. Heat release rate fluctuation images show that significant heat release rate fluctuations occur in the inner shear layer, the outer recirculation zone, and the near wall region and that the primary effect of increasing the forcing frequency or the percent pilot is a shift of the heat release rate fluctuation from the near wall region to the inner shear layer. In addition, an increase in the percent pilot results in lengthening and narrowing of the inner shear layer and the near wall regions. The phase images show that the phase is less uniform as the frequency or percent pilot increase, resulting in greater interference between in phase and out of phase fluctuations which reduces the FTF gain. The phase images also show that the wavelength of the heat release rate perturbation travelling through the inner shear layer decreases with increasing frequency and percent pilot which suggests that the pilot flame alters the recirculation flow field. Flame transfer functions calculated for the heat release rate fluctuations in the inner shear layer, the near wall region and the outer recirculation zone show that the inner shear layer is the largest contributor to the global heat release rate fluctuation in the unpiloted flame and that the primary effect of the pilot flame on the reduction of the global FTF gain is a result of the pilot flame’s effect on the inner shear layer.

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.

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.

scholarly journals Effects of the Injector Design on the Transfer Function of Premixed Swirling Flames

2017 ◽  
Author(s):  
M. Gatti ◽  
R. Gaudron ◽  
C. Mirat ◽  
T. Schuller
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Bluff Body ◽  
Transfer Functions ◽  
Thermal Power ◽  
Phase Lag ◽  
Swirl Number ◽  
Injector Design

This article reports a series of experiments on the dynamics of lean-premixed swirl-stabilized flames submitted to harmonic flowrate modulations. The flame transfer function is analyzed for different injector designs with a specific focus on conditions leading to the lowest heat release rate response for a given flowrate perturbation. Experiments are carried out at a fixed equivalence ratio and fixed thermal power. Transfer functions are measured for radial swirling vanes by modifying the diameter of the swirler injection holes, the diameter of the injection tube at the top of the swirler and the end piece diameter of a central insert serving as a bluff body. It is found that the lowest response depends on the forcing frequency and is obtained when the injector design features the largest swirl number. The transfer function of the studied flames features a minimum gain value which decreases for increasing swirl levels. This minimum value is found to be independent of the velocity forcing level and is only controlled by the level of swirl. An excessive swirl level however leads to flash-back of the perturbed flames inside the injector. The way the flame behaves at this forcing frequency is analyzed for a set of injectors featuring the same radial swirling vane design and different injection tube diameters or conical end pieces. It is found that at the condition corresponding to the lowest FTF gain, i.e. the injector with the largest swirl number, the upper and lower parts of the flame contribute to out of phase heat release oscillations, but they also both feature a reduced level of fluctuations. When the swirl number decreases, the FTF gain increases due to a reduction of the phase lag between heat release rate oscillations in the lower and the upper parts of the flame and more importantly due to a general increase of the level of heat release oscillations in both parts of the flame.

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