Amplitude statistics prediction in thermoacoustics

2018 ◽  
Vol 844 ◽  
pp. 216-246 ◽  
Author(s):  
G. Ghirardo ◽  
F. Boudy ◽  
M. R. Bothien
Keyword(s):  
Acoustic Pressure ◽  
Combustion Noise ◽  
Pressure Amplitude ◽  
Describing Function ◽  
Thermoacoustic Instability ◽  
Design Change ◽  
Describing Functions ◽  
Annular Combustor ◽  
Acoustic Losses ◽  
Thermoacoustic Oscillations

We discuss the statistics of acoustic pressure of thermoacoustic oscillations, either axial or azimuthal in nature. We derive a model where the describing functions of the fluctuating heat release rate of the flame and of the acoustic losses appear directly in the equations. The background combustion noise is assumed to be additive, and we show how one can recover, from the measurement of the acoustic pressure at the flame location, the projected describing function of the flame minus the acoustic losses. Using the same equations, one can predict the statistics of the amplitude of acoustic pressure for a certain system. The theory is then tested on an azimuthal thermoacoustic instability in an industrial annular combustor by measuring the state of the system, predicting the acoustic pressure amplitude statistics after a design change and comparing the prediction with the measured statistics after the design change has been implemented.

scholarly journals Onset of flame-intrinsic thermoacoustic instabilities in partially premixed turbulent combustors

2018 ◽  
Vol 10 (3) ◽  
pp. 171-184 ◽  
Author(s):  
Meenatchidevi Murugesan ◽  
Balasubramanian Singaravelu ◽  
Abhijit K Kushwaha ◽  
Sathesh Mariappan
Keyword(s):  
Network Model ◽  
Model Analysis ◽  
Flow Speed ◽  
Dominant Mode ◽  
Combustion Noise ◽  
Driving Mechanism ◽  
Thermoacoustic Instability ◽  
Thermoacoustic Instabilities ◽  
Partially Premixed ◽  
Thermoacoustic Oscillations

We investigate the onset of thermoacoustic instabilities in a turbulent combustor terminated with an area contraction. Flow speed is varied in a swirl-stabilized, partially premixed combustor and the system is observed to undergo a dynamical transition from combustion noise to instability via intermittency. We find that the frequency of thermoacoustic oscillations does not lock-on to any of the acoustic modes. Instead, we observe that the dominant mode in the dynamics of combustion noise, intermittency and thermoacoustic instability is a function of the flow speed. We also find that the observed mode is insensitive to the changes in acoustic field of the combustor, but it varies as a function of upstream flow time scale. This new kind of thermoacoustic instability was independently discovered in the recent theoretical analysis of premixed flames. They are known as intrinsic thermoacoustic modes. In this paper, we report the experimental observation and the route to flame intrinsic thermoacoustic instabilities in partially premixed flame combustors. A simplified low-order network model analysis is performed to examine the driving mechanism. Frequencies predicted by the network model analysis match well with the experimentally observed dominant frequencies. Intrinsic flame-acoustic coupling between the unsteady heat release rate and equivalence ratio fluctuations occurring at the location of fuel injection is found to play a key role. Further, we observe intrinsic thermoacoustic modes to occur only when the acoustic reflection co-efficients at the exit are low. This result indicates that thermoacoustic systems with increased acoustic losses at the boundaries have to consider the possibility of flame intrinsic thermoacoustic oscillations.

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.

scholarly journals G-equation modelling of thermoacoustic oscillations of partially premixed flames

2017 ◽  
Vol 9 (4) ◽  
pp. 260-276 ◽  
Author(s):  
Bernhard Semlitsch ◽  
Alessandro Orchini ◽  
Ann P Dowling ◽  
Matthew P Juniper
Keyword(s):  
Heat Release ◽  
Limit Cycle ◽  
Heat Release Rate ◽  
Release Rate ◽  
Large Scale ◽  
Describing Function ◽  
Describing Functions ◽  
Wide Range ◽  
Flame Describing Function ◽  
Thermoacoustic Oscillations

Numerical simulations aid combustor design to avoid and reduce thermoacoustic oscillations. Non-linear heat release rate estimation and its modelling are essential for the prediction of saturation amplitudes of limit cycles. The heat release dynamics of flames can be approximated by a flame describing function. To calculate a flame describing function, a wide range of forcing amplitudes and frequencies needs to be considered. For this reason, we present a computationally inexpensive level-set approach, which accounts for equivalence ratio perturbations on flames with arbitrarily complex shapes. The influence of flame parameters and modelling approaches on flame describing functions and time delay coefficient distributions are discussed in detail. The numerically obtained flame describing functions are compared with experimental data and used in an acoustic network model for limit cycle prediction. A reasonable agreement of the heat release gain and limit cycle frequency is achieved even with a simplistic, analytical velocity fluctuation model. However, the phase decay is over-predicted. For sophisticated flame shapes, only the realistic modelling of large-scale flow structures allows the correct phase decay predictions of the heat release rate response.

Characterization and Modeling of a Spinning Thermoacoustic Instability in an Annular Combustor Equipped With Multiple Matrix Injectors

2014 ◽  
Vol 137 (2) ◽  
Author(s):  
Jean-François Bourgouin ◽  
Daniel Durox ◽  
Jonas P. Moeck ◽  
Thierry Schuller ◽  
Sébastien Candel
Keyword(s):  
High Speed ◽  
Stable Limit Cycle ◽  
Turbulent Flames ◽  
Linear Growth Rate ◽  
Stable Limit ◽  
Acoustic Oscillations ◽  
Describing Function ◽  
Azimuthal Mode ◽  
Thermoacoustic Instability ◽  
Annular Combustor

Oscillations in fully annular systems coupled by azimuthal modes are often observed in gas turbine combustors but not well documented. One objective of the present study is to characterize this type of oscillation in a laboratory scale system, allowing detailed pressure measurements and high speed visualization of the flame motion. The experiment is designed to allow detailed investigations of this process at a stable limit cycle and for an extended period of time. Experiments reported in the present article are carried out in the MICCA facility which was used in our previous work to analyze instabilities arising when the chamber backplane was equipped with multiple swirling injectors (Bourgouin et al., 2013, “Self-Sustained Instabilities in an Annular Combustor Coupled by Azimuthal Acoustic Modes,” ASME Paper No. GT2013-95010). In the present study, these units are replaced by a set of matrix injectors. The annular plenum feeds 16 such devices confined by two cylindrical quartz tubes open to the atmosphere. The multiple flames formed by the matrix injectors are laminar and have a well documented describing function. This constitutes an ideal configuration allowing systematic investigations of thermo-acoustic oscillations coupled by longitudinal or azimuthal modes while avoiding complexities inherent to swirling turbulent flames studied previously. Optical access to the chamber allows high speed imaging of light emission from the flames providing instantaneous flame patterns and indications on the heat release rate fluctuations. Eight waveguide microphones record the pressure signal at the combustor injection plane and in the plenum. Among the unstable modes observed in this setup, this analysis focuses on situations where the system features a spinning azimuthal mode. This mode is observed at a frequency which is close to that associated with the 1A mode of the plenum. A theoretical analysis is then carried out to interpret the angular shift between the nodal lines in the plenum and chamber, and the measured flame describing function (FDF) is used to quantify this shift and determine the linear growth rate.

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.

A Selective Fast Fourier Filtering Approach Applied to High Frequency Thermoacoustic Instability Analysis

2017 ◽  
Author(s):  
Felix Grimm ◽  
Jean-Michel Lourier ◽  
Oliver Lammel ◽  
Berthold Noll ◽  
Manfred Aigner
Keyword(s):  
Heat Release ◽  
High Frequency ◽  
Compressible Flows ◽  
Acoustic Pressure ◽  
Inverse Transformation ◽  
Turbulent Heat ◽  
Thermoacoustic Instability ◽  
Acoustic Feedback ◽  
Fourier Filtering ◽  
Filtering Approach

A method for selective, frequency-resolved analysis of spatially distributed, time-coherent data is introduced. It relies on filtering of Fourier-processed signals with periodic structures in frequency-domain. Therefrom extracted information can be analyzed in both, frequency- and time-domain using an inverse transformation ansatz. In the presented paper, the approach is applied to a laboratory scale, twelve nozzle FLOX®-GT-burner for the investigation of high-frequency thermoacoustic pressure oscillations and limit-cycle mechanisms. The burner is operated at elevated pressure for partially premixed combustion of a hydrogen and natural gas mixture with air. At a certain amount of hydrogen addition to fuel injection, the burner exhibits self-sustained high-frequency thermoacoustic oscillation. This unstable operation is simulated with the fractional step approach SICS (Semi Implicit Characteristic Splitting), a pressure based solver extension of the Finite Volume based research code THETA (Turbulent Heat Release Extension for the TAU Code) for the treatment of weakly compressible flows with combustion. A hybrid LES/URANS simulation delivers time-resolved simulation data of the thermoacoustically unstable operation condition, which is analyzed with the presented SFFFA (Selective Fast Fourier Filtering Approach). Acoustic pressure distribution in the combustion chamber is explicitly resolved and assigned to different characteristic modes by signal decomposition. Furthermore, the SFFFA is used for the analysis of acoustic feedback mechanism by investigating filtered transient heat release, acoustic pressure, velocity and mixture fraction. Coherent structures in flow field and combustion as well as periodic convective processes are resolved and linked to transient acoustic pressure, extensively describing the acoustic feedback of the examined burner configuration.

Numerical Simulation of Acoustic Field for the Miniature Cylindrical Transducer with a Rectangular Hole

2011 ◽  
Vol 421 ◽  
pp. 739-742
Author(s):  
Li Li Yu ◽  
Jin Hua Zhao ◽  
Quan Zhou Zhao ◽  
Chun Hui
Keyword(s):  
Numerical Simulation ◽  
Acoustic Field ◽  
Focal Plane ◽  
Optimization Design ◽  
Acoustic Pressure ◽  
Pressure Amplitude ◽  
Hole Size ◽  
Rectangular Hole ◽  
Cylindrical Transducer ◽  
Focusing Properties

In this paper the characteristics of acoustic field for miniature cylindrical focused transducer with a hole was studied in order to instruct the optimization design of the transducer for both realizing visualization and improving the treatment effect. Then the acoustic field was simulated numerically with different parameters of hole. It is found that position of focus is almost unchanged but acoustic pressure amplitude declines. In addition the performance of transverse focusing for the focal plane and levels length of acoustic pressure are lowered. Moreover, if size of transducer and rigidity of material permit, the area and ratio of width to height for the hole should be reduced appropriately to improve the focusing properties. And it is deduced that area and ratio of width to height for the cylinder can be increased to achieve the same therapeutic effect with a fixed hole size.

scholarly journals Effects of Nozzle Helmholtz Number on Indirect Combustion Noise by Compositional Perturbations

2017 ◽  
Vol 140 (3) ◽  
Author(s):  
Luca Magri ◽  
Jeffrey O'Brien ◽  
Matthias Ihme
Keyword(s):  
Velocity Profile ◽  
Combustion Chamber ◽  
Transfer Functions ◽  
Mixture Fraction ◽  
Supersonic Nozzle ◽  
Noise Pollution ◽  
Combustion Noise ◽  
Dipole Source ◽  
Multicomponent Gas ◽  
Thermoacoustic Oscillations

By modeling a multicomponent gas, a new source of indirect combustion noise is identified, which is named compositional indirect noise. The advection of mixture inhomogeneities exiting the gas-turbine combustion chamber through subsonic and supersonic nozzles is shown to be an acoustic dipole source of sound. The level of mixture inhomogeneity is described by a difference in composition with the mixture fraction. An n-dodecane mixture, which is a kerosene fuel relevant to aeronautics, is used to evaluate the level of compositional noise. By relaxing the compact-nozzle assumption, the indirect noise is numerically calculated for Helmholtz numbers up to 2 in nozzles with linear velocity profile. The compact-nozzle limit is discussed. Only in this limit, it is possible to derive analytical transfer functions for (i) the noise emitted by the nozzle and (ii) the acoustics traveling back to the combustion chamber generated by accelerated compositional inhomogeneities. The former contributes to noise pollution, whereas the latter has the potential to induce thermoacoustic oscillations. It is shown that the compositional indirect noise can be at least as large as the direct noise and entropy noise in choked nozzles and lean mixtures. As the frequency with which the compositional inhomogeneities enter the nozzle increases, or as the nozzle spatial length increases, the level of compositional noise decreases, with a similar, but not equal, trend to the entropy noise. The noisiest configuration is found to be a compact supersonic nozzle.

Lean Blowout Phenomena and Prior Detection of Lean Blowout in a Premixed Model Annular Combustor

2019 ◽  
Author(s):  
Arijit Bhattacharya ◽  
Bikash Gupta ◽  
Satyajit Hansda ◽  
Zohadul Haque ◽  
Ashish Kumar ◽  
...  
Keyword(s):  
Bluff Body ◽  
Nox Reduction ◽  
Marked Change ◽  
Nox Emission ◽  
Thermoacoustic Instability ◽  
Lean Blowout ◽  
Lean Combustion ◽  
Annular Combustor ◽  
Lean Limit ◽  
Lift Off

Abstract Strict emission norms in the last few decades have paved the path for adaptation of new low NoX emission alternatives to power generation and aircraft propulsion. Lean combustion is a very promising and practicable technology for reducing NOX reduction and also have very high fuel efficiency. However, lean combustion technology suffers from inherent combustion instabilities that are manifested under different conditions, most importantly, thermoacoustic instability and lean blowout. Lean blowout occurs when a gas turbine combustor operating close to lean limit, for lowest NoX emission, faces abrupt changes in fuel homogeneity, quality or flow rate. While many work have been done in thermo-acoustic instability and flame propagation in annular combustors, studies in lean blowout in annular combustors are very limited. The lean limit of combustors are not fixed and is dependent on fuel characteristics and operating condition including environmental effects. So accurate online prediction of lean limit is very important to keep the combustors operating safely near lean limit. Recent works have demonstrated that single burner combustors leave out a significant amounts of physics including interaction of flames from different burners prior to blowout. In this work, a stepped down swirl and bluff body stabilized annular combustor in CB configuration (having chamber and burner), is used as experimental test rig having 4 number of identical burners. Video and heat release data are taken at different conditions as lean blowout is approached. Frequent attachment and reattachment of the flames prior to lift off was seen. As lean blowout is approached, inherent subtle differences in the different burners get amplified when flame becomes sufficiently weak and flame symmetry is broken. As air fuel mixture is made gradually leaner, one by one the flames from different burners elongates although remains partially attached to burner. Further lowering the equivalence ratio results in lift off and merging of the flame fronts of different burners. Three pixel averaged color ratios are extracted from still camera RGB images as flame stability indicators which are, red by blue, red by green and blue by green. The parameters show marked change at the point of lift off as well as at the lean blowout point.

Multifractal characteristics of combustor dynamics close to lean blowout

2015 ◽  
Vol 784 ◽  
pp. 30-50 ◽  
Author(s):  
Vishnu R. Unni ◽  
R. I. Sujith
Keyword(s):  
Stability Margin ◽  
Static Stability ◽  
Combustion Noise ◽  
Stable Operation ◽  
Nonlinear Behaviour ◽  
Classical Literature ◽  
Unified Framework ◽  
Simple Description ◽  
Thermoacoustic Instability ◽  
Lean Blowout

In classical literature, blowout is described as loss of static stability of the combustion system whereas thermoacoustic instability is seen as loss of dynamic stability of the system. At blowout, the system transitions from a stable reacting state to a non-reacting state, indicating loss of static stability of the reaction. However, this simple description of stability margin is inadequate since recent studies have shown that combustors exhibit complex nonlinear behaviour prior to blowout. Recently, it was shown that combustion noise that characterizes the regime of stable operation is itself dynamically complex and exhibits multifractal characteristics. Researchers have already described the transition from combustion noise to combustion instability as a loss of multifractality. In this work, we provide a multifractal description for lean blowout in combustors with turbulent flow and thus introduce a unified framework within which both thermoacoustic instability and blowout can be described. Further, we introduce a method for predicting blowout based on the multifractal description of blowout.

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