scholarly journals Swirler geometry effects (dh/do ratio) on synthetic gas flames. Part 2: dynamic flame behaviour at external y altered acoustic conditions

Energetika ◽  
2021 ◽  
Vol 67 (1) ◽  
Author(s):  
Harun Yilmaz ◽  
Omer Cam ◽  
Ilker Yilmaz
Keyword(s):  
Boundary Conditions ◽  
Pressure Sensors ◽  
Acoustic Energy ◽  
Acoustic Boundary Conditions ◽  
Acoustic Damping ◽  
Pressure Profiles ◽  
Flame Response ◽  
Scale Variable ◽  
A Minor ◽  
Flame Behaviour

In a combustion device, unsteady heat release causes acoustic energy to increase when acoustic damping (energy loss) is not that effective, and, as a result, thermo-acoustic flame instabilities occur. In this study, effects of the swirler dh/do ratio (at different swirl numbers) on dynamic flame behaviour of the premixed 20%CNG/30%H2/30%CO/20%CO2 mixture under externally altered acoustic boundary conditions and stability limits (flashback and blowout equivalence ratios) of such mixture were investigated in a laboratory-scale variable geometric swirl number combustor. Therefore, swirl generators with different dh/do ratios (0.3 and 0.5) and geometric swirl numbers (0.4, 0.6, 0.8, 1.0 1.2 and 1.4) were designed and manufactured. Acoustic boundary conditions in the combustion chamber were altered using loudspeakers, and flame response to these conditions was perceived using photodiodes and pressure sensors. Dynamic flame behaviour of respective mixture was evaluated using luminous intensity and pressure profiles. Results showed that the dh/do ratio has a minor impact on dynamic flame behaviour.

Energetically Consistent Computation of Combustor Stability with a Model Consisting of a Helmholtz-Fem-Domain and a Low-Order Network

2021 ◽  
Author(s):  
Gerrit Heilmann ◽  
Christoph Hirsch ◽  
Thomas Sattelmayer
Keyword(s):  
Boundary Conditions ◽  
Energy Flux ◽  
Fundamental Problem ◽  
Acoustic Energy ◽  
Special Treatment ◽  
Transfer Matrices ◽  
Mean Flow ◽  
Acoustic Damping ◽  
Spatially Resolved ◽  
Low Order

Abstract An efficient approach for the detection of the acoustic damping of gas turbine combustors is the combination of spatially resolved FEM approaches based on the Helmholtz equation with low-order networks for all elements leading to acoustic damping. A fundamental problem of such hybrid approaches is that the flow is considered in the networks, but not in the spatially resolved FEM area. Without special treatment of the boundary conditions this leads to serious errors in the calculation of the damping rate. The purpose of the paper is the derivation of the required correction procedures, which allow the energetically consistent formulation of such hybrid models and lead to correct damping rates. The time averaged equation of acoustic energy flux is expressed in terms of reflection coefficients and compared to the equivalent formulation for vanishing mean flows. An existing transformation for boundary conditions to obtain equal energy flux at the interface between network and Helmholtz domain is analyzed in detail. The findings are then used to derive energetically consistent transformations of transfer matrices to couple two FEM domains via a network model. The relevance of energetically consistent transfer matrices for stability analysis is demonstrated with a generic test case. The central partition is acoustically characterized via low order models considering mean flow. The resulting acoustic two-port is transformed to obtain an energetically consistent transfer matrix for a subsequent FEM discretized eigenvalue analysis of the remaining geometry. The eigenvalues of energetically consistent calculations are finally compared to eigenvalues of energetically inconsistent setups.

Energetically Consistent Computation of Combustor Stability With a Model Consisting of a Helmholtz-FEM-Domain and a Low-Order Network

2020 ◽  
Author(s):  
Gerrit Heilmann ◽  
Christoph Hirsch ◽  
Thomas Sattelmayer
Keyword(s):  
Boundary Conditions ◽  
Energy Flux ◽  
Fundamental Problem ◽  
Acoustic Energy ◽  
Special Treatment ◽  
Transfer Matrices ◽  
Mean Flow ◽  
Acoustic Damping ◽  
Spatially Resolved ◽  
Low Order

Abstract An efficient approach for the detection of the acoustic damping of gas turbine combustors is the combination of spatially resolved FEM approaches based on the Helmholtz equation with low-order networks for all elements leading to acoustic damping. A fundamental problem of such hybrid approaches is that the flow is considered in the networks, but not in the spatially resolved FEM area. Without special treatment of the coupling plane and the boundary conditions this leads to serious errors in the calculation of the damping rate. The purpose of the paper is the derivation of the required correction procedures, which allow the energetically consistent formulation of such hybrid models and lead to correct damping rates. The time averaged equation of acoustic energy flux for non-uniform fluid flows is expressed in terms of reflection coefficients and compared to the equivalent formulation for vanishing mean flows. An existing transformation for boundary conditions to obtain equal energy flux at the interface between network and Helmholtz domain is analyzed in detail. The findings are then used to derive an energetically consistent transformation of transfer matrices to couple two FEM domains via a network model. The relevance of energetically consistent transfer matrices for stability analysis is demonstrated with a generic test case. The central partition is acoustically characterized via a low order model considering mean flow. The resulting acoustic two-port is transformed to obtain an energetically consistent transfer matrix for a subsequent FEM discretized eigenvalue analysis of the remaining geometry. The eigenvalues of energetically consistent calculations are finally compared to eigenvalues of energetically inconsistent setups.

Protection and Identification of Thermoacoustic Azimuthal Modes

2020 ◽  
Author(s):  
G. Ghirardo ◽  
F. Gant ◽  
F. Boudy ◽  
M. R. Bothien
Keyword(s):  
Acoustic Field ◽  
Invariant Manifolds ◽  
Pressure Sensors ◽  
Fourier Component ◽  
Combustion Noise ◽  
Order Problem ◽  
Acoustic Damping ◽  
Protection Scheme ◽  
Flame Response ◽  
Low Order

Abstract This paper first characterizes the acoustic field of two annular combustors by means of data from acoustic pressure sensors. In particular the amplitude, orientation, and nature of the acoustic field of azimuthal order n is characterized. The dependence of the pulsation amplitude on the azimuthal location in the chamber is discussed, and a protection scheme making use of just one sensor is proposed. The governing equations are then introduced, and a low-order model of the instabilities is discussed. The model accounts for the nonlinear response of M distinct flames, for system acoustic losses by means of an acoustic damping coefficient α and for the turbulent combustion noise, modelled by means of the background noise coefficient σ. Keeping the response of the flames arbitrary and in principle different from flame to flame, we show that, together with α and σ, only the sum of their responses and their 2n Fourier component in the azimuthal direction affect the dynamics of the azimuthal instability. The existing result that only this 2n Fourier component affects the stability of standing limit-cycle solutions is recovered. It is found that this result applies also to the case of a non-homogeneous flame response in the annulus, and to flame responses that respond to the azimuthal acoustic velocity. Finally, a parametric flame model is proposed, depending on a linear driving gain β and a nonlinear saturation constant κ. The model is first mapped from continuous time to discrete time, and then recast as a probabilistic Markovian model. The identification of the parameters {α, β, κ, σ} is then carried out on engine timeseries data. The optimal four parameters {α, σ, β, κ} are estimated as the values that maximize the data likelihood. Once the parameters have been estimated, the phase space of the identified low-order problem is discussed on selected invariant manifolds of the dynamical system.

The Role of Nonlinear Acoustic Boundary Conditions in Combustion/Acoustic Coupled Instabilities

2009 ◽  
Author(s):  
T. Schuller ◽  
N. Tran ◽  
N. Noiray ◽  
D. Durox ◽  
S. Ducruix ◽  
...  
Keyword(s):  
Boundary Conditions ◽  
Perforated Plate ◽  
Mode Switching ◽  
Describing Function ◽  
Complex Flow ◽  
Acoustic Boundary Conditions ◽  
Practical Applications ◽  
Bias Flow ◽  
Flow Interactions ◽  
Flame Response

Triggering, frequency shifting, mode switching and hysteresis are commonly encountered during self-sustained oscillations in combustors. These mechanisms cannot be anticipated from classical linear stability analysis and the nonlinear flame response to incident flow perturbations is often invoked to interpret these features. However, the flame may not be solely responsible for nonlinearities. Recent studies indicate that interactions with boundaries can be influenced by the perturbation level and that this needs to be considered. The nonlinear response of acoustic boundary conditions to flow perturbations is here exemplified in two configurations which typify practical applications. The first corresponds to a perforated plate backed by a cavity conveying a bias flow and the second corresponds to a set of flames stabilized at a burner outlet. These systems are submitted to acoustic perturbations of increasing amplitudes as can be encountered during unstable operation. It shown that these terminations can be characterized by an impedance featuring an amplitude dependent response. The classical linear impedance Z(ω) is then replaced by its nonlinear counterpart an Impedance Describing Function (IDF), which depends on the perturbation level input Z(ω, |p′| or |u′|). Using this concept, it is shown that the passive perforated plate optimized to damp instabilities of small amplitudes may eventually loose its properties when submitted to large sound pressure levels and that the flame response shifts when the amplitude of incoming flow perturbations is amplified. The influence of these nonlinear elements on the stability of a generic burner is then examined using a methodology which extends a previous analysis based on the Flame Describing Function (FDF) to systems with complex flow interactions at the boundaries.

Effect of Cooling Liner on Acoustic Energy Absorption and Flame Response

2010 ◽  
Author(s):  
Umesh Bhayaraju ◽  
Johannes Schmidt ◽  
Karthik Kashinath ◽  
Simone Hochgreb
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Acoustic Waves ◽  
Bluff Body ◽  
Acoustic Energy ◽  
Combustion Instabilities ◽  
Acoustic Damping ◽  
Lean Combustion ◽  
Bias Flow ◽  
Flame Response

Gas turbine combustors with lean combustion injectors are prone to thermo-acoustic/combustion instabilities. Several passive techniques have been developed to control combustion instabilities, such as using Helmholtz resonators or viscous dampers using perforated liners that have potential for broadband acoustic damping. In this paper the role of single-walled cooling liners is considered in the damping of acoustic waves and on the flame transfer function in a sample bluff-body burner. Three liner geometries are considered: no bias flow (solid liner), normal effusion holes, and grazing effusion holes at 25° inclination. Cold flow experiments with speaker forcing are carried out to characterise the absorption properties of the liner and compared with an acoustic network model. The results show that whereas the bulk of the acoustic losses is due to the vortex recirculation zones, the liners contribute significantly to the absorption over a wide area of the frequency range. The flame transfer function gain is measured as a function of bias flow for a given operating condition of the burner. The experiments show that for the geometry considered, the global flame transfer function is little affected by cooling except in the case of the normal flow holes. Further analysis shows that whereas the total flame transfer function is not affected, the flame heat release becomes more spatially distributed along the axial length, and a 1D flame response shows distinct modes corresponding to the modal heat release locations.

Experimentally Determining the Acoustic Damping Rates of a Combustor With a Swirl Stabilized Lean Premixed Flame

2015 ◽  
Author(s):  
Nicolai V. Stadlmair ◽  
Michael Wagner ◽  
Christoph Hirsch ◽  
Thomas Sattelmayer
Keyword(s):  
Probability Distributions ◽  
Dynamic Pressure ◽  
Pressure Sensors ◽  
Stability Margin ◽  
Open Loop ◽  
Decay Rates ◽  
Acoustic Energy ◽  
Test Rig ◽  
Acoustic Damping ◽  
Lean Premixed

In this study, we employ a method based on Bayesian statistics to determine the rate of acoustic decay from dynamic pressure measurements inside a combustor. It is common, that lean premixed flames tend to drive thermoacoustic instabilities at specific eigenfrequencies. Hence, the dissipation of acoustic energy inside the combustor, its absorption at the boundaries and its transfer over the in- and outlets must always exceed the acoustic excitation from the flame to avoid pulsations. Quantitative measures for the level of stability are of high technical relevance. In that context the occurring eigenfrequencies and their damping rates are important indicators for the stability margin of gas turbine combustors. A modular swirl burner is investigated in an atmospheric single burner test rig under lean premixed conditions. For the experimental determination of the damping rates, a siren is used to externally excite resonant frequencies of the combustion system. After interrupting the forcing abruptly, time series of the decaying signals are recorded by dynamic pressure sensors inside the combustion chamber. For the analysis of this data, an algorithm based on a Bayesian network approach, which uses a Gibbs Sampler is employed. Probability distributions of frequencies and decay rates are obtained with the Markov-Chain Monte-Carlo (MCMC) method. For the investigated configuration, the influence of the acoustic boundary conditions and the preheat temperature on eigenmodes and damping rates is evaluated. Finally, the results are compared to a network model of the test rig. With that approach, the Open-Loop Gain (OLG) is evaluated for the frequency range of interest. Eigenfrequencies as well as their corresponding damping rates are obtained from Nyquist analysis.

Low-Order Network Modeling for Annular Combustors Exhibiting Longitudinal and Circumferential Modes

2018 ◽  
Author(s):  
Dong Yang ◽  
Aimee S. Morgans
Keyword(s):  
Boundary Conditions ◽  
Acoustic Waves ◽  
Acoustic Boundary Conditions ◽  
Thermoacoustic Instability ◽  
Longitudinal Modes ◽  
Before And After ◽  
Flame Response ◽  
Annular Combustor ◽  
Set Up ◽  
Low Order

Modern gas turbine combustors often have annular geometries. These are able to sustain thermoacoustic modes which vary in both the longitudinal and circumferential directions. Effects such as nonlinearity of the flame response to perturbations and differing burners around the annulus lead to the coupling of acoustic modes with different circumferential mode numbers. Such coupling renders differing spatial patterns of instability possible — for example purely longitudinal modes, circumferential standing modes, circumferential spinning modes, mixed modes and slanted modes. Accurately predicting the spatial pattern of limit cycle oscillations resulting from thermoacoustic instability remains an open challenge. This work develops a frequency domain low-order thermoacoustic network model for annular combustors which is notable in (i) accounting for both longitudinal and circumferential modes and (ii) allowing for generic acoustic boundary conditions at either end of the network. Linear acoustic waves are considered, with the different circumferential wavenumbers decoupled for sections both before and after the flames. Modal coupling occurs only at the flames, and is accounted for by summing all modal contributions prior to application of the flame models, and decomposing back into circumferential modes after application of flow conservation equations across the flames. By applying acoustic boundary conditions at either end of the network, an eigenvalue system is established which allows the thermoacoustic modes of the whole combustion system to be analysed. This low order modelling approach is applied to a simplified annular combustor set-up and is demonstrated to be able to capture limit cycles exhibiting longitudinal modes, circumferential spinning modes, circumferential standing modes and even the recently identified slanted modes.

Acoustic Energy Balance During the Onset, Growth and Saturation of Thermoacoustic Instabilities

2020 ◽  
Author(s):  
R. Gaudron ◽  
D. Yang ◽  
A. S. Morgans
Keyword(s):  
Energy Balance ◽  
Network Models ◽  
Acoustic Energy ◽  
Dimensionless Numbers ◽  
Weakly Nonlinear ◽  
Acoustic Damping ◽  
Thermoacoustic Instabilities ◽  
Wide Range ◽  
Acoustic Absorption Coefficient ◽  
Flame Describing Function

Abstract Thermoacoustic instabilities can occur in a wide range of combustors and are prejudicial since they can lead to increased mechanical fatigue or even catastrophic failure. A well-established formalism to predict the onset, growth and saturation of such instabilities is based on acoustic network models. This approach has been successfully employed to predict the frequency and amplitude of limit cycle oscillations in a variety of combustors. However, it does not provide any physical insight in terms of the acoustic energy balance of the system. On the other hand, Rayleigh’s criterion may be used to quantify the losses, sources and transfers of acoustic energy within and at the boundaries of a combustor. However, this approach is cumbersome for most applications because it requires computing volume and surface integrals and averaging over an oscillation cycle. In this work, a new methodology for studying the acoustic energy balance of a combustor during the onset, growth and saturation of thermoacoustic instabilities is proposed. The two cornerstones of this new framework are the acoustic absorption coefficient Δ and the cycle-to-cycle acoustic energy ratio λ, both of which do not require computing integrals. Used along with a suitable acoustic network model, where the flame frequency response is described using the weakly nonlinear Flame Describing Function (FDF) formalism, these two dimensionless numbers are shown to characterize: 1) the variation of acoustic energy stored within the combustor between two consecutive cycles, 2) the acoustic energy transfers occurring at the combustor’s boundaries and 3) the sources and sinks of acoustic energy located within the combustor. The acoustic energy balance of the well-documented Palies burner is then analyzed during the onset, growth and saturation of thermoacoustic instabilities using this new methodology. It is demonstrated that this new approach allows a deeper understanding of the physical mechanisms at play. For instance, it is possible to determine when the flame acts as an acoustic energy source or sink, where acoustic damping is generated, and if acoustic energy is transmitted through the boundaries of the burner.

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