Experimental study on self-excited thermoacoustic instabilities and intermittent switching of azimuthal and longitudinal modes in an annular combustor

Thermoacoustic instabilities pose a major threat to modern gas turbines. The use of acoustic dampers, like Helmholtz resonators, has proven useful for the mitigation of such instabilities. However, assessing the effect of acoustic dampers on thermoacoustic modes in annular combustion chambers remains an intricate task. This results from the implicit nature of the thermoacoustic Helmholtz equation associated with the high number of possible parameter values for the positioning of the dampers and their impedance design. In the present work, the principal challenges of the effective placement and the design of the impedance of acoustic dampers in annular chambers are discussed. This includes the choice of an appropriate objective function for the optimization, the combinatorial challenges when dealing with different possible damper arrangements, and the numerical complexities when using the thermoacoustic Helmholtz equation to approach this issue. As a key aspect, the paper proposes a new adjoint-based approach to tackle these problems. The new algorithm establishes algebraic models that predict the effect of acoustic dampers on the growth rates of the thermoacoustic modes. The theory is exemplified on the basis of a generic annular combustor model with 12 burners.

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Numerical and Experimental Flame Stabilization Analysis in the New Spinning Combustion Technology Framework

Volume 4A: Combustion, Fuels, and Emissions ◽

10.1115/gt2020-15035 ◽

2020 ◽

Author(s):

Pasquale Walter Agostinelli ◽

Yi Hao Kwah ◽

Stephane Richard ◽

Gorka Exilard ◽

James R. Dawson ◽

...

Keyword(s):

Film Cooling ◽

Flame Stabilization ◽

Fuel Injector ◽

Azimuthal Component ◽

Thermoacoustic Instabilities ◽

Annular Combustor ◽

Combustion Technology ◽

Experimental Side ◽

Spinning Combustion ◽

Swirled Flames

Abstract Global warming, climate change and pollution are burning environmental issues. To reduce the carbon footprint of the aviation sector, aeronautical companies have been striving to lower engine emissions via the development of reliable lean combustors. In this context, effort has been devoted to the better understanding of various flame dynamics with emphasis on thermoacoustic instabilities, lean blow-off and extinctions. In line with this effort, Safran Helicopter Engines has recently developed and patented the revolutionary spinning combustion technology (SCT) for its next generation of combustors. This technology has indeed great flexibility when it comes to ignition and blow-off capabilities. To better understand the various physical mechanisms occurring in a SCT combustor, a joint numerical and experimental analysis of the flame stabilization in this spinning combustion technology framework has been devised. On the experimental side, the NTNU atmospheric annular combustor has been modified to introduce a relevant azimuthal component of velocity while operating under premixed fuel conditions, following the SCT concept. Note that to reduce temperature at the backplane of the chamber, film cooling is incorporated to avoid fuel injector damage. On the numerical side, high fidelity Large Eddy Simulations of the test bench have been carried out with the AVBP code developed at CERFACS, providing insights on the flame stabilization in this unique SCT geometry. In particular, it is noted that there is a strong interaction between the cooling film and the highly swirled flames exiting from the fuel injector bend. In that respect, changing the injector or global equivalence ratios while operating the SCT is shown to affect the combustion of this design.

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Weakly nonlinear analysis of thermoacoustic instabilities in annular combustors

Journal of Fluid Mechanics ◽

10.1017/jfm.2016.494 ◽

2016 ◽

Vol 805 ◽

pp. 52-87 ◽

Cited By ~ 26

Author(s):

G. Ghirardo ◽

M. P. Juniper ◽

J. P. Moeck

Keyword(s):

Gas Turbines ◽

Practical Importance ◽

Industrial Applications ◽

Local Pressure ◽

Weakly Nonlinear ◽

Acoustic Damping ◽

Thermoacoustic Instabilities ◽

Rotationally Symmetric ◽

Flame Response ◽

Annular Combustor

Rotationally symmetric annular combustors are of practical importance because they generically resemble combustion chambers in gas turbines, in which thermoacoustically driven oscillations are a major concern. We focus on azimuthal thermoacoustic oscillations and model the fluctuating heat release rate as being dependent only on the local pressure in the combustion chamber. We study the dynamics of the annular combustor with a finite number of compact flames equispaced around the annulus, and characterize the flames’ response with a describing function. We discuss the existence, amplitude and the stability of standing and spinning waves, as a function of: (i) the number of the burners; (ii) the acoustic damping in the chamber; (iii) the flame response. We present the implications for industrial applications and the future direction of investigations. We then present as an example the first theoretical study of thermoacoustic triggering in annular combustors, which shows that rotationally symmetric annular chambers that are thermoacoustically unstable do not experience only stable spinning solutions, but can also experience stable standing solutions. We finally test the theory on one experiment with good agreement.

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A Numerical and an Experimental Study for Optimization of a Small Annular Combustor

Journal of Power and Energy Systems ◽

10.1299/jpes.2.921 ◽

2008 ◽

Vol 2 (3) ◽

pp. 921-933 ◽

Cited By ~ 1

Author(s):

Norihiko IKI ◽

Andrea GRUBER ◽

Hiro YOSHIDA

Keyword(s):

Experimental Study ◽

Annular Combustor

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Investigation of Thermoacoustic Stability Limits of an Annular Gas Turbine Combustor Test-Rig With and Without Helmholtz-Resonators

Volume 2: Turbo Expo 2005 ◽

10.1115/gt2005-68246 ◽

2005 ◽

Cited By ~ 16

Author(s):

Joachim Lepers ◽

Werner Krebs ◽

Bernd Prade ◽

Patrick Flohr ◽

Giacomo Pollarolo ◽

...

Keyword(s):

Gas Turbine ◽

Gas Turbines ◽

Thermal Power ◽

Power Input ◽

Test Rig ◽

Combustion Chambers ◽

Thermoacoustic Instabilities ◽

Helmholtz Resonators ◽

Annular Combustor ◽

Computational Analyses

Providing gas turbine combustion chambers with Helmholtz-resonators is a promising approach for extending the operating range of gas turbines towards higher thermal power input whilst minimizing the risk of thermoacoustic instabilities. The work currently being reported gives an overview of experimental and computational analyses carried out for a full annular combustor test-rig located at Gioia del Colle in Italy. The thermoacoustic stability characteristics of this test-rig were thoroughly analyzed both for a base configuration without Helmholtz-resonators and for an extended configuration with 14 Helmholtz-resonators. An increase of power input to the combustor by 8.5–20% can be realized when the test-rig is equipped with resonators. The experimental analyses are reproduced by a computational model.

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High-Frequency Instabilities in Cylindrical Flame Tubes: Feedback Mechanism and Damping

Volume 1A: Combustion, Fuels and Emissions ◽

10.1115/gt2013-94064 ◽

2013 ◽

Cited By ~ 10

Author(s):

Joachim Schwing ◽

Thomas Sattelmayer

Keyword(s):

High Frequency ◽

Thermal Power ◽

Feedback Mechanism ◽

Acoustic Velocity ◽

Decay Rates ◽

Operating Conditions ◽

Experimental Results ◽

Positive Contribution ◽

Thermoacoustic Instabilities ◽

Longitudinal Modes

Thermoacoustic instabilities are a major concern in gas turbine combustion chambers today. In the last decades research interest in thermoacoustic instabilities has focused on low frequencies. The feedback mechanisms related to longitudinal modes are for the most part understood. Transverse modes, though, have not been studied to a large extent in the past. However, interest has been rising in the last few years. But little is known about the thermoacoustic feedback of high-frequency instabilities. Our previous publications characterized the flow and the flame at the eigenfrequency of high-frequency instabilities. There, a feedback mechanism was derived from the experimental results and discussed: the acoustic velocity leads to a periodic displacement of the flame resulting in a positive contribution to the Rayleigh criterion. Thus, the thermoacoustic feedback couples to the acoustic velocity, but not to the pressure or a periodic vortex formation. Different means can be derived from the model to influence high-frequency instabilities: Helmholtz dampers are used to shift the onset of instabilities to increased thermal power. With loudspeakers naturally stable operating points are excited. Stopping the excitation and evaluating the signal, decay rates are analyzed. Decay rates — i.e. stability margins — are compared for different operating conditions. Switching from perfect premixing to technical premixing, the radial profile of the fuel-to-air ratio can be changed. The influence of a lean core flow compared to a homogeneous mixture on the feedback is investigated and its impact on the instabilities is compared to the model. The observations reflect, what is predicted by the model. Velocity coupling, at least a significant part of the feedback mechanism for transverse high-frequency instabilities, is supported by the experimental results.

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Prediction of Thermoacoustic Instabilities With Focus on the Dynamic Flame Behavior for the 3A-Series Gas Turbine of Siemens KWU

Volume 2: Coal, Biomass and Alternative Fuels; Combustion and Fuels; Oil and Gas Applications; Cycle Innovations ◽

10.1115/99-gt-111 ◽

1999 ◽

Cited By ~ 15

Author(s):

Uwe Krüger ◽

Jens Hüren ◽

Stefan Hoffmann ◽

Werner Krebs ◽

Dieter Bohn

Keyword(s):

Gas Turbine ◽

Gas Turbines ◽

Analytical Data ◽

Flow Distribution ◽

Transient Behavior ◽

Axial Direction ◽

Step Function ◽

Lean Premixed Combustion ◽

Thermoacoustic Instabilities ◽

Annular Combustor

Self-induced combustion driven oscillations are a crucial challenge in the design of advanced gas turbine combustors. Lean premixed combustion, typically used in modern gas turbines, has a pronounced tendency to produce these instabilities. Thus, the prediction of these thermoacoustic instabilities in the design phase of an engine becomes more and more important. A method based on linear acoustic four-pole elements to predict the instabilities of the ring combustor of the Siemens 3A-series gas turbines will be presented in this paper. The complex network includes the entire system starting from both compressor outlet and fuel supply system and ending at the turbine inlet. Most of the transfer elements can be described by analytical data. Nevertheless, the most important elements, “flame” and “combustion chamber”, have to be investigated more in detail due to their complex 3D acoustics. For the turbulent, premixed and swirled flame, a numerical simulation of the transient behavior after a sudden jump in mass flow at the inlet (step-function approach) is used to obtain the flame frequency response for axial direction as well as circumferential direction. This method has been verified for numerous different flame types (Krüger et al. (1998), Bohn et al. (1997), Bohn et al. (1996)). The four-pole element of the annular combustor is derived by an eigenfrequency analysis of the chamber, including a numerical predicted temperature and flow distribution. The results show the principle possibilities of the instability analysis described. The frequencies predicted correspond well with experience from engine test fields. The importance of several elements for self-induced combustion driven oscillations is pointed out clearly.

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Multiphase Flow LES Study of the Fuel Split Effects on Combustion Instabilities in an Ultra Low-NOx Annular Combustor

Volume 4B: Combustion, Fuels and Emissions ◽

10.1115/gt2015-44139 ◽

2015 ◽

Cited By ~ 2

Author(s):

M. Bauerheim ◽

T. Jaravel ◽

L. Esclapez ◽

E. Riber ◽

L. Y. M. Gicquel ◽

...

Keyword(s):

Multiphase Flow ◽

Analytical Model ◽

Vortex Shedding ◽

Liquid Fuel ◽

Gaseous Fuel ◽

Combustion Instabilities ◽

Thermoacoustic Instabilities ◽

Flame Dynamics ◽

Global Correlation ◽

Annular Combustor

This paper describes the application of a coupled Acoustic model/LES approach to assess the effect of fuel split on combustion instabilities in an industrial ultra low-NOx annular combustor. Multiphase flow LES and an analytical model (ATACAMAC) to predict thermoacoustic modes are combined to reveal and compare two mechanisms leading to thermoacoustic instabilities: 1) a gaseous type in the multi-point zone where acoustics generates vortex shedding, wrinkling the flame front and 2) a multiphase flow type in the pilot zone where acoustics can modify the liquid fuel transport and the evaporation process leading to gaseous fuel oscillations. The aim of this paper is to investigate these mechanisms by changing the fuel split (from 5% to 20%, mainly affecting the pilot zone and mechanism 2) and therefore assess which mechanism controls the flame dynamics. First, the eigenmodes of the annular chamber are investigated using the analytical model and validated by 3D Helmholtz simulations. Then, multiphase flow LES are forced at the eigenfrequencies of the chamber for three different fuel split values. Key features of the flow and flame dynamics are investigated. Results show that acoustic forcing generates gaseous fuel oscillations which strongly depend on the fuel split parameter. However, the global correlation between heat release fluctuations and acoustics highlights no dependency on the fuel split staging. It suggests that vortex shedding in the multi-point zone, almost not depending on the fuel split here, is the main feature controlling the flame dynamics for this LEMCOTEC engine.

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Acoustic Damper Placement and Tuning for Annular Combustors: An Adjoint-Based Optimization Study

Volume 4B: Combustion, Fuels and Emissions ◽

10.1115/gt2016-57799 ◽

2016 ◽

Author(s):

Georg A. Mensah ◽

Jonas P. Moeck

Keyword(s):

Helmholtz Equation ◽

Gas Turbines ◽

Algebraic Models ◽

Combustion Chambers ◽

Thermoacoustic Instabilities ◽

Helmholtz Resonators ◽

Major Threat ◽

Optimization Study ◽

Annular Combustor ◽

Parameter Values

Thermoacoustic instabilities pose a major threat to modern gas turbines. The use of acoustic dampers, like Helmholtz resonators, has proven useful for the mitigation of such instabilities. However, assessing the effect of acoustic dampers on thermoacoustic modes in annular combustion chambers remains an intricate task. This results from the implicit nature of the thermoacoustic Helmholtz equation associated with the high number of possible parameter values for the positioning of the dampers and their impedance design. In the present work, the principal challenges of the effective placement and the design of the impedance of acoustic dampers in annular chambers are discussed. This includes the choice of an appropriate objective function for the optimization, the combinatorial challenges when dealing with different possible damper arrangements, and the numerical complexities when using the thermoacoustic Helmholtz equation to approach this issue. As a key aspect, the paper proposes a new adjoint-based approach to tackle these problems. The new algorithm establishes algebraic models that predict the effect of acoustic dampers on the growth rates of the thermoacoustic modes. The theory is exemplified on the basis of a generic annular combustor model with 12 burners.

Download Full-text