Influence of the Interaction of Equivalence Ratio and Mass Flow Fluctuations on Flame Dynamics

2005 ◽  
Author(s):  
M. P. Auer ◽  
C. Hirsch ◽  
T. Sattelmayer
Keyword(s):  
Heat Release ◽  
Active Control ◽  
Mass Flow ◽  
Equivalence Ratio ◽  
Feedback Loop ◽  
Combustion Instabilities ◽  
Flame Dynamics ◽  
Secondary Fuel ◽  
Flow Oscillations ◽  
Mass Flows

Modern lean combustion systems are often prone to combustion instabilities — an interaction of acoustic waves, fluid dynamics and heat release oscillations. Mass flow oscillations are one important part of the feedback loop of combustion instabilities. Therefore, modulated mass flows of fuel or/and combustion air are main objectives in many studies on combustion instabilities and their active control (AIC, Active Instability Control). Flame response and flame transfer matrices are often determined by excitation of combustion air at various frequencies by sirens or loudspeakers. For the purpose of active control modulated secondary fuel is usually injected to the mass flow to dampen heat release fluctuations of the flame in order to de-couple the thermoacoustic feedback loop. This paper demonstrates the influence of modulated mass flows on the flame dynamics in an atmospheric test rig with a natural gas fired swirl burner. In the investigated cases the modulation of combustion air also result in equivalence ratio fluctuations due to choked main fuel injection. This combination has a tremendous effect on the flame dynamics. A model was developed to describe the interaction of equivalence ratio fluctuations and total mass flow oscillations and their influence on combustion instabilities. In experiments these equivalence ratio fluctuations were generated by injecting modulated secondary fuel. The derived model provides a deep insight into the driving mechanisms of combustion instabilities.

Active Control of Combustion Instabilities in a Matrix Burner Using Primary and Pilot Fuel and Acoustic Forcing

2010 ◽  
Author(s):  
Dieter Bohn ◽  
Nils Ohlendorf ◽  
Frank Weidner ◽  
James F. Willie
Keyword(s):  
Heat Release ◽  
Mass Flow ◽  
Gas Turbines ◽  
Open Loop ◽  
Nox Emissions ◽  
Combustion Instabilities ◽  
Loop Control ◽  
First Case ◽  
Pilot Fuel ◽  
Acoustic Forcing

Lean premixed flames applied in modern gas turbines leads to reduce NOx emissions, but at the same time they are more susceptible to combustion instabilities than diffusion flames. These oscillations cause pressure fluctuations with high amplitudes and unacceptable noise as well as the risk of component or even engine failure. They can lead to pockets of fuel being formed in the mixing chamber and to bad mixing, which leads to increase in emissions. This paper reports the successful decoupling of the pressure and heat release inside the combustion chamber of a matrix burner using two actuation techniques. This led to the successful attenuation of the dominant instability modes occurring inside the combustor of the matrix burner. In the first case, acoustic forcing was used to decouple the pressure and the heat release inside the combustor. This was achieved by using a loudspeaker to modulate the primary air mass flow. This was followed by using acoustic forcing in CFD to decouple the pressure and heat release inside the combustor. For the action of the loudspeaker, sinusoidal forcing was used to mimic the modulation action of the diaphragm of the loudspeaker. In the second case, a fast gaseous “on-off” injector was used to modulate the primary fuel mass flow. After this, pilot fuel modulation was used to stabilize the flame. The control law governing the primary and pilot fuel modulation is discussed in details. The effect of open loop control on NOx emissions in the burner is also reported and discussed.

Thermoacoustic Modeling of a Gas Turbine Using Transfer Functions Measured Under Full Engine Pressure

2010 ◽  
Vol 132 (11) ◽  
Author(s):  
Bruno Schuermans ◽  
Felix Guethe ◽  
Douglas Pennell ◽  
Daniel Guyot ◽  
Christian Oliver Paschereit
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Gas Turbine ◽  
Mass Flow ◽  
Equivalence Ratio ◽  
Transfer Functions ◽  
Test Facility ◽  
Optical Technique ◽  
Optical Techniques ◽  
Acoustic Technique

Thermoacoustic transfer functions of a full-scale gas turbine burner operating under full engine pressure have been measured. The excitation of the high-pressure test facility was done using a siren that modulated a part of the combustion airflow. Pulsation probes have been used to record the acoustic response of the system to this excitation. In addition, the flame’s luminescence response was measured by multiple photomultiplier probes and a light spectrometer. Three techniques to obtain the thermoacoustic transfer function are proposed and employed: two acoustic-optical techniques and a purely acoustic technique. The first acoustical-optical technique uses one single optical signal capturing the chemiluminescence intensity of the flame as a measure for the heat release in the flame. This technique only works if heat release fluctuations in the flame have only one generic source, e.g., equivalence ratio or mass flow fluctuations. The second acoustic-optical technique makes use of the different response of the flame’s luminescence at different optical wavelengths bands to acoustic excitation. It also works, if the heat release fluctuations have two contributions, e.g., equivalence ratio and mass flow fluctuation. For the purely acoustic technique, a new method was developed in order to obtain the flame transfer function, burner transfer function, and flame source term from only three pressure transducer signals. The purely acoustic method could be validated by the results obtained from the acoustic-optical techniques. The acoustic and acoustic-optical methods have been compared and a discussion on the benefits and limitations of each is given. The measured transfer functions have been implemented into a nonlinear, three-dimensional, time domain network model of a gas turbine with an annular combustion chamber. The predicted pulsation behavior shows a good agreement with pulsation measurements on a field gas turbine.

Influence of Air and Fuel Mass Flow Fluctuations in a Premix Swirl Burner on Flame Dynamics

2006 ◽  
Author(s):  
M. P. Auer ◽  
C. Hirsch ◽  
T. Sattelmayer
Keyword(s):  
Heat Release ◽  
Mass Flow ◽  
High Speed ◽  
Structural Changes ◽  
Flame Structure ◽  
Flame Length ◽  
Turbulent Flames ◽  
Swirl Burner ◽  
Fuel Mass ◽  
Flame Dynamics

This paper discusses the structural changes observed in oscillating premixed turbulent swirling flames and demonstrates the influence of modulated mass flows on the flame dynamics in a preheated atmospheric test rig with a natural gas fired swirl burner. The experimentally investigated self excited and forced combustion oscillations of swirl stabilized premixed flames show varying time delays between the acoustically driven mass flow oscillations and the integral heat release rate of the flame. High speed films of the OH*-chemiluminescence reveal how the flame structure changes with the oscillation frequency and the phase angle between the fuel mass flow oscillation and the total mass flow at the burner exit. These parameters are found determine the spatial and temporal heat release distribution and thus the net heat release fluctuation. Therefore, the spatial and temporal heat release distribution along the flame length has an influence on the thermoacoustic coupling, even in the case of acoustically compact flames. The observed phenomena are discussed further using an 1-d analytical model. It underscores that for swirl stabilized premixed turbulent flames the dynamics of the flow field perturbation play a major role in creating the effective heat release fluctuation.

The Effect of the Degree of Premixedness on Self-Excited Combustion Instability

2020 ◽  
Author(s):  
Adam Howie ◽  
Daniel Doleiden ◽  
Stephen Peluso ◽  
Jacqueline O’Connor
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Equivalence Ratio ◽  
Feedback Loop ◽  
Relative Phase ◽  
Common Strategy ◽  
Rate Oscillations ◽  
Recirculation Zones ◽  
Increased Susceptibility

Abstract The use of lean, premixed fuel and air mixtures is a common strategy to reduce NOx emissions in gas turbine combustors. However, this strategy causes an increased susceptibility to self-excited instability, which manifests as fluctuations in heat release rate, flow velocity, and combustor acoustics that oscillate in-phase in a feedback loop. This study considers the effect of the level of premixedness on the self-excited instability in a single-nozzle combustor. In this system, the fuel can be injected inside the nozzle to create a partially-premixed mixture or far upstream to create a fully-premixed mixture, varying the level of premixedness of the fuel and air entering the combustor. When global equivalence ratio is held constant, the cases with higher levels of premixing have higher instability amplitudes. Highspeed CH* chemiluminescence imaging shows that the flame for these cases is more compact and the distribution of the heat release rate oscillations is more concentrated near the corner of the combustor in the outer recirculation zones. Rayleigh index images, which are a metric for quantifying the relative phase of pressure and heat release rate oscillations, suggest that vortex rollup in the corner region is primarily responsible for determining instability characteristics when premixedness is varied. This finding is further supported through analysis of local flame edge dynamics.

Influence of the Combustor Aerodynamics on Combustion Instabilities From Equivalence Ratio Fluctuations

2002 ◽  
Vol 125 (1) ◽  
pp. 11-19 ◽  
Author(s):  
T. Sattelmayer
Keyword(s):  
Heat Release ◽  
Equivalence Ratio ◽  
Spatial Dispersion ◽  
Combustion System ◽  
Combustion Instabilities ◽  
Additional Mechanism ◽  
Wide Range ◽  
Unrealistic Assumption ◽  
Unstable Case ◽  
The Stability

Gas turbine combustors are often susceptible to self-excited oscillations, which lead to unacceptable levels of pressure, velocity, and heat release fluctuations. Although instabilities can occur in systems with locally constant equivalence ratio, it is very important to take into account the influence of equivalence ratio fluctuations, which are generated in the fuel air mixer in the unstable case. These fluctuations are convected into the flame and lead to an additional mechanism for the generation of heat release fluctuations. Moreover, entropy waves are produced in the flame, which travel through the combustor and generate additional pressure waves during the acceleration of the flow at the combustor exit. To date, available theories use the physically unrealistic assumption that the equivalence ratio waves as well as the entropy waves are convected downstream without any spatial dispersion due to the combustor aerodynamics. An analytical approach is presented, which allows us to take the spatial dispersion into consideration. For that purpose, the response of the burner and the combustor to an equivalence ratio impulse or an entropy impulse is calculated using the Laplace transformation and a more general transfer function for harmonic waves is derived. The obtained expression has three parameters, which represent the influence of the burner or the combustor aerodynamics, respectively. This equation can be used in numerical codes, which represent the combustion system through a network of acoustic multiports, if the equivalence ratio and the entropy are added to the vector of variables considered. The parameters required for the dynamic combustor model can be deduced from a detailed CFD analysis of the combustor flow in case of the application of the theory to a particular combustor design. As an example, a simple model combustor is used to demonstrate the application of the theory. It is highlighted how the spatial dispersion of the equivalence ratio and entropy fluctuations can be included in the stability analysis. The calculated examples reveal that the influence of both variables on the generation of instabilities is highly overpredicted if the spatial dispersion is not taken into account. Furthermore, it can be deduced from the study that burner and combustor designs with a wide range of convective time scales have advantages with respect to the stability of the combustor.

Influence of the Combustor Aerodynamics on Combustion Instabilities From Equivalence Ratio Fluctuations

2000 ◽  
Author(s):  
Thomas Sattelmayer
Keyword(s):  
Heat Release ◽  
Equivalence Ratio ◽  
Spatial Dispersion ◽  
Combustion System ◽  
Combustion Instabilities ◽  
Additional Mechanism ◽  
Wide Range ◽  
Unrealistic Assumption ◽  
Unstable Case ◽  
The Stability

Gas turbine combustors are often susceptible to self excited oscillations, which lead to unacceptable levels of pressure, velocity and heat release fluctuations. Although instabilities can occur in systems with locally constant equivalence ratio, it is very important to take into account the influence of equivalence ratio fluctuations, which are generated in the fuel air mixer in the unstable case. These fluctuations are convected into the flame and lead to an additional mechanism for the generation of heat release fluctuations. Moreover, entropy waves are produced in the flame, which travel through the combustor and generate additional pressure waves during the acceleration of the flow at the combustor exit. To date, available theories use the physically unrealistic assumption that the equivalence ratio waves as well as the entropy waves are convected downstream without any spatial dispersion due to the combustor aerodynamics. An analytical approach is presented, which allows to take the spatial dispersion into consideration. For that purpose, the response of the burner and the combustor to an equivalence ratio impulse or an entropy impulse is calculated using the Laplace transformation and a more general transfer function for harmonic waves is derived. The obtained expression has three parameters, which represent the influence of the burner or the combustor aerodynamics, respectively. This equation can be used in numerical codes, which represent the combustion system through a network of acoustic multiports, if the equivalence ratio and the entropy are added to the vector of variables considered. The parameters required for the dynamic combustor model can be deduced from a detailed CFD analysis of the combustor flow in case of the application of the theory to a particular combustor design. As an example, a simple model combustor is used to demonstrate the application of the theory. It is highlighted how the spatial dispersion of the equivalence ratio and entropy fluctuations can be included in the stability analysis. The calculated examples reveal that the influence of both variables on the generation of instabilities is highly overpredicted if the spatial dispersion is not taken into account. Furthermore, it can be deduced from the study, that burner and combustor designs with a wide range of convective time scales have advantages with respect to the stability of the combustor.

CFD Investigation of Hydrodynamic and Acoustic Instabilities of Bluff-Body Stabilized Turbulent Premixed Flames

2014 ◽  
Author(s):  
C. Y. Lee ◽  
R. S. Cant
Keyword(s):  
Heat Release ◽  
Coherent Structures ◽  
Equivalence Ratio ◽  
Large Scale ◽  
Bluff Body ◽  
Premixed Flames ◽  
Small Scale ◽  
Combustion Instabilities ◽  
Turbulent Premixed Flames ◽  
Turbulent Premixed

Combustion instabilities in propulsion systems are often manifested through high amplitude pressure oscillations that can severely compromise performance and even lead to mechanical failure. Such instability arises from the development of large-scale coherent structures and their breakdown into fine scale turbulence that can alter the flame structure and affect turbulent mixing. When in phase with the pressure, the modulated heat release rate fluctuations can drive the system to the point where it reaches a limit cycle. Using high fidelity CFD, the present investigation describes the occurrence of combustion-driven instability in bluff-body stabilized turbulent premixed flames, in which there is dynamic coupling between the preferred hydrodynamic modes and the acoustics of the duct. A URANS approach is adopted, using a second moment closure to solve for the anisotropic turbulent Reynolds stresses. This is combined with the Bray-Moss-Libby (BML) combustion model with a modified reaction rate closure that aims to capture the changes in the flame surface density due to external flow perturbations. Two different geometries are used for the investigation: the first is a laboratory-scale planar bluff-body flameholder [1]; and the second is the well-known Volvo afterburner experiment [2]. Four different conditions are presented to illustrate the various self-excited instabilities that can appear depending on the coupling mechanisms between the different fluid-mechanical and acoustic phenomena. For the planar geometry, a self-sustained hydrodynamic instability induced by large-scale coherent structures occurs under fuel-lean conditions. When the equivalence ratio is increased, the flame becomes strongly wrinkled due to velocity perturbations arising from the Kelvin-Helmholtz (K-H) instability of the shear layer. The combustion heat release becomes modulated such that its phase relationship with the pressure fluctuations is sufficient to trigger thermoacoustic instability. For the Volvo experiment, symmetric shedding takes place and an acoustic mode of the duct is excited when the mixture strength is lean. At higher equivalence ratio, the flame is perturbed by the hydrodynamic instabilities of the most amplified mode. Small scale structures can be seen in the vicinity of the flameholder, and larger fluctuations in the flame occur further downstream. No appreciable feedback from the acoustic modes is present to sustain combustion instabilities.

Thermoacoustic Modeling of a Gas Turbine Using Transfer Functions Measured at Full Engine Pressure

2009 ◽  
Author(s):  
Bruno Schuermans ◽  
Felix Guethe ◽  
Douglas Pennel ◽  
Daniel Guyot ◽  
Christian Oliver Paschereit
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Gas Turbine ◽  
Mass Flow ◽  
Equivalence Ratio ◽  
Transfer Functions ◽  
Optical Methods ◽  
Test Facility ◽  
Optical Technique ◽  
Acoustic Technique

Thermoacoustic transfer functions have been measured of a full-scale gas turbine burner operating at full engine pressure. Excitation of the high-pressure test facility was done using a siren that modulated part of the combustion airflow. Pulsation probes have been used to record the acoustic response of the system to this excitation. In addition, the flame’s luminescence response was measured by multiple photomultiplier tubes and a light spectrometer. Three techniques to obtain the thermoacoustic transfer function are proposed and employed: two combined acoustical-optical technique and a purely acoustic technique. The first acoustical-optical technique uses one single optical signal capturing the chemiluminescence intensity of the flame as a measure for the heat release in the flame. It only works, if heat release fluctuations in the flame have only one contribution, e.g. equivalence ratio or mass flow fluctuations. The second acoustic-optical acoustic-optical technique makes use of the different response of the flame’s luminescence at different optical wavelengths bands to acoustic excitation. It also works, if the heat release fluctuations have two contributions, e.g. equivalence ratio and mass flow fluctuation. For the purely acoustic technique, a new method was developed in order to obtain the flame transfer function, burner transfer function and flame source term from only three pressure transducer signals. The purely acoustic method could be validated by the results obtained from the acoustic-optical techniques. The acoustic and acoustic-optical methods have been compared and a discussion on the benefits and limitations of the methods is given. The measured transfer functions have been implemented into a non-linear, three-dimensional, time domain network model of a gas turbine with an annular combustion chamber. The predicted pulsation behavior shows a good agreement with pulsation measurements on a field gas turbine.

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