Transfer Function Measurements on a Swirl Stabilized Premix Burner in an Annular Combustion Chamber

2004 ◽  
Author(s):  
Klaas Kunze ◽  
Christoph Hirsch ◽  
Thomas Sattelmayer
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Combustion Chamber ◽  
Acoustic Waves ◽  
Velocity Fluctuation ◽  
Transfer Functions ◽  
Flow Direction ◽  
Near Field ◽  
Thermal Power ◽  
Annular Combustor

A generic swirl stabilized premix burner for natural gas is experimentally investigated in both a single burner test rig and in an annular combustion chamber. Flame transfer functions are measured relating the fluctuation of the flame heat release to the axial velocity fluctuation at the burner outlet. The OH-chemiluminescence signal of the flame, captured with a photomultiplier tube, is taken as an estimate for flame heat release, whereas the velocity fluctuation is measured with a hot wire probe. As integral measurements of the entire flame reveal important differences between the single burner and the annular combustor, locally resolved measurements are performed observing slices of the flame that are perpendicular to the main flow direction at a variable distance from the burner outlet. In both the single and the annular combustor a near field and a far field of the dynamic flame behavior can be distinguished. The annular combustor flame has a larger near field than the single combustor flame and a different shape in the presence of circumferential acoustic waves. Variation of swirl, thermal power and mass flow and comparison of the steady state heat release distribution within the flames lead to the result that the effective swirl in the annular combustor is lower than for the identical burner in the single burner combustor. When the difference in swirl is compensated for by modifying the burner configuration in the annular combustion chamber the flame transfer function is still not equal to the single combustor flame. The remaining difference can be attributed to the circumferential acoustic waves in the annular combustor which influence the flame shape.

Measurement and Analysis of Flame Transfer Function in a Sector Combustor Under High Pressure Conditions

2003 ◽  
Author(s):  
W. S. Cheung ◽  
G. J. M. Sims ◽  
R. W. Copplestone ◽  
J. R. Tilston ◽  
C. W. Wilson ◽  
...  
Keyword(s):  
High Pressure ◽  
Transfer Function ◽  
Heat Release ◽  
Gas Turbine ◽  
Mass Flow ◽  
Atmospheric Pressure ◽  
Acoustic Waves ◽  
Gas Turbines ◽  
Transfer Functions ◽  
Unsteady Pressure

Lean premixed prevaporised (LPP) combustion can reduce NOx emissions from gas turbines, but often leads to combustion instability. A flame transfer function describes the change in the rate of heat release in response to perturbations in the inlet flow as a function of frequency. It is a quantitative assessment of the susceptibility of combustion to disturbances. The resulting fluctuations will in turn generate more acoustic waves and in some situations self-sustained oscillations can result. Flame transfer functions for LPP combustion are poorly understood at present but are crucial for predicting combustion oscillations. This paper describes an experiment designed to measure the flame transfer function of a simple combustor incorporating realistic components. Tests were conducted initially on this combustor at atmospheric pressure (1.2 bar and 550 K) to make an early demonstration of the combustion system. The test rig consisted of a plenum chamber with an inline siren, followed by a single LPP premixer/duct and a combustion chamber with a silencer to prevent natural instabilities. The siren was used to induce variable frequency pressure/acoustic signals into the air approaching the combustor. Both unsteady pressure and heat release measurements were undertaken. There was good coherence between the pressure and heat release signals. At each test frequency, two unsteady pressure measurements in the plenum were used to calculate the acoustic waves in this chamber and hence estimate the mass-flow perturbation at the fuel injection point inside the LPP duct. The flame transfer function relating the heat release perturbation to this mass flow was found as a function of frequency. The same combustor hardware and associated instrumentation were then used for the high pressure (15 bar and 800 K) tests. Flame transfer function measurements were taken at three combustion conditions that simulated the staging point conditions (Idle, Approach and Take-off) of a large turbofan gas turbine. There was good coherence between pressure and heat release signals at Idle, indicating a close relationship between acoustic and heat release processes. Problems were encountered at high frequencies for the Approach and Take-off conditions, but the flame transfer function for the Idle case had very good qualitative agreement with the atmospheric-pressure tests. The flame transfer functions calculated here could be used directly for predicting combustion oscillations in gas turbine using the same LPP duct at the same operating conditions. More importantly they can guide work to produce a general analytical model.

A Finite Element Method for Three-Dimensional Analysis of Thermo-acoustic Combustion Instability

2010 ◽  
Vol 133 (1) ◽  
Author(s):  
S. M. Camporeale ◽  
B. Fortunato ◽  
G. Campa
Keyword(s):  
Finite Element ◽  
Heat Release ◽  
Combustion Chamber ◽  
Growth Rates ◽  
Acoustic Waves ◽  
Transfer Functions ◽  
Three Dimensional ◽  
Complex Eigenvalue ◽  
Governing Equations ◽  
Acoustic Instabilities

A method for predicting the onset of acoustically driven combustion instabilities in gas turbine combustor is examined. The basic idea is that the governing equations of the acoustic waves can be coupled with a flame heat release model and solved in the frequency domain. The paper shows that a complex eigenvalue problem is obtained that can be solved numerically by implementing the governing equations in a finite element code. This procedure allows one to identify the frequencies at which thermo-acoustic instabilities are expected and the growth rate of the pressure oscillations, at the onset of instability, when the hypothesis of linear behavior of the acoustic waves can be applied. The method can be applied virtually to any three-dimensional geometry, provided the necessary computational resources that are, anyway, much less than those required by computational fluid dynamics methods proposed for analyzing the combustion chamber under instability condition. Furthermore, in comparison with the “lumped” approach that characterizes popular acoustics networks, the proposed method allows one for much more flexibility in defining the geometry of the combustion chamber. The paper shows that different types of heat release laws, for instance, heat release concentrated in a flame sheet, as well as distributed in a larger domain, can be adopted. Moreover, experimentally or numerically determined flame transfer functions, giving the response of heat release to acoustic velocity fluctuations, can be incorporated in the model. To establish proof of concept, the method is validated at the beginning against simple test cases taken from literature. Over the frequency range considered, frequencies and growth rates both of stable and unstable eigenmodes are accurately evaluated. Then the method is applied to a much more complex annular combustor geometry in order to evaluate frequencies and growth rates of the unstable modes and to show how the variation in the parameters of the heat release law can influence the transition to instability.

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.

Identification of Flame Transfer Functions From LES of a Premixed Swirl Burner

2010 ◽  
Author(s):  
Luis Tay Wo Chong ◽  
Thomas Komarek ◽  
Roland Kaess ◽  
Stephan Fo¨ller ◽  
Wolfgang Polifke
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Transfer Functions ◽  
Signal To Noise Ratio ◽  
Thermal Power ◽  
Reacting Flow ◽  
Signal To Noise ◽  
Swirl Burner ◽  
Low Frequencies ◽  
Noise Ratio

Large eddy simulations of compressible, turbulent, reacting flow were carried out in order to identify the Flame Transfer Function (FTF) of a premixed swirl burner at different power ratings. The Thickened Flame model with one step kinetics was used to model combustion. Time-averaged simulation results for inert and reacting flow cases were compared with experimental data for velocity and heat release distribution with good agreement. Heat losses at the combustor walls were found to have a strong influence on computed flame shapes and spatial distributions of heat release. For identification of the FTF with correlation analysis, broadband excitation was imposed at the inlet. At low power rating (30 kW), measured and computed FTFs agree very well at low frequencies (corresponding to Strouhal numbers St < 4), showing a pronounced maximum of the gain at St ≈ 2. At higher frequencies, where the flame response weakens, the agreement between experiment and computation deteriorates, presumably due to decreasing signal-to-noise ratio. At higher thermal power (50 kW), a high-frequency instability developed during the simulation runs, resulting in poor overall signal-to-noise ratio and thus to unsatisfactory prediction of the gain of the flame transfer function. The phase of the FTF, on the other hand, was predicted with good accuracy up to St < 5. An analytical expression for the FTF, which models the flame dynamics as a superposition of time-delayed responses to perturbations of mass flow rate and swirl number, respectively, was found to match the experimental results.

Mixture-Forced Flame Transfer Function Measurements and Mechanisms in a Single-Nozzle Combustor at Elevated Pressure

2011 ◽  
Author(s):  
Nick Bunce ◽  
Jong Guen Lee ◽  
Bryan D. Quay ◽  
Domenic A. Santavicca
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Velocity Fluctuation ◽  
Transfer Functions ◽  
Modulation Frequency ◽  
Shear Layers ◽  
Operating Conditions ◽  
Phase Curve ◽  
Destructive Interference ◽  
Resultant Velocity

The mixture-forced flame transfer function of a lean fully premixed single-nozzle research combustor operating on natural gas is determined experimentally at combustor pressures from 1 to 4 atm. Measurements are made over a range of inlet temperatures (100–300°C), mean velocities (25–35 m/s), and equivalence ratios (0.5–0.75). A rotating siren device, located upstream of the nozzle, is used to modulate the flow rate of the premixed fuel-air mixture. The amplitude and phase of the resultant velocity fluctuation are measured near the exit of the nozzle using the two-microphone method. The measured normalized velocity fluctuation serves as the input to the flame transfer function. In this study, the amplitude of the normalized velocity fluctuation is fixed at 5% and the modulation frequency is varied from 100 to 500 Hz. The output of the flame transfer function is the normalized global heat release fluctuation, which is measured using a photomultiplier tube and interference filter which captures the CH* chemiluminescence from the entire flame. In addition, two-dimensional CH* chemiluminescence images are taken for both forced and unforced flames. Forced flame images are phase-synchronized with the velocity fluctuation. The flame transfer functions for all of the operating conditions tested exhibit similar behavior. At low frequencies, the gain is initially greater than one, but then decreases as the frequency increases. After reaching a minimum, the gain increases with increasing frequency to a second peak and then again decreases. At certain operating conditions, the gain exhibits a second minimum. At frequencies corresponding to the minima in gain the phase curve exhibits inflection points. Regions of maximum and minimum gain are explained in terms of the constructive and destructive interference of vorticity fluctuations generated in the inner and outer shear layers. Phase-synchronized images are analyzed to isolate the fluctuating component of heat release. At frequencies where the gain is amplified, this analysis shows that the heat release fluctuations caused by the vorticity fluctuations generated in the inner and outer shear layers are in phase. While when the gain is at its minimum value, the heat release fluctuations are out of phase and therefore destructively interfere.

Optical Measurement of Local and Global Transfer Functions for Equivalence Ratio Fluctuations in a Turbulent Swirl Flame

2013 ◽  
Author(s):  
Bernhard C. Bobusch ◽  
Bernhard Ćosić ◽  
Jonas P. Moeck ◽  
Christian Oliver Paschereit
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Equivalence Ratio ◽  
Transfer Functions ◽  
Optical Measurement ◽  
Low Frequencies ◽  
Fuel Flow ◽  
Calibration Measurement

Equivalence ratio fluctuations are known to be one of the key factors controlling thermoacoustic stability in lean premixed gas turbine combustors. The mixing and thus the spatio-temporal evolution of these perturbations in the combustor flow is, however, difficult to account for in present low-order modeling approaches. To investigate this mechanism, experiments in an atmospheric combustion test rig are conducted. To assess the importance of equivalence ratio fluctuations in the present case, flame transfer functions for different injection positions are measured. By adding known perturbations in the fuel flow using a solenoid valve, the influence of equivalence ratio oscillations on the heat release rate is investigated. The spatially and temporally resolved equivalence ratio fluctuations in the reaction zone are measured using two optical chemiluminescence signals, captured with an intensified camera. A steady calibration measurement allows for the quantitative assessment of the equivalence ratio fluctuations in the flame. This information is used to obtain a mixing transfer function, which relates fluctuations in the fuel flow to corresponding fluctuations in the equivalence ratio of the flame. The current study focuses on the measurement of the global, spatially integrated, transfer function for equivalence ratio fluctuations and the corresponding modeling. In addition, the spatially resolved mixing transfer function is shown and discussed. The global mixing transfer function reveals that despite the good spatial mixing quality of the investigated generic burner, the ability to damp temporal fluctuations at low frequencies is rather poor. It is shown that the equivalence ratio fluctuations are the governing heat release rate oscillation response mechanism for this burner in the low-frequency regime. The global transfer function for equivalence ratio fluctuations derived from the measurements is characterized by a pronounced low-pass characteristic, which is in good agreement with the presented convection–diffusion mixing model.

Experimental Development of On-Line Flame Transfer Function Measurements for Fielded Gas Turbines

2021 ◽  
Author(s):  
Austin Matthews ◽  
Anna Cobb ◽  
Subodh Adhikari ◽  
David Wu ◽  
Tim Lieuwen ◽  
...  
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Gas Turbine ◽  
Gas Turbines ◽  
High Speed ◽  
Transfer Functions ◽  
Full Scale ◽  
Fuel Injector ◽  
Experimental Development ◽  
On Line

Abstract Understanding thermoacoustic instabilities is essential for the reliable operation of gas turbine engines. To complicate this understanding, the extreme sensitivity of gas turbine combustors can lead to instability characteristics that differ across a fleet. The capability to monitor flame transfer functions in fielded engines would provide valuable data to improve this understanding and aid in gas turbine operability from R&D to field tuning. This paper presents a new experimental facility used to analyze performance of full-scale gas turbine fuel injector hardware at elevated pressure and temperature. It features a liquid cooled, fiber-coupled probe that provides direct optical access to the heat release zone for high-speed chemiluminescence measurements. The probe was designed with fielded applications in mind. In addition, the combustion chamber includes an acoustic sensor array and a large objective window for verification of the probe using high-speed chemiluminescence imaging. This work experimentally demonstrates the new setup under scaled engine conditions, with a focus on operational zones that yield interesting acoustic tones. Results include a demonstration of the probe, preliminary analysis of acoustic and high speed chemiluminescence data, and high speed chemiluminescence imaging. The novelty of this paper is the deployment of a new test platform that incorporates full-scale engine hardware and provides the ability to directly compare acoustic and heat release response in a high-temperature, high-pressure environment to determine the flame transfer functions. This work is a stepping-stone towards the development of an on-line flame transfer function measurement technique for production engines in the field.

Flame Response Mechanisms Due to Velocity Perturbations in a Lean Premixed Gas Turbine Combustor

2010 ◽  
Vol 133 (2) ◽  
Author(s):  
Brian Jones ◽  
Jong Guen Lee ◽  
Bryan D. Quay ◽  
Domenic A. Santavicca
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Gas Turbine ◽  
Velocity Fluctuation ◽  
Gas Turbine Combustor ◽  
Velocity Fluctuations ◽  
Inlet Velocity ◽  
Lean Premixed ◽  
Flame Response ◽  
Second Peak

The response of turbulent premixed flames to inlet velocity fluctuations is studied experimentally in a lean premixed, swirl-stabilized, gas turbine combustor. Overall chemiluminescence intensity is used as a measure of the fluctuations in the flame’s global heat release rate, and hot wire anemometry is used to measure the inlet velocity fluctuations. Tests are conducted over a range of mean inlet velocities, equivalence ratios, and velocity fluctuation frequencies, while the normalized inlet velocity fluctuation (V′/Vmean) is fixed at 5% to ensure linear flame response over the employed modulation frequency range. The measurements are used to calculate a flame transfer function relating the velocity fluctuation to the heat release fluctuation as a function of the velocity fluctuation frequency. At low frequency, the gain of the flame transfer function increases with increasing frequency to a peak value greater than 1. As the frequency is further increased, the gain decreases to a minimum value, followed by a second smaller peak. The frequencies at which the gain is minimum and achieves its second peak are found to depend on the convection time scale and the flame’s characteristic length scale. Phase-synchronized CH∗ chemiluminescence imaging is used to characterize the flame’s response to inlet velocity fluctuations. The observed flame response can be explained in terms of the interaction of two flame perturbation mechanisms, one originating at flame-anchoring point and propagating along the flame front and the other from vorticity field generated in the outer shear layer in the annular mixing section. An analysis of the phase-synchronized flame images show that when both perturbations arrive at the flame at the same time (or phase), they constructively interfere, producing the second peak observed in the gain curves. When the perturbations arrive at the flame 180 degrees out-of-phase, they destructively interfere, producing the observed minimum in the gain curve.

OH* Chemiluminescence and OH-PLIF Measurements in a Micro Gas Turbine Combustor

2010 ◽  
Author(s):  
Martina Hohloch ◽  
Rajesh Sadanandan ◽  
Axel Widenhorn ◽  
Wolfgang Meier ◽  
Manfred Aigner
Keyword(s):  
Heat Release ◽  
Combustion Chamber ◽  
Gas Turbine ◽  
Thermal Power ◽  
Gas Turbine Combustor ◽  
Maximum Heat ◽  
Micro Gas Turbine ◽  
Power Load ◽  
Reaction Zones ◽  
Maximum Heat Release

In this work the combustion behavior of the Turbec T100 natural gas/air combustor was analyzed experimentally. For the visualization of the flame structures at various stationary load points OH* chemiluminescence and OH-PLIF measurements were performed in a micro gas turbine test rig equipped with an optically accessible combustion chamber. The OH* chemiluminescence measurements are used to get an impression of the shape and the location of the heat release zones. In addition the OH-PLIF measurements enabled spatially and temporarily resolved information of the reaction zones. Depending on the load point the shape of the flame was seen to vary from cylindrical to conical. With increasing thermal power load the maximum heat release zones shift to a lifted flame. Moreover, the effect of the optically accessible combustion chamber on the performance of the micro gas turbine is evaluated.

scholarly journals Impact of the heat release distribution on high-frequency transverse thermoacoustic driving in premixed swirl flames

2016 ◽  
Vol 9 (3) ◽  
pp. 143-154 ◽  
Author(s):  
Michael Hertweck ◽  
Frederik M Berger ◽  
Tobias Hummel ◽  
Thomas Sattelmayer
Keyword(s):  
Heat Release ◽  
Combustion Chamber ◽  
High Frequency ◽  
Transfer Functions ◽  
Three Dimensional ◽  
Elevated Pressure ◽  
Rayleigh Index ◽  
The Stability ◽  
Stability Limits ◽  
The Impact

Self-excited, high-frequency first transversal thermoacoustic instabilities in a cylindrical combustion chamber equipped with a premixed swirl-stabilized flame are investigated. Phase-locked image analysis of the phenomena shows the displacement of the flame and a higher burning rate in the region of elevated pressure. The impact of diffuser angle and fuel composition on the stability limits and the flame position is investigated. The Rayleigh-Index is computed for a three-dimensional domain based on analytical flame transfer functions for experimentally obtained data of OH*-chemiluminescence as measure for the spatial heat release. Two models from different sources are applied, which describe the interaction between flame and acoustic locally. The axial dependence of the amplitude of the transversal mode is computed by a numerical model, which takes the temperature distribution inside the combustion chamber into account. The comparison of the Rayleigh-Index of different operation points shows a correlation with the stability limits for some, but not for all investigated configurations.

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