Premixed Gas Turbine Combustor Research Articles

Effect of Fuel Reactivity and Operating Conditions on Flame Anchoring in the Premixing Zone of a Swirl Stabilized Gas Turbine Combustor

Abstract Flashback with subsequent flame anchoring (FA) is an inherent risk of lean premixed gas turbine combustors operated with highly reactive fuel. The present study has been performed to characterize flame stabilization in the premixing zone of a lean premixed swirl stabilized burner and to identify critical combustion characteristics. An optically accessible burner was used for experimental investigations under atmospheric pressure and elevated preheat temperatures. The air mass flowrate, global equivalence ratio and preheat temperature were systematically varied to identify critical operating parameters. Hydrogen-natural gas mixtures with hydrogen mass fractions from 15 to 100% were studied to evaluate the impact of fuel reactivity. The air-fuel mixture was ignited with a focused single laser pulse to trigger FA in the premixing zone during steady operation. High speed imaging with OH*-chemiluminescence were applied to observe flame characteristics and evaluate flame anchoring propensity. Flame anchoring limits (FAL) are reported in terms of the minimum global equivalence ratio at which the flame was blown out of the premixing zone within a critical time period. A comparison of characteristic time scales at FAL shows that the main impact during flame anchoring is given by the fuel reactivity and to some ex tent by preheat temperature. A Damköhler criterion is derived from the FAL that allows prediction of FA propensity based on operating conditions and 1-D reacting simulations.

The Lean Blowout Prediction Techniques in Lean Premixed Gas Turbine: An Overview

The lean blowout is the most critical issue in lean premixed gas turbine combustion. Decades of research into LBO prediction methods have yielded promising results. Predictions can be classified into five categories based on methodology: semi-empirical model, numerical simulation, hybrid, experimental, and data-driven model. First is the semi-empirical model, which is the initial model used for LBO limit prediction at the design stages. An example is Lefebvre’s LBO model that could estimate the LBO limit for eight different gas turbine combustors with a ±30% uncertainty. To further develop the prediction of the LBO limit, a second method based on numerical simulation was proposed, which provided deeper information and improved the accuracy of the LBO limit. The numerical prediction method outperformed the semi-empirical model on a specific gas turbine with ±15% uncertainty, but more testing is required on other combustors. Then, scientists proposed a hybrid method to obtain the best out of the earlier models and managed to improve the prediction to ±10% uncertainty. Later, the laboratory-scale combustors were used to study LBO phenomena further and provide more information using the flame characteristics. Because the actual gas turbine is highly complex, all previous methods suffer from simplistic representation. On the other hand, the data-driven prediction methods showed better accuracy and replica using a real dataset from a gas turbine log file. This method has demonstrated 99% accuracy in predicting LBO using artificial intelligence techniques. It could provide critical information for LBO limits prediction at the design stages. However, more research is required on data-driven methods to achieve robust prediction accuracy on various lean premixed combustors.

Experimental and Statistical ANOVA Analysis on Combustion Stability of CH4/O2/CO2 in a Partially Premixed Gas Turbine Combustor

Abstract The application of the oxy-fuel combustion technique could tackle the combustion process's environmental issues. Experiments were conducted on partially premixed air- and oxy-methane combustion flames stabilized over a novel perforated burner in the present work. The burner has a premixing ratio of 7.0. In oxy-fuel combustion, the experiments were performed at oxygen fractions (OF%: volumetric percentage of O2 in the oxidizer mixture) of 29%, 32%, and 36% and over a range of operating conditions necessary for a stable flame. The results of oxy-combustion flames were compared with the corresponding air-combustion flames at the same operating conditions. Two sets of statistical analyses were performed for further confirmation of the experimental results. The first set investigated the operating parameters’ effect, including OF and oxidizer Reynolds number (Re), on the upper flammability limits (UFL). Simultaneously, the second set studied the impact of OF and equivalence ratio on flame length. The experimental results revealed that the flammability limits get wider as the OF increases due to the resulting flame speed rise with O2-enrichment. The statistical analysis is conducted by analysis of variance (ANOVA) technique, which carries innovation and confirms that OF and Re significantly impacted the UFL. The visual flame length of oxy-flames was longer than its correspondents of air-flames due to the reduction of flame speed associated with the negative influence of CO2 dilution in oxy-flames. The statistical analysis showed a significant effect of OF and equivalence ratio on the visible flame appearance.

Modeling of Natural Gas Composition Effect on Low NOx Burners Operation in Heavy Duty Gas Turbine

Abstract This paper addresses the impact of natural gas composition on both the operability and emissions of lean premixed gas turbine combustion system. This is an issue of growing interest due to the challenge for gas turbine manufacturers in developing fuel-flexible combustors capable of operating with variable fuel gases while producing very low emissions at the same time. Natural gas contains primarily methane (CH4) but also notable quantities of higher order hydrocarbons such as ethane (C2H6) can also be present. A deep understanding of natural gas combustion is important to obtain the highest combustion efficiency with minimal environmental impact. For this purpose, Large Eddy Simulations of an annular combustor sector equipped with a partially premixed burner are carried out for two different natural gas compositions with and without including the effect of flame strain rate and heat loss resulting in a more adequate description of flame shape, thermal field, and extinction phenomena. Promising results, in terms of NOx, compared against available experimental data, are obtained including these effects on the flame brush modeling, enhancing the fuel-dependency under nonadiabatic condition.

Thermoacoustic Instability Considerations for High Hydrogen Combustion in Lean Premixed Gas Turbine Combustors: A Review

Hydrogen is receiving increasing attention as a versatile energy vector to help accelerate the transition to a decarbonised energy future. Gas turbines will continue to play a critical role in providing grid stability and resilience in future low-carbon power systems; however, it is recognised that this role is contingent upon achieving increased thermal efficiencies and the ability to operate on carbon-neutral fuels such as hydrogen. An important consideration in the development of gas turbine combustors capable of operating with pure hydrogen or hydrogen-enriched natural gas are the significant changes in thermoacoustic instability characteristics associated with burning these fuels. This article provides a review of the effects of burning hydrogen on combustion dynamics with focus on swirl-stabilised lean-premixed combustors. Experimental and numerical evidence suggests hydrogen can have either a stabilising or destabilising impact on the dynamic state of a combustor through its influence particularly on flame structure and flame position. Other operational considerations such as the effect of elevated pressure and piloting on combustion dynamics as well as recent developments in micromix burner technology for 100% hydrogen combustion have also been discussed. The insights provided in this review will aid the development of instability mitigation strategies for high hydrogen combustion.

Effects of superheated steam on combustion instability and NOx emissions in a model lean premixed gas turbine combustor

This study experimentally explored the effects of preheated steam on combustion instabilities and NOx emissions in a lean-premixed model gas turbine combustor. Three variables were investigated to confirm the effectiveness of preheated steam injection on combustion instability and NOx emissions—the steam flow rate, the injection heights and the inner diameter of injectors. Experimental results denote that optimal the damping ratio of sound pressure can reach 91.73% with preheated steam dilution, the amplitude of sound pressure reduced from 46 Pa to 3.8 Pa. The optimal damping ratio of NOx emissions can reach 83.74% with preheated steam dilution, concentration of NOx emissions reduced from 24.6 ppm to 4.0 ppm. Mode shifting of thermoacoustic instability is appeared as the inner diameter of the injector varied from 1.5 mm to 4.0 mm. The oscillation frequency of the flame CH* chemiluminescence suddenly drops from 256 Hz to 74 Hz as the inner diameter changes from 2.0 mm to 2.5 mm. The lower the relative height of the superheated steam injectors, the better the control effect of combustion instability and NOx emission. However, unreasonable design of the steam injector will trigger thermoacoustic instability. The flame roots, fronts and recirculation zones are all changed under preheated steam, the whole flame become short and compact. The flame length reduced from 75 mm to 37 mm while the preheated steam flow increases. This study explored the dilution effects of preheated steam on lean premixed combustion instability and NOx emissions, the results could be conducive to the design of clean, safe and high-efficient gas turbine combustors.

Effects of Fuel Staging on the Hydrodynamic Stability of Multinozzle Swirl Flows

Abstract Hydrodynamic instability in lean premixed gas turbine combustors can cause coherent flow velocity oscillations. These can in turn drive heat release oscillations that when favorably coupled with combustor acoustic modes can result in combustion instability. The aim of this paper is to understand the impact of fuel staging on the characteristics of hydrodynamic modes in multinozzle combustors. We extend our recent numerical study on the hydrodynamic stability characteristics of a multinozzle combustor having three nozzles in a straight line with uniform fuel–air ratio in each nozzle, to the nonuniform fuel–air ratio case. As before, we construct the base flow model for this study by superposing contributions from individual nozzles, determined using a base flow model for a nominally axisymmetric single nozzle, at every point in the computational domain. The impact of fuel staging is captured by changing the burnt to unburnt gas density ratio parameter in the individual contribution from each nozzle. We investigate the characteristics of the most locally absolutely unstable mode for two cases. The first one is when the middle nozzle is made fuel rich when compared to the side nozzles and the second is when the side nozzles are made fuel rich relative to the middle nozzle. The impact of nonuniform fuel/air ratio on the local absolutely unstable temporal eigenvalues is seen to be small. However, significant changes in the spatial structure of the flow oscillations associated with the hydrodynamic eigenmodes are observed. In the first case, the flow oscillations with a different locally azimuthal nature on the middle nozzle when compared to the side nozzles emerge as the middle nozzle is made richer. In the second case, the oscillations on the two side nozzles are suppressed leaving the middle nozzle in a state that closely matches that of a single unconfined nozzle with the same nominal base flow velocity field. These types of internozzle variations in flow oscillation characteristics can explain the emergence of nonuniformity in heat release oscillation characteristics between individual nozzles in multinozzle combustors.

Effect of fuel line acoustics on the flame dynamics of H2/CH4 syngas in a partially premixed gas turbine combustor

This paper investigates the dynamic response of flames, considering H2-enriched CH4 fuels that are submitted to upstream mechanical perturbations in a model gas turbine combustor, resulting in fuel velocity oscillations. Flame transfer function (FTF) and cold-flow transfer function (CTF), which represent, respectively, the dynamic responses in the flow of the flame and fuel induced by a velocity modulation in the fuel feed line (FFL), are derived. The gain of FTF shows a low-pass behavior and gradually increases until a specific modulation frequency is reached, which is identical to the frequencies of the corresponding peak gains of the CTF. The periodic variation of OH* intensity is observed from the images collected with a high-speed OH-PLIF. The high oscillation near the injector indicates that the fuel velocity perturbation leads to the amplification of the heat release fluctuation at specific modulation frequencies. The singularity frequencies corresponding to the peaks of the gain of both transfer functions coincide with the resonance frequencies in the FFL. The phase of FTF and CTF decreases as the H2 ratio increases and the phase of FTF is well fitted into a single line by nondimensionalization through Strouhal number. Most of these findings on the characteristics of combustion instability and acoustical characteristics of FFL can be used as a crucial methodology of H2 combustor design, such as an anti-resonance fuel/air feed line, which can ensure the stable operation of combustion system by avoiding H2 induced combustion instability failure.

Autoignition and flame lift-off behavior of a fuel jet mixing with turbulent hot air coflow

For advanced lean premixed gas turbine combustors that have high inlet air temperatures, autoignition that occurs during the fuel/air mixing process is becoming an issue, which can cause flame-holding inside the premixing device and burn the hardware. This paper reports an experimental study on the autoignition and flame lift-off behavior of a fuel jet injected into hot air coflow that mimics the fuel/air mixing process of lean-premixed combustors. Autoignition occurred when a fuel jet (diluted with N2 or CO2) was injected concentrically into the hot turbulent air coflow. Three distinctive types of autoignition behavior were observed at increasingly higher jet velocity: (1) attached flame; (2) lifted flame followed by intermittent autoignition spots; (3) lifted flame. Lift-off height of the flame was measured at various fuel jet velocities and dilution ratios. The liftoff height increases monotonically with fuel jet velocity and dilution ratio. The CO2-diluted flame has higher lift-off height than the N2-diluted flame. The lift-off height data were fitted to various models to determine the flame stabilization mechanism. A modified large-scale mixing model can capture the effects of both turbulence mixing and autoignition kinetics on lift-off height, underscoring the importance of both turbulent mixing and autoignition kinetics in flame stabilization.

A numerical investigation of the effects of hydrogen addition on combustion instability inside a partially-premixed swirl combustor

Large eddy simulations (LES) of a hydrogen enriched partially premixed gas turbine combustor are performed for various operating conditions for examining the fuel condition effects. The LES turbulence model and multi-step chemical mechanism of hydrogen-methane combustion are applied for simulating the three-dimensional swirling flame in the combustor. To confirm the adequacy of the numerical simulation, a comparison with the existing experimental data is conducted which indicates that hydrogen-enriched methane fuel conditions trigger unstable pressure fluctuations in the combustor. The analysis on pressure spectra and harmonic mode on the unsteady flame is performed to investigate the driving source of combustion instability. In the unstable case, rigorous vortex burning appears as unburnt mixture is periodically supplied into the recirculation zone. By considering the fuel composition and flow rate modifications, it is shown that hydrogen content and fuel flow rate play a dominant role in determining the flame structure and its combustion instability. If hydrogen content is increased or fuel flow rate is decreased, the flame structure changes and the flow field is stabilized. Then, vortex burning does not appear as flame is formed in a narrow region near the dump plane. Thus, the present investigation has identified the change of flame structure and its interaction with vortex field as the key features for understanding the combustion instability.

A numerical study of combustion and NOX emission characteristics of a lean premixed model gas turbine combustor

The combustion characteristics of a model gas-turbine combustor are investigated by performing 3-D Reynolds-averaged Navier-Stokes equations (RANS) simulations of swril-stabilized lean-premixed flames of methane/air for different equivalence ratio and swirler type. Their NOX emission characteristics and thermal fatigue life are also elucidated with chemical reactor network (CRN) analysis and finite element analysis (FEA). To validate numerical models, the RANS simulations results are compared to experimental data, which shows reasonable agreement between them. The CRN analysis reveals that the amount of NOX can be reduced by low temperature and short residence time. The FEA identifies areas vulnerable to thermal fatigue. The largest strain amplitude is observed at a single nozzle exit because of large temperature variation and complicated structure. The cycles to failure at the vulnerable regions are estimated using the e-N curve of AISI type 304 stainless steel.

Quantitative measurement of density fluctuations with a full-field laser interferometric vibrometer

Modern, lean and premixed gas turbine combustion concepts for low NOx emissions are prone to combustion instabilities. In a previous work it was shown that laser interferometric vibrometry (LIV) can be used to record global as well as local heat release fluctuations in swirl-stabilized premixed methane flames quantitatively, if other effects influencing density are small. In this work a newly developed camera-based full-field LIV system (CLIV) was applied to a lean, confined, premixed and swirl-stabilized methane flame under atmospheric conditions. Instead of time-consuming pointwise scanning of the flame, CLIV records full-field line-of-sight density fluctuations with high spatio-temporal resolution. With a recording rate of 200 kHz, CLIV enables the visualization of highly unsteady processes in fluid dynamics and combustion research. As an example for an unsteady process, the propagation of the flame front through a lean, premixed gas volume is visualized during an ignition process. A discussion of algorithms and assumptions necessary to calculate heat release oscillations from density oscillations is presented and applied to phase-averaged data recorded with CLIV for this type of flame. As reference, OH* chemiluminescence data were recorded simultaneously. While density gradients travelling with the flow are recorded by LIV and CLIV, chemiluminescence imaging will show nothing in the absence of chemical reaction.Graphic abstracta Time-averaged density gradient within the combustor in lateral direction. b Density fluctuations along line-of-sight 7 ms after ignition. c Phase-averaged and local heat release fluctuations at 225 Hz perturbation frequency

Numerical investigation on combustion instability modeling in a lean premixed gas turbine combustor combining finite element analysis with local flame transfer function

The present work suggests a numerical approach using an FEM (finite element method) code for the eigenvalue calculation of the thermoacoustic combustion instabilities coupling with the local flame transfer function in a generic lean premixed combustor. The URANS (unsteady Reynolds average Navier-Stokes) simulation shows a good agreement in capturing steady and unsteady flame geometries with the measurement in the literature. Also, global and local parameters of flame responses to acoustic velocity fluctuations undergo quantitatively similar changes in prediction and in the experiment. The numerically determined flame transfer function and gas properties are incorporated in an in-house Helmholtz solver to compute eigenvalues of the target combustor. In the solver, the Helmholtz equation is discretized with a Galerkin FEM on a multi-dimensional hybrid unstructured mesh. The nonlinearity related with the heat release source term is treated with an iterative methodology, and the large scale eigenvalue calculation is performed using the shift-invert approach available in the ARPACK (ARnoldi PACKage) library. The predictive capabilities of the current modeling approach are validated against the measured mode frequency and instability growth rate. In addition, the prediction results show the effect of spatially-distributed heat release on the thermoacoustic model accuracy.

Formula omitted] enrichment of [formula omitted] blends in lean premixed gas turbine combustion: An experimental study on effects on flame shape and thermoacoustic oscillation dynamics

Thermoacoustic instabilities under lean operation in gas turbine burners hinder the development of lean premixed combustion mode of operation, thus limiting the potential to decrease NOx emissions. A method to improve stability of lean combustion while maintaining low thermal NOx formation is to add hydrogen in typical gas turbine fuels such as natural gas. The present work examines the thermoacoustic dynamic characteristics of hydrogen-enriched methane blends in a swirl stabilized model gas turbine combustor. We blend CH4 with increasing H2 molar content (from 0% to 40%) at a global equivalence ratio of ϕ=0.55 on a constant Reynolds number Re = 19000. The reference case of pure methane is susceptible to blow off at the same equivalence ratio. On increasing the hydrogen content at 10% H2, the flammability limits are extended. However, further increase of the H2 content leads to manifestation of random short bursts of dynamic pressure and heat release. Thermoacoustic dynamics are intermittently injected between a quiescent state and a regime of high amplitude oscillations. On further increasing H2 content, the dynamics are attracted towards a limit cycle; a fully established high amplitude regime, where no requiescence is observed. In this regime, heat release rate and dynamic pressure oscillate in phase and at the same frequency. Based on the observed dynamics, we seek a mixture property to characterize the dynamic state, the combustor operates in. We show that the extinction strain rate associated with each mixture can collapse the dynamic transitions from quiescent to intermittent instabilities and finally to fully established thermoacoustic oscillations. In each dynamic state, we examine the mechanism that affiliates coherent structures of the underlying thermodynamic flow field with the flame through the relation of the spatial distribution of the flow imposed strain rate over the extinction strain rate of each mixture. It is shown that the flame anchoring locations for a given mixture are dictated by the flow imposed strain rate and then that increased extinction resistance couples the flame with naturally excited to swirling flows helical instabilities, that may instigate mechanisms responsible for intermittent heat release rate bursts. Finally, the combustor demonstrates a limit cycle behaviour where the flow-imposed strain rate oscillates about the extinction strain rate along the inner shear layers causing local extinction of the flame at the root close to the centerbody.

Improved Chemical Reactor Network Application for Predicting the Emission of Nitrogen Oxides in a Lean Premixed Gas Turbine Combustor

This study presents the improvement and application of a 24-element chemical reactor network (CRN) model for predicting the emission of nitrogen oxides in a lean premixed gas turbine combustor. The 24-element CRN improvement is based on the results of CFD simulations. The emission of nitrogen oxides predicted by the 24-element CRN is in good agreement with the experimental data obtauned at the Korea Electric Power Research Institute. The new CRN is able to handle complex chemical mechanisms. As there are few requirements for computation time, the CRN can be used as a tool for the analysis of combustion systems and can be integrated into gas turbine combustor designs.

Numerical analysis of the effect of the hydrogen composition on a partially premixed gas turbine combustor

Large-eddy simulations (LESs) of a hydrogen-enriched 1/3-scale GE7EA gas turbine combustor are conducted. Four different fuel compositions are employed to investigate the role of the CH4/H2 syngas composition on the resulting flame structure and pressure oscillations occurring inside the combustor. A comparison with the experimental data is conducted to validate the numerical results. First, imaging processing is performed using an Abel-inversion technique for the accumulated OH mass fraction showing good agreement with the experimental images. Then, the calculated velocity fields are successfully compared to the experimental (particle image velocimetry) results. The results show that the flame structure is readily altered when changing the syngas composition; this strongly affects the flow field and therefore the pressure oscillations inside the combustor. When the hydrogen composition is increased, the flame becomes shorter and thicker, and its effect on the outer recirculation zone is minimized. When the flame length approaches the radial length of the combustor under certain conditions, the flame periodically attaches to the rigid wall and the pressure oscillations inside the combustor become amplified. Overall, the LES combined with the multi-step kinetics successfully predicts the variation in the flow fields due to fuel composition changes and reveals the role of the syngas composition in the combustor.

Improved Chemical Reactor Network Application for Predicting the Emission of Nitrogen Oxides in a Lean Premixed Gas Turbine Combustor

Improved Chemical Reactor Network Application for Predicting the Emission of Nitrogen Oxides in a Lean Premixed Gas Turbine Combustor

Numerical Analysis of Equivalence Ratio Fluctuations in a Partially Premixed Gas Turbine Combustor Using Large Eddy Simulations

In the present work, the fluctuations of equivalence ratio in the PRECCINSTA combustor are investigated via large eddy simulations (LES). Four isothermal flow cases with different combinations of global equivalence ratios (0.7 or 0.83) and grids (1.2 or 1.8 million cells) are simulated to study the mixing process of air with methane, which is injected into the inlet channel through small holes. It is shown that the fluctuations of equivalence ratio are very large, and their ranges are [0.4, 1.3] and [0.3, 1.2] for cases 0.83 and 0.7, respectively. For simulating turbulent partially premixed flames in this burner with the well-known dynamically thickened flame (DTF) combustion model, a suitable multistep reaction mechanism should be chosen aforehand. To do that, laminar premixed flames of 15 different equivalence ratios are calculated using three different methane/air reaction mechanisms: 2S_CH4_BFER, 2sCM2 reduced mechanisms and GRI-Mech 3.0 detailed reaction mechanism. The variations of flame temperature, flame speed and thickness of the laminar flames with the equivalence ratios are compared in detail. It is demonstrated that the applicative equivalence ratio range for the 2S_CH4_BFER mechanism is [0.5, 1.3], which is larger than that of the 2sCM2 mechanism [0.5, 1.2]. Therefore, it is recommended to use the 2S_CH4_BFER scheme to simulate the partially premixed flames in the PRECCINSTA combustion chamber.

NOx-Formation and CO-Burnout in Water-Injected, Premixed Natural Gas Flames at Typical Gas Turbine Combustor Residence Times

With the transition of the power production markets toward renewable energy sources, an increased demand for flexible, fossil-based power production systems arises. Steep load gradients and a high range of flexibility make gas turbines a core technology in this ongoing change. In order to further increase this flexibility research on power augmentation of premixed gas turbine combustors is conducted at the Lehrstuhl für Thermodynamik, TU München. Water injection in gas turbine combustors allows for the simultaneous control of NOx emissions as well as the increase of the power output of the engine and has therefore been transferred to a premixed combustor at lab scale. So far stable operation of the system has been obtained for water-to-fuel ratios up to 2.25 at constant adiabatic flame temperatures. This paper focuses on the effects of water injection on pollutant formation in premixed gas turbine flames. In order to guarantee for high practical relevance, experimental measurements are conducted at typical preheating temperatures and common gas turbine combustor residence times of about 20 ms. Spatially resolved and global species measurements are performed in an atmospheric single burner test rig for typical adiabatic flame temperatures between 1740 and 2086 K. Global measurements of NOx and CO emissions are shown for a wide range of equivalence ratios and variable water-to-fuel ratios. Cantera calculations are used to identify nonequilibrium processes in the measured data. To get a close insight into the emission formation processes in water-injected flames, local concentration measurements are used to calculate distributions of the reaction progress variable. Finally, to clarify the influence of spray quality on the composition of the exhaust gas, a variation of the water droplet diameters is done. For rising water content at constant adiabatic flame temperature, the NOx emissions can be held constant, whereas CO concentrations increase. On the contrary, both values decrease for measurements at constant equivalence ratio and reduced flame temperatures. Further analysis of the data shows the close dependency of CO concentration on the equivalence ratio; however, due to the water addition, a shift of the CO curves can be detected. In the local measurements, changes in the distribution of the reaction progress variable and an increase of the flame length were detected for water-injected flames along with changes of the maximum as well as the averaged CO values. Finally, a strong influence of water droplet size on NOx and CO formation is shown for constant operating conditions.

Finite element analysis of self-excited instabilities in a lean premixed gas turbine combustor

The present study numerically simulates self-excited instabilities of hydrogen-blended natural gas flames in a lean premixed gas turbine combustor with variable chamber length. In order to effectively predict the thermoacoustic instability within the detailed geometry of the combustor, the Helmholtz equation is discretized with a Galerkin finite element method on a three-dimensional hybrid unstructured mesh. The unsteady heat release rate is modeled with an experimentally measured flame transfer function. The nonlinearity associated with the flame response term is handled with an iterative method, and the large-scale eigenvalue problem is solved by means of the shift-invert method of the ARPACK (ARnoldi PACKage) software. The present Helmholtz solver reproduces the experimentally observed instabilities in terms of the mode frequency and the chamber length range of self-excited pressure oscillations which are significantly affected by the hydrogen enrichment of natural gas fuel. The effects of the acoustic boundary condition and the flame response model on the predictive accuracy are also discussed.