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Linear stability analysis by means of low-order network models is widely spread in industry and academia to predict the thermoacoustic characteristics of combustion systems. Even though a vast amount of publications on this topic exist, much less is reported on the predictive capabilities of such stability analyses with respect to real system behavior. In this sense, little effort has been made on investigating if predicted critical parameter values, for which the combustion system switches from stability to instability, agree with experimental observations. Here, this lack of a comprehensive experimental validation is addressed by using a model-based control scheme. This scheme is able to actively manipulate the acoustic field of a combustion test rig by imposing quasiar-bitrary reflection coefficients. It is employed to continuously vary the downstream reflection coefficient of an atmospheric swirl-stabilized combustion test rig from fully reflecting to anechoic. By doing so, the transient behavior of the system can be studied. In addition to that, an extension of the common procedure, where the stability of an operating point is classified solely based on the presence of high amplitude pressure pulsations and their frequency, is given. Generally, the predicted growth rates are only compared with measurements with respect to their sign, which obviously lacks a quantitative component. In contrast to that, in this paper, validation of linear stability analysis is conducted by comparing calculated and experimentally determined linear growth rates of unstable modes. Besides this, experimental results and model predictions are also compared in terms of frequency of the least stable mode. Excellent agreement between computations from the model and experiments is found. The concept is also used for active control of combustion instabilities. By tuning the downstream reflectivity of the combustion test rig, thermoacoustic instabilities can be suppressed. The underlying mechanism is an increase in the acoustic energy losses across the system boundary. [DOI: 10.1115/1.4000806] Keywords: combustion instability, linear stability analysis, active instability control, acoustic boundary condition

In the design process, new burners are generally tested in combustion test rigs. With these experiments, as well as with computational fluid dynamics, finite element calculations, and low-order network models, the burner's performance in the full-scale engine is sought to be predicted. Especially, information about the thermoacoustic behavior and the emissions is very important. As the thermoacoustics strongly depend on the acoustic boundary conditions of the system, it is obvious that test rig conditions should match or be close to those of the full-scale engine. This is, however, generally not the case. Hence, if the combustion process in the test rig is stable at certain operating conditions, it may show unfavorable dynamics at the same conditions in the engine. In previous works, the authors introduced an active control scheme, which is able to mimic almost arbitrary acoustic boundary conditions. Thus, the test rig properties can be tuned to correspond to those of the full-scale engine. The acoustic boundary conditions were manipulated using woofers. In the present study, an actuator with higher control authority is investigated, which could be used to apply the control scheme in industrial test rigs. The actuator modulates an air mass flow to generate an acoustic excitation. However, in contrast to the woofers, it exhibits a strong nonlinear response regarding amplitude and frequency. Thus, the control scheme is further developed to account for these nonlinear transfer characteristics. This modified control scheme is then applied to change the acoustic boundary conditions of an atmospheric swirl-stabilized combustion test rig. Excellent results were obtained in terms of changing the reflection coefficient to different levels. By manipulating its phase, different resonance frequencies could be imposed without any hardware changes. The nonlinear control approach is not restricted to the actuator used in this study and might therefore be of use for other actuators as well [DOI: 10.1115/1.4000599] Keywords: burner development, impedance tuning, acoustic boundary condition, thermoacoustic

A combination of postprocessing tools of O[H.sup.*]-chemiluminescence snapshots is used to characterize the coherent structures of two types of premixed burners: a bluff body and an industrial swirl burner. Two methods are combined to extract the structures: a phase-averaging algorithm and the proper orthogonal decomposition. The first method is based on the estimation of the instantaneous phase of the snapshots relative to a (local) time-resolved signal. A phase-sorting--phase-averaging algorithm then reconstructs the evolution of the flame at a chosen frequency over one cycle. The proper orthogonal decomposition method is used as a filter to smoothen the snapshots. Both methods provide insight into the physical mechanisms of coherent structures in the two premixed flames under consideration. The snapshots of the bluff-body combustion exhibit a symmetric structure. This indicates that the von Karman vortex street in the cold flow is suppressed by the addition of heat in the shear layer. Three coexisting flame structures of the swirl burner in the combustion chamber could be identified: a natural helical structure of the burner and two axisymmetric modes. Increasing the amplitude of acoustic forcing at the natural flow frequency changes the helical structure to an axisymmetric one. DOI: 10.2514/1.J050188

In the design process, new burners are generally tested in combustion test rigs. With these experiments, computational fluid dynamics, and finite element calculations, the burners' performance in the full-scale engine is sought to be predicted. Especially, information about the thermoacoustic behavior and the emissions is very important. As the thermoacoustics strongly depend on the acoustic boundary conditions of the system, it is obvious that test rig conditions should match or be close to those of the full-scale engine. This is, however, generally not the case. Hence, if the combustion process in the test rig is stable at certain operating conditions, it may show unfavorable dynamics at the same conditions in the engine. In previous works, the authors introduced an active control scheme, which is able to mimic almost arbitrary acoustic boundary conditions. Thus, the test rig properties can be tuned to correspond to those of the full-scale engine. The acoustic boundary conditions were manipulated using woofers. In the present study, proportional valves are investigated regarding their capabilities of being used in the control scheme. It is found that the test rig impedance can be tuned equally well. In contrast to the woofers, however, the valves could be used in industrial applications, as they are more robust and exhibit more control authority. Additionally, the control scheme is further developed and used to tune the test rig at discrete frequencies. This exhibits certain advantages compared with the case of control over a broad frequency band. [DOI: 10.1115/1.2969088] Keywords: burner development, impedance tuning, acoustic boundary condition, thermoacoustic

Modern gas turbines need to fulfil increasingly stringent emission targets on the one hand and exhibit outstanding operational and fuel flexibility on the other. Ansaldo Energia GT26 and GT36 gas turbine models address these requirements by employing a combustion system in which two lean premixed combustors are arranged in series. Due to the high inlet temperatures from the first stage, the second combustor stage predominantly relies on autoignition for flame stabilization. In this paper, the response of autoignition flames to temperature, pressure and velocity excitations is investigated. The gas turbine combustor geometry is represented by a backward-facing step. Based on the conservation equations an analytical model is derived by solving the linearized Rankine-Hugoniot conditions. This is a commonly used analytical approach to describe the relation of thermodynamic quantities up- and downstream of a propagation stabilized flame. In particular, the linearized Rankine-Hugoniot jump conditions are derived taking into account the presence of a moving discontinuity as well as upstream entropy inhomogeneities. The unsteady heat release rate of the flame is modelled as a linear superposition of flame transfer functions, accounting for velocity, pressure, and entropy disturbances, respectively. This results in a 3 × 3 flame transfer matrix relating both primitive acoustic variables and the temperature fluctuations across the flame. The obtained analytical expression is compared to large eddy simulations with excellent agreement. A discussion about the contribution of the single terms to the modelling effort is provided, with a focus on autoignition flames. [ABSTRACT FROM AUTHOR]

Sequential combustion architectures rely on auto-ignition flame stabilization in the second stage combustor. Due to the exponential temperature dependence of the ignition delay time the second stage flames can respond to hot spots generated by unsteady first stage combustion. In this paper, the auto-ignition flame response is investigated by extending an existing analytical model for acoustic waves so that it also accounts for entropy waves. Explicit expressions for the flame transfer functions and linear to non-linear transition thresholds are derived. Results for different operating conditions, forcing amplitudes and frequencies are discussed. For this, three computational domains of increasing complexity are considered for model validation: a 2D duct, a 3D backward facing step, and finally a full-scale Ansaldo Energia GT26 sequential combustor. Unsteady Reynold-averaged Navier–Stokes as well as large-eddy simulations are performed. Acoustic and entropic contributions of the flame response are separated by means of system identification techniques. Finally, non-linear flame transfer functions are identified. It is observed that the gains linearly increase with the excitation frequency and non-linear, frequency dependent effects are observed already for small amplitude levels of excitation. [ABSTRACT FROM AUTHOR]

Excess energy generation from renewables can be conveniently stored as hydrogen for later use as a gas turbine fuel. Also, the strategy to sequestrate CO2 from natural gas (NG) will require gas turbines to run with hydrogen-based fuels. In such scenarios, high temperature low emission combustion of hydrogen is a key requirement for the future gas turbine market. Ansaldo Energia's gas turbines featuring sequential combustion have an intrinsic advantage when it comes to fuel flexibility and in particular hydrogen-based fuels. The sequential combustion system is composed of two complementary combustion stages in series: one premix stage followed by an auto-ignited second stage overcoming the limits of traditional premix combustion systems through a highly effective extra tuning parameter, i.e., the temperature between the first and the second stage. The standard constant pressure sequential combustion (CPSC) system as applied in the GT36 engine is tested, at high pressure, demonstrating that a modified operation concept allows stable combustion with no changes in combustor hardware for the whole range of NG and hydrogen blends. It is shown that in the range from 0% to 70% (vol.) hydrogen, stable combustion is achieved at full nominal exit temperature, i.e., without any derating and thus clearly outperforming other available conventional premixed combustors. Operation between 70% and 100% is possible as well and only requires a mild reduction of the combustor exit temperature. By proving the transferability of the single-can high pressure results to the engine, this paper demonstrates the practicality of operating the Ansaldo Energia GT36 H-Class gas turbine on fuels containing unprecedented concentrations of hydrogen while maintaining excellent performance and low emissions both in terms of NOx and CO2. [ABSTRACT FROM AUTHOR]

Abstract A three-dimensional direct numerical simulation (DNS) is performed for a turbulent hydrogen-air flame, represented with detailed chemistry, stabilized in a model gas-turbine combustor. The combustor geometry consists of a mixing duct followed by a sudden expansion and a combustion chamber, which represents a geometrically simplified version of Ansaldo Energia's GT26/GT36 sequential combustor design. In this configuration, a very lean blend of hydrogen and vitiated air is prepared in the mixing duct and convected into the combustion chamber, where the residence time from the inlet of the mixing duct to the combustion chamber is designed to coincide with the ignition delay time of the mixture. The results show that when the flame is stabilized at its design position, combustion occurs due to both autoignition and flame propagation (deflagration) modes at different locations within the combustion chamber. A chemical explosive mode analysis (CEMA) reveals that most of the fuel is consumed due to autoignition in the bulk-flow along the centerline of the combustor, and lower amounts of fuel are consumed by flame propagation near the corners of the sudden expansion, where the unburnt temperature is reduced by the thermal wall boundary layers. An unstable operating condition is also identified, wherein periodic auto-ignition events occur within the mixing duct. These events appear upstream of the intended stabilization position, due to positive temperature fluctuations induced by pressure waves originating from within the combustion chamber. The present DNS investigation represents the initial step of a comprehensive research effort aimed at gaining detailed physical insight into the rate-limiting processes that govern the sequential combustor behavior and avoid the insurgence of the off-design auto-ignition events. [ABSTRACT FROM AUTHOR]

We investigate the effect of increasing levels of hydrogen enrichment on the nonlinear response and saturation of premixed bluff-body stabilized methane/hydrogen flames submitted to acoustic forcing. The thermal power is kept approximately constant to preserve the nozzle velocity while increasing the flame speed through hydrogen enrichment. The flame describing function (FDF) is measured for a fixed frequency and three hydrogen-methane blends ranging from 10% to 50% by power, corresponding to 25% to 75% by volume. We show that when the flame is forced at the same frequency at similar power and bulk velocities, increasing levels of hydrogen enrichment increase the saturation amplitude of the flame. To provide insight into the flame dynamics responsible for the change in the global nonlinear response and saturation amplitude, the flames were investigated using high-speed imaging in combination with OH planar laser-induced fluorescence (OH-PLIF) at a range of forcing amplitudes. At lower hydrogen concentrations, the flame is stabilized along the inner shear layer and saturation in the heat release rate (HRR) occurs at lower forcing amplitudes due to large-scale flame-vortex interactions causing flame annihilation as observed in several previous studies. At increased levels of hydrogen enrichment, distinctly different flame dynamics are observed. In these cases, the flame accelerates and propagates across to the outer shear layer, which acts to suppress large-scale flame annihilation during roll-up of both the inner and outer shear layers. This results in a coherent increase in flame surface area with forcing amplitudes significantly increasing the saturation amplitude of the flame. These results show that high levels of hydrogen increase the amplitude response to acoustic forcing leading to higher saturation amplitudes. This suggests that substituting natural gas with hydrogen in gas turbines increases the risk of much higher limit-cycle amplitudes if self-excited instabilities occur. [ABSTRACT FROM AUTHOR]

Modem gas turbine combustors operating in lean-premixed mode are prone to thermoacoustic instabilities. In annular combustion chambers, usually azimuthal acoustic modes are the critical ones interacting with the flame. In case of constructive interference, high amplitude oscillations might result. In this paper, the azimuthal acoustic field of a fullscale engine is investigated in detail. The analyses are based on measurements in a fullscale gas turbine, analytical models to derive the system dynamics, as well as simulations performed with an in-house 3d nonlinear network model. It is shown that the network model is able to reproduce the behavior observed in the engine. Spectra, linear growth rates, as well as the statistics of the system's dynamics can be predicted. A previously introduced algorithm is used to extract linear growth rates from engine and model time domain data. The method's accuracy is confirmed by comparison of the routine's results to analytically determined growth rates from the network model. The network model is also used to derive a burner staging configuration, resulting in the decrease of linear growth rate and thus an increase of engine operation regime; model predictions are verified by full-scale engine measurements. A thorough investigation of the azimuthal modes statistics is performed. Additionally, the network model is used to show that an unfavorable flame temperature distribution with an amplitude of merely 1% of the mean flame temperature can change the azimuthal mode from dominantly rotating to dominantly standing. This is predicted by the network model that only takes into account flame fluctuations in axial direction. [ABSTRACT FROM AUTHOR]

A Helmholtz resonator is an important passive control device to abate excessive sound pressure levels. Because of its effectiveness and simplicity (in principle, it consists of an arbitrarily shaped volume and a small duct), it is of high relevance for industrial applications relying on robustness, availability, and reliability. It is the method of choice to counteract high-amplitude combustion-induced pressure oscillations often exhibited in lean premixed gas turbine combustors. Traditional Helmholtz resonators feature an efficient damping performance only in a narrow frequency bandwidth. To derive an efficient design for the full-scale engine, it is key to accurately predict its resonance frequency. In addition to the geometrical dimensions and the speed of sound, this is a function of a length correction term that accounts for inertia effects of the oscillating fluid. In case the Helmholtz resonator is connected to an enclosure of equal density, several empirical correlations for this correction term are available in the literature. However, if density differences between resonator and enclosure exist, as is the case for gas turbine combustion or acoustic liners in aeroengines, the end correction is affected. To the knowledge of the authors, the influence of nonisopycnic conditions on the length correction was not systematically investigated before. In this work, dedicated cold-flow experiments featuring density differences are conducted to investigate this phenomenon, and a density-dependent correction term is derived. A wide range of different densities are realized by blending air and krypton. [ABSTRACT FROM AUTHOR]

Damping of thermoacoustically induced pressure pulsations in combustion chambers is a major focus of gas turbine operation. Conventional Helmholtz resonators are an excellent means to attenuate thermoacoustic instabilities in gas turbines. Usually, however, the damping optimum is in a narrow frequency band at one operating condition. The work presented here deals with a modification of the basic Helmholtz resonator design overcoming this drawback. It consists of a damper body housing multiple volumes that are connected to each other. Adequate adjustment of the governing parameters results in a broadband damping characteristic for low frequencies. In this way, changes in operating conditions and engine-to-engine variations involving shifts in the combustion pulsation frequency can conveniently be addressed. Genetic algorithms and optimization strategies are used to derive these parameters in a multidimensional parameter space. The novel damper concept is described in more detail and compared with cold-flow experiments. In order to validate the performance under realistic conditions, the new broadband dampers were implemented in a full-scale test engine. Pulsation amplitudes could be reduced by more than 80%. In addition, it is shown that, due to sophisticated damper placement in the engine, two unstable modes can be addressed simultaneously. Application of the damper concept allowed a considerable increase of the engine operating range, thereby reducing NOx emissions by 55%. Predictions obtained with the physics-based model excellently agree with experimental results for all tested damper geometries, bias flows, excitation amplitudes, and most importantly with the measurements in the engine. [ABSTRACT FROM AUTHOR]

The aim of this study is to eliminate unimportant steps from a detailed chemical-kinetic mechanism in order to identify a skeletal kinetic mechanism that can predict with sufficient accuracy ignition delay times and laminar premixed-flame velocities for H 2 - C H 4 mixtures under conditions of practical interest in gas-turbine applications, which pertain to high pressure, high reactant temperature, and primarily lean-to-stoichiometric mixture compositions (although somewhat rich conditions also are considered for completeness). The accuracy of selected detailed chemical-kinetic mechanisms that are suited to represent combustion of hydrogen-methane mixtures in air was evaluated through comparison of computed and measured ignition delay times and laminar flame velocities, and because of its relative simplicity and sufficient accuracy, the San Diego mechanism was selected for the needed chemical-kinetic reduction. Under the pressure and temperature conditions of the mixture composition addressed, thirty nine reversible elementary steps involving eighteen species were found to suffice to describe with acceptable accuracy both the ignition delay time and the laminar burning velocities. The skeletal mechanism is given here, along with discussion of its derivation and characteristics, as well as comparison of its predictions with those of the detailed mechanism and, where possible, with experiment. Image 1 • A skeletal mechanism is developed for CH 4 /H 2 combustion at gas turbine conditions. • The prediction is validated over a wide range of initial conditions. • Good agreement for ignition delay time and laminar burning velocity. [ABSTRACT FROM AUTHOR]