Gas Turbine Combustor Research Articles

Experiments on symmetry breaking of azimuthal combustion instabilities and their analysis combining acoustic energy balance and flame describing functions

Combustion instabilities in annular systems raise fundamental issues that are also of practical importance to aircraft engines and ground-based gas turbine combustors. Recent studies indicate that the injector plays a significant role in the stability of combustors by defining the flame dynamical response and setting the inlet impedance of the system. The present investigation examines the effects of combinations of injectors of two different types ( $U$ and $S$ ) on thermoacoustic instabilities in a laboratory-scale annular combustor and compares different circumferential staging strategies. The combustor operates in a stable fashion when all injection units belong to the $S$ -family, but exhibits large amplitude pressure oscillations when all these units are of the $U$ -type. When the system comprises a mix of $U$ - and $S$ -injectors, it is possible to determine the number of $S$ -injectors leading to stable operation. For a fixed proportion of $U$ - and $S$ -injectors, some arrangements give rise to stable operation while others do not. Results also show that introducing symmetry-breaking elements affects the system's modal dynamics. These experimental observations are interpreted in an acoustic energy balance framework used to derive an expression for the growth rate as a function of the describing functions of the flames formed by the different injectors and their respective azimuthal locations. Growth rates are determined for the different configurations and used to explain the various observations, estimate the system damping rate and predict the location of the nodal line when the standing mode prevails.

FGM vs ATF: A comparative LES analysis in predicting the flame characteristics of an industrial lean premixed burner for gas turbine applications

In the pursuit of decarbonization, the reduction of greenhouse gas emissions from power generation through gas turbine (GT) engines plays a crucial role in the whole industrial sector. As industries strive to transition towards cleaner energy sources, the design and optimization of novel GT burners require a deep comprehension of the complex interaction between fluid dynamics and combustion processes embedded within the system. Computational Fluid Dynamics (CFD) plays a pivotal role in these processes by providing valuable insights into the complex flow patterns, flame topology, and stability limits within the combustor. Concurrently, the burner design phase necessitates a considerable number of simulations to ascertain flame stability limits under various burner designs and operating conditions. Therefore, it is imperative to control computational costs while ensuring a high level of accuracy. The present work is focused on a comprehensive comparative analysis of two widely employed turbulent combustion closure models: the Flamelet Generated Manifold (FGM) and the Artificially Thickened Flame (ATF). Both models utilize extended versions with specific modifications aimed at effectively addressing their respective limitations. The investigation is performed through a Large Eddy Simulation (LES) based CFD analysis within the context of a lean premixed burner designed by Baker Hughes and operated with methane at atmospheric pressure. The primary benchmark for numerical validation will be provided by detailed chemiluminescence images from a test campaign conducted by the University of Florence, thereby yielding valuable insights into flame topology and positioning. Furthermore, potential disparities in the flow field and fuel concentration at the burner exit between the two models will be revealed.

Analysis of Thermoacoustic Instabilities Using the Helmholtz Method in a Swirled Premixed Combustor

The Helmholtz method is developed to predict the self-excited thermoacoustic instabilities in a gas turbine combustor, combining flame describing functions, the measured damping rates under the firing condition, and the non-uniform spatial distributions of the physical parameters. The impact of the hydrodynamic and geometrical parameters on the thermoacoustic instabilities is investigated. The measured damping rates show lower values under a hot condition compared with those in a cold state. The experimental results indicate that the relative errors of the predicted eigenfrequencies and the velocity fluctuation levels are below 10%. The pressure amplitude decreases and the phase increases in the axial direction, indicating a typical 1/4-wavelengh mode. At a higher equivalence ratio, the mode shape in the axial direction becomes steeper due to the elevated fluctuation amplitude at the pressure antinode after enhancing the thermal power. When the air flow rate increases, the discrepancies between the pressure shape on the flame tube side and that on the plenum side are reduced. The velocity fluctuation level increases as the combustor length increases at a constant damping rate. In fact, the velocity fluctuation level first increases and then declines, caused by more significant damping rates when employing longer flame tubes. Self-excited thermoacoustic instabilities can be well predicted using the proposed method.

Experimental investigation of high-frequency transverse instability in Helmholtz resonator-coupled lean-premixed hydrogen combustor

Recent experimental studies suggest that lean-premixed pure hydrogen combustion plays a central role in generating self-sustained pressure oscillations, preferentially coupled with high-frequency tangential modes in land-based gas turbine combustors. The triggering of such high-frequency combustion dynamics is known to be closely connected with relatively small, flashback-resistant, characteristic nozzle dimensions, and the structural and kinematic properties of the premixed hydrogen flames. To identify important trends in the transverse modal dynamics of hydrogen flames, here we conduct detailed measurements of multislit injector-driven hydrogen flame dynamics. We analyze the measured data using the wave decomposition technique for PDF-based statistical assessments of bimodality and nodal line orientation. Experimentally, the planar-to-transverse mode transformation is revealed to be controlled by the combined influence of flame temperature and thermal power, which ultimately leads to the development of well-defined standing modes at around 4.69 kHz under the most severe test condition. Using analytic modeling and rigorous validation measurements, we also investigate Helmholtz resonator-type passive stabilization and its impact on the integrated system's thermoacoustic states with respect to the change in purge air flowrate. Our results show that the coupled system does not retain many of the initial thermoacoustic properties, but instead undergoes prominent changes in the first tangential modal dynamics, progressively evolving from a large-amplitude bimodal to a low-amplitude unimodal state.

Investigating the Mixture Quality in Multi-Injector Burner Systems, Part II: Model Application

Abstract Multi-injector burner systems offer a high spatial-temporal mixture homogeneity due to their small size and thus have a high NOx emission reduction potential at increasing flame temperatures. This potential is reduced due to the sensitivity of the mixing quality to inflow distortions caused by the flow path in the burner head. A deep understanding of the link between the inflow conditions and the mixture quality helps to optimize the flow field upstream of the injectors within the spatial and pressure loss constraints of gas turbine combustors to obtain minimal NOx emissions. This paper presents a new model approach for determining the mixture quality in multi-injector burners with output-based proper orthogonal decomposition (O-POD). The sensitivity to the inflow distortion is considered with the so-called observable vector, which describes the injector inflow conditions and is given as model input. The target quantity is the probability mass function (PMF) of the equivalence ratio at the injector outlet. This model allows a fast estimation of the mixing PMF for arbitrary inflow conditions, which is otherwise only accessible with complex time-resolved experimental or numerical approaches. The performance of the model was demonstrated with 21 reference datasets, for which a good agreement between the experimental results and the model output was obtained.

Numerical Simulation of Gas Self-Oscillations in Gas Turbine Combustors

Numerical Simulation of Gas Self-Oscillations in Gas Turbine Combustors

Thermo-elastic Characteristics of Thermal Barrier Coating in a Gas Turbine Combustion Chamber with an Intermediate FGM Layer

This study is focused on the analysis of thermos-elastic characteristics of a gas turbine combustion chamber, where the inner surface experiences a higher temperature while the outer region of the wall should be thermally conductive to dissipate the heat of combustion. Using a single material is not suitable to ensure the above two requirements simultaneously. Instead of using a single material, a layered combustion chamber with high-temperature material at the inner region and thermally conductive material at the outer region can be a solution. This produces the problem of a sharp interface where disbonding occurs due to high thermal stresses. The problem of sharp interface can be eliminated by using a functionally graded material (FGM) layer in between the inner and outer layers of the combustion chamber. The present study considers a gas turbine combustion chamber consisting of three layers: the inner layer is used as a thermal barrier coating, the intermediate layer is a smooth transition from the inner layer to the outer layer, and the outer layer is a thermally conductive material. ANSYS simulation is used for the analysis of thermos-elastic characteristics of the combustion chamber with homogeneous and FGM intermediate layers. The effect of linear and nonlinear material distributions in the FGM layer on the thermos-elastic characteristics is studied. A comparison of results reveals that thermal stress smoothly changes from the inner layer to the outer layer in the case of the FGM intermediate layer compared to the homogeneous intermediate layer. This suggests that a combustion chamber with an FGM intermediate layer is more reliable than a homogeneous intermediate layer.

Comparison of Response Surface Based Preliminary Design Methodologies for a Gas Turbine Combustor

Preliminary design of gas turbine combustor is a multi-objective optimization problem. The methodology to be used at the preliminary design stage depends on the freedom of design choices available. In this article, we explore three preliminary design methodologies for gas turbine combustor - M1: combustion liner design for a given casing; M2: combustion liner design without the casing and M3: coupled design of combustion liner and casing. A workflow for the automated design space exploration of gas turbine combustor using response surface methodology is presented. Computational fluid dynamics studies along with central composite design for design of experiments and genetic aggregation for response surface generation are used to quantify the combustor performance in design space. Comparison of three different design methodologies (M1, M2 and M3) is made to show how the choice of design methodology changes the available design space and limits/expands combustor performance. Candidate optimal designs and associated trade-offs from the optimization study are also presented. This study can aid combustor design engineers choose the most suitable preliminary design methodology for their specific use case.

Experimental and kinetic modeling study on laminar flame speeds and emission characteristics of oxy-ammonia premixed flames

The low flame speed and NOx emissions of ammonia are the key to affecting combustion temperature, stability, flame structure, and pollutant control, limiting its application in power generation devices. This study proposes oxy-ammonia combustion (oxygen content in oxidant = 100%, NH3/O2 combustion) as a novel method for significantly enhancing ammonia combustion. A method for measuring the laminar flame speed using the NH* distribution of the Bunsen burner flame is proposed, and the measured data is close to the constant volume combustion bomb. 21 kinetic models were collected and optimized after comparison with the experimental data of constant volume combustion bombs, and results show that Han et al.'s model has higher prediction accuracy for NH3/O2 flames. The preheating temperature was relevant to gas turbine combustors (298.15–500 K), and the equivalence ratio ranged from 0.7 to 1.6. At this extreme condition (oxygen content in oxidant = 100%), the results show that the maximum laminar flame speed reaches 1.25 m/s at ambient pressure and temperature, which is approximately 18 times that of NH3/air combustion, mainly due to the increased reaction rates of OH, H, O and NH2 radicals in the reaction zone (primary ammonia decomposition and H/O radical pool). The NO emission of NH3/O2 combustion at stoichiometric conditions is about 7216 ppmv@15%O2, 1.5 and 13 times that of NH3/air and CH4/air combustion, respectively. NO production and consumption may follow the N2O and NNH mechanism, and HNO is an important precursor for NO formation. Increasing the preheating temperature has less effect on the reaction pathway directions in the reaction network of nitrogen conversion but affects the path flux significantly.

Investigation of ignition process of a multi-swirl staged model combustor in low-temperature environment

The ignition reliability of lean combustion is fundamental to make sure that gas turbine combustor can operate in low-temperature environment reliably. It is important to investigate the influence of key factors on ignition process to improve ignition performance. Based on the multi-swirl staged model combustor, the effect of low-temperature inlet and fuel to air ratio (FAR) on the behaviors of the initial flame kernel and flame propagation. In this work, Large Eddy Simulation (LES) and Dynamic Thickened Flame (DTF) model coupled with skeletal chemical reaction mechanism of kerosene are used to capture flamelet information during the ignition process. The results show that the numerical method can capture the ignition process accurately. The axial velocity, droplet temperature and local equivalence ratio of the ignition position decrease as the inlet air temperature decreases, while the local equivalence ratio increases with the increase of FAR. Ignition fails when inlet air temperature reducing to 253 K, as local equivalence ratio and local FAR decrease, and the Karlovitz number (Ka) of the ignition position increases. The ignition with low-temperature inlet air can be successful by raising the FAR to 0.04 at the same time, but the ignition delay time is extended by 26.72 % and the flame propagation path is changed.

Plasma-sprayed GNP/CNT reinforced lanthanum-cerate coatings: Hot corrosion assessment in Na2SO4-V2O5 environment

This study investigates the hot-corrosion behavior of Carbon nanotube/Graphene-nanoplatelets (CNT/GNP)-reinforced lanthanum cerate (LC) coatings in a Na2SO4–V2O5 molten salt environment at 1350 °C for varying durations. Coatings were prepared using APS and characterized using XRD, FESEM, Raman, XPS, etc., techniques. The CNT/GNP-reinforced LCGC hybrid composite coating exhibited superior hot-corrosion resistance compared to LC and LCG coatings. The uniform dispersion of GNPs/CNTs within the coating matrix and the creation of protective LC oxides enhanced corrosion resistance. These findings highlight the potential of GNP/CNT-reinforced coatings in protecting high-temperature components like combustion chambers and gas turbine engines from hot corrosion.

Exploring NH3 combustion in environments with CO2 and H2O via reactive molecular dynamics

In existing power units such as internal combustion engines and gas turbines, the combustion of ammonia mixed with hydrocarbon fuels can effectively enhance the combustion intensity of ammonia. In the duel-fuel combustion scenario, the effect mechanism of the main components of combustion exhaust gas i.e., CO2 and H2O on ammonia combustion is still unclear and needs to be studied. Therefore, this paper uses reactive molecular dynamics simulations to study the combustion and emission processes of ammonia in environments with CO2 and H2O. The effects of the diverse gas environments (N2/O2, O2/CO2, and O2/CO2/H2O environments), high CO2 levels, varying H2O proportions, and O2 concentrations on ammonia combustion and emission were systematically explored. It was found that ammonia consumption was accelerated in the O2/CO2/H2O environments compared to N2/O2 environments, resulting in more NH2, H2, and H2O production. The effect of high CO2 concentration on ammonia oxidation was mainly related to temperature. The simulations found that the high CO2 concentration inhibited ammonia consumption at T < 2800K while enhancing it at T ⩾ 2800K. In addition, the high CO2 concentration could inhibit NO production under stoichiometric conditions but promote the conversion of nitrogen-containing intermediates to NO with the oxidizing capacity of CO2 enhanced significantly under extremely high temperature conditions. It was found that the increasing content of H2O molecules in the environment promoted NO production and inhibited CO production under stoichiometric conditions. In addition, the reaction paths for the conversion of HNO and HNO2 molecules into NO molecules were significantly enhanced with increasing H2O concentration. The increase in oxygen concentration can promote OH radical production and NO formation at high temperatures.

Large eddy simulation of combustion characteristics during dual fuel switching process in gas turbine combustor

The present study aims to investigate the combustion characteristics, fuel switching strategy, and flame stability associated with fuel switching processes in a dual-fuel gas turbine combustor. Specifically, the Large Eddy Simulation method is employed in conjunction with a skeletal chemical reaction mechanism to simulate the fuel switching process. It is revealed that the presence of a local rich combustion zone, formed by an increased accumulation of fuel in the head region, leads to the displacement of the heat release position towards the downstream inner shear layer. Additionally, this condition causes a reduction in the recirculation zone area and a deterioration in the uniformity of fuel blending. Simultaneously, the combustor experiences low-frequency, high-amplitude pressure pulsations, as well as pulsations in the heat release rate. Furthermore, a comparative analysis is conducted to assess the discrepancies among three distinct fuel supply strategies employed during fuel switching. The fast-opening strategy is found to demonstrate superior performance indicators, as it accomplishes filling the combustor performance indicators within 80% of the allocated time due to its lower initial fuel changing rate.

Overview of safety challenges associated with integration of hydrogen-based propulsion systems for climate neutral aviation

Electrification through hydrogen-based fuel cells as well as hydrogen combustion in gas turbines is a key strategy in aviation for achieving substantial reduction of emissions. However, this transition presents multifaceted challenges. Besides the development and improvement of technologies required for such hydrogen-fuelled aero engines, the safety of hydrogen storage and distribution systems on aircraft is paramount. Challenges associated with hydrogen in terms of its material properties, the design and selection of components for the conditioning and distribution, as well as the system design are being presented and discussed in this work. This includes the consideration of high diffusivity, flammability and reactivity of hydrogen and the consequences of these traits: hydrogen embrittlement, hydrogen-induced cracking and leakage, for instance. The challenges elaborated in this work are pertinent to both hydrogen fuel cell-based propulsion systems and hydrogen combusting gas turbines. Design considerations were derived and are being outlined in this work. These are transferable to applications in other industries such as automotive and stationary power plants. The need for novel rigorous safety protocols to enable a sustainable future in aviation is being highlighted.

Effects of uncontrolled combustion in a power plant gas turbine on fracture surface features and microstructural evolutions of a nickel-based superalloy blades

The present work investigates the fracture surface features and microstructural evolutions of nickel-based superalloy (Udimet 500) blades after uncontrolled combustion and overheating in a power plant gas turbine. In normal operation of the turbine, flameout occurred, but fuel continued to flow to hot section of turbine. The injected fuel failed to ignite as intended, but combusted upon contact with hot surfaces, including blades and the exhaust duct. This led to an explosion and the turbine failed after 753 equivalent operating hours (EOH) in normal operation. The majority of the damage was in the second-stage blades, primarily due to their proximity to the exhaust duct. The explosion in the exhaust duct led to the dynamic fracture of the second-stage blades and an increase in temperature, which caused the microstructural degradation of the airfoil part of the blades. The microstructure of the blades was investigated by an optical microscope (OM), a field-emission scanning electron microscope (FE-SEM) and energy-dispersive X-ray spectroscopy (EDS). The results revealed that the most of microstructure deterioration of the blades occurred in the airfoil of the blades and the microstructure of the root of the blades did not deteriorated since the airfoil experienced a higher temperature in comparison with the root. The degeneration of MC carbide to M23C6 carbide was observed at the airfoil grain boundaries, where M represents Cr and Mo. Furthermore, the morphology of γ' precipitates in the airfoil changed to cauliflower-morphology by growing and splitting mechanisms. The spalling nature of the fracture surface suggested that the dynamic fracture, which is produced by interaction between shock waves, as well as occurred. The failure of gas turbine blades was a consequence of uncontrolled combustion in the turbine section, leading to dynamic fracture and deterioration of the microstructure in the airfoil parts of the blades.

Thermal runaway effect of the centerbody on boundary layer flashback in a swirl-stabilized gas turbine burner operated with hydrogen

The effect of centerbody heating by the flame on flashback propensity is experimentally investigated. The burner is technically premixed with 100% hydrogen as fuel under atmospheric pressure. The centerbody temperature was monitored by thermocouples revealing a thermal runaway of the centerbody temperature at the onset of flashback. Post-processed OH*-chemiluminescence imaging data of the flame show an increase in OH*-chemiluminescence intensity during thermal runaway indicating either less quenching or auto-ignition events of hydrogen/air mixtures being in contact with the heated centerbody surface. The authors recorded the formation of an auto-ignited flame kernel upstream of the flame front. An analytical model involving an auto-ignition Damköhler number DaAI, the Reynolds number Re and the normalized burner pressure drop Δpbur/Δpburref is introduced predicting whether thermal runaway takes place under certain operating conditions. Experimental data from two different burners are applied to the model’s safety map and good agreement is found.

Experimental investigation on the effects of transverse injection distribution scheme on dual flame dynamics subjected to flow disturbances

Combustion instability has become a major problem in low-emission annular gas turbine combustors. It is of practical importance to investigate the effects of transverse nozzle distribution scheme on multi-flame dynamics subjected to external acoustic perturbations. In this study, the responses of heat release rate perturbations from dual propane/air laminar premixed flames to axial acoustic perturbations have been experimentally measured. Specifically, the velocity disturbance upstream the flame base has been measured by two-microphone method and the flame heat release rate perturbation is obtained by the photomultiplier tube with CH* filter lens. The dynamic evolution of dual flames is captured by the high-speed camera and then analyzed with the proper orthogonal decomposition (POD) method. Results indicate that there is a critical distribution distance, where the peak gain of the dual flame transfer function is the largest. And the direct injection dual flame transfer function can behave as the low-pass filter. Furthermore, the dual flame transfer function behaves nonlinearly with the increase of the injection angle. The dual flame transfer function for large injection angle can behave as the hybrid mode of the low-pass filter and the band-pass filter. The POD analysis reveals that the coupling effect of gas thermal expansion and stretch effect would contribute to the interaction between dual flame front dynamics. The transverse injection distribution scheme (dual flame distance and injection angle) has a significant effect on the dual flame front dynamics, which in turn affects the flame heat release rate disturbance. The bulk mean flow velocity can enhance the interaction effect between dual flame front dynamics.

Numerical investigation on mixing of heated confined swirling coaxial jets with blockage

Abstract A coaxial jet that has one application in gas turbine combustion chambers where there is a requirement for very good mixing of air and fuel for complete and stable combustion. A study has been carried out to improve the mixing of annular and central jets in the confined space of the combustor. The combustor model is used with two confined coaxial air-swirling jets under non-combustion conditions. The influence of a heated central jet and the effect of blockage at different locations in confined spaces on mixing has been studied. To understand mixing in jets in confinement, the center line axial velocity profile and streamlines have been presented at different locations. The computational results are obtained using commercial CFD software, ANSYS FLUENT 18.0, with a standard k–ω turbulence model. After validation with the existing literature, parametric studies have been carried out with co-swirl and counter-swirl conditions of annular and central jets at different blockage locations in confinement. The results show a good understanding of the mixing process of two jets in confinement.

Hybrid rich- and lean-premixed ammonia-hydrogen combustion for mitigation of NOx emissions and thermoacoustic instabilities

The large-scale direct utilization of two carbon-free fuels, ammonia and hydrogen, is currently attracting significant interest in the context of the development of new gas turbine combustion technologies, with paying close attention to the reduction of nitrogen oxides and unburned ammonia emissions. Motivated by recent observations that rich-premixed conditions tend to mitigate excessive NOx emissions from ammonia combustion, and that uniformly blended ammonia-hydrogen fuel/air mixtures tend to increase NOx production exponentially, here we propose a hybrid configuration of hydrogen-doped rich-premixed ammonia-air flames (inner stage) and lean-premixed pure hydrogen-air flames (outer stage) in a radially stratified primary reaction zone. This fuel staging scheme offers a possible mechanism for chemical kinetics-controlled NOx abatement and asymmetry-induced thermoacoustic instability suppression. The relevance and feasibility of the unconventional premixing approach are rigorously evaluated based on detailed measurements of exhaust gas concentrations and self-excited pressure fluctuations, in conjunction with OH*/NH2*/NH* chemiluminescence and OH PLIF imaging measurements. We observe that drastically different non-homogeneous reaction regions can be stably established in a comparatively compact combustion volume without producing negative flame stabilization effects – such as blowoff of less reactive ammonia flames and flashback of more reactive hydrogen flames. As compared with the uniform-blend baseline condition, the hybrid staging method is shown to significantly mitigate total nitrogen oxides emissions, from 7764 to 310 ppmvd (96 % reduction), and achieving an approximately two-fold reduction in dynamic pressure amplitude. Interestingly, hydrogen-enriched rich-premixed ammonia flames are revealed to exhibit anomalous oscillatory states originating from the preferential diffusion of hydrogen molecules and reaction rate-dependent separation of reactive layers, enabling interacting non-homogeneous reaction zones with markedly different characteristic time scales to resist the growth of intense pressure perturbations.

Experimental study on spray characteristics of aviation kerosene from a pressure-swirl nozzle in high-speed airflow with elevated temperatures

Spray characteristics of aviation kerosene are critical to the performance of gas turbine combustors. Actual gas turbine combustors are mainly operated at high temperatures and high-speed airflow environments, whereas almost all research on spray characteristics in high-speed airflow is limited to room temperature in the literature. In this paper, the spray characteristics of aviation kerosene from a pressure-swirl nozzle are investigated experimentally in the airflow temperature range of 313–500 K and the airflow speed range of 108–136 m/s. The information on spray characteristics such as droplet flux, velocity, and diameter was obtained by phase Doppler anemometry. Planar Mie scatter was performed to obtain the information on the concentration distribution of spray. The results show that the droplet flux, velocity, concentration, and Sauter mean diameter (D32) change significantly with airflow temperatures. In special, D32 in the center recirculation zone shows an increasing trend with increasing airflow temperature. The difference in the change of D32 along the propagation direction at elevated airflow temperatures is mainly due to the difference in evaporation processes. It was found that the evaporation models of a droplet that existed in literature do not predict well the D32 variation of spray in this experiment. A new spray evaporation model considering turbulence and fuel vapor fraction is proposed, which significantly reduces the prediction errors of variation in D32 in this experiment. This paper can provide experimental data and preliminary theoretical references for subsequent investigation of spray characteristics in high-speed airflow with elevated temperatures.