Sequential Combustion Research Articles

Feedback control of thermoacoustic instabilities in sequential combustor with nanosecond repetitively pulsed discharges

Controlling thermoacoustic instabilities is a necessary task for the development and operation of gas turbines. With the increasing proportion of renewable energy sources, gas turbines must have flexibility in terms of both fuel and load. This implies that thermoacoustic instabilities must be controlled under different operating conditions. Active control strategies are indeed very suitable in this case, as they can adapt to the changes in operating conditions. Nanosecond repetitively pulsed discharges (NRPD) has shown to be promising active control actuators because they require no moving parts. In this study, the effectiveness of feedback and open-loop control law with NRPD actuation to suppress thermoacoustic instabilities in a lab-scale sequential combustor is evaluated. The effect of the controller parameters on the NO emissions of the combustor is also quantified. The results indicate that there is a trade-off between acoustic pulsation reduction and NO emission.

Spontaneous ignition and flame propagation in hydrogen/methane wrinkled laminar flames at reheat conditions: Effect of pressure and hydrogen fraction

Direct numerical simulations (DNS) with detailed chemical kinetics and chemical explosive mode analysis (CEMA) are performed to investigate the effect of operating pressure and hydrogen–methane blending on laminar wrinkled flames at reheat combustion conditions. Building upon previous three-dimensional DNS datasets of reheat hydrogen flames, the present work considers two-dimensional configurations that render computationally-feasible simulations of high-pressure combustion and significantly more complex hydrocarbon chemistry. A geometrically-simplified representation of the reheat combustor is adopted and the thermodynamic states considered in the simulations are carefully chosen to mimic gas turbine operation conditions, which are constrained to achieve a target flame position and flame temperature. The two-dimensional DNS results are first assessed against the available three-dimensional data and then analyzed to extract the main trends exhibited by the reheat combustion process with respect to the fraction of the fuel consumed by spontaneous ignition and flame propagation, for increasing pressures and hydrogen fractions. Due to significant differences in the characteristic features of the velocity and thermal boundary layers between the three-dimensional and the two-dimensional configurations, a different global flame shape is observed (V-shaped flame vs W-shape flame) together with a ∼10% bias in the fraction of fuel consumed by spontaneous ignition. Crucially, results from the scaling studies reveal very clear trends with a significant decrease in the fuel consumption by spontaneous ignition for increasing pressures and hydrogen fractions, at the conditions investigated. Finally, this study highlights the important role that first-principle direct numerical simulations, even when conducted in computationally affordable two-dimensional simplified geometrical configurations, can play in obtaining detailed insights and quantitative trends useful for the characterization of complex reactive flows.Novelty and significanceThe reheat burners of the two-stage sequential gas turbine combustors are designed to stabilize flames primarily due to spontaneous ignition. However, prior numerical simulations revealed the presence of an additional assisted-ignition mode aided by recirculation zones. In this study, we used practically feasible two-dimensional direct numerical simulations of premixed hydrogen–air mixtures under lean conditions to quantify the roles of the two flame stabilization modes at different operating pressures. Achieving fundamental understanding of the effect of pressure and hydrogen fraction on flame stabilization is crucial to the development of two-stage sequential combustion and the present two-dimensional calculations provide important new insights. We show that at high pressures, both modes consume fuel to a similar extent. We also investigated the effect of methane blending on flame stabilization. This study also discusses how two-dimensional direct numerical simulations can be leveraged in the design optimization of reheat burners at low technology readiness levels.

Sulfur isotope analyses using 3× elemental analysis/isotope ratio mass spectrometry: Saving helium and energy while reducing analytical time and costs.

Helium (He) and energy shortages have caused price increases and reduced their availability. Using three combustion reactions per acquisition of carbon and nitrogen isotope ratios saves 50% He and energy during the elemental analysis/isotope ratio mass spectrometry (EA/IRMS). This approach needs to be tested for sulfur isotope (δ34S) analyses. A new method to measure δ34S in three sequential combustion reactions within one EA/IRMS acquisition was developed. The same material or blank samples could be used in the three reactions. After SO2 was used, a N2 purging method was employed to prolong the lifetime of the valves in the EA/IRMS interface. The 3×EA/IRMS was applied to measure δ34S in precious samples, such as Ag2S from acid-volatile and chromium-reducible sulfur extracted with a multiple-port setup. The 3×EA/IRMS-δ34S method was validated with replicate analyses of international reference materials and laboratory standards with a wide range of mineralogical compositions and δ34S values. The method provided a strategic advantage for the δ34S measurements of small precious samples (measured between blanks). The accuracy and precision of the 3×EA/IRMS values effectively matched those obtained using conventional EA/IRMS, with good agreement between the mean ± SD values and the recommended values with their uncertainties. Compared with the conventional EA/IRMS, the proposed method provides accurate and precise δ34S measurements of the sulfate and sulfide samples while saving approximately 50% of He, energy, SO2 reference gas, O2, analysis time, and cost. Notably, 3×EA/IRMS can provide up to three δ34S values unaffected by memory effects.

Potential of Miscanthus × giganteus as fuel in a suspended furnace

ABSTRACT Miscanthus is one of the potential bioenergy crops in the world. This non-woody biomass can grow almost anywhere and does not require high cultivation costs. Its energy content is approximately 525 GJ/ha/year, the highest among other non-woody biomass. This research aims to study the potency of Miscanthus × giganteus as fuel in a suspended furnace. Some combustion characteristics of Miscanthus particles were determined experimentally and numerically. At a 40°C/min heating rate, the particle's initial and active decomposition temperatures are 197 and 352°C, respectively. The study also reveals that the combustion mode of the Miscanthus particles was affected by the furnace temperature. At furnace temperatures of 300–400°C, the combustion mode of the Miscanthus particles refers to the simultaneous combustion model. While, at temperatures 450–600°C, its combustion produces extensive flames and is called the sequential combustion model. The extensive flames are useful in cyclone furnaces to maintain combustion stability. Under cold modelling, the average drag time of the representative particles with an equivalent diameter of 1384 µm, in a typical furnace, is 26.82 s, much higher than its combustion time of 11.25 s. This indicates that the particles can be burned out in a suspended state.

Numerical Study of Ignition and Combustion of Hydrogen-Enriched Methane in a Sequential Combustor

Ignition and combustion behavior in the second stage of a sequential combustor are investigated numerically at atmospheric pressure for pure CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{CH}}_{4}$$\\end{document} fueling and for two CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{CH}}_{4}$$\\end{document}-H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{H}}_{2}$$\\end{document} fuel blends in 24:1 and 49:1 mass ratios , respectively, using Large Eddy Simulation (LES). Pure CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{CH}}_{4}$$\\end{document} fueling results in a turbulent propagating flame anchored by the hot gas recirculation zones developed near the inlet of the sequential combustion chamber. As the H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{H}}_{2}$$\\end{document} content increases, the combustion process changes drastically, with multiple auto-ignition kernels produced upstream of the main flame brush. Analysis of the explosive modes indicates that, for the highest H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{H}}_{2}$$\\end{document} amount investigated, flame stabilization in the combustion chamber is strongly supported by auto-ignition chemistry. The analysis of fuel decomposition pathways highlights that radicals advected from the first stage flame, in particular OH, induce a rapid fuel decomposition and cause the reactivity enhancement that leads to auto-ignition upstream of the sequential flame. This behavior is promoted by the relatively large mass fraction of OH radicals found in the flow reaching the second stage, which is approximately one order of magnitude greater than it would be at chemical equilibrium. The importance of the out-of-equilibrium vitiated air on the ignition behavior is proven via an additional LES that features weak auto-ignition kernel formation when equilibrium is artificially imposed. It is therefore concluded that parameters affecting the relaxation towards chemical equilibrium of the vitiated flow can have an important influence on the operability of sequential combustors fueled with varying fractions of H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext{H}}_{2}$$\\end{document} blending.

BOATS: Bayesian Optimization for Active control of ThermoacousticS

This investigation presents novel adaptive control algorithms specifically designed to address and mitigate thermoacoustic instabilities. Gas turbines are limited in their operational range due to thermoacoustic instability. Two control strategies are available to alleviate this issue: active and passive. Active control strategies have a wider flexibility than passive control strategies because they can adapt to the operating conditions of the gas turbine. However, optimizing the control parameters remains a challenge, especially if additional constraints have to be fulfilled, such as e.g. pollutant emission levels. To address this issue, we propose three adaptive control strategies based on Bayesian optimization. The first and foundational algorithm is the safeOpt algorithm, and the two adaptations that have been made are stageOpt and shrinkAlgo. The algorithms facilitate safe exploration within the control parameter space, ensuring compliance with the constraint function, while simultaneously optimizing the objective function. The Gaussian Process Regressor (GPR) is employed to approximate both the objective and constraint functions, with continuous updates occurring during iterations. The algorithms also enable the transfer of knowledge obtained from one operating point to another, thereby reducing the number of iterations needed to reach the optimal point. We demonstrate the effectiveness of the algorithms both numerically and through two distinct experimental validations. In the numerical demonstration, we employ a low-order thermoacoustic network model to simulate a single-stage combustor setup equipped with loudspeaker actuation and a gain-delay (n−τ) controller for active stabilization. In the first experimental validation, we optimize the control parameters of a single-stage turbulent combustor with loudspeaker actuation and a gain-delay controller. For the second experimental validation, we apply the framework to a sequential combustor configuration utilizing nanosecond repetitively pulsed discharges (NRPD) as the control actuator. This demonstrates the framework’s adaptability to various control actuation methods in turbulent combustors where control parameter optimization is required.

Comparative radiative property measurements of single biomass and coal particles burning at high reactor temperatures

This research measured the emissivity, temperature and soot loading of pulverized bituminous coal and three types of renewable biomass. Combustion occurred in a laboratory-scale drop tube furnace (DTF), where the fuels experienced high heating rates and ignited and burned in two distinct phases: devolatilization accompanied by volatile matter combustion in envelope flames, and sequential combustion of the char residues. Multi-wavelength spectrometry was used to concurrently determine the emissivity and the temperature of burning particles at discrete points in their history. Three-wavelength pyrometry was used to obtain entire temperature-time histories, and high-speed photography was used to provide both visual observations of phenomena and to obtain two-dimensional temperature distributions. Results showed that soot in volatile matter flames of burning single particles of the bituminous coal and of the three types of torrefied biomass had spectral emissivities in the range of 0.03–0.10, in the wavelength domain of 600–1000 nm. Emissivities were nearly constant with wavelength, i.e., soot mantles in these flames radiated as graybodies. Spectral emissivities of the chars of the bituminous coal and the three types of torrefied biomass were in the range of 0.29–0.5 and varied with wavelength, i.e., they radiated as non-graybodies. The highest measured temperatures of soot in volatile flames for all fuels were between 2150 and 2360 K and maximum temperatures of chars were between 1700 and 2000 K, based on the three measurement techniques. The graybody assumption caused negligible (± 10 K) deviations in volatile matter flame temperatures; however, it caused significant deviations (± 20–100 K) in char temperatures. Envelope flame diameters of torrefied biomass particles were larger than those of coal. To the contrary, soot volume fractions in the envelope flames of coal particles were found to be higher than those in biomass particle flames. Finally, the soot volume fractions in the flames of these fuels were inversely proportional to particle size.

Numerical Assessment of a Rich-Quench-Lean Staging Strategy for Clean and Efficient Combustion of Partially Decomposed Ammonia in the Constant Pressure Sequential Combustion System

Abstract In a future energy system prospective, predictably dominated by (often) remote and (always) unsteady, nondispatchable renewable power generation from solar and wind resources, hydrogen (H2) and ammonia (NH3) have emerged as logistically convenient, chemically simple and carbon-free chemicals for energy transport and storage. Moreover, the reliability of supply of a specific fuel feedstock will remain unpredictable in the upcoming energy transition period. Therefore, the ability of gas turbine combustion systems to seamlessly switch between very disparate types of fuels must be ensured, aiming at intrinsically fuel-flexible combustion systems, i.e., capable of operating cleanly and efficiently with novel carbon-free energy vectors like H2 and NH3 as well as conventional fossil fuels, e.g., natural gas or fuel oils (back-up feedstock). In this context, a convenient feature of Ansaldo's constant pressure sequential combustion (CPSC) system, resulting in a fundamental advantage compared to alternative approaches, is the possibility of controlling the amount of fuel independently fed to the two combustion stages, depending on the fuel reactivity and combustion characteristics. The fuel-staging strategy implemented in the CPSC system, due to the intrinsic characteristics of the auto-ignition stabilized reheat flame, has already been proven able of handling fuels with large hydrogen fractions without significant penalties in efficiency and emissions of pollutants. However, ammonia combustion is governed by widely different thermo-chemical processes compared to hydrogen, requiring a considerably different approach to mitigate crucial issues with extremely low flame reactivity (blow-out) and formation of significant amounts of undesired pollutants and greenhouse gases (NOx and N2O). In this work, we present a fuel-flexible operational concept for the CPSC system and, based on unsteady Reynolds-Averaged Navier–Stokes (uRANS) and large eddy simulation (LES) performed in conjunction with detailed chemical kinetics, we explore for the first time full-load operation of the CPSC architecture in a Rich-Quench-Lean (RQL) strategy applied to combustion of partially-decomposed ammonia. Results from the numerical simulations confirm the main features of ammonia-firing in RQL operation already observed from previous work on different combustion systems and suggests that the CPSC architecture has excellent potential to operate in RQL-mode with low NOx and N2O emissions and good combustion efficiency.

Numerical study of nitrogen oxides chemistry during plasma assisted combustion in a sequential combustor

Plasma Assisted Combustion (PAC) is a promising technology to enhance the combustion of lean mixtures prone to instabilities and flame blow-off. Although many PAC experiments demonstrated combustion enhancement, several studies report an increase in NOx emissions. The aim of this study is to determine the kinetic pathways leading to NOx formation in the second stage of a sequential combustor assisted by Nanosecond Repetitively Pulsed Discharges (NRPDs). For this purpose, Large Eddy Simulation (LES) associated with an accurate description of the combustion/NOx chemistry and a phenomenological model of the plasma kinetics is used. Detailed kinetics 0-Dimensional reactors complement the study. First, the LES setup is validated by comparison with experiments. Then, the NOx chemistry is analyzed. For the conditions of operation studied, it is shown that the production of atomic nitrogen in the plasma by direct electron impact on nitrogen molecules increases the formation of NO. Then, the NO molecules are transported through the turbulent flame without being strongly affected. This study illustrates the need to limit the diatomic nitrogen dissociation process in order to mitigate harmful emissions. More generally, the very good agreement with experimental measurements demonstrates the capability of LES combined with accurate models to predict the NRPD effects on both turbulent combustion and NOx emissions.

Thermoacoustic stabilization of a sequential combustor with ultra-low-power nanosecond repetitively pulsed discharges

This study demonstrates the stabilization of a sequential combustor with Nanosecond Repetitively Pulsed Discharges (NRPD). Sequential combustors offer advantages such as enhanced fuel flexibility and broader operational range compared to the conventional combustors. However, thermoacoustic instabilities limit their potential. While passive control strategies like Helmholtz dampers have been utilized, active control methods have been hindered by the lack of robust actuators for harsh conditions with sufficient control authority. This study demonstrates successful suppression of instabilities using NRPD in a lab-scale atmospheric sequential combustor, even at low plasma power (1.1 W, about 1.5×10−3 percent of thermal power of 73.4 kW). We also examine the effect of pulse repetition frequency and plasma generator voltage on NO emissions and the sequential flame topology. Notably, higher plasma power can further enhance suppression, albeit with a slight increase in NO emissions. However, the highest plasma power in our study (81 W, 1.1×10−1 percent of flame thermal power) shows a slight improvement in acoustic suppression while significantly elevating NO emissions. Intriguingly, specific plasma parameter combinations can excite another acoustic mode, necessitating further exploration for optimized control. This pioneering study highlights the potential of ultra-low-power plasma-based thermoacoustic control in sequential combustors, paving the way for enhanced stability and performance.

Computation and prediction of intrinsic thermoacoustic oscillations associated with autoignition fronts

A sequential combustor architecture consists of two longitudinally staged combustion zones featuring a propagation-stabilized flame in the first stage and an autoignition-stabilized flame in the second (reheat) stage. This layout allows for increased fuel flexibility compared to single-stage systems and enables clean and efficient combustion of carbon-free fuels such as hydrogen. Interactions between the autoignition-stabilized flame and the acoustic field created by the unsteady ignition front and the surrounding boundaries can result in self-sustained (thermoacoustic) flame oscillations in the reheat stage of the sequential burner, causing performance losses and structural damage. In this paper, an intrinsic flame-acoustic feedback mechanism causing self-sustained thermoacoustic oscillations in a reheat combustor is studied. The intrinsic thermoacoustic feedback is established when an acoustic wave generated by the unsteady ignition front travels upstream and introduces local temperature, pressure and velocity perturbations in the unburnt mixture. These modulate the ignition chemistry and the front kinematics and thus, in turn, act as a new source of perturbation for the ignition front that generates heat release rate oscillations and upstream-traveling acoustic disturbances, closing the feedback loop. Compressible reactive Euler equation computations of autoignition fronts in a simple one-dimensional reheat combustor configuration with fully non-reflecting boundaries are first performed to demonstrate and characterize the intrinsic thermoacoustic oscillations associated with autoignition fronts. These computations show that the autoignition front exhibits distinct harmonic thermoacoustic oscillations which tend to become unstable at some conditions. Next, frequencies and growth rates of the intrinsic thermoacoustic oscillations in the linear regime are predicted using a hybrid approach with flame response transfer functions taken from a forced Euler calculation and using a linearized Euler equation framework to describe the combustor acoustic field. The predictions of the intrinsic thermoacoustic eigenvalue from the linearized Euler equation framework show good agreement with the linear behaviour of these oscillations observed in the Euler computations. The findings of the present work are useful both from a fundamental standpoint for prediction of intrinsic thermoacoustic instabilities and from an industrial perspective for prevention of thermoacoustic instabilities in staged combustion systems.

Combustion modelling of sequential combustion in steam-methane reformation (SMR) furnace using adiabatic flamelet generated manifold

Combustion modelling of sequential combustion in steam-methane reformation (SMR) furnace using adiabatic flamelet generated manifold

Effect of turbulent mixing on combustion behaviours of a single biomass pellet

Biomass co-firing in coal-fired power plants is an appealing technology for reducing greenhouse gas emissions due to the abundant biomass resource and low cost of retrofitting the existing power plants. Burning larger biomass particles is inevitable in real boilers due to the energy-consuming grinding process. A profound understanding of the combustion characteristics of larger biomass particles in a strong turbulent gas environment inside the boiler is essential for designing and operating biomass conversion facilities. This paper presents a detailed experimental study of the effects of turbulent mixing on the combustion of biomass particles. An impinging arranged four-fan stirred turbulent furnace was established in the laboratory. Particle image velocimetry (PIV) was adopted to measure the turbulent flow field characteristics under various fan speeds. The combustion behaviours of a 5 mm homemade cylindrical cellulose pellet were investigated under natural convection and turbulent flow at 700 °C. The centre and surface temperature of the particle, the shape and peak temperature of the volatile flame, and the sequential combustion history were measured and analysed. The results show that the turbulent fluctuation velocity Velrms increases linearly with increasing fan speed. Whether the gas environment is natural convection or turbulence has an essential impact on the combustion behaviours of the biomass particle. Increasing turbulence intensity accelerates the heating rate and significantly shortens the burnout time from 94 s to 55 s. The volatile flame area fluctuates randomly and generally decreases with the turbulent intensity due to the dilution of gas-phase volatiles by the turbulent eddies motion. Volatile flame has higher local luminosity and peak combustion temperature at a low turbulent fluctuation velocity of 0 ∼ 0.8 m/s but gradually weakens due to heat dissipation at a higher turbulent fluctuation velocity of 0.8 ∼ 1.6 m/s. The present results are the first to study the combustion behaviours of a single biomass pellet in different turbulent intensities, indicating that the effect of enhanced heat and mass transfer in turbulent environments on biomass combustion is essential and deserves further study.

Effect of Mixing on the Anchoring and Combustion Regimes of Pure Hydrogen Flames in Sequential Combustors

Abstract In this work, we perform an experimental study of the combustion of pure hydrogen in the sequential stage of a generic combustor. This academic test rig is a simplified model of an industrial sequential combustor. The sequential fuel is injected using different injector geometries. The composition and temperature of the hot stream at the inlet of the sequential burner are defined by the mass flows of the hot combustion products from the first stage (30 kW natural gas–air flame with equivalence ratio of 0.7) and of the dilution air. This temperature is varied between 1100 K and 850 K by modifying the dilution air mass flow in order to study the different combustion regimes of the sequential hydrogen flame. High-speed imaging of OH radicals chemiluminescence is performed with optical emission spectroscopy to measure vitiated gas temperatures. In particular, we investigate the transition from a flame anchored in the sequential combustion chamber, to the situation where it stabilizes upstream into the mixing section, when the inlet flow temperature is increased. Of particular interest is the increasing rate of formation of auto-ignition kernels in this transition process. The underlying combustion regime change is analyzed with 0D reactor simulations, and the limitations of such a simplified low-order model of the flame location are discussed. The effects and importance of the mixing process between fresh fuel and the hot vitiated coflow are examined. Two different injectors are compared under the same operating conditions that create different flow structures along the mixing section. As a result of that, they provide different degrees of mixing between the hydrogen and the hot vitiated flow and allow to demonstrate the impact of mixing quality on the flame morphology.

Computation of Intrinsic Instability and Sound Generation From Auto-Ignition Fronts

Abstract Burning carbon-free fuels such as hydrogen in gas turbines promise power generation with minimal emissions of greenhouse gases. A two-stage sequential combustor architecture with a propagation-stabilized flame in the first stage and an auto-ignition-stabilized flame in the second stage allows for efficient combustion of hydrogen fuels. However, interactions between the auto-ignition-stabilized flame and the acoustic modes of the combustor may result in self-sustained thermoacoustic oscillations, which severely affect the stable operation of the combustor. In this paper, we study an “intrinsic” thermoacoustic feedback mechanism in which acoustic waves generated by unsteady heat release rate oscillations of the auto-ignition front propagate upstream and induce flow perturbations in the incoming reactant mixture, which, in turn, act as a disturbance source for the ignition front. We first perform detailed reactive Navier–Stokes (direct numerical simulation (DNS)) and Euler computations of an auto-ignition front in a one-dimensional setting to demonstrate the occurrence of intrinsic instability. Self-excited ignition front oscillations are observed at a characteristic frequency and tend to become more unstable as the acoustic reflection from the boundaries is increased. The Euler computations yield identical unsteady ignition front behavior as the DNS computations, suggesting that diffusive mechanisms have a minor effect on the instability. In the second part of this work, we present a simplified framework based on the linearized Euler equations (LEE) to compute the sound field generated by an unsteady auto-ignition front. Unsteady auto-ignition fronts create sources of sound due to local fluctuations in gas properties, in addition to heat release oscillations, which must be accounted for. The LEE predictions of the fluctuating pressure field in the combustor agree well with the DNS data. The findings of this work are essential for understanding and modeling thermoacoustic instabilities in reheat combustors with auto-ignition-stabilized flames.

Stabilization of Auto-Igniting Flames Within a Gas Turbine Sequential Combustor, Through the Control of Static Temperature Variation—Detailed Numerical Investigation

Abstract Axially staged combustion systems offer both enhanced operability and fuel flexibility for gas turbines, allowing stable operation and low emissions across a wide range of engine loads. The sequential combustion concept, where the first combustion stage is supported by a standard lean premixed flame and the second stage relies on an auto-ignition-dominated flame, forms the focus of the present contribution. Within the present state of the art, the still-oxygen-rich exhaust gases from the first stage are mixed with second stage fuel within a sequential burner. Within the presently investigated concept, the flame is anchored by establishing a positive static temperature gradient within the burner. The advantage of such a concept is that it potentially allows for very small combustor residence times and can be easily incorporated into an integrated combustor–nozzle guide vane. The concept does however present significant challenges, which are investigated within then present contribution. A critical challenge is that, in order to setup the static temperature gradient, the flow has to be accelerated to a relatively high Mach number, ca. 0.7, and then decelerated in a diffusing section where the flame is located. Achieving fuel/air premixing and combustion, while achieving acceptable pressure drops is not trivial at the high velocities. Additionally, the dynamic stability of the concept is not clear and needs to be investigated. Within the present work, compressible computational fluid dynamics (CFD) is used to investigate the pressure drop characteristics within the system. It is demonstrated that for the system a total pressure drop of less than 6% can be achieved. To realize this, the premixing section includes multipoint fuel injection coupled with mixing devices. The arrangement is designed to both limit excessive pressure losses by focusing losses within regions of the flow where they contribute effectively to fuel/air mixing as well as locating the flame where Rayleigh losses are acceptable. The dynamic behavior of the system is studied by way of two-dimensional (2D) fully premixed CFD. Investigation of the flame response to harmonic perturbations in inlet temperature shows that the flame transfer function (FTF) is characterized by amplitude growing, in line with the concept of auto-ignition at low Mach number, linearly with frequency. The rate of growth with frequency of the FTF amplitude is rather high reaching up to 60 times the imposed relative fluctuation of inlet temperature at a frequency of 600 Hz. This rapid growth is in line with the behavior of auto-ignition at low Mach number. A substantial difference with the low Mach number concept is given by upstream traveling acoustic waves generated by the flame that going through high Mach number locations, can affect, in respect of the conservation of entropy transported by convection, the upstream temperature distribution and therefore auto-ignition itself.

Topological transitions of Jet A-1 lean azimuthal flames (LEAF)

Prior studies about liquid fuel combustion in a vitiated air environment have shown increased combustion efficiency with reduced NOx, CO, and soot emissions. The concept of lean azimuthal flame (LEAF), which can be associated to the latter combustion mode, is based on opposed injections of air and liquid fuel sprays in an axisymmetric chamber with a central outlet, which can result in a highly turbulent toroidal reaction zone. The mixture of fresh air and hot combustion products of each spray provides a vitiated cross-flow configuration to the next spray distributed along the chamber circumference, leading to ignition and sequential combustion of the sprays by the others. The present paper deals with a LEAF combustor with air-assisted spray atomization, which has not been investigated so far. The combustor is fueled with Jet A-1 and operated from 15 to 25 kW with variations in the atomization-air to liquid mass flow ratio (ALR). This study focuses on the flame topology transitions as a function of atomizer ALR. Experimental results based on flame chemiluminescence and OH planar laser-induced fluorescence show two flame topologies: tubular and LEAF topology for ratio of 2 and 4, respectively (denoted ALR2 and ALR4). The spray Mie scattering indicates a significant presence of unburnt droplets for ALR2, whereas quick evaporation is observed for ALR4 cases. In this paper, we propose and validate a basic model based on the spray droplet size distribution, and the evaporation and convection timescales, which are the prominent factors governing the flame topology. Indeed, for ALR2, the evaporation timescale is longer than the convective timescale, which causes incomplete spray evaporation and insufficient vitiated environment, leading to a tubular flame topology and preventing a LEAF to develop. In contrast, for ALR4, the spray evaporation timescales are smaller than the convective timescales, which aids the LEAF topology.

Combustion regime transition of H[formula omitted] flames during steady and transient operation of a sequential combustor

The combustion regime transition in a sequential burner (SB) supplied with H2 (48 kW) is experimentally studied during steady and transient operation. The test rig is a simplified model of an industrial sequential combustor featuring two-staged combustion chambers separated by a mixing section in which dilution air and fuel are injected. The temperature, velocity and composition of the hot vitiated gas flowing through the SB are defined by the products from the first stage (30 kW natural gas-air flame at equivalence ratio 0.7), and by the mass flow of dilution air m˙DA. To study the combustion regime transition during steady operation of the combustor, m˙DA is fixed at several values between 22 g/s and 7 g/s. For transient operation investigations, m˙DA is suddenly changed between 20 and 7 g/s, which triggers a fast transition of the combustion mode. High-speed hydroxyl radicals OH* chemiluminescence is used to characterize the combustion process, and optical emission spectroscopy (OES) and tunable diode laser absorption spectroscopy (TDLAS) are respectively used to extract mean and time-resolved temperatures of the vitiated gas in the SB. In particular, we investigate the transition from a propagation-driven turbulent flame anchored at the inlet of the sequential combustion chamber, to a flame stabilized by autoignition inside the mixing section of the burner when the dilution air mass flow is suddenly reduced. Zero-dimensional (0D) simulations are used to analyze the underlying combustion regime transition. A 0D reactor network is developed and calibrated with the experimental data. This simplified low-order model predicts well the flame location for both steady and transient operation. Moreover, the good agreement between the numerical results and the experimental data demonstrates that time-resolved TDLAS successfully enables measurement of small temperature variations in the vitiated flow associated with non-perfect mixing of the different streams in the SB.

Effect of plasma-flow coupling on the ignition enhancement with non-equilibrium plasma in a sequential combustor

The effect of the regime of nanosecond repetitively pulsed discharges (NRPDs) on ignition and stabilization of a natural-gas/hydrogen/air flame in the sequential stage of a lab-scale atmospheric pressure sequential combustor is investigated experimentally. Electrical parameters of the NRPDs are characterized by measuring voltage, current, and deposited energy. Fast gas heating (FGH) of the nanosecond discharges is measured in a single pulse regime and validated by means of 0D kinetic modelling. It was found that the conventional scheme for energy release from internal degrees of freedom adequately describes the dynamics of FGH in vitiated hot environment diluted with air. Short-gated ICCD imaging and spatially-resolved emission spectroscopy are used to identify the coupling between the NRPDs and the vitiated hot flow. The effectiveness of the NRPDs actuation is assessed through the chemiluminescence images of the sequential flame. The distance of the center of gravity of the sequential flame to the outlet of the mixing channel is evaluated, with and without plasma actuation. The effect of fuel reactivity on plasma effectiveness is also studied by varying the fraction of hydrogen in the fuel blend of the second stage of the combustor. The results show that the glow NRPDs regime allows strengthening the flame anchoring for the most reactive blends considered in this work, while the spark NRPDs is required for the ignition and prevention of lean blow-out of the flame for the least reactive fuel blends which exhibit low fractions of hydrogen.

Numerical study of plasma assisted combustion in a sequential combustor

The effects of Nanosecond Repetitively Pulsed Discharges (NRPDs) on the second stage flame of a sequential combustor is investigated using numerical tools supported by experiments. In this work, Large Eddy Simulations (LESs) with an accurate description of the combustion chemistry and a simplified model of the plasma kinetics are performed and successfully reproduce experimental observations of the combustion enhancement obtained with NRPDs. Detailed plasma kinetics calculations and experimental data demonstrate that the simplified plasma modelling retrieves the main energetic routes for the dissipation of the electron energy during the discharges. This model initially developed for discharges in pure air, is still valid for the highly air-diluted mixture considered in this study. The results of the LESs are analyzed to explain the underlying physics. Chemical Explosive Mode Analysis (CEMA) is used to identify the combustion modes of the ignition kernels, which result from the fast gas heating and the radical production by the NRPDs. These ignition kernels quickly give rise to propagating flames, advected through the sequential burner. Moreover, it is shown that the oxygen atoms produced by the Non-Equilibrium Plasma (NEP) dramatically accelerate the oxidation of the fuel.