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This study numerically investigates the combustion characteristics in a swirl burner for CO-H2 syngas by large eddy simulation. Firstly, experimental data are used to verify the applicability of the Boivin mechanism and the Sandia mechanism. The simulation results show that the Boivin mechanism predicts a more reasonable flame structure. Then, Syngas25, Syngas45, and Syngas100 (composed of 25%, 45%, and 100% H2 by volume, respectively) are set to numerically investigate the effect of hydrogen addition ratio on combustion characteristics by using the dynamic Smagorinsky model and the partially stirred reactor model. The detailed simulation data shows that the addition of hydrogen greatly influences the flame structure and flow field; with an increase in hydrogen addition, the flame length shortens, the temperature rises, and flashback caused by combustion induced vortex breakdown occurs. Furthermore, the precessing vortex core cannot exist stably in Syngas100, though it can be clearly observed in other cases.

Based on an experimental system involving a pulsating airflow burner and gliding arc generator, the characteristics of gliding arc plasma at different flow rates and its control effect on the static instability of the swirl flame have been studied. The current, voltage, and power wave forms, as well as the simultaneous evolution of plasma topology, were measured to reveal the discharge characteristics of the gliding arc. A bandpass filter was used to capture the chemiluminescence of CH in the flame, and pressure at the burner outlet was acquired to investigate the static instability. Experimental results showed that there were two different discharge types in gliding arc plasma. With the low flow rate, the glow type discharge was sustained and the current was nearly a sine wave with hundreds of milliamperes of amplitude. With the high flow rate, the spark type discharge appeared and spikes which approached almost 1 ampere in 1 &mu, s were found in the current waveform. The lean blowout limits increased when the flame mode changed from stable to pulsating, and decreased significantly after applying the gliding arc plasma. In pulsating flow mode, the measured pressure indicated that static instability was generated at the frequency of 10 Hz, and the images of flame with plasma showed that the plasma may have acted as the ignition source which injected the heat into the flame.

This work predicts the evolution of self-excited thermo-acoustic instabilities in a gas turbine model combustor using large eddy simulation. The applied flow solver is fully compressible and comprises a transported sub-grid probability density function approach in conjunction with the Eulerian stochastic fields method. An unstable operating condition in the PRECCINSTA test case—known to exhibit strong flame oscillations driven by thermo-acoustic instabilities—is the chosen target configuration. Good results are obtained in a comparison of time-averaged flow statistics against available measurement data. The flame’s self-excited oscillatory behaviour is successfully captured without any external forcing. Power spectral density analysis of the oscillation reveals a dominant thermo-acoustic mode at a frequency of 300 Hz; providing remarkable agreement with previous experimental observations. Moreover, the predicted limit-cycle amplitude is found to closely match its respective measured value obtained from experiments with rigid metal combustion chamber side walls. Finally, a phase-resolved study of the oscillation cycle is carried out leading to a detailed description of the physical mechanisms that sustain the closed feedback loop.

The present article investigates the interactions between the pilot and main flames in a novel stratified swirl burner using both experimental and numerical methods. Experiments are conducted in a test rig operating at atmospheric conditions. The system is equipped with the BASIS (Beihang Axial Swirler Independently-Stratified) burner fuelled with premixed methane-air mixtures. To illustrate the interactions between the pilot and main flames, three operating modes are studied, where the burner works with: (i) only the pilot flame, (ii) only the main flame, and (iii) the stratified flame (with both the pilot and main flames). We found that: (1) In the pilot flame mode, the flame changes from V-shape to M-shape when the main stage is switched from closed to supplying a pure air stream. Strong oscillations in the M-shape flame are found due to the dilution of the main air to the pilot methane flame. (2) In the main flame mode, the main flame is lifted off from the burner if the pilot stage is supplied with air. The temperature of the primary recirculation zone drops substantially and the unsteady heat release is intensified. (3) In the stratified flame mode, unique beating oscillations exhibiting dual closely-spaced frequencies in the pressure spectrum. is found. This is observed within over narrow window of equivalence ratio combinations between the pilot and main stages. Detailed analysis of the experimental data shows that flame dynamics and thermoacoustic couplings at these two frequencies are similar to those of the unstable pilot flame and the attached main flame cases, respectively. Large Eddy Simulations (LESs) are carried out with OpenFOAM to understand the mechanisms of the time averaged flame shapes in different operating modes. Finally, a simple acoustic analysis is proposed to understand the acoustic mode nature of the beating oscillations., Comment: https://doi.org/10.1016/j.combustflame.2019.11.020

The recirculation zone and the swirl flame behavior can be influenced by the burner exit shape, and few studies have been made into this structure. Large eddy simulation was carried out on 16 cases to distinguish critical geometry factors. The time series of the heat release rate were decomposed using seasonal-trend decomposition procedure to exclude the effect of short physical time. Dynamic mode decomposition (DMD) was performed to separate flame structures. The frequency characteristics extracted from the DMD modes were compared with those from the flame transfer functions. Results show that the flame cases can be categorized into three types, all of which are controlled by a specific geometric parameter. Except one type of flame, they show nonstationary behavior by the Kwiatkowski&ndash, Phillips&ndash, Schmidt&ndash, Shin test. The frequency bands corresponding to the coherent structures are identified. The flame transfer function indicates that the flame can respond to external excitation in the frequency range 100&ndash, 300 Hz. The DMD modes capture the detailed flame structures. The higher frequency bands can be interpolated as the streamwise vortices and shedding vortices. The DMD modes, which correspond to the bands of flame transfer functions, can be estimated as streamwise vortices at the edges.

We have performed a detailed analysis of the temperature field in a turbulent swirl flame operating with a self-excited thermo-acoustic instability. The temperature field was measured using 5 kHz chirped-probe-pulse (CPP) femtosecond (fs) coherent anti-Stokes Raman scattering (CARS). The measurements are described in detail in the part 1 companion article. In this paper, part 2, a detailed analysis of the time-resolved temperature measurements and simultaneous pressure measurements is performed to provide insight into the dynamics and structure of the swirl-stabilized flame. This work is the first to capture the dynamics of the flame, flow, and coupled flow-flame processes using high-fidelity, spatially- and temporally-resolved thermometry in a flame of practical relevance. The time-averaged contour plot of the temperature field indicates that the flame is very flat and stabilizes approximately 10 mm downstream of the burner face. In this region, there are very significant temperature fluctuations indicating a very high level of unsteadiness. The temperature probability distribution functions (PDFs) are clearly bimodal in this region near the injector face. A Fourier analysis of the temperature time series revealed multiple coherent oscillatory modes. The strongest oscillation was found to be coherent and in-phase with an acoustic resonance at 314 Hz, as expected from the Rayleigh criteria for the unstable flame. An analysis of the phase-conditioned average temperature fields clearly shows an axial pumping of low-temperature reactants, which are consumed after a convective delay and result in a spike in the global heat-release rate. Continued analysis also revealed a 438 Hz oscillation that was found to correspond with the dynamics of convective transport by a helical precessing vortex core (PVC). The structure of the PVC, and its interaction with the flame, were studied based on the presence of this characteristic frequency in the power spectral densities computed throughout the flow. The precision and time-resolution of the CPP fsCARS measurements was also sufficient to enable computation of the integral time-scales as well as the PDFs of the temporal temperature gradients. A sample of state space trajectories were used to provide insight into the nature of coupling between the narrowband acoustic resonance and the broadband spectrum of turbulent flame processes.

International audience; The reliable prediction of the turbulent combustion processes in lean flames is nowadays of crucial importance in the design of gas turbine combustors. This work presents an assessment of the capabilities of Flamelet Generated Manifold (FGM) in the framework of Large-Eddy Simulation (LES), as implemented in the commercial CFD solver CONVERGE. One of the main characteristic of the code is the Adaptive Mesh Refinement (AMR) technique, namely the use of a dynamic mesh where elements size varies during the simulation. For validation purposes, the TECFLAM swirl burner, consisting of a strongly swirling, unconfined natural gas flame, has been chosen. Results highlight the advantages of AMR in describing turbulent flames, leading to a successful prediction of the main characteristics of the reacting flow field.

International audience; Large eddy simulations (LES) of combustion instabilities are often performed with simplified thermal wall boundary conditions, typically adiabatic walls. However, wall temperatures directly affect the gas temper- atures and therefore the sound speed field. They also control the flame itself, its stabilization characteris- tics and its response to acoustic waves, changing the flame transfer functions (FTFs) of many combustion chambers. This paper presents an example of LES of turbulent flames fully coupled to a heat conduc- tion solver providing the temperature in the combustor walls. LES results obtained with the fully coupled approach are compared to experimental data and to LES performed with adiabatic walls for a swirled turbulent methane/air burner installed at Engler-Bunte-Institute, Karlsruhe Institute of Technology and German Aerospace Center (DLR) in Stuttgart. Results show that the fully coupled approach provides rea- sonable wall temperature estimations and that heat conduction in the combustor walls strongly affects both the mean state and the unstable modes of the combustor. The unstable thermoacoustic mode ob- served experimentally at 750 Hz is captured accurately by the coupled simulation but not by the adiabatic one, suggesting that coupling LES with heat conduction solvers within combustor walls may be necessary in other configurations in order to capture flame dynamics.

A lean, swirl-stabilized gas turbine model combustor is simulated with a stochastic approach for combustion noise prediction. The employed hybrid and particle based method, FRPM-CN (Fast Random Particle Method for Combustion Noise Prediction) reconstructs temperature variance based direct combustion noise sources from local CFD-RANS turbulence and flow field statistics. Those monopole sound sources are used as right hand side forcing of the Linearized Euler Equations. First, findings from steady state CFD simulations are validated with experimental results. It is shown that the employed RANS models accurately reproduce the experimental flow field and combustion. Turbulence is treated with a two equation model and a global reaction mechanism is utilized for combustion. Subsequently, the specifications of the CCA (Computational Combustion Acoustics) setup is introduced and selected pressure spectra of the acoustics simulations are compared to experimental results, showing that FRPM-CN is able to deliver absolute combustion noise levels for the investigated burner at low computational costs.

Single-laser-shot temperature measurements at 5 kHz were performed in a gas turbine model combustor using femtosecond (fs) coherent anti-Stokes Raman scattering (CARS). The combustor was operated at two conditions; one exhibiting a low level of thermoacoustic instability and the other a high level of instability. Measurements were performed at 73 locations within each flame in order to resolve the spatial flame structure and compare to previously published studies. The measurement procedures, including the procedure for calibrating the laser system parameters, are discussed in detail. Despite the high turbulence levels in the combustor, signals were obtained on virtually every laser shot, and these signals were strong enough for spectral fitting analysis for determination of flames temperatures. The spatial resolution of the single-laser shot temperature measurements was approximately 600 µm, the precision was approximately ±2%, and the estimated accuracy was approximately ±3%. The dynamic range was sufficient for temperature measurements ranging from 300 K to 2200 K, although some detector saturation was observed for low temperature spectra. These results demonstrate the usefulness of fs-CARS for the investigation of highly turbulent combustion phenomena. In a companion paper, the time-resolved fs CARS data are analyzed to provide insight into the temporal dynamics of the gas turbine model combustor flow field.

Numerical investigation of turbulent swirling flames is performed in a model gas turbine combustor. The calculations are performed using the CFD code OpenFOAM. Large Eddy Simulation (LES) approach based on the Smagorinsky model is used as the main turbulence modelling strategy, whereas Unsteady Reynolds Averaged Numerical Simulations (URANS) are also applied, employing the Shear Stress Transport model as the turbulence model. Turbulence-chemistry interactions are modelled by the Eddy Dissipation Concept (EDC) and the Laminar Flamelet Model (LFM). In EDC, a three-step global reaction mechanism is used. In LFM, limitations of the standard non-premixed approach, based on the mixture fraction and the scalar dissipation rate, for lifted flames like the present one, is overcome by adding the progress variable as an additional dimension to the flamelet libraries. URANS is applied only with combination with LFM. LES is applied in combination with EDC and LFM. Special attention is paid to obtaining an adequate grid resolution. Predictions are compared with measurements. It is observed that LES provides a better accuracy compared to URANS, whereas the latter may still be seen useful, since its computational time is shorter. For LES, it is observed that EDC provides a similar, or even slightly better overall-accuracy compared to LFM. On the other hand, it is observed that LFM requires substantially shorter computational times compared to EDC. This makes LFM attractive especially for LES of real combustors requiring much larger grids and/or for cases where a detailed reaction mechanism is of interest.

Lean blowout (LBO) of a partially premixed swirl flame is studied using chemiluminescence imaging and simultaneous stereo-PIV and OH-PLIF measurements at repetition rates up to 5 kHz. The flame, which is operated with methane and air in a gas turbine model combustor at atmospheric pressure, features a pronounced precessing vortex core (PVC) at the inner shear layer. In the first part of the study, the stabilization mechanism of the flame close to LBO is investigated. The fields of velocity and OH show that near LBO there are essentially two regions where reaction takes place, namely the helical zone along the PVC and the flame root around the lower stagnation point. The zone along the PVC is favorable to the flame due to low strain rates in the vortex center and accelerated mixing of burned and fresh gas. The flame root, which is located close to the nozzle exit, is characterized by an opposed flow of hot burned gas and relatively fuel-rich fresh gas. Due to the presence of high strain rates, the flame root is inherently unstable near LBO, featuring frequent extinction and reignition. The blowout process, discussed in the second part of the study, starts when the extinction of the flame root persists over a critical length of time. Subsequently, the reaction in the helical zone can no longer be sustained and the flame finally blows out. The results highlight the crucial role of the flame root, and suggest that well-aimed modifications of flow field or mixture fraction in this region might shift the LBO limit to leaner conditions.

Velocity fields and flame structures of a partially premixed swirl flame in a gas turbine model combustor are measured in axial and transverse sections using simultaneous (Stereo-)PIV and OH-PLIF. The flame, operated under atmospheric pressure with air and methane at a thermal power of 10.3 kW and a global equivalence ratio of ϕ = 0.75, features thermoacoustic oscillations at a frequency f ≈ 295 Hz. The averaged flow field with inner and outer recirculation zones is typical of swirl-stabilized flames, and the instantaneous measurements show the presence of a helical vortex (PVC) located in the inner shear layer. The PVC, which rotates with a different frequency than the thermoacoustic oscillation, leads to an enhanced mixing of burned and unburned gas and thus to stabilization of the flame. Two distinct large-scale structures of velocity and OH are found in the transverse cross-sections. The first type is characterized by a roughly annular region of inflowing unburned gas and distinct inner and outer recirculation zones. In the second type, the region of positive axial velocity forms a spiral, and the recirculation zone consists of an inner region that is connected to the outer parts by a narrow curved zone along the spiral arm. Whereas the first type is a typical scenario of vortex breakdown with a PVC, the transient spiral recirculation zone observed here has, to our knowledge, not yet been reported. A phase-resolved analysis shows that the annular form correlates with low, and the spiral form with high rates of global heat release.

Topology transitions of swirl-stabilized flames in a cylindrical combustion chamber are analyzed for lean CH4/H2/air combustible mixtures. It is demonstrated that when the tip of a swirl flame is quenched at the combustor side wall, V to M flame topology transitions are triggered by flashback of the flame front along this wall. An analysis of the power spectral densities of the chemiluminescence luminosity reveals that these topological transitions are not consecutive to the appearance or disappearance of any axisymmetric or helical periodic flow structures interacting with the flame. It is shown that the appropriate space to analyze the stabilization regimes of these swirling flames corresponds to a diagram based on Karlovitz and Lewis numbers. Transitions from V to M flame shapes are found for a critical Karlovitz number below which the flame takes an M shape. The value of this critical Karlovitz number decreases when the Lewis number of the combustible mixture increases. The same type of law is found for M to V shape flame transitions, but this boundary does not coincide with the V to M shape transition boundary. In between these boundaries, the flame takes intermittently V or M shapes randomly. It is then shown that the critical Karlovitz number corresponding to the transition of V to M shape may be expressed by the product of a velocity gradient g calculated with the bulk injection velocity U and a characteristic scale of the outer recirculation zone, the laminar flame thickness α/SL, and the inverse of a vortex induced flame displacement speed S T θ . This displacement speed is related to the maximal azimuthal velocity of the swirling jet at the injector outlet and to the laminar burning velocity. Using this definition of the Karlovitz number, V to M flame topology transitions collapse roughly on the same curve for all flow conditions explored when the swirl number (0.3 ≤ S ≤ 0.68), injection bulk velocity (7 ≤ U ≤ 25 m · s − 1 ), equivalence ratio (0.5 ≤ ϕ ≤ 0.9), and hydrogen enrichment in the fuel blend ( 0 ≤ X H 2 fuel ≤ 0.37 ) are modified for rod stabilized and aerodynamically stabilized swirl flames. Results presented in this study might be used to ease predictions of the shape taken by swirl flames interacting with the chamber side wall when the flow operating conditions are modified.

This paper demonstrates a method for calculating thermoacoustic energy transfer (viz. Rayleigh Index) fields in complex swirl-stabilized flames having asymmetric 3D flow structures using high-repetition-rate OH* chemiluminescence measurements. Measurements were acquired in a variety of perfectly premixed methane-air flames, each of which contained a helical velocity disturbance that was coupled with a precessing vortex core (PVC). The azimuthal position of the PVC and helical disturbance relative to the OH* chemiluminescence viewing angle was determined by tracking the position of the chemiluminescence centroid. Tomographic reconstruction of multiply-phase-conditioned mean chemiluminescence fields then was performed to determine the mean 3D shape of the helically-perturbed heat release field at different phases of the thermoacoustic cycle. These fields, in combination with measured pressured signals, allowed calculation of the thermoacoustic energy transfer distribution. Complex patterns were found, which generally involved considerable energy transfer in the periphery of the burner (i.e. towards the outer recirculation zone). The method described provides a relatively simple and robust diagnostic for determining combustor regions driving thermoacoustic oscillations.

Sooting ethylene/air flames were investigated experimentally in a dual swirl gas turbine model combustor with good optical access at atmospheric pressure. The goals of the investigations were a detailed characterization of the soot formation and oxidation processes under gas turbine relevant conditions and the establishment of a data base for the validation of numerical combustion simulations. The flow field was measured by stereoscopic particle image velocimetry, the soot volume fractions by laser-induced incandescence, the heat release by OH chemiluminescence imaging and the temperatures by coherent anti-Stokes Raman scattering. Two flames are compared: a fuel-rich partially premixed flame with moderate soot concentrations and a second one with the same parameters but additional injection of secondary air. Instantaneous as well as average distributions of the measured quantities are presented and discussed. The measured soot distributions exhibit a high temporal and spatial dynamic. This behaviour correlates with broad temperature probability density functions. With injection of secondary air downstream of the flame zone the distributions change drastically. The data set, including PDFs of soot concentration, temperature and flow velocity, is unique in combining different laser diagnostics with a combustor exhibiting a more challenging geometry than existing validation experiments.Copyright © 2011 by ASME

The relation between flow field and flame structure of a turbulent swirl flame is investigated using simultaneous particle image velocimetry (PIV) and planar laser-induced fluorescence of OH (OH-PLIF). The measurements are performed in one axial and three transverse sections through the combustion chamber of a gas turbine model combustor, which is operated with methane and air under atmospheric pressure. Analysis of the velocity fields using proper orthogonal decomposition (POD) shows that the dominant unsteady flow structure is a so-called precessing vortex core (PVC). In each of the four sections, the PVC is represented by a characteristic pair of POD eigenmodes, and the phase angle of the precession can be determined for each instantaneous velocity field from its projection on this pair. Phase-conditioned averages of velocity field and OH distribution are thereby obtained and reveal a pronounced effect of the PVC in the form of convection-enhanced mixing. The increased mixing causes a rapid ignition of the fresh gas, and the swirling motion of the PVC leads to an enlarged flame surface due to flame roll-up. A three-dimensional representation shows that the PVC is accompanied by a co-precessing vortex in the outer shear layer, which, however, has no direct impact on the flame. As an alternative to phase averaging, a low-order representation of the phase-resolved dynamics is calculated based on the first pair of POD modes. It is found that small-scale structures are represented more accurately in the phase averages, whereas the low-order model has a considerable smoothing effect and therefore provides less detailed information. The findings demonstrate that the combined application of POD, PIV, and PLIF can provide detailed insights into flow–flame interaction in turbulent flames.