Search

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.

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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–Phillips–Schmidt–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 100–300 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.

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 temperatures and therefore the sound speed field. They also control the flame itself, its stabilization characteristics 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 conduction 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 reasonable 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 observed 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. [ABSTRACT FROM AUTHOR]

Self-excited spinning mode azimuthal instability in an annular combustor with non-swirling flow is investigated using large eddy simulation (LES). Compressible Navier–Stokes equations are solved with a flamelet combustion model to describe the subgrid chemistry-turbulence interactions. Two flamelet models, with and without heat loss effects, are compared to elucidate the non-adiabatic wall effects on the thermoacoustic instability. The azimuthal modes are captured well by both models with only marginal differences in the computed frequencies and amplitudes. By comparing with the experimental measurements, the frequencies given by the LES are approximately 10% higher and the amplitudes are well predicted. Further analysis of the experimental and LES data shows a similar dominant anti-clockwise spinning mode, under which a good agreement is observed for the phase-averaged heat release rate fluctuations. Dynamic mode decomposition (DMD) is applied to shed more light on this spinning mode. The LES and experimental DMD modes reconstructed for their azimuthal mode frequencies agree very well for the heat release fluctuations. The DMD mode structure for the acoustic pressure from the LES shows a considerable non-zero profile at the combustor outlet, which could be essential for azimuthal modes to establish in this annular combustor. Finally, a low-order modelling study was conducted using an acoustic network combined with the flame transfer function extracted from LES. The results show that the dominant mode is associated with the plenum showing a first longitudinal and azimuthal mixed mode structure. By tuning the plenum length to match the effective volume, the predicted frequency becomes very close to the measured value. • A full annular combustor with 12 burners is simulated using compressible LES. • Self-excited thermoacoustic instability is reproduced well in the LES. • Low-order model is combined with LES to reveal physical insights for control. [ABSTRACT FROM AUTHOR]

Abstract: The nonlinear heat release response of a premixed swirl flame to velocity perturbations is investigated using Large Eddy Simulation. The nonlinear heat release response is required for the prediction of thermo-acoustic limit cycle pressure amplitudes and is represented here by the Flame Describing Function (FDF). As test case a premixed atmospheric swirl flame, for which experimental data on the FDF is available, was used. First a steady reacting LES solution was obtained and compared to experimental data. The simulation was then excited by superimposing a mono-frequency harmonic wave on the inlet boundary condition. Both the frequency and amplitude of the acoustic wave were varied to obtain the FDF. The calculated FDF was in good agreement with experimental data. At a frequency of 115Hz, the flame was found to saturate for larger excitation amplitudes. A detailed analysis of the LES results revealed that the mechanism causing the saturation was a nonlinear evolution of the area of the flame surface with increasing perturbation amplitudes. Furthermore it was shown that for the present swirl flame, the heat release and flame surface fluctuations were linearly dependent on the tangential component of velocity fluctuations upstream of the flame, while they increased nonlinearly with the axial component of velocity fluctuations. [Copyright &y& Elsevier]

Thermo-acoustic characterization of gas turbine combustion systems is crucial for a successful development of new gas turbine engines to meet emission and efficiency targets. In this context, it becomes more and more necessary to predict the limit cycle amplitudes of thermo-acoustic induced combustion instabilities in order to figure out if they can be tolerated or if they are above the critical design limit and will cause damage to the engine. For the prediction of limit cycle amplitudes, the nonlinear flame response of the combustion system is needed, which is represented in this work by the flame describing function (FDF). In this article, the identification of the FDF from a large eddy simulation (LES) is validated. The test case used was a premixed atmospheric swirl flame, for which experimental data on the FDF were available. First a steady reacting LES solution was obtained and compared to experimental data. The simulation was then excited by superimposing a mono-frequency harmonic wave on the velocity inlet boundary condition. Both the frequency and amplitude of the acoustic wave were varied to obtain the FDF. The calculated FDF was in good agreement with experimental data. At a frequency of 115 Hz, the heat release rate of the flame was found to saturate for larger excitation amplitudes. [ABSTRACT FROM AUTHOR]

• Large eddy simulation using flamelet generated manifold modelling considering NO transport effect was performed. • LES results are validated with the experimental results obtained by OH-PLIF and PIV techniques. • The NO production mechanism by the residence time and local temperature are analyzed. • The key components and chemical reactions for the NOx at with equivalence ratio are revealed. Ammonia, made up of 17.8% hydrogen, is one of carbon free fuels which can be used in large-scale power plants. However, ammonia combustion faces some challenges due to the high NOx emission. This study investigates the emission characteristics of CH 4 /NH 3 /air co-firing premixed swirling flames with NH 3 mole fraction up to 60% in a model combustor. Large eddy simulation (LES) coupled with flamelet generated manifold (FGM) model which considers the transport equations of slow chemical emission component was performed, with experimental validation in both flame structure and global emission. The simulation shows an overall good agreement with experimental results. It is found that the NO emission is improved with the NO transport equation calculated during the simulation. The results imply that the nitrogenous components, residence time and temperature are important factors that influence NO concentration profile. The residence time of inner recirculation indicates the NO level for CH 4 /NH 3 /air flames, and local high NO concentration is mainly caused by local long residence time. The OH and NO are correlative in premixed CH 4 /NH 3 /air flames because these components are both strongly related to the temperature. N 2 O has a correlation with NH and HNO components. The radical components such as HNO, O, OH etc. also influence the emission formation process. When the NH 3 ratio is closed to 40%, the concentration of H, OH, O2 and HNO are all adequate to produce maximum NO. [ABSTRACT FROM AUTHOR]

Large Eddy Simulation is applied to a non-premixed bluff-body stabilized swirled methane-air flame of the Sydney flame series. The combustion chemistry is included via the so-called premixed flamelet-generated manifolds, being a progress variable approach based on steady premixed laminar flamelets. As turbulent mixing and chemistry interact on the subgrid scales, additional modeling of the probability density function of the mixture fraction and the progress variable is required. Two different approaches considering the statistical independence of these two quantities are presented. No large differences between these approaches were observed in a first computation, therefore, only one method was investigated in more detail. Radial profiles of velocity components and species obtained in the latter computation are compared with experimental data. [ABSTRACT FROM AUTHOR]