Turbulent Lean Premixed Flames Research Articles

Can flamelet manifolds capture the interactions of thermo-diffusive instabilities and turbulence in lean hydrogen flames?—An a-priori analysis

Flamelet-based methods are extensively used in modeling turbulent hydrocarbon flames. However, these models have yet to be established for (lean) premixed hydrogen flames. While flamelet models exist for laminar thermo-diffusively unstable hydrogen flames, for which consideration of curvature effects has resulted in improved model predictions [1], it is still unclear whether these models are directly applicable to turbulent hydrogen flames. Therefore, a detailed assessment of stretch effects on thermochemical states in a turbulent lean premixed hydrogen-air slot flame through finite-rate chemistry simulations is conducted. Strain and curvature are examined individually using a composition space model, revealing their distinct influences on thermochemical states. An a-priori analysis confirms that the previously developed tabulated manifolds fall short of capturing all turbulent flame phenomena, necessitating a novel manifold incorporating both strain and curvature variations. These results underscore the significance of these variations in developing manifold-based combustion models for turbulent lean hydrogen flames.

A Numerical Study of Flow Structures and Flame Shape Transition in Swirl-Stabilized Turbulent Premixed Flames Subject to Local Extinction

ABSTRACT Large Eddy Simulations (LES) of turbulent lean-premixed flames of V- and M-shape are presented. A simple algebraic closure with the ability to capture finite-rate chemistry effects is used for subgrid reaction rate modeling. The V-shaped flame is stabilized in the inner shear layer between a swirling annular jet and a central recirculating bubble in a sudden expansion duct. The M-shaped flame is stabilized in the inner and outer shear layer, adjoining the corner recirculation zone induced by the vertical step. The focus of the study is on the flow fields and shapes of the flames, which distinguish themselves through different heat load and sensitivity to local extinction. Good agreement with measurements is observed for the cold and the reacting flow cases. The numerical results suggest that the entrainment of hot gases into the outer recirculation zone occurs close to the impingement point of the swirling annular jet on the wall and this process is strongly dependent on intense vortical structures in the outer shear layer. The results further suggest that local extinction influences the position of the flame in the inner shear layer and, thereby, also the intensity of the local entrainment process.

Stretch Effects in Large Eddy Simulation of Turbulent Premixed Bunsen Flames

ABSTRACT The stretch effects in large eddy simulation (LES) of a turbulent lean propane-air premixed Bunsen flame in the wrinkled flamelet regime are investigated. A simple approach to model the subgrid-scale flame stretch is proposed. It is based on the analysis which shows that, when the surface-filtered strain rate in a subgrid-scale stretch model is approximated using the volume-filtered velocity field solved for in LES, heat release and associated gas expansion in the filtered flame brush tend to artificially alter the wrinkling of resolved flame fronts. To minimize such artifacts, it is suggested that the volume-filtered strain rate on the unburned side of the filtered flame brush be used to approximate the surface-filtered strain rate and projected through the filtered flame brush. The results show the importance of mitigating the artificial heat release effects when considering the strain effects in LES. The relative importance of the curvature stretch is also investigated in terms of the mean and local effects. For a Bunsen flame with a positive Markstein number, the mean flame curvature effect tends to increase the total burning rate by enhancing the laminar flame speed near the flame tip, while the local flame curvature effect tends to decrease the total burning rate by suppressing the wrinkling of the resolved flames. It is found that the competition between the two makes the overall curvature effects not influence the total burning rate much for the flame investigated here, as compared with the strain effects.

Impact of non-symmetric confinement on the flame dynamics of a lean-premixed swirl flame

The impact of confinement on a turbulent lean premixed swirl flame is investigated using a finite-volume large-eddy simulation method to solve the compressible Navier-Stokes equations and a combined G-equation progress variable approach to model the flame. The geometry is an experimentally investigated burner by Moeck et al. [Combust. Flame, 159, 2650-2668 (2012)] in which a precessing vortex core (PVC) and a self-excited thermoacoustic instability occur. To analyze the effect of confinement on the M-shaped flame, three configurations are investigated, i.e., an unconfined configuration, a symmetric confined configuration, and a non-symmetric confined configuration. The symmetric confined configuration corresponds to the experimental burner and the numerical results are in good agreement with the measurements. The flow fields of the confined and unconfined configurations differ significantly due to a more pronounced PVC downstream of the injection tube in the confined configurations. The numerical results confirm experimental findings from the literature, i.e., the confinement defines the recirculation zones and the turbulence intensity of the swirling jets. Furthermore, the present results show that the limit-cycle amplitude of the thermoacoustic instability, which occurs in the confined configurations due to a resonant coupling of the flame with the acoustic quarter-wave mode of the combustion chamber, is significantly reduced in the non-symmetric confined configuration. The mode determined by a dynamic mode analysis (DMD) that describes the impact of the acoustic quarter-wave mode on the velocity field only occurs in the symmetric confined configuration. Consequently, a lower coupling of the acoustic oscillations due to the thermoacoustic instability with the flame is evident. The results emphasize the sensitivity of the thermoacoustic instabilities on the confinement configuration and indicate the dependence of their oscillation amplitudes on the location of the swirl flame in the combustion chamber.

Erratum to “impact of burner plenum acoustics on the sound emission of a turbulent lean premixed open flame”

Erratum to “impact of burner plenum acoustics on the sound emission of a turbulent lean premixed open flame”

Lean blow-out prediction in an industrial gas turbine combustor through a LES-based CFD analysis

The use of lean premixed flames to control combustion temperature and limit nitric oxides (NOx) formation is a common practice in all the applications of land-based gas turbines irrespective of their size and power production capacity. Despite the capability of lean premixed combustion to nominally reduce NOx emissions well below the most strict international regulations, such technology still requires improvements to overcome the impact on whole engine operability due to possible onset of combustion instabilities and flame lean blow-out (LBO). Both phenomena involve highly unsteady processes resulting in challenging numerical modeling. Particularly critical is the prediction of lean blow-out mechanisms where the role of finite rate chemistry and turbulence cannot be easily simplified. The current study aims at proving the capability of a CFD methodology based on Large Eddy Simulation (LES) and on an extended version of the Turbulent Flame Closure (TFC), to describe and properly catching the dynamics of flame extinction at the leanest operating conditions in typical turbulent lean premixed flames adopted in gas turbine combustors. The methodology is first validated on a laboratory test case where detailed experimental results are available. The extended TFC model well describes the LBO process observed in an atmospheric swirl stabilized burner investigated at the University of Cambridge. The proposed approach is then applied to the investigation of a real scale combustor operated with a swirl lean burner, representing the standard device adopted by Baker-Hughes on the heavy-duty 16 MW class engine Nova LT16. An accelerated numerical procedure is used to trigger the LBO, considering both constant firing temperature and constant pilot split conditions. An excellent agreement is observed with respect to the data obtained on a full-annular test rig, confirming the validity of the developed LBO modeling strategy which can be used in design phase to improve the overall engine operability.

LES of a turbulent swirl flame using a mesh adaptive level-set method with dynamic load balancing

A turbulent lean premixed swirl flame is numerically investigated by solving the Navier–Stokes equations with a finite-volume large-eddy simulation (LES) method. A combined G-equation progress variable approach is applied to model the combustion process using a solution adaptive level-set solver. The finite-volume and the level-set solver are parallelized and coupled on a joint hierarchical Cartesian mesh, where each solver can individually use and adapt a subset of the mesh cells. The level-set solver adapts the mesh locally in a band close to the flame front location to automatically satisfy the high accuracy requirements of the level-set solution. The numerical results for the turbulent swirl flame are in excellent agreement with experimental findings. Since the flame shape varies during the computation, the number of cells associated with the flame shifts between the individual subdomains which changes the workload distribution on the parallel computing processes. To achieve a high parallel efficiency, a dynamic load balancing method is applied which determines a new partitioning of the grid using estimated cell weights based on measured computing times to redistribute the cells among all subdomains accordingly. The efficiency of the dynamic load balancing scheme to reduce load imbalances is demonstrated for a large-scale simulation of the investigated swirl flame on 170,000 compute cores. That is, the dynamic load balancing scheme reduces the computing time of this simulation by approximately 30%.

Erratum to "impact of burner plenum acoustics on the sound emission of a turbulent lean premixed open flame"

Erratum to "impact of burner plenum acoustics on the sound emission of a turbulent lean premixed open flame"

Effect of hydrogen enrichment on flame broadening of turbulent premixed flames in thin reaction regime

An experimental study to identify the effect of hydrogen enrichment and differential diffusion on the flame broadening is conducted. Turbulent lean premixed flames in the Broadened Preheat–Thin Reaction (BP-TR) regime are obtained. The flames are stabilized on a Bunsen burner and CH4/H2/air mixtures are adopted with three hydrogen fractions of 0, 30% and 60%. The preheat zone and heat release zone are captured with the multi-species Planar Laser-Induced Fluorescence (PLIF) of OH and CH2O radicals. Flame thicknesses of the preheat and heat release layers are measured. Results show broadened preheat zone and thin heat release layers for the flames, as predicted by the BP-TR regime. The preheat zone thickness can be increased to about 3–6 times compared to the laminar preheat thickness. An apparently decreased preheat zone thickness with hydrogen addition is observed. The differential diffusion is anticipated to locally thicken the heat release zone along the flame front. The mean heat release thickness is nearly not affected by the turbulence or hydrogen addition.

Effect of differential diffusion on turbulent lean premixed hydrogen enriched flames through structure analysis

This study presents the flame structure influenced by the differential diffusion effects and evaluates the structural modifications induced by the turbulence, thus to understand the coupling effects of the diffusively unstable flame fronts and the turbulence distortion. Lean premixed CH4/H2/air flames were conducted using a piloted Bunsen burner. Three hydrogen fractions of 0, 30% and 60% were adopted and the laminar flame speed was kept constant. The turbulence was generated with a single-layer perforated plate, which was combined with different bulk velocities to obtain varied turbulence intensities. Quasi-laminar flames without the plate were also performed. Explicit flame morphology was obtained using the OH-PLIF. The curvature, flame surface density and turbulent burning velocity were measured. Results show that the preferential transport of hydrogen produces negatively curved cusps flanked with positively curved bulges, which are featured by skewed curvature pdfs and consistent with the typical structure caused by the Darrieus-Landau instability. Prevalent bulge-cusp like wrinkles remain with relatively weak turbulence. However, stronger turbulence can break the bulges to be finer, and induce random positively curved cusps, therefore to destroy the bulge-cusp structures. Evident positive curvatures are generated in this process modifying the skewed curvature pdfs to be more symmetric, while the negative curvatures are not affected seriously. From low to high turbulence intensities, the hydrogen addition always strengthens the flame wrinkling. The augmentation of flame surface density and turbulent burning velocity with hydrogen is even more obvious at higher turbulence intensity. It is suggested that the differential diffusion can persist and even be strengthened with strong turbulence.

Mutual synchronization of two flame-driven thermoacoustic oscillators: Dissipative and time-delayed coupling effects.

Low-emissions can-annular gas turbines are prone to develop low-frequency self-excited thermoacoustic oscillations. Such oscillations arise from the coupling between adjacent combustors and can increase wear and thermal stresses. In this experimental study, we explore the mutual synchronization of two thermoacoustic oscillators (i.e., two model combustors) interacting via dissipative and time-delayed coupling, as introduced via a cross-talk section. Unlike most previous studies, our study makes use of a turbulent lean-premixed flame in each combustor, bringing the system configuration closer to that of practical gas turbines. Using stationary and transient measurements, we examine the effect of the cross-talk diameter and length so as to gain insight into the effect of dissipative and time-delayed coupling. We find that strengthening the dissipative coupling promotes mutual synchronization, but that weakening the dissipative coupling leads to weakly coupled or desynchronized oscillations. On operating the two combustors at different conditions, we find a significant reduction in their overall oscillation amplitude for some coupling conditions. On varying the combustor length and examining the transient response, we find elaborate changes in the pressure-heat-release-rate coupling, spontaneous mode transitions between coupled thermoacoustic modes, and the emergence of a rhomboid structure in the phase plane owing to the coexistence of in-phase and out-of-phase synchronization. In the combustion community, these two types of synchronization are known to be associated with push-push modes and push-pull modes. These findings offer new insight into the mutual synchronization of low-frequency, self-excited thermoacoustic oscillations in can-annular gas turbines, paving the way for the development of improved control strategies.

Impact of burner plenum acoustics on the sound emission of a turbulent lean premixed open flame

The acoustic field of a turbulent lean cpremixed open flame is numerically investigated by a hybrid method solving the Navier-Stokes equations in a large-eddy simulation formulation and the acoustic perturbation equations. The interaction of acoustic modes of a burner plenum and the turbulent flame is analyzed with respect to the sound emission of the flame. It is investigated if a simplified computation yields a good broadband agreement of the sound pressure spectrum with experimental measurements. The results of two numerical setups, i.e., the first configuration consists of the burner plus the plenum geometry while in the second configuration the plenum is neglected, which is often done in technical applications due to computational efficiency reasons, are compared with experimental findings. It can be concluded that the plenum has a pronounced impact on the dynamics and combustion noise of the open flame. To be more precise, the comparative juxtaposition of the numerical and experimental results shows a good agreement only for the full burner-plenum computation since the interaction of the acoustic quarter-wave modes of the burner plenum with the jet flow has to be captured. The interaction of these quarter-wave modes with the flow is analyzed and the acoustic response to heat release fluctuations of the flame of the full burner-plenum computation is compared to that of the simplified burner computation, in which the plenum acoustics is neglected. Due to the excitation by the plenum acoustics, the jet flow of the full burner plenum contains higher turbulent kinetic energy and the flame is excited at several additional frequencies which result in distinct peaks in the acoustic spectrum and a higher overall sound pressure level.

Effects of non-thermal plasma on the lean blowout limits and CO/NOx emissions in swirl-stabilized turbulent lean-premixed flames of methane/air

This study investigates experimentally the effects of non-thermal plasma (NTP) induced by a dielectric barrier discharge (DBD) reactor on the characteristics of swirl-stabilized turbulent lean-premixed methane/air flames in a laboratory scale combustor by systematically varying the applied AC voltage, VAC, and frequency, fAC. Especially, it is elucidated how the NTP influences the lean blowout (LBO) limits and the characteristics of CO/NOx emissions depending on flame configuration. Without applying the NTP as the mixture equivalence ratio, ϕ, decreases from the stoichiometry to an LBO limit, the flame configuration changes from an M-flame (Regime I) to a conical flame (Regime II) and to a columnar flame (Regime III) for the whole range of the mixture nozzle exit velocity, U0, (4–10 m/s). With the NTP, however, it exhibits only Regimes I and II at relatively-low U0 range (4–6 m/s), while all three regimes at relatively-high U0 range (7–10 m/s). For both velocity ranges, the LBO limits are significantly extended by the NTP enhancing the flame stability. Under the relatively-low U0 range, streamers induced by the DBD reactor play a critical role in stabilizing the flames such that the degree of extension of the LBO limit depends linearly on VAC and fAC. Under the relatively-high U0 range, however, ozone generated by the DBD reactor in Regime III is found to be a major reason in extending the LBO limit, which is substantiated by another flame regime diagram with ozone addition only, and hence, the extension of LBO limit minimally depends on fAC. Simultaneously, the NTP considerably reduces CO emission, while slightly increases NOx emission near the LBO limits due to the enhanced combustion by ozone.

Analysis of Shear Effects on Mixing and Reaction Layers in Premixed Turbulent Stratified Flames using LES coupled to Tabulated Chemistry

ABSTRACT In this work, the effect of shear on turbulent lean premixed flames under stratified burning conditions including the flame TSF-A-r and TSF-D-r of the Darmstadt stratified burner, respectively, is numerically investigated. For this purpose, the large eddy simulation (LES) technique, coupled with tabulated chemistry and an artificially thickening approach, is applied. First, numerical predictions of time averaged data are compared with available experimental data in order to establish the prediction capability of the numerical approach. Then, the simulation results are exploited to analyze and characterize the stratified mixing layers and their interaction with the flame. In particular, coherent patterns in the mixing layers and their influence on the flame are pointed out. Finally, the onset and intensity of stratification of the two test cases are evaluated and compared in terms of equivalence ratio gradient, strength of the mixing and reaction layers, and scalar dissipation rates. It turns out that (1) a higher shear tends to attenuate the level of stratification, (2) the mixing and reaction layers correlate earlier and stronger with increasing turbulence, (3) a dominant premixed mode can rather be promoted with a stronger turbulence as can be observed from the ratio of the dissipation rate of the mixture fraction and that of the reaction progress variable.

The effects of differential diffusion in counter-flow premixed flames with dilution and hydrogen enrichment

The effects of differential diffusion on local flamelet velocities, turbulent burning rates, and structure of lean turbulent premixed flames in the thin reaction zone regime are investigated using aerodynamically stabilized flames in a counter-flow apparatus. Various fuel–oxidizer–inert mixtures with different transport properties, representative of distinct effective Lewis numbers, are studied. In order to minimize the effects of mixture reactivity in these experiments, unstretched laminar flame speed is kept constant during mixture dilution, and hydrogen enrichment of hydrocarbon flames, through changing the mixture equivalence ratio. Furthermore, bulk-flow properties and stagnation surface temperature are kept constant; hence, the study focuses on the effects of differential diffusion, which is the change in transport properties of the mixture, i.e., fuel and heat diffusivities, in the context of fuel flexibility. Highly strained laminar flame measurements are also reported as a reference of comparison. Local instantaneous statistics of various flame parameters within the imaged plane, such as flame location, flame velocity, and flame-front topology, are quantified using high-speed particle image velocimetry (PIV) and Mie scattering flame tomography at two levels of turbulence intensity. These parameters are presented as probability density functions using sufficiently large data sets to ensure statistical accuracy. The results for various flame-parameter statistics, which are measured over a wide range of Lewis numbers, show that the effects of differential diffusion are important in turbulent flames in the thin reaction zone regime. At constant turbulence intensity, differential diffusion increases the burning rates of turbulent flames in thermo-diffusively unstable mixtures through two main mechanisms: (1) increasing local flamelet displacement velocity, and (2) increasing flame-surface area. The relative contribution of these two parameters in increasing turbulent burning rates is approximately 76% and 24%, respectively, which is not dependent on the fuel, oxidizing-gas mixture, or turbulence intensity, and the results overlap over a wide range of Lewis numbers.

Role of intermediate temperature kinetics and radical transport in the prediction of leading edge structure of turbulent lean premixed flames

The stabilization of turbulent lean premixed flames in a backward-facing step combustor is investigated using large eddy simulations, with a reduced 2-step reaction kinetics mechanism and one of two detailed mechanisms. Although all reaction mechanisms predict similar laminar flame velocities and adiabatic flame temperatures, significant instabilities are observed only when the detailed mechanisms are employed, as supported by experimental observations. The flame structure, location of its leading edge away from the step, and primary recirculation zone are generally in better agreement with the experimental data when using the detailed mechanisms. These observations suggest that key kinetic steps missing in a reduced mechanism may be crucial for modeling the complex interactions between the turbulent flow and chemical reactions in these reacting flows. Indeed, by examining the flame structure in some detail, we find that intermediate temperature chemistry in the recirculation zone region contributes to the flame response and hence the evolution of instabilities downstream. We examine the results obtained using the detailed mechanisms to show that radical transport within the flame anchoring standoff distance play an important role in intermediate temperature reactions, initiated by local mixing of reactants and products. While flamelet like behavior was observed downstream, departure from that structure in the upstream recirculation region where reactants mix with products makes the flames more receptive to large-scale vorticity fluctuations. While global reduced mechanisms reproduce macro targets such as flame speed and temperature, they need not necessarily reproduce the correct flame anchoring location, and the interactions with shear layers.

Investigation of Reactive Scalar Mixing in Transported PDF Simulations of Turbulent Premixed Methane-Air Bunsen Flames

Transported probability density function (TPDF) simulations have been performed in conjunction with DNS data to investigate the mixing characteristics of reactive scalars in two turbulent lean premixed methane-air Bunsen flames with Case A being close to the corrugated flamelet regime and Case C being close to the broken reaction zones regime. The study shows that with an accurate mixing timescale of progress variable being provided, TPDF simulations using the EMST mixing model predict scalar mixing and flame characteristics reasonably well. Modeling reactive scalar mixing rate remains one key challenge. For turbulent flames close to the flamelet regime, i.e. Case A, the turbulent flame structure represented by the scatter of OH, as well as the resemblance of the flame induced dissipation rate to the actual dissipation rate, highlights the necessity to account for flame structure when modeling reactive scalar mixing in flamelet region. A posteriori tests show that the hybrid mixing timescale model, which accounts for both turbulence and flame structure effects on the scalar mixing timescale, yields better performance than the constant mechanical-to-scalar timescale model for turbulent premixed flames close to the flamelet regime. Moreover, the hybrid model shows potential for modeling differential mixing rates of intermediate species featuring their own characteristic timescales. The effects of progress variable definition and turbulence modeling on the computed flame characteristics are investigated, and the significance of turbulence modeling in RANS-TPDF simulation is illustrated.

The dissipation and dispersion of the entropy wave caused by a turbulent lean premixed flame in a combustor

One of the most important defects in gas turbines during the last three decades is the combustion instabilities due to diluting the mixture of air and fuel. The combustion instability occurs because of mutual effects of combustion and flow dynamical processes. Currently, entropy wave is one of the challenging reasons to start combustion instability in the combustors. Nonetheless, there are no adequate or consistent results for decaying or survival of the entropy waves. The current study aims to find the effects of hydrodynamic and thermal effects on the entropy waves using numerical simulation. The effects of inlet velocity, equivalence ratio and excitation frequency in both thermally convective and adiabatic combustor are investigated. The maximum wave changeis found in the excitation frequency. Further, the cooling convective heat transfer on the walls causes more deterioration in the wave. It is found that the low frequency entropy waves have more chance to survive in comparison to high frequency ones. The results clearly show that survival of the entropy waves majorly depends on the combustor thermal and hydrodynamic conditions and this is, therefore, hard to say any universal conclusion for survival of the entropy waves.

Micromixing Models for PDF Simulations of Turbulent Premixed Flames

ABSTRACTTransported probability density function (TPDF) methods are attractive for modeling turbulent flames as the highly nonlinear chemical reactions appear in closed form. The ability of the TPDF methods to capture complex phenomena such as extinction and re-ignition has been well demonstrated for turbulent non-premixed flames. However challenges remain when applying the TPDF methods to turbulent premixed flames, for which modeling molecular diffusion is difficult because the local species gradients may be strongly influenced by chemical reactions. The specification of a constant mechanical-to-scalar timescale ratio to relate the scalar mixing timescale to the turbulence timescale is questionable for premixed flames in the flamelet regime. In this paper, recent progress on scalar mixing is reviewed with particular focus on the analysis and modeling of the scalar mixing timescales in turbulent premixed flames using direct numerical simulation (DNS) datasets. The scalar dissipation rate of the progress variable from temporally evolving turbulent lean premixed H2-air flames in the thin reaction zone regime is analyzed using the chemical explosive mode analysis to understand its dependence on combustion modes and transient flame features. Moreover, models for the mixing timescale of the progress variable are discussed, with a focus on a recently developed hybrid mixing timescale model. This work concludes with a discussion on modeling differential diffusion in turbulent premixed flames.

An a priori analysis of a DNS database of turbulent lean premixed methane flames for LES with finite-rate chemistry

Abstract An a priori analysis of a DNS database of turbulent lean premixed methane flames is presented considering the relative effects of turbulence and LES filtering, along with a potential modelling approach for LES with finite-rate chemistry. The leading-order effect was found to be due to the filter operation; flame response to turbulence was a secondary effect, and manifested primarily as an increase in standard deviations about conditional means. It was found that the radicals O, H and OH were less impacted by the filter than other high-temperature radicals, which were significantly reduced in magnitude by the filter. By considering reaction path diagrams, key reactions have been identified that are responsible for disparities between the desired filtered reaction rates and the reaction rates evaluated using quantities available in LES calculations (i.e. the filtered species and temperature). More specifically, the hydrogen abstraction reactions that take CH 4 to CH 3 (by O, H and OH) were found to have particularly enhanced reaction rates, and dominate the reaction path. Under the conditions presented, reaction paths were found to be independent of turbulence intensity. In general, the filtered reaction rates from the DNS were found to align more closely with the filtered laminar profile than the reaction rate of filtered species and temperature, (but disparities were found to decrease with increasing Karlovitz number). A simple model for scaling reaction rates is considered based on filtered laminar flame profiles, and the resulting reaction paths demonstrate proof-of-concept of a simple approach for formulating a reaction rate model for LES with finite-rate chemistry.