Heat Release Fluctuations Research Articles

Advanced Identification of Coherent Structures in Swirl-Stabilized Combustors

We present an application of a newly introduced method to analyze the time-resolved experimental data from the flow field of a swirl-stabilized combustor. This method is based on the classic proper orthogonal decomposition (POD) extended by a temporal constraint. The filter operation embedded in this method allows for continuous fading from the classic POD to the Fourier mode decomposition. This new method—called spectral proper orthogonal decomposition (SPOD)—allows for a clearer separation of the dominant mechanisms due to a clean spectral separation of phenomena. In this paper, the fundamentals of SPOD are shortly introduced. The actual focus is put on the application to a combustor flow. We analyze high-speed particle image velocimetry (PIV) measurements from flow fields in a combustor at different operation conditions. In these measurements, we consider externally actuated, as well as natural dynamics and reveal how the natural and actuated modes interact with each other. As shown in the paper, SPOD provides detailed insight into coherent structures in the swirl flames. Two distinct PVC structures are found that are very differently affected by acoustic actuation. The coherent structures are related to the heat release fluctuations, which are derived from simultaneously acquired OH* chemiluminescence measurements. Besides the actuated modes, a low frequency mode was found that significantly contribute to the global heat release fluctuations.

Interaction between velocity fluctuations and equivalence ratio fluctuations during thermoacoustic oscillations in a partially premixed swirl combustor

The combined effects of velocity fluctuations (VF) and equivalence ratio fluctuations (ERF) on thermoacoustic flame oscillations are studied experimentally in a gas turbine model combustor. The analysis is based on time-resolved simultaneous measurements of flow field, flame structure and fuel distribution using combined PIV, OH-PLIF and tracer-PLIF at a rate of 10 kHz. In order to separate the effects of VF and ERF, the combustor has been operated in two configurations: a technically (i.e. partially) premixed (TP) configuration where both ERF and VF occur, and a perfectly premixed (PP) configuration where only VF are present. A particular operating condition is selected where the VF are similar for both premixing modes. The corresponding fluctuations of heat release, by contrast, exhibit considerably different spatial patterns for the two modes. In particular, periodic widening and narrowing as well as extinction and re-ignition of the flame base, and a convective motion of the flame zone are observed in the TP flame but not in the PP case. An evaluation of periodic variations of flux rates and heat release shows that the feedback loop of the TP flame includes an additional convective delay of 50°. The time-resolved measurements reveal that this delay leads to a coincidence of fuel-lean unburned gas and high velocity that induces local flame extinction and narrowing of the flame base. When the equivalence ratio later increases, the flame base re-ignites and a widened zone of increased heat release forms that is convected downstream. For the PP case, by contrast, reaction at the flame base is stable and accordingly the flame responds earlier to VF compared to the TP case. The results show that interacting VF and ERF have a strong and complex impact on the thermoacoustic response of turbulent swirl flames, and this impact depends largely on their time-delay.

Investigation of intermittent oscillations in a premixed dump combustor using time-resolved particle image velocimetry

The present work deals with time-resolved investigation of the flow field during acoustic self-excitation by a lean premixed flame in a dump combustor. Simultaneous measurements of unsteady pressure, velocity fields using time-resolved particle image velocimetry (TR-PIV) and CH* chemiluminescence are performed. As the equivalence ratio is varied, conditions of maximum pressure amplitude correspond to the prevalence of intermittent bursts in all the measured quantities. The intermittency is quantified by means of the permutation entropy of the pressure signals. The most significant frequencies are identified with the fundamental natural duct acoustic mode and the large-scale vortex roll-up (“wake mode”) frequency. The simultaneous occurrence of these multiple frequencies and shifts in their relative dominance of frequencies over time, shown by wavelet transform, result in the observed intermittent burst oscillations. Regimes of these ordered oscillations alternate with those of relative silence as well. The dynamic mode decomposition (DMD) of the TR-PIV data during windows of decay and growth of intermittent oscillations in the pressure signal quantifies the contribution of the above dominant modes in terms of their temporal growth rates. Besides the above two modes, the flapping mode and the shear layer mode of the flow field are also indicated. Consecutive PIV realisations and chemiluminescence images in a time-resolved sequence during excitation of high pressure amplitude show the large-scale vortex roll-up to substantially convolute the flame, leading to high heat release fluctuations. A further sequence of consecutive flow/flame fields is also examined as to how oscillations emerge out of a relatively silent time interval. The Kelvin–Helmholtz structures of the shear layer mode prevalent during the silent regime are seen to undergo a collective interaction to form a large-scale structure that excites the vortex and acoustic modes in turn.

Influence of atomization quality modulation on flame dynamics in a hypergolic rocket engine

For the numerical evaluation of the thermoacoustic stability of rocket engines often hybrid methods are applied, which separate the computation of wave propagation in the combustor from the analysis of the flame response to acoustic perturbations. Closure requires a thermoacoustic feedback model which provides the heat release fluctuation in the source term of the employed wave transport equations. The influence of the acoustic fluctuations in the combustion chamber on the heat release fluctuations from the modulation of the atomization of the propellants in a hypergolic upper stage rocket engine is studied. Numerical modeling of a single injector provides the time mean reacting flow field. A network of transfer functions representing all aspects relevant for the feedback model is presented. Analytical models for the injector admittances and for the atomization transfer functions are provided. The dynamics of evaporation and combustion are studied numerically and the numerical results are analyzed. An analytical approximation of the computed flame transfer function is combined with the analytical models for the injector and the atomization quality to derive the feedback model for the wave propagation code. The evaluation of this model on the basis of the Rayleigh index reveals the thermoacoustic driving potential originating from the fluctuating spray quality.

Combustion instability feedback mechanisms in a lean-premixed swirl-stabilized combustor

Constructive interference between heat release rate and acoustic perturbation is responsible for the growth of acoustic pressure amplitudes, leading to high-amplitude combustion instabilities in combustion systems. This is referred to as the Rayleigh criterion. Even though the knowledge of the heat release-acoustic pressure coupling processes is critical in the description of self-excited combustion instabilities, little is known about how the unsteady coupling processes are determined in response to the variation of time and length scales of acoustic and convective waves present in turbulent reacting flow fields. To address this issue, we performed a large number of measurements of self-excited instabilities, using two different tunable gas turbine combustors, over a wide range of operating conditions. The initiation, evolution, and saturation of pressure disturbances in swirl-stabilized combustion systems were systematically investigated by integrated analyses of pressure-heat release-velocity feedback coupling processes. It was found that self-excited combustion instabilities are governed by the Rayleigh criterion over the entire parameter space. We also observed that at certain inlet conditions, high amplitude velocity and heat release fluctuations coexist even without pressure-heat release coupling, meaning that the Rayleigh criterion is not necessarily associated with the onset of the self-excited instability. The Rayleigh criterion, however, does play an essential role in the linear to nonlinear transition that is necessary for the system to evolve toward a final state of stable limit cycles. We also found that the presence of equivalence ratio nonuniformities exerts a profound influence on instability feedback mechanisms, and consequently the Rayleigh criterion becomes a weak necessary condition for the occurrence of self-excited instabilities. A feedback mechanism that controls the relationship between velocity and acoustic pressure fluctuations was investigated to understand the effect of swirl number on limit cycle behavior. Acoustic admittance analysis reveals that at sufficiently high disturbance amplitude, nonlinear gas dynamic processes become so significant that the nonlinear saturation behavior cannot be defined by the acoustic velocity amplitude normalized by mean flow velocity.

Pressure wave generation from perturbed premixed flames

Numerical simulations and perturbation analysis of a radially imploding laminar premixed flame are used to study the mechanisms responsible for the generation of pressure fluctuations at flame fronts for various Lewis numbers. The relative importance of mechanisms based on unsteady heat release and on vorticity is investigated using an optimization methodology. Particular attention is paid to the influence of non-axisymmetric conditions and local flame curvature. It is shown that vorticity-based noise generation prevails for high-wavenumber, non-axisymmetric disturbances at all curvatures, while heat-release-driven noise generation dominates the axisymmetric and low-wavenumber regimes. These results indicate that short-wavelength vorticity waves actively participate in flame acoustic activity and can surpass acoustic output mechanisms based on heat-release fluctuations in the vicinity of the flame front.

Investigation of combustion dynamics in a cavity-based combustor with high-speed laser diagnostics

The dynamics of the flame/flow interaction produced in an optically accessible, premixed, and staged cavity-based combustor was investigated with high-speed particle image velocimetry (PIV) and OH-planar laser-induced fluorescence (OH-PLIF) . The combined PIV and OH-PLIF images were recorded at 2.5 kHz to assess stabilization mechanisms occurring between the cavity and the mainstream. Dynamic pressure and global heat-release rate fluctuations were complementary measured. Important characteristics were identified for two operating conditions, differing from the ratio of momentum J (taken between the mainstream and the cavity jet): a high ratio of momentum (\(J = 7.1\)) produced a “stable” flow, whereas a lower one (\(J = 2.8\)) displayed “unstable” conditions. Analysis of the “unstable” case revealed an intense flow instability, primarily due to premixed flow rate fluctuations inside the cavity. This effect is confirmed from a proper orthogonal decomposition analysis of PIV data, which illustrates the prominent role of large-scale flow oscillations in the whole combustor. Furthermore, the simultaneous analysis of flow velocities and gas state (either unburned or burned) displayed important fluctuations inside the shear layer, reducing effective flame-holding capabilities. By contrast, the increase in the ratio of momentum in the “stable” case reduces significantly the penetration of the cavity flow into the mainstream and consequently produces stable properties of the shear layer, being valuable to considerably improve flame stabilization.

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Combustion oscillation is investigated for a rapidly-mixed tubular flame burner to clarify what kind of influence abrupt pipe expansion causes. The OH^* chemiluminescence of the tubular flame is detected for the fluctuation of heat release by an optical wave length demultiplexer. It is found from the experimental results that the sound pressure level of the rapidly-mixed type is lower than that of the premixed type by about 20 dB near stoichiometric condition. This is why heat release oscillation, which increasing the sound pressure level in the premixed type, doesn't exist in the rapidly-mixed type.

Nonlinear Combustion Instabilities Analysis of Azimuthal Mode in Annular Chamber

Thermo-acoustic combustion instabilities are due to the coupling mechanism between acoustic pressure oscillations and heat release rate fluctuations. This phenomenon is of great interest because combustion instabilities cause recurrent problems in modern gas turbines. The case of annular chambers largely diffused in gas turbines for power generation is of particular interest. In this work, the azimuthal modes of a three-dimensional annular combustion chamber are studied by means of a finite element (FEM) code. For each burner, the coupling between heat release fluctuations ▪ and the velocity fluctuations ▪ is modeled by means of a non-linear analytical Flame Describing Function (FDF) consisting of a third order and fifth order polynomial expression. A delay time between ▪and ▪is considered. The dissipation of acoustic energy of the system due to viscous and thermal boundary layer is directly modeled in the code. The weakly nonlinear analysis is performed to determining the bifurcation diagrams for these flame models, considering the acoustic-combustion interaction index n as the control parameter. The influence on the bifurcations diagram of the time delays (τ) and of the damping coefficient (ζ) is analyzed.

Multiphase Flow Large-Eddy Simulation Study of the Fuel Split Effects on Combustion Instabilities in an Ultra-Low-NOx Annular Combustor

This paper describes the application of a coupled acoustic model/large-eddy simulation approach to assess the effect of fuel split on combustion instabilities in an industrial ultra-low-NOx annular combustor. Multiphase flow LES and an analytical model (analytical tool to analyze and control azimuthal modes in annular chambers (ATACAMAC)) to predict thermoacoustic modes are combined to reveal and compare two mechanisms leading to thermoacoustic instabilities: (1) a gaseous type in the multipoint zone (MPZ) where acoustics generates vortex shedding, which then wrinkle the flame front, and (2) a multiphase flow type in the pilot zone (PZ) where acoustics can modify the liquid fuel transport and the evaporation process leading to gaseous fuel oscillations. The aim of this paper is to investigate these mechanisms by changing the fuel split (from 5% to 20%, mainly affecting the PZ and mechanism 2) to assess which mechanism controls the flame dynamics. First, the eigenmodes of the annular chamber are investigated using an analytical model validated by 3D Helmholtz simulations. Then, multiphase flow LES are forced at the eigenfrequencies of the chamber for three different fuel split values. Key features of the flow and flame dynamics are investigated. Results show that acoustic forcing generates gaseous fuel oscillations in the PZ, which strongly depend on the fuel split parameter. However, the correlation between acoustics and the global (pilot + multipoint) heat release fluctuations highlights no dependency on the fuel split staging. It suggests that vortex shedding in the MPZ, almost not depending on the fuel split, is the main feature controlling the flame dynamics for this engine.

Reduced-Order Modeling of Aeroacoustic Systems for Stability Analyses of Thermoacoustically Noncompact Gas Turbine Combustors

A methodology is presented to model noncompact thermoacoustic phenomena using reduced-order models (ROMs) based on the linearized Navier–Stokes equations (LNSEs). The method is applicable to geometries with a complex flow field as in a gas turbine combustion chamber. The LNSEs, and thus the resulting ROM, include coupling effects between acoustics and mean fluid flow and are hence capable of describing propagation and (e.g., vortical) damping of the acoustic fluctuations within the considered volume. Such an ROM then constitutes the main building block for a novel thermoacoustic stability analysis method via a low-order hybrid approach. This method presents an expansion to state-of-the-art low-order stability tools and is conceptually based on three core features: First, the multidimensional and volumetric nature of the ROM establishes access to account spatial variability and noncompact effects on heat-release fluctuations. As a result, it is particularly useful for high-frequency phenomena such as screech. Second, the LNSE basis grants the ROM the capability to reconstruct complex acoustic performances physically accurate. Third, the formulation of the ROM in state-space allows convenient access to the frequency and time domain. In the time domain, nonlinear saturation mechanisms can be included, which reproduce the nonlinear stochastic limit cycle behavior of thermoacoustic oscillations. In order to demonstrate and verify the ROM's underlying methodology, a test case using an orifice-tube geometry as the acoustic volume is performed. The generation of the ROM of the orifice tube is conducted in a two-step procedure. As the first step, the geometrical domain is aeroacoustically characterized through the LNSE in frequency domain and discretized via the finite element method (FEM). The second step concerns the actual derivation of the ROM. The high-order dynamical system from the LNSE discretization is subjected to a modal reduction as order reduction technique. Mathematically, this modal reduction is the projection of the high-order (N∼ 200,000) system into its truncated left eigenspace. An order reduction of several magnitudes (ROM order: Nr∼ 100) is achieved. The resulting ROM contains all essential information about propagation and damping of the acoustic variables, and efficiently reproduces the aeroacoustic performance of the orifice tube. Validation is achieved by comparing ROM results against numerical and experimental benchmarks from LNSE–FEM simulations and test rig measurements, respectively. Excellent agreement is found, which grants the ROM modeling approach full eligibility for further usage in the context of thermoacoustic stability modeling. This work is concluded by a methodological demonstration of performing stability analyses of noncompact thermoacoustic systems using the herein presented ROMs.

Numerical study on intrinsic thermoacoustic instability of a laminar premixed flame

A study on the velocity sensitivity and intrinsic thermoacoustic stability of a laminar, premixed, Bunsen-type flame is carried out. Direct numerical simulation (DNS) of the flame, placed in an acoustically anechoic environment and subjected to broad-band, low-amplitude acoustic forcing, generates time series of fluctuating heat release rate, velocities and pressure. The time series data is post-processed with system identification to estimate the impulse response and transfer function of the flame. The associated frequency response is validated against experiment with good accuracy. DNS results obtained with acoustic excitation from the inlet or outlet boundary, respectively, confirm that the flame responds predominantly to perturbations of velocity. The stability of eigenmodes related to intrinsic thermoacoustic feedback is investigated with a network model. Both stable and unstable intrinsic thermoacoustic modes are predicted, depending on details of the configuration. The predicted modes are directly observed in direct numerical simulations, with good agreement in frequencies and stability.

Two-time correlation of heat release rate and spectrum of combustion noise from turbulent premixed flames

The spectral characteristics of combustion noise are dictated by the temporal correlation of the overall change of heat release rate fluctuations which has not received sufficient attention in prior studies. In this work, the two-time correlation of the volumetric heat release rate fluctuations within the flame brush and its role in modeling combustion noise spectrum are investigated by analyzing direct numerical simulation (DNS) data of turbulent premixed V-flames. This two-time correlation can be well represented by Gaussian-type functions and it captures the slow global variation of the fluctuating heat release rate and hence the low-frequency noise sources of unsteady combustion. The resulting correlation model is applied to predict the far-field noise spectrum from test open flames, and different reference time scales are used to scale this correlation from the DNS data to the test flames. The comparison between predictions and measurements indicates that the correlation models of all reference time scales are capable of reproducing the essential spectral shape including the low- and high-frequency dependencies. Reasonable agreement in the peak frequency, peak sound pressure level, and the Strouhal number scaling of peak frequency is also achieved for two turbulent time scales. A promising convective time scale shows great potential for characterizing the spectral features, yet its predictive capabilities are to be further verified through a longer DNS signal of a bounded flame configuration.

Finite Volume Time-Domain Solver to Estimate Combustion Instabilities

Modeling tools used to estimate thermoacoustic combustion instabilities include classic network models, three-dimensional frequency-domain acoustic solvers, and computational fluid dynamics. Motivated by the large gap in both computational cost and predictive capability between the first two tools and computational fluid dynamics, the present work discusses and tests an approach that bridges this gap: a three-dimensional finite volume, acoustic solver in the time domain. Distinguishing features of the newly developed solver include the ability to capture both linear and nonlinear acoustics, the use of a solution algorithm based on an approximate Riemann solver, and the ability to handle complex geometries with unstructured meshes. This new solver produces results that agree well with analytical solutions for a two-dimensional isothermal cavity and a one-dimensional Rijke tube, as well as with the experimental data of a reheat buzz. For this last problem, a limit cycle is produced with a physical model that bounds heat-release fluctuations. In addition, results from the new acoustic solver for an annular combustor compare well with those of a three-dimensional frequency-domain acoustic solver, demonstrating the capabilities of the new solver to capture multidimensional acoustics.

Combustion noise

Combustion noise is becoming increasingly important as a major noise source in aeroengines and ground based gas turbines. This is partially because advances in design have reduced the other noise sources, and partially because next generation combustion modes burn more unsteadily, resulting in increased external noise from the combustion. This review reports recent progress made in understanding combustion noise by theoretical, numerical and experimental investigations. We first discuss the fundamentals of the sound emission from a combustion region. Then the noise of open turbulent flames is summarized. We subsequently address the effects of confinement on combustion noise. In this case not only is the sound generated by the combustion influenced by its transmission through the boundaries of the combustion chamber, there is also the possibility of a significant additional source, the so-called ‘indirect’ combustion noise. This involves hot spots (entropy fluctuations) or vorticity perturbations produced by temporal variations in combustion, which generate pressure waves (sound) as they accelerate through any restriction at the exit of the combustor. We describe the general characteristics of direct and indirect noise. To gain further insight into the physical phenomena of direct and indirect sound, we investigate a simple configuration consisting of a cylindrical or annular combustor with a low Mach number flow in which a flame zone burns unsteadily. Using a low Mach number approximation, algebraic exact solutions are developed so that the parameters controlling the generation of acoustic, entropic and vortical waves can be investigated. The validity of the low Mach number approximation is then verified by solving the linearized Euler equations numerically for a wide range of inlet Mach numbers, stagnation temperature ratios, frequency and mode number of heat release fluctuations. The effects of these parameters on the magnitude of the waves produced by the unsteady combustion are investigated. In particular the magnitude of the indirect and direct noise generated in a model combustor with a choked outlet is analyzed for a wide range of frequencies, inlet Mach numbers and stagnation temperature ratios. Finally, we summarize some of the unsolved questions that need to be the focus of future research.

Flow Field and Flame Dynamics of Swirling Methane and Hydrogen Flames at Dry and Steam Diluted Conditions

The majority of recent stationary gas turbine combustors employ swirling flows for flame stabilization. The swirling flow undergoes vortex breakdown (VB) and exhibits a complex flow field including zones of recirculating fluid and regions of high shear intensities. Often, self-excited helical flow instabilities, which manifest in a precession of the vortex core, are found in these flows and may influence the combustion process in beneficial and adverse ways. In the present study, we investigate the occurrence and shape of self-excited hydrodynamic instabilities and their impact on heat release fluctuations and mixing characteristics over a wide range of operating conditions. We employ high-speed stereoscopic particle image velocimetry (S-PIV) and simultaneous OH*-chemiluminescence imaging to resolve the flow velocities and heat release distribution, respectively. The results reveal four different flame shapes: A detached annular flame, a long trumpet shaped flame, a V flame, and a very short flame anchored near the combustor inlet. The flame shapes were found to closely correlate with the reactivity of the mixture. Highly steam-diluted or very lean flames cause a detachment, whereas hydrogen fuel leads to very short flames. The detached flames feature a helical instability, which, in terms of frequency and shape, is similar to the isothermal case. A complete suppression of the helical structure is found for the V flame. Both the trumpet shaped flame and the very short flame feature helical instabilities of different frequencies and appearances. The phase-averaged OH*-chemiluminescence images show that the helical instabilities cause large-scale heat release fluctuations. The helical structure of the fluctuations is exploited to use a tomographic reconstruction technique. Furthermore, it is shown that the helical instability significantly enhances the mixing between the emanating jet and the central recirculation zone.

Flame Leading Edge and Flow Dynamics in a Swirling, Lifted Flame

Flames in high swirl flow fields with vortex breakdown often stabilize aerodynamically in front of interior flow stagnation points. In contrast to shear layer stabilized flames with a nearly fixed, well-defined flame attachment point, the leading edge of aerodynamically stabilized flames can move around substantially as a result of both the inherent dynamics of the vortex breakdown region and externally imposed oscillations. Motion of this flame stabilization point relative to the flow field may have an important dynamical role during combustion instabilities, as it creates flame front wrinkles and heat release fluctuations. For example, a prior study has shown that nonlinear dynamics of the flame response at high forcing amplitudes were related to these leading edge dynamics. This heat release mechanism exists alongside other flame wrinkling processes, arising from such processes as shear layer rollup and swirl fluctuations. This article describes an experimental investigation of flow forcing effects on the dynamics of the leading edge of a swirl stabilized flame. Flame and flow dynamics were characterized using high-speed particle image velocimetry (PIV) and CH* chemiluminescence imaging. A range of forcing conditions was achieved by varying the forcing frequency, amplitude, and acoustic field symmetry. These results show that the flame leading edge motion is dominated by the natural flow instabilities, particularly the precession of the recirculation zone around the centerline. The fluctuations in leading edge motion at the excitation frequency are much smaller than these natural motions, but are still on the order of the fluid particle displacement associated with the external excitation, indicating that they exhibit an important influence on local flame wrinkling and heat release. Flame response modeling shows that the global, spatially integrated heat release response is controlled by three factors—vortical disturbances, acoustic flow disturbances, and flame leading edge motion. The vortical flow motions dominate the flame response, and the flame leading edge motion is a minor contributor to the overall heat release response. This is an important result, as it shows that the significant motions of the flame leading edge actually have little dynamical significance for understanding the spatially integrated, forced response of the flame.

Propagation, dissipation, and dispersion of disturbances on harmonically forced, non-premixed flames

This paper describes the dynamics of non-premixed flames responding to bulk velocity fluctuations, and compares the dynamics of the flame sheet position and spatially integrated heat release to a similarly excited premixed flame. Bulk axial or transverse excitation, in either case, leads to the excitation of wrinkles on the flame that propagate axially. Inclusion of axial diffusion in the non-premixed case, and burning velocity stretch sensitivity in the premixed case, cause wrinkle dissipation and dispersion. There are important differences in spatially integrated unsteady heat release dynamics between premixed and non-premixed flames. For general Strouhal numbers, mass burning rate fluctuations are the dominant contributor to non-premixed flame heat release fluctuations, while area fluctuations are the dominant contributor to premixed flame heat release fluctuations. Moreover, the heat release response of non-premixed flames rolls off much slower with frequency, O(St−1/2) compared to O(St−1) for premixed flames, and, hence, are more sensitive to flow perturbations than premixed flames at high Strouhal numbers. The asymptotic tendencies of the non-premixed flame, however, are largely controlled by the near burner exit region with high transverse gradients and, thus, are expected to be quite sensitive to burner exit details and finite chemistry effects.

Thermo-acoustic instabilities in lean premixed swirl-stabilized combustion and their link to acoustically coupled and decoupled flame macrostructures

We investigate the onset of thermo-acoustic instabilities and their link to the mean flame configurations – or macrostructures – under acoustically coupled and decoupled conditions. Methane–hydrogen mixtures are used to explore the role of the fuel in changing the flame macrostructure, as determined by chemiluminescence, as the equivalence ratio (ϕ) varies. We observe four different configurations: a columnar flame (I); a bubble-columnar flame (II); a single conical flame (III); and a double conical flame (IV). We also observe different thermo-acoustic modes in the lean regime investigated, ϕ∈[0.5-0.75], that correspond to different flame configurations. By changing the combustor length without affecting the underlying flow, the resonant modes of the combustor are shifted to higher frequencies allowing for the decoupling of heat release fluctuations and the acoustic field over a range of equivalence ratio. We find that the same flame macrostructures observed in the long, acoustically coupled combustor arise in the short, acoustically decoupled combustor and transition at similar equivalence ratios in both combustors. The onset of the first fully unstable mode in the long combustor occurs at similar equivalence ratio as the flame transition from configuration III to IV. In the acoustically decoupled case, this transition occurs gradually starting with the intermittent appearance of a flame in the outer recirculation zone (ORZ). Spectral analysis of this phenomenon, referred to as “ORZ flame flickering” shows the existence of an unsteady event occurring over a narrow frequency band centered around 28Hz along with a weaker broadband region at lower frequency in the range [1–10]Hz. The tone at 28Hz is shown to be associated with the azimuthal advection of the flame by the outer recirculation zone flow. Changes in the fuel composition, by adding hydrogen (up to 20%), do not affect the correspondence between the thermo-acoustic modes and the flame macrostructures, but shift the transition points to lower equivalence ratio.

Influence of flow and ignition fluctuations on cycle-to-cycle variations in early flame kernel growth

Engine Cycle-to-Cycle Variations (CCVs) constitute a restriction for numerous concepts aimed at improving fuel consumption in spark-ignited engines. In consequence, engine CCV modeling has become an active research topic. Nevertheless, few fundamental studies have thus far examined the individual influences of the key parameters. In the present study, we performed 2D Direct Numerical Simulations (DNSs) of early flame kernel growth with semi-detailed chemistry solving to identify the controlling parameters of heat release fluctuations that are relevant to engine CCVs. Our objective was to assess how certain turbulence and flame parameters – namely turbulent flow motion pattern, turbulence intensity, integral length scale, mixture strength and initial kernel size – influence combustion variability. Two extreme mixture conditions were selected (stable operation at stoichiometry and unstable operation at the lean flammability limit), and computation conditions were derived from actual low load engine operating points. Results provide a classification of the key parameters, indicating initial kernel size and turbulent flow motion pattern as the main contributors. Additionally, DNSs highlighted the crucial role of mixture strength in the occurrence of local flame quenching and in the related amplification of heat release fluctuations.