Lean Premixed Combustion Systems Research Articles

Effect of flame response asymmetries on the modal patterns and collective states of a can-annular lean-premixed combustion system

We experimentally study the effect of rotational asymmetries in the flame response distribution on the thermoacoustic oscillations of four turbulent lean-premixed combustors coupled in a ring network. The asymmetries are created via different combinations of high-swirl (HS) and low-swirl (LS) nozzles. By analyzing the inter-combustor acoustic interactions in terms of discrete thermoacoustic modes, we find a variety of modal patterns: (i) global alternating push–pull modes emerge for most pair-wise asymmetric nozzle combinations, (ii) 2-can push–pull modes emerge for an alternating 2-fold symmetric nozzle combination, and (iii) strong mode localization and global push–push modes emerge when the HS nozzles outnumber the LS nozzles. Using a complex systems framework, we reinterpret these modal patterns as collective states, such as a weak breathing chimera, a weak anti-phase chimera, and in-phase/anti-phase synchronization. This study shows that changing the flame response distribution of a multi-combustor system, via changes in the nozzle swirl distribution, can induce a variety of modal patterns and collective states. This sets the stage for the potential use of rotational asymmetries in the passive control of thermoacoustic modes in can-annular combustion systems.

Bayesian inference and uncertainty quantification for hydrogen-enriched and lean-premixed combustion systems

Development of probabilistic modelling tools to perform Bayesian inference and uncertainty quantification (UQ) is a challenging task for practical hydrogen-enriched and low-emission combustion systems due to the need to take into account simultaneously simulated fluid dynamics and detailed combustion chemistry. A large number of evaluations is required to calibrate models and estimate parameters using experimental data within the framework of Bayesian inference. This task is computationally prohibitive in high-fidelity and deterministic approaches such as large eddy simulation (LES) to design and optimize combustion systems. Therefore, there is a need to develop methods that: (a) are suitable for Bayesian inference studies and (b) characterize a range of solutions based on the uncertainty of modelling parameters and input conditions. This paper aims to develop a computationally-efficient toolchain to address these issues for probabilistic modelling of NOx emission in hydrogen-enriched and lean-premixed combustion systems. A novel method is implemented into the toolchain using a chemical reactor network (CRN) model, non-intrusive polynomial chaos expansion based on the point collocation method (NIPCE-PCM), and the Markov Chain Monte Carlo (MCMC) method. First, a CRN model is generated for a combustion system burning hydrogen-enriched methane/air mixtures at high-pressure lean-premixed conditions to compute NOx emission. A set of metamodels is then developed using NIPCE-PCM as a computationally efficient alternative to the physics-based CRN model. These surrogate models and experimental data are then implemented in the MCMC method to perform a two-step Bayesian calibration to maximize the agreement between model predictions and measurements. The average standard deviations for the prediction of exit temperature and NOx emission are reduced by almost 90% using this method. The calibrated model then used with confidence for global sensitivity and reliability analysis studies, which show that the volume of the main-flame zone is the most important parameter for NOx emission. The results show satisfactory performance for the developed toolchain to perform Bayesian inference and UQ studies, enabling a robust and consistent process for designing and optimising low-emission combustion systems.

Modeling of Natural Gas Composition Effect on Low NOx Burners Operation in Heavy Duty Gas Turbine

Abstract This paper addresses the impact of natural gas composition on both the operability and emissions of lean premixed gas turbine combustion system. This is an issue of growing interest due to the challenge for gas turbine manufacturers in developing fuel-flexible combustors capable of operating with variable fuel gases while producing very low emissions at the same time. Natural gas contains primarily methane (CH4) but also notable quantities of higher order hydrocarbons such as ethane (C2H6) can also be present. A deep understanding of natural gas combustion is important to obtain the highest combustion efficiency with minimal environmental impact. For this purpose, Large Eddy Simulations of an annular combustor sector equipped with a partially premixed burner are carried out for two different natural gas compositions with and without including the effect of flame strain rate and heat loss resulting in a more adequate description of flame shape, thermal field, and extinction phenomena. Promising results, in terms of NOx, compared against available experimental data, are obtained including these effects on the flame brush modeling, enhancing the fuel-dependency under nonadiabatic condition.

Influence of Turbulence Schmidt Number on Exit Temperature Distribution of an Annular Gas Turbine Combustor using Flamelet Generated Manifold

The Reynolds analogy concept has been used in almost all turbulent reacting flow RANS (Reynolds-averaged Navier-Stokes) simulations, where the turbulence scalar transfers in flow fields are calculated based on the modeled turbulence momentum transfer. This concept, applied to a lean premixed combustion system, was assessed in this paper in terms of exit temperature distribution. Because of the isotropic assumption involved in this analogy, the prediction in some flow condition, such as jet cross flow mixing, would be inaccurate. In this study, using Flamelet Generated Manifold as reaction model, some of the numerical results, obtained from an annular combustor configuration with the turbulent Schmidt number varying from 0.85 to 0.2, were presented and compared with a benchmark atmospheric test results. It was found that the Schmidt number σt in mean mass fraction f transport equation had significant effect on dilution air mixing process. The mixing between dilution air and reaction products from the primary zone obviously improved as σt decreased on the combustor exit surface. Meanwhile, the sensitivity of σt in three turbulence models including Realizable k-e, SST (Shear Stress Transport) and RSM (Reynolds Stress Model) has been compared as well. Since the calculation method of eddy viscosity was different within these three models, RSM was proved to be less sensitive than another two models and can guarantee the best prediction of mixing process condition. On the other hand, the results of dilution air mixing were almost independent of Schmidt number Sct in progress variable c transport equation. This study suggested that for accurate prediction of combustor exit temperature distribution in steady state reacting flow simulation, the turbulent Schmidt number in steady state simulation should be modified to cater to dilution air mixing process.

CFD predictions of Swirl burner aerodynamics with variable outlet configurations

Swirl stabilised combustion is one of the most widely used techniques for flame stabilisation in gas turbine combustors. Lean premixed combustion systems allow the reduction of NOx coupled with fair flame stability. The swirl mechanism produces an aerodynamic region known as central recirculation zone (CRZ) providing a low velocity region where the flame speed matches the flow velocity, thus anchoring the flame whilst serving to recycle heat and active chemical species to the root of the former. Another beneficial feature of the CRZ is the enhancement of the mixing in and around this region. However, the mixing and stabilisation processes inside of this zone have shown to be extremely complex. The level of swirl, burner outlet configuration and combustor expansion are very important variables that define the features of the CRZ. Therefore, in this paper swirling flame dynamics are investigated using computational fluid dynamics (CFD) with commercial software (ANSYS). A new generic swirl burner operated under lean-premixed conditions was modelled. A variety of nozzles were analysed using several gaseous blends at a constant power output. The investigation was based on recognising the size and strength of the central recirculation zones. The dimensions and turbulence of the Central Recirculation Zone were measured and correlated to previous experiments. The results show how the strength and size of the recirculation zone are highly influenced by the blend and infer that it is governed by both the shear layer surrounding the Central Recirculation Zones (CRZ) and the gas composition.

Thermoacoustic Instability Model With Porous Media: Linear Stability Analysis and the Impact of Porous Media

Lean premixed (LPM) combustion systems are susceptible to thermoacoustic instability, which occurs when acoustic pressure oscillations are in phase with the unsteady heat release rates. Porous media has inherent acoustic damping properties and has been shown to mitigate thermoacoustic instability; however, theoretical models for predicting thermoacoustic instability with porous media do not exist. In the present study, a one-dimensional (1D) model has been developed for the linear stability analysis of the longitudinal modes for a series of constant cross-sectional area ducts with porous media using a n-Tau flame transfer function (FTF). By studying the linear regime, the prediction of acoustic growth rates and subsequently the stability of the system is possible. A transfer matrix approach is used to solve for acoustic perturbations of pressure and velocity, stability growth rate, and frequency shift without and with porous media. The Galerkin approximation is used to approximate the stability growth rate and frequency shift, and it is compared to the numerical solution of the governing equations. Porous media is modeled using the following properties: porosity, flow resistivity, effective bulk modulus, and structure factor. The properties of porous media are systematically varied to determine the impact on the eigenfrequencies and stability growth rates. Porous media is shown to increase the stability domain for a range of time delays (Tau) compared to similar cases without porous media.

Experimental characterizing combustion emissions and thermodynamic properties of a thermoacoustic swirl combustor

Many practical lean-premixed combustion systems involved in land-based power plants, gas turbines and boilers are susceptible to self-excited combustion instability, which is characterized by detrimental periodic pressure oscillations. Little attention has been paid on experimentally characterizing the chemical emissions and thermodynamic properties of a thermoacoustic swirl combustor. In this work, the effects of (1) fuel-air equivalence ratio Φ and fuel flow rate QCH4 on generating such combustion instability and its impact on chemical emissions and thermodynamic properties in a swirling combustor are experimentally studied. For this, a methane-fueled lab-scale swirl combustor is designed and tested. To monitor the thermodynamic properties of the combustor, an acoustic pressure sensor, an infrared thermal imaging camera, K-type thermal couples, and an infrared flue gas analyzer are applied. It is found that the fuel-air ratio plays an important role on generating combustion instability at different frequencies and amplitudes. This is confirmed by conducting autocorrelation and frequency spectrum analyses of the acoustic pressure time trace. Furthermore, the dominant mode swap (mode-switching) between a low frequency ω1 and a high non-harmonic one is experimentally observed. Whether the mode switching from high (harmonic or non-harmonic) to low frequency or low to high frequency is found to depend strongly on QCH4. As the equivalence ratio is changed from lean to rich, i.e. 0.8 ≤ Φ ≤ 1.2, NOx emission is increased from 1 ppm to 37 ppm. However, CO emission is decreased by 2 order of magnitudes from 1000 ppm first and then increased. The minimum CO emission is approximately 3.0 ppm. In addition, O2 concentration is decreased by more than 80% with increased Φ, depending on the methane flow rate. This means that the combustion efficiency characterized by the O2 emission is decreased dramatically from 99.5% to 68% with Φ increased from 0.6 to 1.2. The present work sheds light on the characteristics of chemical emissions and thermodynamic properties, when a thermoacoustic swirl combustor is operated with methane-air equivalence ratio Φ varied from lean to rich condition and different QCH4. It opens up a practical means to design a stably operated but low-emission thermoacoustic swirl combustor.

Investigation of the Heat Loss Effect on Cellular Flames via Proper Orthogonal Decomposition

ABSTRACTFor better control of a lean premixed combustion system, it is necessary to understand the mechanism of unsteady cellular flame. The objective of the present study is to investigate how heat loss influences the cellular instability of lean premixed flame. The lean premixed CH4/H2/air cellular flames generated on a McKenna burner were studied, and heat loss was responsible for the manifestation of unsteady cellular structure. A method of applying proper orthogonal decomposition in scale analysis is presented, which has proven helpful in characterizing the cellular flame. The multiple scale nature of unstable cellular flame was identified, and the small scales were found to act in the random movement of cells. The contribution of each scale is obtained. The energy distribution on modes indicates that the energy on higher-order modes, equivalently the contribution of smaller scales, is decreased with decreasing heat loss. Through analyzing the principal modes and temporal coefficients, heat loss is found to influence a flame’s unstable behavior by promoting the oscillation of large scales, and the wrinkling factor is estimated. It is found that there is a cut-off length scale, below which the wrinkles make little contribution to the enhancement of combustion intensity.

The Influence of Carrier Air Preheating on Autoignition of Inline-Injected Hydrogen–Nitrogen Mixtures in Vitiated Air of High Temperature

The use of highly reactive hydrogen-rich fuels in lean premixed combustion systems strongly affects the operability of stationary gas turbines (GT) resulting in higher autoignition and flashback risks. The present study investigates the autoignition behavior and ignition kernel evolution of hydrogen–nitrogen fuel mixtures in an inline co-flow injector configuration at relevant reheat combustor operating conditions. High-speed luminosity and particle image velocimetry (PIV) measurements in an optically accessible reheat combustor are employed. Autoignition and flame stabilization limits strongly depend on temperatures of vitiated air and carrier preheating. Higher hydrogen content significantly promotes the formation and development of different types of autoignition kernels: More autoignition kernels evolve with higher hydrogen content showing the promoting effect of equivalence ratio on local ignition events. Autoignition kernels develop downstream a certain distance from the injector, indicating the influence of ignition delay on kernel development. The development of autoignition kernels is linked to the shear layer development derived from global experimental conditions.

Premixed jet flame behavior in a hot vitiated crossflow of lean combustion products

Improvement in NOx emissions from gas turbine combustors requires development of safe and reliable lean-premixed combustion systems. In this paper, the flame stabilization behavior of a premixed ethylene–air jet injected normal to a hot vitiated crossflow (JICF) of lean combustion products was studied experimentally. The equivalence ratio of the premixed jet was varied from lean to rich conditions. Measurements of the flame were conducted using high-speed chemiluminescence imaging and simultaneous particle image velocimetry (PIV), hydroxyl (OH) and formaldehyde (CH2O) planar laser-induced fluorescence (PLIF). From the PLIF measurements, pixel-by-pixel multiplication of OH and CH2O fluorescence signals was conducted to estimate the heat release zone of the JICF flame front. The unsteady windward flame exhibited both attached and lifted flame behavior, while the leeward flame branch remained consistently attached to the jet exit. When the windward flame was lifted, chemiluminescence imaging showed that both flame propagation and auto-ignition kernel formation contributed to flame anchoring. From the PLIF imaging, formaldehyde signal was observed upstream of the lifted windward flame base, suggesting pre-ignition behavior due to mixing between the jet reactants and the hot crossflow. The windward flame base was always in the jet shear layer and the trailing flame remained in the shear layer under lean conditions but traversed the shear layer into the bulk jet flow for stoichiometric and rich conditions. Alignment between 2D dilatation and heat release was seen to vary depending on the location of the flame relative to the shear layer. Two-dimensional dilatation and heat release location aligned best when the flame resided away from the shear layer, where jet and crossflow mixing and out-of-plane motion are minimal.

Turbulent flame stabilization modes in premixed swirl combustion: Physical mechanism and Karlovitz number-based criterion

Modern low-emission, lean premixed gas turbine combustion systems often rely on confined swirling flows associated with sudden expansions to enhance flame anchoring. Through the establishment of multiple recirculation zones and shear layers, such complex reacting flows give rise to several possible average flame shapes or macrostructures. Among these, the single conical flame stabilized along the inner shear layer (ISL) and the double conical flame stabilized along both the inner as well as the outer shear layers (OSL) and the outer recirculation zone (ORZ) are of special interest. One of the reasons is that the transition between these two flames has been previously linked to the onset of thermo-acoustic instabilities under acoustically coupled conditions. In this study we investigate the mechanism underlying the flame transition to the ORZ/OSL and propose a criterion for its occurrence, in an acoustically uncoupled combustion system. To reach this goal, the effects of the fuel composition (CH4–H2), Reynolds number, swirler blade angle and heat loss were experimentally analyzed. We find evidence that the transition starts with an intermittent inflammation of the ORZ caused by a flame kernel detaching from the ISL. Above a critical equivalence ratio, the flame kernel expands and spins along with the ORZ flow. Next, we explore the effect of the operating conditions on the onset of an ORZ flame. We propose a Strouhal number to describe the ORZ flame spinning frequency (fORZ) also shown to be a predominately hydrodynamic quantity. Finally, we show that the flame transition to the ORZ is governed by a balance between a flame time to a flow time that can be expressed in a form of a Karlovitz number (KaORZ) defined as the ratio of the ORZ spinning frequency and extinction strain rate; the former is a surrogate for the bulk azimuthal strain rate in the ORZ.

배기가스 재순환을 적용한 희박-과농 연소시스템의 공해물질 배출특성 연구

In this study, the CH4/air lean-rich combustion system with exhaust gas recirculation (EGR) was investigated to explore the potential for lowering pollutant emissions. To achieve this purpose, experiments of lean-rich combustion system with EGR were conducted to measure the changes in the characteristics of the pollutant emission and flame shape with various equivalence ratios and EGR rates. Here, this study was applied to the fuel distribution ratio of 3:1 for the formation of the lean and rich flames. Additionally, the results were compared with CH4/air lean premixed combustion system. The results show that flame shape of lean-rich combustion system was determined by lean and rich equivalence ratios (ΦL and ΦR) and stratified flame was formed with increasing ΦR. According to the pollutant emission characteristics based on experimental results, the NOx and CO emission index (EINOx and EICO) decreased with increasing EGR rate. Especially, in the range needed to form a stable flame, the reduction rates of EINOx and EICO were approximately 47% and 48% for an EGR rate of 25%, global equivalence ratio of 0.85 and ΦL of 0.80 compared with lean premixed combustion system (Φ = 0.78).

Time-Resolved Particle Image Velocimetry Measurements of Nonreacting Flow Field in a Swirl-Stabilized Combustor Without and With Porous Inserts for Acoustic Control

Combustion noise and thermo-acoustic instabilities are of primary importance in highly critical applications such as rocket propulsion systems, power generation, and jet propulsion engines. Mechanisms for combustion instabilities are extremely complex because they often involve interactions among several different physical phenomena such as unsteady flame propagation leading to unsteady flow field, acoustic wave propagation, natural and forced hydrodynamic instabilities, etc. In the past, we have utilized porous inert media (PIM) to mitigate combustion noise and thermo-acoustic instabilities in both lean premixed (LPM) and lean direct injection (LDI) combustion systems. While these studies demonstrated the efficacy of the PIM concept to mitigate noise and thermo-acoustic instabilities, the actual mechanisms involved have not been understood. The present study utilizes time-resolved particle image velocimetry (PIV) to measure the turbulent flow field in a nonreacting swirl-stabilized combustor without and with PIM. Although the flow field inside the annulus of the PIM cannot be observed, measurements immediately downstream of the PIM provide insight into the turbulent structures. Results are analyzed using the proper orthogonal decomposition (POD) method and show that the PIM alters the flow field in an advantageous manner by modifying the turbulence structures and eliminating the corner recirculation zones and precessing vortex core (PVC), which would ultimately affect the acoustic behavior in a favorable manner.

Time-resolved PIV of lean premixed combustion without and with porous inert media for acoustic control

In the past, we have documented the efficacy of the ring-shaped porous inert media (PIM) to passively mitigate combustion noise and thermo-acoustic instabilities in lean premixed (LPM) swirl-stabilized combustion systems, although the underlying mechanisms involved have not been understood. The present study utilizes time-resolved particle image velocimetry (PIV) to measure the turbulent flow field in a LPM combustion system without and with PIM placed on the dump plane. Measurements of instantaneous and average flow fields, vorticity, and turbulent kinetic energy are presented to gain insight into the flow structure. Results are analyzed using the proper orthogonal decomposition (POD) to identify the effect of PIM on the coherent turbulent structures in the flow field. The addition of PIM removes the recirculation zones and the dominant coherent turbulent structures formed in the shear layers are convected downstream of the reaction zone. Harmonic reconstruction of the flow field is performed at the frequency of thermo-acoustic instability to identify the relationship between the flow and acoustic fields of the system. Without PIM, the vortical modes in the corner recirculation zone are shown to be the driving source for the instability. PIM alters the flow field from global instability to convective instability, which effectively eliminates the feedback mechanism for thermo-acoustic instabilities in the present LPM swirl-stabilized combustion system.

Hydrodynamic instability and shear layer effect on the response of an acoustically excited laminar premixed flame

Combustion instabilities can cause serious problems which limit the operating envelope of low-emission lean premixed combustion systems. Predicting the onset of combustion instability requires a description of the unsteady heat release driving the instability, i.e., the heat release response transfer function of the system. This study focuses on the analysis of fully coupled two-way interactions between a disturbance field and a laminar premixed flame that incorporates gas expansion effects by solving the conservation equations of a compressible fluid. Results of the minimum and maximum flame front deflections are presented to underline the impact of the hydrodynamic instability on the flame and the shear layer effect on the initial flame front wrinkling which is increased at decreasing gas expansion. These phenomena influence the magnitude of the burning area and burning area rate response of the flame at lower frequency excitation more drastically than reduced-order model (ROM) predictions even for low temperature ratios. It is shown that the general trend of the flame response magnitudes can be well captured at higher frequency excitation, where stretch effects are dominant. The phase response is influenced by the DL mechanism, which cannot be captured by the ROM, and by the resulting discrepancy in the flame pocket formation and annihilation process at the flame tip.

Assessment of a Gas Turbine NOx Reduction Potential Based on a Spatiotemporal Unmixedness Parameter

The paper presents a one-dimensional approach to assess the reduction potential of NOx emissions for lean premixed gas turbine combustion systems. NOx emissions from these systems are known to be mainly caused by high temperatures, not only from an averaged perspective but especially related to poor mixing quality of fuel and air. The method separates the NOx chemistry in the flame front zone and the postflame zone (slow reaction). A one-dimensional treatment enables the use of detailed chemistry. A lookup table parameterized by reaction progress and equivalence ratio is used to improve the computational efficiency. The influence of mixing quality is taken into account by a probability density function of the fuel element–based equivalence ratio, which itself translates into a temperature distribution. Hence, the NOx source terms are a function of reaction progress and equivalence ratio. The reaction progress is considered by means of the two-zone approach. Based on unsteady computational fluid dynamics (CFD) data, the evolution of the probability density function with residence time has been analyzed. Two types of definitions of an unmixedness quantity are considered. One definition accounts for spatial as well as temporal fluctuations, and the other is based on the mean spatial distribution. They are determined at the location of the flame front. The paper presents a comparison of the modeled results with experimental data. A validation and application have shown very good quantitative and qualitative agreement with the measurements. The comparison of the unmixedness definitions has proven the necessity of unsteady simulations. A general emissions-unmixedness correlation can be derived for a given combustion system.

Premixed flame response to equivalence ratio fluctuations: Comparison between reduced order modeling and detailed computations

Combustion instability events in lean premixed combustion systems can cause spatio-temporal variations in unburnt mixture fuel/air ratio. This provides a driving mechanism for heat-release oscillations when they interact with the flame. Several Reduced Order Modelling (ROM) approaches to predict the characteristics of these oscillations have been developed in the past. The present paper compares results for flame describing function characteristics determined from a ROM approach based on the level-set method, with corresponding results from detailed, fully compressible reacting flow computations for the same two dimensional slot flame configuration. The comparison between these results is seen to be sensitive to small geometric differences in the shape of the nominally steady flame used in the two computations. When the results are corrected to account for these differences, describing function magnitudes are well predicted for frequencies lesser than and greater than a lower and upper cutoff respectively due to amplification of flame surface wrinkling by the convective Darrieus-Landau (DL) instability. However, good agreement in describing function phase predictions is seen as the ROM captures the transit time of wrinkles through the flame correctly. Also, good agreement is seen for both magnitude and phase of the flame response, for large forcing amplitudes, at frequencies where the DL instability has a minimal influence. Thus, the present ROM can predict flame response as long as the DL instability, caused by gas expansion at the flame front, does not significantly alter flame front perturbation amplitudes as they traverse the flame.

Autoignition Limits of Hydrogen at Relevant Reheat Combustor Operating Conditions

The use of highly reactive fuels in the lean premixed combustion systems employed in stationary gas turbines can lead to many practical problems, such as unwanted autoignition in regions not designed for combustion. In the present study, autoignition characteristics for hydrogen, diluted with up to 30 vol. % nitrogen, were investigated at conditions relevant to reheat combustor operation (p = 15 bar, T >1000 K, hot flue gas, relevant residence times). The experiments were performed in a generic, optically accessible reheat combustor, by applying high-speed imaging and particle image velocimetry. Autoignition limits for different mixing section (temperature, velocity) and fuel jet (N2 dilution) parameters are described. The dominant factor influencing autoignition was the temperature, with an increase of around 2% leading to a reduction of the highest possible H2 concentration without “flame-stabilizing autoignition kernels” of approximately 16 vol. %. Furthermore, the onset and propagation of the ignition kernels were elucidated using the high-speed measurements. It was found that the ability of individual autoignition kernels to develop into stable flames depends on the initial position of the kernel and the corresponding axial velocity at that position. While unwanted autoignition occurred prior to reaching the desired operating point for most investigated conditions, for certain conditions the reheat combustor could be operated stably with up to 80 vol. % H2 in the fuel.

Nonlinear simulations of combustion instabilities with a quasi-1D Navier–Stokes code

As lean premixed combustion systems are more susceptible to combustion instabilities than non-premixed systems, there is an increasing demand for improved numerical design tools that can predict the occurrence of combustion instabilities with high accuracy. The inherent non-linearities in combustion instabilities can be of crucial importance, and we here propose an approach in which the one-dimensional Navier-Stokes and scalar transport equations are solved for geometries of variable cross-section. The focus is on attached flames, and for this purpose a new phenomenological model for the unsteady heat release from a flame front is introduced. In the attached flame method (AFM) the heat release occurs over the full length of the flame. The non-linear code with the use of the AFM approach is validated against results from an experimental study of thermoacoustic instabilities in oxy-fuel flames by Ditaranto and Hals [Combustion and Flame, 146, 493-512 (2006)]. The numerical simulations are in accordance with the experimental measurements and both the frequencies and the amplitudes of the resonant acoustic pressure modes are reproduced with good accuracy.

Detailed measurements of equivalence ratio modulations in premixed flames using laser Rayleigh scattering and absorption spectroscopy

Equivalence ratio non-uniformities may give rise to some of the instabilities observed in modern lean premixed combustion systems. The present work intends to investigate the influence of equivalence ratio perturbations on the dynamics of premixed flames. A burner equipped with a secondary injection system is used to generate equivalence ratio perturbations which are convected by the flow and impinge on a conical flame. Two laser-diagnostics, based on Rayleigh scattering and hydrocarbon infrared absorption, respectively, are employed to give insight into the spatial and temporal evolution of the mixture composition field. Rayleigh scattering images also reveal the flame front dynamics providing an indication on the response of a weakly turbulent flame subject to mixture composition inhomogeneities. Laser light absorption provides a time resolved signal which is used to estimate the equivalence ratio perturbation level. A theoretical model based on the G-equation is used to interpret the experimental data and compare the relative effects of velocity and equivalence ratio perturbations.