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Abstract This paper presents a hybrid and unsupervised approach to flame front detection for low signal-to-noise planar laser-induced fluorescence (PLIF) images. The algorithm combines segmentation and edge detection techniques to achieve low-cost and accurate flame front detection in the presence of noise and variability in the flame structure. The method first uses an adaptive contrast enhancement scheme to improve the quality of the image prior to segmentation. The general shape of the flame front is then highlighted using segmentation, while the edge detection method is used to refine the results and highlight the flame front more accurately. The performance of the algorithm is tested on a dataset of high-speed PLIF images and is shown to achieve high accuracy in finely wrinkled turbulent hydrogen-enriched flames with order of magnitude improvements in computation speed. This new algorithm has potential applications in the experimental study of turbulent flames subject to intense wrinkling and low signal-to-noise ratios. Graphic abstract

Bluff-body flame instabilities are experimentally investigated under varying turbulence conditions during lean blowout. For all turbulence conditions, the blowout process is induced through a temporal reduction of the fuel flow rate to capture the flame-flow dynamics approaching blowout. Simultaneous high-speed particle image velocimetry (PIV), stereoscopic PIV, and C2*/CH* chemiluminescence imaging are employed, along with an independent CH* imaging system, to capture flame-flow instabilities. Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) techniques are used to identify prominent flame oscillations and evaluate recurring spatiotemporal modes during blowout. The results reveal that the dominant flame oscillations and wrinkling characteristics are directly dependent on the turbulence conditions in the combustor. Specifically, the flame-flow oscillations are strongly coupled with the integral length scales, which were able to collapse the oscillation frequencies to a unified value. The turbulence-driven flame-flow oscillations are shown to largely impact the magnitude, temporal evolution, and oscillatory behavior of the flame strain rate. As the turbulence intensity is increased, the oscillation of the flame strain rate increases in frequency, making it more likely for localized extinctions to occur. Additionally, the magnitude of the flame strain rate increases at high turbulence intensities and accelerates the lean blowout process.

The heat release distribution, combustion instability characteristics and flow dynamics of lean swirl-stabilized flames of hydrogen enriched natural gas were studied using time-resolved (10 kHz) stereo PIV, OH* chemiluminescence imaging and acoustic pressure measurements. The technically premixed gas turbine model combustor was operated at elevated pressure up to 5 bar with preheated air. The H2 vol. fraction in the fuel was varied up to 50%. An M-shaped aerodynamically stabilized flame persisted at 1 bar up to low H2 enrichment ratios. Increasing H2 enrichment caused the M-flame to first transition to a bistable flame then to a shear layer stabilized V-flame. Increasing pressure and H2 enrichment both decreased the flame length. The combustion instability frequency and thermoacoustic amplitude varied and were mapped across operating conditions. The effect of H2 enrichment on the thermoacoustic amplitude was explained by examining the convective coupling between the Helmholtz acoustics and heat release. H2 enrichment was shown to increase the phase delay between the pressure and heat release by decreasing the pressure wave travel times in the plenum and swirler sections of the burner and by decreasing the flame length while simultaneously increasing the pressure wave speed within the burner. Thus H2 enrichment increased the thermoacoustic amplitude when the phase delay at the initial condition was negative, and decreased it when the initial phase delay was positive. Flow dynamics were studied by characterizing the precessing vortex core frequency and energy. The PVC structure existed across all conditions, and its frequency reduced to a different Strouhal number for the M-flame and V-flame. H2 enrichment consistently slightly decreased the PVC energy.

The lean blowout mechanisms of premixed bluff-body flames are experimentally investigated at various turbulence intensities. Turbulence levels are varied using a novel turbulence generator, which combines static grid and fluidic jet impingement techniques. Three different turbulence levels are probed to study their effects on lean blowout. The three conditions span across the combustion regime diagram, from flamelets to broken reactions. For all three turbulence levels, the lean blowout process is induced through controlled fuel flow rate reduction. The transient blowout process is captured using three simultaneous high-speed diagnostic systems: particle image velocimetry (PIV), stereoscopic PIV (SPIV), and C2*/CH* species measurements. The two PIV systems are used to resolve the instantaneous velocity and vorticity fields, and the C2*/CH* species diagnostics allow for global equivalence ratios to be evaluated throughout the duration of blowout. The results show that the dynamics of lean blowout vary with turbulence intensity. At low turbulence levels, the flame experiences a global effect where the flame boundary interacts with the shear layer vorticity. This imparts high strain rates along the length of the flame, leading to blowout. As turbulence levels increase, the blowout mechanism becomes less dependent on flame and shear layer interactions and more driven by flame–turbulence interactions. At high turbulence conditions, flame-eddy interactions within the freestream augment flame stretching via increased flame straining and small-scale flame corrugations. Increased flame stretching disrupts the flame stabilization process, and ultimately results in blowout.

The effect of hydrogen ($$\mathrm {H}_{\mathrm {2}}$$ H 2 ) enrichment on the flame-holding characteristics of two natural gas jet flames in crossflow is investigated here, experimentally. The flame and flowfield measurements are analyzed using simultaneously acquired high-speed (10 kHz) stereoscopic particle image velocimetry, planar laser-induced fluorescence of the hydroxyl radical, and OH* chemiluminescence. The flames, enriched with 20% and 40% $$\mathrm {H}_{\mathrm {2}}$$ H 2 , by volume, are studied at conditions typical of the mixing duct of a modern gas turbine engine; specifically in confinement, at 10 bars, and with a crossflow preheat of 530 K. Consistent with previous findings, the 40% $$\mathrm {H}_{\mathrm {2}}$$ H 2 flame was found to be stabilized on the windward and leeward side of the jet, while the 20% $$\mathrm {H}_{\mathrm {2}}$$ H 2 flame was stabilized only on the leeward side. Analysis of mean and instantaneous velocity fields showed no major differences in the trajectories and principal compressive strain fields of the two flames. The presence of the windward stabilized flame in the 40% $$\mathrm {H}_{\mathrm {2}}$$ H 2 case was, however, found to decrease the centerline velocity decay and greatly reduce or eliminate large scale vortices along the windward shear layer. The difference in the flame-holding here was attributed to the difference in the extinction strain rate from the addition of hydrogen, which would impact the local and global extinction of the flame along the high shear windward region of the flame.

Abstract In this paper, an experimental study of the non-reacting turbulent flow field characteristics of a piloted premixed Bunsen burner designed for operational at elevated pressure conditions is presented. The generated turbulent flow fields were experimentally investigated at atmospheric and elevated pressure by means of high-speed particle image velocimetry (PIV). The in-nozzle flow through the burner was computed using large-eddy simulation (LES), and the turbulent flow field predicted at the burner exit was compared against the experimental results. The findings show that the burner yields a reasonably homogeneous, nearly isotropic turbulence at the nozzle exit with highly reproducible boundary conditions that can be well predicted by numerical simulations. Similar levels of turbulence intensities and turbulent length scales were obtained at varied pressures and bulk velocities with turbulent Reynolds numbers up to 5300. This work demonstrates the burner’s potential for the study of premixed flames subject to intermediate and extreme turbulence at the elevated pressure conditions found in gas turbine combustors. Graphical abstract

Large Eddy Simulation (LES) is a fundamental research tool to study gas turbines and aero-engine combustors. In LES, although rarely addressed systematically, it is known that thermal boundary conditions control the heat transfer between the flow and the combustor walls. This work presents a study on the impact of thermal wall boundary conditions for the PRECCINSTA test bench, operated by the German Space Agency (DLR). Two approaches are tested: Heat Resistances Tuning (HRT), where a local resistance is tuned using experimental temperature data, and full Conjugate Heat Transfer (CHT), where the chamber wall-temperature is solved and coupled to the flow computation. Results reveal that the HRT method captures the mean flame correctly but the predicted flame becomes unstable and responds to a thermoacoustic oscillation which is not observed experimentally. On the contrary, using CHT, the flame is correctly predicted and stable as in the experiments. Finally, to understand the differences between the HRT and the CHT simulations, Dynamic Mode Decomposition (DMD) analysis is performed showing that the correct response of the flame branches to the pressure oscillations is recovered only in the CHT simulations for which thermoacoustically stable operations are retrieved.

Large eddy simulations (LES) of premixed hydrogen-enriched swirling flames were performed to investigate the flame topology and combustion instabilities with different hydrogen concentrations. A compressible LES approach is utilised to account for the self-excited combustion dynamics. A transported probability density function (pd f) approach is adopted to account for sub-grid scale (sgs) turbulence-chemistry interaction, and the solution to the joint sgs – pd f evolution equation of the scalars is obtained by the stochastic field method. The chemistry is represented using a reduced chemical reaction mechanism containing 15 reaction steps and 19 species. The results revealed that as the concentration of hydrogen increases, the flame is shortened in the injecting direction and more confined in the cross-sectional direction, which is consistent with experimental observations. The self-excited limit-cycle oscillations for all considered cases were successfully reproduced, with the predicted peak frequencies of the chamber pressure spectra in excellent agreement with the measured values. The feedback loop of the oscillations is successfully captured and analysed with the temporal evolution of axial velocity and heat release presented.

We study the impact of H2 enrichment on the unsteady flow dynamics and thermoacoustic instability in the single nozzle PRECCINSTA swirl combustor. We analyze data from two operating modes, premixed (PM) and technically premixed (TPM). The experiments were performed at atmospheric conditions with H2/CH4 fuel mixtures at a global equivalence ratio of 0.65 while maintaining a constant thermal power of 20 kW. We examine the effect of H2 addition on the flow dynamics by analyzing cases with three fuel compositions: 0% H2, 20% H2 and 50% H2 in both operating modes. A new multi resolution modal decomposition method, using a combination of wavelet transforms and proper orthogonal decomposition (WPOD) of the experimental time resolved high speed flow velocity and OH-PLIF measurements is performed. Thermoacoustic oscillations are observed in the TPM operating mode alone. WPOD results for the 0% H2 TPM operating mode case reveals intermittent helical PVC oscillations along with axi-symmetric hydrodynamic flow oscillations due to the thermoacoustic oscillation. These oscillations cause local flame extinction near the nozzle centrebody resulting in liftoff. A precessing vortex core (PVC) oscillation develops in the flow that enables intermittent flame reattachment and results in intermittent thermoacoustic oscillations in this case. In the 0% H2 PM case, the flame remains lifted off of the centrebody despite the presence of PVC oscillations in this case as well. H2 enrichment results in the suppression of flame lift-off and the PVC in both operating modes. We show from flow strain rate statistics and extinction strain rate calculations that the increase of the latter with H2 addition, allows the flame to stabilize in the region near the centrebody where the pure CH4 cases show lift off. The lack of thermoacoustic oscillations in the PM operating mode shows that the primary heat release driving mechanism is due to fuel-air ratio oscillation that the thermoacoustic oscillation generates. The time averaged flow fields and the emergence of the PVC when the flame is lifted off, together suggest that PVC oscillations are caused by the separation between the vortex breakdown bubble and the wake behind the centrebody, as suggested by prior computational studies.

Combustion instability, which is the result of a coupling between combustor acoustic modes and unsteady flame heat release rate, is a severely limiting factor in the operability and performance of modern gas turbine engines. This coupling can occur through different coupling pathways, such as flow field fluctuations or equivalence ratio fluctuations. In realistic combustor systems, there are complex hydrodynamic and thermo-chemical processes involved, which can lead to multiple coupling pathways. In order to understand and predict the mechanisms that govern the onset of combustion instability in real gas turbine engines, we consider the influences that each of these coupling pathways can have on the stability and dynamics of a partially-premixed, swirl-stabilized flame. In this study, we use a model gas turbine combustor with two concentric swirling nozzles of air, separated by a ring of fuel injectors, operating at an elevated pressure of 5 bar. The flow split between the two streams is systematically varied to observe the impact on the flow and flame dynamics. High-speed stereoscopic particle image velocimetry, OH planar laser-induced fluorescence, and acetone planar laser-induced fluorescence are used to obtain information about the velocity field, flame, and fuel-flow behavior, respectively. Depending on the flow conditions, a thermoacoustic oscillation mode or a hydrodynamic mode, identified as the precessing vortex core, is present. The focus of this study is to characterize the mixture coupling processes in this partially-premixed flame as well as the impact that the velocity oscillations have on mixture coupling. Our results show that, for this combustor system, changing the flow split between the two concentric nozzles can alter the dominant harmonic oscillation modes in the system, which can significantly impact the dispersion of fuel into air, thereby modulating the local equivalence ratio of the flame. This insight can be used to design instability control mechanisms in real gas turbine engines.

This paper presents a data-driven identification framework with the objective to retrieve a flame model from the nonlinear limit cycle. The motivation is to identify a flame model for configurations, which do not allow the determination of the flame dynamics: that is commonly for industrial applications where (i) optical access for non-intrusive measurements of velocity and heat release fluctuations are not feasible and (ii) unstable combustion is monitored via multiple pressure recordings. To demonstrate the usefulness of the method, we chose three test cases: (i) a classical Rijke tube; (ii) an experiment of a laminar flame (EM2C case) (iii) and a high-pressure, turbulent premixed flame (DLR case). The procedure is as follows: First, acoustic network models for the three cases are generated for which the in-house software taX is employed. Next, the acoustic network models are embedded in an optimization framework with the objective to identify flame parameters that match the experimental limit cycle data: based on the instability frequency and pressure amplitudes, we formulate physical constraints and an objective function in order to identify the flame model parameters gain nopt and time delay τopt in the nonlinear regime. We demonstrate for the three cases that the identified flame parameters reproduce the unstable combustion processes and highlight the usefulness of the method for control purposes.

The mechanisms of flame blowout under pressure gradient effects are explored for a bluff-body stabilized flame. The blowout process is induced through equivalence ratio reduction from a lean stabilized flame to complete blowout. Simultaneous high-speed particle image velocimetry (PIV) and C2*/CH* chemiluminescence imaging diagnostics are used to obtain the instantaneous flame structure, vorticity field, equivalence ratio, and local strain rate during the extinction process. The goal is to elucidate the effect of flame-generated vorticity on lean flame extinction. Three test-sections configured as a nozzle, a rectangular duct, and a diffuser, are used to alter the downstream pressure gradient yielding high, nominal, and low magnitudes of flame-generated baroclinic torque. For all three configurations, the flame brush narrows and the shear layer vorticity expands in the transverse direction resulting in flame-shear interactions and extinction. The flame-shear layer interaction increases the strain rate along the flame; however, the strong flame-generated vorticity for the nozzle case delayed the strain rate increase by keeping the flame away from the shear layer the longest. The sharp increase in the Karlovitz number above unity caused by the sudden increase in the strain rate corresponds to the time of flame brush contraction and shear layer width expansion. It is shown that the downstream pressure gradient can either augment or attenuate the time required for the Karlovitz number to reach a critical value of unity, which is associated with local extinctions along the flame. In all of the test-section configurations, the flame-generated vorticity has a weak influence on the Benard von Karman (BVK) instability mode and its harmonics. The Strouhal number during blow-out remained relatively constant in all of the cases showing greater sensitivity to the shear layer length than to the BVK frequency.

Pocket formation is an important characteristic of turbulent premixed flames and understanding pocket behavior is key to developing high-fidelity numerical combustion models. In this study, a dual-burner experiment is used to study pockets in single- and dual-flame configurations and synchronized high-speed OH-planar laser-induced fluorescence and stereoscopic-particle image velocimetry imaging techniques are implemented to track flame pockets and the surrounding flow field. Statistical analysis of pocket origin and fate is performed using a novel tracking algorithm incorporating non-rigid image registration. Results show that pocket formation rates increase as a function of increasing inlet turbulence level; reactant pocket formation increases as a function of downstream distance, whereas product pocket formation decreases. Tracking reactant pocket lifetime shows that a majority of these pockets burn out and displacement speeds are characterized. Product pockets usually merge with the main flame surface, which could have an impact on local flame structure and propagation. Results presented in this study show that pocket behavior in turbulent flames can change local flame dynamics and it is important to capture these effects in sub-grid scale combustion models to accurately predict flame behavior.

Premixed methane–air turbulent flame is generated in a single-nozzle jet-stabilized combustor designed based on the FLOX ® concept 1 . Confinement-induced, self-sustained jet oscillation is observed. Its influence on combustion stability near lean blow-out (LBO) is investigated using simultaneous particle imaging velocimetry (PIV), planar laser-induced fluorescence of OH radicals (OH PLIF), and OH chemiluminescence imaging at 5-kHz repetition rate. Via proper orthogonal decomposition (POD) of the velocity field and extended POD of the scalar fields, pronounced variations in the flame shape are observed during a cycle of jet oscillation. In extreme cases, flame is partially blown out in the combustor due to jet impingement on the wall during the first half of its oscillation cycle. In the subsequent half cycle following jet detachment, flame is restabilized after robust flashback and re-light. Statistical analysis shows that such pattern is by far the most prevalent mechanism for blow-out and restabilization to take place at the operating condition. Additionally, these events are found with much higher probability during slow-paced jet oscillations.

Flame–flame interaction events occur frequently in turbulent premixed flames and change the local structure and dynamics of flames. It is essential to understand these flame–flame interaction events to develop high-fidelity combustion models for use in modern combustion devices. In this study, we experimentally investigate the topology of flame–flame interaction events in single- and multi-flame configurations. A dual-burner experiment is probed with high-speed OH-planar laser-induced fluorescence and stereoscopic-particle image velocimetry to obtain simultaneous flame front locations and velocity fields. A non-rigid image registration technique is implemented to track the topological changes occurring in these flames. In both single- and dual-flame configurations, small-scale interactions occur more frequently compared to large-scale interactions, and statistics show that most of the reactant-side interactions contribute to large flame surface destructions than the product-side interactions. It is also found that turbulence length- and velocity-scales can play an important role in facilitating the interaction events and pocket formations from these events. Filamentarity is used to quantify the two-dimensional shape of these interactions and comparisons are made between the orientation and shape of interaction events and the local turbulence in the flowfield. Alignment between the orientation of the interaction shapes and the principal strain rates show that compressive fluid forces drive both types of interaction events.

The PRECCINSTA GTMC was studied at elevated pressure and power density with 6 kHz stereoscopic particle image velocimetry (SPIV), OH* chemiluminescence (CL), and 100 kHz dynamic pressure measurements. This technically premixed, swirl stabilized flame exhibited self-excited thermoacoustic oscillations with limit-cycle behavior. A helical precessing vortex core (PVC) was detected within the inner shear layer, between the central recirculation bubble (CRB) and the reactant jets. The PVC was found to be the delineating flow feature for combustion dynamics even at elevated pressure. Sparse dynamic mode decomposition (DMD) of the velocity fields deconvolved the dynamics into a thermoacoustic and PVC mode. The precession of the PVC was at a non-harmonic frequency to the thermoacoustic oscillations, and at least twice that of findings at atmospheric conditions. Nevertheless, the continuous and persistent structure of the PVC allows it promote unsteady heat release to sustain the thermoacoustic cycle. The three dimensional structure of the reactant jets, central recirculation bubble, and PVC was reconstructed by double phase conditioning the reconstructed velocity field. The surface of the CRB was observed to transition between asymmetric and symmetric states depending on the thermoacoustic phase. Analysis of the swirling strength values on the CRB surface indicates the interaction strength between the hydrodynamic structures of the PVC and CRB. When this coupling is large, the heat release determined by the mean OH*-CL intensity is maximum. These findings indicate a critical role of the PVC and CRB interaction on combustion in unstable swirl flames at conditions closer to those found in a modern gas turbine engine.

The mechanisms of transient formation and oxidation of soot in an aero-engine model combustor at elevated pressure are studied for the first time using a combination of high-speed simultaneous stereo-PIV and OH-PLIF and results from a recent detailed LES. A combined analysis of experiment and LES shows that the highly transient and intermittent evolution of soot in this combustor is governed by an unsteady interplay of distinct pockets of burned gas in the inner recirculation zone (IRZ) with either relatively rich or relatively lean composition. The former originate from reaction of fuel-rich unburned gas, whereas the latter result from additional admixture of secondary air further downstream. The analysis further enables distinction and localization of premixed and diffusion-type flame fronts within the flame zone. The time-resolved complementary measurements of velocity field and flame structure allow accurate tracking of both the burned gas pockets and soot filaments. It is seen that soot generally forms in the rich pockets if their residence time in the IRZ is sufficient, whereas oxidation occurs in the lean zones carrying OH. Correlating the dynamics of flow field and soot indicates that the intermittency of soot is driven by an intermittent flow of lean burned gas into the IRZ that affects the residence time of rich pockets. The results suggest that the formation of soot might be further reduced by a proper adjustment of secondary air injection aiming at a sufficient and more constant recirculation of lean burned gas. A remarkably good agreement of measured and simulated instantaneous flame structures is observed, indicating that flow field and gas-phase reactions are well predicted by the LES. The experimental insights into the transient mechanisms of soot formation and oxidation, on the other hand, may provide useful input for LES soot models where deviations from measurements are generally larger.

A dual burner experiment is used to investigate how flow interactions affect local flame-flame interaction in turbulent premixed flames. The presence of adjacent flows influences the local structure of these flames and understanding the sensitivity of these flames to adjacent flows is essential for multi-nozzle combustion devices. To study this sensitivity, a high-aspect-ratio Bunsen flame operating at a constant flow velocity is placed adjacent to an identical burner with non-reacting flow at varying velocities. High-speed OH-planar laser induced fluorescence and stereoscopic-particle image velocimetry measurements are performed to capture flame-front locations and velocity fields. A non-rigid image registration technique is used to calculate the local flame-area variations that occur due to topological differences, and conditional statistics are extracted to relate the local behavior to changes observed in the global behavior of the flames. Extracted flame curvatures and time-averaged progress variables conditioned on flame-flame interactions show differences existing in the inner and outer flame branches near the flame-attachment region. Statistics of these results are presented and compared for all test cases.

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

Self-sustained jet flapping is observed in a confined, premixed and preheated methane-air turbulent flame, generated in a single-nozzle jet-stabilized gas turbine model combustor designed based on the FLOX Ⓡ concept. The flapping frequency and its complex motion within the confinement of the combustor are characterized in detail using proper orthogonal decomposition (POD) of the flow fields measured by particle imaging velocimetry (PIV). The influence of jet flapping on combustion stability is examined using simultaneous PIV/OH chemiluminescence imaging and PIV/planar laser-induced fluorescence of OH radicals (OH PLIF) at 5 kHz repetition rate. By influencing the size and location of the recirculation zones, jet flapping modifies the flame shape and flame lift-off height. It also controls the amount of hot gas entrainment into the recirculation zones. In extreme cases, jet flapping is found to cause temporary local extinction of the flame, due to jet impingement on the combustor wall and partial blockage of burned gas entrainment. The flame is only able to recover after the jet detaches from the wall and reopens the back flow channel. The results suggest that jet flapping could play a key role in the stabilization mechanisms in similar jet-stabilized combustors.

The Turbomeca gas turbine model combustor was studied to understand the effect of increased thermal power density on the structure and dynamics of premixed swirl flames. The burner was operated under two flame conditions: one stable, and one with self-excited thermoacoustic oscillations. Simultaneous measurements of scalar and three-component velocity fields were acquired at 3 and 6 kHz frequencies. Nonreacting flow measurements were used to approximate the spatial and temporal resolution with respect to the scales of the incoming flow. Time-resolved velocimetry revealed the presence of periodic fluctuations in both flames, occurring at shifted frequencies that did not correspond to a resonant acoustic mode or any corresponding harmonics. Proper orthogonal decomposition analysis revealed stably forced inner shear-layer oscillations that periodically formed and ejected symmetric vortex pairs under stable flame operation. The unstable flame was found to exhibit a single spatiotemporally evolving flow struct...

The capabilities of the joint temperature-velocity imaging technique, thermographic particle image velocimetry (TPIV), are examined in H2 laminar diffusion flames by seeding the fuel stream with BAM: Eu 2 + particles. The upper measurable temperature is found to be 900–1000 K. For these phosphor particles this limit is primarily imposed by thermal quenching of the laser-induced luminescence emission that is used for thermometry. Within this limit, the measured temperature distribution quantitatively agrees with a numerical simulation. It is also shown that the luminescence signal per phosphor particle is not influenced by passing the particles through a high temperature reaction zone.

A 12-nozzle FLOX® combustor is used to generate a full-premixed methane-air flame at ϕ = 0.86 at atmospheric pressure. Combustion stability is examined using 5-kHz simultaneous stereo Planar Image Velocimetry (PIV), OH Planar Laser-induced Fluorescence (OH PLIF) and OH chemiluminescence imaging. Proper Orthogonal Decomposition (POD) of the PIV results from various measurement planes reveals that slow jet oscillations with a characteristic Strouhal number of 0.012 are the dominant fluctuations in the flow field. Jet impingement on and detachment from the walls during jet oscillations are shown to cause the liftoff heights of the flames to increase and decrease. Such changes in flame lift-off heights are also primarily asymmetric among geometrically-symmetric flame pairs. In addition, direct flame-flame interactions are observed as jets collide during oscillations. Dynamic Mode Decomposition (DMD) of the same flow fields is shown able to capture not only the same low-frequency jet-oscillation mode, but also a series of modes spanning the whole resolvable spectrum, which are potential origins of higher-frequency peaks observed in the power spectra of integrated OH chemiluminescence signals.

This study was designed to test the feasibility of acquiring simultaneous PIV/OH-PLIF measurements at multi-kHz rates in a turbulent swirl flame at pressures relevant to modern industrial gas turbine combustors. To accomplish this, particle image velocimetry (PIV) and planar laser-induced fluorescence of the hydroxyl radical (OH-PLIF) were applied simultaneously at 3 kHz to study the dynamics of a lean partially-premixed turbulent swirl-stabilized flame of natural gas in an optically accessible, high-pressure combustion test rig at 5 bars. With 0.25 mJ/pulse at 283 nm for the OH-PLIF measurements, an average signal-to-noise ratio (SNR) of 4.1 was achieved over a region measuring 20 × 80 mm. Absorption of the excitation laser proved to be the greatest challenge in this study, resulting in a significant variation in SNR from one side of the OH-PLIF images to the other. A procedure based on modeling the absorption according to the mean OH-distribution was used to semi-quantitatively correct for this effect. A gradient-based edge-detection algorithm was used to identify reaction zone locations in the resulting images. These were used to compute mean distributions of the flame surface density. With 2.5 mJ/pulse at 532 nm for the PIV system, velocity fields measuring 20 × 80 mm were measured at a resolution of 1.25 mm. Consistent with prior measurements in the burner, the flame shows strong thermo-acoustic pulsation, with a peak frequency of 388 Hz. Phase-averages of the PIV and OH∗ data indicate these pulsations are driven by the same resonant feedback mechanism responsible for thermo-acoustic pulsation in the burner at atmospheric pressure. No evidence of a precessing vortex core, known to dominate the flow-field of the burner at atmospheric-pressure conditions, was observed.

Single-laser-shot temperature measurements at 5 kHz were performed in a model gas turbine combustor using femtosecond (fs) coherent anti-Stokes Raman scattering (CARS). The combustor was operated at a global equivalence ratio of 0.65 and 10 kW thermal power. Measurements were performed at various locations within the flame in order to resolve the spatial flame structure and compare to previously published studies. Power spectral density analysis of the temperature measurements yielded the characteristic thermo-acoustic pulsation frequency previously reported at 308 Hz. These results demonstrate the usefulness of fs-CARS for the investigation of highly turbulent combustion phenomena. The spatial resolution of the single-laser shot temperature measurements was approximately 600 μm, the precision was approximately ±2%, and the estimated accuracy was approximately ±3%. The dynamic range was sufficient for temperature measurements ranging from 300 K to 2400 K, although some detector saturation was observed for low temperature spectra.

The goal of this study was to select and test some analysis tools to identify and characterize precursors to the onset of thermo-acoustic pulsation in a gas turbine combustor. The target flame was a turbulent, swirl-stability ethylene-air flame operated at ϕ = 0.91, and 5 bars pressure. 3-component stereo-particle image velocimetry (PIV) measurements, acquired at 9.3 kHz over periods of approximately 4 seconds were used to characterize the flow-field near the exit plane of the combustor. Acoustic measurements and OH*-chemiluminescence images were acquired synchronously, with OH* images acquired at every third cycle of the PIV measurement system. Analysis revealed the presence of two characteristic frequencies in the data for the stable flame; a 475 Hz oscillation associated with the shear-layer dynamics and a weak thermo-acoustic oscillation 610 Hz. In the excited state, a 720 Hz self-excited thermo-acoustic oscillation dominated combustor dynamics. A possible precursor event was identified in the form of a 635 Hz oscillation that appeared in the data 0.15s prior to transition. Bandpass filtering of the velocity data at this frequency showed this oscillation originates in the outer recirculation zone of the combustor.

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

N anticipation of increased regulation of soot emissions from aviation engines there has been considerable effort in recent years to better understand, model and predict soot formation in gas turbine combustors. Experimental studies in this effort generally fall into one of two categories: 1) detailed studies of chemical kinetics and mechanisms of specific sub-processes of soot formation, agglomeration and oxidation in simple, well-characterized test flames (e.g., laminar Bunsen or diffusion flames) and 2) studies focusing on system-level parameters such as global soot emissions vs fuel grade and combustor pressure at the expense of detailed understanding of specific subprocesses. Although both categories have specific strengths, the understanding yielded by each is of limited use for the purpose of predictive modelling of gas turbine combustors. The German Aerospace Center (Deutsches Zentrum fur Luftund Raumfahrt, DLR) has led an effort in recent years to bridge the gap between fundamental scientific studies of soot formation and system-level characterization of gas turbine combustors. This effort has focused on acquiring detailed measurements of a series of soot-generating flames in a generic, swirl-stabilized combustor at elevated pressure. These flames are designed to capture much of the complexity of a modern, swirl-stabilized gas turbine combustor, while maintaining excellent optical access for pointand planar laser measurement techniques. As part of this effort, Lammel et al. (2007) applied laser-induced incandescence (LII) and coherent anti-Stokes Raman scattering (CARS) spectroscopy to quantify the effects of pressure, equivalence ratio and secondary oxidation air on mean soot distribution in swirl-stabilized, sooting ethylene-air flames at pressures up to 9 bars and thermal loads up to 45 kW. Their results showed the highest soot concentrations were to be found in the lower part of the inner recirculation zone (IRZ) of the combustor, where residence times and local fluid temperatures are high. The combustor used by Lammel et al. (2007) was based on the well-characterized DLR Dual-swirl Gas Turbine Model Combustor or “DS-burner” [2,3] applied LII, CARS and particle image velocimetry (PIV) in a similar combustor at atmospheric pressure and found that injection of secondary air downstream of the flame zone results in drastic changes to soot distribution in the combustor. Geigle et al. (2014) extended these measurements (in a slightly modified burner geometry) to include flames at pressures up to 5 bars and observed that, in addition to pressure and global equivalence ratio, soot concentration and distribution were sensitive to the ratio of air flow supplied to the innerand outer swirl nozzles. Geigle et al. (2015) applied simultaneous LII and planar laser-induced fluorescence of OH (OH-PLIF) to study the spatial correlation of soot and high temperature combustion products in these flames. Their results showed the addition of air downstream of the main flame zone resulted in secondary combustion zones that consume the soot (previously observed in the IRZ) or even prevent soot formation in the first place. These studies produced a rich database of experimental measurements on sooting, swirl-stabilized flames in a gas turbine model combustor. This database, however, is limited to single-shot measurements which yield only mean and fluctuating quantities of interest. Soot formation, agglomeration and oxidation are highly dynamic processes, dependent upon multiple tightly coupled parameters. It is therefore of considerable interest to acquire time-resolved measurements of quantities such as velocity, soot distribution and reaction zone location. Furthermore, these studies have focused primarily upon globally fuel-rich flame conditions. Although this is certainly of key interest in understanding soot-dynamics, it has been observed that even (globally) lean flames can

The dynamics of flame-hole reignition were studied experimentally in a turbulent non-premixed CH 4 / H 2 / N 2 jet flame at Re d = 22,800 (flame ‘DLR-B’ from the TNF workshop). Simultaneous measurements of the OH combustion radical and velocity field were performed using planar laser induced fluorescence (PLIF) and particle image velocimetry (PIV) at a sustained rate of 10 kHz. The dynamics of the reignition process were tracked through time and two reignition mechanisms were identified. Particular care was taken to reduce the influence of out-of-plane motion on the analyzed events by simultaneously measuring the OH distribution in crossed planes. Flame-holes reignited due to both edge-flame propagation and turbulent transport of burning flame segments. However, the edge-flame propagation mechanism was dominant and accounted for over 90% of the flame-hole reignition rate on average. Furthermore, the presence of large scale turbulent structures adjacent to a flame-hole did not necessarily result in reignition due to turbulent transport. Instead, the edge-flames propagated around the perimeter of such structures, indicating intervening regions of well mixed gas. The range of measured edge-flame propagation speeds agreed well that of highly-preheated premixed flames, with a mode of approximately 4 m/s and a mean of approximately 7 m/s.

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

This paper presents observations and analysis of the time-dependent behavior of a 10 kW partially pre-mixed, swirl-stabilized methane–air flame exhibiting self-excited thermo-acoustic oscillations. This analysis is based on a series of measurements wherein particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) of the OH radical were performed simultaneously at 5 kHz repetition rate over durations of 0.8 s. Chemiluminescence imaging of the OH * radical was performed separately, also at 5 kHz over 0.8 s acquisition runs. These measurements were of sufficient sampling frequency and duration to extract usable spatial and temporal frequency information on the medium to large-scale flow-field and heat-release characteristics of the flame. This analysis is used to more fully characterize the interaction between the self-excited thermo-acoustic oscillations and the dominant flow-field structure of this flame, a precessing vortex core (PVC) present in the inner recirculation zone. Interpretation of individual measurement sequences yielded insight into various physical phenomena and the underlying mechanisms driving flame dynamics. It is observed for this flame that location of the reaction zone tracks large-scale fluctuations in axial velocity and also conforms to the passage of large-scale vortical structures through the flow-field. Local extinction of the reaction zone in regions of persistently high principal compressive strain is observed. Such extinctions, however, are seen to be self healing and thus do not induce blowout. Indications of auto-ignition in regions of unburned gas near the exit are also observed. Probable auto-ignition events are frequently observed coincident with the centers of large-scale vortical structures, suggesting the phenomenon is linked to the enhanced mixing and longer residence times associated with fluid at the core of the PVC as it moves through the flame.

Planar laser-induced fluorescence (PLIF) and stereoscopic particle image velocimetry (PIV) were simultaneously applied at 1.5 kHz repetition-rate to acquire time-resolved, cinematographic planar measurements of the flamebase stabilization region of a turbulent lifted jet flame. The current work focuses on the development and demonstration of the diagnostic system and presents key examples of the new capabilities it provides. OH-PLIF is used to identify and track the spatial position and shape of the flamefront near the flamebase, while the stereoscopic PIV is used to acquire three-component planar velocity measurements over the same region. Initial results indicate that the significant temporal histories and experimental access to the Vz-velocity component afforded by this system are frequently essential for accurate interpretation of planar measurements in the flow field of a lifted turbulent jet flame.

Time-resolved particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF), both at 5 kHz, were applied simultaneously on extinguishing turbulent opposed jet flames. This repetition rate allowed tracking of transient extinction events in turbulent combustion. The additional information acquired about time history enabled a study of the evolution of vortex-flame interactions leading to extinction from individual events. A newly introduced multidimensional conditioning technique to avoid spatial- and temporal-smearing of important flow field information was developed in order to compare individual extinction events in a meaningful, statistical manner. The conditional statistics show that vortices tend to align around the flame and generate regions of high strain in the region where the flame is about to extinguish.

A single-nozzle FLOX® model combustor was used to produce a confined, premixed CH4-air flame with an equivalence ratio of ϕ = 0.74 and a jet exit velocity of vjet = 150m/s with a preheat temperature of T0=300°C. For the first time for this combustor, surface thermometry was performed on the chamber walls. In addition, particle imaging velocimetry (PIV) and planar laser-induced fluorescence of hydroxyl radical (OH PLIF) were acquired simultaneously in this flame at 5 kHz repetition rate. The interface between burnt and unburnt gas mixture were identified from instantaneous OH PLIF images and were compared with corresponding PIV results for flame-turbulence interaction analysis. Combustion instabilities were analyzed via proper orthogonal decomposition and phase-averaged flow field and OH distribution. A pronounced flapping motion of the jet was identified and its impact on the recirculation of hot burnt gas was characterized.Copyright © 2015 by ASME

Laser-induced incandescence (LII) was performed at 3 kHz in an optically accessible cylinder of a fired diesel engine using a commercially available diode-pumped solid-state laser and an intensified CMOS camera. The resulting images, acquired every 3° of crank angle, enabled the spatiotemporal tracking of soot structures during the expansion/exhaust stroke of the engine cycle. The image sequences demonstrate that soot tends to form in thin sheets that propagate and interact with the in-cylinder flow. These sheets tend to align parallel to the central axis of the cylinder and are frequently wrapped into conical spirals by aerodynamic swirl. Most of the soot is observed well away from the cylinder walls. Quantitative soot measurements were beyond the scope of this study but the results demonstrate the practical utility of using kHz-rate LII to acquire ensemble-averaged statistical data with high crank angle resolution over a complete engine cycle. Based on semi-quantitative measures of soot distribution, it was possible to identify soot dynamics related to incomplete charge exchange. This study shows that long-duration, multi-kHz acquisition rate LII measurements are viable in a fired diesel engine with currently available laser and camera technology, albeit only in the expansion and exhaust phase of the cycle at present. Furthermore, such measurements yield useful insight into soot dynamics and therefore constitute an important new tool for the development and optimization of diesel engine technology.

Statistics of the alignment of fluid-dynamic principal strain-rates and the local flamelet-normal in a premixed turbulent V-flame (methane-air, Re t = 450, φ = 0.8) were measured experimentally using simultaneous stereoscopic particle image velocimetry (SPIV) and planar laser-induced fluorescence of OH (OH-PLIF). The use of a second OH-PLIF sheet, oriented in a crossed-plane imaging configuration enabled conditioning of the statistics with respect to through-plane flame orientation. The statistics show the geometric alignment changes significantly with the distance between the flame and the location where the strain-rate field is evaluated. It was observed that approximately 30 η upstream of the flame, the fluid-dynamic principal strain-rates show no preferential alignment with the flamelet. With increasing proximity to the flame, the most extensive principal strain-rate is observed to align preferentially perpendicular to the local flamelet-normal. In the immediate vicinity of the flame, where local fluid-dynamics are dominated by dilatation, the principal extensive strain-rate is observed to align preferentially parallel to the local flamelet-normal. The realignment of the principal strain-rates in the immediate vicinity of the flame is clearly the result of local flow acceleration caused by heat-release at the reaction zone. As the most extensive principal strain-rate tends to align preferentially perpendicular to the local flamelet-normal outside the region of heat-release, the data indicate that high scalar gradients observed ahead of the flamelet are produced by the local turbulent flow-field, rather than destroyed by it.

A scaled model of a gas turbine (GT) burner with coaxially mounted swirlers has been used to study the effects of fuel staging on the behavior of lean premixed methane air flames. Lean flames are known to be susceptible to instabilities that can lead to unsteady operation, flame extinction, and thermo-acoustic oscillations. High speed (10 kHz) laser and optical diagnostic techniques have been used to investigate the fuel staging effect on the mechanisms involved in such instabilities. Methane air flames at atmospheric pressure have been investigated at a constant thermal power of 58 kW. The global equivalence ratio was kept constant, while the fuel staging was varied. The bulk flow velocity at the exit plane was kept constant at 20 m/s. Simultaneous high speed OH PLIF, OH* CL, and acoustic measurements were performed at kHz repetition rate to characterize the flames and determine the operability limits of the combustor. The characterization measurements reveal significant changes in flame shape for variou...

Tomographic particle image velocimetry (tomographic-PIV) is a recently developed measurement technique used to acquire volumetric velocity field data in liquid and gaseous flows. The technique relies on line-of-sight reconstruction of the rays between a 3D particle distribution and a multi-camera imaging system. In a turbulent flame, however, index-of-refraction variations resulting from local heat-release may inhibit reconstruction and thereby render the technique infeasible. The objective of this study was to test the efficacy of tomographic-PIV in a turbulent flame. An additional goal was to determine the feasibility of acquiring usable tomographic-PIV measurements in a turbulent flame at multi-kHz acquisition rates with current-generation laser and camera technology. To this end, a setup consisting of four complementary metal oxide semiconductor cameras and a dual-cavity Nd:YAG laser was implemented to test the technique in a lifted turbulent jet flame. While the cameras were capable of kHz-rate image acquisition, the laser operated at a pulse repetition rate of only 10 Hz. However, use of this laser allowed exploration of the required pulse energy and thus power for a kHz-rate system. The imaged region was 29 × 28 × 2.7 mm in size. The tomographic reconstruction of the 3D particle distributions was accomplished using the multiplicative algebraic reconstruction technique. The results indicate that volumetric velocimetry via tomographic-PIV is feasible with pulse energies of 25 mJ, which is within the capability of current-generation kHz-rate diode-pumped solid-state lasers.

Local discontinuities or ‘holes’ in the flame sheet of turbulent jet diffusion flames have been the focus of considerable experimental and theoretical interest in recent years [1 – 5]. Using single and two-shot CH-PLIF, Watson et al. [2] observed local discontinuities in the flame front of a turbulent, lifted CH

Gas turbine (GT) flames at lean operating conditions are susceptible to instabilities that can lead to unsteady operation, flame extinction, and thermoacoustic oscillations. High speed (10 kHz) laser and optical diagnostic techniques have been used to investigate the effect of fuel staging on the mechanisms involved in such instabilities and the overall performance of a gas turbine model combustor. The GT burner used in this study consists of coaxial swirlers which allow for fuel staging capability, where the fuel is varied from 100% to 20% fuel injection in the inner swirler. The burner is equipped with a combustion chamber with large quartz windows, allowing for the application of optical and laser diagnostics. Simultaneous high speed OH Planar Laser Induced Fluorescence (PLIF) and OH* chemiluminescence (CL) imaging, exhaust gas sampling and acoustic measurements were applied to characterize the flames and determine the operability limits of the combustor. Methane air flames at atmospheric pressure have been investigated at a constant thermal power of 58 kW. The global equivalence ratio was kept constant, while the fuel staging was varied. The bulk flow velocity at the exit plane was kept constant at 20 m/s. Simultaneous high speed particle image velocity (PIV) and OH PLIF measurements were performed at a repetition rate of 10 kHz on specifically chosen flames with a fixed staging and equivalence ratio. This paper will present the flame and the flow field structure resolved using the kHz measurement technique. The interaction between the velocity field and the flame front marked by the OH LIF will be presented. The mean PIV image provides the location of the inner and outer recirculation zones. The flame structure presented in this paper will also show the effectiveness of fuel mixing as the staging is varied. The changes in flame shape with variation in fuel staging is determined using the OH* chemiluminescence images. As the fuel flow in the inner swirler is reduced, the NOx and CO emissions also reduce and reach a minimum at a staging of 45% fuel being injected in the inner swirler. As fuel injection in the outer swirler increases beyond 60% the NOx and CO emissions start also increasing.

An image-processing routine was developed to autonomously identify and statistically characterize flame-kernel events, wherein OH (from a planar laser-induced fluorescence, PLIF, measurement) appears in the probe region away from the contiguous OH layer. This routine was applied to datasets from two gas turbine model combustors that consist of thousands of joint OH-velocity images from kHz framerate OH-PLIF and particle image velocimetry (PIV). Phase sorting of the kernel centroids with respect to the dominant fluid-dynamic structure of the combustors (a helical precessing vortex core, PVC) indicates through-plane transport of reacting fluid best explains their sudden appearance in the PLIF images. The concentration of flame-kernel events around the periphery of the mean location of the PVC indicates they are likely the result of wrinkling and/or breakup of the primary flame sheet associated with the passage of the PVC as it circumscribes the burner centerline. The prevailing through-plane velocity of the swirling flow-field transports these fragments into the imaging plane of the OH-PLIF system. The lack of flame-kernel events near the center of the PVC (in which there is lower strain and longer fluid-dynamic residence times) indicates that auto-ignition is not a likely explanation for these flame kernels in a majority of cases. The lack of flame-kernel centroid variation in one flame in which there is no PVC further supports this explanation.