Gain Of Flame Transfer Function Research Articles

Experimental investigation on the effects of transverse injection distribution scheme on dual flame dynamics subjected to flow disturbances

Combustion instability has become a major problem in low-emission annular gas turbine combustors. It is of practical importance to investigate the effects of transverse nozzle distribution scheme on multi-flame dynamics subjected to external acoustic perturbations. In this study, the responses of heat release rate perturbations from dual propane/air laminar premixed flames to axial acoustic perturbations have been experimentally measured. Specifically, the velocity disturbance upstream the flame base has been measured by two-microphone method and the flame heat release rate perturbation is obtained by the photomultiplier tube with CH* filter lens. The dynamic evolution of dual flames is captured by the high-speed camera and then analyzed with the proper orthogonal decomposition (POD) method. Results indicate that there is a critical distribution distance, where the peak gain of the dual flame transfer function is the largest. And the direct injection dual flame transfer function can behave as the low-pass filter. Furthermore, the dual flame transfer function behaves nonlinearly with the increase of the injection angle. The dual flame transfer function for large injection angle can behave as the hybrid mode of the low-pass filter and the band-pass filter. The POD analysis reveals that the coupling effect of gas thermal expansion and stretch effect would contribute to the interaction between dual flame front dynamics. The transverse injection distribution scheme (dual flame distance and injection angle) has a significant effect on the dual flame front dynamics, which in turn affects the flame heat release rate disturbance. The bulk mean flow velocity can enhance the interaction effect between dual flame front dynamics.

Experimental study on conical flame transfer functions considering velocity profiles

This paper investigates the linear thermoacoustic dynamic response of laminar premixed stoichiometric propane/air flames under different oncoming flow velocity profiles. Experiments focused on four nozzle thickness-to-diameter ratios: 0.75, 1.5, 2.5 and 3.75, and three different mean bulk velocities: 1.1m/s, 1.2m/s and 1.3m/s. As the nozzle thickness-to-diameter ratio increases, the velocity profile transitions from a developing velocity profile to a fully developed velocity profile. The flame transfer function (FTF) gain exceeds unity at low frequencies. Notably, the effect of the velocity profile is mainly manifested in the moderate frequency range where the FTF gain decreases. When the velocity profile approaches the resonance frequency of the corresponding small orifice, the decline in FTF gain becomes less pronounced. For fully developed velocity profiles, the decrease in mean bulk velocity leads to an increase in FTF gain at low frequencies, but a more rapid decrease at higher frequencies. Additionally, this paper also investigates segmented flame transfer functions (segFTFs) at various axial positions along the flame. With the increase in frequency, the effect of flame front disturbance on FTF changes from the overall flame displacement caused by flame root stand-off distance, to the effect of flame front wrinkle counteracting effect. The different velocity profiles significantly impact the flame dynamic response, thereby affecting the flame heat release rate and consequently the FTF.

On the flame transfer function models for laminar premixed conical and V- flames considering the stretch effect

This paper investigates a predictive model that considers the impact of stretch on the dynamic response of laminar premixed conical and V- flames; the flame stretch consists of two components: the flame curvature and flow strain. The steady and perturbed flame fronts are determined via the linearized G-equation associated with the flame stretch model. Parameter analyses of the effects of Markstein length L, flame radius R and unstretched flame aspect ratio β are also conducted. Results show that the flame stretch reduces the steady flame height, with this effect being more significant for larger Markstein lengths and smaller flame sizes. The effects of flame stretch on perturbed flames are evaluated by comparing the flame transfer function (FTF) considering the flame stretch and not. For flames of different sizes, the impact of flame stretch on FTF gain can be divided into three regions. When both β and R are relatively small, due to the decrease in steady flame height and the impact of flow strain, the FTF gain increases. As β and R gradually increase, the FTF gain of the conical flame oscillates periodically while the FTF gain of the V-flame decreases, primarily due to the flame curvature enhancing the flame front disturbance and the wrinkle counteracting effect. When β and R are large, a disruption in wrinkle counteracting effect ensues, leading to a significant increase in FTF gain. Furthermore, as the actual flame height is reduced, the flame stretch also reduces the FTF phase lag which is related to the disturbance propagation time from the flame root to the tip.

Experimental Study on Flame Response Characteristics of a Non-Premixed Swirl Model Combustor

Non-premixed swirl combustion has been widely used in pieces of industrial combustion equipment such as industrial boilers, furnaces, and certain specific gas turbine combustors. In recent years, the combustion instability of non-premixed swirl flames has begun receiving attention, yet there is still a lack of related research in academia. Therefore, in this study, we conducted experimental research on a swirl stabilized gas flame model combustor and studied the heat release response characteristics of the swirl combustor through the flame transfer function. Firstly, the flame transfer function (FTF) was measured under different inlet velocities and equivalence ratios, and the experimental results showed that the FTF gain curve of the non-premixed swirl flame exhibited a significant “bimodal” shape, with the gain peaks located around 230 Hz and 330 Hz, respectively. Secondly, two oscillation modes of the flame near the two gain peaks were identified (the acoustic induced vortex mode Mv and the thermoacoustic oscillation mode Ma), which have not been reported in previous studies on swirl non-premixed flames. In addition, we comprehensively analyzed the flame pulsation characteristics under the two oscillation modes. Finally, the coupling degrees between velocity fluctuations, fuel pressure fluctuations, and heat release fluctuations were analyzed using the Rayleigh Index (RI), and it was found that in the acoustic-induced vortex mode, a complete feedback loop was not formed between the combustor and the fuel pipeline, which was the main reason for the significant difference in the pressure fluctuation amplitude near 230 Hz and 330 Hz.

Influence of pilot H[formula omitted] injection on methane-air swirled flame stabilization and acoustic response

Large Eddy Simulation is used to investigate the effect of localized pilot H2 injection on the Flame Transfer Function (FTF) of a premixed CH4-air swirled flame. The response of a perfectly premixed methane-air flame is compared to the one with an additional pilot hydrogen injection that supplies 10% of the original power. Numerical simulations are validated against experiments in terms of global FTF values at selected forcing frequencies, acoustic pressure and velocity signals, CH* flame images and flame root position dynamics. The unforced cases are first considered showing that, when H2 is injected in the center, the flame becomes slightly lifted with a localized diffusion front above the pilot injection. As in the experiments, LES retrieves that hydrogen pilot injection leads to a global redistribution of the heat release rate towards the flame root due to higher burning rates which translates to an overall FTF gain reduction over the entire frequency range explored. Then, the flame acoustic response for the two injection strategies is scrutinized at two distinct forcing frequencies: 240 Hz where the FTF gain difference is maximum, and 590 Hz where the FTF phase shift is maximum. LES reveals that, despite H2 pilot injection does not modify the dynamics of the large vortical structures shed in the external shear layer of the swiling jet which are synchronized by the acoustic forcing, the redistribution of the heat release rate towards the flame base weakens their interaction with the flame tip, explaining partly the FTF gain reduction at the two selected frequencies. In addition to that, a marked axial movement of the internal recirculation zone is observed at 240 Hz during the forcing cycle. For the pilot injection, it leads to an oscillation of the lifted flame root while, for the CH4-air case, the flame anchoring point is not affected. This additional oscillation leads for the pilot case to heat release rate fluctuations acting in phase opposition with respect to those observed at the flame tip, generating a further drop of the FTF gain at this specific frequency. The increased burning rate at the flame root and the flame length reduction of the pilot hydrogen flame also affect the characteristic time lag of the flame response. For both frequencies f = 240 Hz and 590 Hz, the phase shift between the two injection strategies is proportional to the flame length reduction caused by hydrogen injection. These simulations confirm that pilot hydrogen injection is an efficient way to reduce the acoustic response of swirled flames over a large frequency bandwidth.

The Effect of Rectangular Confinement Aspect Ratio on the Flame Transfer Function of a Turbulent Swirling Flame

Abstract The flame transfer function (FTF) has been experimentally investigated for a premixed, turbulent and highly swirled flame in rectangular enclosures with varying aspect ratio. A range of equivalence ratios and inlet bulk velocities are considered to vary the length of the flame. Increasing confinement is also shown to increase flame length. More confined flames become increasingly asymmetric, with significant changes to the mean flame shape. Characteristic peaks and dips are observed in the FTF gain for several configurations, caused by constructive and destructive interference between different sources of flow perturbations. Changes in the geometrical confinement distinctly affects these interactions, with FTFs in the most confined chamber containing more pronounced peaks and dips. Analysis of the phase-averaged dynamics has been conducted for two limiting operational conditions in the least and most confined enclosures: a short flame at a low bulk velocity and high equivalence ratio condition; and a long flame at a high velocity and low equivalence ratio condition. The analysis of the short flames shows similar behavior in both enclosures, both in terms of the global response and the local structure of the heat release rate oscillations. Small differences in local response symmetry in the flame due to the close confinement do not affect the global flame response. By comparison, close confinement significantly affects the symmetry of the flame dynamics in the long flame. However, the changes in symmetry do not significantly modify the response, and more important is the change in flame length, which significantly alters the cutoff frequency, reducing the gain at high frequencies.

Generation of low frequency flame oscillations for the development of passive acoustic flame arresters

The development of acoustic-based flame arresters needs a profound understanding of low frequency flame fluctuations. The present study attempts the synthesis and analysis of such low frequency flame oscillations in a Rijke tube combustor subjected to external acoustic forcing. A high frequency (fn = 437±3 Hz) self-excited laminar premixed conical flame is considered. This flame is forced (ff)around its natural frequency (fn) to generate beating (fb=|ff−fn|). The results show that the system falls into lock-in synchronization for a range of forcing frequencies (0.97≤ff/fn≤1.03) at a forcing amplitude (εf) of 10% wherein quenching of natural mode is achieved. A strong flame beating is observed on either side of this synchronization region. The flame transfer function (FTF) at fb is then estimated to delineate the strength of the beating. The FTF gain at fb shows different behavior on either side of the synchronization region. For ff >fn, the gain exhibits a local peak at low beating frequencies (∼20 Hz) which is consistent with the response of a stable laminar conical flame subjected to acoustic forcing at these frequencies. However, for ff<fn, negligible FTF gain is obtained at low beating frequencies, followed by a continuous increase as the beating frequency increases. This asymmetric behavior is further analyzed using proper orthogonal decomposition technique (POD). For ff>fn, the POD results at low fbshow that the flame base undergoes a strong flapping-like oscillation at the beating frequency. However for ff <fn, the flame base oscillates only at the natural mode of the combustor. This different beating behavior of the system can be attributed to the strong and weak thermoacoustic coupling, after and before the synchronization region, respectively.

Effects of hydrogen addition on the forced response of H2/CH4 flames in a dual-nozzle swirl-stabilized combustor

The effects of hydrogen addition on the forced response of H2/CH4 flames are analyzed in a dual-nozzle swirl-stabilized combustor. The hydrogen volumetric content in the fuel is varied from 0% to 40%. Flame transfer function (FTF) is used to compare the forced response of the flames. The FTF gain featuring the local maximum and minimum values, which occurred commonly in the FTFs under all hydrogen contents, is determined by two different mechanisms: the change in the flame angle and the flame roll-up phenomenon. Among two mechanisms, the flame roll-up phenomenon has a more important role in determining the FTF characteristics. In addition, hydrogen addition attenuates the local maximum gains and decreases the FTF phase slope. The change in the flame roll-up behavior, which is induced by a short and compact flame distribution at high hydrogen contents, is the primary reason of these differences in the FTF.

Comparison of flame response characteristics between Non‐premixed and premixed flames under acoustic excitation

A comparative study was conducted on the response characteristics according to the increase in velocity perturbation intensity (u'/u) under acoustic excitation conditions of non-premixed and premixed flames. In addition, the flame dynamic characteristics and flame structure according to acoustic excitation were analyzed, and the response characteristics were compared according to the two flame types through the flame transfer function (FTF) and flame height perturbation analysis. In a non-premixed flame, the number of waveforms (undulations) on the flame surface increased according to the forcing frequency in the flame structure according to the acoustic excitation. On the other hand, as the velocity perturbation intensity increased, the flame height decreased. Thus, in the low forcing frequency range, the magnitude of the flame fluctuation increases, and in the high forcing frequency, the magnitude of the flame fluctuation tends to decrease. Therefore, the Strouhal number and FTF gain are nonlinear properties, and the nonlinear effect depends on the magnitude of the gain and the critical Strouhal number. In a premixed flame, various flame structures are shown according to acoustic excitation. The average flame height does not change much with increasing (u'/u¯). The correlation between the Strouhal number and FTF gain is linear. The flame height perturbation parameters were introduced to examine the flame dynamic characteristics according to the acoustic excitation, and non-premixed and premixed flames were analyzed. As a result, it shows the trend and similarity of the FTF gain, suggesting its potential as a parameter for simple analysis of the heat-release rate. In the correlation between the Strouhal number and FTF gain, the non-premixed flame has a nonlinear characteristic and agrees with the analytical study. On the other hand, the premixed flame is not consistent with analytic studies when the Strouhal number is greater than 1. The reason for various flame structures is to consider parameters such as the unusual flame structure and laminar flame speed by external perturbation.

Thermoacoustic instability and flame transfer function in a lean direct injection model gas turbine combustor

This paper discusses the combustion dynamic characteristics of hydrogen/methane flames and the flame response to fuel flow fluctuation in a model lean-direct injection gas turbine combustor. The results show that a higher frequency longitudinal thermoacoustic instability is triggered as hydrogen/methane ratios increase. The mode shift is accompanied by shortening flame length and hence decreasing convection time delay between fuel injection and flame location. The flame transfer function (FTF) subject to fuel flow rate modulation is determined over a range of frequencies for hydrogen/methane volume ratios of 25:75, 50:50, and 75:25 at a fixed equivalence ratio of 0.55. The input and output of the FTF are the velocity fluctuation of fuel-mixture at the inlet of the burner and the heat release fluctuation of flame in the combustion zone, respectively. Because of the difficulty of measuring fuel flow velocity at the inlet of the combustor while the flame exists, the fuel transfer function (FLTF) is measured and used along with the intermediate flame transfer function (ITF) to determine the FTF. The transfer functions are measured at frequencies up to 600 Hz and the FTF above 600 Hz is determined from the flame response at higher harmonic frequencies. Results of the FLTF measurement show that an acoustic resonance is formed within the fuel injector and the resonance frequency changes with the hydrogen/methane ratio as well as the length of the fuel feedline. The gain of FTF at a given frequency decreases as the hydrogen content increases and the absolute value of the slope of the phase plot, which is related to the convection time delay between fuel injector tip and flame location, decreases as the hydrogen enrichment ratio increases. Using the measured FTF as input to an open-source simulator, the occurrence of instability is predicted and compared with experimental results. The modeling results agree very well with the experimental data in predicting the occurrence of unstable combustion, the frequency at which unstable combustion occurs, and the acoustic mode inside the combustor.

Effect of fuel line acoustics on the flame dynamics of H2/CH4 syngas in a partially premixed gas turbine combustor

This paper investigates the dynamic response of flames, considering H2-enriched CH4 fuels that are submitted to upstream mechanical perturbations in a model gas turbine combustor, resulting in fuel velocity oscillations. Flame transfer function (FTF) and cold-flow transfer function (CTF), which represent, respectively, the dynamic responses in the flow of the flame and fuel induced by a velocity modulation in the fuel feed line (FFL), are derived. The gain of FTF shows a low-pass behavior and gradually increases until a specific modulation frequency is reached, which is identical to the frequencies of the corresponding peak gains of the CTF. The periodic variation of OH* intensity is observed from the images collected with a high-speed OH-PLIF. The high oscillation near the injector indicates that the fuel velocity perturbation leads to the amplification of the heat release fluctuation at specific modulation frequencies. The singularity frequencies corresponding to the peaks of the gain of both transfer functions coincide with the resonance frequencies in the FFL. The phase of FTF and CTF decreases as the H2 ratio increases and the phase of FTF is well fitted into a single line by nondimensionalization through Strouhal number. Most of these findings on the characteristics of combustion instability and acoustical characteristics of FFL can be used as a crucial methodology of H2 combustor design, such as an anti-resonance fuel/air feed line, which can ensure the stable operation of combustion system by avoiding H2 induced combustion instability failure.

Relationship of gain and phase in the transfer function of swirling flames

Gain and phase are two key characteristics of flame transfer function (FTF) in evaluating unsteady flame response. Few studies have investigated the correlation between gain and phase of FTF compared to the extensive literature on the gain characteristics. In this study, we measured the gain and phase of acoustic-excited swirling flames with different flow rates, fuels, equivalence ratios, and burner structures on a single nozzle premixed swirl burner. We identified, for the first time, the consistent variation of FTF gain and derivative of phase over a wide range of acoustic frequencies, and revealed an essential linear relationship between the extremal frequencies of gain and phase derivative, demonstrating that the gain and phase of FTF are not independent. We further proposed an analytical decomposition of the periodically oscillated swirling flame in which FTF equals the combined complex vector of all the perturbing mechanisms. The synchronized variation characteristics of the gain and phase derivative for different flames and acoustic excitations can be explained using a two vector model based on vector decomposition. The distinct time lags and angular velocities of the perturbing vectors were determined by the different spatial positions of the flame tip and base. The local extrema of the FTF modulus and phase derivative are caused by the phase interference of the sub-vectors. Multiple complex vectors affecting heat release rate should be considered under high frequency.

A numerical study on the acoustic forcing of a laminar premixed saturated spray flame

This paper presents a numerical investigation into the interaction between a laminar premixed saturated spray flame and periodic acoustic forcing. A two-phase model is adopted that implements the Euler–Euler governing equations for the liquid and gaseous phases, enabling the flame dynamics to be examined in detail, particularly in terms of the flame transfer function (FTF). Three distinct mechanisms are identified: (1) a direct perturbation of the flame by a velocity-coupled mechanism, (2) a secondary effect acting via perturbations of the advection velocity of the liquid fuel droplets and (iii) a tertiary effect caused by local variations in the droplet concentration due to droplet clustering. The FTF gain at low-frequency approaching is $$ \left| {\text{FTF}} \right| \approx 2 $$. The phase difference between the processes acting in the second and third coupling mechanisms is found to be a key parameter governing the flame response. At low forcing frequencies, the third coupling mechanism is found to dominate over the first two.

Impact of swirl and bluff-body on the transfer function of premixed flames

The frequency response of three lean methane/air flames submitted to flowrate perturbations is analyzed for flames featuring the same equivalence ratio and thermal power, but a different stabilization mechanism. The first flame is stabilized by a central bluff body without swirl, the second one by the same bluff body with the addition of swirl and the last one only by swirl without central insert. In the two last cases, the swirl level is roughly the same. These three flames feature different shapes and heat release distributions, but their Flame Transfer Function (FTF) feature about the same phase lag at low frequencies. The gain of the FTF also shows the same behavior for the flame stabilized by the central insert without swirl and the one fully aerodynamically stabilized by swirl. Shedding of vortical structures from the injector nozzle that grow and rollup the flame tip controls the FTF of these flames. The flame stabilized by the swirler-plus-bluff-body system features a peculiar response with a large drop of the FTF gain around a frequency at which large swirl number oscillations are observed. Velocity measurements in cold flow conditions reveal a strong reduction of the size of the vortical structures shed from the injector lip at this forcing condition. The flame stabilized aerodynamically only by swirl and the one stabilized by the bluff body without swirl do not exhibit any FTF gain drop at low frequencies. In the former case, large swirl number oscillations are still identified, but large vortical structures shed from the nozzle also persist at the same forcing frequency in the cold flow response. These different flame responses are found to be intimately related to the dynamics of the internal recirculation region, which response strongly differs depending upon the injector used to stabilize the flame.

Effect of pressure on the transfer functions of premixed methane and propane swirl flames

This paper reports on the effect of pressure on the response of methane–air and propane–air swirl flames to acoustic excitation of the flow. These effects are analyzed on the basis of the flame transfer function (FTF) formalism, experimentally determined from velocity and global OH* chemiluminescence measurements at pressures up to 5 bar. In parallel, phase-locked images of OH* chemiluminescence are collected and analyzed in order to determine the associated flame dynamics. Flame transfer functions and visual flame dynamics at atmospheric pressure are found to be similar to previous studies with comparable experimental conditions. Regardless of pressure, propane flames exhibit a much larger FTF gain than methane flames. For both fuels, the effect of pressure primarily is to modify the gain response at the local maximum of the FTF, at a Strouhal number around 0.5 (176 Hz). For methane flames, this gain maximum increases monotonically with pressure, while for propane flames it increases from 1 to 3 bar and decreases from 3 to 5 bar. At this frequency and regardless of pressure, the flame motion is driven by flame vortex roll-up, suggesting that pressure affects the FTF by modifying the interaction of the flame with the vortex detached from the injector rim during a forcing period. The complex heat transfer, fluid dynamics, and combustion coupling in this configuration does not allow keeping the vortex properties constant when pressure is increased. However, the different trends of the FTF gain observed for methane and propane fuels with increasing pressure imply that intrinsic flame properties and fuel chemistry, and their variation with pressure, play an important role in controlling the response of these flames to acoustic forcing.

Two-step approach for pressure oscillations prediction in gas turbine combustion chambers

Currently, gas turbine manufacturers frequently face the problem of strong acoustic combustion-driven oscillations inside combustion chambers. These combustion instabilities can cause extensive wear and sometimes even catastrophic damage of combustion hardware. This requires prevention of combustion instabilities, which, in turn, requires reliable and fast predictive tools. We have developed a two-step method to find a set of operating parameters under which gas turbines can be operated without going into self-excited pressure oscillations. As the first step, an unsteady Reynolds-averaged Navier–Stokes simulation with the flame speed closure model implemented in the OpenFOAM® environment is performed to obtain the flame transfer function of the combustion set-up. As the second step time-domain simulations employing low-order network model implemented in Simulink® are executed. In this work, we apply the proposed method to the Beschaufelter RingSpalt test rig developed at the Technische Universität München. The sensitivity of thermoacoustic stability to the length of a combustion chamber, flame position, gain and phase of flame transfer function and outlet reflection coefficient are studied.

Effects of convection time on the high harmonic combustion instability in a partially premixed combustor

The fuel composition effects of H2/CH4 syngas in a partially premixed model combustor (PP-MC) were examined for the unique phenomenon of combustion instability (CI) frequency/mode shifting (FMS), which is a transition of mode as well as frequency. The increase in the H2 composition of the fuel altered FMS from a longitudinal fundamental mode (≈250Hz) to a 7th harmonic mode (≈1750Hz). The cause and characteristics of this FMS were investigated using OH planar laser-induced fluorescence (OH-PLIF) measured at 10Hz, particle-image velocimetry (PIV), and the flame transfer function (FTF).The convection time (τconv) was assumed to be the key parameter of the FMS. Thus, tests were conducted to determine the air flow rate (V˙air) and equivalence ratio (φ) variation, which are vital parameters of the τconv in terms of the flame length and mixing time. The φ variation caused obvious changes in the flame length and instability frequency/mode, while the V˙air variation did not. The τconv was analyzed by calculating the global convection time (τconv_global) and the real convection time (τconv_real) from the length of the OH-PLIF-based unburned mixture length divided by the averaged mixture nozzle exit velocity. The τconv_real was calculated from the integral of the real velocity determined from PIV. Both calculations showed an inverse correlation between τconv and CI frequency, which particularly signifies that the FMS is controllable and a specific mode of CI can be generated by adjusting the τconv. The FTF was measured to determine the intrinsic characteristics of the flame. The FTF phase was normalized by the Strouhal number (St) and identified a direct relationship between FTF gain and τconv variation.In conclusion, the τconv is the main reason for the FMS. The importance of τconv in understanding the CI characteristics was confirmed in a PP-MC using high H2 fuels.

Interaction Between Swirl Number Fluctuations and Vortex Shedding in a Single-Nozzle Turbulent Swirling Fully-Premixed Combustor

Flame response to imposed velocity fluctuations is experimentally measured in a single-nozzle turbulent swirling fully-premixed combustor. The flame transfer function is used to quantify the flame's response to imposed velocity fluctuations. Both the gain and phase of the flame transfer function are qualitatively similar for all operating conditions tested. The flame transfer function gain exhibits alternating regions of decreasing gain with increasing forcing frequency followed by regions of increasing gain with increasing forcing frequency. This alternating behavior gives rise to gain extrema. The flame transfer function phase magnitude increases quasi-linearly with increasing forcing frequency. Deviations from the linear behavior occur in the form of inflection points. Within the field, the current understanding is that the flame transfer function gain extrema are caused by the constructive/destructive interference of swirl number fluctuations and vortex shedding. Phase-synchronized images of forced flames are acquired to investigate the presence/importance of swirl number fluctuations, which manifest as fluctuations in the mean flame position and vortex shedding in this combustor. An analysis of phase-synchronized flame images reveals that mean flame position fluctuations are present at forcing frequencies corresponding to flame transfer function gain minima but not at forcing frequencies corresponding to flame transfer function gain maxima. This observation contradicts the understanding that flame transfer function gain maxima are caused by the constructive interference of mean flame position fluctuations and vortex shedding, since mean flame position fluctuations are shown not to exist at flame transfer function gain maxima. Further analysis of phase-synchronized flame images shows that the variation of the mean flame position fluctuation magnitude with forcing frequency follows an inverse trend to the variation of flame transfer function gain with forcing frequency, i.e., when the mean flame position fluctuation magnitude increases, the flame transfer function gain decreases and vice versa. Based on these observations it is concluded that mean flame position fluctuations are a subtractive effect. The physical mechanism through which mean flame position fluctuations decrease flame response is through the interaction of the flame with the Kelvin–Helmholtz instability of the mixing layer in the combustor. When mean flame position fluctuations are large the flame moves closer to the mixing layer and damps the Kelvin–Helmholtz instability due to the increased kinematic viscosity, fluid dilatation, and baroclinic production of vorticity with the opposite sign associated with the high temperature reaction zone.

Forced and self-excited oscillations in a natural gas fired lean premixed combustor

An experimental study of the flame response in a premixed gas turbine combustor has been conducted at room temperature and under atmospheric pressure inlet conditions using natural gas. The fuel is premixed with the air upstream of a choked inlet to avoid equivalence ratio fluctuations. Therefore the observed flame response is only the result of the imposed velocity fluctuations, which are produced using a variable-speed siren. Also, a variable length combustor is designed for investigating characteristics of self-excited instabilities. Measurements are made of the velocity fluctuation in the mixing section using hot wire anemometry and of the heat release fluctuation in the combustor using chemiluminescence emission. The results are analyzed to determine the phase and gain of the flame transfer function. The results show that the gain of flame transfer function is closely associated both with inlet flow forcing conditions such as frequency and amplitude of modulation as well as the operating conditions such as equivalence ratio. In order to predict the operating conditions where the combustor goes stable or unstable at given combustor and nozzle designs, time-lag analysis was tried using convection time delay measured from the phase information of the transfer function. The model prediction was in very good agreement with the self-excited instability measurement. However, spatial heat release distribution became more significant in long flames than in short flames and also had an important influence on the system damping procedure.

Spatially distributed flame transfer functions for predicting combustion dynamics in lean premixed gas turbine combustors

The present paper describes a methodology to improve the accuracy of prediction of the eigenfrequencies and growth rates of self-induced instabilities and demonstrates its application to a laboratory-scale, swirl-stabilized, lean-premixed, gas turbine combustor. The influence of the spatial heat release distribution is accounted for using local flame transfer function (FTF) measurements. The two-microphone technique and CH ∗ chemiluminescence intensity measurements are used to determine the input (inlet velocity perturbation) and the output functions (heat release oscillation), respectively, for the local flame transfer functions. The experimentally determined local flame transfer functions are superposed using the flame transfer function superposition principle, and the result is incorporated into an analytic thermoacoustic model, in order to predict the linear stability characteristics of a given system. Results show that when the flame length is not acoustically compact the model prediction calculated using the local flame transfer functions is better than the prediction made using the global flame transfer function. In the case of a flame in the compact flame regime, accurate predictions of eigenfrequencies and growth rates can be obtained using the global flame transfer function. It was also found that the general response characteristics of the local FTF (gain and phase) are qualitatively the same as those of the global FTF.