Equivalence Ratio Oscillations Research Articles

Blowout dynamics and plasma-assisted stabilization of premixed swirl flames under fuel pulsations

The present work investigates blowout dynamics of premixed swirl flames under low-frequency high-amplitude flow pulsations from the fuel feedline, and applies microsecond repetitively pulsed discharges with extremely-low power consumption to precisely control the stability of pulsating flames. First, both positive and negative fuel pulsations deteriorate the flame stability, causing much-narrowed lean blowout regimes. The fuel pulsation causes the flame to remain at the fuel-lean condition for a long period of time, which is considered to be the main cause of flame blowout. The flame response can be further explained by the convection process of the equivalence ratio oscillation. Secondly, the microsecond pulsed discharge with the same repetition rate as the fuel pulsation is used to alter the lean blowout via thermal, kinetic, and hydrodynamics effects. More importantly, the time delay between the discharge pulse and the fuel pulsation determines whether the plasma has an enhancement effect or not. It is found that the discharge prior to the convection of the equivalence ratio pulsation reaches the optimal effectiveness for flame stabilization, extending the blowout limit by approximately 10%. Finally, the lean blowout limit can be continuously extended with the increasing discharge repetition rate until saturation.

Optical Equivalence Ratio Measurement of a Dual Fuel Burner for Natural Gas and Kerosene

A measurement technique for determination of the global and local equivalence ratios from the flame chemiluminescence for a swirl-stabilized lean premixed combustion of natural gas and kerosene is presented. First, we conducted spectrally resolved chemiluminescence studies using an imaging spectrometer to correlate the ratio of individual chemiluminescence signals to the equivalence ratio. Flame spectra were recorded for a multitude of different lean operating conditions for natural gas and kerosene combustion. The spectra show that, without background correction, the CH*/CO2* ratios for both natural gas and kerosene combustion exhibited a monotonic relationship to the equivalence ratio in the investigated range. Subsequently, bandpass-filtered images of CH* and CO2* chemiluminescence were acquired simultaneously on one camera chip using an image doubler to investigate the local relationship of the CH*/CO2* ratio with the equivalence ratio. The ratio images corroborate the monotonic relationship of the CH*/CO2* ratio to the equivalence ratio. Furthermore, the ratio was found to be influenced by the local reaction zone temperature. The presented technique allows high temporal resolution determination of the local equivalence ratio in lean premixed natural gas and kerosene flames and can thus be applied to quantify equivalence ratio oscillations during unstable combustion.

The effects of ring-shaped porous inert media on equivalence ratio oscillations in a self-excited thermoacoustic instability

Gas turbine operation increasingly relies on lean premixed (LPM) combustion to reduce harmful emissions, which is susceptible to thermoacoustic instabilities. Most combustion systems are technically premixed and exhibit a degree of equivalence ratio inhomogeneity. Thermoacoustic pressure oscillations can couple with the heat release oscillations through the generation of equivalence ratio fluctuations at fuel injection sites, which are then convected to the flame front. Previous experimental studies have shown that porous inert media (PIM) can passively mitigate these instabilities by adding acoustic damping and by reducing the thermoacoustic feedback mechanism. To understand the role of PIM on these equivalence ratio oscillations, spatially resolved, phased averaged equivalence ratio fluctuations are measured using the ratio of OH*/CH* chemiluminescence. Spatial imaging of OH* or CH* radicals produce integrated line of sight intensity values and an Abel transformation is used to obtain spatially resolved values. Phase averaged images are synced with dynamic pressure measurements, and an axisymmetric atmospheric burner is used to study the effects of ring-shaped PIM on the spatially resolved equivalence ratio field with self-excited thermoacoustic instabilities. The results show that PIM significantly reduces these fluctuations, and the effects on the stability of the system are discussed.

Combustion Instabilities With Different Degrees of Premixedness in a Separated Dual-Swirl Burner

Abstract The effects of premixedness degrees on combustion instabilities of separated dual-swirl flames have been investigated experimentally in the Beihang Axial Swirler Independently Stratified (BASIS) burner. The degree of premixedness is modulated by the fuel split between two injection positions in the outer stream. In the spectra of pressure oscillations, both the frequency and amplitude are positively correlated with fuel split ratios under partially premixed conditions, and the mode transition between perfectly and partially premixed conditions has been observed. The location of perfectly premixed flames shows no obvious variation at different phase angles, only with a slightly wrinkling of the flame surface along the shear layer. Under partially premixed conditions, however, the flame is found to feature a large-scale periodic convective motion, accompanied by the obvious variation of heat releases due to the equivalence ratio oscillations. The local Rayleigh index map compares the thermoacoustic driving factors under perfectly and partially premixed conditions. The development of above convective motions under partially premixed conditions is explained by combining the variations of pressure oscillations and heat releases. An analysis of the thermoacoustic network and convective path is applied to explain the cause of the mode transition. The results show that the appearance of equivalence ratio oscillations and the elongated convective path under partially premixed conditions brings a longer delay time of the flame response, which could be the reason for the mode transition.

Characteristics of counterflow premixed flames with low frequency composition fluctuations

The response of laminar methane/air counterflow premixed flames under sinusoidal equivalence ratio oscillation was investigated numerically. The timescales of the oscillation were chosen to be sufficiently longer than the flame timescale so that the flame responds quasi-steadily. The response of periodically stratified flame (SF) with a detailed reaction mechanism exhibited the “back-support” effect, in that the consumption speed Sc response deviated increasingly from Sc of steady homogeneous flames (HFs) at higher oscillation frequencies. It was shown that even when the imposed oscillation timescale is much longer than the flame timescale, the flame response can still be delayed under a sufficiently large equivalence ratio gradient. Subsequently, the above results were compared with those obtained with a global four-step mechanism that omits back-diffusion radicals into the reaction zone. As a result, SFs with the global mechanism displayed a much smaller back-support effect in both lean and rich mixtures. Further analysis with modified diffusion coefficients revealed the dominant roles of H2 and radical species diffusion in inducing the back-support effect. Contrary to the previous findings, variations in burned gas temperature were found to play a negligible role in modifying Sc. Additionally, the hysteresis of the back-support effect under periodical stratification was found to be more prominent on the richer side because of the presence of a larger H2 pool.

Effect of simultaneous velocity and equivalence ratio oscillations on a laminar premixed stagnating flame

The dynamics of a laminar premixed stagnating flame subjected to simultaneous velocity and equivalence ratio oscillations are investigated experimentally. A flame of premixed methane and air is formed in a wall-stagnating flow. The oscillations of velocity and equivalence ratio are imposed independently by two sets of dual cylinder–piston oscillators. To impose the equivalence ratio oscillation, methane/air mixtures with equivalence ratios of $$\phi =1.0$$ and $$0.4$$ are supplied to different cylinder–piston units. The pistons move in anti-phase and create a sinusoidal equivalence ratio oscillation while keeping the volume flow rate constant. To impose the velocity oscillation, a mixture with $$\phi =0.7$$ is supplied to the single unit of another oscillator. The phase difference between the two oscillations is set by changing the relative position of the pulley teeth of the two oscillators. The oscillator frequency is varied between 2 and 20 Hz, meaning that the oscillation wavelengths are much longer than the flame thickness. When frequencies of the velocity and equivalence ratio fluctuations are in a quasi-steady state condition, the flame motion is affected independently by velocity oscillation and equivalence ratio oscillation and the linear-superposition assumption can be applied. When the velocity and equivalence ratio oscillation frequencies are increased and exceed the transition frequency from the quasi-steady state to unsteady state, the flame motion gradually deviates from linear-superposition assumption. This is due to the variations in the unsteady molecular diffusion and the back-support effect.

Background-Oriented Schlieren of Fuel Jet Flapping Under Thermoacoustic Oscillations in a Sequential Combustor

This study deals with thermoacoustic instabilities in a generic sequential combustor. The thermoacoustic feedback involves two flames: the perfectly premixed swirled flame anchored in the first stage and the sequential flame established downstream of the mixing section, into which secondary fuel is injected in the vitiated stream from the first stage. It is shown that the large amplitude flapping of the secondary fuel jet in the mixing section plays a key role in the thermoacoustic feedback. This evidence is brought using high-speed background-oriented Schlieren (BOS). The fuel jet flapping is induced by the intense acoustic field at the fuel injection point. It has two consequences: first, it leads to the advection of equivalence ratio oscillations toward the sequential flame; second, it modulates the residence time of the ignitable mixture in the mixing section, which periodically triggers autoignition kernels developing upstream of the chamber. In addition, the BOS images are processed to quantify the flow velocity in the mixing section and these results are validated using particle image velocimetry (PIV). This study presents a new type of thermoacoustic feedback mechanism, which is peculiar to sequential combustion systems. In addition, it demonstrates how BOS can effectively complement other diagnostic techniques that are routinely used for the study of thermoacoustic instabilities.

Dynamic response of wall-stagnating lean methane-air premixed flame to equivalence ratio oscillation

Dynamic response of wall-stagnating lean methane-air premixed flame to equivalence ratio oscillation

Experimental investigation of the response of laminar premixed flames to equivalence ratio oscillations

A detailed experimental analysis of laminar premixed methane/air flames subject to equivalence ratio oscillations is presented. The equivalence ratio of methane/air flames was periodically modulated by the addition of methane into the stationary fuel/air flow of a lean premixed flame. The reaction of the flame as a function of amplitude and frequency of the modulation was investigated in a parametric study. The flames were operated at reduced pressure (70 mbar) in order to increase the spatial resolution across and within the flame front and to approximate the time and length scales of the flame and the induced equivalence ratio oscillations in an order of magnitude of 10 Hz. The detection and evaluation of OH*-chemiluminescence revealed the shape and position of the flame front. One-dimensional laser Raman scattering was applied in order to simultaneously and quantitatively determine the concentration profiles of the major species and the temperature along the symmetry axis of the flame. The results show that several interacting effects occur, which have different dependencies on the oscillation frequency: first, a “macroscopic” reaction of the flame arises from the variation of the laminar flame speed and the flow velocity and, hence, its location. Second, the structure of the flame front is affected by the diffusional transport processes and the influence of the oscillation on the species concentrations. At low frequencies, the flame can follow the oscillations reaching temporarily quasi-steady states at the minimum and maximum fuel fraction levels. At sufficiently high frequencies, the flame tip location remains virtually unchanged. However, the species profiles across the flame front show a significant phase shift relatively to each other and cannot be described by steady states throughout the oscillation period.

周期的濃度変動を伴う混合気流に対する平面予混合火炎の応答

The response characteristics of a lean methane-air premixed flame to equivalence ratio oscillation was investigated experimentally. In order to clarify unsteady flame response from the actual measured values of the burning velocity, we developed a new burner that allowed measurement of the flow field by using particle image velocimetry (PIV). Chemiluminescent emission intensity of CH radical, which is correlated with the combustion intensity of the lean flame, was also measured by using high speed camera with a band pass filter. The equivalence ratio was varied sinusoidally only in the direction of the main flow at burner exit by two speakers. For an oscillation with mean equivalence ratio of 0.85, amplitude of 0.05 and the oscillation frequency ranging from 5 to 10 Hz, following results were obtained: The variation widths of burning velocity and CH radical emission intensity in fluctuating flame are increased as oscillation frequency increases. These variation widths are larger than those of a steady flame in the equivalence ratio range corresponding to the fluctuation. This means that when the flame moves against (along) the fuel flow, the mass flux at the flame front increases (decreases) as compared with that of the steady flame.

STAGNATION LAMINAR PREMIXED CH4/AIR FLAME SUBJECTED TO THE EQUIVALENCE RATIO OSCILLATIONS

The effect of the equivalence ratio oscillation on a premixed laminar CH4/air flame motion was studied experimentally with equivalence ratio oscillation frequencies of 2 to 15 Hz at lean equivalence ratio using stagnation flow field burner. Novel oscillator does the oscillation conditions and turbulence reduction method is used to suppress the velocity perturbation. The flame position variations at 2, 5, 10 and 15 Hz oscillation frequencies were significantly small when the amplitude of the equivalence ratio oscillation was zero. On the other hand, increase in amplitudes of the equivalence ratio oscillation increased the flame position variation significantly. The flame moved in sinusoidal shape and it can be clearly seen that the flame movement’s amplitude was proportional to the amplitudes of the equivalence ratio variations. This result showed that the velocity perturbation is significantly suppressed by turbulence reduction method in the examination range.

Experimental investigations on combustion characteristics of a cavity pilot augmentor of the turbine-based combined cycle engine

A cavity flame holder has been a very promising novel concept to be applied to the augmentor of a turbine-based combined cycle (TBCC) engine as it offers improvements in ignition performance, altitude relight, lean blowout limit, operating range, as well as combustion efficiency compared to conventional bluff body flame holders. The present paper describes the detailed designs of a cavity pilot augmentor and provides a laboratory-scale test rig. A series of experiments has been conducted to investigate the combustion characteristics of the test rig under atmospheric pressure. Experimental results show that for the bypass inlet temperature 343–473 K and inlet Mach number 0.25–0.40, ignition equivalence ratio and ignition pressure oscillation obtained is between 0.94–1.72 and 1.1–2.4% respectively, and lean blowout equivalence is between 0.42 and 0.92, indicating good ignition and lean blowout performances; as the total fuel-to-air ratio increases, the combustion efficiency decreases in the main at both afterburner operating mode and ramjet operating mode, while the increase of the bypass inlet Mach number or temperature is beneficial to high combustion efficiency, especially as inlet temperature over 423 K, and the combustion efficiency got in present experiment is between 70% and 95%. This paper demonstrates the feasibility for the cavity pilot flame holder to be applied to a realistic augmentor of TBCC engine preliminarily and provides reference for TBCC augmentor design.

Model of flame dynamics of laminar premixed flame subject to the low frequency equivalence ratio oscillations

The effect of the non-uniform profile of scalar variables, such a fuel at the upstream and temperature at the downstream of the flame zone was discussed theoretically to elucidate; (1) the deviation of motion from the steady state case and (2) the hysteresis of premixed flames response to the equivalence ratio oscillations seen in an experimental and numerical works. One-dimensional integral model for the non-uniform scalar variable profile with low frequency equivalence ratio oscillation has been developed. Here, the wavelength of the oscillation is assumed to be larger than the nominal flame thickness. Through the integral analysis, we obtained the relation of the flame propagation speed for steady and unsteady cases depending on the non-uniform scalar profile at the upstream and downstream of the flame zone. Hysteresis of the flame propagation speed is found due to the transport of fuel and heat by the non-uniform scalar profile at the upstream and downstream of the flame zone. This result qualitatively agreed with the numerical results of a response of the stagnation laminar CH4/air premixed flames for a low equivalence ratio oscillation frequency.

Numerical Studies on the Impact of Equivalence Ratio Oscillations on Lean Premixed Flame Characteristics and Emissions

Laminar and turbulent one-dimensional lean premixed methane-air flames subject to equivalence ratio oscillations are investigated numerically. For the turbulent simulations, the Linear Eddy Model is employed. Harmonic perturbations at various frequencies and with various amplitudes are considered and their influence on CO and NO emissions, heat release rate fluctuations, and burning velocity is evaluated. The results indicate a strongly nonlinear behavior of the flame response for high forcing amplitudes attributed to nonlinear effects due to the interaction of burning velocity and equivalence ratio oscillations leading to nonsinusoidal oscillations at the flame front. Furthermore, the turbulent cases reveal decreasing mean burning velocities and heat release rates with increasing amplitudes due to damping of turbulent fluctuations induced by the oscillations.

Flame dynamics of equivalence ratio oscillations in a laminar stagnating lean methane/air premixed flame

This study investigates the effect of fuel concentration oscillation on laminar stagnating premixed flames by both experiment and numerical simulation. The numerical analysis is conducted using ANSYS Fluent 14.5. The equivalence ratio oscillation in the experiments is formed by a novel oscillator with two cylinder piston units that can produce alternating ejections of leaner and richer pre-mixtures. Velocity fluctuation is well suppressed by installing screens on the burner exit. The fuel concentration oscillation between the stagnation plate and the burner exit is visualized and analyzed by acetone ultraviolet light-induced fluorescence in the isothermal condition. The oscillator frequency is varied in the range 2–20Hz, and the oscillation wavelength is much longer than the flame thickness. The flame oscillates with the fuel concentration, and in the experiment, the amplitude of the flame oscillation attenuates as the frequency of fuel concentration oscillation increases above 5Hz, which corresponds to a Strouhal number of unity. This indicates that the Strouhal number distinguishes quasi-steadiness for St<1 and unsteadiness for St>1. The flame oscillation pattern is a closed loop, which might be attributable to variation of the back support effect on the flame. The numerical results show a similar trend for the flame response to oscillations in fuel concentration. This study finds the flame motion is significantly affected by fuel concentration oscillations, even at low frequencies; in other words, the oscillation wavelength is much longer than the flame thickness, as a result of the back support effect.

Optical Measurement of Local and Global Transfer Functions for Equivalence Ratio Fluctuations in a Turbulent Swirl Flame

Equivalence ratio fluctuations are known to be one of the key factors controlling thermoacoustic stability in lean premixed gas turbine combustors. The mixing and thus the spatiotemporal evolution of these perturbations in the combustor flow is, however, difficult to account for in present low-order modeling approaches. To investigate this mechanism, experiments in an atmospheric combustion test rig are conducted. To assess the importance of equivalence ratio fluctuations in the present case, flame transfer functions for different injection positions are measured. By adding known perturbations in the fuel flow using a solenoid valve, the influence of equivalence ratio oscillations on the heat release rate is investigated. The equivalence ratio fluctuations in the reaction zone are measured spatially and temporally resolved using two optical chemiluminescence signals, captured with an intensified camera. A steady calibration measurement allows for the quantitative assessment of the equivalence ratio fluctuations in the flame. This information is used to obtain a mixing transfer function, which relates fluctuations in the fuel flow to corresponding fluctuations in the equivalence ratio of the flame. The current study focuses on the measurement of the global, spatially integrated, transfer function for equivalence ratio fluctuations and the corresponding modeling. In addition, the spatially resolved mixing transfer function is shown and discussed. The global mixing transfer function reveals that, despite the good spatial mixing quality of the investigated generic burner, the ability to damp temporal fluctuations at low frequencies is rather poor. It is shown that the equivalence ratio fluctuations are the governing heat release rate oscillation response mechanism for this burner in the low-frequency regime. The global transfer function for equivalence ratio fluctuations derived from the measurements is characterized by a pronounced low-pass characteristic, which is in good agreement with the presented convection–diffusion mixing model.

Combustion instability of a lean premixed prevaporized gas turbine combustor studied using phase-averaged PIV

A strong, naturally-occurring “growl” combustion instability was studied for the case of a lean premixed prevaporized (LPP) combustor that shows great promise in reducing pollutant emissions. Phase-averaged particle image velocimetry (PIV) was applied for the first time to an LPP device to measure phase lags and spatial correlations. The extensive data set includes spatial and temporal correlations between six parameters: combustor pressure, plenum pressure, injection velocity, heat release rate (Rayleigh index), flame liftoff distance and flame centroid. Measured phase angles and time lags are consistent with the MIT model of Ghoniem et al., along with the concept of “equivalence-ratio oscillation” discussed by Lieuwen et al. Frequency and phase data prove that a dual-mode Helmholtz resonance is driven by an equivalence ratio oscillation. One common modeling assumption is shown to be not valid; the length of an attached flame is not what is oscillating; instead the flame base oscillates violently due to periodic liftoff and flashback and this presents modeling challenges. Growl boundaries and the effects of varying some geometric lengths were recorded.

Interference mechanisms of acoustic/convective disturbances in a swirl-stabilized lean-premixed combustor

Interference mechanisms of acoustic/convective disturbances were experimentally investigated in a swirl-stabilized lean-premixed gas turbine combustor operated with natural gas fuel and air at atmospheric pressure and elevated temperature. Interference between azimuthal and acoustic velocity disturbances at high-amplitude limit cycle oscillations is characterized in detail as a function of axial swirler location, oscillation frequency, and mean nozzle velocity. We show that both the frequency and the intensity of self-excited instabilities in a model gas turbine combustor are correlated with axial swirler position, which indicates that a vorticity wave generated at the swirl vanes is a primary source of convective disturbances in the absence of equivalence ratio nonuniformities. Flame transfer function measurements confirm that the linear/nonlinear heat release response is a strong function of axial swirler location, even when unforced flame structures remain unchanged. The key parameter controlling this phenomenon is the phase difference between the azimuthal and acoustic velocity perturbations at the combustor dump plane; the phase difference is affected by swirler location, frequency, mean velocity, and the speed of sound. It was found that out-of-phase interference between azimuthal and acoustic velocity disturbances at the combustor inlet yields large flame angle fluctuations in relation to swirl number fluctuations, and therefore the formation of a coherent structure is hindered due to high kinematic viscosity within the vortex formation region. In-phase interference mechanisms, on the other hand, lead to high-amplitude limit cycle oscillations. This interference mechanism is then explored in the presence of temporal equivalence ratio nonuniformities, in which two different sources of convective mechanisms should be considered simultaneously in connection with acoustic velocity perturbations and the vortex dynamics. Results reveal that equivalence ratio oscillation has a significant effect on the strength of combustion-acoustic interactions. Strong self-excited instabilities of partially premixed flames are produced by in-phase interactions between acoustic velocity and equivalence ratio oscillations, which are governed by fuel injection location, frequency, mean nozzle velocity, and fuel injector impedance. At this phase condition, unburned reactants with high equivalence ratio impinge on the flame front with high inlet velocity, potentially causing large fluctuations of heat release rate.

The Response of a Conical Laminar Premixed Flame to Equivalence Ratio Oscillations in Rich Conditions

The Response of a Conical Laminar Premixed Flame to Equivalence Ratio Oscillations in Rich Conditions

The Response of Transient Inhomogeneous Flames to Pressure Fluctuations and Stretch: Planar and Outwardly Propagating Methane/Air Flames

Transient outwardly propagating premixed methane/air flames subjected to joint pressure and equivalence ratio oscillations are investigated using an implicit direct method that couples the compressible flow with realistic chemistry and multicomponent transport properties. Chemistry is a detailed 30 species C 1–C 2 mechanism. The chemical kinetic–transport coupling is compared with results from hydrogen–air flames obtained in earlier work (Malik and Lindstedt, 2010). Increasing flow divergence (positive stretch) decreases the relaxation time (the time that a flame takes to return to equilibrium conditions after initial disturbance) from 6.0 ms (planar), to 4.5 ms (cylindrical) and 4.0 ms (spherical)—these are about six times longer than from the hydrogen/air flames. However, the nondimensional relaxation numbers (where is a characteristic flame time scale based upon the thickness of the fuel consumption rate profile) are similar in both types of flames, n R ≈ 4.4(P), 3.3(C), and 3.0(S) indicating a similar level of stability to applied disturbances. n R deceases by about 40% with increasing positive flow divergence in both flame types under the conditions studied, demonstrating the strong coupling of the heat release to the flame stretch and geometry. Spectral analysis of the pressure fluctuations shows that the spectra scale approximately like E p ∼ ω−3 in the frequency range 2–10 kHz, and a weak spectral broadening in the spherical case. This is a much weaker kinetic-pressure coupling than in hydrogen/air flames where E p ∼ ω−2. It was found that increasing flow divergence (from planar to spherical flames) shifts the range of excited frequencies to higher frequencies. It is confirmed that individual chemical species show markedly different responses to the disturbances. Species formed in thin reaction layers are generally not disturbed except with respect to their peak concentration. By contrast, species with broader profiles are generally significantly affected. These findings suggest that combustion processes may be usefully viewed as a spectrum of length scales rather than as just a single global scale. Although the flame adjusts to the local unsteady conditions, some evidence of a time lag or “memory effect” between fuel consumption and the rate of heat relase induced by positive flow divergence is observed.