Self-excited Flame Research Articles

Study on the acoustic disturbance characteristics of the spray flame

The axial sound field was added to the ethanol spray flame as a perturbation element to study the dynamic response and combustion characteristics. A forced acoustic field is applied to a small oscillating flame. In cases where the frequency of the external sound field matches the eigenfrequency, even small amplitude self-excited flames demonstrate significant instability. As the intensity of the sound field increases, the amplitude of pressure oscillations sharply rises, and the pressure and flame heat release oscillations are in-phase. When the sound field operates at frequencies other than the eigenfrequency, it has an inhibitory effect on the evaporation and combustion process of ethanol droplets. Consequently, intensifying the sound field leads to a reduction in both the area of the evaporation combustion zone and the axial temperature of the flame. The CO formation is influenced by both temperature and the sound field, showing a pattern of increasing and then decreasing with the increase of sound intensity. Additionally, NOx generation exhibits higher emission concentrations in low-temperature and short flames, which is more likely to prompt NOx and is not closely related to frequency.

Investigation on self-excited thermoacoustic instability and emission characteristics of premixed CH4/NH3/air flame

Ammonia is one of the promising carbon-free alternative fuels for low-carbon power generation. This study investigated the self-excited thermoacoustic instability of premixed CH4/NH3/air flame in a swirl burner. The work aims to determine the effects of ammonia mole fraction (χNH3), equivalence ratio (Ф) and combustion chamber length (CL) on flame morphology and self-excited flame dynamics. In addition, the emission characteristics of CH4/NH3/air flame were also investigated. Results show that the thermoacoustic instability of the 1/4 wave mode is excited when 0.8≤Ф≤1.2 and χNH3 ≤ 0.3. There are apparent pressure fluctuations and heat release fluctuations in the combustion chamber. The fluctuating frequency decreases with increasing CL. The maximum fluctuating frequency and oscillation intensity appear near Ф=1.0. When χNH3 increases, the amplitude of fluctuating pressure and heat release rate decrease obviously, and the instability range narrows to Ф=1.0–1.1 simultaneously. The low-order acoustic network model successfully predicted the instability transition and frequency decreasing trend caused by increasing ammonia. The increase in the convective time delay is responsible for this transition. Emission analysis indicates that NOx emission can be controlled within 100 ppm under fuel-rich conditions, but the CO emission increases sharply. Thermoacoustic instability and nitrogen oxides can be simultaneously controlled under fuel-rich conditions when χNH3>0.3. However, this will be limited by unburned reactants.

Longitudinal and azimuthal thermo-acoustic instabilities in an industrial gas turbine combustor operating at elevated pressure

This work numerically investigates longitudinal and azimuthal thermo-acoustic instabilities in the swirl-stabilised can-type industrial SGT-100 gas turbine combustor operated at elevated pressures of 3 and 6 bar. Previous experiments have shown that the combustor is susceptible to self-excited flame oscillations sustained by a thermo-acoustic feedback loop at specific operating conditions. In order to gain a better understanding of this feedback loop, a fully compressible large eddy simulation method is applied. The unknown sub-grid scale turbulence-chemistry interactions are modelled via a transported probability density function approach solved by the Eulerian stochastic fields method. First, the reaction zones and global flame topology at both operating pressures are analysed and compared to experimental images providing good qualitative agreement. Radial profiles of time-averaged and root-mean-square quantities furthermore demonstrate good quantitative agreement with the available measurement data. The applied simulation approach is capable of successfully reproducing self-excited thermo-acoustic instabilities in the longitudinal direction. The fundamental frequency of the predicted limit-cycle oscillation matches the experimentally measured frequency with high accuracy. Similar to the experimental observations, the fluctuation amplitudes of the pressure and global heat release rate increase significantly upon increasing the mean operating pressure from 3 to 6 bar. In addition to the dominant longitudinal mode, a high-frequency, low-amplitude azimuthal mode is also identified at both pressures. This azimuthal mode is periodically amplified and attenuated by the superposed longitudinal mode and induces small asymmetric (around the burner circumference) fluctuations of the local fuel and total mixture mass flow rates entering the flame region.

Flame features and oscillation characteristics in near-blowout swirl-stabilized flames using high-speed OH-PLIF and mode decomposition methods

Flame features and dynamics are important to the explanation and prediction of a lean blowout (LBO) phenomenon. In this paper, recognition of near-LBO flame features and oscillation characterization methods were proposed based on flame spectroscopic images. High-speed planar laser-induced fluorescence measurements of OH were used to capture unique dynamic features such as the local extinction and reignition feature and entrained reactant pockets. The Zernike moment demonstrated a good performance in recognition of stability and near-LBO conditions, though the geometric moment had more advantages to characterize frequency characteristics. Low-frequency oscillations, especially at the obvious self-excited oscillation frequency around 200 Hz, were found when approaching an LBO condition, which can be expected to be used as a novel prediction characteristic parameter of the flameout limit. Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) were used to conduct dynamic analysis of near-LBO flames. POD modes spectra showed the unique frequency characteristics of stable and near-LBO flames, which were basically in line with those at the heat-release frequency. The primary POD modes demonstrated that the radial vibration mode dominated in a stable flame, while the rotation mode was found to exist in a near-LBO flame. Analysis of modal decomposition showed that flame shedding and agminated entrained reactant pockets were responsible for generating self-excited flame oscillations.

Investigation into the coherence of flame intensity oscillations in a model multi-element rocket combustor using complex networks

Capturing the complex spatiotemporal flame dynamics inside a rocket combustor is essential to validate high-fidelity simulations for developing high-performance rocket engines. Utilizing tools from a complex network theory, we construct positively and negatively correlated weighted networks from methylidyne (CH*) chemiluminescence intensity oscillations for different dynamical states observed during the transition to thermoacoustic instability (TAI) in a subscale multi-element rocket combustor. We find that the distribution of network measures quantitatively captures the extent of coherence in the flame dynamics. We discover that regions with highly correlated flame intensity oscillations tend to connect with other regions exhibiting highly correlated flame intensity oscillations. This phenomenon, known as assortative mixing, leads to a core group (a cluster) in the flow-field that acts as a “reservoir” for coherent flame intensity oscillations. Spatiotemporal features described in this study can be used to understand the self-excited flame response during the transition to TAI and validate high-fidelity simulations essential for developing high-performance rocket engines.

A Lattice-Boltzmann study of premixed flames thermo-acoustic instabilities

We present possibly for the first time Lattice-Boltzmann numerical simulations of thermo-acoustic instabilities of premixed flames. We study flames interacting with an imposed acoustic field where flames submitted to a parametric instability can be observed, as well as plane flames re-stabilized by the acoustic forcing. Self-induced thermo-acoustic oscillations of flames propagating in narrow channels are also studied, indicating an unexpected dependency with the channel width. For both excited and self-excited flames, results confirm that Lattice-Boltzmann method can capture the complex coupling between flame dynamics and acoustics.

Mitigating self-excited flame pulsating and thermoacoustic oscillations using perforated liners

Open-loop control of self-excited flame pulsating oscillations and thermo-acoustic instability is considered in this work. The performance of the control strategy is numerically evaluated in a 2D Rijke-type combustor with a perforated pipe implemented. It is found that approximately 38 dB sound pressure level (SPL) reduction can be achieved by actively tuning the cooling flow through the perforated pipe. Furthermore, the vorticity-induced damping performance is contributing to the breaking up of flame-acoustics coupling. However, the shedding of vortices is not uniformly distributed along the perforated pipe. To apply the control strategy in practice and to validate the findings, experimental studies are performed on a customer-designed Rijke-type combustor with a perforated liner implemented. To mimic practical engines, a cooling flow generated by a centrifugal pump is provided to pass through the perforated pipe. Properly tuning the cooling flow rate is found to lead to the unstable combustor being successfully stabilized. SPL is reduced by approximately 35 dB at ω1/2π≈245 Hz, and harmonic thermoacoustic modes are completely attenuating. Further study is conducted by suddenly removing the perforated pipe section. The combustion system is found to be associated with not only classical thermo-acoustic limit cycle oscillations with a dominant mode at 2.45×102 Hz, but also beating oscillations at 1.4×100 Hz. It is revealed that increasing acoustic losses by implementing the perforated pipe is another critical mechanism contributing to attenuating flame pulsating instability. The present work opens up an applicable means to attenuate both self-excited high-frequency thermoacoustic and low-frequency flame pulsating oscillations.

Instability Control by Actuating the Swirler in a Lean Premixed Combustor

A detailed study concerning a novel, dynamic control strategy for mitigating thermoacoustic instability in a swirl-stabilized lean premixed, laboratory combustor configuration is presented in this paper. The mitigation strategy is realized by rotating the otherwise static swirler, which is primarily meant for stabilizing the lean premixed flame. The proposed strategy is tested over a range of bulk flow velocities, mixture equivalence ratios, and swirler rotation rates for validating the robustness of this concept. A prominent reduction in the fundamental acoustic mode amplitude by about 25 dB is observed with this control technique for the cases that are studied. The physical mechanism responsible for the instability mitigation due to the rotating swirler is investigated by observing the distinct changes associated with the reacting flowfield using particle image velocimetry. An attempt is made to probe into the self-excited flame dynamics using high-speed intensified, chemiluminescence imaging and identifying the instability driving source locations from a spatial Rayleigh indices map. The rotating swirler induces vortex breakdown and increased turbulence intensity to decimate strongly positive Rayleigh indices regions (and eventually the acoustic energy source) to render quiet instability mitigated swirling flames.

Lock-in and quasiperiodicity in hydrodynamically self-excited flames: Experiments and modelling

Hydrodynamically self-excited flames are often assumed to be insensitive to low-amplitude external forcing. To test this assumption, we apply acoustic forcing to a range of jet diffusion flames. These flames have regions of absolute instability at their base and this causes them to oscillate at discrete natural frequencies. We apply the forcing around these frequencies, at varying amplitudes, and measure the response leading up to lock-in. We then model the system as a forced van der Pol oscillator.Our results show that, contrary to some expectations, a hydrodynamically self-excited flame oscillating at one frequency is sensitive to forcing at other frequencies. When forced at low amplitudes, it responds at both frequencies as well as at several nearby frequencies, indicating quasiperiodicity. When forced at high amplitudes, it locks into the forcing. The critical forcing amplitude for lock-in increases both with the strength of the self-excited instability and with the deviation of the forcing frequency from the natural frequency. Qualitatively, these features are accurately predicted by the forced van der Pol oscillator. There are, nevertheless, two features that are not predicted, both concerning the asymmetries of lock-in. When forced below its natural frequency, the flame is more resistant to lock-in, and its oscillations at lock-in are stronger than those of the unforced flame. When forced above its natural frequency, the flame is less resistant to lock-in, and its oscillations at lock-in are weaker than those of the unforced flame. This last finding suggests that, for thermoacoustic systems, lock-in may not be as detrimental as it is thought to be.

Acoustic-wave interactions with counterflow single- and twin-premixed flames: Finite-rate kinetics, heat release and phase effects

The interaction of acoustic waves with planar methane–air counterflow premixed flames is investigated numerically employing a detailed and several global kinetic models. The quasi one-dimensional fully unsteady governing equations for laminar counterflow flames are integrated based on a MacCormack predictor–corrector scheme with the implementation of Navier–Stokes characteristic boundary conditions. For well-resolved simulations, the occurrence of self-excited flame–acoustics instabilities is analyzed in both twin-premixed and single-premixed counterflow flames for a range of flow strain rates and flame locations. While previous unsteady counterflow work required external perturbations, the resonant unsteady phenomenon predicted in this study is self-sustained under favorable boundary conditions. It is shown that the acoustic response of the flame with the detailed model varies depending on geometry and phase effects. In contrast, one-step global models with large activation energy (126 kJ mol −1) always promote the amplification of acoustic pressure fluctuations. However, two-step global models developed with the same activation energy for the initiation reaction and with zero activation energy for the second propagation reaction exhibit a wide range of flame–acoustics interactions, depending on the distribution of heat release among the two reactions. Detailed analyses of the difference in phase between oscillatory pressure and velocity due to flame location, as well as of characteristic time-scales associated with transport, chemistry and acoustics are presented to provide a better understanding of the exact coupling mechanisms.

Self-sustained acoustic-wave interactions with counterflow flames

The interaction of acoustic waves with a planar counterflow flame is investigated numerically employing a detailed kinetic model and one-step global kinetic models. The mathematical formulation of quasi-one-dimensional fully unsteady laminar counterflow flames is presented and the governing equations are integrated numerically based on a MacCormack predictor–corrector scheme with second-order accuracy in space. Navier–Stokes characteristic boundary conditions are implemented to accurately represent perfect and partial reflection of acoustic waves at the boundaries. For well-resolved simulations, the occurrence of self-excited flame–acoustics instabilities is analysed in both non-premixed and premixed flames for a range of flow strain rates and flame locations, and employing two finite-rate kinetic models. Unlike the detailed kinetic model, one-step global models with large activation energy and overall reaction order greater than unity promote the amplification of acoustic pressure fluctuations in counterflow non-premixed flames. In contrast, premixed counterflow flames exhibit flame–acoustics instabilities with both kinetic models. While previous unsteady counterflow studies required external perturbations, the resonant unsteady phenomena predicted in this study are self-sustained under favourable boundary conditions. Detailed analyses of the characteristic time scales associated with convection, diffusion, chemistry and acoustics are presented to provide a better understanding of the exact coupling mechanisms.

Application of Active Combustion Instability Control to a Heavy Duty Gas Turbine

During the prototype shop tests, the Model V84.3A ring combustor gas turbine unexpectedly exhibited a noticeable “humming” caused by self-excited flame vibrations in the combustion chamber for certain operating conditions. The amplitudes of the pressure fluctuations in the combustor were unusually high when compared to the previous experience with silo combustor machines. As part of the optimization program, the humming was investigated and analyzed. To date, combustion instabilities in real, complex combustors cannot be predicted analytically during the design phase. Therefore, and as a preventive measure against future surprises by “humming,” a feedback system was developed which counteracts combustion instabilities by modulation of the fuel flow rate with rapid valves (active instability control, AIC). The AIC achieved a reduction of combustion-induced pressure amplitudes by 86 percent. The Combustion instability in the Model V84.3A gas turbine was eliminated by changes of the combustor design. Therefore, the AIC is not required for the operation of customer gas turbines.

Stability of laminar-jet flames

A self-excited flame oscillation of low frequency appears often, when laminar fuel jets burn in open air. The flame oscillation is axially symmetrical and has no connection with the size of combustion chambers and the length of fuel pipes. The present paper reports the results of the study of this phenomenon on the basis of the hypothesis that the flame oscillation is caused by the instability of the laminar-jet flows. The instability of axially symmetrical parallel flows of inviscid gases is studied in the case of axially symmetrical disturbances, and the criteria for the instability are derived. A detailed study based on the instability theory is conducted on an oscillating city-gas laminar-jet flame, measuring its mean-velocity and mean-temperature fields. As the results of the study, it is shown that the observed facts, such as the occurrence of the oscillation during combustion, the aspects of streamlines of the flame, and the frequency of the oscillation, can be explained reasonably with the instability theory. Thus, it is concluded that the hypothesis that the flame oscillation of this type is caused by the instability of the laminar jet flows is reasonable.