Flame Fluctuations Research Articles

Darrieus-Landau instability effect on the flame topology and brush thickness for premixed turbulent flames

The effect of Darrieus-Landau (DL) instability on flame topology and brush thickness was systematically investigated on a planar and Bunsen flame, with the latter used for direct comparison with experimental results from PLIF technique. For planar flame, the flame brush thickness significantly increases with turbulence intensity v′, then the increase slows down and finally levels off to a certain value for all cases. This is tributed to the fact that DL instability plays a primary role on flame topology at weak turbulence intensities while this effect is shaded by turbulence perturbation at high intensities ultimately leading to an indistinguishable flame brush. In Bunsen configuration, flame fluctuations are superimposed on an average conical flame. The brush thickness shows a monotonous increase with downstream distance and follows the linear relation as predicted by Taylor’s diffusion law. The centerline flame brush thickness significantly increases with v′ at weak intensity range and then levels off to a constant value with sub-critical flame being thicker, which is opposite with planar flame. The normalized value collapses to a common curve, indicating this can be an appropriate parameter for Bunsen flame. The good qualitative match with C3H8/air flames indicates the prediction capabilities of the hydrodynamic model.

Investigation of the Jet-Flame Interaction by Large Eddy Simulation and Proper Decomposition Method

ABSTRACTLarge eddy simulation (LES) results are presented for a premixed methane/air turbulent flame arising from a confined laboratory-scale single-nozzle burner. The jet issuing from an off-centered nozzle facilitates the development of a large-scale, dominant lateral recirculation zone that stabilizes the flame. A self-sustained jet oscillation is present, which intermittently causes extreme flame fluctuations such as blowout and relight events in the bottom section of the combustion chamber. The combined probability density function transport approach with the Eulerian stochastic fields method is used to numerically investigate the influence of this jet oscillation on combustion stability at the operating condition near lean blowout. The general structure of the flow, including the formation of the recirculation zones depending on the location of the flapping jet, is well-reproduced together with the mean and fluctuating velocity profiles. The behavior of the jet oscillation is investigated using a popular decomposition method known as proper orthogonal decomposition (POD) based on the predicted three-dimensional flow fields. Thanks to POD, the evolution of the simulated flame structure featuring a pronounced flame fluctuation is compared against that experimentally measured according to the phase angles of the low-order modeled jet motion. The absence of the most dominant coherent structure at a single frequency is due to a feedback mechanism between the jet oscillation and combustion process. The simulation shows that a low-frequency jet flapping causes the flame blowout and flashback in the bottom section of the combustor and a stable flame persists as long as the jet flapping rate exceeds a critical value.

Combustion Process Analysis and Diagnostic Using Optical Flame Scanners in Front-Fired Pulverized Coal Boiler

The paper presents a novel concept and method of coal combustion process analysis using flame scanners supervision system. The combustion process analysis and diagnostic has a crucial influence on boiler effectiveness, especially in high variance of load demand, which is nowadays a top challenge for coal-fired power plants. The first indicator of combustion inefficiency is flame stability, which can be observed as variation of flame intensity. Nowadays, there are no validated measuring methods dedicated for industrial usage, which are able to give complete information about flame condition. For this reason, the research activity was launched and focused on usage of commercial flame scanners for fast combustion analysis based on on-line flame parameters measuring. The analysis of combustion process was performed for 650 t/h live steam power boiler, which is supplied by five coal mill units. Each coal mill supplies four pulverized coal burners pulverized fuel ((PF) burners). The boiler start-up installation consists of 12 heavy oil burners placed in PF burners equipped with individual supervisory system based on Paragon 105f-1 flame scanners, which gave the possibility to observe and analyze the PF burner flame and oil burner flame individually. The research included numerous tests in which the combustion conditions inside the combustion chamber were changed. During stable load of selected mills, the primary air flow, secondary air dampers, air–coal mixture temperature, and balance were changed. The results of the changes were observed by flame scanners and the available optical parameters of the flame were analyzed: power spectral density, average amplitude (AA) of flame fluctuation, and flame temperature.

Restrain of fluctuation of Bunsen flame to a transient change in fuel components

Restrain of fluctuation of Bunsen flame to a transient change in fuel components

Dynamics of lean premixed flames stabilized on a meso-scale bluff-body in an unconfined flow field

Two-dimensional direct numerical simulations were conducted to investigate the dynamics of lean premixed flames stabilized on a meso-scale bluff-body in hydrogen-air and syngas-air mixtures. To eliminate the flow confinement effect due to the narrow channel, a larger domain size at twenty times the bluff-body dimension was used in the new simulations. Flame/flow dynamics were examined as the mean inflow velocity is incrementally raised until blow-off occurs. As the mean inflow velocity is increased, several distinct modes in the flame shape and fluctuation patterns were observed. In contrast to our previous study with a narrow channel, the onset of local extinction was observed during the asymmetric vortex shedding mode. Consequently, the flame stabilization and blow-off behavior was found to be dictated by the combined effects of the hot product gas pocket entrained into the extinction zone and the ability to auto-ignite the mixture within the given residence time corresponding to the lateral flame fluctuations. A proper time scale analysis is attempted to characterize the flame blow-off mechanism, which turns out to be consistent with the classic theory of Zukoski and Marble.

Flame fluctuations in Oxy-CO2-methane mixtures in swirl assisted distributed combustion

Swirl assisted oxy-methane combustion investigated here has focused on the impact of carbon dioxide dilution. Carbon dioxide dilution is essential as it lowers the flame adiabatic temperature and flame speed of oxy-fuel combustion leading to a more stable operation. However, carbon dioxide dilution may result in flame fluctuations, under fuel lean conditions. Experiments using a swirl burner examined the flame fluctuations with different amounts of CO2 dilution, with the goal to achieve stable oxy-CO2-methane combustion via colorless distributed combustion (CDC). This novel CDC technique has shown significant performance gains in gaseous fuel-air flames on emission reduction, enhanced thermal field uniformity, noise reduction, and improved flame stability. Results have shown that for oxy-CO2-fuel combustion, most of the heat release fluctuations had frequencies below 60Hz. The results obtained revealed that increase in CO2 dilution increased the magnitude of flame fluctuation till a maximum of 28% O2 concentration in the oxidizer. Further increase in dilution gases and reduction in O2 concentration led to a more stable flame, along with more favorable conditions for colorless distributed combustion. Data analysis of flame images revealed the source of oscillations under different conditions. At maximum fluctuations with 28% O2, the flame oscillated between two modes leading to unstable operation and large fluctuation in the heat release signal. Decrease in oxygen concentration to below 27%, promoted a more stable flame behavior, occupying a larger flame volume, which is characteristic of distributed combustion. Further reduction in O2 led to flame blow off at around 21% O2. This work outlines the importance CO2 dilution in oxy-fuel flames for achieving stable distributed combustion and mitigate the increased oscillations normally encountered in higher oxygen concentration flames.

The evolution of local instability regions in turbulent non-premixed flames

Unsteady turbulent flame evolution in non-premixed combustion has been computationally investigated using large eddy simulations. A simple coaxial combustion chamber, subjected to highly unsteady, turbulent recirculating flow is considered, following the experimental study of Owen et al. (Proc. Combust. Inst., vol. 16, 1976, pp. 105–117). Large-scale flame fluctuations, reported in the above experimental study, such as pulsating flames in swirling and non-swirling conditions, were identified here in our computation. New criteria for flame three-dimensional inhomogeneity are suggested and implemented in the present study, providing the ability to quantify the flame unsteadiness. Using this technique, it is shown that local, large quenched regions develop in the flame’s mixing area and rotate continuously, even when swirl is not imposed on the inlet. However, this rotation appears to be disordered, abruptly changing its direction. On the other hand, our study shows that when swirl is imposed on the inlet, a larger quenched region is identified, rotating in steady ordered rotation in the direction of the imposed swirl. In addition, large-scale radial flame fluctuations are increased downstream with the increase of swirl number. Consequently, significant correlations between radial and circumferential flame fluctuation frequencies were retrieved. Proper orthogonal decomposition analysis reveals coherent flame structures of five dominant modes that contain most of the energy in the fluctuating flame. A simplified analytical stability model is derived and implemented here to assess the hydrodynamic contribution to the flame instability; it is shown that radial fluctuations are excited by circumferential perturbations in the mixing region, providing new insight into the mechanism responsible for the onset of radial fluctuations. The computed radial flame fluctuation spectrum is predicted well using the linear stability analysis. Thus, our findings may therefore be applicable to a large class of non-premixed turbulent combustion problems.

Combustion behaviors of GO2/GH2 swirl-coaxial injector using non-intrusive optical diagnostics

This research evaluates the combustion behaviors of a single-element, swirl-coaxial injector in an atmospheric combustion chamber with gaseous oxygen and gaseous hydrogen (GO2/GH2) as the propellants. A brief simulated flow field schematic comparison between a shear-coaxial injector and the swirl-coaxial injector reveals the distribution characteristics of the temperature field and streamline patterns. Advanced optical diagnostics, i.e., OH planar laser-induced fluorescence and high-speed imaging, are simultaneously employed to determine the OH radical spatial distribution and flame fluctuations, respectively. The present study focuses on the flame structures under varying O/F mixing ratios and center oxygen swirl intensities. The combined use of several image-processing methods aimed at OH instantaneous images, including time-averaged, root-mean-square, and gradient transformation, provides detailed information regarding the distribution of the flow field. The results indicate that the shear layers anchored on the oxygen injector lip are the main zones of chemical heat release and that the O/F mixing ratio significantly affects the flame shape. Furthermore, with high-speed imaging, an intuitionistic ignition process and several consecutive steady-state images reveal that lean conditions make it easy to drive the combustion instabilities and that the center swirl intensity has a moderate influence on the flame oscillation strength. The results of this study provide a visualized analysis for future optimal swirl-coaxial injector designs.

Structure and Dynamics of Premixed Swirl Flames at Elevated Power Density

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 structure consistent with a helical precessing vortex core. A reconstructed time series elucidated the interactions of the reactant jets, the precessing vortex core, and the central recirculation bubble over a thermoacoustic cycle. This coupled interaction, and the resultant modulation of transport within the inner shear layer, were identified as a mechanism by which the precessing vortex core was linked to elevated power density flame dynamics.

Investigation of non-premixed flame combustion characters in GO2/GH2 shear coaxial injectors using non-intrusive optical diagnostics

Single-element combustor experiments are conducted for three shear coaxial geometry configuration injectors by using gaseous oxygen and gaseous hydrogen (GO2/GH2) as propellants. During the combustion process, several spatially and timeresolved non-intrusive optical techniques, such as OH planar laser induced fluorescence (PLIF), high speed imaging, and infrared imaging, are simultaneously employed to observe the OH radical concentration distribution, flame fluctuations, and temperature fields. The results demonstrate that the turbulent flow phenomenon of non-premixed flame exhibits a remarkable periodicity, and the mixing ratio becomes a crucial factor to influence the combustion flame length. The high speed and infrared images have a consistent temperature field trend. As for the OH-PLIF images, an intuitionistic local flame structure is revealed by single-shot instantaneous images. Furthermore, the means and standard deviations of OH radical intensity are acquired to provide statistical information regarding the flame, which may be helpful for validation of numerical simulations in future. Parameters of structure configurations, such as impinging angle and oxygen post thickness, play an important role in the reaction zone distribution. Based on a successful flame contour extraction method assembled with non-linear anisotropic diffusive filtering and variational level-set, it is possible to implement a fractal analysis to describe the fractal characteristics of the non-premixed flame contour. As a result, the flame front cannot be regarded as a fractal object. However, this turbulent process presents a self-similarity characteristic.

Investigation of unsteady phenomena in turbulent diffusion flames with hybrid fuel of methane and hydrogen in high pressure and temperature conditions

The objective of the present study is to understand the effects of added hydrogen and operating conditions on turbulent fluctuation and dynamic flame stability in a turbulent diffusion flame with methane/hydrogen hybrid fuel. A large eddy simulation (LES) technique is used to predict turbulent statistics accurately. A parametric study is performed by changing hydrogen concentration, background pressure and temperature, which include the conditions similar as in a direct injection internal combustion (IC) engine. Turbulent statistics such as RMS flow fields, flame stability index and combustion noise are analyzed to evaluate the flame stability. A normalized Rayleigh index proves to be a useful indicator for the stability of turbulent diffusion flame, as compared to the RMS of flow variables. From the combustion noise spectra, hydrogen addition stabilizes mainly the primary vortex shedding in the standard condition but reduces the noise level in the entire frequency range as the compression ratio increases. Also, it is concluded that stabilization of unsteady fluctuation is highly correlated to the reduced flame length.

Experimental and numerical analysis of oxy-fuel combustion in a porous plate reactor

The present work focuses on studying experimentally and numerically the oxy-fuel combustion characteristics inside a porous plate reactor towards the application of oxy-combustion carbon capture technology. Initially, non-reactive flow experiments are performed to analyze the permeation rate of oxygen in order to obtain the desired stoichiometric ratios. A numerical model is developed for non-reactive and reactive flow cases. The model is validated against the presently recorded experimental data for the non-reacting flow cases, and it is validated against the available literature data for oxy-fuel combustion for the reacting flow cases. A modified two-step oxy-combustion reaction kinetics model for methane is implemented in the present model. Simulations are performed over wide range of operating oxidizer ratios (O2/CO2 ratio), from OR = 0.2 to OR = 0.4, and over wide range of equivalence ratios, from φ = 0.7 to φ = 1.0. The flame length was decreased as a result of the increase of the oxidizer ratio. Effects of CO2 recirculation amount on the oxy-combustion flame stability are examined. A reduction in combustion temperature and increase in flame fluctuations are encountered while increasing CO2 concentration inside the reactor. At high equivalence ratio, the combustion temperature and flame stability are improved. At low equivalence ratio, the flame length is increased, and the flame was moved towards the reactor center line. Copyright © 2015 John Wiley & Sons, Ltd.

Mixing-related low frequency oscillation of combustion in an ethylene-fueled supersonic combustor

The present work investigated combustion instabilities inside an ethylene-fueled scramjet combustor mounted on a Mach 2.1 direct-connect test facility with an inflow stagnation temperature of 846K. Effects of fueling schemes on the combustion stability characteristics were examined. The experimental results suggest that the oscillation modes correlate with mixing status closely. For the cases with a quasi-steady thermal throat or stable shock trains, flame fluctuation exists in a mode of thermo-acoustic type oscillation with a broad frequency range. For the cases with a transient thermal throat, if a fuel/air premixed region from the injection to the cavity flameholder exists, the cavity pilot flame could reignite the fuel/air mixture and undergo a process similar to deflagration–detonation transition (DDT). This process couples with the flame quenching upstream of the injection location, and a DDT-type low frequency oscillation can be formed.

Spatial Analysis on Forced Heat Release Response of Turbulent Stratified Flames

The local equivalence ratio distribution in a flame affects its shape and response under velocity perturbations. The forced heat release response of stratified lean-premixed flames to acoustic velocity fluctuations is investigated via chemiluminescence measurements and spatial Fourier transfer analysis. A laboratory scale burner and its boundary conditions were designed to generate high-amplitude acoustic velocity fluctuations in flames. These flames are subject to inlet radial equivalence ratio distributions created via a split annular fuel delivery system outfitted with a swirling stabilizer. Simultaneous measurements on the oscillations of inlet velocity and heat release rate were carried out via a two-microphone technique and OH* chemiluminescence. The measurements show that, for a given mean total power and equivalence ratio (φg=0.60), the flame responses vary significantly on the equivalence ratio split, forcing frequency, and velocity fluctuation amplitude, with significant nonlinearities with respect to forcing amplitude and stratification ratio (SR). The spatial Fourier transfer analysis shows how the dependence is affected by the underlying changes in the rate of heat release, including the direction and speed of the perturbation within the flame.

Analysis of Azimuthal Thermo-acoustic Modes in Annular Gas Turbine Combustion Chambers

Modern gas turbine combustors operating in lean-premixed mode are prone to thermo-acoustic instabilities. In annular combustion chambers, usually azimuthal acoustic modes are the critical ones interacting with the flame. In case of constructive interference, high amplitude oscillations might result. In this paper, the azimuthal acoustic field of a full-scale engine is investigated in detail. The analyses are based on measurements in a full-scale gas turbine, analytical models to derive the system dynamics, as well as simulations performed with an in-house 3d nonlinear network model. It is shown that the network model is able to reproduce the behavior observed in the engine. Spectra, linear growth rates, as well as the statistics of the system's dynamics can be predicted. A previously introduced algorithm is used to extract linear growth rates from engine and model time domain data. The method's accuracy is confirmed by comparison of the routine's results to analytically determined growth rates from the network model. The network model is also used to derive a burner staging configuration, resulting in the decrease of linear growth rate and thus an increase of engine operation regime; model predictions are verified by full-scale engine measurements. A thorough investigation of the azimuthal modes statistics is performed. Additionally, the network model is used to show that an unfavorable flame temperature distribution with an amplitude of merely 1% of the mean flame temperature can change the azimuthal mode from dominantly rotating to dominantly standing. This is predicted by the network model that only takes into account flame fluctuations in axial direction.

Spatio-temporal characteristics of temperature fluctuations in turbulent non-premixed jet flames

We report new results from high-repetition-rate (10kHz) 2D temperature imaging to characterize the temporally-fluctuating nature of the well-known DLR series of turbulent non-premixed jet flames. Specifically, we present temporally-based statistics of temperature fluctuations at multiple simultaneous spatial positions with a specific focus on characterizing the effects of Reynolds number. DLR B (Re=22,800) was found to exhibit a much higher probability of large temporal gradients as compared to DLR A (Re=15,200). While the higher Reynolds number should promote higher fluctuations, the levels were commensurate with an increased effect due to high levels of local flame extinction and re-ignition in DLR B as compared to DLR A. The joint probability density function between the temperature fluctuation and the natural logarithm of the square of the temporal gradient was examined across flame conditions. At an axial position of forty nozzle diameters downstream (x/d=40), DLR A exhibited statistical independence, while DLR B exhibited a negative correlation between the two quantities. These results suggest a Reynolds number-dependence on the spatial development of statistical independence. Integral time scales also are reported for both flames as a function of axial and radial position. The integral time scales are seen to increase with increasing axial and radial position and decrease with increasing Reynolds number as expected from theory.

Premixed Flames Excited by Helical Disturbances: Flame Wrinkling and Heat Release Oscillations

This paper describes an analysis of the response of swirling premixed flames to helical disturbances, i.e., where the flow fluctuations have an azimuthal dependence of the form and denotes the helical mode number. Results elaborate the nature of local flame wrinkling and heat release fluctuations. Significantly, these results show that these different induced fluctuations exhibit very different sensitivities to helical mode number , swirl strength, and dimensionless frequency. In addition, the degree of axisymmetry of the time averaged flame plays a crucial role in these interactions, particularly in how flow disturbances translate into heat release oscillations. Thus, the helical mode , with the dominant contribution to local flame wrinkling and heat release, , and spatially integrated heat release fluctuations, , is generally different. For example, helical mode , leading to the largest amplitude of local flame wrinkling and heat release in a solid body swirl flowfield, is given by , where is the ratio of swirl angular rotation rate to forcing frequency, is the component of mean tangential velocity resolved along the axial forcing direction, and is the phase speed of the convecting vortex. In contrast, only the axisymmetric mode, , leads to global surface area fluctuations in axisymmetric flames, which are also completely insensitive to swirl number. Because the maximum amplitude of local flame wrinkling is, in general, excited by helical modes with , care must be taken in interpreting the significance of large scale helical flame flapping, as often is captured from planar experimental data or visualized in computations.

Characterization and Comparison of Flame Fluctuation Range of a Turbulent Buoyant Jet Diffusion Flame under Reduced- and Normal Pressure Atmosphere

Experiments have been conducted to investigate the magnitude of flame fluctuations of a turbulent buoyant jet diffusion flame in both reduced- (0.64 atm) and normal pressure (1 atm) atmosphere conditions. By image processing, the flame intermittency distribution contour is obtained. The magnitude of flame fluctuations is determined as the vertical flame extent from intermittency of 0.95 to that of 0.05. It is found that the fluctuation range is significantly larger in the reduced pressure than that in the normal pressure condition. The magnitude of the vertical flame fluctuations normalized by the nozzle diameter (D) and the air to fuel mass stoichiometric ratio (1+S) are correlated non-dimensionally with the flame Froude number (Frf) for both of these two pressures showing about a 2/5 power law function, although the normalized values are still a bit higher in the reduced pressure atmosphere.

Hydrogen addition effects on high intensity distributed combustion

Distributed combustion provides significant improvements under high intensity conditions characteristic of gas turbine to provide uniform thermal field (improved pattern factor), ultra-low pollution, enhanced stability and higher efficiency. Mixing between fresh air/fuel stream with hot reactive species is critical to result in distributed reactions and spontaneous ignition. Hydrogen enrichment of fuel is examined with emphasis on combustion stability and emissions under swirling flow conditions. Results are presented on the role of hydrogen enrichment to methane (4–15% by mass, 25–58.5% by volume) on the combustion characteristics under fuel-lean conditions. CO emission was substantially reduced with hydrogen enrichment, with minimal effect on NO emission under premixed combustion. Hydrogen addition extended the lean operational limits of the combustor with stable combustion and no flame fluctuations or flashback. Results obtained on pollutants emission and flame marking via OH* chemiluminescence revealed near volume distributed high intensity combustion with ultra-low emission (<3PPM NO and <9PPM CO) and high performance at lower equivalence ratio. Hybrid numerical–experimental approach can provide more realistic prediction of NO emission from hydrogen enriched methane combustion.

Study on Ignition-Like Behavior Induced by Interaction of Curved Non-Premixed (Diffusion) Flames

An abrupt temperature increase induced by the interaction of “curved” diffusion flames is studied experimentally. The main purpose is to reproduce and measure the numerically predicted unsteady behavior of interacting flames experimentally. A pair of curved non-premixed (i.e., diffusion) flames, which was made by four slot burners (two combined counter flow burners), is utilized. Curved flames are either formed over the fuel stream (normal diffusion flame) or oxidizer stream (inverse diffusion flame) depending on the condition, and their interaction is demonstrated. The characteristics of the ignition-like behavior, namely, a time-sequential change in sound and luminescence from the image generated by the flame fluctuation, are acquired by a microphone and a high-speed camera, respectively. From the periodic unsteady behavior of interacting flames, the ignition-like behavior is successfully confirmed in the experiment. The observed change of frequency of the ignition-like behavior against the bulk velocity gradient agrees with that predicted previously in the numerical analysis.