Instantaneous Flame Front Research Articles

Deep learning-based image segmentation for instantaneous flame front extraction

This paper delves into the methodology employed in examining lean premixed turbulent flame fronts extracted from Planar Laser Induced Fluorescence (PLIF) images at elevated pressures. In such flow regimes, the PLIF signal suffers from significant collisional quenching, typically resulting in image data with low signal-to-noise ratio (SNR). This poses severe difficulties for conventional flame front extraction algorithms based on intensity gradients and requires intense user intervention to yield acceptable results. In this work, we propose Convolutional Neural Network (CNN)-based Deep Learning (DL) models as an alternative to problem specific conventional methods. The pretrained DL models were fine-tuned, employing data augmentation, on a small annotated dataset including a variety of conditions between SNR ≈\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\approx$$\\end{document} 1.6 to 2.6 and subsequently evaluated. All DL models significantly outperformed the best-scoring conventional implementation both quantitatively and visually, while having similar inference times. IoU-scores and Recall values were found to be up to a factor ≈\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\approx$$\\end{document} 1.2 and ≈\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\approx$$\\end{document} 2.5 higher, respectively, with ≈\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\approx$$\\end{document} 1.15 times improved Precision. Small-scale structures were captured much better with fewer erroneous predictions, becoming particularly pronounced for the lower SNR data investigated. Moreover, by applying artificially modeled noise, it was shown that the range of image conditions in terms of SNR that can be reliably processed extends well beyond the images included in the training data, and satisfactory segmentation performances were found for SNR as low as ≈\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\approx$$\\end{document} 1.1. The presented DL-based flame front detection algorithm marks a methodology with significantly increased detection performance, while a similar computational effort for inference is achieved and the need for user-based parameter tuning is eliminated. It enables a very accurate extraction of instantaneous flame fronts in large image datasets where supervised processing is infeasible, unlocking unprecedented possibilities for the study of flame dynamics and instability mechanisms at industry-relevant conditions.

Flame front visualization in highly turbulent jet flames using CH3 photofragmentation laser-induced fluorescence

Flame front visualization using methyl photofragmentation laser-induced fluorescence (CH3-PF-LIF) was demonstrated in turbulent premixed methane/air flames. A pump–probe method was used to detect CH3, where CH3 was first photolyzed to CH (X2П) fragments using a pump laser (212.8 nm), and the fragments were subsequently excited to CH in the C2Σ+ state by a probe laser at 314.4 nm. By detecting fluorescence from CH (C-X), instantaneous two-dimensional flame front imaging with a high signal-to-noise (SNR) ratio was achieved in fuel-lean and turbulent flames. A laser excitation scan was conducted to ensure that the detection of CH3 did not interfere with the other emissions. Visualization using CH3-PF-LIF was compared with visualization with conventional CH planar laser-induced fluorescence (CH-PLIF). The SNR of CH3-PF-LIF imaging is almost ten times higher than those of CH-PLIF, and flame front visualization using CH3-PF-LIF shows more topological structures than CH-PLIF in fuel-lean and highly turbulent flames.

Planar laser-induced imaging of CH3 for high resolution single-shot reaction-zone visualization in premixed methane/air flames over broad stoichiometric ratios

We report a novel approach for single-shot planar imaging of CH3 radicals in premixed methane and air flames. A 213 nm beam from the 5th harmonic of an Nd:YAG laser was resonantly absorbed by the CH3 radicals, which were excited to the highly pre-dissociative upper level and dissociated to H2 and CH (X), as the main dissociation channel. The CH radicals were consequently excited by a 388 nm beam from an alexandrite laser, and the fluorescence from the excited CH radicals was collected off-resonant at 431 nm. Using this Photo-Fragmentation Planar Laser-Induced Fluorescence (PF-PLIF) technique, instantaneous flame front structures, represented by CH3 radicals, can be visualized with high spatial resolution over a broad range of stoichiometric ratios. Signal-to-noise ratios up to 50 were observed for premixed methane/air flame with stoichiometric ratio as low as 0.26. The CH radicals naturally presented in flame front are more than 400 times lower in concentration than the CH3 radicals in premixed methane/air flames even at the conditions close to stoichiometric or slightly fuel rich cases where the highest CH concentrations exist, and the CH3/CH concentration ratios increase dramatically moving towards fuel lean conditions. By adopting a structured illumination of the 213 nm pump beam, the naturally presented CH radicals were visualized simultaneously with CH3 at slightly fuel rich laminar flames, where the CH signal intensity was 5 times lower than that from CH3. The results indicate that the CH3 PF-PLIF technique can provide much stronger signal than the CH PLIF and presented a much promising potential for applications in fuel-lean flames. Finally, the CH3 PF-PLIF was performed in premixed turbulent flames to demonstrate the feasibility of this technique for flame front visualization in turbulent premixed flames.

Instantaneous flame front identification by Mie scattering vs. OH PLIF in low turbulence Bunsen flame

Simultaneous OH-PLIF and Mie scatter imaging were used to investigate turbulent premixed flame edge detection under a range of turbulence characteristics on a stabilised piloted Bunsen burner. A 527 nm wavelength laser beam is used to generate a Mie scattering sheet at 500 Hz, and a 283 nm wavelength laser sheet is created using 355 nm wavelength laser to pump an optical parametric oscillator to induce florescence from OH radicals at 5 Hz. A phase-locking technique is applied to synchronize and lock the two laser systems. The number density method has been used to detect flame edges in Mie scattering images, and three algorithms were applied to OH-PLIF images as a reference. A comparison of the methods and different parameter setting is made by using the metrics of location difference, flame surface density and curvature of flame edges. The processed data show that once a well-tuned window size is determined by applying the number density method, averaged spatial differences between Mie scattering images and OH-PLIF images are of the order of or smaller than the laminar flame thickness, demonstrating that under these conditions, high frequency Mie scatter measurements can be used as well as OH-PLIF images to define the flame edge at that spatial resolution. The positive result confirms that double-frame Mie scattering allows the measurement of high frequency conditional velocity distributions and flame properties simultaneously using solely Mie scattering, provided that the particle density is suitably designed to be around 16 px^2 per particle.Graphic

Classification of flame prehistory and quenching topology in a side-wall quenching burner at low-intensity turbulence by correlating transport effects with CO[formula omitted], CO and temperature

Thermochemical states of a turbulent, lean premixed dimethyl ether/air flame were assessed for the first time using simultaneous measurements of gas temperature T, CO2 and CO mole fractions with locations as close as 120 μm above the quenching wall. This is realized by combined dual-pump coherent anti-Stokes Raman spectroscopy (DP-CARS) and two-photon laser-induced fluorescence (LIF) of CO. In addition to thermochemical states, the flow and flame dynamics were measured separately using a combined two component particle image velocimetry (PIV) and planar LIF of the OH radical at high (4 kHz) and low (50 Hz) repetition rates. The data from the independent measurements was linked by the instantaneous flame front topologies, determined by the qualitative OH-LIF in both experiments. The grid-generated turbulence intensity was found to be relatively low in the bulk flow (∼4.5%, streamwise velocity component) with the turbulent flame classified within the regime of wrinkled flamelets (w′/sL between 0.3 and 0.75, for streamwise velocity component). The flame-wall interaction could be assigned to either a side-wall quenching (SWQ)-like (∼50%) or a head-on quenching (HOQ)-like scenario (∼50%), with a transition between these scenarios taking place within a few milliseconds. The thermochemical states depend significantly on whether a SWQ-like or a HOQ-like scenario is present. Here, the thermochemistry of the SWQ-scenario is studied in detail, and three zones, A, B, C, could be distinguished in the state space on the basis of the (CO2,T) correlations. Zone A is characterized by strong wall heat losses and mixing influences, while zone B features less pronounced wall heat losses and mixing processes. In zone C the impacts of turbulence almost completely disappear and conditions comparable to a laminar near-wall flow are observed. The distinguishability of the three zones in the (CO,T) or (CO,CO2) correlations is less clear, which underlines the importance of the additional CO2 measurement in the DP-CARS methodology.

Effect of non-ambient pressure conditions and Lewis number variation on direct numerical simulation of turbulent Bunsen flames at low turbulence intensity

The instantaneous flame front structure of high pressure turbulent premixed Bunsen flames has been analyzed for a wide range of characteristic Lewis numbers using a new Direct Numerical Simulation (DNS) database. High pressure turbulent premixed flames of lean-light fuels are likely to feature both thermo-diffusive and Darrieus–Landau instabilities. As the effects of these instabilities are eclipsed by intense turbulence, the present analysis focuses on flames located at the border of the wrinkled and corrugated flamelets regimes. The flame morphology has been characterized by the skewness and kurtosis of the probability density function (PDF) profiles of the mean and Gaussian curvatures. While skewness alone is not sufficient as a marker to distinguish between thermo-diffusive and Darrieus–Landau instabilities, it has been found that excess kurtosis of Gaussian curvature possibly can be used to distinguish between both instabilities. Further, the inner cut-off length and the fractal dimension have been computed for characterization of flame scales and for parameterization of wrinkling factor models. It has been observed that increasing pressure and decreasing Lewis number give rise to flame instabilities which results in an increased fractal dimension and a decreased inner cut-off scale. For high pressure flames with Lewis numbers around unity, the inner cut-off scale scales very well with the critical wavelength for flame instabilities determined from the theoretical analysis by Matalon and Matkowsky (1982). For small sub-unity Lewis numbers, the flames become unconditionally unstable and the critical wavelength loses its meaning, while the inner cut-off scales continues to decrease with Lewis number and the fractal dimension continues to increase up to a limit of about 7/3. The present findings are in excellent agreement with experimental observations from literature and theoretical analysis of flame instabilities.

Local flame and flow properties of propagating premixed turbulent flames during light-round process in a MICCA-type annular combustor

In this paper, local flame and flow properties of propagating premixed turbulent propane/air flames were derived by two-dimensional time-resolved Mie scattering and PIV methodology in a MICCA-type annular combustor. The curvature of flame front, local flame speed and the evolution of leading points during the light-round process were analyzed in detail. All experiments were performed under a constant equivalence ratio of 0.67, with the bulk flow velocity set from 1.95 to 5.85 m/s in five cases to achieve different turbulent intensities. The instantaneous flame fronts and velocity fields of unburned gases were measured to investigate the flame-flow interactions corresponding to four regions of the annular combustor. The results indicate that the slight asymmetry of the propagation in two flame branches is mainly due to the competition between the local swirling injection and the overall circumferential flow along the inner or outer parts of the combustor. Based on the statistical analysis of the local radius of flame curvature, the whole flame front tends to be convex to the unburned mixture. As a characteristic length scale, the inner-cutoff scale εi of the flame front is about 2–3 times the flame thickness δL, which decreases with the increase of turbulent intensity. The value of fractal dimension increases with the increase of turbulence intensity, and its value is about 2.15–2.25 for all cases. Besides, in order to analyze the flame properties during the ignition process, the local flame speed wn was calculated using statistical method as well as the velocity of unburned mixture gas. Moreover, the experimental results (Ub=3.90 m/s) of the leading points’ trajectory were consistent with the large-eddy simulations at the height of h =25 mm, and the leading points of the flame fronts were generally concentrated near (about 5 mm away) the inner tube for both flame branches. The researches of ignition mechanisms in annular combustors have been carried out for almost ten years, but the quantitative measurements of flow and flame fronts are barely reported. Motivated by this, it will provide new insights into the ignition process in annular combustors from this work.

Influence of a Standing Wave Flow-Field on the Dynamics of a Spray Diffusion Flame

A theoretical investigation of the influence of a standing wave flow-field on the dynamics of a laminar two-dimensional spray diffusion flame is presented for the first time. The mathematical analysis permits mild slip between the droplets and their host surroundings. For the liquid phase, the use of a small Stokes number as the perturbation parameater enables a solution of the governing equations to be developed. Influence of the standing wave flow-field on droplet grouping is described by a specially constructed modification of the vaporization Damkohler number. Instantaneous flame front shapes are found via a solution for the usual Schwab–Zeldovitch parameter. Numerical results obtained from the analytical solution uncover the strong bearing that droplet grouping, induced by the standing wave flow-field, can have on flame height, shape, and type (over- or under-ventilated) and on the existence of multiple flame fronts.

Flame- and flow-conditioned vorticity transport in premixed swirl combustion

This paper presents an experimental analysis of flame-induced enstrophy transport in premixed swirl combustion at Karlovitz numbers between 20 and 50. Such flames possess a large-scale pressure field that – in addition to the pressure fields associated with small-scale turbulent vortices – can interact with density gradients to produce baroclinic torque. Simultaneous tomographic particle image velocimetry and formaldehyde planar laser induced fluorescence measurements are used to obtain high-resolution velocity and progress-variable fields. This allows statistical evaluation of the various terms in the enstrophy transport equation. The impact of small- and large-scale pressure gradients is assessed by conditioning the baroclinic torque on the position of the fluid within the instantaneous flame front, within the flame brush, and axially within the combustor. At all conditions studied, the baroclinic torque was a significant contributor to enstrophy transport, with a comparable magnitude to vortex stretching and viscous diffusion. Enstrophy attenuation and production by baroclinic torque tended to occur towards the reactant and product sides of the instantaneous flame surface, respectively. However, the value of the baroclinic torque also depended equally strongly on the position in the combustor. Hence, both small- and large-scale pressure fields can result in significant enstrophy changes through baroclinic torque. A scaling is proposed to quantify the significance of swirl-induced pressure field on baroclinic torque enstrophy production; it is shown to increase with increasing swirl number and decreasing Damköhler numbers. This is evidence both that flame-induced vorticity dynamics are significant in swirl combustion, and that large-scale geometry-dependent flow fields can impact flame-generated turbulence.

Measurements on flame structure of bluff body and swirl stabilized premixed flames close to blow-off

The structure of unconfined lean premixed CH4/air flames stabilized with an axisymmetric bluff-body and a swirler was studied for conditions far from and close to blow-off. Blow-off was approached by gradually reducing the equivalence ratio while keeping bulk velocity constant at about 20 m/s. The objective is to examine the critical regions of flame structure that initiate and develop to blow-off for relatively short flames. OH-PLIF and CH* chemiluminescence were adopted to capture the instantaneous flame front and time-averaged flame structure. Statistics of the flame root-lift-off, flame front curvature and flame surface density (FSD) were obtained. Local burning velocity was estimated based on the FSD field. The burning intensity was also demonstrated with CH* chemiluminescence. As blow-off was approaching, the flame structure was seen to be more compacted. Lift-off at the flame roots was observed to be more frequent and the lift-off height was increased at leaner conditions. The local burning intensity was significantly decreased at upstream stages including the flame root close to blow-off. The wrinkling ratio was growing obviously at downstream, with finer flame structure observed through the short and intensive probability density function (PDF) of curvatures. The enhanced wrinkling by the recirculating flow at downstream is assumed to be favorable for the flame stabilization prior to the final blow-off. This indicates minor effects of downstream on blow-off. Comparatively, upstream of the flame including the main shear layer and the flame root is more unstable and unfavorable for ignition and flame stabilization, which dominantly promotes the blow-off. The measurements in present study are useful for model validation in terms of flame structures, and also significant for exploring the blow-off behavior of shorter flames as proposed in previous studies.

Consequences of flame geometry for the acoustic response of premixed flames

This study investigates the consequences of flame geometry for the linear response of laminar premixed flames to acoustic perturbations, as expressed by the flame transfer function (FTF). Analytical G-equation-based response models are derived for Slit, Bunsen and Wedge type flames; their respective characteristics are analyzed and validated against data obtained from high fidelity numerical simulations. Motivated by the poor agreement between numerical and analytical flame response predictions, particularly for Slit flames, an extension to the well-established incompressible-convective velocity model is proposed, which employs a Gaussian kernel function. Such a kernel disperses the flame response in time and leads to very good agreement with high fidelity numerical simulations. The validity of the model is further confirmed by comparing model predictions with experimental data taken from the literature. Analyzing the analytical flame response modeling concepts in detail, we find that the surface integration, which is required to compute the change of the global heat-release rate from the instantaneous flame front deflections, constitutes the most significant geometry-related property affecting the FTF. The linearized global heat-release rate of stiffly anchored Slit flames reacts only to movements of the flame tip and, hence, these flames respond time delayed to imposed flame front perturbations. Bunsen flames continuously transform convected flame front deflections to changes in the heat-release rate and, therefore, show a more pronounced low-pass behavior than Slit flames. The heat-release rate of Wedge flames reacts to both movements of the flame tip and the integral of instantaneous flame front deflections. Hence, perturbations of the flame front initially have a rather weak effect until they reach the flame tip, where a sudden and strong response is provoked. This leads to the occurrence of high peak gain values in the corresponding FTF.

PIV/OH PLIF investigation of flow and flame front dynamics of acoustically perturbed conical flame

The paper reports on the investigation of the flow and flame front dynamics in a premixed conical methane/air flame perturbed by an acoustic field. The investigation is carried out by using stereoscopic particle image velocimetry and planar laser-induced fluorescence of HCHO to measure simultaneously the instantaneous velocity field and flame front location in the axial plane. The optical techniques provided information about local velocity and velocity gradients, stretching and curvature of the flame front. Flow conditions when the flame is covered from the top by a cooled metallic flat plate are also considered. It is found that the flow and flame front lose axial symmetry when the obstacle is present because of disruption of the global instability mechanism of the combustion products plume driven by bouncy force.

The structure of swirl-stabilized turbulent premixed CH4/air and CH4/O2/CO2 flames and mechanisms of intense burning of oxy-flames

The objective of this work is to examine the structure of lean turbulent premixed CH4/air (air-flames) and CH4/O2/CO2 (oxy-flames) in a swirl-stabilized combustor at the large (macro) and small (micro) scales, and to explain why the latter burns more intensely despite its lower laminar burning velocity. Measurements of the instantaneous flame front and flow field using OH-PLIF and PIV, respectively, are analyzed. The CO2 dilution in the oxy-flame was adjusted to achieve the same adiabatic temperature for both flames while keeping the equivalence ratio (=0.65) and Reynolds number (=20,000) the same. Results show that at the large scale, the overall length of the oxy-flame is notably shorter than that of the air-flame. We use a strained flame model with detailed kinetics to show that the strained consumption speed of oxy-flames is lower than that of air-flames, mainly due to the chemical role of CO2, and hence laminar flame properties cannot explain the difference between the two flames in the turbulent case. Instead, the turbulent dynamics properties such as the small scale wrinkling of the flame, its surface area density and radius of curvature are obtained from the data and used to explain the observed trends. Measurements show that the flame surface area density of oxy-flames is higher because more flame segments with smaller radius of curvature appear. This is particularly noticeable downstream, and is consistent with the Lewis number effects, i.e., the response for the flame to thermo-diffusive instability. The local average burning intensity, being high under oxy-combustion conditions, explain the shorter overall average flame length.

Effects of mixture composition and turbulence intensity on flame front structure and burning velocities of premixed turbulent hydrocarbon/air Bunsen flames

The influences of the equivalence ratio, turbulence intensity, and different thermo-diffusive characteristics on the flame brush characteristics, instantaneous flame front structures, and burning velocities of premixed turbulent methane/–, ethane/–, and propane/air Bunsen flames were investigated systematically. Particle image velocimetry and Mie scattering techniques were utilized to measure the turbulence statistics and to visualize flame front corrugations, respectively. All experiments were performed under a constant bulk flow velocity of 21.0 m/s. The equivalence ratio range was from 0.7 to 1.35 for methane/air flames, 0.7–1.45 for ethane/air flames, and 0.8–1.35 for propane/air flames. Two perforated plates were used to produce different turbulence levels. A series of comprehensive parameters including the characteristic flame height, mean flame brush thickness, mean volume of the turbulent flame region, mean fuel consumption rate, two-dimensional flame front curvature, local flame front angle, two-dimensional flame surface density, wrinkled flame surface area, turbulent burning velocity, mean flamelet consumption velocity, and mean turbulent flame stretch factor were obtained. The mean turbulent flame stretch factor displayed a dependence on the equivalence ratio and turbulence intensity. Results show that the mean turbulent flame stretch factors for lean/stoichiometric and rich mixtures were not equal when the unstrained premixed laminar burning velocity, non-dimensional bulk flow velocity, non-dimensional turbulence intensity, and non-dimensional longitudinal integral length scale were kept constant.

Flame front structure of turbulent premixed flames of syngas oxyfuel mixtures

In order to investigate oxyfuel combustion characteristics of typical composition of coal gasification syngas connected to CCS systems. Instantaneous flame front structure of turbulent premixed flames of CO/H2/O2/CO2 mixtures which represent syngas oxyfuel combustion was quantitatively studied comparing with CH4/air and syngas/air flames by using a nozzle-type Bunsen burner. Hot-wire anemometer and OH-PLIF were used to measure the turbulent flow and detect the instantaneous flame front structure, respectively. Image processing and statistical analyzing were performed using the Matlab Software. Flame surface density, mean progress variable, local curvature radius, mean flame volume, and flame thickness, were obtained. Results show that turbulent premixed flames of syngas possess wrinkled flame front structure which is a general feature of turbulent premixed flames. Flame surface density for the CO/H2/O2/CO2 flame is much larger than that of CO/H2/O2/air and CH4/air flames. This is mainly caused by the smaller flame intrinsic instability scale, which would lead to smaller scales and less flame passivity response to turbulence presented by Markstain length, which reduce the local flame stretch against turbulence vortex. Peak value of Possibility Density Function (PDF) distribution of local curvature radius, R, for CO/H2/O2/CO2 flames is larger than those of CO/H2/O2/air and CH4/air flames at both positive and negative side and the corresponding R of absolute peak PDF is the smallest. This demonstrates that the most frequent scale is the smallest for CO/H2/O2/CO2 flames. Mean flame volume of CO/H2/O2/CO2 flame is smaller than that of CH4/air flame even smaller than that of CO/H2/O2/air flame. This would be due to the lower flame height and smaller flame wrinkles.

Measurement on instantaneous flame front structure of turbulent premixed CH4/H2/air flames

Instantaneous flame front structure of turbulent premixed CH4/H2/air flames (hydrogen fraction of 0%, 5%, 10% and 20% by mole fraction) was investigated quantitatively using a nozzle-type Bunsen burner. Hot wire anemometer and OH-PLIF were used to measure the turbulent flow and detect the instantaneous flame front structure, respectively. Turbulent burning velocity, ST, flame surface density, Σ, and mean flame volume, Vf, were calculated by processing the OH-PLIF images. Results show that the flame front structures of the turbulent premixed flames are the wrinkled flame front and it becomes much finer with the increase of turbulence intensity as well as hydrogen fraction. The value of ST/SL significantly increases with the increase of u′/SL and it slightly increases with the increase of hydrogen fraction. Flame surface density profile are symmetric and gives its maximum value at about 〈c〉=0.5. Hydrogen addition slightly enhances the Σ and the tendency is more obvious under higher turbulence intensity. The decrease of Σ with the increase of turbulence intensity is mainly due to the effect of flame volume. The mean flame volume of flame region obviously increases with the increase of turbulence intensity within the experimental range due to the increase in depth of the large scale flame wrinkles and flame height. Hydrogen addition is not a predominant factor within the hydrogen fraction range in this study.

Flame front structure and burning velocity of turbulent premixed CH4/H2/air flames

Flame front structure of turbulent premixed CH4/H2/air flames at various hydrogen fractions was investigated with OH-PLIF technique. A nozzle-type burner was used to achieve the stabilized turbulent premixed flames. Hot-wire anemometer measurement and OH-PLIF observation were performed to measure the turbulent flow and detect the instantaneous flame front structure, respectively. The hydrogen fractions of 0%, 5%, 10% and 20% were studied. Results show that the flame front structures of the turbulent premixed flames are wrinkled flame front with small scale convex and concave structures compared to that of the laminar-flame front. The wrinkle intensity of flame front is promoted with the increase of turbulence intensity as well as hydrogen fraction. Hydrogen addition promotes the flame intrinsic instability which leads to the active response of laminar flame to turbulence and results in the much more wrinkled flame front structure. The value of ST/SL increases monotonically with the increase of u′/SL and hydrogen fraction. The increase of ST/SL with the increase of hydrogen fraction is mainly attributed to the diffusive-thermal instability effects represented by the effective Lewis number, Leeff. A general correlation between ST/SL and u′/SL is provided from the experimental data fitting in the form of ST/SL∝a(u′/SL)n, and the exponent, n, gives the constant value of 0.35 for all conditions and at various hydrogen fractions.

Measurement of the instantaneous flame front structure of syngas turbulent premixed flames at high pressure

Instantaneous flame front structure of syngas turbulent premixed flames including the local radius of curvature, the characteristic radius of curvature, the fractal inner cutoff scale and the local flame angle were derived from the experimental OH-PLIF images. The CO/H2/CO2/air flames as a model of syngas/air combustion were investigated at pressure of 0.5MPa and compared to that of CH4/air flames. The convex and concave structures of the flame front were detected and statistical analysis including the PDF and ADF of the local radius of curvature and local flame angle were conducted. Results show that the flame front of turbulent premixed flames at high pressure is a wrinkled flame front with small scale convex and concave structures superimposed with large scale flame branches. The convex structures are much more frequent than the concave ones on flame front which reflects a general characteristic of the turbulent premixed flames at high pressure. The syngas flames possess much wrinkled flame front with much smaller fine cusps structure compared to that of CH4/air flames and the main difference is on the convex structure. The effect of turbulence on the general wrinkled scale of flame front is much weaker than that of the smallest wrinkled scale. The general wrinkled scale is mainly dominated by the turbulence vortex scale, while, the smallest wrinkled scale is strongly affected by the flame intrinsic instability. The effect of flame intrinsic instability on flame front of turbulent premixed flame is mainly on the formation of a large number of convex structure propagating to the unburned reactants and enlarge the effective contact surface between flame front and unburned reactants.

Correlation of turbulent burning velocity for syngas/air mixtures at high pressure up to 1.0 MPa

Instantaneous flame front structures of the turbulent premixed flames of syngas/air and CH4/air mixtures were investigated using OH-PLIF technique at high pressure up to 1.0MPa, through which the turbulent burning velocities were derived and correlated with the turbulence intensity. Results show that both syngas/air and CH4/air mixtures, ST/SL increases remarkably with the increase of u′/SL particularly in the weak turbulence region. For the syngas/air mixture, the intensity of flame front wrinkle is promoted with the increase of hydrogen fraction in the syngas due to the increased preferential diffusive-thermal instability. Compared to CH4/air mixture, the syngas flames possess much wrinkled flame front with much smaller fine cusps structure, and with increasing u′/SL, the rate of the increase of ST/SL for the syngas/air mixtures is more significant than that of CH4/air mixtures. This demonstrates that the increase of flame front area due to turbulence wrinkling is promoted by flame intrinsic instability for syngas/air mixtures. The values of ST/SL for all mixtures increase with the increase of pressure because of the decrease of flame thickness which promotes the hydrodynamic instability. A general correlation of turbulent burning velocity for the syngas/air and CH4/air mixtures was obtained in the form of ST/SL∝a[(P/P0)(u′/SL)]n.

Examining flow-flame interaction and the characteristic stretch rate in vortex-driven combustion dynamics using PIV and numerical simulation

In this paper, we experimentally investigate the combustion dynamics in lean premixed flames in a laboratory scale backward-facing step combustor in which flame-vortex driven dynamics are observed. A series of tests was conducted using propane/hydrogen/air mixtures for various mixture compositions at the inlet temperature ranging from 300K to 500K and at atmospheric pressure. Pressure measurements and high speed particle image velocimetry (PIV) are used to generate pressure response curves and phase-averaged vorticity and streamlines as well as the instantaneous flame front, respectively, which describe unsteady flame and flow dynamics in each operating regime. This work was motivated in part by our earlier study where we showed that the strained flame consumption speed Sc can be used to collapse the pressure response curves over a wide range of operating conditions. In previous studies, the stretch rate at which Sc was computed was determined by trial and error. In this study, flame stretch is estimated using the instantaneous flame front and velocity field from the PIV measurement. Independently, we also use computed strained flame speed and the experimental data to determine the characteristic values of stretch rate near the mode transition points at which the flame configuration changes. We show that a common value of the characteristic stretch rate exists across all the flame configurations. The consumption speed computed at the characteristic stretch rate captures the impact of different operating parameters on the combustor dynamics. These results suggest that the unsteady interactions between the turbulent flow and the flame dynamics can be encapsulated in the characteristic stretch rate, which governs the critical flame speed at the mode transitions and thereby plays an important role in determining the stability characteristics of the combustor.