Laminar Premixed Methane Research Articles

The effect of hydrogen enrichment on the conical premixed methane–air flame response and thermoacoustic modes coupling

This paper investigates the thermoacoustic dynamic responses of hydrogen-enriched laminar premixed conical methane–air flames under dual-mode coupling. Utilizing the level set method and the G-equation model, one meticulously derives the flame describing function (FDF) in relation to variations in hydrogen enrichment levels (ηH). This precisely derived FDF is then integrated into the low-order network model of the Rijke tube to analyze the thermoacoustic unstable modes of the system. Finally, dual-frequencies incoming flow velocity perturbations are reintroduced as inputs to obtain the flame response under thermoacoustic modes coupling. Results show that while keeping the unstretched steady flame aspect ratio constant, an increase in ηH not only raises the FDF’s cut-off frequency but also enhances the FDF gain within the nonlinear frequency region, leading to more unstable modes, especially high frequency modes within the Rijke tube system. Furthermore, the flame response is further altered under the excitation of dual unstable modes. When both modes are within the linear frequency region of the FDF, the flame response is co-controlled by the two modes, predominantly by the mode with a larger velocity perturbation amplitude, with weaker modes coupling leading to the additional frequency perturbation having a suppressive effect on the flame response at the original frequency. Conversely, when one or two modes are within the nonlinear frequency region of the FDF, the flame response is dominated by the lower-frequency mode, with higher nonlinear modes coupling allowing the additional frequency perturbation to both promote and suppress the response at the original frequency and also couple to produce a significant difference frequency response. Novelty and significance The novelty of this paper lies in the determination of the flame describing function (FDF) with varying hydrogen enrichment levels and the stability of thermoacoustic systems, and more significantly, conducting an in-depth and comprehensive investigation into the nonlinear dynamic responses of hydrogen-enriched flames to different types of dual-mode coupling perturbations. The dual-mode perturbations result in either attenuation or amplification of the flame response, depending on whether the corresponding frequencies lie within the linear or nonlinear frequency regions of the FDF, which innovatively incorporates the influence of the FDF phase and examines the responses at the difference frequencies generated by the coupling. This lays a foundation for further exploration into the mechanisms of modes coupling in hydrogen-enriched and other thermoacoustic systems.

Premixed combustion and emission characteristics of methane diluted with ammonia under F-class gas turbine relevant operating condition

Ammonia has been used on a small scale in other industrial equipment, such as gas turbines, as a carbon-free fuel. However, ammonia fuel suffers disadvantages such as high ignition temperature, low flame velocity and high NOx emissions. Doping with ammonia using a more reactive fuel, such as methane, can solve the above problems. Therefore, under the relevant operating conditions of the gas turbine (T = 723 K, p = 16.5 atm), the effect of ammonia content on the combustion and emission characteristics of laminar premixed methane flames was numerically investigated. This research uses the PREMIX code from ANSYS CHEMKIN-PRO 2020 and Okafor chemical kinetic mechanisms and provides a reference for our subsequent analysis of gas turbine operating conditions. Firstly, the emission data of major pollutants under different ammonia content (XNH3 = 0–1.0) and equivalent ratio (Φ = .6–1.4) were calculated. Then, the laminar premixed flame structure is analyzed under the lean fuel conditions associated with gas turbines (Φ = .6, .8). Finally, the effect of ammonia addition on the chemical reaction path of NO and CO emission was studied. The results show that ammonia/methane mixture fuel is more suitable for combustion at .6 < Φ < .8 under high temperature and pressure. High ammonia content (XNH3 > .6) and low equivalent ratio can reduce NO and CO emissions. The molar fractions of H, O, and OH radicals and flame temperature decreased with the increase in ammonia content. In addition, high temperature and high pressure conditions and ammonia content greatly influence the reaction path of NO and CO production. The increase in pressure resulted in a change in the primary reaction that produced NO. In conclusion, this study guides reducing the emission of NO and CO from lean side of gas turbine plants.

Effect of hydrogen addition on the combustion and emission characteristics of methane under gas turbine relevant operating condition

The influence of hydrogen addition (0–100%) on the combustion and emission characteristics (NO and CO formation) of laminar-premixed methane flame under gas turbine relevant operating conditions was numerically studied. The PREMIX code of ANSYS CHEMKIN-PRO and the GRI-Mech 3.0 were utilized. The study of the effect of increasing hydrogen content in the methane-hydrogen mixtures on combustion and emissions at T = 298 K, P = 1 atm can provide a reference for our subsequent research on gas turbine conditions (T = 723 K, P = 16.5 atm). The LBV (laminar burning velocity) and AFT (adiabatic flame temperature) with different hydrogen contents (XH2=0–100%) and equivalence ratio (Φ = 0.6–1.4) were investigated. In addition, under gas turbine relevant lean burning conditions (Φ = 0.6, 0.8), the effect of hydrogen addition on the NO and CO emissions were investigated. The rate and sensitivity coefficients of NO/CO production were also examined. The results show that the LBV and AFT increase with the hydrogen content. Both the mole fraction of H, O and OH radicals and the flame temperature were observed to increase with the hydrogen content. Furthermore, the hydrogen addition can significantly increase the NO emissions and on the other hand suppress the CO formation. Reducing the equivalence ratio (from 1.2 to 0.6) can significantly reduce the NO emission. The addition of hydrogen can also increase the combustion stability of methane fuel under fuel-lean conditions. This is a good hint for gas turbine device to reduce NO emissions at fuel-lean side (Φ < 1).

Hydrogen as a fuel additive in laminar premixed methane flames: Impact on the nucleation and growth of soot particles

This work reports new experimental data regarding the impact of H2 on slightly sooting methane flames (Φ–1.91) characterized by the formation of soot particles undergoing growth processes. This study has been carried out in three atmospheric sooting premixed methane flames with added H2 (2% of the total flowrate) or substituted (10% molar comparing to CH4 fuel).The mole fraction profiles of selected aliphatic and aromatic compounds (C1-C16) have been measured by Gas Chromatography and Jet-Cooled Laser-Induced Fluorescence. Concerning the soot particles, we have implemented for the first time an analytical methodology based on in-situ measurements by Laser-induced Incandescence and Cavity Ring-Down Spectroscopy coupled with ex-situ measurements by Scanning Mobility Particle Sizing (SMPS). This original approach allows to determine the soot volume fraction profiles and corresponding evolution of the particle number density and particles size density function without requiring accurate knowledge of the dilution ratio during SMSP measurements.The reported data highlight that the introduction of H2 strongly influences the formation of aromatic species and soot depending on the operating conditions (H2 addition or substitution). The impact of H2 was evaluated on the nucleation step and the soot growth processes. In that context, we notably show that the H2 substitution inhibits while the H2 addition favors the formation of nascent soot particles, and furthermore the H2 addition generates substantial modifications in the dynamic of the soot growth processes. This work also supports the idea that the onset of the particle nucleation might be controlled by specific concentration thresholds of aromatic precursors.

Development and validation study of a 1D analytical model for the response of reheat flames to entropy waves

Numerical simulations of laminar premixed flames burning hydrogen and methane in spontaneous ignition mode are performed by harmonically exciting the reactants’ temperature at the domain inlet. The results are compared to an analytical model representing the same reactive flow configuration. The model provides a simplified but nevertheless accurate representation of reheat combustion taking place in sequential gas turbine combustors. An analytic expression for autoignition flames transfer functions to entropy waves is derived and used to extend transfer function models from the literature. For validation purposes, results from fully compressible Direct Numerical Simulations (DNS), including a complete representation of the fluctuating acoustic and entropic fields of the reactive flow, are analyzed and compared to incompressible Unsteady Reynolds-Averaged Navier–Stokes (URANS) simulations that only take into account the fluctuating entropic field. Methane flames are found to be more sensitive to entropic forcing than hydrogen flames, featuring nonlinear phenomena even for low excitation amplitudes. In the linear regime, all flames behave as predicted by the analytical model and the URANS simulations are found to correctly predict the fluctuating entropic field. The transition from linear to nonlinear flame response is described in detail and its physical mechanisms are explained. Comparisons with results available in the literature show good prediction capabilities, both in terms of flame describing function and integrated heat release rate. Limitations of the proposed analytical model with respect to real combustion systems are discussed and a simple correction is proposed.

Influence of Transverse Slot Jet on Premixed Flame Acceleration

This work aims to identify the key flow parameters that influence flame acceleration in a semiconfined square channel. A transverse fluidic jet was used as an active flow blockage mechanism and to introduce turbulence into the propagating flame. Three experimental parameters were used to examine the relative influence of 1) mixture reactivity (defined here as system equivalence ratio, SER), 2) jet mixture composition (JMC), and 3) the momentum ratio (MR) on the acceleration of laminar premixed methane flame. High-speed particle image velocimetry and schlieren photography were used to characterize the instantaneous flowfield conditions throughout the flame–jet interaction. Using these diagnostic techniques, flame front positions and local velocity vector fields have been spatially and temporally resolved. Changes in flame properties, including flame structure, velocity, and vorticity, were tracked as functions of time. Stoichiometric equivalence ratios were more effective in the production of vorticity and the promotion of flame acceleration. The stoichiometric condition accelerated the flame to the highest final flame velocity of the three parameters examined. Different compositions of the jet mixture demonstrated that the flame acceleration is primarily affected by the jet turbulence and not on the reactivity of the jet compositions. The MR parameter had the least amount of influence on the flowfield and flame acceleration. The increase of 33% in the MR had negligible effect on the final flame front velocity and implies that the jet turbulence is the main driving mechanism for flame acceleration.

Three-dimensional simulation of ionic wind in a laminar premixed Bunsen flame subjected to a transverse DC electric field

The role of the ionic wind effects on modifying flame dynamics was demonstrated by detailed computational models. Full three-dimensional simulations were conducted to reproduce and describe the response of a laminar premixed methane–air Bunsen flame subjected to a transverse DC electric field at saturation condition. The chemical kinetic mechanism employed a methane–air skeletal mechanism with an optimized ionization model to predict the positive and negative ions that are important for generating the electric currents. Given the strong dependence of the ionic wind on the amount of charged species created by chemi-ionization, the ion production rate was optimized to match the measured saturation current. The simulation successfully reproduced the flame tilt toward the cathode. The ionic winds blowing from the flame toward the electrodes in both rightward and leftward directions were also captured. The calculated flow field is qualitatively consistent with the PIV experimental data. Accurate description of the three-body electron attachment to oxygen and the charge transfer reactions generating heavy anions was found to be critical in simulating the flame–electric field interaction. This is a first demonstration of the ionic wind effect by full three-dimensional simulations. Further investigation by both experiment and modeling are required in future work to address quantitative differences.

Head-on quenching of laminar premixed methane flames diluted with hot combustion products

Transient head-on quenching of laminar premixed methane flames diluted with hot combustion products is analyzed using full-chemistry 1D DNS. The impact of the dilution level, pressure and wall temperature on carbon monoxide (CO) emissions is investigated. Increasing dilution level and pressure reduce peak average near-wall CO concentrations, and reduce the near-wall CO reduction rate. However, the peak average near-wall CO and near-wall CO reduction rate increase with increasing wall temperature. Analysis of the species transport budget for CO near the wall before, during and after quenching indicates that there are conditions where diffusion is the dominant transport term. As a consequence, it may be possible to model the near-wall CO using only the integrated diffusion term within certain spatial regions. Dilution increases the size of these regions, whereas increasing pressure reduces this size.

Modelling analysis of PAH and soot measured in a premixed toluene-doped methane flame

A detailed gas-phase kinetic model coupled with a multi-sectional approach for the particle phase was used for inferring the effect of toluene addition on the sooting structure of a fuel-rich laminar premixed methane flame. Premixed methane flames at different C/O ratios established the reference environments to gain insight on the influence of toluene addition.Experimental data derived from a detailed analysis of the combustion aerosol sampled along the methane flame and the toluene-doped flame, burning in similar temperature conditions, were compared with the predictions of the model to assess its sensitivity to small changes in the fed mixture composition.Modeling results predicted reasonably well the gas- and the particle-phase concentrations. Overall, the effect of small amounts of toluene on the sooting yield and structure of the methane flame strongly tending toward the sooting structure typical of aromatic fuels, was observed. Experimental evidences and modeling results suggest that the relative importance of soot inception and surface growth by C2H2 and PAH onto existing particles or aggregates is significantly affected by toluene addition.

Effect of different wall materials and thermal-barrier coatings on the flame-wall interaction of laminar premixed methane and propane flames

We investigate the flame-wall interaction of laminar, premixed methane/air and propane/air flames in a stationary, sidewall-quenching configuration over a wide range of equivalence ratios. Both, the wall material (stainless-steel, cast iron, aluminum), the type of thermal-barrier coating (soot, zirconium dioxide, titanium dioxide) and its thickness (only for zirconium dioxide) were systematically varied. The flame-wall interaction is characterized by the measurement of quenching distances, which are based on the separate analysis of chemiluminescence images of electronically excited OH* and CH* radicals. A method has been developed to determine the position of the wall in the chemiluminescence images with sub-pixel accuracy. It turns out, that while the quenching distance strongly changes with stoichiometry and fuel, it only weakly depends on the wall material and type of coating, although the quenching distance tends to decreases with increasing thickness of the ceramic coating. Overall, the data confirms present scaling laws with a quenching Peclet number of PeQ ≈ 7.5. Quenching distances determined independently for OH* and CH* differ more strongly than the distances of peak mole fractions in an undisturbed flame, without the influence of the wall.

Spatially resolved laser-induced breakdown spectroscopy in laminar premixed methane–air flames

Laser-induced breakdown spectroscopy was evaluated for the analysis of the structure of laminar premixed methane–air flames. Firstly, breakdown threshold pulse energy and plasma energy in different areas of the flame were measured simultaneously, and an approximate linear relation between them was detected. Secondly, a new approach was proposed to qualitatively characterize the flame temperature distributions based on the plasma energy distributions. Finally, combination of the spatial analysis of the spectrum intensity, plasma energy and equivalence ratio, the laminar premixed flames structure was investigated deeply, including the distribution of the flame temperature, the width and distribution of different flame region (e.g. premixed combustion regions, high temperature regions.),as well as the location of the flame front.

Methyl Radical Imaging in Methane-Air Flames Using Laser Photofragmentation-Induced Fluorescence.

Imaging detection of methyl radicals has been performed in laminar premixed methane-air flames at atmospheric pressure. A nanosecond Q-switched neodymium-doped yttrium aluminum garnet (Nd : YAG) laser was employed to provide the fifth-harmonic-generated 212.8 nm laser beam. The intense ultraviolet (UV) laser pulse was sent through the flame front to photodissociate the methyl (CH3) radicals in the reaction zone of the flames stabilized on a piloted jet flame burner. The emission spectra from the photodissociated fragments were collected using an imaging spectrometer with the flame-front structure spatially resolved. Combining the spatial and spectral information, we recognized that the emission from the (A-X) methine (CH) transitions located at 431 nm was generated from the CH3 photolysis and could be used to visualize the distribution of CH3 radicals. With proper filtering, the high-power UV laser (around 15 mJ/pulse) provided by the compact Nd : YAG laser makes it possible to visualize CH3 distribution naturally generated in the reaction zone of laminar methane-air premixed flames.

Analysis of chemiluminescence, density and heat release rate fluctuations in acoustically perturbed laminar premixed flames

Laser interferometric vibrometry (LIV) has recently been proposed as an alternative mean to obtain time-resolved density and heat release rate measurements at relatively low cost and experimental effort. This technique is sensitive to fluctuations of the refractive index of gases resulting from density and composition changes along the laser beam intersecting the reacting flow. It yields a line-of-sight integrated signal of the probed flow from which density and heat release rate disturbances may be inferred. The link between these signals with chemiluminescence is examined in the present study by first considering a theoretical analysis to determine the relationships between the LIV, density and heat release rate perturbation signals in a multi-species reactive mixture of gases. For air combustion systems interacting with sound waves, low frequency density perturbations in the flame zone, result mainly from heat release rate fluctuations below a certain frequency threshold. An experimental analysis is then conducted with confined conical laminar premixed flames submitted to harmonic flow modulations. Measurements are presented for methane–air mixtures at different equivalence ratios 0.8 ≤ ϕ ≤ 1.2 and thermal powers. It is shown that fluctuations of the chemiluminescence signal examined in different wavelength bands, including the OH*, CH* or the entire visible emission bands, always capture the same dynamics. This indicates that heat release rate fluctuations can be deduced without specific filters for the laminar premixed methane–air flames investigated. It is then shown that heat release rate measurements deduced from LIV and chemiluminescence data match well. A proportional relation is found that does not depend on the measurement position, modulation frequency and modulation level for fixed injection conditions. This linear relation slightly depends on the mean flow operating conditions partly due to the difficulty to interpret chemiluminescence emission for rich flames.

Development and validation of a chemical reaction solver coupled to the FLASH code for combustion applications

We report on modifications to the widely used astrophysical code, FLASH (Fryxell, Olson et al., 2000) that enable accurate simulations of chemically reacting flows with heat addition. The enhancements to FLASH include the implementation of extensive hydrogen–air and methane–air chemistry through multiple, detailed mechanisms (Smooke, 1991; Katta and Roquemore, 1995; Mueller, Kim et al., 1999; Billet, 2005), accomplished by building on the existing infrastructure of nuclear reaction network solvers. The chemical reaction network is represented as a system of coupled ODEs, that are then solved either through the Kaps–Rentrop (Rosenbrok) method (Kaps and Rentrop, 1979) or the Bader–Deuflhard method (Bader and Deuflhard, 1983), supplemented by a sparse matrix package for solving linear systems of equations. Furthermore, an existing gamma-law equation of state solver was modified to describe multiple species, each with temperature-dependent properties necessary for realistic simulations of combustion applications. The calculation of temperature-dependent transport properties of constituent species is accomplished through a comprehensive expansion of the materials database. We take advantage of the capability in FLASH to handle the diffusion of heat, mass, and momentum either through an update of the fluxes of each quantity across cell faces, or by directly solving a diffusion equation for each, and the relative merits of each approach for reacting flows are discussed. The capabilities of the modified tool are extensive and in some instances unique, and are documented in detail, along with numerical properties. We also present results from validation of the above capabilities through comparison with analytical solutions, and published numerical and experimental data of chemically reacting flows. Our validation cases include comparison of temporal evolution of species and temperature in a well stirred reactor, comparison of adiabatic flame temperature data, advection of reacting and non-reacting 1D fronts, 2D laminar premixed methane–air flame in a Bunsen burner configuration, shock-driven combustion of an initially circular hydrogen bubble, and a reacting Richtmyer–Meshkov instability not studied previously.

Experimental study and kinetics modeling of partial oxidation reactions in heavily sooting laminar premixed methane flames

The partial oxidation (POX) of methane in heavily sooting laminar premixed methane/oxygen flames was studied with an emphasis on acetylene formation and depletion. The flame temperature profiles were measured with a Pt/Pt–Rh thermocouple coated with Y2O3–BeO ceramic. Gas species along the flame axis were sampled by a quartz probe for their concentrations to be measured by a mass spectrometer. The problem of soot deposition on the sampling probe was overcome by in situ cleaning of the nozzle orifice. The mole ratios of O2/CH4 in the experiments were 0.55, 0.60, 0.65 and the STP (standard temperature and pressure) reactant flow velocity was fixed at 4cm/s. Computational results based on the Curran, Wang–Frenklach and GRI 3.0 detailed chemistry mechanisms were compared with the experimental results. The values predicted by the Curran and Wang-Frenklach mechanisms for the reaction conditions of this study were within the acceptable range. The maximum concentrations of acetylene were positioned in the flame area at 4–8mm distance from the burner, and were behind the positions of the maximum mole fractions of ethane and ethylene. Much more diacetylene and benzene were generated in the post-flame area than in the flame. Recombination reactions to larger hydrocarbon molecules and oxidation with hydroxyl radicals in the post-flame region were the main reactions responsible for acetylene depletion in the fuel rich methane flame.

Experimental and modeling study of the oxidation of n-butylbenzene

New experimental results for the oxidation of n-butylbenzene, a component of diesel fuel, have been obtained using three different devices. A rapid compression machine has been used to measure autoignition delay times after compression at temperatures in the range 640–960K, at pressures from 13 to 23bar, and at equivalence ratios from 0.3 to 0.5. Results show low-temperature behavior, with the appearance of cool flames and a negative temperature coefficient (NTC) region for the richest mixtures. To investigate this reaction at higher temperatures, a shock tube has been used. The shock tube study was performed over a wide range of experimental temperatures, pressures, and equivalence ratios, with air used as the fuel diluent. The ignition temperatures were recorded over the range 980–1740K, at reflected shock pressures of 1, 10, and 30atm. Mixtures were investigated at equivalence ratios of 0.3, 0.5, 1.0 and 2.0 in order to determine the effects of fuel concentration on reactivity over the entire temperature range. Using a jet-stirred reactor, the formation of numerous reaction products has been followed at temperatures from 550 to 1100K, at atmospheric pressure, and at equivalence ratios of 0.25, 1.0, and 2.0. Slight low-temperature reactivity (below 750K) with a NTC region has been observed, especially for the leanest mixtures. A detailed chemical kinetic model has been written based on rules similar to those considered for alkanes by the system EXGAS developed at Nancy. Simulations using this model have been compared to the experimental results presented in this study, but also to results in the literature obtained in a jet-stirred reactor at 10bar, in the same rapid compression machine for stoichiometric mixtures, in a plug flow reactor at 1069K and atmospheric pressure, and in a low-pressure (0.066bar) laminar premixed methane flame doped with n-butylbenzene. The observed agreement is globally better than that obtained with models from the literature. Flow rate and sensitivity analyses have revealed a preponderant role played by the addition to molecular oxygen of resonantly stabilized, 4-phenylbut-4-yl radicals.

Comprehensive Theoretical and Experimental Studies on the CF3H Fire‐extinguishing Mechanism

AbstractIn order to clarify the chemical suppression mechanisms of CF3H, experimental and theoretical studies were conducted respectively in this paper. Firstly, the combustion species in low pressure laminar premixed flat methane flames with CF3H addition is measured by synchrotron radiation molecular beam mass spectrometry (SR‐MBMS) experimentally. Fire suppression chemistry of CF3H is investigated by selective detection of combustion radicals and intermediates in experimental process. Secondly, quantum chemistry calculations are performed to calculate the potential energy surfaces (PES) for the CF3H unimolecular dissociation reaction and reactions of CF3H with free radical OH and H at the B3LYP/6‐311++G** and QCISD(T)/6‐311++G** levels. Finally, the chemical suppression mechanism of CF3H was discussed by comparing the theoretical calculation with experimental measurement.

Sodium and Potassium Released from Burning Particles of Brown Coal and Pine Wood in a Laminar Premixed Methane Flame Using Quantitative Laser-Induced Breakdown Spectroscopy

A quantitative point measurement of total sodium ([Na](total)) and potassium ([K](total)) in the plume of a burning particle of Australian Loy Yang brown coal (23 ± 3 mg) and of pine wood pellets (63 ± 3 mg) was performed using laser-induced breakdown spectroscopy (LIBS) in a laminar premixed methane flame at equivalence ratios ( U ) of 1.149 and 1.336. Calibration was performed using atomic sodium or potassium generated by evaporation of droplets of sodium sulfite (Na(2)SO(3)) or potassium sulfate (K(2)SO(4)) solutions seeded into the flame. The calibration compensated for the absorption by atomic alkalis in the seeded flame, which is significant at high concentrations of solution. This allowed quantitative measurements of sodium (Na) and potassium (K) released into the flame during the three phases of combustion, namely devolatilization, char, and ash cooking. The [Na](total) in the plume released from the combustion of pine wood pellets during the devolatilization was found to reach up to 13 ppm. The maximum concentration of total sodium ([Na](max)M(total)) and potassium ([K](max)(total)) released during the char phase of burning coal particles for φ = 1.149 was found to be 9.27 and 5.90 ppm, respectively. The [Na](max)(total) and [K](max)(total) released during the char phase of burning wood particles for φ = 1.149 was found to be 15.1 and 45.3 ppm, respectively. For the case of φ = 1.336, the [Na](max)(total) and [K](max)(total) were found to be 13.9 and 6.67 ppm during the char phase from burning coal particles, respectively, and 21.1 and 39.7 ppm, respectively, from burning wood particles. The concentration of alkali species was higher during the ash phase. The limit of detection (LOD) of sodium and potassium with LIBS in the present arrangement was estimated to be 29 and 72 ppb, respectively.

Impact of Turbulence Intensity and Equivalence Ratio on the Burning Rate of Premixed Methane–Air Flames

Direct Numerical Simulations (DNS) have been conducted to study the response of initially laminar spherical premixed methane–air flame kernels to successively higher turbulence intensities at five different equivalence ratios. The numerical experiments include a 16-species/25-step skeletal mechanism for methane oxidation and a multicomponent molecular transport model. Highly turbulent conditions (with integral Reynolds numbers up to 4513) have been accessed. The effect of turbulence on the physical properties of the flame, in particular its consumption speed Sc, which is an interesting measure of the turbulent flame speed ST has been investigated. Local quenching events are increasingly observed for highly turbulent conditions, particularly for lean mixtures. The obtained results qualitatively confirm the expected trend regarding correlations between u′/SL and the consumption speed: Sc first increases, roughly linearly, with u′/SL (low turbulence zone), then levels off (bending zone) before decreasing again (quenching limit) for too intense turbulence. For a fixed value of u′/SL, Sc/SL varies with the mixture equivalence ratio, showing that additional parameters should probably enter phenomenological expressions relating these two quantities.

Modeling the oxidation-induced fragmentation of soot aggregates in laminar flames

While the formation and growth of soot particles has received much attention, the subsequent destruction of the particles is less well understood. Soot particles are destroyed though two parallel processes: oxidation and fragmentation. Oxidation is the removal of mass from particles due to chemical reactions with molecular oxygen and hydroxyl radicals. Fragmentation is the break-up of large aggregates into smaller aggregates. Here, a new model for fragmentation inspired by previous experimental investigations is proposed and formulated within the Hybrid Method of Moments (HMOM). With the formulation, the rate of particle loss due to oxidation is closed, resolving a long-standing problem with the Method of Moments. Less important, secondary unclosed terms are introduced, and models for these terms are proposed. The oxidation and fragmentation models are validated using a set of laminar premixed methane flames and then applied to a series of laminar counterflow diffusion acetylene flames. The role of fragmentation is distinctly different in the two flame types. In the premixed flames, fragmentation only occurs in lean flames with a high oxygen concentration. In the diffusion flames, fragmentation is virtually absent, for soot passes through an OH radical layer and is completely oxidized before reaching high oxygen concentrations.