Extinction of premixed H2/air flames: Chemical kinetics and molecular diffusion effects

Laminar flame speed has traditionally been used for the partial validation of flame kinetics. In most cases, however, its accurate determination requires extensive data processing and/or extrapolations, thus rendering the measurement of this fundamental flame property indirect. Additionally, the presence of flame front instabilities does not conform to the definition of laminar flame speed. This is the case for Le<1 flames, with the most notable example being ultralean H₂/air flames, which develop cellular structures at low strain rates so that determination of laminar flame speeds for such mixtures is not possible. Thus, this low-temperature regime of H₂ oxidation has not been validated systematically in flames. In the present investigation, an alternative/supplemental approach is proposed that includes the experimental determination of extinction strain rates for these flames, and these rates are compared with the predictions of direct numerical simulations. This approach is meaningful for two reasons: (1) Extinction strain rates can be measured directly, as opposed to laminar flame speeds, and (2) while the unstretched lean H₂/air flames are cellular, the stretched ones are not, thus making comparisons between experiment and simulations meaningful. Such comparisons revealed serious discrepancies between experiments and simulations for ultralean H₂/air flames by using four kinetic mechanisms. Additional studies were conducted for lean and near-stoichiometric H₂/air flames diluted with various amounts of N₂. Similarly to the ultralean flames, significant discrepancies between experimental and predicted extinction strain rates were also found. To identify the possible sources of such discrepancies, the effect of uncertainties on the diffusion coefficients was assessed and an improved treatment of diffusion coefficients was advanced and implemented. Under the conditions considered in this study, the sensitivity of diffusion coefficients to the extinction response was found to be significant and, for certain species, greater than that of the kinetic rate constants.     

Characterization of the effects of hydrogen addition in premixed methane/air flames

The use of hydrogenated fuels shows considerable promise for applications in gas turbines and internal combustion engines. In the present work, the effects of hydrogen addition in methane/air flames are investigated using both a laminar flame propagation facility and a high-pressure turbulent flame facility. The aim of this research is to contribute to the characterization of lean methane/hydrogen/air premixed turbulent flames at high pressures, by studying the flame front geometry, the flame surface density and the instantaneous flame front thermal thickness distributions. The experiments and analyses show that a small amount of hydrogen addition in turbulent premixed methane–air flames introduces changes in both instantaneous and average flame characteristics.     

Tubular premixed and diffusion flames: Effect of stretch and curvature

Tubular flames are ideal for the study of stretch and curvature effects on flame structure, extinction, and instabilities. Tubular flames have uniform stretch and curvature and each parameter can be varied independently. Curvature strengthens or weakens preferential diffusion effects on the tubular flame and the strengthening or weakening is proportional to the ratio of the flame thickness to the flame radius. Premixed flames can be studied in the standard tubular burner where a single premixed gas stream flows radially inward to the cylindrical flame surface and products exit as opposed jets. Premixed, diffusion and partially premixed flames can be studied in the opposed tubular flame where opposed radial flows meet at a cylindrical stagnation surface and products exit as opposed jets. The tubular flame flow configurations can be mathematically reduced to a two-point boundary value solution along the single radial coordinate. Non-intrusive measurements of temperature and major species concentrations have been made with laser-induced Raman scattering in an optically accessible tubular burner for both premixed and diffusion flames. The laser measurements of the flame structure are in good agreement with numerical simulations of the tubular flame. Due to the strong enhancement of preferential diffusion effects in tubular flames, the theory-data comparison can be very sensitive to the molecular transport model and the chemical kinetic mechanism. The strengthening or weakening of the tubular flame with curvature can increase or decrease the extinction strain rate of tubular flames. For lean H₂-air mixtures, the tubular flame can have an extinction strain rate many times higher than the corresponding opposed jet flame. More complex cellular tubular flames with highly curved flame cells surrounded by local extinction can be formed under both premixed and non-premixed conditions. In the hydrogen fueled premixed tubular flames, thermal-diffusive flame instabilities result in the formation of a uniform symmetric petal flames far from extinction. In opposed-flow tubular diffusion flames, thermal-diffusive flame instabilities result in cellular flames very close to extinction. Both of these flames are candidates for further study of flame curvature and extinction.     

Extinction studies of near-limit lean premixed syngas/air flames

Extinction studies of weakly-stretched near-limit lean premixed syngas/air flames were conducted in a twin-flame counterflow configuration. Experiments showed that buoyancy-induced natural convection at normal gravity strongly disturbed these flames. In order to validate the simulation, accurate extinction data was obtained at micro-gravity. Experimental data obtained from the 3.6 s micro-gravity drop tower showed that the extinction equivalence ratio increased with the increasing global stretch rate and decreased with the increasing H₂ mole fraction in the fuel. Numerical simulation was conducted with CHEMKIN software using GRI 3.0 and USC-Mech II mechanisms. The predicted extinction limit trend was in agreement with the micro-gravity experimental data. Sensitivity analyses showed that the competition between the main branching reaction H + O₂ ⇔ O + OH and the main termination reaction H + O₂ + M ⇔ HO₂ + M in the H₂/O₂ chemistry determined the extinction limits of the flames. The dominant species for syngas/air flame extinction was the H radical. The key exothermal reaction changed from OH + CO ⇔ H + CO₂ to OH + H₂ ⇔ H + H₂O with the increasing H₂ mole fraction in the fuel. Also, the mass diffusion played a more important role than chemical kinetics in the flame extinction. When the H₂ mass diffusion was suppressed, the reaction zone was pushed to the stagnation plane and the flame became weaker; while H mass diffusion is suppressed, the reaction zone slightly shifted towards the upstream and the flame was slightly strengthened.     

Effects of position and frequency of obstacles on turbulent premixed propagating flames

This paper studies the effects of the number and location of solid obstacles on the rate of propagation of turbulent premixed flames. A vented explosion chamber is constructed where controlled premixed flames are ignited from rest to propagate past grids or baffles plates as well as other solid obstacles strategically positioned in the chamber. Laser Induced Fluorescence (LIF) is used to image OH which is used as an indicator of the reaction zone while pressure transducers are used to obtain pressure-time traces. Single grids or baffle plates located at different distances from the ignition source are tested. Two as well as three baffle plates are also investigated in varying configurations. It is found that while the peak overpressure increases with increasing number of grids or baffle plates, a limit is reached where the pressure starts to decrease. The location of the obstacles is found to have a significant effect on the overpressure and the flame structure. Higher overpressures are obtained when the baffle plates and obstacles are stacked closer together hence not allowing turbulence to decay. LIF images for OH show that the reaction zones become more contorted with increasing number of baffle plates in the flame path.     

Numerical simulations of premixed flame cellular instability for a simple chain-branching model

Two-dimensional direct numerical simulations are performed to investigate the non-linear dynamics of low Lewis number premixed flames, in the context of a two-step chain-branching chemistry model. This consists of a thermally-neutral, but temperature sensitive, chain-branching step which produces intermediates such as radicals and an exothermic, zero activation energy chain-completion step which converts the intermediates into products. Emphasis is on examining the role of intermediates in the flame structure on the cellular instability and in comparing and contrasting with previous one-step chemistry model solutions. When intermediates are present only in small concentrations in the underlying one-dimensional flame structure, the two-step cellular dynamics are qualitatively similar to those of the one-step model, including cell-splitting and re-merging, symmetry breaking bifurcations and formation of asymmetric cells, localized quenching of the flame front and a significant enhancement of the flame speed. However, a higher peak value of the intermediates concentration, corresponding to a more distributed heat release, is shown to have a significant stabilizing effect, e.g., in a domain of fixed transverse size, the fully developed cellular structure and flame speed remain closer to those of the one-dimensional flame.     

Flame brush characteristics and burning velocities of premixed turbulent methane/air Bunsen flames

The flame brush characteristics and turbulent burning velocities of premixed turbulent methane/air flames stabilized on a Bunsen-type burner were studied. Particle image velocimetry and Rayleigh scattering techniques were used to measure the instantaneous velocity and temperature fields, respectively. Experiments were performed at various equivalence ratios and bulk flow velocities from 0.7 to 1.0, and 7.7 to 17.0 m/s, respectively. The total turbulence intensity and turbulent integral length scale were controlled by the perforated plate mounted at different positions upstream of the burner exit. The normalized characteristic flame height and centerline flame brush thickness decreased with increasing equivalence ratio, total turbulence intensity, and longitudinal integral length scale, whereas they increased with increasing bulk flow velocity. The normalized horizontal flame brush thickness increased with increasing axial distance from the burner exit and increasing equivalence ratio. The non-dimensional leading edge and half-burning surface turbulent burning velocities increased with increasing non-dimensional turbulence intensity, and they decreased with increasing non-dimensional bulk flow velocity when other turbulence statistics were kept constant. Results show that the non-dimensional leading edge and half-burning surface turbulent burning velocities increased with increasing non-dimensional longitudinal integral length scale. Two correlations to represent the leading edge and half-burning surface turbulent burning velocities were presented as a function of the equivalence ratio, non-dimensional turbulence intensity, non-dimensional bulk flow velocity, and non-dimensional longitudinal integral length scale. Results show that the half-burning surface turbulent burning velocity normalized by the bulk flow velocity decreased as the normalized characteristic flame height increased.     

Approximating the chemical structure of partially premixed and diffusion counterflow flames using FPI flamelet tabulation

Various strategies have been proposed to tabulate complex chemistry for subsequent introduction into fluid mechanics computations. Some of them are grounded on laminar flame calculations, which are useful to seek out key relations linking a few control parameters with relevant species responses. The objective of this paper is to estimate whether approaches based on premixed flamelets (FPI or FGM) can be extended to partially premixed and diffusion flames. Prototypes of nonpremixed laminar and strained counterflow flames are simulated using fully detailed chemistry. The configuration studied is a jet of methane/air mixture opposed to an air stream. A set of reference flames is then obtained, to which FPI results are compared. By varying the equivalence ratio of the free stream of methane/air mixture, from stoichiometry up to pure methane, premixed, partially premixed, and diffusion flames are analyzed. When the fresh fuel/oxidizer mixture equivalence ratio takes values within the flammability limits, excellent results are obtained with FPI. When this equivalence ratio is outside the flammability limits, diffusive fluxes across isomixture fraction surfaces lead to a departure between the FPI tabulation and the reference detailed chemistry flames. This is associated mainly with the appearance of a double-flame structure, progressively evolving into a single diffusion flame when the fuel side equivalence ratio is further increased. Using an improved flame index to distinguish between premixed and diffusion flame burning, hybrid partially premixed combustion is reproduced from a combination of FPI and diffusion flamelets.     

Onset of cellular instabilities in spherically propagating hydrogen-air premixed laminar flames

Using high-speed Schlieren and Shadow photography, the instabilities of outwardly propagating spherical hydrogen-air flames have been studied in a constant volume combustion bomb. Combustion under different equivalence ratios (0.2 ∼ 1.0), temperatures (298 K ∼ 423 K) and pressures (1.0 bar ∼ 10.0 bar) is visualized. The results show that flames experience both unequal diffusion and/or hydrodynamic instabilities at different stages of propagation. The critical flame radius for such instabilities is measured and correlated to the variations of equivalence ratio, temperature and pressure. Analysis revealed that equivalence ratio affects unequal diffusion instability via varying the Lewis number, Le$Le$; increased temperature can delay both types of instabilities in the majority of tests by promoting combustion rate and changing density ratio; pressure variation has minor effect on unequal diffusion instability but is responsible for enhancing hydrodynamic instability, particularly for stoichiometric and near-stoichiometric flames.     

Propagation of symmetric and non-symmetric premixed flames in narrow channels: Influence of conductive heat-losses

Two-dimensional symmetric and non-symmetric premixed flames propagating in a narrow channel subject to a Poiseuille flow are investigated within the context of a diffusive-thermal model. Attention is focussed on the impact of the flame-wall heat exchange on the structure and stability of flames. It is found that an increase in the heat-losses leads to a discontinuity of the steady-state response curves: for Le < 1 the flame extinguishes inside a finite interval of flow rate values while for Le > 1 the flame cannot exist for flow rates larger or smaller than some critical values.     

Effect of ferrocene addition on sooting limits in laminar premixed ethylene-oxygen-argon flames

The critical sooting C/O ratio was measured for a series of atmospheric-pressure, laminar premixed ethylene-oxygen-argon flames doped with a small amount of ferrocene. Comparisons of the doped and undoped flames show marked increases in sooting tendency for flames doped with ferrocene. Under the same flame conditions, the critical C/O ratios in doped flames were uniformly lower than those of undoped flames. A five-step reaction mechanism of ferrocene decomposition, leading to the formation of the cyclopentadienyl, was proposed. Detailed kinetic modeling of the experimental flames showed little changes in the flame temperature and the aromatic concentration upon ferrocene doping. The experimental and computational results support the early suggestion that in premixed flames the increase in sooting tendency due to ferrocene addition is the result of induced nucleation by iron oxide nanoparticles. These particles provide a surface to initiate soot surface growth.     

Lewis number effect on the propagation of premixed flames in narrow adiabatic channels: Symmetric and non-symmetric flames and their linear stability analysis

The propagation of premixed flames with different Lewis numbers in a planar channel subject to a Poiseuille flow is considered within the diffusive-thermal model for steady and time-dependent cases. It was found that, depending on the Lewis number and the flow rate, symmetric and non-symmetric flames are possible. The existence of multiple steady solutions in cases of the low Lewis number is demonstrated. The time-dependent simulations carried out for high Lewis number flames also showed the symmetric and non-symmetric oscillatory solutions.Linear stability analysis of two-dimensional steady-states was performed using a practical method developed in the paper and applied to calculate the main eigenvalue. It was shown that for symmetric flames with a low Lewis number the increase in the flow rate leads to a loss of stability with subsequent formation of non-symmetric solutions. For flames with a high Lewis number the Poiseuille flow produces a stabilization effect. The results of the stability analysis were successfully compared with the results of direct numerical simulations.     

Flame speed and tangential strain measurements in widely stratified partially premixed flames interacting with grid turbulence

Methane-air partially premixed flames subjected to grid-generated turbulence are stabilized in a two-slot burner with initial fuel concentration differences leading to stratification across the stoichiometric concentration. The fuel concentration gradient at the location corresponding to the flame base is measured using planar laser induced fluorescence (PLIF) of acetone in the non-reacting mixing field. Simultaneous PLIF of the OH radical and particle image velocimetry (PIV) measurements are performed to deduce the flow velocity and the flame front. These flames exhibit a convex premixed flame front and a trailing diffusion flame, with flow divergence upstream of the flame, as indicated by the instantaneous OH–PLIF, Mie scattering images, and PIV data. The mean streamwise velocity profile attains a global minimum just upstream of the flame front due to expansion of a gases caused by heat release. The flame speed measured just upstream of the flame leading edge is normalized with respect to the turbulent stoichiometric flame speed that takes into account variations in turbulent intensity and integral length scale. The turbulent edge flame speed exceeds the corresponding stoichiometric premixed flame speed and reaches a peak at a certain concentration gradient. The mean tangential strain at the flame leading edge locally peaks at the concentration gradient corresponding to the peak flame speed. The strain varies non-monotonically with the flame curvature unlike in a non-stratified curved premixed flame. The mechanism of peak flame speed is explained as the competition between availability of hot excess reactants from the premixed flame branches to the flame stretch induced due to flame curvature. The results suggest that the stabilization of lifted turbulent partially premixed flames occurs through an edge flame even at a relatively gentle concentration gradient. The strain is also evaluated along the flame front; it peaks at the flame leading edge and decreases gradually on either side of the leading edge. The present results also show qualitatively similar trends as those of laminar triple flames.     

Tabulation of complex chemistry based on self-similar behavior of laminar premixed flames

Detailed mechanisms describing complex phenomena of combustion chemistry, such as flame propagation or pollutant formation, involve hundreds of species and thousands of elementary reactions and cannot be handled in practical simulations of turbulent combustion. A widely used way to reduce chemistry is to build look-up tables where chemical parameters such as reaction rates and/or species mass fractions are determined from a reduced set of coordinates (ILDM, FPI, or FGM methods). Nevertheless, these tables may require large memory spaces and nonnegligible access times, especially when running on massively parallel computers. In this work, the self-similarity behavior of laminar premixed flames is first put into evidence and then theoretically sustained. This property provides a way to reduce the size of chemical databases, especially for computations on massively parallel machines, under the FPI (flame prolongation of ILDM) framework. The database is reduced to similarity profiles for the species reaction rates (or the species mass fractions), stored together with scaling rules. This new formulation is then implemented in the PREMIX code and numerical simulations of laminar premixed flames successfully compare with full chemistry computation, validating this promising approach.     

A computational study of combustion and extinction of opposed-jet syngas diffusion flames

This paper reports a numerical study on the combustion and extinction characteristics of opposed-jet syngas diffusion flames. A model of one-dimensional counterflow syngas diffusion flames was constructed with constant strain rate formulations, which used detailed chemical kinetics and thermal and transport properties with flame radiation calculated by statistic narrowband radiation model. Detailed flame structures, species production rates and net reaction rates of key chemical reaction steps were analyzed. The effects of syngas compositions, dilution gases and pressures on the flame structures and extinction limits of H₂/CO synthetic mixture flames were discussed. Results indicate the flame structures and flame extinction are impacted by the compositions of syngas mixture significantly. From H₂-enriched syngas to CO-enriched syngas fuels, the dominant chain reactions are shifting from OH + H₂→H + H₂O for H₂O production to OH + CO→H + CO₂ for CO₂ production through the key chain-branching reaction of H + O₂→O + OH. Flame temperature increases with increasing hydrogen content and pressure, but the flame thickness is decreased with pressure. Besides, the study of the dilution effects from CO₂, N₂, and H₂O, showed the maximum flame temperature is decreased the most with CO₂ as the dilution gas, while CO-enriched syngas flames with H₂O dilution has highest maximum flame temperature when extinction occurs due to the competitions of chemical effect and radiation effect. Finally, extinction limits were obtained with minimum hydrogen percentage as the index at different pressures, which provides a fundamental understanding of syngas combustion and applications.     

Flame chemiluminescence in premixed combustion of hydrogen-enriched fuels

The growing interest in the use of syngas, produced from gasification of a number of fuels such as coal, biomass or waste, has increased the need of developing new strategies of flame control and monitoring in order to ensure a quiet and efficient combustion in the current combustion systems, which had been optimized for conventional fuels.     

Effects of premixed flames on turbulence and turbulent scalar transport

Experimental data and results of direct numerical simulations are reviewed in order to show that premixed combustion can change the basic characteristics of a fluctuating velocity field (the so-called flame-generated turbulence) and the direction of scalar fluxes (the so-called countergradient or pressure-driven transport) in a turbulent flow. Various approaches to modeling these phenomena are discussed and the lack of a well-elaborated and widely validated predictive approach is emphasized. Relevant basic issues (the transition from gradient to countergradient scalar transport, the role played by flame-generated turbulence in the combustion rate, the characterization of turbulence in premixed flames, etc.) are critically considered and certain widely accepted concepts are disputed. Despite the substantial progress made in understanding the discussed effects over the past decades, these basic issues strongly need further research.