An experimental and numerical study on characteristics of laminar premixed H2/CO/CH4/air flames

The effects of variations in the fuel composition on the characteristics of H₂/CO/CH₄/air flames of gasified biomass are investigated experimentally and numerically. Experimental measurements and numerical simulations of the flame front position and temperature are performed in the premixed stoichiometric H₂/CO/CH₄/air opposed-jet flames with various H₂ and CO contents in the fuel. The adiabatic flame temperatures and laminar burning velocities are calculated using the EQUIL and PREMIX codes of Chemkin collection 3.5, respectively. Whereas the flame structures of the laminar premixed stoichiometric H₂/CO/CH₄/air opposed-jet flames are simulated using the OPPDIF package with the GRI-Mech 3.0 chemical kinetic mechanisms and detailed transport properties. The measured flame front position and temperature of the stoichiometric H₂/CO/CH₄/air opposed-jet flames are closely predicted by the numerical calculations. Detailed analysis of the calculated chemical kinetic structures reveals that the reaction rate of reactions (R38), (R46), and (R84) increase with increasing H₂ content in the fuel mixture. It is also found that the increase in the laminar flame speed with H₂ addition is most likely due to an increase in active radicals during combustion (chemical effect), rather than from changes in the adiabatic flame temperature (thermal effect). Chemical kinetic structure and sensitivity analyses indicate that for the stoichiometric H₂/CO/CH₄/air flames with fixed H₂ concentration in the fuel mixture, the reactions (R99) and (R46) play a dominant role in affecting the laminar burning velocity as the CO content in the fuel is increased.     

Pressure dependence of NO formation in laminar fuel-rich premixed CH4/air flames

Effects of pressure on NO formation in CH₄/air flames at a fixed equivalence ratio of 1.3 are investigated. The axial profiles of temperature, OH, CH, and NO mole fractions are measured using laser-induced fluorescence and compared with one-dimensional flame calculations. The measured and calculated temperature, CH, and NO profiles in free flames are observed to vary upon increasing the pressure from 40 to 75 Torr, following a scaling law derived for a chemical mechanism containing only second-order reactions. At pressures 300–760 Torr, the measurements and calculations in burner-stabilized flames show increasing flame temperature and NO mole fractions when the mass flux is increased linearly with pressure, while the CH profiles remain unchanged. The observed deviation from the scaling law in the temperature profiles arises from the increasing contribution of three-body reactions to the flame front propagation velocity, leading to a decrease in the degree of burner stabilization. The deviation from the pressure scaling law for the NO mole fractions is due to the temperature dependence of the rate coefficient for the reaction between CH and N₂ and the fact that the temperature profiles themselves do not scale. In contrast, the surprisingly good scaling of the CH mole fractions with pressure indicates the dominant role of two-body reactions participating in the chain of chemical reactions leading to CH formation. The calculations using GRI-Mech 3.0 substantially overpredict (up to 50%) the measured nitric oxide concentrations for all pressures studied. The observed differences in the NO mole fraction may be addressed by improving the CH prediction.     

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.     

Laminar flame speed studies of lean premixed H2/CO/air flames

Laminar flame speeds of lean premixed H₂/CO/air mixtures were measured in the counterflow configuration over a wide range of H₂ content at lean conditions. The values were determined by extrapolating the referenced flame speed to zero stretch rate using the non-linear extrapolation method to reduce the systematic error. Detailed calculation of laminar flame speed was also conducted using PREMIX code coupled with three different kinetic models. In general, simulation results agreed well with the experimental data. Both the experimental and calculation results revealed that the laminar flame speeds of lean premixed H₂/CO/air mixtures increased with H₂ content significantly when H₂ content was small (?15%) and gradually when H₂ content was large (>15%).     

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.     

Influence of H2 on the response of lean premixed CH4 flames to high strained flows

A combined experimental and numerical investigation on the effects of H₂ addition to lean-premixed CH₄ flames in highly strained counterflow fields (with strain rates up to 8000 s⁻¹) using preheated flows indicate significant enhancement of lean flammability limits and extinction strain rates for relatively small amounts of H₂ addition. Numerical modeling of the counterflow opposed jet configuration used in this study indicated extinction strain rates which were within 5% of experimentally measured values for equivalence ratios ranging from 0.75 to less than 0.4. Both experimental and numerical results indicate that increasing H₂ in the fuel significantly increases flame speeds and thus extinction strain rates. Furthermore, increasing H₂ decreases the dependency of extinction equivalence ratio on the strain rate of the flow. For all of the mixtures investigated, extinction temperatures depend primarily on equivalence ratio and not fuel composition for the range of H₂ content studied, which suggests that extinction can be correlated to flame temperature and O₂ concentration. Nonetheless, H₂ addition greatly increases the maximum allowable strain rate before extinction temperatures are reached. Inspection of the model-predicted species profiles suggest that the enhancement of CH₄ burning rates with H₂ addition is driven by early H₂ breakdown increasing radical production rates early in the flame zone to enhance CH₄ ignition under conditions where otherwise CH₄ combustion might be prone to undergo extinction.     

Studies on properties of laminar premixed hydrogen-added ammonia/air flames for hydrogen production

In order to evaluate the potential of burning and reforming ammonia as a carbon-free fuel in production of hydrogen, fundamental unstretched laminar burning velocities, and flame response to stretch (represented by the Markstein number) for laminar premixed hydrogen-added ammonia/air flames were studied both experimentally and computationally. Freely (outwardly)-propagating spherical laminar premixed flames at normal temperature and pressure were considered for a wide range of global fuel-equivalence ratios, flame stretch rates (represented by the Karlovitz number) and the extent of hydrogen substitution. Results show the substantial increase of laminar burning velocities with hydrogen substitution, particularly under fuel-rich conditions. Also, predicted flame structures show that the hydrogen substitution enhances nitrogen oxide (NO_x) and nitrous oxide (N₂O) formation. At fuel-rich conditions, however, the amount of NO_x and N₂O emissions and the extent of the increase with the hydrogen substitution are much lower than those under fuel-lean conditions. These observations support the potential of hydrogen as an additive for improving the burning performance with low NO_x and N₂O emissions in fuel-rich ammonia/air flames and hence the potential of using ammonia as a clean fuel. Increasing the amount of added hydrogen tends to enhance flame sensitivity to stretch.     

Effects of hydrogen addition on soot formation and oxidation in laminar premixed C2H2/air flames

In an effort to elucidate the influence of hydrogen addition on soot formation and oxidation, a series of numerical investigations was performed for fuel rich laminar C₂H₂/air premixed flat flames using a modified CHEMKIN-II PREMIX code with a detailed soot chemistry mechanism. To clarify the influence of hydrogen addition, the hydrogen content (in volume %) in the fuel mixture was gradually increased from 10 to 50%. The hydrogen addition was found to slow the oxidation of C₂H₂ near the burner surface. The lowered rate of C₂H₂ oxidation coupled with lower C₂H₂ concentration near the burner surface impedes the formation of benzene. However, the formation of benzene was enhanced with the hydrogen addition as the height above burner (HAB) was increased. This was due to the increased reverse rate of the H abstraction reaction that prevents the radical formation process. Through the identical mechanism, the hydrogen addition slows further growth of benzene to larger polycyclic aromatic hydrocarbons (PAHs), eventually lowering the rate of particle inception. Numerical results also indicated that reductions in the soot emissions were mainly attributed to a significant reduction in the mass growth of soot particles. The abundance of hydrogen in the flames deactivated the surface site of soot particles covered with C-H bonds, lowering the surface growth rate (which leads to reductions in the mass growth of soot particles).     

Counter-gradient transport in the combustion of a premixed CH4/air annular jet by combined PIV/OH-LIF

A combination of PIV/OH laser induced fluorescence technique is used to measure the conditional - burned and unburned - gas velocity in a turbulent premixed CH₄/air annular bluff-body stabilized burner. By changing the equivalence ratio from lean to almost stoichiometric, the energy budget of the recirculating region anchoring the flame is altered in such a way to increasingly lift the flame away from the jet exit. The overall turbulence intensity interacting with each flame is thus systematically varied in a significant range, allowing for a parametric study of its effect on turbulent scalar transport under well controlled conditions, always well within the flamelet regime. The component of the flux normal to the average front is found to reverse its direction, confirming the Bray number as a good indicator of gradient/counter-gradient behavior, once the actual incoming turbulence level felt locally by the flame is assumed as the proper control parameter.     

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.     

NOx emission characteristics of partially premixed laminar flames of H2/CO/CO2 mixtures

NO_x emission indices were experimentally measured for partially premixed laminar flames of five different H₂/CO/CO₂ fuels over a wide range of equivalence ratios. Through those fuels, the effects of H₂/CO ratio and CO₂ concentration on NO_x emissions, flame appearance, visible flame height and flame temperature are presented. EINO_x values increase when 1.0 ≤ Φ ≤ 1.6, then remain near the highest value, before decreasing slowly when 3.85 ≤ Φ ≤ ∞. The increase of the CO₂ concentration reduces the EINO_x for the whole range of equivalence ratios, while the increase in the H₂/CO ratio reduces the EINO_x when Φ ≤ 2.0 and is inconsequential for richer mixtures. The variation in flame temperatures approximates EINO_x trends. The variation of flame color from blue to orange when the H₂/CO ratio is increased might be explained by higher CO levels in by-product combustion.     

Effects of hydrogen peroxide on combustion enhancement of premixed methane/air flames

Hydrogen peroxide is generally considered to be an effective combustion promoter for different fuels. The effects of hydrogen peroxide on the combustion enhancement of premixed methane/air flames are investigated numerically using the PREMIX code of Chemkin collection 3.5 with the GRI-Mech 3.0 chemical kinetic mechanisms and detailed transport properties. To study into the enhancement behavior, hydrogen peroxide is used for two different conditions: (1) as the oxidizer substituent by partial replacement of air and (2) as the oxidizer supplier by using different concentrations of H₂O₂. Results show that the laminar burning velocity and adiabatic flame temperature of methane flame are significantly enhanced with H₂O₂ addition. Besides, the addition of H₂O₂ increases the CH₄ consumption rate and CO production rate, but reduces CO₂ productions. Nevertheless, using a lower volumetric concentration of H₂O₂ as an oxidizer is prone to reduce CO formation. The OH concentration is increased with increasing H₂O₂ addition due to apparent shifting of major reaction pathways. The increase of OH concentration significantly enhances the reaction rate leading to enhanced laminar burning velocity and combustion. As to NO emission, using H₂O₂ as an oxidizer will never produce NO, but NO emission will increase due to enhanced flame temperature when air is partially replaced by H₂O_2.     

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.     

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.     

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.     

A numerical study on NOx formation in laminar counterflow CH4/air triple flames

Formation of NO_x in counterflow methane/air triple flames at atmospheric pressure was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. Results indicate that in a triple flame, the appearance of the diffusion flame branch and the interaction between the diffusion flame branch and the premixed flame branches can significantly affect the formation of NO_x, compared to the corresponding premixed flames. A triple flame produces more NO and NO₂ than the corresponding premixed flames due to the appearance of the diffusion flame branch where NO is mainly produced by the thermal mechanism. The contribution of the N₂O intermediate route to the total NO production in a triple flame is much smaller than those of the thermal and prompt routes. The variation in the equivalence ratio of the lean or rich premixed mixture affects the amount of NO formation in a triple flame. The interaction between the diffusion and the premixed flame branches causes the NO and NO₂ formation in a triple flame to be higher than in the corresponding premixed flames, not only in the diffusion flame branch region but also in the premixed flame branch regions. However, this interaction reduces the N₂O formation in a triple flame to a certain extent. The interaction is caused by the heat transfer and the radical diffusion from the diffusion flame branch to the premixed flame branches. With the decrease in the distance between the diffusion flame branch and the premixed flame branches, the interaction is intensified.     

Numerical investigation of the effects of H2/CO/syngas additions on laminar premixed combustion characteristics of NH3/air flame

Co-firing NH₃ with H₂/CO/syngas (SYN) is a promising method to overcome the low reactivity of NH₃/air flame. Hence, this study aims to systematically investigate the laminar premixed combustion characteristics of NH₃/air flame with various H₂/CO/SYN addition loadings (0–40%) using chemical kinetics simulation. The numerical results were obtained based on the Han mechanism which can provide accurate predictions of laminar burning velocities. Results showed that H₂ has the greatest effects on increasing laminar burning velocities and net heat release rates of NH₃/air flame, followed by SYN and CO. CO has the most significant effects on improving NH₃/air adiabatic flame temperatures. The H₂/CO/SYN additions can accelerate NH₃ decomposition rates and promote the generation of H and NH₂ radicals. Furthermore, there is an evident positive linear correlation between the laminar burning velocities and the peak mole fraction of H + NH₂ radicals. The reaction NH₂ + NH <=> N₂H₂ + H and NH₂ + NO <=> NNH + OH have remarkable positive effects on NH₃ combustion. The mole fraction of OH × NH₂ radicals positively affects the net heat release rates. Finally, it was discovered that H radicals play an important role in the generation of NO. The H₂/CO/SYN additions can reduce the hydrodynamic and diffusional-thermal instabilities of NH₃/air flame. The NH₃ reaction pathways for NH₃–H₂/CO/SYN-air flames can be categorized mainly into NH₃–NH₂–NH–N–N₂, NH₃–NH₂–HNO–NO(?N₂O)–N₂ and NH₃–NH₂(?N₂H₂)–NNH–N₂. CO has the greatest influence on the proportions of three NH₃ reaction routes.     

Multidimensional mathematical modeling and numerical investigation of co-flow partially premixed methane/air laminar flames

The goal of this paper is to analyze, by means of detailed numerical simulations, the influence of the partial premixing level and the adequacy of different mathematical submodels on the modeling of co-flow partially premixed methane-air laminar flames. Five levels of premixing of the primary inlet are considered from an equivalence ratio of Φ=∞ (non-premixed flame) to Φ=2.464. Main flame properties are provided, giving special emphasis to the analysis of pollutant formation. Different mathematical formulation aspects (several chemical mechanisms, radiation effects, mass transport models, and inlet boundary conditions) are tested and validated against experimental data available in the literature. Finite volume techniques over staggered grids are used to discretize the governing equations. A parallel multiblock algorithm based on domain decomposition techniques running with loosely coupled computers has been used to obtain a competitive ratio between computational cost and resources. To assess the quality of the numerical solutions presented in this article, a verification process based on the generalized Richardson extrapolation technique and on the grid convergence index (GCI) has been applied.