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.     

Investigation of turbulent premixed methane/air and hydrogen-enriched methane/air flames in a laboratory-scale gas turbine model combustor

Methane and hydrogen-enriched (25 vol% and 50 vol% H₂-enriched CH₄) methane/air premixed flames were investigated in a gas turbine model combustor under atmospheric conditions. The flame operability ranges were mapped at different Reynold numbers (Re), showing the dependence on Re and H₂ concentrations. The effects of equivalence ratio (Φ), Re, and H₂ enrichment on flame structure were examined employing OH-PLIF measurement. For CH₄/air cases, the flame was stabilized with an M shape; while for H₂-enriched cases, the flame transitions to a П shape above a specific Φ. This transition was observed to influence significantly the flashback limits. The flame shape transition is most likely a result of H₂ enrichment, occurring due to the increase in flame speed, higher resistance of the flame to the strain rate, and change in the inner recirculation zone. Flow fields of CH₄/air flames were compared between low and high Re cases employing high-speed PIV. The flashback events, led by two mechanisms (combustion-induced vortex breakdown, CIVB, and boundary-layer flashback, BLF), were observed and recorded using high-speed OH chemiluminescence imaging. It was found that the CIVB flashback occurred only for CH₄ flames with M shape, whereas the BLF occurs for all H₂-enriched flames with П shape.     

Onset of cellular instability in adiabatic H2/O2/N2 premixed flames anchored to a flat-flame heat-flux burner

The onset of cellular instability in adiabatic H₂/O₂/N₂ premixed flames anchored to a heat-flux burner is investigated numerically. Both hydrodynamic instability and diffusional-thermal instability are shown to play an important role in the onset of cellular flames. The burner can effectively suppress cellular instability when the flames are close to the burner, otherwise the burner can suppress the instabilities only at large wavenumbers. Because of differential diffusion, local extinction can occur in lean H₂/O₂/N₂ flames. When the flames develop to take on cellular shapes, the surface length, the overall heat release rate and the mean burning velocity are all increased. For near stoichiometric fuel-rich flames the mean burning velocity can increase by as much as 20%–30%. For lean flames with an equivalence ratio of 0.56, the mean burning velocity can be 2–3 times of the burning velocity of the corresponding planar flame.     

Effects of ammonia substitution on extinction limits and structure of counterflow nonpremixed hydrogen/air flames

The potential of partial ammonia substitution to improve the safety of hydrogen use was evaluated computationally, using counterflow nonpremixed ammonia/hydrogen/air flames at normal temperature and pressure. The ammonia-substituted hydrogen/air flames were considered using a recent kinetic mechanism and a statistical narrow-band radiation model for a wide range of flame strain rates and the extent of ammonia substitution. The effects of ammonia substitution on the extinction limits and structure, including nitrogen oxide (NO_x) and nitrous oxide (N₂O) emissions, of nonpremixed hydrogen/air flames were investigated. Results show reduction of the high-stretch extinction (i.e., blow-off) limits, the maximum flame temperature and the concentration of light radicals (e.g., H and OH) with ammonia substitution in hydrogen/air flames, supporting the potential of ammonia as a carbon-free, clean additive for improving the safety of hydrogen use in nonpremixed hydrogen/air flames. For high-stretched flames, however, NO_x and N₂O emissions substantially increase with ammonia substitution even though ammonia substitution reduces flame temperature, implying that chemical effects (rather than thermal effects) of ammonia substitution on flame structure are dominant. Radiation effects on the extinction limits and flame structure are not remarkable particularly for high-stretched flames.     

Effect of hydrogen ratio on turbulent flame structure of oxyfuel syngas at high pressure up to 1.0 MPa

The CO/H₂/CO₂/O₂, CO/H₂/CO₂/air turbulent premixed flames as the model of syngas oxyfuel and syngas/air combustion were studied experimentally and compared to that of CH₄/air mixtures at high pressures up to 1.0 MPa. Hydrogen ratio in syngas was set to be 35%, 50% and 65% in volumetric fraction. Four perforated plates are used to generate wide range of turbulence intensity and scales. The instantaneous flame structure was measured with OH-PLIF technique and then statistic flame structure parameters and turbulent burning velocity were derived to interpret the multi scale turbulence-flame interaction. Results show that the flame structure of syngas is wrinkled and convex cusps to the unburned mixtures are sharper and deeper comparing to that of CH₄ flames. Pressure has a dominating effect on flame wrinkling other than mixtures composition at high pressure of 1.0 MPa. The flame surface density, Σ of syngas is larger than that of CH₄. The Σ of syngas flames is almost independent on pressure and hydrogen ratio especially when hydrogen ratio is over 50% which is a significant feature of syngas combustion. Larger flame surface density for syngas flames mainly comes from the finer structure with smaller wrinkles which is the result of more intensive flame intrinsic instability. The S_T/S_L of syngas is larger than CH₄ and it slightly increases with the pressure rise. The S_T/S_L of syngas oxyfuel is similar to that of syngas/air flames in the present study. The S_T/S_L increases with the increase of hydrogen ratio and keeps almost constant when hydrogen ratio is over 50%.     

Effects of CO addition on the characteristics of laminar premixed CH4/air opposed-jet flames

The effects of CO addition on the characteristics of premixed CH₄/air opposed-jet flames are investigated experimentally and numerically. Experimental measurements and numerical simulations of the flame front position, temperature, and velocity are performed in stoichiometric CH₄/CO/air opposed-jet flames with various CO contents in the fuel. Thermocouple is used for the determination of flame temperature, velocity measurement is made using particle image velocimetry (PIV), and the flame front position is measured by direct photograph as well as with laser-induced predissociative fluorescence (LIPF) of OH imaging techniques. The laminar burning velocity is calculated using the PREMIX code of Chemkin collection 3.5. The flame structures of the premixed stoichiometric CH₄/CO/air opposed-jet flames are simulated using the OPPDIF package with GRI-Mech 3.0 chemical kinetic mechanisms and detailed transport properties. The measured flame front position, temperature, and velocity of the stoichiometric CH₄/CO/air flames are closely predicted by the numerical calculations. Detailed analysis of the calculated chemical kinetic structures reveals that as the CO content in the fuel is increased from 0% to 80%, CO oxidation (R99) increases significantly and contributes to a significant level of heat-release rate. It is also shown that the laminar burning velocity reaches a maximum value (57.5 cm/s) at the condition of 80% of CO in the fuel. Based on the results of sensitivity analysis, the chemistry of CO consumption shifts to the dry oxidation kinetics when CO content is further increased over 80%. Comparison between the results of computed laminar burning velocity, flame temperature, CO consumption rate, and sensitivity analysis reveals that the effect of CO addition on the laminar burning velocity of the stoichiometric CH₄/CO/air flames is due mostly to the transition of the dominant chemical kinetic steps.     

Adiabatic burning velocity of H2–O2 mixtures diluted with CO2/N2/Ar

Global warming due to CO₂ emissions has led to the projection of hydrogen as an important fuel for future. A lot of research has been going on to design combustion appliances for hydrogen as fuel. This has necessitated fundamental research on combustion characteristics of hydrogen fuel. In this work, a combination of experiments and computational simulations was employed to study the effects of diluents (CO₂, N₂, and Ar) on the laminar burning velocity of premixed hydrogen/oxygen flames using the heat flux method. The experiments were conducted to measure laminar burning velocity for a range of equivalence ratios at atmospheric pressure and temperature (300 K) with reactant mixtures containing varying concentrations of CO₂, N₂, and Ar as diluents. Measured burning velocities were compared with computed results obtained from one-dimensional laminar premixed flame code PREMIX with detailed chemical kinetics and good agreement was obtained. The effectiveness of diluents in reduction of laminar burning velocity for a given diluent concentration is in the increasing order of argon, nitrogen, carbon dioxide. This may be due to increased capabilities either to quench the reaction zone by increased specific heat or due to reduced transport rates. The lean and stoichiometric H₂/O₂/CO₂ flames with 65% CO₂ dilution exhibited cellular flame structures. Detailed three-dimensional simulation was performed to understand lean H₂/O₂/CO₂ cellular flame structure and cell count from computed flame matched well with the experimental cellular flame.     

Effect of high hydrogen enrichment on the outer-shear-layer flame of confined lean premixed CH4/H2/air swirl flames

In this study, we investigated the H₂-induced transition of confined swirl flames from the “V” to “M” shape. H₂-enriched lean premixed CH₄/H₂/air flames with H₂ fractions up to 80% were conducted. The flame structure was obtained with Planar Laser-Induced Fluorescence (PLIF) of the OH radical. Flow fields were measured with Particle Image Velocimetry (PIV). It was observed that the flame tip in the outer shear layer gradually propagated upstream and finally anchored to the injector with the hydrogen fractions increase, yielding the transition from the “V” to “M” flame. We examined the flame structures and the flame flow dynamics during the transition. The shape transition was directly related to the evolution of the corner flame along the outer shear layer. With H₂ addition, the outer recirculation zone first appeared downstream where the corner flame started to propagate upstream; then, the recirculation zone expanded upward to form a stable “M” flame gradually. The flow straining was observed to influence the stabilization of the outer shear layer flame significantly. This study can be useful for the understanding of recirculation-stabilized swirling flames with strong confinement. The flame structure and the flow characteristics of flames with a high H₂ content are also valuable for model validation.     

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.     

Laminar burning velocities and flame instabilities of diluted H2/CO/air mixtures under different hydrogen fractions

An experimental study was conducted using outwardly propagating flame to evaluate the laminar burning velocity and flame intrinsic instability of diluted H₂/CO/air mixtures. The laminar burning velocity of H₂/CO/air mixtures diluted with CO₂ and N₂ was measured at lean equivalence ratios with different dilution fractions and hydrogen fractions at 0.1 MPa; two fitting formulas are proposed to express the laminar burning velocity in our experimental scope. The flame instability was evaluated for diluted H₂/CO/air mixtures under different hydrogen fractions at 0.3 MPa and room temperature. As the H₂ fraction in H₂/CO mixtures was more than 50%, the flame became more unstable with the decrease in equivalence ratio; however, the flame became more stable with the decrease in equivalence ratio when the hydrogen fraction was low. The flame instability of 70%H₂/30%CO premixed flames hardly changed with increasing dilution fraction. However, the flames became more stable with increasing dilution fraction for 30%H₂/70%CO premixed flames. The variation in cellular instability was analyzed, and the effects of hydrogen fraction, equivalence ratio, and dilution fraction on diffusive-thermal and hydrodynamic instabilities were discussed.     

Flame front evolution and laminar flame parameter evaluation of buoyancy-affected ammonia/air flames

For flames with very low burning speed, the flame propagation is affected by buoyancy. Flame front evolution and laminar flame parameter evaluation methods of buoyancy-affected flame have been proposed. The evolution and propagation process of a center ignited expanding ammonia/air flame has been analyzed by using the methods. The laminar flame parameters of ammonia/air mixture under different equivalence ratio (ER) and initial pressure have been studied. At barometric pressure, with the increase of ER, the laminar burning velocity (LBV) of ammonia/air mixture undergoes a first increase and then decrease process and reaches its maximum value of 7.17 cm/s at the ER of 1.1, while the Markstein length increases monotonously. For ammonia/air flames with ER less than unity, the flame velocity shows a decreasing trend with stretch rate, resulting in the propensity to flame instability, but no cellular structure was observed in the process of flame propagation. As the initial pressure increases, the LBV decreases monotonously as well as the Markstein length. The flame thicknesses of ammonia/air mixtures decrease with initial pressure and are much thicker than those of hydrogen flames, which makes a stronger stabilizing effect of curvature on the flame front. The most enhancement of LBV is contributed by the dehydrogenation reaction of NH₃ with OH. The NO concentration decreases significantly with the increase of ER.     

Piloted jet flames of CH4/H2/air: Experiments on localized extinction in the near field at high Reynolds numbers

Measurements of temperature and major species concentrations, based on the simultaneous line-imaged Raman/Rayleigh/CO-LIF technique, are reported for piloted jet flames of CH₄/H₂ fuel with varying amounts of partial premixing with air (jet equivalence ratios of ?_j = 3.2, 2.5, 2.1 corresponding to stoichiometric mixture fraction values of ξ_st = 0.35, 0.43, 0.50, respectively) and varying degrees of localized extinction. Each jet flame is operated at a fixed and relatively high exit Reynolds number (60,000 or 67,000), and the probability of localized extinction is increased in several steps by progressively decreasing the flow rate of the pilot flame. Dimensions of the piloted burner, originally developed at Sydney University, are the same as for previous studies. The present measurements complement previous results from piloted CH₄/air jet flames as targets for combustion model calculations by extending to higher Reynolds number, including more steps in the progression of each flame from a fully burning state to a flame with high probability of local extinction, and adding the degree of partial premixing as an experimental parameter. Local extinction in these flames occurs close to the nozzle near a downstream location of four times the jet exit diameter. Consequently, these data provide the additional modeling challenge of accurately representing the initial development of the reacting jet and the near-field mixing processes.     

Effects of different preheated CO2/O2 jet in cross-flow on combustion instability and emissions in a lean-premixed combustor

To reveal the suppression mechanism of thermoacoustic instability flames under CO₂/O₂ jet in cross flow. Experiments on the effects of different preheated CO₂/O₂ jet in cross-flow (JICF) on combustion instability and NO_x emissions in a lean-premixed combustor were conducted in a model gas turbine combustor. Two variables of the JICF were investigated—the flow rate and the temperature. Results indicate that combustion instability and NOx emissions could be suppressed when the JICF flow rate increases from 1 to 5 L/min. The average pressure amplitude decreases from 18.6 Pa to 1.6 Pa, and the average NOx emission decreases from 26.4 ppm to 12.1 ppm. But the average pressures amplitude and NOx emissions increase as the JICF temperature grows up. The sound pressure and the flame heat release rate exhibits different mode-shifting characteristics. The oscillation frequency of the sound pressure almost unchanged under JICF injection. However, the oscillation frequency of the heat release rate jumps from 95 Hz to 275 Hz under different JICF temperatures. As the CO₂/O₂ JICF flow rate arrived 3 L/min, the oscillation frequency of flame heat release rate jumps from 85 Hz to 265 Hz. The color of the flame fronts and roots were changed by the JICF injection. The average length of flame under CO₂/O₂ JICF cases is shorter than the N₂/O₂ JICF cases. There are three different modes of flames when the CO₂/O₂ JICF flow rate varies, and two different modes of flames when the CO₂/O₂ JICF temperature varies. This article explored the joint effects of different CO₂/O₂ or N₂/O₂ JICF on combustion instability and NOx emissions, which could be instructive to the designing of safely and clean combustors in industrial gas turbines.     

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.     

Effects of ammonia substitution on hydrogen/air flame propagation and emissions

In order to evaluate the potential of partial ammonia substitution to improve the safety of hydrogen use and the effects on the performance of internal combustion engines, the propagation, development of surface cellular instability and nitrogen oxide (NO_x) and nitrous oxide (N₂O) emissions of spark-ignited spherical laminar premixed ammonia/hydrogen/air flames were studied experimentally and computationally. With ammonia being the substituent, the fundamental unstretched laminar burning velocities and Markstein numbers, the propensity of cell formation and the associated flame structure were determined. Results show substantial reduction of laminar burning velocities with ammonia substitution in hydrogen/air flames, similar to hydrocarbon (e.g., methane with a similar molecular weight to ammonia) substitution. In all cases, ammonia substitution enhances the NO_x and N₂O formation. At fuel-rich conditions, however, the amount of NO_x emissions increases and then decreases with ammonia substitution and the increased amount of NO_x and N₂O emissions with ammonia substitution is much lower than that under fuel-lean conditions. These observations support the potential of ammonia as a carbon-free, clean additive for improving the safety of hydrogen use with low NO_x and N₂O emissions in fuel-rich hydrogen/air flames. The potential of ammonia as a suppressant of both preferential-diffusional and hydrodynamic cellular instabilities in hydrogen/air flames was also found particularly for fuel-lean conditions, different from methane substitution. However, it should be noted that the use of ammonia also imposes considerable technological challenges and public concerns, particularly those associated with toxicity and the specific properties such as high reactivity with container materials and water, which should be completely resolved.     

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.     

Hydrogen addition effect on NO formation in methane/air lean-premixed flames at elevated pressure

The present study investigates freely propagating methane/hydrogen lean-premixed laminar flames at elevated pressures to understand the hydrogen addition effect of natural gas on the NO formation under the conditions of industrial gas turbine combustors. The detailed chemical kinetic model which was used in the previous study on the NO formation in high pressure methane/air premixed flames was adopted for the present study to analyze NO formation of methane/hydrogen premixed flames. The present mechanism shows good agreement with experimental data for methane/hydrogen mixtures, including ignition delay times, laminar burning velocities, and NO concentration in premixed flames. Hydrogen addition to methane/air mixtures with maintaining methane content leads to the increase of NO concentration in laminar premixed flames due to the higher flame temperature. Methane/hydrogen/argon/air premixed flames are simulated to avoid the flame temperature effect on NO formation over a pressure range of 1–20atm and equivalence ratio of 0.55. Kinetic analyses shows that the N₂O mechanism is important on NO formation for lean flames between the reaction zone and postflame region, and thermal NO is dominant in the postflame zone. The hydrogen addition leads to the increase of NO formation from prompt NO and NNH mechanisms, while NO formation from thermal and N₂O mechanisms are decreased. Additionally, the NO formation in the postflame zone has positive pressure dependencies for thermal NO with an exponent of 0.5. Sensitivity analysis results identify that the initiation reaction step for the thermal NO and the N₂O mechanism related reactions are sensitive to NO formation near the reaction zone.     

Effects of density ratio and differential diffusion on flame accelerative propagation of H2/O2/N2 mixtures

The effects of density ratio and differential diffusion on premixed flame propagation of H₂/O₂/N₂ mixtures are investigated by constant volume combustion chamber. The density ratio and differential diffusion are controlled independently by adjusting the O₂/N₂ ratio and equivalence ratio. Results show that the density ratio has no effect on turbulent burning velocity while the differential diffusion has a promotion effect on turbulent burning velocity. The onsets of laminar flame acceleration are promoted by both density ratio and differential ratio. The turbulent flames perform a continuous acceleration propagation and the dependence between flame propagation speed and flame radius can be characterized as (dR/dt)/(σ·S_L) ～ R^0.33～0.37, which is lower than the 1/2 power law. The acceleration parameters of laminar flames and turbulent flames (u^’/S_L = 1) are around 0.17 and 0.36 respectively, and both of them are not affected by density ratio and differential diffusion. The empirical formula m = 0.19·(u^’/S_L)^0.4+0.17 is concluded to quantitatively describe the accelerative characteristics of laminar and turbulent flames. The current study indicates that the acceleration of laminar flames is mainly induced by flame intrinsic instability, and the latter can affect the acceleration onset but not affect the fractal excess. The acceleration of turbulent flames is dominated by turbulent stretch, while the effects of density ratio and differential diffusion can be ignored.     

Effects of water vapor addition on the laminar burning velocity of oxygen-enriched methane flames

The effects of steam addition on the laminar burning velocity of premixed oxygen-enriched methane flames are investigated at atmospheric pressure. Experiments are carried out with an axisymmetric burner on which laminar conical flames are stabilized. A newly devised steam production system is used to dilute the reactants with water vapor. The oxygen-enrichment ratio in the oxidizer, defined as O₂/(O₂ + N₂) (mol.), is varied from 0.21 (air) to 1.0 (pure oxygen). The equivalence ratio ranges from 0.5 to 1.5 and the steam molar fraction in the reactive mixture is varied from 0 to 0.50. For all compositions examined, the reactive mixture is preheated to a temperature T_u = 373 K. Laminar flame speeds are determined with the flame area method using a Schlieren apparatus. The deviations induced by stretch effects due to aerodynamic strain and flame curvature are assessed using Particle Imaging Velocimetry measurements and flame images, and these data are used to estimate the uncertainty of the flame speed measurements. The experiments are completed by numerical simulations conducted with the PREMIX code using different detailed kinetic mechanisms. It is shown that the laminar flame speed of CH₄/O₂/N₂/H₂O_(v) mixtures features a quasi-linear decrease with increasing steam molar fraction, even at high steam dilution rates. Numerical predictions are in good agreement with experimental data for all compositions explored, except for low dilution rates X_H2O<0.10 in methane–oxygen mixtures, where the flame speed is slightly underestimated by the calculations. It is also shown that steam addition has a non-negligible chemical impact on the flame speed for methane–air flames, mainly due to water vapor high chaperon efficiency in third-body reactions. This effect is however strongly attenuated when the oxygen concentration is increased in the reactive mixture. For highly oxygen-enriched flames, steam can be considered as an inert diluent.     

The effects of composition on burning velocity and nitric oxide formation in laminar premixed flames of CH4 + H2 + O2 + N2

Experimental measurements of adiabatic burning velocity and NO formation in (CH₄ + H₂) + (O₂ + N₂) flames are presented. The hydrogen content in the fuel was varied from 0 to 35% and the oxygen content in the air from 20.9 to 16%. Nonstretched flames were stabilized on a perforated plate burner at 1 atm. The heat flux method was used to determine burning velocities under conditions when the net heat loss of the flame is zero. Adiabatic burning velocities of methane + hydrogen + nitrogen + oxygen mixtures were found in satisfactory agreement with the modeling. The NO concentrations in these flames were measured in the burnt gases at a fixed distance from the burner using probe sampling. In lean flames, enrichment by hydrogen has little effect on [NO], while in rich flames, the concentration of nitric oxide decreases significantly. Dilution by nitrogen decreases [NO] at any equivalence ratio. Numerical predictions and trends were found in good agreement with the experiments. Different responses of stretched and nonstretched flames to enrichment by hydrogen are demonstrated and discussed.