Experimental and numerical study on the effect of composition on laminar burning velocities of H2/CO/N2/CO2/air mixtures

Experimental and numerical study on laminar burning velocity of H₂/CO/N₂/CO₂/air mixtures was conducted by using a constant volume bomb and Chemkin package. Good agreement between experimental measurements and numerical calculations by using USCII Mech is achieved. Diffusional-thermal instability is enhanced but hydrodynamic instability is insensitive to the increase of hydrogen fraction in fuel mixtures. For mixtures with different hydrogen fractions, the adiabatic flame temperature is not the dominant influencing factor while high thermal diffusivity of hydrogen obviously enhances the laminar burning velocity. Laminar burning velocities increase with increasing hydrogen fraction and equivalence ratio (0.4–1.0). This is mainly due to the high reactivity of H₂ leading to high production rate of H and OH radicals. Reactions  and  play the dominant role in the production of H radical for mixtures with high hydrogen fraction, and reaction R31 plays the dominant role for mixtures with low hydrogen fraction.     

Enhancement of biogas/air combustion by hydrogen addition at elevated temperatures

In this study, combustion characteristics of various biogas/air mixtures with hydrogen addition at elevated temperatures were experimentally investigated using bunsen burner method. Methane, CH₄, was diluted with different concentrations of carbon dioxide, CO₂, 30 to 40% by volume, to prepare the biogas for testing. It is followed by the hydrogen, H₂, enrichment within the range of 0 to 40% by volume and the temperature elevation of unburned gas till 440 K. Blowoff velocities were measured by lowering the jet velocity until a premixed flame could be stabilized at the nozzle exit, while laminar burning velocities were calculated by analyzing the shape of the directly captured premixed bunsen flames. The results showed that hydrogen had a positive effect on the blowoff velocity for all three fuel samples. Nonlinear growth of the blowoff velocity with hydrogen addition was associated to the dominance of methane-inhibited hydrogen combustion process. It was also observed that the increase in the initial temperature of the unburned mixture led to a linear increase of the blowoff velocity. Moreover, specific changes in flame structure such as flame height, standoff distance, and the existence of tip opening were attributed to the change in the blowoff velocity. The effect of CO₂ content in the mixture was examined with regards to laminar burning velocity for all compositions. The outcome of the experiment showed that the biogas mixture with higher content of CO₂ possessed lower values of laminar burning velocity over the wide range of equivalence ratios. A reduced GRI-Mech 3.0 was used to simulate the combustion of biogas/air mixtures with different compositions using ANSYS Fluent. The numerically simulated stable conical flames were compared with the experimental flames, in terms of flame structure, showing that the reduced GRI-Mech 3.0 was suitable for modeling the combustion of biogas/air mixtures.     

Laminar burning velocities and flame stability analysis of H2/CO/air mixtures with dilution of N2 and CO2

Experimental measurement of the laminar burning velocities of H₂/CO/air mixtures and equimolar H₂/CO mixtures diluted with N₂ and CO₂ up to 60% and 20% by volume, respectively, were conducted at different equivalence ratios and conditions near to the sea level, 0.95 atm and 303 ± 2 K. Flames were generated using contoured slot-type nozzle burners and Schlieren images were used to determine the laminar burning velocity with the angle method. Numerical calculations were also conducted using the most recent detailed reaction mechanisms for comparison with the present experimental results. Additionally, a study was conducted to analyze the flame stability phenomenology that was found in the present experiments. The increase in the N₂ and CO₂ dilution fractions considerably reduced the laminar burning velocity due to the decrease in heat release and increase in heat capacity. At the same dilution fractions this effect was higher for the case of CO₂ due to its higher heat capacity and dissociation effects during combustion. Flame instabilities were observed at lean conditions. While the presence of CO in the fuel mixture tends to stabilize the flame, H₂ has a destabilizing effect which is the most dominant. A higher N₂ and CO₂ dilution fraction increased the range of equivalence ratios where unstable flames were obtained due to the increase in the thermal-diffusive instabilities.     

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.     

The features of ignition and combustion of composite propane-hydrogen fuel: Modeling study

The extensive numerical analysis of the features of self ignition and formation of NO and CO during combustion of blended fuel, consisting of propane and hydrogen, with air is considered on the basis of extended detailed kinetic model involving both high and low temperature submechanisms of propane oxidation. It has been shown that for the blended C₃H₈–H₂ fuel there exists the temperature region, where the ignition of the C₃H₈–H₂–air mixture occurs faster compared to pure propane. However, this region is not broad enough and has low and high temperature boundaries (T_b and T_h, respectively). At the initial temperature of fuel–air mixture T₀ < T_b, the induction time of blended C₃H₈–H₂ fuel is greater than that of pure propane and, at T₀ > T_h, the admixture of a small amount of propane (1 ∼ 5% per volume) to hydrogen accelerates the ignition. The values of T_b and T_h depend on the composition of blended fuel and initial pressure. It has been revealed that the addition of hydrogen to propane increases the flame speed and extends the flammability thresholds both in fuel-lean and in fuel-rich regions, but doesn't result in the substantial change of the concentrations of main pollutants NO and CO in the combustion exhaust. However, the addition of hydrogen to fuel-lean propane–air mixture allows one to provide the stable combustion of leaner fuel–air mixture and, thus, to reduce notably the emission of NO and CO compared to that for the combustion of pure propane–air mixture.     

Explosion behavior of CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures

In this work, the explosion behavior of stoichiometric CH₄/O₂/N₂/CO₂ and H₂/O₂/N₂/CO₂ mixtures has been studied both experimentally and theoretically at different CO₂ contents and oxygen air enrichment factors. Peak pressure, maximum rate of pressure rise and laminar burning velocity were measured from pressure time records of explosions occurring in a closed cylindrical vessel. The laminar burning velocity was also computed through CHEMKIN–PREMIX simulations.     

Determination of laminar burning velocities for lean low calorific H2/N2 and H2/CO/N2 gas mixtures

Combustion of lean and ultra-lean synthetic H₂/CO mixtures that are highly diluted in inert gases is of great importance in several fields of technology, particularly in the field of post combustion for combined heat and power (CHP) systems based on fuel cell technology. In this case H₂/CO mixtures occur via hydrocarbon reforming and their complete conversion requires efficient, compact and low emission combustion systems. In order to design such systems, knowledge of global flame properties like the laminar burning velocity, is essential. Using the heat flux burner method, laminar burning velocities were experimentally determined for highly N₂ diluted synthetic H₂ and H₂/CO mixtures with low calorific value, burning with air, at ambient temperature and atmospheric pressure. Furthermore, numerical 1-D simulations were performed, using a series of different chemical reaction mechanisms. These numerical predictions are analysed and compared with the experimental data.     

Adiabatic burning velocity and cellular flame characteristics of H2–CO–CO2–air mixtures

The objective of this work was to study the effect of dilution with carbon dioxide on the adiabatic burning velocity of syngas fuel (with various H₂/CO ratios)-air(21% O₂–79% N₂ by volume) mixtures along with detailed understanding of cellular flame structures. Heat flux method with a setup similar to that of de Goey and co-workers [1] was used for measurement of burning velocities. Validation experiments were done for H₂ (5%)–CO (95%)–air and H₂ (5%)–CO (45%)–CO₂ (50%)–air mixtures at various equivalence ratios and the results were in good agreement with published data in the literature. The mixtures considered in this work had 1:4, 1:1 and 4:1 H₂/CO ratio in the fuel and 40%, 50% and 60% CO₂ dilution. The burning velocity increased significantly with the increase in H₂ content in mixture of H₂–CO with fixed CO₂ dilution. The burning velocity reduced remarkably with carbon dioxide dilution in H₂–CO mixture due to reduction in heat release, flame temperature and thermal diffusivity of the mixture. The location of peak adiabatic burning velocity shifted from ? = 1.6 for 40% CO₂ to ? = 1.2 for 60% CO₂, whereas it remained unchanged with variation of H₂:CO ratio (4:1, 1:1 and 1:4) at a given CO₂ dilution. A comparison of experiments and simulations indicated that the Davis et al. [2] mechanism predicted burning velocities well for the most of experimental operating conditions except for rich conditions. For some lean mixtures, flames exhibited cellular structures. In order to explain the structures and generate profiles of various field variables of interest, computations of three dimensional porous burner stabilized cellular flames were performed using commercial CFD software FLUENT. Simulations for lean H₂ (25%)–CO (25%)–CO₂ (50%)–air mixtures (? = 0.6 and 0.8) produced cellular flame structures very similar to those observed in the experiments. It was found that the in the core region of a typical cell, stretch rate was positive, the volumetric heat release rate was high and the net reaction rate for the reaction O + H₂ ? H + OH and the net consumption rate of H₂ were both high.     

Experimental study on cellular instabilities in hydrocarbon/hydrogen/carbon monoxide–air premixed flames

To investigate cell formation in methane (or propane)/hydrogen/carbon monoxide-air premixed flames, the outward propagation and development of surface cellular instabilities of centrally ignited spherical premixed flames were experimentally studied in a constant pressure combustion chamber at room temperature and elevated pressures. Additionally, unstretched laminar burning velocities and Markstein lengths of the mixtures were obtained by analyzing high-speed schlieren images. In this study, hydrodynamic and diffusional-thermal instabilities were evaluated to examine their effects on flame instabilities. The experimentally-measured unstretched laminar burning velocities were compared to numerical predictions using the PREMIX code with a H₂/CO/C₁-C₄ mechanism, USC Mech II, from Wang et al. [22]. The results indicate a significant increase in the unstretched laminar burning velocities with hydrogen enrichment and a decrease with the addition of hydrocarbons, whereas the opposite effects for Markstein lengths were observed. Furthermore, effective Lewis numbers of premixed flames with methane addition decreased for all of the cases; meanwhile, effective Lewis numbers with propane addition increase for lean and stoichiometric conditions and increase for rich and stoichiometric cases for hydrogen-enriched flames. With the addition of propane, the propensity for cell formation significantly diminishes, whereas cellular instabilities for hydrogen-enriched flames are promoted. However, similar behavior of cellularity was obtained with the addition of methane, which indicates that methane is not a candidate for suppressing cell formation in methane/hydrogen/carbon monoxide-air premixed flames.     

Effect of preferential diffusion and flame stretch on flame structure and laminar burning velocity of syngas Bunsen flame using OH-PLIF

By using OH-PLIF technique, experiments were conducted for laminar Bunsen flame of premixed CO/H₂/air mixtures with equivalence ratio ranging from 0.5 to 1.8. Reynolds number was varied from 800 to 2200, X_H2 = H₂/(H₂+CO) in the mixture was varied from 20% to 100% to study the effects of both preferential diffusion and flame curvature on flame structures and laminar flame burning velocities. Results showed that the combined effects of preferential diffusion and curvature gave an interesting phenomenon of the flame OH radical distributions on high hydrogen content flames. Furthermore, with the increase of H₂ fraction in fuel mixture, the effects of both preferential diffusion and flame curvature were increased. Interpretation of flame stretch effect on laminar burning velocity is also provided in this paper.     

Experimental and numerical study on the laminar burning velocity of hydrogen enriched biogas mixture

The laminar burning velocities of biogas-hydrogen-air mixture at different fuel compositions and equivalence ratios were determined and studied using the spherical flame method. The combined effects of H₂ and CO₂ on the laminar burning velocity were investigated quantitatively based on the kinetic effects and the thermal effects. The results show that the laminar burning velocities of the BG40, BG50 and BG60 are increased almost linearly with the H₂ addition owing to the improved fuel kinetics and the increased adiabatic flame temperature. The dropping trend of laminar burning velocity from the BG60-hydrogen to the BG40-hydrogen is primarily attributed to the decreased adiabatic flame temperature (thermal effects). The GRI 3.0 mechanism can predict the laminar burning velocity of biogas-hydrogen mixture better than the San Diego mechanism in this study. Whereas, the GRI mechanism still needs to be modified properly for the hydrogen-enriched biogas as the CO₂ proportion exceeds 50% in the biogas at the fuel-rich condition. The increased CO₂ exerts the stronger suppression on the net reaction rate of H + O₂=OH + O than that of H + CH₃(+M) = CH₄(+M), which contributes to that the rich-shift of peak laminar burning velocity of biogas-hydrogen mixture requires higher H₂ addition as the CO₂ content is enhanced. For the biogas-hydrogen fuel, the H₂ addition decreases the flame stability of biogas fuel effectively due to the increased diffusive-thermal instability and hydrodynamic instability. The improved flame stability of biogas-hydrogen fuel with the increased CO₂ content is resulted from the combined effects of diffusive-thermal instability and hydrodynamic instability.     

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.     

Structures and burning velocity of biomass derived gas flames

Biomass derived gases produced via gasification, pyrolysis, and fermentation are carbon neutral alternative fuels that can be used in gas turbines, furnaces, and piston engines. To make use of these environmentally friendly but energy density low fuels the combustion characteristics of these fuels have to be fully understood. In this study the structure and laminar burning velocity of biomass derived gas flames are investigated using detailed chemical kinetic simulations. The studied gaseous fuels are the air-blown gasification gas, co-firing of gasification gas with methane, pyrolysis gases, landfill gases, and syngas, a mixture of carbon monoxide and hydrogen. The simulated burning velocities of reference fuel mixtures using two widely used chemical kinetic mechanisms, GRI Mech 3.0 and the San Diego mechanism, are compared with the experimental data to explore the uncertainties and scattering of the simulation data. The different chemical kinetic mechanisms are shown to give a reasonable agreement with each other and with experimental data, with a discrepancy within 7% over most of the conditions. The results show that the structures of typical landfill gas flames and co-firing of methane/gasification gas flames share essential similarity with methane flames. The reaction zones of these flames consist of a thin inner layer and a relatively thick CO/H₂ oxidation layer. In the inner layer hydrocarbon fuel (methane) is converted through chain reactions to intermediates such as CH₃, CH₂O, CO, H₂, etc. The structures of gasification gas flames, pyrolysis gas flames, syngas flames share similarity with the oxidization layer of the methane/air flames. Overall, the chemical reactions of all biomass derived gas flames occur in thin zones of the order of less than 1 mm. The thickness of all BDG gas flames is inversely proportional to their respective laminar burning velocity. The laminar burning velocities of landfill gases are found to increase linearly with the mole fraction of methane in the mixtures, whereas for gasification gas, syngas and pyrolysis gas where hydrogen is present, the laminar burning velocities scale linearly with the mole fraction of hydrogen.     

Comparative study on the effect of CO2 and H2O dilution on laminar burning characteristics of CO/H2/air mixtures

A study on the effect of CO₂ and H₂O dilution on the laminar burning characteristics of CO/H₂/air mixtures was conducted at elevated pressures using spherically expanding flames and CHEMKIN package. Experimental conditions for the CO₂ and H₂O diluted CO/H₂/air/mixtures of hydrogen fraction in syngas from 0.2 to 0.8 are the pressures from 0.1 to 0.3 MPa, initial temperature of 373 K, with CO₂ or H₂O dilution ratios from 0 to 0.15. Laminar burning velocities of the CO₂ and H₂O diluted CO/H₂/air/mixtures were measured and calculated using the mechanism of Davis et al. and the mechanism of Li et al. Results show that the discrepancy exists between the measured values and the simulated ones using both Davis and Li mechanisms. The discrepancy shows different trends under CO₂ and H₂O dilution. Chemical kinetics analysis indicates that the elementary reaction corresponding to peak ROP of OH consumption for mixtures with CO/H₂ ratio of 20/80 changes from reaction R3 (OH + H₂ = H + H₂O) to R16 (HO₂+H = OH + OH) when CO₂ and H₂O are added. Sensitivity analysis was conducted to find out the dominant reaction when CO₂ and H₂O are added. Laminar burning velocities and kinetics analysis indicate that CO₂ has a stronger chemical effect than H₂O. The intrinsic flame instability is promoted at atmospheric pressure and is suppressed at elevated pressure for the CO₂ and H₂O diluted mixtures. This phenomenon was interpreted with the parameters of the effective Lewis number, thermal expansion ratio, flame thickness and linear theory.     

Burning velocities and kinetics of H2/NF3/N2, CH4/NF3/N2, and C3H8/NF3/N2 flames

The combustion characteristics and reaction mechanism of mixtures containing nitrogen trifluoride (NF₃) were investigated. Burning velocities for H₂/NF₃/N₂, CH₄/NF₃/N₂, and C₃H₈/NF₃/N₂ flames were determined for the first time at various equivalence ratios and N₂ mole fractions. The burning velocities of the latter two flames were similar and showed peaks at equivalence ratios of ∼1.0, while those of the H₂/NF₃/N₂ flames had the pronounced peak at low equivalence ratios where the formation of the wrinkled flames was observed. A detailed kinetic model was constructed to simulate the laminar burning velocities of H₂/NF₃/N₂ and CH₄/NF₃/N₂ flames. The model accurately reproduced the experimental results. Analyses of the reaction mechanism revealed the major reaction pathways that involve the decomposition of NF₃, the oxidation and chain-fluoridation of H₂ and CH₄, and the formation of N₂.     

Fabrication of AlLi and Al2Li3/Al4Li9 intermetallic compounds by molten salt electrolysis and their application for hydrogen generation from water

The concept of “hydrogen on demand” has been widely publicized. More importantly, the materials used to produce hydrogen on demand should be in themselves safe to handle. In present work, Al–Li intermetallic compounds (IMC) were fabricated in air by electrolysis from LiCl–KCl molten salt at 480 ± 25 °C. Bulk AlLi IMC and the bulk compound with mixture of Al₂Li₃ and Al₄Li₉ (Al₂Li₃/Al₄Li₉ IMC) were not pyrophoric and can be safely handled in air. When both compounds in contact with water, vigorous reaction occurred and H₂ was directly produced. The by-products after H₂ generation from AlLi IMC were a mixture of Li-containing α-Al and Li–Al hydrotalcite (hereafter referred to as Li–Al LDH). The by-product after H₂ generation from Al₂Li₃/Al₄Li₉ compound was a mixture of LiOH·H₂O and Li–Al LDH. Those by-products can be easily separated from water and may be reused as a resource. Approximately 500–860 ml of H₂ per weight (g) of the IMC compounds was generated in deionized water at room temperature. Experimentally, AlLi IMC powder and Al₂Li₃/Al₄Li₉ compound exhibit gravimetric hydrogen capacity of 7.0 wt.% and 5.4 wt.%, respectively. Although the energy consumed for fabricating Al–Li IMC compounds is a little larger than the energy provided by the generated H₂, the Al–Li IMC compounds are promising materials for producing hydrogen on demand without the necessity of hydrogen storage.     

Laminar burning characteristics of ammonia/hydrogen/air mixtures with laser ignition

Ammonia, as a zero-carbon fuel, is drawing more and more attention. The major challenge of using ammonia as a fuel for the combustion engines lies in its low chemical reactivity, and therefore more fundamental researches on the combustion characteristics of ammonia are required to explore effective ways to burn ammonia in engines. In this study, the laminar burning characteristics of the premixed ammonia/hydrogen/air mixtures are investigated. In the experiment, the laser ignition was used to achieve stable ignition of the ammonia/air mixtures with an equivalence ratio range from 0.7 to 1.4. The propagating flame was recorded with the high-speed shadowgraphy. Three different processing methods were introduced to calculate the laminar burning velocity with a consideration of the flame structure characteristics induced by the laser ignition. The effects of initial pressure (0.1 MPa–0.5 MPa), equivalence ratio (0.7–1.4), hydrogen fraction (0–20%) on the laminar burning velocity were investigated under the initial ambient temperature of 360 K. The state-of-the-art kinetic models were used to calculate the laminar burning velocities in the CHEMKIN-pro software. Both the simulation and experimental results show that the laminar burning velocity of the ammonia mixtures increases at first, reaches the peak around ϕ of 1.1, and then decreases with the equivalence ratio increasing from 0.7 to 1.4. The peak laminar burning velocities of the ammonia mixture are lower than 9 cm/s and are remarkably lower than those of hydrocarbon fuels. The laminar burning velocity of the ammonia mixture decreases with the increase of the initial ambient pressure, and it can be drastically speeded up with the addition of hydrogen. While the models except for those by Miller and Bian can give reasonable predictions compared to the experimental results for the equivalence ratio from 0.7 to 1.1 in the ammonia (80%)/hydrogen (20%)/air mixtures, all the kinetic models overpredict the experiments for the richer mixtures, indicating further work necessary in this respect.     

Characterization of biogas-hydrogen premixed flames using Bunsen burner

Experimental study is conducted to clarify the effects of hydrogen addition to biogas and hydrogen fraction in the biogas-H₂ mixture on the stability, thermal and emission characteristics of biogas-H₂-air premixed flames using a 9 mm-ID-tube Bunsen burner. Variation in biogas composition is allowed to range from BG₆₀ (60%CH₄–40%CO₂), down to BG₅₀ (50%CH₄–50%CO₂) and to BG₄₀ (40%CH₄–60%CO₂). For each biogas, the fraction of hydrogen in the biogas-H₂ mixture is varied from 10% to 50%. The results show that upon hydrogen addition and increasing hydrogen fraction in the fuel mixture, there are corresponding changes in flame stability, laminar burning velocity, flame tip temperature and CO pollutant emission.     

A comparison of the emission and impingement heat transfer of LPG–H2 and CH4–H2 premixed flames

Experiments were performed to add hydrogen to liquefied petroleum gas (LPG) and methane (CH₄) to compare the emission and impingement heat transfer behaviors of the resultant LPG–H₂–air and CH₄–H₂–air flames. Results show that as the mole fraction of hydrogen in the fuel mixture was increased from 0% to 50% at equivalence ratio of 1 and Reynolds number of 1500 for both flames, there is an increase in the laminar burning speed, flame temperature and NO_x emission as well as a decrease in the CO emission. Also, as a result of the hydrogen addition and increased flame temperature, impingement heat transfer is enhanced. Comparison shows a more significant change in the laminar burning speed, temperature and CO/NO_x emissions in the CH₄ flames, indicating a stronger effect of hydrogen addition on a lighter hydrocarbon fuel. Comparison also shows that the CH₄ flame at α = 0% has even better heat transfer than the LPG flame at α = 50%, because the longer CH₄ flame configures a wider wall jet layer, which significantly increases the integrated heat transfer rate.     

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.