Effects of buffer gas composition on autoignition

This work quantifies the chemical kinetic and thermal effects of buffer gas composition on autoignition of three fuels at conditions relevant to engines, combustors, and experimental facilities used to study ignition kinetics. Computational simulations of autoignition of iso-octane, n-heptane, and of n-butanol were used to characterize the effects of buffer gas composition on ignition delay time and heat release rate. Stoichiometric mixtures, ? = 1.0, and a temperature range of 600–1100 K were considered. Iso-octane and n-heptane were studied at initial pressures of 9.0 atm and 60.0 atm, and n-butanol was studied at initial pressures of 3.2 atm and 60.0 atm. Two dilution levels of buffer gas to O₂ of 3.76:1 (mole basis) and 5.64:1 were considered (∼21% and ∼15% O₂ respectively, mole basis). The fuels and simulation conditions were selected based on the relevance to engine operating conditions and previously published ignition studies. The buffer gases considered were argon, nitrogen, water, and carbon dioxide. Simulation results predicted changes of greater than a factor of 2 in ignition delay time and heat release rate as a function of buffer gas composition in the negative temperature coefficient (NTC) region for n-heptane and iso-octane. Outside the NTC region, the predicted effects of changes in buffer gas composition were small (<20%); however, experimental data for n-heptane indicate larger effects of buffer gas composition on ignition delay time at higher temperatures (>a factor of 2). The heat release rates were also sensitive to buffer gas composition, with carbon dioxide exhibiting relatively high levels of early and late heat release relative to the other buffer gases. Sensitivity analysis of the third-body collision efficiencies for the buffer gases showed the effects of uncertainties in the third body collision efficiencies on ignition delay time and heat release rate. The results highlight the significance of buffer gas composition on low-temperature combustion chemistry, particularly via H₂O₂ and HO₂ decomposition and recombination reactions.     

Autoignition of methylcyclohexane at elevated pressures

Autoignition of methylcyclohexane (MCH) is investigated in a rapid compression machine (RCM). Experiments are conducted for homogeneous MCH/O₂/N₂/Ar mixtures with equivalence ratios varying from 0.5 to 1.5, at compressed charge pressures of 15.1 and 25.5 bar, and over a compressed gas temperature range of 680-905 K. In the RCM experiments, the mole percentage of MCH is kept constant at 1.047%, while the equivalence ratio is varied through variation in oxygen mole percentage. Ignition delay measurements demonstrate the features of two-stage ignition characteristics and strong negative temperature coefficient (NTC) behavior. The NTC behavior is also noted to shift to higher temperatures at higher pressures and for oxygen-rich mixtures. In addition, both the first-stage and total ignition delays show significant dependence on oxygen mole fraction. Furthermore, the current ignition delay measurements provide additional benchmark data for the validation of the oxidation kinetics of MCH. Comparison of the experimental ignition delays with the simulated results using an available detailed kinetic mechanism is conducted and discussed.     

Autoignition of n-decane under elevated pressure and low-to-intermediate temperature conditions

An experimental investigation of the autoignition for various n-decane/oxidizer mixtures is conducted using a rapid compression machine, in the range of equivalence ratios of ?=0.5-2.2, dilution molar ratios of N₂/(O₂ + N₂) = 0.79-0.95, compressed gas pressures of PC=7-30 bar, and compressed gas temperatures of TC=635-770 K. The current experiments span a temperature range not fully investigated in previous autoignition studies on n-decane. Two-stage ignition, characteristic of large hydrocarbons, is observed over the entire range of conditions investigated, as demonstrated in the plots of raw experimental pressure traces. In addition, experimental results reveal the sensitivity of the first-stage and total ignition delays to variations in fuel and oxygen mole fractions, pressure, and temperature. Predictability of two kinetic mechanisms is compared against the present data. Discrepancies are noted and discussed, which are of direct relevance for further improvement of kinetic models of n-decane at conditions of elevated pressures and low-to-intermediate temperatures.     

Two-stage ignition in HCCI combustion and HCCI control by fuels and additives

A Rapid Compression Machine (RCM) has been used to study the effects of fuel structure and additives on the Homogeneous Charge Compression Ignition (HCCI) of pure hydrocarbon fuels and mixtures under well-determined conditions. Such information is needed for understanding ignition delays and burning rates in HCCI engines, and “knock” in spark-ignition engines. It is also valuable for validating basic chemical kinetic models of hydrocarbon oxidation. The pure fuels used in the study include: paraffins (n-heptane, iso-octane), cyclic paraffins (cyclohexane, methylcyclohexane), olefins (1-heptene, 2-heptene, 3-heptene), cyclic olefins (cyclohexene, 1,3-cyclohexadiene), and an aromatic hydrocarbon (toluene). The additives were 2-ethyl-hexyl-nitrate and di-tertiary-butyl-peroxide. It was found that fuels which contained the structure -CH₂-CH₂-CH₂- showed two-stage ignition with relatively short ignition delays and that the ignition delay depended strongly on the energy released during the first-stage. For primary reference fuel mixtures (n-heptane + iso-octane), the ignition delay depended only on the molar ratio of n-heptane to oxygen and was independent of the octane number (percent iso-octane). On the other hand, the burn rate depended on both these parameters, which uniquely determine the equivalence ratio. When additives were included in the air/fuel mixtures, the ignition delay was reduced but the burn rate was not affected. These results indicate that for HCCI combustion, the ignition delay and the burn rate can be independently controlled using various fuel mixtures and additives.     

A shock tube study of iso-octane ignition at elevated pressures: The influence of diluent gases

The ignition of iso-octane/air and iso-octane/O₂/Ar (∼20% O₂) mixtures was studied in a shock tube at temperatures of 868-1300 K, pressures of 7-58 atm, and equivalence ratios Φ=1.0, 0.5, and 0.25. Ignition times were determined using endwall OH^∗ emission and sidewall piezoelectric pressure measurements. Measured iso-octane/air ignition times agreed well with the previously published results. Mixtures with argon as the diluent exhibited ignition times 20% shorter, for most conditions, than those with nitrogen as the diluent (iso-octane/air mixtures). The difference in measured ignition times for mixtures containing argon and nitrogen as the diluent gas can be attributed to the differing heat capacities of the two diluent species and the level of induction period heat release prior to ignition. Kinetic model predictions of ignition time from three mechanisms are compared to the experimental data. The mechanisms overpredict the ignition times but accurately capture the influence of diluent gas on iso-octane ignition time, indicating that the mechanisms predict an appropriate amount of induction period heat release.     

An experimental and modeling study of iso-octane ignition delay times under homogeneous charge compression ignition conditions

Autoignition of iso-octane was examined using a rapid compression facility (RCF) with iso-octane, oxygen, nitrogen, and argon mixtures. The effects of typical homogeneous charge compression ignition (HCCI) conditions on the iso-octane ignition characteristics were studied. Experimental results for ignition delay times, τ_ign, were obtained from pressure time-histories. The experiments were conducted over a range of equivalence ratios (?=0.25-1.0), pressures (P=5.12-23 atm), temperatures (T=943-1027 K), and oxygen mole fractions (χO₂=9-21%), and with the addition of trace amounts of combustion product gases (CO₂ and H₂O). It was found that the ignition delay times were well represented by the expression

     

Chemical effects of added CO2 on the extinction characteristics of H2/CO/CO2 syngas diffusion flames

Chemical effects of added CO₂ on flame extinction characteristics are numerically studied in H₂/CO syngas diffusion flames diluted with CO₂. The two representative syngas flames of 80% H₂ + 20% CO and 20% H₂ + 80% CO are inspected according to the composition of fuel mixture diluted with CO₂ and global strain rate. Particular concerns are focused on impact of chemical effects of added CO₂ on flame extinction characteristics through the comparison of the flame characteristics between well-burning flames far from extinction limit and flames at extinction. It is seen that chemical effects of added CO₂ reduce critical CO₂ mole fraction at flame extinction and thus extinguish the flame at higher flame temperature irrespective of global strain rate. This is attributed by the suppression of the reaction rate of the principal chain branching reaction through the augmented consumption of H-atom from the reaction CO₂ + H→CO + OH. As a result the overall reaction rate decreases. These chemical effects of added CO₂ are similar in both well-burning flames far from extinction limit and flames at extinction. There is a mismatching in the behaviors between critical CO₂ mole fraction and maximum flame temperature at extinction. This anomalous phenomenon is also discussed in detail.     

Extinction of premixed methane/air flames in microgravity by diluents: Effects of radiation and Lewis number

Laminar burning velocities and flammability limits of premixed methane/air flames in the presence of various diluents were investigated by combined use of experiments and numerical simulations. The experiments used a 1-m free-fall spherical combustion chamber to eliminate the effect of buoyancy, enabling accurate measurements of near-limit burning velocities and flammability limits. Burning velocities were measured for CH₄/air flames with varying concentrations of He, Ar, N₂ and CO₂ at NTP. The limiting concentration of each diluent was measured by systematically varying the composition and ignition energy and finding the limiting condition through successive experiment trials. The corresponding freely-propagating, planar 1-D flames were simulated using PREMIX. The transient spherically-expanding flames were simulated using the 1-D Spherical Flame & Reactor Module of COSILAB considering detailed radiation models. The results show that helium exhibits more complex limit behavior than the other diluents due to the large Lewis number of helium mixtures. The near-limit helium-diluted flames require much higher ignition energy than the other flames. In addition, for the spherically expanding helium-diluted flames studied here (Le > 1), stretch suppresses flame propagation and may cause flame extinction. For the CO₂-diluted flames, the flame speed predicted by the optically-thick model based on the Discrete Transfer Method (DTW) and a modified wide band model has better agreement with measurements in the near-limit region. A significant amount of heat is absorbed by the dilution gas CO₂, resulting in elevation of temperature of the ambient gases. The optically-thick model, however, still overpredicts flame speed, indicating a more sophisticated radiation property model may be needed. Finally, the chemical effect of CO₂ on flame suppression was quantified by a numerical analysis. The results show that the chemical effect of CO₂ is more important than the other diluents due to its active participation in the reaction CO₂ + H = CO + OH, which competes for H radicals with the chain-branching reactions and thus reduces flame speed.     

Ignition characteristics of laser-ionized fuel injected into a hypersonic crossflow

This paper presents an experimental investigation on the ignition characteristics of laser-ionized fuel injected into a hypersonic air-crossflow. A Q-switched laser causes breakdown in the sonic H₂ injector stream before interaction with the air-crossflow traveling at 2 km/s with a total specific enthalpy of 2.5 MJ/kg. The flowfield is visualized using schlieren imaging and planar laser-induced fluorescence (PLIF) on the NO molecule. The temporal evolution of the ignition process is visualized using PLIF on the OH molecule. We compare the ignition effectiveness, in terms of the OH PLIF signal, between a series of test cases with different injector pressures and laser energies. These results are also compared to a previous study, where the laser-spark was generated in the jet interaction region, outside the fuel injector. The influence of using H₂ fuel diluted with Ar, which serves as a plasma buffer gas, to extend plasma lifetimes is also investigated. The ignition technique is found to be effective in terms of post-plasma hydroxyl production, with a significant increase in performance observed when Ar is used as a plasma buffer gas.     

Experiments and modeling of the autoignition of methylcyclohexane at high pressure

New experimental data are collected for methyl-cyclohexane (MCH) autoignition in a heated rapid compression machine (RCM). Three mixtures of MCH/O₂/N₂/Ar at equivalence ratios of ? = 0.5, 1.0, and 1.5 are studied and the ignition delays are measured at compressed pressure of 50 bar and for compressed temperatures in the range of 690–900 K. By keeping the fuel mole fraction in the mixture constant, the order of reactivity, in terms of inverse ignition delay, is measured to be ? = 0.5 > ? = 1.0 > ? = 1.5, demonstrating the dependence of the ignition delay on oxygen concentration. In addition, an existing model for the combustion of MCH is updated with new reaction rates and pathways, including substantial updates to the low-temperature chemistry. The new model shows good agreement with the overall ignition delays measured in this study, as well as the ignition delays measured previously in the literature using RCMs and shock tubes. This model therefore represents a strong improvement compared to the previous version, which uniformly over-predicted the ignition delays. Chemical kinetic analyses of the updated mechanism are also conducted to help understand the fuel decomposition pathways and the reactions controlling the ignition. Combined, these results and analyses suggest that further investigation of several of the low-temperature fuel decomposition pathways is required.     

Single particle ignition and combustion of anthracite,semi-anthracite and bituminous coals in air and simulated oxy-fuel conditions

A fundamental investigation has been conducted on the combustion behavior of single particles (75–150 μm) of four coals of different ranks: anthracite, semi-anthracite, medium-volatile bituminous and high-volatile bituminous. A laboratory-scale transparent laminar-flow drop-tube furnace, electrically-heated to 1400 K, was used to burn the coals. The experiments were performed in different combustion atmospheres: air (21%O₂/79%N₂) and four simulated dry oxy-fuel conditions: 21%O₂/79%CO₂, 30%O₂/70%CO₂, 35%O₂/65%CO₂ and 50%O₂/50%CO₂. The ignition and combustion of single particles was observed by means of three-color pyrometry and high-speed high-resolution cinematography to obtain temperature–time histories and record combustion behaviors. On the basis of the observations made with these techniques, a comprehensive examination of the ignition and combustion behaviors of these fuels was achieved. Higher rank coals (anthracite and semi-anthracite) ignited heterogeneously on the particle surface, whereas the bituminous coal particles ignited homogeneously in the gas phase. Moreover, deduced ignition temperatures increased with increasing coal rank and decreased with increasing oxygen concentrations. Strikingly disparate combustion behaviors were observed depending on the coal rank. The combustion of bituminous coal particles took place in two phases. First, volatiles evolved, ignited and burned in luminous enveloping flames. Upon extinction of these flames, the char residues ignited and burned. In contrast, the higher rank coal particles ignited and burned heterogeneously. The replacement of the background N₂ gas of air with CO₂ (i.e., changing from air to an oxy-fuel atmosphere) at the same oxygen mole fraction impaired the intensity of combustion. It reduced the combustion temperatures and lengthened the burnout times of the particles. Increasing the oxygen mole fraction in CO₂ to 30–35% restored the intensity of combustion to that of air for all the coals studied. Volatile flame burnout times increased linearly with the volatile matter content in the coal in both air and all oxygen mole fractions in CO₂. On the other hand, char burnout times increased linearly or quadratically versus carbon content in the coal, depending on the oxygen mole fraction in the background gas.     

Shock tube evaluation on C2H4 ignition delay differences among N2, Ar,He, CO2 diluent gases

To reveal the fuel ignition delay differences among diluent gases, non-reactive gas Ar, He, N₂ and reactive gas CO₂ four gases were selected to compare the ethylene ignition delay difference on a shock tube rig. The tested condition temperature and pressure range is 1074–1832 K and 2 atm. Comparison results show that the ethylene ignition delay discrepancy was not pronounced among Ar, He and N₂ these non-reactive dilution gases. In terms of CO₂ dilution, a critical CO₂ concentration was observed. When the CO₂ concentration was lower than this critical value, no distinguished ignition delay difference was found. Conversely, the ignition delay difference was pronounced once the CO₂ concentration was above this critical value. Particularly, the difference was specially apparent in low temperature conditions. Reaction dynamic analysis shows that the heat effect is predominant in low temperature while the chemical effect is predominant in high temperature in the reaction of CO₂ participation. Additionally, the CH* and OH* radical emission signal intensity turns smaller and profile curve turns irregular as the CO₂ ratio increases. The CO₂ shows a strong inhibition effect which is closely related with H radical consumption reaction CO₂+HCO + OH.     

Hydrate-based pre-combustion carbon dioxide capture process in the system with tetra-n-butyl ammonium bromide solution in the presence of cyclopentane

Effects of 0.29 mol% tetra-n-butyl ammonium bromide (TBAB) solution in conjunction with cyclopentane (CP) on the hydrate-based pre-combustion CO₂ capture are investigated by the measurements of the gas uptakes, CO₂ separation efficiencies and induction time of the hydrate formation at the different temperature-pressure conditions. The results show that the volume of the TBAB has an effect on the CO₂ separation and the induction time, and the addition of the CP into the TBAB solution remarkably enhances the CO₂ separation and shortens the induction time. The system with the CP/TBAB solution volume ratio of 5 vol% and TBAB solution/reactor effective volume ratio of 0.54 is optimum to obtain the largest gas uptake and the highest CO₂ separation efficiency at 274.65 K and 4.0 MPa. Compared to the results with tetrahydrofuran (THF) as an additive [1], the gas uptake is enhanced by at least 2 times and the induction time is shortened at least 10 times at the similar temperature-pressure condition. In addition, the CO₂ concentration in the decomposed gas from the hydrate slurry phase reaches approximately 93 mol% after the first-stage separation at 274.65 K and 2.5 MPa. The gas uptakes of more than 80 mol% are obtained after 400 s at the temperature range of 274.65-277.65 K and the pressure range of 2.5-4.5 MPa.     

Experiments and modeling of propane combustion with vitiation

The chemical species composition of a vitiated oxidizer stream can significantly affect the combustion processes that occur in many propulsion and power generation systems. Experiments were performed to investigate the chemical kinetic effects of vitiation on ignition and flame propagation of hydrocarbon fuels using propane. Atmospheric-pressure flow reactor experiments were performed to investigate the effect of NO_x on propane ignition delay time at varying O₂ levels (14–21 mol%) and varying equivalence ratios (0.5–1.5) with reactor temperatures of 875 K and 917 K. Laminar flame speed measurements were obtained using a Bunsen burner facility to investigate the effect of CO₂ dilution on flame propagation at an inlet temperature of 650 K. Experimental and modeling results show that small amounts of NO can significantly reduce the ignition delay time of propane in the low- and intermediate-temperature regimes. For example, 755 ppmv NOx in the vitiated stream reduced the ignition delay time of a stoichiometric propane/air mixture by 75% at 875 K. Chemical kinetic modeling shows that H-atom abstraction reaction of the fuel molecule by NO₂ plays a critical role in promoting ignition in conjunction with reactions between NO and less reactive radicals such as HO₂ and CH₃O₂ at low and intermediate temperatures. Experimental results show that the presence of 10 mol% CO₂ in the vitiated air reduces the peak laminar flame speed by up to a factor of two. Chemical kinetic effects of CO₂ contribute to the reduction in flame speed by suppressing the formation of OH radicals in addition to the lower flame temperature caused by dilution. Overall, the detailed chemical kinetic mechanism developed in the current work predicts the chemical kinetic effects of vitiated species, namely NO_x and CO₂, on propane combustion reasonably well. Moreover, the reaction kinetic scheme also predicts the negative temperature coefficient (NTC) behavior of propane during low-temperature oxidation.     

Numerical study on CH4 laminar premixed flames for combustion characteristics in the oxidant atmospheres of N2/CO2/H2O/Ar-O2

The freely-propagating laminar premixed flames of CH₄–N₂/CO₂/H₂O/Ar-O₂ mixtures were conducted with the PREMIX code. The effects of the equivalence ratio and various oxidant atmospheres on the basic combustion characteristics were analyzed with the initial pressure and temperature of 1 atm and 398 K, respectively, O₂ content in the oxidant of 21%. The chemical reaction mechanism GRI-Mech 3.0 was chosen to determine the effects of the oxidant atmospheres of N₂/O₂, CO₂/O₂, H₂O/O₂, and Ar/O₂ on the adiabatic flame temperature, laminar burning velocity, flame structure, free radicals, intermediate species, net heat release rate and specific heat of the fuel/oxidant mixtures. The numerical results show that the maximum adiabatic flame temperatures and laminar burning velocities are at Ar/O₂ atmosphere. The mole fractions of CO and H₂ increased fastest at CO₂/O₂ atmosphere and H₂O/O₂, respectively. The mole fractions of CH₃ and H follow the order Ar/O₂> N₂/O₂>H₂O/O₂>CO₂/O₂. In addition, for 4 oxidant atmospheres, the peak mole fraction of C₂H₂ is following the order H₂O/O₂>Ar/O₂>N₂/O₂>CO₂/O₂ and the net heat release rate is following the order Ar/O₂>N₂/O₂>H₂O/O₂>CO₂/O₂ for all equivalence ratios.     

An experimental investigation of iso-octane ignition phenomena

High-speed digital imaging has been used in rapid compression facility (RCF) studies to investigate ignition phenomena of iso-octane/air mixtures. Sequential images were captured for each experiment. The results indicate the existence of two ignition regimes. In one domain, ignition is rapid, typically less than 76 μs, and ignition occurs simultaneously throughout the test volume. In the other domain, reaction fronts form and propagate within the test volume prior to volumetric ignition. The data span equivalence ratios from ?=0.20 to 1.98, with inert/O₂ gas ratios from 1.38 to 5.89, pressures from 8.7 to 16.6 atm, and temperatures from 903 to 1020 K. The transition between the two regimes is discussed in the context of the mixture composition and experimental conditions. The analysis shows that the fuel mole fraction is a key parameter dictating the boundary between the modes of ignition. Below a critical mole fraction limit, volumetric ignition is observed; above the critical limit, reaction fronts are consistently present prior to volumetric ignition. The ignition delay times for both ignition regimes are well reproduced using a homogeneous simulation with detailed reaction chemistry, when the state conditions are modified to account for the presence of the reaction fronts. The results are discussed in terms of proposed reaction chemistry, ignition theory, and previous studies of iso-octane ignition.     

Effects of coal syngas major compositions on Ni/YSZ anode-supported solid oxide fuel cells

This paper presents a systematical evaluation of the effects of CO₂, H₂O, CO, N₂ and CH₄ in the coal syngas on the properties of typical Ni/YSZ anode-supported solid oxide fuel cells (SOFCs). The results show that CO₂, H₂O, CO, N₂ and CH₄ have complicated effects on the cell performance and the electrochemical impedance spectra (EIS) analysis reveals the addition of these gases influences electrode processes such as the oxygen ion exchange from YSZ to anode TPBs, the charge transfer at the anode TPBs, gas diffusion and conversion at the anode. Two kinds of mixture gases with different compositions are thus constituted and used as fuel for aging test on two cells at 750 °C. No degradation or carbon deposition is observed for the cell fueled with 40% H₂-20% CO-20% H₂O-20% CO₂ for 360 h while the cell fueled with 50% H₂-30% CO-10% H₂O-10% CO₂ exhibits an abrupt degradation after 50 h due to the severe carbon deposition.     

Effects of hydrogen preconversion on the homogeneous ignition of fuel-lean H2/O2/N2/CO2 mixtures over platinum at moderate pressures

The impact of fractional hydrogen preconversion on the subsequent homogeneous ignition characteristics of fuel-lean (equivalence ratio φ = 0.30) H₂/O₂/N₂/CO₂ mixtures over platinum was investigated experimentally and numerically at pressures of 1, 5 and 8 bar. Experiments were performed in an optically accessible channel-flow reactor and involved Raman measurements of major species over the catalyst boundary layer and planar laser induced fluorescence (LIF) of the OH radical. Simulations were carried out with a 2-D elliptic code that included detailed hetero-/homogeneous chemistry. The predictions reproduced the LIF-measured onset of homogeneous ignition and the Raman-measured transport-limited catalytic hydrogen consumption. For 0% preconversion and wall temperatures in the range 900 K ? T_w ? 1100 K, homogeneous ignition was largely suppressed for p ? 5 bar due to the combined effects of intrinsic gas-phase hydrogen kinetics and the competition between the catalytic and gas-phase pathways for fuel consumption. A moderate increase of preconversion to 30% restored homogeneous combustion for p ? 5 bar, despite the fact that the water formed due to the upstream preconversion inhibited homogeneous ignition. The catalytically-produced water inhibited gas-phase combustion, particularly at higher pressures, and this kinetic inhibition was exacerbated by the diffusional imbalance of hydrogen that led to over-stoichiometric amounts of water in the near-wall hot ignitable regions. Radical adsorption/desorption reactions hindered the onset of homogeneous ignition and this effect was more pronounced at 1 bar. On the other hand, over the post-ignition reactor length, radical adsorption/desorption reactions significantly suppressed gas-phase combustion at 5 and 8 bar while their impact at 1 bar was weaker. By increasing hydrogen preconversion, the attained superadiabatic surface temperatures could be effectively suppressed. An inverse catalytically stabilized thermal combustion (CST) concept has been proposed, with gas-phase ignition achieved in an upstream porous burner via radiative and heat conduction feedback from a follow-up catalytic reactor. This arrangement moderated the superadiabatic surface temperatures and required modest or no preheat of the incoming mixture.     

The effects of hydrogen addition on Fenimore NO formation in low-pressure,fuel-rich-premixed,burner-stabilized CH4/O2/N2 flames

We investigate the effects of hydrogen addition on Fenimore NO formation in fuel-rich, low-pressure burner-stabilized CH₄/O₂/N₂ flames. Towards this end, axial profiles of temperature and mole fractions of CH and NO are measured using laser-induced fluorescence (LIF). The experiments are performed at equivalent ratios of 1.3 and 1.5, using 0.25 mole fraction of hydrogen in the fuel, while varying the mass flux through the burner. The results are compared with those reported previously for burner-stabilized CH₄/O₂/N₂ flames. The increased burning velocity caused by hydrogen addition is seen to result in a lower flame temperature as compared to methane flame stabilized at the same mass flux. This increase in burner stabilization upon hydrogen addition results in significantly lower CH mole fractions at φ = 1.3, but appears to have little effect on the CH profile at φ = 1.5. In addition, the results show that not only the maximum flame temperature is reduced upon hydrogen addition, but the local gas temperature in the region of the CH profile is lowered as well. The measured NO mole fractions are seen to decrease substantially for both equivalence ratios. Analysis of the factors responsible for Fenimore NO formation shows the reduction in temperature in the flame front to be the major factor in the decrease in NO mole fraction, with a significant contribution from the decrease in CH mole fraction at φ = 1.3. At φ = 1.5, the results suggest that the lower flame temperature upon hydrogen addition further retards the conversion of residual fixed-nitrogen species to NO under these rich conditions as compared to the equivalent methane flames.