Thermal NO Mechanism Research Articles

Measurements of the laminar burning velocities and NO concentrations in neat and blended ethanol and n-heptane flames

Adiabatic laminar burning velocities and post-flame NO mole fractions for neat and blended ethanol and n-heptane premixed flames were experimentally determined using a heat flux burner and laser-induced fluorescence. The flames were stabilized at atmospheric pressure and at an initial temperature of 338 K, over equivalence ratios ranging from 0.6 to 1.5. These experiments are essential for the development, validation and optimization of chemical kinetic models, e.g. for the combustion of gasoline-ethanol fuel mixtures. It was observed that the addition of ethanol to n-heptane leads to an increase in laminar burning velocity that is not proportional to the ethanol content and to a decrease of NO formation. Such a NO reduction is due to the slightly lower flame temperatures of ethanol, which decrease the production of thermal-NO at 0.6 < Φ < 1.2, while under fuel-rich conditions this behavior is due to the lower concentrations of CH radicals, which decrease the production of prompt-NO. At Φ > 1.3, the lower NO formation through the prompt mechanism in the ethanol flames is partially offset by a lower rate of NO consumption through the reburning mechanism. New experimental results were compared with predictions of the POLIMI detailed chemical kinetic mechanism. An excellent agreement between measurements and simulated results was observed for the laminar burning velocities over the equivalence ratio range investigated; however, discrepancies were found for the NO mole fractions, especially under rich conditions. Further numerical analyses were performed to identify the main causes of the observed differences. Differences found at close-to stoichiometric conditions were attributed to an uncertainty in the thermal-NO mechanism. In addition, disagreement under rich conditions could be explained by the relative importance of reactions in hydrogen cyanide consumption pathways.

The effect of air/fuel ratio on the CO and NOx emissions for a twin-spark motorcycle gasoline engine under wide range of operating conditions

In the present study, the possibility and limitation of lean burn in a motorcycle engine were explored, from the aspect of exhaust emissions. The CO and NOx emissions were studied by experimental measurements and combined simulations on a twin-spark motorcycle gasoline engine under wide range of operating conditions. The results illuminated that, the CO emissions drop sharply when air/fuel mixture changed from rich to lean. Particularly, at full load the CO emission decreases by about 4 times when λ changed from 0.85 to 1.1, and it keeps decreasing to only about one-tenth at λ = 1.2. Besides, at λ = 1.1 and full engine load the NO emission is about 2.4 times of that at λ = 0.85. Not so much variation has been observed between λ = 1.1 and λ = 1.2. This indicates enlarging the lean burn range will not further deteriorate NOx emission, but benefit from CO emission by a large margin. It is further found the variation in CO emission amount between λs is not mainly due to which reaction is in favor, but due to the amount of its precursors. Besides, almost all the NO is produced by the thermal-NO mechanism.

Mechanisms of NO formation in MILD combustion of CH4/H2 fuel blends

The mechanisms of formation and destruction of NO in MILD combustion of CH4/H2 fuels blends are investigated both experimentally and numerically. Experiments are carried out at a lab-scale furnace with the mass fraction of hydrogen in fuel ranging from 0% to 15%; furnace temperature, extracted heat and exhaust NOx emissions are measured. Detailed chemical kinetics calculations utilizing computational fluid dynamics (CFD) and well-stirred reactor (WSR) are performed to better analyze and isolate the different mechanisms.When the MILD combustion of the CH4/H2 fuel is established in experiments, the thermal field is quasi uniform and the high temperature zone is located at the junction of the fuel and air jets. As the mass fraction of hydrogen in fuel is increased from 5.7% to 14.4%, although the furnace average temperature is increased, the NOx emission remains unchanged. This cannot be explained by the thermal NO mechanism. CFD and WSR simulations both suggest that, when equivalence ratio ≤0.8, the N2O-intermediate route controls the NO formation and the NO-reburning reaction is also strong. With the hydrogen addition, the importance of the NNH route is increased but that of the prompt route is decreased, consequently non-affecting the NOx emission as measured.Chemical kinetics calculations indicate that the conversion from NO to NO2 becomes significant and thus the relative importance of NO2 is increased in the total NOx emission under low temperature MILD conditions. As the reactor temperature is increased from 1100 K to 1600 K, the importance of N2O route decreases while that of thermal route increases. In contrast, as the initial mass fraction of oxygen is increased from 3% to 9%, the importance of N2O route increases but that of the prompt and NNH routes decreases. Likewise, as the equivalence ratio increases, the NO-reburning reaction becomes strong. Worth noting is that the N2O-intermediate route controls the NO production under fuel lean conditions whereas the prompt route is dominant in rich ones.

Computational Fluid Dynamics Study of Alternative Nitric-Oxide Emission Mechanisms in a Spark-Ignition Engine Fueled with Hydrogen and Operating in a Wide Range of Exhaust Gas Recirculation Rates for Load Control

AbstractNitric oxide (NO) emissions are practically the only ones emitted from spark-ignition (SI), hydrogen-fueled engines, and their reliable prediction is important in engine simulation codes. In this work, the reaction mechanisms of nitric oxide are investigated in such engines during load variation by using a very wide range of exhaust gas recirculation (EGR) rates, up to 47%. For that purpose, a three-dimensional computational fluid dynamics code is applied, which has been developed by the authors and validated for its main sub-models, such as the heat transfer and combustion. The latter one includes the thermal NO mechanism, widely known as “Zeldovich mechanism,” whereas two alternative production paths have been included, viz. through the NNH and N2O species formation in order to improve the numerical predictions. The NNH path has been shown to be favored under lean and low-temperature combustion conditions, especially for hydrogen flames, whereas the N2O path becomes important for lean flames irr...

Investigation on combustion characteristics and NO formation of methane with swirling and non-swirling high temperature air

Combustion characteristics of methane jet flames in an industrial burner working in high temperature combustion regime were investigated experimentally and numerically to clarify the effects of swirling high temperature air on combustion. Speziale-Sarkar-Gatski (SSG) Reynolds stress model, Eddy-Dissipation Model (EDM), Discrete Ordinates Method (DTM) combined with Weighted-Sum-of-Grey Gases Model (WSGG) were employed for the numerical simulation. Both Thermal-NO and Prompt-NO mechanism were considered to evaluate the NO formation. Temperature distribution, NO emissions by experiment and computation in swirling and non-swirling patterns show combustion characteristics of methane jet flames are totally different. Non-swirling high temperature air made high NO formation while significant NO prohibition were achieved by swirling high temperature air. Furthermore, velocity fields, dimensionless major species mole fraction distributions and Thermal-NO molar reaction rate profiles by computation interpret an inner exhaust gas recirculation formed in the combustion zone in swirling case.

A numerical study on extinction and NOx formation in nonpremixed flames with syngas fuel

The flame structure, extinction, and NOx emission characteristics of syngas/air nonpremixed flames, have been investigated numerically. The extinction stretch rate increased with the increase in the hydrogen proportion in the syngas and with lower fuel dilution and higher initial temperature. It also increased with pressure, except for the case of highly diluted fuel at high pressure. The maximum temperature and the emission index of nitric oxides (EINOx) also increased in aforementioned conditions. The EINOx decreased with stretch rate in general, while the decreasing rate was found to be somewhat different between the cases of N2 and CO2 dilutions. The reaction paths of NOx formation were analyzed and represented as NO reaction path diagram. The increase in N radical resulted in larger NOx production at high initial temperature and pressure. As the pressure increases, EINOx increases slower due to the third-body recombination. The thermal NO mechanism is weakened for high dilution cases and non-thermal mechanisms prevail. The combustion conditions achieving higher extinction stretch rate can be lead to more NOx emission, therefore that the selection of optimum operation range is needed in syngas combustion.

A comparison of the thermal, emission and heat transfer characteristics of swirl-stabilized premixed and inverse diffusion flames

Abstract Two swirl-stabilized flames, a premixed flame (PMF-s) and an inverse diffusion flame (IDF-s), were investigated experimentally in order to obtain information on their thermal, emission and heat transfer characteristics. The two flames, having different global air/fuel mixing mechanisms, were compared under identical air and fuel flow rates. Results showed that the two flames have similar visual features such as flame shape, size and structure because the Reynolds number and the swirl number which are important parameters representative of the aerodynamic characteristics of a swirling jet flow, are almost the same for the two flames. The minor dissimilarity in flame color and flame length indicates that the IDF-s is more diffusional. Both the PMF-s and IDF-s are stabilized by the internal recirculation zone (IRZ) and the IDF-s is more stable. Flame temperature is uniformly distributed in the IRZ due to the strong mixing caused by flow recirculation. The highest flame temperature is achieved at the main reaction zone and it is higher for the PMF-s due to more rapid and localized heat release. For the IDF-s, the thermal NO mechanism dominates the NO x formation. For the PMF-s, both the thermal and prompt mechanisms tend to play important roles in the global NO x emission under rich conditions. The comparison of EINO x and EICO shows that the PMF-s has lower level of NO x emission under lean combustion and lower level of CO emission under all conditions. The reason is that the air/fuel premixing in the PMF-s significantly enhances the mixedness of the supplied air/fuel mixture. The analysis of the behaviors of the impinging PMF-s and IDF-s heat transfer reveals that because the PMF-s has more rapid and localized heat release at the main reaction zone, the peak heat flux is higher than that of the IDF-s and the IDF-s has more uniform heating effect. A comparison of the overall heat transfer rates shows that, due to more complete combustion, the PMF-s has higher overall heat transfer.

Correlations for dependence of NO x emissions on heat loss in premixed CH 4/air combustion

The present study represents an effort to correlate the dependence of NO x emissions on heat losses to the atmospheric environment in a CH 4/air fueled combustor. To this end, the numerical analysis was performed over a wide range of residence times, equivalence ratios and heat losses using a perfectly stirred reactor (PSR) code. The numerical results showed that the calculated NO x concentration initially increased, reached a maximum value and then decreased with increasing residence time when the heat loss was present. The similar variation was observed in changes in the thermal NO concentration that was evaluated by only considering the reactions associated with the thermal (Zeldovich) NO mechanism. With the heat loss increased, the calculated NO x concentration was substantially reduced for all equivalence ratios investigated. In addition, the reductions in the NO x concentration with respect to residence time became faster with increasing the equivalence ratio particularly for fuel rich conditions. The observed variations in the calculated NO x concentration over the residence time (NO x / τ) were found to fit well to the following correlation: ln ( NO x / τ ) = a ( HLI ) + b . In the correlation, HLI is the dimensionless heat loss parameter and coefficients a and b are constants expressed as a function of adiabatic flame temperature (for a given equivalence ratio) and equivalence ratio, respectively.

Laser-induced fluorescence measurements in venturi cascaded propane gas jet flames

<title/>A propane jet diffusion flame at a burner exit Reynolds number of 5100 was examined with a set of venturis of specific sizes and spacing arrangement. The venturi cascading technique resulted in a decrease of 33% in NO emission index along with a 24% decrease in soot emission from the flame, compared to the baseline condition (same flame without venturis). The temperature measurements showed that, in the near-burner region, the venturi cascaded flame has an average temperature lower by 5% than the baseline flame. However, in the mid-flame and far-burner regions, it has a temperature 13 and 12% higher, respectively. The NO composition measurements revealed that in the near-burner, mid-flame and far-burner regions, the venturi cascaded flame has lower NO concentrations by 16, 5 and 3%, in average values respectively, compared to the baseline case. In order to understand the above results and observations, Laser induced fluorescence (LIF) spectroscopy was employed to detect the minor species (OH, CH and CN) in the baseline and venturi cascaded flames. The LIF measurements, in the near-burner region of the venturi cascaded flame, indicated an average decrease of 18, 24 and 12% in OH, CH and CN radical species respectively, from their baseline values. However, in the mid-flame region, a 40% average increase in OH, from its baseline value, was observed. In this particular region, the CH or CN radicals were not detected. The LIF results were used to understand the thermochemical processes involved including soot and NO formation. It showed that OH radical, in the downstream locations, was mostly affected by soot rather than by temperature. In addition, prompt NO mechanism appeared to play a significant role besides the conventional thermal NO mechanism.

Thermal and emission characteristics of a turbulent swirling inverse diffusion flame

Abstract The thermal and emission characteristics of a swirl-stabilized turbulent inverse diffusion flame (IDF) burning liquefied petroleum gas (LPG) were studied experimentally and the results of visible flame lengths, flame temperatures, in-flame gaseous species concentrations and global pollutant emissions were reported. The flame shape and length of the swirling IDF and a non-swirling IDF were compared. The swirling IDF is featured by a large internal recirculation zone (IRZ), which plays an important role in stabilizing and shortening the flame. Compared with the non-swirling IDF, the swirling IDF is shorter, wider and more stable. For the swirling IDF, both temperature and species distributions are uniform in the IRZ. Comparison of the radial NOx/temperature distributions indicates that the thermal NO mechanism plays a leading role in NOx formation, since the high-temperature IRZ favors thermal NO production. The effects of air jet Reynolds number (Re) and overall equivalence ratio (Ф) on centerline temperature and emission index were examined. The main finding is that the IRZ which is large in size and high in temperature dominates the thermal and emission characteristics of the swirling flame. Efforts were made to compare the global NOx and CO emissions of the swirling and non-swirling IDFs. It was found that strong swirl and lean combustion are two key factors for reducing NOx emission. However, the decreasing NOx emission is compromised by increasing CO. Under stoichiometric and rich conditions, EINOx of the swirling IDFs is slightly higher, but the EICO is significantly lower. Further comparison of EINOx with other studies indicates that the swirling IDF can achieve low NOx emission.

완전혼합 반응기에서 CH4/Air 예혼합화염의 열손실율에 따른 Nox 생성특성

Abstract The effect of heat loss rate on NOx formation of CH 4 /air premixed flame were examined numerically in a perfectly stirred reactor. The following conclusions were drawn. Under the adiabatic wall condition, an increase in the residence time causes a remarkable increases in NOx emission. Under the heat loss conditions, however, NOx decreases significantly as the heat transfer coefficient and residence time increase. As the heat loss rate increases, Thermal NO mechanism and Re-burning NO mechanism play an important role in the NOx reduction, but Prompt NO mechanism and N 2 O-intermediate NO mechanism lead to the increase in NOx production. Although the NOx formation is actually related to complex NOx mechanism with the changes in the heat transfer coefficient and residence time, it was found that NOx concentration can be represented by independent Thermal NO mechanism. From these results, new NOx correlation combined with the heat loss rate and residence time was suggested for predicting the NOx concentration in a practical CH

Numerical study on laminar burning velocity and NO formation of premixed methane–hydrogen–air flames

Abstract Numerical study on laminar burning velocity and NO formation of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity, adiabatic flame temperature, and radical mole fractions of H, OH and NO are obtained at various equivalence ratios and hydrogen fractions. The results show that the unstretched laminar burning velocity is increased with the increase of hydrogen fraction. Methane-dominated combustion is presented when hydrogen fraction is less than 40%, where laminar burning velocity is slightly increased with the increase of hydrogen addition. When hydrogen fraction is larger than 40% , laminar burning velocity is exponentially increased with the increase of hydrogen fraction. A strong correlation exists between burning velocity and maximum radical concentration of H + OH radicals in the reaction zone of premixed flames. High burning velocity corresponds to high radical concentration in the reaction zone. With the increase of hydrogen fraction, the overall activation energy of methane–hydrogen mixture is decreased, and the inner layer temperature and Zeldovich number are also decreased. All these factors contribute to the enhancement of combustion as hydrogen is added. The curve of NO versus equivalence ratio shows two peaks, where they occur at the stoichiometric mixture due to Zeldovich thermal-NO mechanism and at the rich mixture with equivalence ratio of 1.3 due to the Fenimore prompt-NO mechanism. In the stoichiometric flames, hydrogen addition has little influence on NO formation, while in rich flames, NO concentration is significantly decreased. Different NO formation responses to stretched and unstretched flames by hydrogen addition are discussed.

Effects of Heat Loss on NOx Emission in the Postflame Region of Premixed CH4−Air Combustion

The nitrogen oxides (NOx) formation characteristics of a one-dimensional premixed CH4/air flame using detailed-kinetic chemistry were examined numerically. The combustor length and the amount of heat loss were varied to investigate the effects of the heat loss and the residence time on NOx formation in the postflame region of the combustor. The following conclusions were drawn. Under the adiabatic wall condition, in the flame region, NO is primarily produced by the prompt NO mechanism; however, the thermal NO mechanism is more important than other mechanisms in the postflame region. Thus, an increase in the combustor length leads to an increase of residence time and causes a remarkable increase of the NOx emission index. Under the heat loss conditions in the postflame region, NO production does not occur additionally at the postflame region, and the NO level decreases considerably compared to that under adiabatic condition as the heat loss rate increases. Specifically, in the case of ϕ = 1.0, even when the temperature decreases by approximately 15% compared to that of the adiabatic condition in a combustor exit of 10 cm, the NO concentration is reduced significantly to 70%. From these results, it can be concluded that control of both the residence time and the heat loss in a practical combustor should be recognized as important factors for NOx reductions.

The effects of composition on the burning velocity and NO formation in premixed flames of C 2H 4 + O 2 + N 2

Experimental measurements of the adiabatic burning velocity and NO formation in C 2H 4 + O 2 + N 2 flames are presented. The oxygen content in synthetic air varied from 18% down to 14%. Non-stretched flames were stabilized on a perforated plate burner at 1 atm. A heat flux method was used to determine the burning velocities under the conditions when the net heat loss of the flame is zero. Adiabatic burning velocities of ethylene + nitrogen + oxygen mixtures were found in good agreement with the modeling. Sampling measurements were performed at 10–20 mm from the burner and concentrations of stable species, CO, CO 2, O 2 and NO x were recorded. The concentrations of CO, CO 2 and O 2 were compared with the modeling to reveal that the range of the flame conditions was less affected by the ambient air entrainment. The dependencies of [NO] as a function of equivalence ratio clearly possess two maxima: in stoichiometric mixtures due to Zeldovich thermal-NO mechanism and in rich mixtures at equivalence ratio close to 1.4 due to Fenimore prompt-NO mechanism. Dilution by nitrogen decreases [NO] at any equivalence ratio. Numerical predictions of the concentrations of NO in a post-flame zone of lean and stoichiometric flames are in good agreement with the experiments when downstream heat losses to the environment were taken into account. The predictions of the Konnov mechanism in rich ethylene flames reproduce trends of the experimental data with an under-prediction of about 10–15 ppm.

Role of the Chemical Kinetics on Modeling NOx Emissions in Diesel Engines

New diesel engine strategies (involving high injection pressure and multiple injections) have been proposed in recent years aiming to reduce pollutant emissions (mainly NOx and particulate matter). These strategies have led to very fast combustion processes as a consequence of the improvement on the fuel atomization, evaporation, and air entrainment phenomena. Although NOx emissions models for diesel engines usually assume equilibrium and/or the steady state hypothesis together with the consideration of very simplified kinetic reaction mechanisms (such as the Lavoie method based on the extended Zeldovich reaction mechanism), modern diesel engine combustion models require more complex chemical kinetic approaches due mainly to the lack of time to reach the equilibrium state. These kinetic considerations are even more important for simulating new diesel combustion concepts (such as homogeneous charge compression ignition (HCCI) and low temperature combustion (LTC)), which are well-known to be kinetically controlled. In such a frame, this work shows, through a reaction mechanism which considers 83 reactions and 38 chemical compounds, the role of the kinetics on the local NO formation/destruction paths under typical diesel engine conditions. The analysis has been carried out for different combustion and dilution rates, showing that equilibrium assumptions overestimate the engine NO emissions. Reactions controlling NO formation during both the combustion and dilution processes have been identified through a sensitivity analysis carried out by using the chemical package CHEMKIN 4.0. This analysis suggests a coupling between the fuel oxidation process and the thermal NO mechanism, which is not considered in most of the diesel engine NOx models. Finally, the differences in modeled engine NO emissions caused by the uncertainties derived from the different kinetic rate values used in the literature are also presented, showing a significant need for more reliable experimental kinetic data obtained under engine pressure and temperature conditions.

CH4/Air 예혼합화염의 하류영역에서 체류시간 및 열손실에 의한 NOx의 생성특성

In this study, the NOx formation characteristics of one-dimensional CH4/Air premixed flameusing detailed-kinetic chemistry are examined numerically. The combustor length and the amount of heat loss are varied to investigate the effect of residence time and heat loss on the NOx formation in a post-flame region. In the flame region, NO is mainly produced by the Prompt NO mechanism including N2O-intermediate NO mechanism over all equivalence ratios. However, thermal NO mechanism is more important than Prompt NO mechanism in the post-flame region. In the case of adiabatic condition, the increase of combustor length causes the remarkable increase of NO emission at the exit due to the increase of residence time. On the other hand, NO reaches the equilibrium state in the vicinityof flame region, considering radiation and conduction heat losses. Furthermore the NO, in the case of φ=1.2, is gradually reduced in the downstream region as the heat loss is increased. From these results, it can be concluded that the controls of residence time and heat loss in a combustor should be recognized as an important NOx reduction technology.

Experimental study of air dilution in oxy-liquid fuel flames

The influence of oxidizer dilution in oxy-liquid ethanol flames is experimentally investigated by using a coaxial air-assisted injector positioned in a vertical combustion chamber. This study accounts for the influence of a two-phase mode since two different injector geometries are used: for the first configuration, a vaporization mode is observed at nominal power in oxy conditions, while for the second one, a brush mode is observed. Dilution with air is applied by keeping oxidizer velocity constant. Flame structure is observed through CH emission: dilution leads to an increase in the flame diameter, and collective effects of two-phase combustion are encouraged. The effect of dilution on oxy flame stability is also studied: for a given oxygen mass fraction in the oxidizer, the oxidizer flow rate is increased until extinction occurs. Dilution leads to a less stable flame, which may be essentially explained by the decrease in laminar flame speed with dilution. For high oxidizer dilution levels, the change in flame structure might be another parameter to consider. Finally, species concentrations are measured using a standard gas sampling technique. NO and CO evolutions with dilution are different between both two-phase combustion regimes. An empirical approach based on thermal NO mechanism and CO oxidation reaction enables one to explain the evolutions for brush mode. For vaporization mode, the residence time in burned gases is also to be considered.

Numerical Study on Flame Structure and NOx Formation of Counterflow Flame Using Oxygen-Enriched Air (Effect of Strain Rate on Reduction of NOx Formation)

In this study, numerical simulations for the flame structure and NOx formation of counterflow diffusion flame with oxygen-enriched air were made by using the GRI chemical reaction mechanism (NOx included) in order to elucidate the effect of strain rate on reduction of NOx formation. The following conclusions were reached : (1) As the strain rate increases, the maximum heat release rate increases, and the width of high-temperature zone becomes narrow. (2) In the case where the strain rate is small, the thermal NO mechanism is dominant of NO formation, and NO formation by the prompt NO mechanism is negative. (3) As the strain rate increases, NO formation by the prompt NO mechanism increases, and it becomes positive when strain rate exceeds 400s-1. At the same time, since the width of high-temperature zone becomes narrow, that by the thermal NO mechanism decreases, and it becomes smaller than that by prompt NO mechanism for the case where the strain rate exceeds 1200 s-1. The decrease rate of thermal NO is bigger than the increase rate of prompt NO, so that the total NO formation is reduced by the increase in the strain rate.

Kinetic modeling of the decomposition and flames of hydrazine

The combustion of lean, stoichiometric and rich hydrogen–air mixtures in well-stirred reactors has been modeled in the temperature range 1500–2200 K employing a detailed H/N/O reaction scheme, and a H/O reaction scheme together with algebraic expressions for the NO production rate as a function of local gas composition and temperature. Thermal-NO, nitrous oxide, and NNH mechanisms of nitric oxide formation have been taken into account. It is demonstrated that explicit expressions of the NO formation rate correctly reproduce the prediction of the detailed mechanism for residence times longer than about 100 μs at all temperatures of this study in rich, stoichiometric and lean mixtures. With time increase the reduced mechanism deviates from the full one in rich mixtures at residence times longer than about 10 ms. The evaluation of the NNH route showed that this pathway is important at all temperatures at short residence times. At temperatures of 2100 K and higher the thermal-NO mechanism becomes dominant after about 1 ms. Sensitivity analysis clearly demonstrates the validity of the assumptions employed in the development of the explicit expressions for the instantaneous NO formation rates in hydrogen combustion in the range of residence times from 1 to 25 ms. A possible new route for NO formation in rich hydrogen mixtures via N2H3 radicals is identified.

Flame Structure and NOx Formation of Counterflow Flame Using Oxygen-Enriched Air.

In this study, numerical simulations of the counterflow diffusion flame and the counterflow double flame with oxygen-enriched air were made by using the GRI chemical reaction mechanism (NOx included) in order to elucidate the effects of oxygen concentration and equivalence ratio of mixture on the flame structure and NOx formation. The following conclusions were reached: (1) In the case of counterflow diffusion flame, as the oxygen concentration increases, the width of high-temperature zone becomes larger, so that NO formation by the thermal NO mechanism increases, but that by the prompt NO mechanism decreases and becomes negative. (2) In the case of counterflow double flame, as the equivalence ratio becomes large, the NO formation by the prompt NO mechanism at the position of the fuel-rich premixed flame decreases, and that by the thermal NO mechanism at the position of the diffusion flame keeps approximately constant. (3) In the both flames, NO formation has a maximum at about YO2=0.8, and it decreases steeply above YO20.8, because the nitrogen concentration decreases.