Soot formation and temperature field structure in co-flow laminar methane-air diffusion flames at pressures from 10 to 60 atm

The effects of pressure on soot formation and the structure of the temperature field were studied in co-flow methane-air laminar diffusion flames over a wide pressure range, from 10 to 60 atm in a high-pressure combustion chamber. The selected fuel mass flow rate provided diffusion flames in which the soot was completely oxidized within the visible flame envelope and the flame was stable at all pressures considered. The spatially resolved soot volume fraction and soot temperature were measured by spectral soot emission as a function of pressure. The visible (luminous) flame height remained almost unchanged from 10 to 100 atm. Peak soot concentrations showed a strong dependence on pressure at relatively lower pressures; but this dependence got weaker as the pressure is increased. The maximum conversion of the fuel’s carbon to soot, 12.6%, was observed at 60 atm at approximately the mid-height of the flame. Radial temperature gradients within the flame increased with pressure and decreased with flame height above the burner rim. Higher radial temperature gradients near the burner exit at higher pressures mean that the thermal diffusion from the hot regions of the flame towards the flame centerline is enhanced. This leads to higher fuel pyrolysis rates causing accelerated soot nucleation and growth as the pressure increases.     

Steady large-scale modulation of a moderately turbulent co-flow jet

The effects of a spatial modulation acting at the inflow of a moderately turbulent planar jet surrounded by a faster co-flow are investigated using direct numerical simulation of the Navier–Stokes equations. We adopt a superposition of spatially filtered small-scale random perturbations and a structured large-scale flow modulation. The large-scale modulation is characterised in terms of a Beltrami flow, specified by a wavenumber K. These large-scale modulations are steady and spatially periodic, while the random small-scale perturbations fluctuate in time and in space. The flow configuration studied in this paper is agitated by this combined large- and small-scale agitation at the inflow plane of a rectangular domain of size L × L × 2L in the x-, y- and streamwise z-directions. The inflow perturbation is focused on a strip of size L × D in the x- and y-directions. A parametric variation is carried out considering different choices for the wavenumber of the large-scale modulation. We focus on effects that the inflow modulation has on global characteristics of the flow, e.g. the width of the mixing region formed between the two streams and the dissipation rate, ?. Results show that the width of the mixing region increases faster compared to the case without the large-scale perturbation, when the flow is agitated by structures of size comparable to the integral scales of the flow. For the dissipation rate, results show the presence of a maximum response at a certain wavenumber K in case we apply a large-scale modulation. This maximum is attained at modulation scales that vary locally with respect to the distance from the inflow plane. Close to the inflow, the maximum response occurs at small modulation scales, while further into the domain a maximum response is present at comparably large modulation scales.     

Polydisperse effects in jet spray flames

A laminar jet polydisperse spray diffusion flame is analysed mathematically for the first time using an extension of classical similarity solutions for gaseous jet flames. The analysis enables a comparison to be drawn between conditions for flame stability or flame blow-out for purely gaseous flames and for spray flames. It is found that, in contrast to the Schmidt number criteria relevant to gas flames, droplet size and initial spray polydispersity play a critical role in determining potential flame scenarios. Some qualitative agreement for lift-off height is found when comparing predictions of the theory and sparse independent experimental evidence from the literature.     

Effect of ammonia addition on suppressing soot formation in methane co-flow diffusion flames

Due to issues surrounding carbon dioxide emissions from carbon-containing fuels, there is growing interest in ammonia (NH₃) as an alternative combustion fuel. One attractive method of burning NH₃ is to co-fire it with hydrocarbons, such as natural gas, and in this case soot formation is possible. To begin understanding the influence of NH₃ on soot formation when co-fired with hydrocarbons, soot volume fractions and mole fractions of gas-phase species were computationally and experimentally interrogated for CH₄ flames with up to 40% NH₃ by volumetric fuel fraction. Mole fractions of gas-phase species, including C₂H₂ and C₆H₆, were measured with on-line electron impact mass spectrometry, and soot volume fractions were obtained via color-ratio pyrometry. The simulations employed a detailed chemical mechanism developed for capturing nitrogen interactions with hydrocarbons during combustion. The results are compared to findings in N₂CH₄ flames, in order to separate thermal and dilution effects from the chemical influence of NH₃ on soot formation. Experimentally, C₂H₂ concentrations were found to decrease slightly for the NH₃CH₄ flames relative to N₂CH₄ flames, and a stronger suppression of C₆H₆ was found for NH₃ relative to N₂ additions. The measured results show a strong suppression of soot with the addition of NH₃, with soot concentrations reduced by over a factor of 10 with addition of up to 20% or more NH₃ by mole fraction. The model satisfactorily captured relative differences in maximum centerline C₂H₂, C₆H₆, and soot concentrations with addition of N₂, but was unable to match measured differences in NH₃CH₄ flames. These results highlight the need for an improved understanding of fuel-nitrogen interactions with higher hydrocarbons to enable accurate models for predicting particulate emissions from NH₃/hydrocarbon combustion.     

Interferometric visualization of jet flames

This paper presents visualizations of reacting, round jets of the premixed and nonpremixed type realized by using interferometry and, complementarily, direct photography. The available interferometer, proposed by Carlomagno (1986), employs low-cost components and is flexible and robust to geometrical misalignments, allowing the drawbacks limiting the application of traditional interferometric systems to be overcome. Several flames are produced by varying the non-dimensional, governing parameters (Reynolds number, equivalence ratio, Grashof number). The results discussion is organized considering laminar, transitional and turbulent flows. In the steady, laminar case, in view of the radial symmetry of the fringes pattern, the temperature field is reconstructed by the interferograms. The structure of the transitional and turbulent combusting jets, primarily determined by shear layer destabilization mechanisms and large-scale vortices formation due to buoyancy, is analyzed and differences with isothermal flows are pointed out. In turbulent regime, studied only for premixed combustion case, qualitative insights into the structure of the reaction zone as a function of the equivalence ratio and turbulence properties in the incoming fresh mixture are also deduced.     

Air/fuel mixing in jet flames

The paper examines eight diverse regimes in which fuels can mix and react with air. These comprise: (i) Lifted subsonic; and (ii) supersonic jet flames, with (iii) and without (iv) cross flows; (v) Rim-attached flames; (vi) Early Downwash flames; (vii) Downwash-attached jet flames; and (viii) Fire Whirls.Correlations of characteristics within these regimes are principally in terms of a dimensionless Flow Number, U*, Cross Flow Reynolds number, Re_c, and, for Fire Whirls, a dimensionless Critical Velocity, CV. Boundaries of seven of the eight regimes are identified, through plots of U*, against Re_c, and of the eighth through a plot of CV against U*. The circumstances of transitions between regimes are identified. The study involves a variety of CH₄ cross flow flame measurements, in a wind tunnel. Cross flows can initially create a small lee-side flame downwash, due to the depression in pressure. With increasing fuel flow this might extend 1.3 m downwards from the horizontal tip of the vertical burner. Jet flames can attach to the downwash, which can become significant above Re_c ≈ 2000. More extensive downwash might further delay blow-off. Regime boundaries are constructed on the U*/Re_c diagram covering lifted flames, early downwash, and downwash-attached flames. The most powerful flames tend to be lifted, choked, flames, with cross flow, and fire whirls. Combustion becomes less efficient at high Re_c and low U*, although CH₄ was efficiently reacted.Experimental values of the ratio of fuel to air velocity, u/u_c, of CH₄ flames ranged between about 10 and 30 for lifted flames, and between 0.3 and 3.6, at blow-off, for rim-attached flames. The latter comprise an important category, often intermediate between lifted flames and downwash-attached flames.     

Soot studies of laminar diffusion flames with recirculation zones

This paper describes the unusual sooting structure of three flames established by the laminar recirculation zones of a centerbody burner. The vertically mounted burner consists of an annular air jet and a central fuel jet separated by a bluff-body. The three ethylene fueled flames are identified as: fully sooting, donut-shape, and ring-shape sooting flames. Different shapes of the soot structures are obtained by varying the N₂ dilution in the fuel and air jets while maintaining a constant air and fuel velocity of 1.2 m/s. All three flames have the unusual characteristic that the soot, entrained into the recirculation zone, follows discrete spiral trajectories that terminate at the center of the vortex. The questions are what cause: (1) the unusual sooting structures and (2) the spiral trajectories of the soot? Flame photographs, laser sheet visualizations, and calculations with a 2D CFD-based code (UNICORN) are used to answer these questions. The different sooting structures are related to the spiral transport of the soot, the spatial location of the stoichiometric flame surface with respect to the vortex center, and the burnout of the soot particles. Computations indicate that the spiral trajectories of the soot particles are due to thermophoresis.     

A numerical study of auto-ignition in turbulent lifted flames issuing into a vitiated co-flow

This paper presents a numerical study of auto-ignition in simple jets of a hydrogen–nitrogen mixture issuing into a vitiated co-flowing stream. The stabilization region of these flames is complex and, depending on the flow conditions, may undergo a transition from auto-ignition to premixed flame propagation. The objective of this paper is to develop numerical indicators for identifying such behavior, first in well-known simple test cases and then in the lifted turbulent flames. The calculations employ a composition probability density function (PDF) approach coupled to the commercial CFD code, FLUENT. The in-situ-adaptive tabulation (ISAT) method is used to implement detailed chemical kinetics. A simple k–ε turbulence model is used for turbulence along with a low Reynolds number model close to the solid walls of the fuel pipe.

The first indicator is based on an analysis of the species transport with respect to the budget of convection, diffusion and chemical reaction terms. This is a powerful tool for investigating aspects of turbulent combustion that would otherwise be prohibitive or impossible to examine experimentally. Reaction balanced by convection with minimal axial diffusion is taken as an indicator of auto-ignition while a diffusive–reactive balance, preceded by a convective–diffusive balanced pre-heat zone, is representative of a premixed flame. The second indicator is the relative location of the onset of creation of certain radical species such as HO₂ ahead of the flame zone. The buildup of HO₂ prior to the creation of H, O and OH is taken as another indicator of autoignition.

The paper first confirms the relevance of these indicators with respect to two simple test cases representing clear auto-ignition and premixed flame propagation. Three turbulent lifted flames are then investigated and the presence of auto-ignition is identified. These numerical tools are essential in providing valuable insights into the stabilization behaviour of these flames, and the demarcation between processes of auto-ignition and premixed flame propagation.     

The effect of co-flow stream velocity on turbulent non-premixed jet flame stability

The stability behaviour of non-premixed jet flames in a co-flowing air stream was investigated experimentally. The experimental data obtained indicate that there exists a range of co-flow velocity where two distinctly different extinction limits can occur at the same co-flow velocity depending on whether the flame is lifted or attached at ignition. Results show that co-flow velocity has a much greater effect on the blowout limits of lifted flames than on the blowoff limits of attached flames. The blowout limit of lifted flames initially increase linearly with co-flow velocity independent of nozzle diameter until a peak value is reached, after which it decreases rapidly with increasing co-flow velocity. Such behaviour appears to be governed by two different mechanisms. A model for predicting lifted flame blowout limits has been developed. It is based on the ratio of the Kolmogorov time scale and the chemical time scale as a function of a jet similarity parameter. The model was used to predict the blowout limits for methane as well as the effect of diluents in either fuel or co-flow stream. Results show very good agreement with experimental data in the current investigation.     

Investigation of the transition from lightly sooting towards heavily sooting co-flow ethylene diffusion flames

Laminar, sooting, ethylene-fuelled, co-flow diffusion flames at atmospheric pressure have been studied experimentally and theoretically as a function of fuel dilution by inert nitrogen. The flames have been investigated experimentally using a combination of laser diagnostics and thermocouple-gas sampling probe measurements. Numerical simulations have been based on a fully coupled solution of the flow conservation equations, gas-phase species conservation equations with complex chemistry and the dynamical equations for soot spheroid growth. Predicted flame heights, temperatures and the important soot growth species, acetylene, are in good agreement with experiment. Benzene simulations are less satisfactory and are significantly under-predicted at low dilution levels of ethylene. As ethylene dilution is decreased and soot levels increase, the experimental maximum in soot moves from the flame centreline toward the wings of the flame. Simulations of the soot field show similar trends with decreasing dilution of the fuel and predicted peak soot levels are in reasonable agreement with the data. Computations are also presented for modifications to the model that include: (i) use of a more comprehensive chemical kinetics model; (ii) a revised inception model; (iii) a maximum size limit to the primary particle size; and (iv) estimates of radiative optical thickness corrections to computed flame temperatures.     

PDF calculations of piloted premixed jet flames

A series of piloted premixed jet flames with strong finite-rate chemistry effects is studied using the joint velocity-turbulence frequency-composition PDF method. The numerical accuracy of the calculations is demonstrated, and the calculations are compared to experimental data. It is found that all calculations show good agreement with the measurements of mean and rms mixture fraction fields, while the reaction progress is overpredicted to varying degrees depending on the jet velocity. In the calculations of the flame with the lowest jet velocity, the species and temperature show reasonable agreement with the measurements, with the exception of a small region near the centerline where products and temperature are overpredicted and fuel and oxidizer are underpredicted. In the calculations of the flame with the highest jet velocity, however, the overprediction of products and temperature and underprediction of fuel and oxidizer is far more severe. An extensive set of sensitivity studies on inlet boundary conditions, turbulence model constants, mixing models and constants, radiation treatment, and chemical mechanisms is conducted to show that any parameter variation offers little improvement from the base case. To shed light on these discrepancies, diagnostic calculations are performed in which the chemical reactions are artificially slowed. These diagnostic calculations serve to validate the experimental data and to quantify the amount by which the base case calculations overpredict reaction progress. Improved calculations of this flame are achieved only through artificially slowing down the chemical reaction by a factor of about 10. The mixing model behavior in this combustion regime is identified as a likely cause for the observed discrepancy in reaction progress.     

Methane flames in a jet impinging onto a wall

Traditionally, research has focused on positive stretch in the stagnation flow and negative stretch along the Bunsen flame. Only a very limited amount of research has been devoted to studying the behavior of a conical Bunsen flame established in a stagnation flow, which is significantly affected by the combined effects of the curvature stretch and the aerodynamic straining. This investigation is aimed at studying the characteristics of laminar conical premixed flames in an impinging jet flow experimentally and theoretically. First, we analyze the transport processes of a nonreactive impinging jet flow numerically. For lower burner-to-plate distance, the potential core becomes concave at the top. Hence, a conical Bunsen flame established in such a flow field may suffer positive flow stretch. The predicted flame shapes using a simple model incorporated with the numerical results agree well with the experimental observations. Flame shapes exhibit double-solution characteristics in a certain range of methane concentrations. Experimentally, by following different paths of adjusting methane concentration (decreasing from rich to lean or increasing from lean to rich), two different flame configurations (planar or conical flame) may exist at the same flow conditions, namely burner-to-plate distance, inlet velocity, and methane concentration. At the higher (or lower) critical methane concentration, the transition from a flat flame to a conical flame (or from a conical flame to a flat flame) occurs. The calculation of stretch and measurement of flame temperature for the low inlet velocity, 0.8 m/s, show that the stretch of a conical flame established in a stagnation flow is negative (dominated by the flame curvature). However, it is important to emphasize that at high velocity, e.g., U_in = 1.6 m/s, a negatively stretched flame tip can suffer positive flow stretch. This significant finding has been verified in the experiment since the conical flame tip is higher than the positively stretched flat flame.     

Influence of pressure on near nozzle flow field and soot formation in laminar co-flow diffusion flames

Soot formation from combustion devices, which tend to operate at high pressure, is a health and environmental concern, thus investigating the effect of pressure on soot formation is important. While most fundamental studies have utilised the co-flow laminar diffusion flame configuration to study the effect of pressure on soot, there is a lack of investigations into the effect of pressure on the flow field of diffusion flames and the resultant influence on soot formation. A recent work has displayed that recirculation zones can form along the centreline of atmospheric pressure diffusion flames. This present work seeks to investigate whether these zones can form due to higher pressure as well, which has never been explored experimentally or numerically. The CoFlame code, which models co-flow laminar, sooting, diffusion flames, is validated for the prediction of recirculation zones using experimental flow field data for a set of atmospheric pressure flames. The code is subsequently utilised to model ethane-air diffusion flames from 2 to 33 atm. Above 10 atm, recirculation zones are predicted to form. The reason for the formation of the zones is determined to be due to increasing shear between the air and fuel steams, with the air stream having higher velocities in the vicinity of the fuel tube tip than the fuel stream. This increase in shear is shown to be the cause of the recirculation zones formed in previously investigated atmospheric flames as well. Finally, the recirculation zone is determined as a probable cause of the experimentally observed formation of a large mass of soot covering the entire fuel tube exit for an ethane diffusion flame at 36.5 atm. Previously, no adequate explanation for the formation of the large mass of soot existed.     

Soot and NO formation in counterflow ethylene/oxygen/nitrogen diffusion flames

Formation of soot and NO in counterflow ethylene/oxygen/nitrogen diffusion flames was numerically investigated. Detailed chemistry and complex thermal and transport properties were used. A simplified two-equation soot model was adopted. The results indicate that NO emission has negligible influence on soot formation. However, soot formation affects the emission of NO through the radiation induced thermal effect and the reaction induced chemical effect. When the oxygen index of the oxidant stream is lower, the relative influence of chemical reaction caused by soot on NO emission is more important, while the relative influence of the radiation induced thermal effect becomes more important for the flame with a higher oxygen index in the oxidant stream.     

Blowout limits of inclined nonpremixed turbulent jet flames

The blowout behavior of inclined nonpremixed turbulent jet flames is investigated by varying the jet inclined angle in the range of -90° to 90° The critical jet velocity at blow-out limit is quantified experimentally for various nozzle diameters, different fuels and inclined angles. Numerical simulations are performed to emphasize the flow field difference for the positive and negative inclined angles. Physical modeling is conducted to incorporate the effect of the inclined angle on blow-out behavior. Major findings include: (1) The negatively inclined jet flames show more intense yellow luminosity with larger sooting zones than the positively inclined jet flames; (2) The blowout limit decreases appreciably with the jet inclined angle for the negatively inclined flames, while for the positively inclined jet flames, this decrease is relatively small; (3) Physical analysis of the flow development of inclined jets is conducted, indicating the centerline velocity along the jet trajectory decreases faster for the flame with smaller inclined angle. And the decrease rate is relatively larger for the negatively inclined jet flames; (4) Based on the analysis of the flow development as well as the characteristic velocity with the inclined angle variation, a model based on the Damköhler number (Da) accounting for the effect of jet inclined angle is developed to characterize the blowout limits of inclined jet flames. The proposed model successfully correlates the experimental data. The present findings provide new data and a basic scaling law for the blowout limit of nonpremixed inclined turbulent jet flames, revealing the effect of the relative angle between the jet momentum and buoyancy.     

Three-dimensional numerical simulations of cellular jet diffusion flames

Recent experimental investigations have demonstrated that the appearance of particular cellular states in circular non-premixed jet flames significantly depends on a number of parameters, including the initial mixture strength, reactant Lewis numbers, and proximity to the extinction limit (Damköhler number). For CO₂-diluted H₂/O₂ jet diffusion flames, these studies have shown that a variety of different cellular patterns or states can form. For given fuel and oxidizer compositions, several preferred states were found to co-exist, and the particular state realized was determined by the initial conditions. To elucidate the dynamics of cellular instabilities, circular non-premixed jet flames are modeled with a combination of three-dimensional numerical simulation and linear stability analysis (LSA). In both formulations, chemistry is described by a single-step, finite-rate reaction, and different reactant Lewis numbers and molecular weights are specified. The three-dimensional numerical simulations show that different cellular flames can be obtained close to extinction and that different states co-exist for the same parameter values. Similar to the experiments, the behavior of the cell structures is sensitive to (numerical) noise. During the transient blow-off process, the flame undergoes transitions to structures with different number of cells, while the flame edge close to the nozzle oscillates in the streamwise direction. For conditions similar to the experiments discussed, the LSA results reveal various cellular instabilities, typically with azimuthal wavenumber m = 1–6. Consistent with previous theoretical work, the propensity for the cellular instabilities is shown to increase with decreasing reactant Lewis number and Damköhler number.     

Transported scalar PDF calculations of autoignition of a hydrogen jet in a heated turbulent co-flow

The joint scalar PDF method, as implemented in FLUENT, was used to simulate the autoignition of a jet of hydrogen in a turbulent co-flow of heated air. While the autoignition phenomenon is intermittent in the experiment, ensemble-averaged data on the effect of the flow on ignition length are available, which enables us to compare them with the steady state calculations.Results of sensitivity tests showed that the choice of chemical mechanism affects the calculation more than the mixing model and model constants. Further calculations for different initial conditions (i.e. temperature and velocity of the jet T _jet and U _jet and the co-flow T _air and U _air) have been done using a set of parameters selected after the sensitivity study. Scatter plots and conditional scalar profiles confirmed that the ignition is always initiated in lean mixture fractions. The ignition length was predicted with good accuracy for the case of U _jet>U _air but not so well for the case of U _jet≈U _air. For the equal velocity case, increasing the velocity resulted in delayed autoignition time (defined as the ignition length divided by the mean velocity), in agreement with the experimental trend. The results give credence to the use of the joint scalar PDF method for autoignition in non-premixed flows.     

Multiple mapping conditioning of turbulent jet diffusion flames

The multiple mapping conditioning (MMC) approach is applied to two non-piloted CH₄/H₂/N₂ turbulent jet diffusion flames with Reynolds numbers of Re = 15,200 and 22,800. The work presented here examines primarily the suitability of MMC to simulate CH₄/H₂ flames with varying Re numbers. The equations are solved in a prescribed Gaussian reference space with only one stochastic reference variable emulating the fluctuations of mixture fraction. The mixture fraction is considered as the only major species on which the remaining minor species are conditioned. Fluctuations around the conditional means are ignored. It is shown that the statistics of the mapped reference field are an accurate model for the statistics of the physical field for both flames. A transformation of the Gaussian reference space introduced in previous work on MMC is used to express the MMC model in the same form as CMC. The most important advantage of this transformation is that the conditionally averaged scalar dissipation term is in a closed form. The corresponding temperature and reactive species predictions are generally in good agreement with experimental data. The application to real laboratory flames and the assessment of the new conditional scalar dissipation model for the closure of the singly conditioned CMC equation is the major novelty of this paper. The results are therefore primarily examined with respect to changes of the conditionally averaged quantities in mixture fraction space.     

Cell formation in non-premixed,axisymmetric jet flames near extinction

Systematic experiments with CO₂ diluted H₂–O₂ circular jet diffusion flames have been undertaken to study the formation of cellular flames, which occur for relatively low reactant Lewis numbers and near the extinction limit. The jet Reynolds number for all experiments was about 500, based on the centreline velocity, jet diameter and ambient fuel properties. The Lewis numbers, based on the initial mixture strength φ and ambient conditions of the investigated near-extinction mixtures, vary in the range 1.1–1.3 for oxygen and 0.25–0.29 for hydrogen (φ is defined here as the fuel-to-oxygen molar ratio normalized by the stoichiometric value). Various conditions near the extinction limit were investigated by fixing the fuel composition (H₂–CO₂ mixture), and systematically lowering the oxygen concentration in the co-flowing oxidizer stream past the point where cellular structures formed, until extinction occurred. The observed different instability states were correlated with the initial mixture strength and the proximity to the extinction limit.

The parameter space for cellularity was found to increase with decreasing initial mixture strength. For a given initial mixture strength, several cellular states were found to co-exist near the extinction limit, and the preferred number of cells (the azimuthal wave number) was observed to decrease with decreasing oxygen concentration (Damköhler number). These trends are consistent with previous theoretical work and our own stability analysis that will be reported elsewhere.     

Numerical simulation of laminar co-flow methane–oxygen diffusion flames: effect of chemical kinetic mechanisms

Laminar co-flow methane–oxygen flames issuing into the unconfined atmosphere have been studied. A numerical model, which employs different chemical kinetics sub-models, including a skeletal mechanism with 43 reaction steps and 18 species and four global reaction mechanisms (two 2-steps and two 4-steps mechanisms), and an optically thin radiation sub-model, has been employed in the simulations. Numerical model has been validated against the experimental results available in literature. The numerical predictions from the global kinetic mechanisms have been compared with the 43-steps mechanism predictions. At all oxygen flow rates, the predictions of the distributions of temperature, mass fractions of CH₄, O₂ and CO₂ by the 2-steps mechanisms are closer to 43-steps mechanism. The overall distribution of H₂O predicted by 2-steps mechanisms is close to that of 43-steps except for the maximum value. Especially at higher oxygen flow rates, the modified 2-steps mechanism predicts these quantities much closer to those predicted by the 43-steps mechanism. Further, the 2-steps mechanisms predict location of the reaction zone accurately. However, they can just give an idea of overall CO distribution in terms of the axial and radial locations within which CO will almost be consumed, but not its maximum value in the domain. The 4-steps mechanisms predict the trend of variation of these quantities quite reasonably. However, they under-predict the location of the reaction zone. At higher oxygen flow rates, the predictions by 4-steps mechanisms becomes better, especially in the prediction of maximum CO and H₂O. Over all, the modified 2-steps mechanism can be recommended for reasonable and economical predictions of oxy-rich methane flames.