Heat release rate as represented by [OH] × [CH2O] and its role in autoignition

Data from a recent instantaneous, simultaneous, high-resolution imaging experiment of Rayleigh temperature and laser induced fluorescence (LIF) of OH and CH₂O at the base of a turbulent lifted methane flame issuing into a hot vitiated coflow are analysed and contrasted to reference flames to further investigate the stabilization mechanisms involved. The use of the product of the quantified OH and semi-quantified CH₂O images as a marker for heat release rate is validated for transient autoigniting laminar flames. This is combined with temperature gradient information to investigate the flame structure. Super-equilibrium OH, the nature of the profiles of heat release rate with respect to OH mole fraction, and comparatively high peak heat release rates at low temperature gradients is found in the kernel structures at the flame base, and found to be indicative of autoignition stabilization.     

Inhibition of hydrogen–oxygen flames by iron pentacarbonyl at atmospheric pressure

The chemistry of inhibition of laminar premixed hydrogen–oxygen flames by iron pentacarbonyl at atmospheric pressure was studied experimentally and by numerical simulation. Flame speed and chemical structure were analyzed. Flame burning velocities and inhibition effectiveness were measured and simulated for various equivalence ratios. The concentration profiles of a number of Fe-containing products of Fe(CO)₅ combustion, including Fe, FeO₂, FeOH, and Fe(OH)₂, were first measured using probing molecular beam mass spectrometry in an atmospheric-pressure H₂/O₂/N₂ flame. A comparison of the experimental and modeling results shows that they are in satisfactory agreement with each other, indicating that the reaction mechanism proposed previously for flame inhibition by iron pentacarbonyl is adequate for predicting the chemical structure of flames. The key recombination stages of active species catalyzed by Fe-containing species for flames of various stoichiometries can be determined by calculations of the production rates of H and O atoms and OH radicals as well as by analysis of the kinetic model.     

An investigation of flame zones and burning velocities of laminar unconfined methane-oxygen premixed flames

Numerical and experimental investigations of unconfined methane-oxygen laminar premixed flames are presented. In a lab-scale burner, premixed flame experiments have been conducted using pure methane and pure oxygen mixtures having different equivalence ratios. Digital photographs of the flames have been captured and the radial temperature profiles at different axial locations have been measured using a thermocouple. Numerical simulations have been carried out with a C2 chemical mechanism having 25 species and 121 reactions and with an optically thin radiation sub-model. The numerical results are validated against the experimental and numerical results for methane-air premixed flames reported in literature. Further, the numerical results are validated against the results from the present methane-oxygen flame experiments. Visible regions in digital flame photographs have been compared with OH isopleths predicted by the numerical model. Parametric studies have been carried out for a range of equivalence ratios, varying from 0.24 to 1.55. The contours of OH, temperature and mass fractions of product species such as CO, CO₂ and H₂O, are presented and discussed for various cases. By using the net methane consumption rate, an estimate of the laminar flame speed has been obtained as a function of equivalence ratio.     

A skeletal mechanism for flame inhibition by trimethylphosphate

On the basis of a multi-step kinetic mechanism for flame inhibition by organophosphorus compounds including more than 200 reactions, a skeletal mechanism for flame inhibition by trimethylphosphate was developed. The mechanism consists of 22 irreversible elementary reactions, involving nine phosphorus-containing species. Selection of the crucial steps was performed by analysing P-element fluxes from species to species and by calculating net reaction rates of phosphorus-involving reactions versus the flames zone. The developed mechanism was validated by comparing the modelling results with the measured and simulated (using the starting initial mechanism) speed and the chemical structure of H₂/O₂, CH₄/O₂ and syngas/air flames doped with trimethylphosphate. The mechanism was shown to satisfactorily predict the speed of H₂/O₂/N₂ flames with various dilution ratios, CH₄/air and syngas/air flames doped with trimethylphosphate. The skeletal mechanism was also shown to satisfactorily predict the spatial variation of H and OH radicals and the final phosphorus-containing products of the inhibitor combustion. Further reduction of the skeletal mechanism without modification of the rate constants recommended in the starting mechanism was shown to result in noticeable disagreement of the flame speed and structure.     

Laminar flame propagation of acetone and 2-butanone at normal to high pressures: Insight into fuel molecular structure effects of ketones

This work reports an experimental and kinetic modeling investigation on the laminar flame propagation of acetone and 2-butanone at normal to high pressures. The experiments were performed in a high-pressure constant-volume cylindrical combustion vessel at 1–10 atm, 423 K and equivalence ratios of 0.7–1.5. A kinetic model of acetone and 2-butanone combustion was developed from our recent pentanone model [Li et al., Proc. Combust. Inst. 38 (2021) 2135–2142] and validated against experimental data in this work and in literature. Together with our recently reported data of 3-pentanone, remarkable fuel molecular structure effects were observed in the laminar flame propagation of the three C₃C₅ ketones. The laminar burning velocity increases in the order of acetone, 2-butanone and 3-pentanone, while the pressure effects in laminar burning velocity reduces in the same order. Modeling analysis was performed to provide insight into the key pathways in flames of acetone and 2-butanone. The differences in radical pools are concluded to be responsible for the observed fuel molecular structure effects on laminar burning velocity. The favored formation of methyl in acetone flames inhibits its reactivity and leads to the slowest laminar flame propagation, while the easiest formation of ethyl in 3-pentanone flames results in the highest reactivity and fastest laminar flame propagation. Furthermore, the LBVs of acetone and 3-pentanone exhibit the strongest and weakest pressure effects respectively, which can be attributed to the influence of fuel molecular structures through two crucial pressure-dependent reactions CH₃ + H (+M) = CH₄ (+M) and C₂H₄ + H (+M) = C₂H₅ (+M).     

Framework for simulating stationary spherical flames

Understanding and quantifying the effects of flame stretch rate on the laminar flame speed and flame structure plays an important role from interpreting experimentally-measured laminar burning velocities to characterizing the impact of turbulence on premixed flames. Unfortunately, accounting for these effects often requires an unsteady reacting flow solver and may be computationally expensive. In this work, we propose a mathematical framework to perform simulations of stationary spherical flames. The objective is to maintain the flame at a constant radius (and hence a constant stretch rate) by performing a coordinate change. The governing equations in the new flame-attached frame of reference resemble the original equations for freely-propagating spherical flames. The only difference is the presence of additional source terms whose purpose is to drive the numerical solution to a steady state. These source terms involve one free parameter: the flame stretch rate, which may either be computed in real time or imposed by the user. This parameter controls ultimately the steady state flame radius and the steady state flame speed. That is why, at a given stretch rate, the results of the stationary spherical flame simulations match those of a freely-expanding spherical flame. As an illustration, the dependence of the laminar flame speed on the stretch rate is leveraged to extract Markstein lengths for hydrogen/air mixtures at different equivalence ratios, as well as for hydrocarbon/air mixtures (CH₄ and C₇H₁₆). Numerical predictions are in good agreement with experimental measurements (within experimental uncertainties). Finally, the proposed methodology is implemented in the chemical kinetic software FlameMaster. The use of a dedicated steady-state solver with a non-uniform optimized mesh leads to significant reductions in the computational cost, highlighting that the proposed methodology is ideally suited for other chemical kinetic software such as Chemkin/Premix and Cantera.     

Experimental and modeling study on the auto-ignition properties of ammonia/methane mixtures at elevated pressures

Ammonia (NH₃) is considered as a promising carbon free energy carrier for energy and transportation systems. However, its low flammability and high NO_x emission potential inhibit the implementation of pure NH₃ in these systems. On the other hand, methane is a favorable low emission fuel that can be used as a co-firing fuel in ammonia combustion to promote the reactivity and control the emission levels. However, knowledge of the ignition properties of NH₃/CH₄ mixtures at intermediate temperatures and elevated pressures is still scarce. This study reports ignition delay times of NH₃/CH₄/O₂ mixtures diluted in Ar or Ar/N₂ over a temperature range of 900–1100 K, pressures of 20 and 40 bar, and equivalence ratios of 0.5, 1.0, and 2.0. The results demonstrate that a higher CH₄ mole fraction in the fuel mixture increases its reactivity, and that the reactivity decreases with increasing the fuel-oxygen equivalence ratio. The most recent mechanisms of Glarborg et al. (2018) and Li et al. (2019) were compared against the experimental data for validation purposes. Both mechanisms can predict the measurements fairly well, and key elementary reactions applied in both mechanisms were compared. A modified mechanism is provided, which can reproduce the measurements with smaller discrepancies in most cases. Detailed modeling for emissions indicated that adding CH₄ to the fuel mixture increases the emission of NO_x.     

Spectroscopic observations of methane-pulverized coal flames

A premixed laminar flame burner was used to study physical and chemical effects of burning methane and pulverized coal simultaneously. Spectral emissions obtained as a function of height in the flame, for OH, CH, C₂, and CO were used to calculate excited state species number densities. Premixed methane-air flames with a number of equivalence ratios and methane-air-coal dust flames with three equivalence ratios were studied and differences were noted. Spatial temperatures were measured in all flames by means of a thermocouple probe.The introduction of a pulverized coal additive (30mg/min) leads to changes in OH, Ch, C₂, and CO number densities and temperature at various heights, which depend on the stoichiometry of the methane-air flame. Possible chemical and physical mechanisms responsible for these changes are discussed qualitatively. The data pose a severe test for any quantitative model which includes the fluid dynamics, gas-phase chemistry, heterogeneous processes, and radiation transport.     

Experimental and kinetic modelling studies of flammability limits of partially dissociated NH3 and air mixtures

This paper presents a set of experimental and kinetic modelling studies of the flammability limits of partially dissociated NH₃ in air at 295 K and 1 atm. The experiments were carried out using a Hartmann bomb apparatus. The kinetic modelling was performed using Ansys Chemkin-Pro with opposed-flow premixed flame model employing three detailed reaction mechanisms, namely, the Mathieu and Petersen, Otomo et al., and Okafor et al. mechanisms. The degree of NH₃ dissociation was varied from 0 to 25% (0 to 20%v/v H₂ in the fuel mixture with a fixed H₂/N₂ ratio of 3). It was found that the lower (LFL) and upper (UFL) flammability limits of pure NH₃ in air were 15.0%v/v and 30.0%v/v, respectively, consistent with the literature data. The flammability limits of the mixture widened significantly with increasing the degree of NH₃ dissociation. At 25% NH₃ dissociation, LFL decreased to 10.1%v/v and UFL increased to 36.6%v/v. All tested mechanisms were able to predict the extinction characteristics exhibited by the lean and rich mixtures of partially dissociated NH₃ in air with non-unity Lewis numbers. While all three mechanisms predict well LFL, the Otomo et al. mechanism showed the best agreement with the experimental data of UFL. The rate of production of radicals, sensitivity, and reaction path analyses were performed to identify the key elementary reactions and radicals during combustion of partially dissociated NH₃. The production of key radicals including OH, H, O, and NH₂ was enhanced in the presence of H₂ and thus the conversion of NH to NO and then NO to N₂ near LFL and the conversion of NH₂ and NO to N₂ near UFL leading to wider flammability limits.     

Numerical investigation of flame–vortex interactions in laminar cross-flow non-premixed flames in the presence of bluff bodies

Flame stabilisation in a combustor having vortices generated by flame holding devices constitutes an interesting fundamental problem. The presence of vortices in many practical combustors ranging from industrial burners to high speed propulsion systems induces vortex–flame interactions and complex stabilisation conditions. The scenario becomes more complex if the flame sustains after separating itself from the flame holder. In a recent study [P.K. Shijin, S.S. Sundaram, V. Raghavan, and V. Babu, Numerical investigation of laminar cross-flow non-premixed flames in the presence of a bluff-body, Combust. Theory Model. 18, 2014, pp. 692–710], the authors reported details of the regimes of flame stabilisation of non-premixed laminar flames established in a cross-flow combustor in the presence of a square cylinder. In that, the separated flame has been shown to be three dimensional and highly unsteady. Such separated flames are investigated further in the present study. Flame–vortex interactions in separated methane–air cross flow flames established behind three bluff bodies, namely a square cylinder, an isosceles triangular cylinder and a half V-gutter, have been analysed in detail. The mixing process in the reactive flow has been explained using streamlines of species velocities of CH₄ and O₂. The time histories of z-vorticity, net heat release rate and temperature are analysed to reveal the close relationship between z-vorticity and net heat release rate spectra. Two distinct fluctuating layers are visible in the proper orthogonal decomposition and discrete Fourier transform of OH mass fraction data. The upper fluctuating layer observed in the OH field correlates well with that of temperature. A detailed investigation of the characteristics of OH transport has also been carried out to show the interactions between factors affecting fluid dynamics and chemical kinetics that cause multiple fluctuating layers in the OH.     

Simulations of laminar non-premixed flames of kerosene with hot combustion products as oxidiser

The effects of hot combustion product dilution in a pressurised kerosene-burning system at gas turbine conditions were investigated with laminar counterflow flame simulations. Hot combustion products from a lean (φ = 0.6) premixed flame were used as an oxidiser with kerosene surrogate as fuel in a non-premixed counterflow flame at 5, 7, 9 and 11 bar. Kerosene-hot product flames, referred to as ‘MILD’, exhibit a flame structure similar to that of kerosene–air flames, referred to as ‘conventional’, at low strain rates. The Heat Release Rate (HRR) of both conventional and MILD flames reflects the pyrolysis of the primary and intermediate fuels on the rich side of the reaction zone. Positive HRR and OH regions in mixture fraction space are of similar width to conventional kerosene flames, suggesting that MILD flames are thin fronts. MILD flames do not exhibit typical extinction behaviour, but gradually transition to a mixing solution at very high rates of strain (above A = 160, 000 s^?1 for all pressures). This is in agreement with literature that suggests heavily preheated and diluted flames have a monotonic S-shaped curve. Despite these differences in comparison with kerosene–air flames, MILD flames follow typical trends as a function of both strain and pressure. Further still, the peak locations of the overlap of OH and CH₂O mass fractions in comparison with the peak HRR indicate that the pixel-by-pixel product of OH- and CH₂O-PLIF signals is a valid experimental marker for non-premixed kerosene MILD and conventional flames.     

Experimental and kinetic modeling study of α-methylnaphthalene laminar flame speeds

α-Methylnaphthalene (AMN) is the primary reference bicyclic aromatic compound of diesel, and it is commonly used as a component of diesel, kerosene and jet-fuel surrogates formulated to describe real fuel combustion kinetics. However, few experimental data on neat AMN combustion are available in the literature. This work provides the first measurements of laminar flame speed profiles of AMN/air mixtures at 1 bar varying the initial temperature from 425 to 484 K, and equivalence ratio (φ) between 0.8 and 1.35 paving the way for the kinetic study of AMN combustion chemistry at high temperatures (>1800 K). The experimental data obtained in a spherical reactor are compared with kinetic model simulations. Specifically, the AMN kinetics is implemented from its analogous monocyclic aromatic compound, i.e., toluene, through the analogy and rate rule approach. This method allows to develop kinetic mechanisms of large species from the kinetics of smaller ones characterized by analogous chemical features, namely the aromaticity and the methyl functionality in the case of toluene and AMN. In doing so, it is possible to overcome the need of high-level electronic structure calculations for the evaluation of rate constants, as their computational cost increases exponentially with the number of heavy atoms of the selected species. To assess the validity of this approach, ab initio calculations are performed to derive the rate constants of the H-atom abstraction reactions by H, OH and CH₃ radicals from both toluene and AMN. The kinetic model obtained satisfactorily agrees with the measured laminar flame speed profiles. Sensitivity and flux analyses are performed to investigate similarities and differences between the main reaction channels of toluene and AMN combustion, with the former leading to ∼6 cm/s faster flame speed at almost identical conditions (P=1 bar, T∼425 K), as evidenced by both kinetic model simulations and experimental findings.     

Addition of NO2 to a laminar premixed ethylene-air flame: Effect on soot formation

Experiments were conducted on a laminar premixed ethylene-air flame at equivalence ratios of 2.34 and 2.64. Comparisons were made between flames with 5% NO₂ added by volume. Soot volume fraction was measured using light extinction and light scattering and fluorescence measurements were also obtained to provide added insight into the soot formation process. The flame temperature profiles in these flames were measured using a spectral line reversal technique in the non-sooting region, while two-color pyrometry was used in the sooting region. Chemical kinetics modeling using the PREMIX 1-D laminar flame code was used to understand the chemical role of the NO₂ in the soot formation process. The modeling used kinetic mechanisms available in the literature. Experimental results indicated a reduction in the soot volume fraction in the flame with NO₂ added and a delay in the onset of soot as a function of height above the burner. In addition, fluorescence signals—often argued to be an indicator of PAH—were observed to be lower near the burner surface for the flames with NO₂ added as compared to the baseline flames. These trends were captured using a chemical kinetics model that was used to simulate the flame prior to soot inception. The reduction in soot is attributed to a decrease in the H-atom concentration induced by the reaction with NO₂ and a subsequent reduction in acetylene in the pre-soot inception region.     

A comparison of computational and experimental lift-off heights of coflow laminar diffusion flames

As a sensitive marker of changes in flame structure, the number densities of excited-state CH (denoted CH^*), and excited-state OH (denoted OH^*) are imaged in coflow laminar diffusion flames. Measurements are made both in normal gravity and on the NASA KC-135 reduced-gravity aircraft. The spatial distribution of these radicals provides information about flame structure and lift-off heights that can be directly compared with computational predictions. Measurements and computations are compared over a range of buoyancy and fuel dilution levels. Results indicate that the lift-off heights and flame shapes predicted by the computations are in excellent agreement with measurement for both normal gravity (1g) and reduced gravity flames at low dilution levels. As the fuel mixture is increasingly diluted, however, the 1g lift-off heights become underpredicted. This trend continues until the computations predict stable flames at highly dilute fuel mixtures beyond the 1g experimental blow-off limit. To better understand this behavior, an analysis was performed, which indicates that the lift-off height is sensitive to the laminar flame speed of the corresponding premixed mixture at the flame edge. By varying the rates of two key “flame speed” controlling reactions, we were able to modify the predicted lift-off heights so as to be in closer agreement with the experiments. The results indicate that reaction sets that work well in low dilution systems may need to be modified to accommodate high dilution flames.     

Flame studies of C2 hydrocarbons

The oxidation characteristics of C₂ hydrocarbons were revisited in flames established in the counterflow configuration. Laminar flame speeds of ethane/air, ethylene/air, and acetylene/oxygen/nitrogen mixtures as well as extinction strain rates of non-premixed ethane/air flames were measured using digital particle image velocimetry. The experiments were modeled using three different kinetic models. While the experimental and computed laminar flame speeds agreed closely for all C₂ hydrocarbons under fuel-lean conditions, notable discrepancies were identified under fuel-rich conditions. Using the computed flame structures, insight was provided into the controlling mechanisms that could be responsible for the observed discrepancies. More specifically, the uncertainties associated with the kinetics of the thermal decomposition of the ethyl radical were found to be a potential source of the observed discrepancies for ethane flames. It was shown also by using alternative rate constants for the ethyl radical decomposition, the rate of flame propagation and the extinction propensity are affected notably. Furthermore, the values of the branching ratio of acetylene consumption reactions involving atomic oxygen were found to have a significant effect on the propagation of rich acetylene flames.     

Experimental and modeling study on the ignition delay times of ammonia/methane mixtures at high dilution and high temperatures

Ammonia is a promising alternative clean fuel due to its carbon-free character and high hydrogen density. However, the low reactivity of ammonia and the potential high NO_x emissions hinder its applications. Blending methane into ammonia can effectively improve the reactivity of pure NH₃. In addition, lean combustion, as a high-efficiency and low-pollution combustion technology, is an effective measure to control the potential increase in NO_x emissions. In the present work, the ignition delay times (IDTs) of NH₃/CH₄ mixtures highly diluted in Ar (98%) with CH₄ mole fractions of 0%, 10%, and 50% were measured in a shock tube at an equivalence ratio of 0.5, pressures of 1.75 and 10 bar and a temperature range of 1421 K - 2149 K. A newly comprehensive kinetic model (named as HUST-NH₃ model) for the NH₃/CH₄ mixtures oxidation was developed based on our previous work. Four kinetic models, the HUST-NH₃ model, Glarborg model [19], Okafor model [7], and CEU model [10], were evaluated against the ignition delay times, laminar flame speeds, and species profiles of pure ammonia and ammonia/methane mixtures from the present work and literature. The simulation results indicated that the HUST-NH₃ model shows the best performance among the above four models. Kinetic analysis results indicated that the absence of NH₃ + M = NH₂ + H + M (R819) and N₂H₂ + M = H + NNH + M (R902) in the CEU model and Okafor model cause the deviations between the experimental and simulation results. The overestimation of the rate constants of NH₂ + NO = NNH + OH (R838) in the Glarborg model is the main reason for the overprediction of the NH₃ laminar flame speeds.     

Resolving flame thickness using high-speed chemiluminescence imaging of OH* and CH* in spherically expanding methane–air flames

The coupling of CFD simulations with detailed chemical kinetics presents great progress in predicting the complex behavior of reacting flows, but also requires validated input parameters in the form of experimental data. The spatial profile of a combustion wave represents one such parameter, which can be directly measured using chemiluminescence imaging of a spherically expanding flame. In this work, emission signals from electronically excited methylidyne (CH*) and hydroxyl (OH*) radicals near 434 nm and 315 nm, respectively, from spherically expanding methane–air flames at 1 atm and 298 K were recorded for equivalence ratios of 0.8, 1.0, and 1.2. Spatial profiles of normalized intensity were compared to predicted profiles from AramcoMech2.0. The effect of image resolution was investigated by repeating experiments for three levels of image pixel density. An Abel inversion was employed to extract intensity profiles of CH* and OH* at flame radii up to 6.5 cm. Measured flame thickness increased as flames grew in size, but this behavior diminished as image resolution increased. A linear stretch correlation was used to extrapolate measured thicknesses to an unstretched thickness for each experimental condition. Radical-based flame thicknesses and corresponding spatial profiles were found to be highly dependent on image resolution, and at high resolution, measured flame thickness appeared to approach the kinetically predicted radical-based thicknesses. This paper lays the foundation for future, comprehensive measurements of spherical, laminar flames that can resolve the flame zone details to a level of precision not typically seen in the literature, providing benchmark data for both kinetics model validation and CFD model inputs. As a result, the measurements thus far indicate that the measured flame zone thickness based on electronically excited species is much closer to the length scale typically predicted by kinetics models than what has been seen in most experiments to date.     

Soot emission reduction in oxygenated co-flow jet flames

A computational study was performed for ethylene/air non-premixed laminar co-flow jet flames using an axisymmetric CFD code to explore the effect of oxygenation on PAH and soot emissions. Oxygenated flames were established using N₂ diluted fuel stream along with O₂ enriched air stream such that the stoichiometric mixture fraction (Ζ_st) is varied but the adiabatic flame temperature is not materially changed. Simulations were carried out using a spatially and temporally accurate algorithm with detailed chemistry and transport. A detailed kinetic model involving 111 species and 784 reactions and a fairly detailed soot model were incorporated into the code. Two different approaches, one with constant flame height and other with constant inlet velocity are comprehensively examined to bring out the effects of changes in flame structure and residence time on soot emissions with respect to Z_st. With increase in Ζ_st, a drastic reduction in the formation of soot precursors (acetylene and benzene) and thus in soot emissions are observed. In the present study, oxygenated flames with Ζ_st ≥ 0.424 are considered as blue flames or completely soot free. For various oxygenated flames a C/O ratio between 0.45 and 0.6 is found to be most favorable for soot formation.     

Laminar flame speed of methane/air stratified flames under elevated temperature and pressure

Flame propagation under mixture stratification is relevant to a wide range of applications including gas turbine combustors and internal combustion engines. One of the local stratification effects is known as the back-support effect, where the laminar flame speed is modified when a premixed flame propagates into gradually richer or leaner mixtures. A majority of previous studies have focused on the propagation of methane/air stratified flames under standard temperature and pressure. However, stratified combustion often occurs under elevated temperature and pressure in practical applications, which may influence the characteristics of the back support effect through modified reaction pathways. This study performs numerical simulations of stratified laminar counterflow flames under an Atmospheric Temperature and Pressure (ATP) condition and an Elevated Temperature and Pressure (ETP) condition and examines the influence of elevated temperature and pressure on the back-support effect. Reaction flow analyses were extensively conducted to elucidate the difference in the primary reaction pathway between the two conditions. When scaled by the stratification Damköhler number, the back-support effect on the rich-to-lean stratified flame is weaker under the ETP condition than the ATP condition in the stoichiometric to lean region. This is due to increased contribution from reactions involved with OH radicals under the ETP condition, which leads to lower H₂ reproduction in the reaction zone than under the ATP condition. The contribution from OH radicals is increased under the ETP condition because the conversion of H into OH is enhanced. These results suggest that the back-support effect may become negligibly small in practical combustors operating under elevated temperature and pressure due to (1) the flame being less sensitive to stratification because of the thinner flame, and (2) the lower H₂ reproduction that deteriorates the radical production that drives the back-support effect.