Fuel-lean Flames Research Articles

Temperature and Thermal Diffusivity Diagnostics in Laminar Methane Flames Using Infrared Four-Wave Mixing Techniques.

Four-wave mixing techniques, such as coherent anti-Stokes Raman spectroscopy (CARS), laser-induced grating spectroscopy (LIGS), and degenerate four-wave mixing (DFWM), have been widely used in combustion diagnostics due to their advantages of high signal-to-noise ratio (S/N), coherent signal, and spatial resolution. In this work, a nano-second pulsed laser is utilized to generate mid-infrared (near 3 µm) pump beams, exciting the rovibrational transitions of nascent water in flames. Combined LIGS and DFWM measurements are demonstrated in premixed laminar CH4/O2/N2 flames with equivalence ratios from 0.6 to 1.5, to achieve precise thermometry in a wide range of flame conditions. The flame temperatures were also measured by thermocouple as a reference, and the results from LIGS and DFWM align well with the trends shown in the thermocouple measurements. In fuel-lean flames, where the mass-to-specific-heat ratio variation is minimal, LIGS provides temperature data with a precision better than 16 K (0.8%). In fuel-rich flames, where the increased H2 concentration in the flame introduces uncertainty in gas constants thus affecting the accuracy of LIGS thermometry, DFWM is instead employed for temperature measurement since it is less sensitive to the gas composition within the measured volume. The high-precision LIGS temperatures in lean flames serve as temperature reference during the DFWM calibration of the degree of saturation, and a precision better than 90 K (4.5%) is achieved for DFWM thermometry. In addition to temperature, a theoretical model is employed to fit LIGS signal time waveforms, extracting the local speed of sound and thermal diffusivity with precisions better than 0.5% and 1.3%, respectively. These high-precision measurements contribute additional data for flame research and simulation calculations. This way, the combined use of DFWM and LIGS proves the potential for accurate thermometry and diagnostics of other thermodynamic parameters across a wide range of flame conditions.

Laminar flame stability analysis of ammonia-methane and ammonia-hydrogen dual-fuel combustion

In recent years, dual-fuel combustion of ammonia blended with reactive fuels like methane or hydrogen has gained attention for reduced carbon emissions. This study investigates the stability of NH3-CH4 and NH3-H2 laminar propagating flames, utilizing experimental and numerical methods. It covers an extensive span of equivalence ratios (0.7–1.6), fuel compositions, and a variety of initial temperatures (300–600 K), as well as pressures ranging from 1 to 5 bar·NH3-CH4 blends exhibit stability across all equivalence ratios, with Markstein lengths of 0.02–2.49. In contrast, NH3-H2 flames display significant variations in Markstein lengths, ranging from 3.73 to −3.36 at an equivalence ratio of 0.7, particularly destabilizing fuel-lean flames due to heightened preferential-diffusional instabilities. Increasing ammonia content stabilizes fuel-rich flames but destabilizes fuel-lean ones. As ammonia content rises, both NH3-CH4 and NH3-H2 flames become more hydrodynamically stable due to flame thickening, although this can exacerbate flame instabilities when preferential-diffusional instabilities dominate. Additionally, preheating destabilizes both ammonia-methane and ammonia-hydrogen flames hydrodynamically, while pressure affects NH3-CH4 and NH3-H2 mixtures oppositely, destabilizing the former and stabilizing the latter due to reduced flame thickness. The addition of diluents stabilizes NH3-CH4 flames but destabilizes NH3-H2 flames, influencing Le and Ze numbers. Diluent addition significantly reduces the flame temperature gradient (up to 64 %), resulting in thicker flames and enhanced hydrodynamic stabilities. This research provides insights into the complexities of ammonia-blended dual-fuel combustion and its impact on flame stability under various conditions.

Flame Structure at Elevated Pressure Values and Reduced Reaction Mechanisms for the Combustion of CH4/H2 Mixtures

Understanding and controlling the combustion of clean and efficient fuel blends, like methane + hydrogen, is essential for optimizing energy production processes and minimizing environmental impacts. To extend the available experimental database on CH4 + H2 flame speciation, this paper reports novel measurement data on the chemical structure of laminar premixed burner-stabilized CH4/H2/O2/Ar flames. The experiments cover various equivalence ratios (φ = 0.8 and φ = 1.2), hydrogen content amounts in the CH4/H2 blend (XH2 = 25%, 50% and 75%), and different pressures (1, 3 and 5 atm). The flame-sampling molecular-beam mass spectrometry (MBMS) technique was used to detect reactants, major products, and several combustion intermediates, including major flame radicals. Starting with the detailed model AramcoMech 2.0, two reduced kinetic mechanisms with different levels of detail for the combustion of CH4/H2 blends are reported: RMech1 (30 species and 70 reactions) and RMech2 (21 species and 31 reactions). Validated against the literature data for laminar burning velocity and ignition delays, these mechanisms were demonstrated to reasonably predict the effect of pressure and hydrogen content in the mixture on the peak mole fractions of intermediates and adequately describe the new data for the structure of fuel-lean flames, which are relevant to gas turbine conditions.

Impact of Fe-doped H2/O2 flame equivalence ratio on the fate and temperature history of early particles

The temperature and species concentration history experienced by the gas-borne nanoparticles during their evolution in the flame has a major impact on their size, morphology, composition, and crystallinity. In our recent work (Combust. Flame, 244 (2022) 112251), we have reported optical emission measurements of a Fe(CO)5-doped H2/O2/Ar fuel-lean (ɸ = 0.5) flame, revealing that the temperature of the early-formed nanoparticles exceeds the gas temperature by several hundred degrees, while the particle volume fraction increases sharply, followed by rapid disintegration in the reaction zone. This behavior, modeled by single particle Monte-Carlo simulations indicates involvement of heterogeneous reactive processes at the particle surface, such as particle reduction and oxidation, growth and etching. Within the refined approach of the current study, reactive and non-reactive collisions were treated consistently, assuming rapid thermalization between the impinging molecule and the particle, with subsequent random energy sampling to determine reactivity. In the present work, we test the limits and validity of the heterogeneous flame-particle interaction model by manipulating the oxidation–reduction and growth-etching balance by varying the equivalence ratio (0.25<ɸ<1.5). For the entire range of equivalence ratios studied in experiments and simulations, we find a deviation between the particle and gas phase temperatures with significantly higher particle temperature, which is continued until a full degree of iron oxidation within the particle (O/Fe ratio=3/2) is reached. Validating the simulations against the measurements of particle temperature and volume fraction over a wide range of equivalence ratios, emphasized the necessity to account for gas-phase Fe-atom concentration depletion. We incorporated nucleation theory to estimate initial cluster population, linking Fe-concentration variation in the gas phase and the stochastic particle evolution model. The surface reaction parameters in our current work were updated using density functional theory literature data, and validation of the model predictions against experimental data, across the entire range of equivalence ratios.

Experimental studies on the OH∗ chemiluminescence and structure characteristics in NH3/H2 and NH3/cracked gas swirl flames

Ammonia (NH3) has been gaining popularity as a carbon-free alternative fuel in recent years. OH∗ chemiluminescence is a promising method to study the structure of ammonia flames, but research on this aspect is still lacking. In this study, the OH∗ chemiluminescence distributions and structure characteristics in NH3/H2 and NH3/cracked gas swirl flames are experimentally investigated. The OH∗ distributions of NH3/H2 and NH3/cracked gas under different addition levels and equivalence ratios are compared. The results show that the increase of equivalence ratio and hydrogen doping level significantly increased the OH∗ chemiluminescence peak of NH3/air swirling flame. Especially in fuel-lean flames, the equivalence ratio effect is greater than the NH3 dilution effect, while the two effects are comparable in equivalence and fuel-rich conditions. The stretching effect of NH3/cracked gas is stronger, and the flame width is narrower than that of NH3/H2 flame, but the peak value of OH∗ radiation is higher.

Numerical investigation of hydrogen addition effects to a methane-fueled high-pressure combustion chamber

To get a general understanding of hydrogen fuel usage in a real-condition high-pressure industrial gas turbine burner operating at 15 atm pressure, the effects of hydrogen addition to methane fuel are studied numerically. Methane-hydrogen premixed fuel-lean flame is investigated while hydrogen is substituted for methane at a fixed fuel and air mass flow rate at the burner inlet. Steady-state reacting and swirling flow equations (RANS) are solved using the standard k−ε model as the turbulence model. A detailed methane combustion mechanism GRI 2.11 is used to simulate methane-hydrogen combustion with Finite Rate formulation more accurately. Temperature and axial velocity variation and important pollutant species emissions such as CO, CO2, and NOx were investigated at the combustion chamber exit. Flame shape, flow pattern, and flame location variation with hydrogen addition were studied due to hydrogen flashback limits. The hydrogen addition leads to higher temperatures inside the burner and the hot zone inside the combustion chamber extends and moves toward the fuel inlet. Flow structure including recirculation zones change with hydrogen addition. CO emission increases by hydrogen addition to the fuel mixture near the burner exit while CO2 mass fraction is reduced. A significant increase in NO mass fraction with hydrogen addition was observed near the burner exit due to higher temperature.

Experimental and numerical study on the explosion suppression of hydrogen/dimethyl ether/methane/air mixtures by water mist containing NaHCO3

The explosion suppression effect of NaHCO3 water mist on the H2/DME/CH4/air mixtures was experimentally investigated. Pressure data and flame images were recorded by a pressure sensor and a high-speed camera, respectively. The suppression mechanism of NaHCO3 water mist was further studied using a developed kinetic model. Results showed that the normalized laminar flame speed calculated by kinetic model agreed well with the experimental data. The suppression effect of NaHCO3 water mist on the H2/DME/CH4/air mixtures explosion was significantly affected by fuel compositions, equivalence ratios and NaHCO3 concentrations. For stoichiometric mixtures (Φ = 1.0), the decreasing rate of the average flame propagation speed of CH4-enriched flames can be up to 84.9%, which is higher than that of DME-enriched flames and the H2-enriched flames. For DME-enriched flames, the decreasing rate of the average flame propagation speed of fuel-lean flames can also be up to 70%. Meanwhile, the maximum explosion pressure of the CH4-enriched flames and fuel-lean flames are more sensitive to the NaHCO3 water mist. The NaHCO3 water mist can suppress hydrodynamic instability while it has little effect on diffusional-thermal instability. The chemical kinetic model revealed that both the adiabatic flame temperature and the maximum mole fraction of (H + OH) can be decreased noticeably by NaHCO3 water mist. Sensitivity analysis indicated that NaOH + H = Na + H2O and Na + OH + M = NaOH + M are the dominant reactions of sodium-containing species for suppressing laminar flame speed. Reaction pathway analysis indicated that Na <=> NaOH suppression cycle is effective to reduce H and OH free radicals.

Visible chemiluminescence of ammonia premixed flames and its application for flame diagnostics

We report a spatially resolved spectroscopic study of the visible chemiluminescence emission from different premixed ammonia-air-oxygen flames stabilized on a laminar flat flame burner, with equivalence ratio ranging from 0.7 to 1.35 and an O2/N2 ratio of 0.4. In the reaction zone of the observed flames, the visible emission was recognized to be the chemiluminescence of excited NH2* radicals, while in the post-flame zone, two types of chemiluminescence were observed: NO2* chemiluminescence dominated in the fuel-lean flames and NH2* chemiluminescence dominated in the fuel-rich flames. The high-resolution spectra of the NO2* and NH2* chemiluminescence in the visible region (400-700 nm) were recorded. The intensity of both spectra increased gradually with wavelength. However, the NO2*-chemiluminescence spectrum appeared to be continuous and unstructured, while the NH2*-chemiluminescence spectrum consisted of groups of distinct emission lines. Based on the spectral feature, the ratios of the integrated chemiluminescence intensities over the 598-603 nm wavelength range to the intensities over the 586-592 nm range and 447-453 nm range were used to sense equivalence ratio. In addition, slightly different colors of the fuel-lean and fuel-rich flames were observed, due to the fact that NO2* chemiluminescence had a relatively stronger signal in the blue region than NH2* chemiluminescence. The difference was used to infer flame equivalence ratio using the flame images recorded by a RGB digital camera, where the ratio of the signal from the red channel to the signal from the blue channel was calculated.

Planar laser-induced imaging of CH3 for high resolution single-shot reaction-zone visualization in premixed methane/air flames over broad stoichiometric ratios

We report a novel approach for single-shot planar imaging of CH3 radicals in premixed methane and air flames. A 213 nm beam from the 5th harmonic of an Nd:YAG laser was resonantly absorbed by the CH3 radicals, which were excited to the highly pre-dissociative upper level and dissociated to H2 and CH (X), as the main dissociation channel. The CH radicals were consequently excited by a 388 nm beam from an alexandrite laser, and the fluorescence from the excited CH radicals was collected off-resonant at 431 nm. Using this Photo-Fragmentation Planar Laser-Induced Fluorescence (PF-PLIF) technique, instantaneous flame front structures, represented by CH3 radicals, can be visualized with high spatial resolution over a broad range of stoichiometric ratios. Signal-to-noise ratios up to 50 were observed for premixed methane/air flame with stoichiometric ratio as low as 0.26. The CH radicals naturally presented in flame front are more than 400 times lower in concentration than the CH3 radicals in premixed methane/air flames even at the conditions close to stoichiometric or slightly fuel rich cases where the highest CH concentrations exist, and the CH3/CH concentration ratios increase dramatically moving towards fuel lean conditions. By adopting a structured illumination of the 213 nm pump beam, the naturally presented CH radicals were visualized simultaneously with CH3 at slightly fuel rich laminar flames, where the CH signal intensity was 5 times lower than that from CH3. The results indicate that the CH3 PF-PLIF technique can provide much stronger signal than the CH PLIF and presented a much promising potential for applications in fuel-lean flames. Finally, the CH3 PF-PLIF was performed in premixed turbulent flames to demonstrate the feasibility of this technique for flame front visualization in turbulent premixed flames.

Direct numerical simulation of turbulent premixed ammonia and ammonia-hydrogen combustion under engine-relevant conditions

The combustion characteristics of ammonia and ammonia-hydrogen fuel blends under spark-ignited turbulent premixed engine-relevant conditions were investigated by means of direct numerical simulation and detailed chemistry. Several test cases were investigated for an outwardly expanding turbulent premixed flame configuration covering pure ammonia and ammonia-hydrogen fuel blends with 10% and 15% hydrogen content by volume for different equivalence ratio values of 0.9, 1.0 and 1.1. The results showed that the fuel-lean flames exhibit strong wrinkled structures at flame front compared to stoichiometric and fuel-rich flames. The heat release rate plots indicate that adding hydrogen into ammonia improves the reactivity of the flame and enhances the combustion process. The scatter plots of heat release rate versus local curvature coloured by NO formation, show that high heat release rate values occur in the concave structures and low heat release rate values occur in the convex structure, which is consistent with NO distribution. The highest turbulent burning velocity values were found for the fuel-lean cases due to more wrinkled flame front with lower effective Lewis number compared to fuel-rich cases. The results show a bending effect for the ratio between turbulent to laminar burning velocities with respect to hydrogen addition at all equivalence ratios with 10% hydrogen addition into ammonia exhibiting a highest value for the burning velocity ratio. Two distinct flame structures (concave and convex) were analysed in terms of local equivalence ratio based on the elements of N and O as well as H and O. They revealed an opposite distribution of NO formation normal to the flame front within concave and convex structures. Elementary chemical reactions involved in NO formation have shown that hydrogen addition into ammonia influences the reactivity of certain specific chemical reactions.

Laminar burning velocity and cellular instability of 2-butanone-air flames at elevated pressures

2-butanone has been exploited as a promising next-generation biofuel candidate for its impressive knock resistant properties. However, its combustion characteristics and intrinsic instabilities have not been well explored. In this study, an experimental and theoretical investigation on laminar burning velocity (LBV) and onset of flame instability in spherically expanding flames of 2-butanone-air mixtures was conducted in a constant-volume chamber at temperature of 423 K, pressure of 1–8 bar, and equivalence ratio ϕ of 0.7–1.5. The measured LBVs and Markstein lengths of 2-butanone were observed to decrease noticeably as the pressure increased. Four recently established kinetic models were applied to predict the experimental data. Stability analysis was performed to investigate the effect of pressure and ϕ on the onset of the cellular instability of 2-butanone flames. Results showed that the hydrodynamic instability monotonically increased as pressure increased and non-monotonically varied with increasing ϕ. Thermal-diffusive instability increased dramatically as ϕ increased while showed less sensitivity to the variation in pressure. The critical conditions at which the flame status turn stable into unstable were also evaluated. The critical Peclet number decreased monotonically as ϕ increased, illustrating that fuel-rich flames suffer more severe cellular instability than fuel-lean flames. The critical flame radius decreased as pressure increased in both experimental measurements and theoretical calculations, reflecting that the onset of cellular instabilities of 2-butanone-air mixtures advanced to smaller radius at higher initial pressures.

Hybrid fs/ps vibrational coherent anti-Stokes Raman scattering for simultaneous gas-phase N2/O2/CO2 measurements

Single-shot, 1-kHz measurements of temperatures and mole fraction ratios along with theoretical modeling for gas-phase N2, O2, and CO2 are demonstrated using hybrid femtosecond/picosecond (fs/ps) vibrational coherent anti-Stokes Raman scattering (CARS). The combination of broadband pump and Stokes pulses covers a spectral range over 1800cm-1 while the narrowband probe pulses generated from a quasi-common-path second harmonic bandwidth compressor (QCP-SHBC) resolves the molecular structures with a bandwidth of ∼7cm-1. Temperature results of 1700-2000 K in methane/air fuel-lean flames show state-of-the-art inaccuracies of less than ∼3% and precision less than 2%. Mole fraction ratios inaccuracy at room temperature is ∼5%, and precision at flame temperatures are 6%-8%.

Experimental and kinetic modeling study of NO formation in premixed CH4+O2+N2 flames

The nitric oxide (NO) formation in methane (CH 4 ) flames has been widely investigated, with quite a few kinetic mechanisms available in the literature. However, studies have shown that there are often discrepancies between the simulations using various mechanisms and the experimental results. To elucidate reactions leading to these discrepancies, experiments were designed to measure the NO formation in the post flame zone of CH 4 +O 2 +N 2 flames with the oxygen ratio, x O 2 = O 2 /(O 2 +N 2 ), varying from 0.2 to 0.27. The experiments were carried out on a heat flux burner at atmospheric pressure and 298 K using saturated Laser-induced fluorescence. The equivalence ratio, ϕ , was changed from 0.7 to 1.6. The corresponding laminar burning velocity, S L , for each condition was also measured using the heat flux method. A comparison was made for the present experimental data and simulation results using the Konnov, Glarborg, NOMecha 2.0, and San Diego mechanisms, and none of them well reproduced the new NO experimental data for all investigated conditions. Numerical analyses show that the increment of NO mole fraction in stoichiometric and fuel-lean flames when the x O 2 increases is mostly defined by the thermal-NO production, which is found to be over-predicted, especially by the Konnov and San Diego mechanisms. The rate constant of reaction NO+N = N 2 +O was derived as k = 1.529 × 10 13 T − 0.0027 exp ( 185.41 [ c a l / m o l e ] R T ) cm 3 / mol s over 225–2400 K temperature range. The rate constants of four reactions controlling CH mole fraction profiles and prompt-NO formation were updated based on the analysis of the literature data that yields an improved performance of the Konnov mechanism.

Numerical Studies of the Pilot Flame Effect on a Piloted Jet Flame

ABSTRACT In a piloted jet flame, the pilot flame has an effect of stabilizing the main flame. Detailed mechanisms of pilot flame/main flame interaction are however not well studied. It is expected that the pilot flame affects the main flame through the following mechanisms: (a) the pilot flame provides the heat and radicals to the reaction zone of the main flame, (b) the pilot flame prevents the cold ambient air from being entrained into the main flame, and (c) the pilot flame modifies the stretch rate of the main flame. In this paper, detailed numerical simulations of piloted laminar methane/air jet flames are carried out to elucidate the effect of pilot flame on the structure and burning velocity of the main jet flame. One-dimensional (1D) freely propagating flame is also simulated to investigate the effect of hot gas mixing with the unburned fuel/air mixture, and 1D counter-flow flame is simulated to study the diffusion of the hot gas from the pilot flame to the reaction zone of the main flame and the effect of flame stretch. The results showed that heat transfer from the pilot flame to the main flame has a more significant effect on the structures and propagation of the main flame than the mass transfer from the pilot flame to the main flame. The heat and mass transfer from the pilot flame affects the local equivalence ratio and temperature of the unburned mixture, which gives rise to a significant enhancement of burning velocity. When the hot gas from the pilot flame is at sufficiently high temperatures, an ultra-lean fuel/air mixture can burn at equivalence ratio below the flammability limit. The reaction rate and burning velocity of ultra fuel-lean flames are enhanced by the strain rate, whereas for a main flame with the equivalence ratio closer to that of pilot flame, the reactivity and burning velocity of the main flame decrease with increasing strain rate.

A one-step reduced mechanism for near-limit hydrogen combustion with general stoichiometry

A one-step reduced mechanism for hydrogen combustion, previously developed for fuel-lean flames, is extended to apply for all equivalence ratios under near-limit conditions by taking into consideration two additional recombination steps that are important under fuel-rich conditions. It is found that the crossover temperature that appears in the cutoff factor is smaller under fuel-rich conditions. Besides improving insights, the results can be beneficial in speeding computations.

Modelling and Numerical Simulation of Dual Fuel Lean Flames Using Local Burning Velocity and Critical Chemical Timescale

Modelling and Numerical Simulation of Dual Fuel Lean Flames Using Local Burning Velocity and Critical Chemical Timescale

Laminar combustion regimes for hybrid mixtures of coal dust with methane gas below the gas lower flammability limit

Understanding flame propagation in dust clouds and hybrid mixtures requires knowledge of the fundamental combustion processes and their coupling interaction. The objective of this work is to use computational fluid dynamics to classify laminar flame structure in hybrid mixtures where the initial gas concentration is below the lower flammability limit. Particular focus is given to the role of reaction chemistry and overall equivalence ratio on flame structure and burning velocity. Through this study, five flame regimes were determined: fuel-lean flames (Type I), volatile-lean flames (Type II), volatile-rich flames (Type III), transition flames (Type IV), and kinetic-limited flames (Type V). Gas-phase chemistry was found to play a critical role in burning velocity for Type III, IV, and V flames. Burning velocities at hybrid volatile component equivalence ratios less than 0.9, were found to be less sensitive to reaction kinetics. Further research using this model will focus on initial gas concentrations above the lower flammability limit, exploring the flammability limits of hybrid mixtures, and extending the results to turbulent flames in system geometries relevant to industrial safety.

Extinction Limits of Swirl Nonpremixed Methane Flames and CH4/CxHy/N2 Mixtures: Experimental Evaluation and Thermodynamic Calculations

In this study, we aimed to evaluate the extinction limits of fuel-rich and fuel-lean flames created in non-premixed coaxial swirl burner of combustion substrates. The experiments were performed with three types of fuels: methane and two mixtures of CH4/CxHy/N2. The analyzed fuels reflected varied composition of the natural gas containing methane, ethane, propane, butane, and nitrogen. The examination of the extinction limits of flames isolated from the surroundings with a quartz tube was based on the limit ratios of equivalence (Φ). Results showed that an increase in the stream of methane directed toward the burner improves blowout limits as determined by the equivalence ratios. However, this pattern was not observed for the other studied fuels. A concurrent increase in the content of CxHy and N2 in natural gas implies worsening of extinction limits of fuel-lean flames and improvement of extinction limits for fuel-rich flames. Also performed thermodynamic calculations using the FactSage 6.3 software packa...

Heat release rate variations in high hydrogen content premixed syngas flames at elevated pressures: Effect of equivalence ratio

Three-dimensional direct numerical simulations with detailed chemistry were performed to investigate the effect of equivalence ratio on spatial variations of the heat release rate and flame markers of hydrogen/carbon monoxide syngas expanding spherical premixed flames under turbulent conditions at elevated pressures. The flame structures and the heat release rate were analysed and compared between fuel-lean, stoichiometric and fuel-rich centrally ignited spherical flames. The equivalence ratio changes the balance among thermo-diffusive effects, Darrieus–Landau instability and turbulence, leading to different flame dynamics and the heat release rate distribution, despite exhibiting similar cellular and wrinkling flames. The Darrieus–Landau instability is relatively insensitive to the equivalence ratio while the thermo-diffusive process is strongly affected by the equivalence ratio. As the thermo-diffusive effect increases as the equivalence ratio decreases, the fuel-lean flame is more unstable than the fuel-rich flame with the stoichiometric flame in between, under the joint effects of the thermo-diffusive instability and the Darrieus–Landau instability. The local heat release rate and curvature display a positive correlation for the lean flame, no correlation for the stoichiometric flame, and negative correlation for the rich flame. Furthermore, for the fuel-lean flame, the low and high heat release rate values are found in the negative and positive curvature zones, respectively, while for the fuel-rich flame, the opposite trends are found. It is found that heat release rate markers based on species concentrations vary strongly with changing equivalence ratio. The results suggest that the HCO, HO2 concentrations and product of OH and CH2O concentrations show good correlation with the local heat release rate for H2/CO premixed syngas-air stoichiometric flame under turbulent conditions at elevated pressures.

Measurements of the concentrations of positive and negative ions along premixed fuel-rich flames of methane and oxygen

The positive and negative ions in fuel-rich, pre-mixed flames of CH4+O2+Ar (0.4 ≤ C/O (atomic basis) ≤0.75) burning at 1 atm have been studied mass spectrometrically. The ions in the most oxygen-rich of these non-turbulent flames were found to be similar to those in more fuel-lean flames, previously studied by Goodings et al. In particular, in this flame, ions are produced in only one chemi-ionisation step in the reaction zone, i.e. by CH+O→CHO++e−. Subsequently a pool of positive ions is then formed rapidly, mainly by protons being transferred from CHO+ to intermediates and final products of combustion. Some negative ions were detected in the reaction zone, but none were found in this flame more than 1mm downstream of the reaction zone, i.e. in the burnt gas. These negatively charged species, like the positive ions, reflect the presence of intermediates in and near the reaction zone. In the burnt gas, the dominant positive ion, H3O+, recombines with free electrons, the principal negatively charged species, in H3O++e−→products. The recombination coefficient was measured to be 2.9 ± 1.5×10−7ml ion−1 s−1 at ∼2250K. However, in the richer oxy-methane flames with C/O ≥ 0.50, another ion-producing step was found to operate from 1 to at least ∼5mm downstream of the reaction zone. This new process is deduced to be CH+C2H2→C3H3++e−, and maybe involves electronically excited CH radicals. Certainly, C3H3+ becomes the major positive ion in the more fuel-rich flames. The richest flame studied, with C/O=0.75, is weakly sooting; in its burnt gas, both positive and negative ions with masses exceeding 170 a.m.u. were formed throughout the first 8 or so mm of the burnt gas. It is likely that the heavy positive ions are produced by proton transfer from a pool of positive ions (with the primary ion being C3H3+) to precursors of soot, including polyaromatic species. Likewise, the pool of massive negative ions reflects these precursors of soot.