Transport budgets in turbulent lifted flames of methane autoigniting in a vitiated co-flow

Autoignition of hydrocarbon fuels is an outstanding research problem of significant practical relevance in engines and gas turbine applications. This paper presents a numerical study of the autoignition of methane, the simplest in the hydrocarbon family. The model burner used here produces a simple, yet representative lifted jet flame issuing in a vitiated surrounding. The calculations employ a composition probability density function (PDF) approach coupled to the commercial CFD package, FLUENT. The in situ adaptive tabulation (ISAT) method is used to implement detailed chemical kinetics. An analysis of species concentrations and transport budgets of convection, turbulent diffusion, and chemical reaction terms is performed with respect to selected species at the base of the lifted turbulent flames. This analysis provides a clearer understanding of the mechanism and the dominant species that control autoignition. Calculations are also performed for test cases that clearly distinguish autoignition from premixed flame propagation, as these are the two most plausible mechanisms for flame stabilization for the turbulent lifted flames under investigation. It is revealed that a radical pool of precursors containing minor species such as CH₃, CH₂O, C₂H₂, C₂H₄, C₂H₆, HO₂, and H₂O₂ builds up prior to autoignition. The transport budgets show a clear convective-reactive balance when autoignition occurs. This is in contrast to the reactive-diffusive balance that occurs in the reaction zone of premixed flames. The buildup of a pool of radical species and the convective-reactive balance of their transport budgets are deemed to be good indicators of the occurrence of autoignition.     

Chemical structure of autoignition in a turbulent lifted H2/N2 jet flame issuing into a vitiated coflow

In the present paper autoignition is studied as the main stabilization mechanism in turbulent lifted H₂/N₂ jet flames issuing into a vitiated hot coflow. The numerical study is performed using the joint scalar PDF approach with detailed chemistry in a two dimensional axisymmetric domain. The SSG Reynolds stress model is used as a turbulence model in the simulation. Chemical structure and characteristics of autoignition are investigated using various methods and parameters. Reaction rate analysis is made to analyze the ignition process at the flame base. The results show the occurrence of a chain branching reaction preceding thermal runaway, which boosts the chain branching process in the flame. This demonstrates the large impact of autoignition at the flame base on the stabilization of the lifted turbulent flame. Further investigation using the scatter-plots of scalars reveals the characteristics of the ignition. The relation between the behavior of temperature and of key intermediate species demonstrates the formation of OH through consumption of HO₂ at nearly isothermal conditions in a very lean-fuel mixture at the flame base. Flux analyses in the conservation equations of species are used to explore the impacts of mass transport on ignition process. Ignition is found to be mainly controlled by chemical features rather than the mixing processes near the flame base. Characteristics of autoignition are also investigated in terms of Damköhler number and progress variable.     

Effects of hydrogen enrichment on CH4/Air turbulent swirling premixed flames in a cuboid combustor

Effects of H₂-enrichment on structures of CH₄/air turbulent swirling premixed flames affected by high intensity turbulence in a gas turbine model combsutor are investigated by conducting direct numerical simulations. Two stoichiometric mixture conditions, of which volume ratio of CH₄:H₂ = 50:50 and 80:20, are simulated by considering a reduced chemistry (25 species and 111 reactions). Results showed qualitatively different flame shapes and reaction zone characteristics between the cases. For the higher H₂-ratio case, the flame is stabilized both in the inner and outer shear layers. For the lower H₂-ratio case, the flame is stabilized only in the inner shear layer and extinction occurs in the outer shear layer. Comparison of the reaction zone characteristics with unstrained and strained laminar flames in phase space showed that H₂ mass fraction for the lower H₂-ratio case and reaction rate profiles for both cases deviate from the corresponding laminar values. Analysis of fuel species conservation equation suggests that the turbulent transports are substantially influential to determine local and global flame structures. These findings would be useful for designing practical H₂-enriched gas turbine combustor in the aspect of flame structures under high intensity turbulence.     

An improved H2/O2 mechanism based on recent shock tube/laser absorption measurements

An updated H₂/O₂ reaction mechanism is presented that incorporates recent reaction rate determinations in shock tubes from our laboratory. These experiments used UV and IR laser absorption to monitor species time-histories and have resulted in improved high-temperature rate constants for the following reactions: H+O₂=OH+OH₂O₂(+M)=2OH(+M)OH+H₂O₂=HO₂+H₂OO₂+H₂O=OH+HO₂ The updated mechanism also takes advantage of the results of other recent rate coefficient studies, and incorporates the most current thermochemical data for OH and HO₂. The mechanism is tested (and its performance compared to that of other H₂/O₂ mechanisms) against recently reported OH and H₂O concentration time-histories in various H₂/O₂ systems, such as H₂ oxidation, H₂O₂ decomposition, and shock-heated H₂O/O₂ mixtures. In addition, the mechanism is validated against a wide range of standard H₂/O₂ kinetic targets, including ignition delay times, flow reactor species time-histories, laminar flame speeds, and burner-stabilized flame structures. This validation indicates that the updated mechanism should perform reliably over a range of reactant concentrations, stoichiometries, pressures, and temperatures from 950 to greater than 3000 K.     

Numerical investigation on effects of high initial temperatures and pressures on flame behavior of CO/H2/Air mixtures near the dilution limit

This study investigates effects of initial temperatures and pressures on dilution limits of CO/H₂/air mixtures by numerical simulation of one-dimensional laminar premixed flames of CO/H₂/air mixtures (50%CO–50%H₂). Maximum flame temperatures, laminar flame speeds, mass burning rates and flame thickness near the dilution limits are analyzed. Results reveal that the dilution limits are extended at the elevated initial temperatures. The laminar flame speeds and mass burning rates at the dilution limits increase with the elevation of initial temperature, however, the flame thickness at the dilution limits decreases with increasing pressures and increases slightly with elevated initial temperature. The decreased flame thickness renders the flamelet modeling more favorable for turbulent combustion at elevated pressure conditions. The ratio of the flame thickness to the reaction thickness and the Zeldovich number increase first and then decrease with increasing pressure, but the non-monotonic trend of ratio of flame thickness to reaction thickness with the increasing pressures is unnoticeable. Sensitivity analysis suggested that the non-monotonic trend of the Zeldovich number could be caused by the combined effects of following elementary reactions: H + O₂ + M → HO₂ + M, 2HO₂ → H₂O₂ + O₂ and H₂O₂ + M → 2OH + M.     

Detailed and reduced chemistry for methanol ignition

Simplified chemical-kinetic mechanisms are sought that can provide agreement with measured shock-tube autoignition times and counterflow critical ignition conditions for methanol (CH₃OH) oxidation. Existing detailed chemistry over-predicts measured counterflow ignition temperatures by 100 K or more. It was found that the elementary step CH₃OH + HO₂ → CH₂OH + H₂O₂ most strongly affects the predictions. Increasing the pre-factor in the Arrhenius expression for the rate of this step from different available literature values by a factor ranging from 2 to 13, namely to 8 × 10¹³ cm³/(mol s), within existing uncertainty, produces agreement of predictions with experiment. Using this revised rate, unimportant steps are deleted from the San Diego mechanism to obtain a set of 26 irreversible elementary steps (augmented to 27 by including fuel dissociation to CH₃ + OH for high-temperature shock-tube conditions) that predict ignition nearly as well as the detailed mechanism. In this mechanism, the intermediate species CH₂OH, CH₃O, HCO, H, O, and OH accurately obey steady states, while the intermediates CH₂O, HO₂, H₂O₂, CO, and H₂ do not. The result is a six-step overall reduced mechanism that describes ignition well at the lower temperatures. At higher temperatures, the aforementioned fuel decomposition becomes important, increasing the six-step mechanism to a seven-step mechanism. Expressions for the reaction rates, branching ratios, and steady-state species concentrations in the six-step reduced mechanism are given to facilitate future methanol ignition computations. Higher alcohols, which are less dependent on HO₂ attack in ignition, are indicated to nevertheless possibly benefit from an increase of the rate of the corresponding step.     

An evaluation of detailed reaction mechanisms for hydrogen combustion under gas turbine conditions

Chemical kinetics in hydrogen combustion for elevated pressures have recently become more relevant because of the implementation of hydrogen as a fuel in future gas turbine combustion applications, such as IGCC or IRCC systems. The aim of this study is to identify a reaction mechanism that accurately represents H₂/O₂ kinetics over a large range of conditions, particularly at elevated pressures as present in a gas turbine combustor. Based on a literature review, six mechanisms of different research groups have been selected for further comparisons within this study. Reactor calculations of ignition delay times show that the mechanisms of Li et al. and Ó Conaire et al. yield the best agreement with data from shock tube experiments at pressures up to 33 bar. The investigation of one-dimensional laminar hydrogen flames indicate that these two mechanisms also yield the best agreement with experimental data of laminar flame speed, particularly for elevated pressures. The present study suggests that the Li mechanism is best suited for the prediction of H₂/O₂ chemistry since it includes more up-to date data for the range of interest.     

Photofragmentation laser-induced fluorescence imaging in premixed flames

Two-dimensional measurements of primarily hydroperoxyl radicals (HO₂) are, for the first time, demonstrated in flames. The measurements are performed in different Bunsen-type premixed flames (H₂/O₂, CH₄/O₂, and CH₄/air) using photofragmentation laser-induced fluorescence (PF-LIF). Photofragmentation is done by laser radiation at 266 nm, and the generated OH photofragments are probed through fluorescence induced by a laser tuned to the Q₁(5) transition at 282.75 nm. The signal due to naturally occurring OH radicals, recorded by having the photolysis laser blocked, is subtracted, providing an image that reflects the concentration of OH fragments generated by photolysis, and hence the presence of primarily HO₂, but also smaller contributions from H₂O₂ and, for the methane flames, CH₃O₂. For the methane flames the measured radial profiles of OH photofragments and natural OH agree well with corresponding profiles calculated for laminar, one-dimensional, premixed flames using CHEMKIN-II with the Konnov detailed C/H/N/O reaction mechanism. An interfering signal contribution is observed in the product zone of the methane flames. It is concluded that the major source for the interference is most likely hot CO₂, from which O atoms are produced by photolysis, and OH is rapidly formed as the O atoms react with H₂O and H₂. This conclusion is supported by the fact that the interference is absent for the hydrogen flame, but appears when CO₂ is seeded into the flame. Another strong indication is that the Konnov mechanism predicts a similar buildup of OH after photolysis.     

Autoignited laminar lifted flames of methane, ethylene, ethane, and n-butane jets in coflow air with elevated temperature

The autoignition characteristics of laminar lifted flames of methane, ethylene, ethane, and n-butane fuels have been investigated experimentally in coflow air with elevated temperature over 800 K. The lifted flames were categorized into three regimes depending on the initial temperature and fuel mole fraction: (1) non-autoignited lifted flame, (2) autoignited lifted flame with tribrachial (or triple) edge, and (3) autoignited lifted flame with mild combustion.For the non-autoignited lifted flames at relatively low temperature, the existence of lifted flame depended on the Schmidt number of fuel, such that only the fuels with Sc > 1 exhibited stationary lifted flames. The balance mechanism between the propagation speed of tribrachial flame and local flow velocity stabilized the lifted flames. At relatively high initial temperatures, either autoignited lifted flames having tribrachial edge or autoignited lifted flames with mild combustion existed regardless of the Schmidt number of fuel. The adiabatic ignition delay time played a crucial role for the stabilization of autoignited flames. Especially, heat loss during the ignition process should be accounted for, such that the characteristic convection time, defined by the autoignition height divided by jet velocity was correlated well with the square of the adiabatic ignition delay time for the critical autoignition conditions. The liftoff height was also correlated well with the square of the adiabatic ignition delay time.     

Numerical analysis on influence of hydrogen peroxide (H2O2) addition on the combustion and emissions characteristics of NH3/CH4-air (O2/N2)/ H2O2 mixture

In this work, extensive chemical kinetic modeling is performed to analyze the combustion and emissions characteristics of premixed NH₃/CH₄–O₂/N₂/H₂O₂ mixtures at different replacement percentages of air with hydrogen peroxide (H₂O₂). This work is comprehensively discusses the ignition delay time, flame speed, heat release rate, and NOx & CO emissions of premixed NH₃/CH₄–O₂/N₂/H₂O₂ mixtures. Important intermediate crucial radicals such as OH, HO₂, HCO, and HNO effect on the above-mentioned parameters is also discussed in detail. Furthermore, correlations were obtained for the laminar flame speed, NO, and CO emissions with important radicals such as OH, HO₂, HCO, and HNO. The replacement of air with H₂O₂ increases flame speed and decreases the ignition delay time of the mixture significantly. Also, increases the CO and NOx concentration in the products. The CO and NOx emissions can be controlled by regulating the H₂O₂ concentration and equivalence ratios. Air replacement with H₂O₂ enhances the reactions rate and concentration of intermediate radicals such as O/H, HO₂, and HCO in the mixture. These intermediate radicals closely govern the combustion chemistry of the NH₃/CH₄– O₂/N₂/H₂O₂ mixture. A linear correlation is observed between the flame speed and peak mole fraction of OH + HO₂ radicals, and 2nd degree polynomial correlation is observed for the peak mole fraction of NO and CO with HNO + OH and HCO + OH radicals, respectively.     

A two-step chemical scheme for kerosene-air premixed flames

A reduced two-step scheme (called 2S_KERO_BFER) for kerosene-air premixed flames is presented in the context of Large Eddy Simulation of reacting turbulent flows in industrial applications. The chemical mechanism is composed of two reactions corresponding to the fuel oxidation into CO and H₂O, and the CO − CO₂ equilibrium. To ensure the validity of the scheme for rich combustion, the pre-exponential constants of the two reactions are tabulated versus the local equivalence ratio. The fuel and oxidizer exponents are chosen to guarantee the correct dependence of laminar flame speed with pressure. Due to a lack of experimental results, the detailed mechanism of Dagaut composed of 209 species and 1673 reactions, and the skeletal mechanism of Luche composed of 91 species and 991 reactions have been used to validate the reduced scheme. Computations of one-dimensional laminar flames have been performed with the 2S_KERO_BFER scheme using the CANTERA and COSILAB softwares for a wide range of pressure ([1; 12] atm), fresh gas temperature ([300; 700] K), and equivalence ratio ([0.6; 2.0]). Results show that the flame speed is correctly predicted for the whole range of parameters, showing a maximum for stoichiometric flames, a decrease for rich combustion and a satisfactory pressure dependence. The burnt gas temperature and the dilution by Exhaust Gas Recirculation are also well reproduced. Moreover, the results for ignition delay time are in good agreement with the experiments.     

Laminar flame speeds and kinetic modeling of hydrogen/chlorine combustion

In this study, laminar flame speeds at atmospheric pressure are accurately measured for H₂/Cl₂/N₂ mixtures at different equivalence ratios and N₂ mole fractions by the counterflow flame technique. A kinetic mechanism based on recently published and evaluated rate constants is developed to model these measured laminar flame speeds as well as the literature data on the concentrations of H₂, Cl₂, and HCl species in flat-burner flames and the ignition delay times from shock tube experiments. The kinetic model yields satisfactory comparison with these experimental data, and suggests that the reactions involving excited HCl(v) species and energy branching are not of substantial significance in combustion situations, and that the use of accurate elementary rate constants is instead crucial to the accuracy of the reaction mechanism.     

Modeling ignition and chemical structure of partially premixed turbulent flames using tabulated chemistry

Correctly reproducing the autoignition and the chemical composition of partially premixed turbulent flames is a challenge for numerical simulations of industrial applications such as diesel engines. A new model DF-PCM (diffusion flame presumed conditional moment) is proposed based on a coupling between the FPI (flame prolongation of ILDM) tabulation method and the PCM (presumed conditional moment) approach. Because the flamelets used to build the table are laminar diffusion flames, DF-PCM cannot be used for industrial applications like Diesel engines due to excessive CPU requirements. Therefore two new models called AI-PCM (autoignition presumed conditional moment) and ADF-PCM (approximated diffusion flames presumed conditional moment) are developed to approximate it. These models differ from DF-PCM because the flamelet libraries used for the table rely on PSR calculations. Comparisons between DF-PCM, AI-PCM, and ADF-PCM are performed for two fuels, n-heptane, representative of diesel fuels, and methane, which does not exhibit a “cool flame” ignition regime. These comparisons show that laminar diffusion flames can be approximated by flamelets based on PSR calculations in terms of autoignition delays and steady state profiles of the progress variable. Moreover, the evolution of the mean progress variable of DF-PCM can be correctly estimated by the approximated models. However, as discussed in this paper, errors are larger for CO and CO₂ mass fractions evolutions. Finally, an improvement to ADF-PCM, taking into account ignition delays, is proposed to better reproduce the ignition of very rich mixtures.     

Improvement of H2/O2 chemical kinetic mechanism for high pressure combustion

An updated H₂/O₂ kinetic mechanism was proposed by incorporating carefully selected reaction rate coefficient and great progress in radical chain mechanisms, in which the uncertainties of rate coefficient were discussed. The performance of the current mechanism was compared to other H₂ mechanism and validated against a wide range kinetic targets, including oxidation, decomposition in shock waves, ignition, flame speed and flame structure. Results show that the current mechanism obtains an overall improvement of performance, especially for the flame speed. By using the updated binary diffusion coefficient from ab initio calculations and the chemically termolecular reactions, the current mechanism presents better agreement with the new experimental flame speed at atmospheric pressure and obtains the improved performance with respect to the negative pressure dependence of high-pressure H₂ flame. Furthermore, the flame speed predictions are strongly sensitive to the H₂O third body efficiency in the H₂ mechanism, affecting the water-contained H₂ flame. The modeling results of rapid compression machine ignition show that present mechanism can more accurately predicts the ignition delay under engine-like conditions. However, all three mechanisms cannot accurately reproduce the negative pressure dependence behavior of mass burning rate in high-pressure H₂ flame, which may be attributed to the fact that the important reaction O + OH(+M) = HO₂(+M) that significantly affects lean high-pressure H₂ flame is not included in current mechanism. Consequently, continuous works should be emphasized on the reactions that are important but neglected in H₂ mechanism. All these not only develop an improved H₂ reaction mechanism for high-pressure combustion, but also point out the direction for refining the H₂ mechanism.     

Extinction of premixed H2/air flames: Chemical kinetics and molecular diffusion effects

Laminar flame speed has traditionally been used for the partial validation of flame kinetics. In most cases, however, its accurate determination requires extensive data processing and/or extrapolations, thus rendering the measurement of this fundamental flame property indirect. Additionally, the presence of flame front instabilities does not conform to the definition of laminar flame speed. This is the case for Le<1 flames, with the most notable example being ultralean H₂/air flames, which develop cellular structures at low strain rates so that determination of laminar flame speeds for such mixtures is not possible. Thus, this low-temperature regime of H₂ oxidation has not been validated systematically in flames. In the present investigation, an alternative/supplemental approach is proposed that includes the experimental determination of extinction strain rates for these flames, and these rates are compared with the predictions of direct numerical simulations. This approach is meaningful for two reasons: (1) Extinction strain rates can be measured directly, as opposed to laminar flame speeds, and (2) while the unstretched lean H₂/air flames are cellular, the stretched ones are not, thus making comparisons between experiment and simulations meaningful. Such comparisons revealed serious discrepancies between experiments and simulations for ultralean H₂/air flames by using four kinetic mechanisms. Additional studies were conducted for lean and near-stoichiometric H₂/air flames diluted with various amounts of N₂. Similarly to the ultralean flames, significant discrepancies between experimental and predicted extinction strain rates were also found. To identify the possible sources of such discrepancies, the effect of uncertainties on the diffusion coefficients was assessed and an improved treatment of diffusion coefficients was advanced and implemented. Under the conditions considered in this study, the sensitivity of diffusion coefficients to the extinction response was found to be significant and, for certain species, greater than that of the kinetic rate constants.     

Negative pressure dependence of mass burning rates of H2/CO/O2/diluent flames at low flame temperatures

Experimental measurements of burning rates, analysis of the key reactions and kinetic pathways, and modeling studies were performed for H₂/CO/O₂/diluent flames spanning a wide range of conditions: equivalence ratios from 0.85 to 2.5, flame temperatures from 1500 to 1800 K, pressures from 1 to 25 atm, CO fuel fractions from 0 to 0.9, and dilution concentrations of He up to 0.8, Ar up to 0.6, and CO₂ up to 0.4. The experimental data show negative pressure dependence of burning rate at high pressure, low flame temperature conditions for all equivalence ratios and CO fractions as high as 0.5. Dilution with CO₂ was observed to strengthen the pressure and temperature dependence compared to Ar-diluted flames of the same flame temperature. Simulations were performed to extend the experimentally studied conditions to conditions typical of gas turbine combustion in Integrated Gasification Combined Cycle processes, including preheated mixtures and other diluents such as N₂ and H₂O.Substantial differences are observed between literature model predictions and the experimental data as well as among model predictions themselves – up to a factor of three at high pressures. The present findings suggest the need for several rate constant modifications of reactions in the current hydrogen models and raise questions about the sufficiency of the set of hydrogen reactions in most recent hydrogen models to predict high pressure flame conditions relevant to controlling NO_x emissions in gas turbine combustion. For example, the reaction O + OH + M = HO₂ + M is not included in most hydrogen models but is demonstrated here to significantly impact predictions of lean high pressure flames using rates within its uncertainty limits. Further studies are required to reduce uncertainties in third body collision efficiencies for and fall-off behavior of H + O₂(+M) = HO₂(+M) in both pure and mixed bath gases, in rate constants for HO₂ reactions with other radical species at higher temperatures, and in rate constants for reactions such as O + OH + M that become important under the present conditions in order to properly characterize the kinetics and predict global behavior of high-pressure H₂ or H₂/CO flames.     

Measurements of laminar flame speeds of acetone/methane/air mixtures

The effect of acetone on the laminar flame speed of methane/air mixtures is investigated over a range of stoichiometries at atmospheric pressure and room temperature. The liquid acetone is vaporised and seeded into the methane/air mixture at 5%, 9% and 20% of the total fuel by mole. The experiment is performed using the jet-wall stagnation flame configuration and the particle imaging velocimetry (PIV) technique. Laminar flame speeds are derived by extrapolating the reference flame speed back to zero strain rate. Experimental results are compared to numerically calculated values using a base methane chemical kinetic mechanism (GRI-Mech 3.0) extended with acetone oxidation and pyrolysis reactions from the literature. The experimental results show that acetone addition does not affect the laminar flame speed of methane significantly within the range of concentrations considered, with a stronger effect on the rich range than under fuel-lean conditions, and that the peak laminar flame speed of acetone in air is ∼42.5 cm/s at ? = 1.2. Simulation results reveal that the most important reactions determining acetone laminar flame speeds are H + O₂ → O + OH, OH + CO → H + CO₂, HO₂ + CH₃ → OH + CH₃O and H + O₂ + H₂O → HO₂ + H₂O. Comparison of the expected disappearance of acetone relative to methane shows that the former is a good fluorescent marker for the latter.     

A bifurcation analysis for limit flame phenomena of DME/air in perfectly stirred reactors

A bifurcation analysis was developed to systematically detect limit flame phenomena, including ignition, extinction and changes in flame stability, and to understand the underlying physicochemical processes that control the limit phenomena. The bifurcation analysis was demonstrated with steady-state perfectly stirred reactors (PSRs) using dimethyl ether (DME) with the negative temperature coefficient (NTC) chemistry. Flame stability was first analyzed to identify ignition and extinction states based on the eigenvalues of the Jacobian of the governing equations. It was found that for DME–air mixtures, extinction may not occur at the turning points on the S-curves. A bifurcation index (BI) was then defined at each bifurcation point on the S-curves to quantify the contribution of each reaction and the mixing process to the limit flame phenomenon. Results show that extinction of the strong flames of DME–air is primarily controlled by the reactions involving small molecules, such as HCO and CO, while extinction of the cool flames is primarily controlled by the NTC chemistry involving larger molecules. To validate this method, the pre-exponential “A”-factors of the selected reactions were perturbed. It was found that the perturbations in reactions with large BI values have significant effects, while those with small BI values have minor effects, on the ignition and extinction states. The BI-based method was further compared to sensitivity analysis, and overall-consistent results were observed on the importance of the reactions at the bifurcation points, indicating that the bifurcation analysis is effective in identifying controlling reactions for limit flame phenomena. The BI values were then employed to guide the refinement of the rate constants in the DME mechanism. A skeletal model with substantially reduced reaction set and systematically tuned rate constants was obtained to accurately capture both steady-state and transient ignition and extinction behaviors of DME–air in PSR.