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.     

Studies of C4 and C10 methyl ester flames

The oxidation of three model biodiesel fuels, namely methyl butanoate (C₅H₁₀O₂, CAS No. 623-42-7), methyl crotonate (C₅H₈O₂, CAS No. 623-43-8), and methyl decanoate (C₁₁H₂₂O₂, CAS No. 110-42-9) was investigated in laminar premixed and non-premixed flames. The experiments were conducted in the counterflow configuration at atmospheric pressure, for a wide range of equivalence or inert-dilution ratios, and elevated reactant temperatures. Laminar flame speeds and local extinction strain rates were determined by measuring the flow velocities using digital particle image velocimetry. The experimental data were compared against those derived for flames of n-alkanes of similar carbon number, in order to assess the effects of saturation, the length of carbon chain, and the presence of the ester group. Several recent chemical kinetic models were tested against the experimental data, and major differences were identified and assessed. The accuracy of the Lennard–Jones potential parameters assigned to the methyl esters in the transport databases of the different models was evaluated and new values were estimated. Insight was provided into the high-temperature kinetic pathways of methyl esters in flame environments. Additionally, the reduced sooting propensity of methyl ester flames compared to n-alkane flames was investigated computationally.     

Propagation and extinction of benzene and alkylated benzene flames

Laminar flame speeds and extinction strain rates of benzene, n-propylbenzene, toluene, o-, m-, and p-xylene, and 1,2,4- and 1,3,5-trimethylbenzene flames were studied experimentally in the counterflow configuration under atmospheric pressure and at the elevated temperature of 353 K for the unreacted fuel-containing stream. The experimental data revealed that the aromatic fuel structure plays a critical role on flame propagation, with the laminar flame speed decreasing with an increase in methylation of benzene. Numerical simulations suggest that the aromatics flames are highly sensitive to fuel-specific chemistry and more specifically to the reaction kinetics of the first few intermediates in the oxidation process following the fuel consumption, and that the different flame propagation speeds relate strongly to radical–radical termination facilitated by benzyl or benzyl-like intermediates. The tendencies of stretch-induced extinction of non-premixed flames was found to follow a trend that is identical to the laminar flame speed, but the extinction data revealed a more discriminative effect arising from fuel-structure differences. Comparisons between kinetic model predictions and experimental data showed that there exist significant discrepancies among these models and uncertainties in the oxidation and pyrolysis kinetics of one-ring aromatics.     

Laminar flame propagation of atmospheric iso-cetane/air and decalin/air mixtures

Laminar flame speeds of iso-cetane/air and decalin/air mixtures were measured in the counterflow configuration at atmospheric pressure and an elevated unburned mixture temperature of 443 K. Axial flow velocities were measured along the stagnation streamline using the digital particle image velocimetry. The laminar flame speeds were determined by determining the variation of a reference flame speed as a function of strain rate and computationally assisted non-linear extrapolations. The data are the first to be reported in the literature, and they were modeled using a recently developed kinetic model that includes 187 species and 6086 elementary reactions. In general, the computed results were found to be in close agreement with the data. In order to get insight into kinetic effects on flame propagation, detailed sensitivity and reaction path analyses were performed using the computed flame structures. The results revealed that at the same equivalence ratio, laminar flame speeds of iso-cetane/air mixtures are lower than those of n-hexadecane/air mixtures. Additionally, it was found that the laminar flame speeds of iso-cetane/air and decalin/air mixtures are sensitive largely to C₀–C₄ kinetic subset, and that the lower reactivity of iso-cetane compared to n-hexadecane could be attributed to the higher production of relatively stable intermediates.     

Laminar mass burning and entrainment velocities and flame instabilities of i-octane,ethanol and hydrous ethanol/air aerosols

The paper reports experiments employing the cloud chamber technique for creating fuel aerosols, in studies of premixed laminar flames. Spherical explosion flames were initiated at different times after the start of expansion of the original gaseous mixture to lower pressure. Flame speeds were measured close to atmospheric pressure, over a range of equivalence ratios of iso-octane, ethanol and hydrous ethanol with air. A methodology was developed for deriving mass burning velocities and entrainment velocities, as well as mass burning fluxes, from the measurements of aerosol number densities, droplet sizes and flame speeds. It was vital to estimate whether droplet evaporation was completed in the flame preheat zone. This was done by calculating the spatial progress of droplet evaporation for the different aerosols from values of the evaporation rate constants of the different fuels.     

A numerical study of laminar flame speed of stratified syngas/air flames

Numerical simulations are performed to study the flame propagation of laminar stratified syngas/air flames with the San Diego mechanism. Effects of fuel stratification, CO/H₂ mole ratio and temperature stratification on flame propagation are investigated through comparing the distribution of flame temperature, heat release rate and radical concentration of stratified flame with corresponding homogeneous flame. For stratified flames with fuel rich-to-lean and temperature high-to-low, the flame speeds are faster than homogeneous flames due to more light H radical in stratified flames burned gas. The flame speed is higher for case with larger stratification gradient. Contrary to positive gradient cases, the flame speeds of stratified flames with fuel lean-to-rich as well as with temperature low-to-high are slower than homogeneous flames. The flame propagation accelerates with increasing hydrogen mole ratio due to higher H radical concentration, which indicates that chemical effect is more significant than thermal effect. Additionally, flame displacement speed does not match laminar flame speed due to the fluid continuity. Laminar flame speed is the superposition of flame displacement speed and flow velocity.     

Laminar burning velocities and flame instabilities of butanol isomers-air mixtures

Laminar burning velocities and flame instabilities of the butanol-air premixed flames and its isomers are investigated using the spherically expanding flame with central ignition at initial temperature of 428 K and initial pressures of 0.10 MPa, 0.25 MPa, 0.50 MPa and 0.75 MPa. Laminar burning velocities and sensitivity factor of n-butanol-air mixtures are computed using a newly developed kinetic mechanism. Unstretched laminar burning velocity, adiabatic temperature, Lewis number, Markstein length, critical flame radius and Peclet number are obtained over a wide range of equivalence ratios. Effect of molecular structure on laminar burning velocity of the isomers of butanol is analyzed from the aspect of CH bond dissociation energy. Study indicates that although adiabatic flame temperatures of the isomers of butanol are the same, laminar burning velocities give an obvious difference among the isomers of butanol. This indicates that molecular structure has a large influence on laminar burning velocities of the isomers of butanol. Branching (CH3) will decrease laminar burning velocity. Hydroxyl functional group (OH) attaching to the terminal carbon atoms gives higher laminar burning velocity compared to that attaching to the inner carbon atoms. Calculated dissociation bond energies show that terminal CH bonds have larger bond energies than that of inner CH bonds. n-Butanol, no branching and with hydroxyl functional group (OH) attaching to the terminal carbon atom, gives the largest laminar burning velocity. tert-Butanol, with highly branching and hydroxyl functional group (OH) attaching to the inner carbon atom, gives the lowest laminar burning velocity. Laminar burning velocities of iso-butanol and sec-butanol are between those of n-butanol and tert-butanol. The instant of transition to cellularity is experimentally determined for the isomers of butanol and subsequently interpreted on the basis of hydrodynamic and diffusion-thermal instabilities. Little effect on flame instability is observed for the isomers of butanol. Critical flame radii are the same for the isomers of butanol. Peclet number decreases with the increase in equivalence ratio.     

Effects of radiation and compression on propagating spherical flames of methane/air mixtures near the lean flammability limit

Large discrepancies between the laminar flame speeds and Markstein lengths measured in experiments and those predicted by simulations for ultra-lean methane/air mixtures bring a great concern for kinetic mechanism validation. In order to quantitatively explain these discrepancies, a computational study is performed for propagating spherical flames of lean methane/air mixtures in different spherical chambers using different radiation models. The emphasis is focused on the effects of radiation and compression. It is found that the spherical flame propagation speed is greatly reduced by the coupling between thermal effect (change of flame temperature or unburned gas temperature) and flow effect (inward flow of burned gas) induced by radiation and/or compression. As a result, for methane/air mixtures near the lean flammability limit, the radiation and compression cause large amounts of under-prediction of the laminar flame speeds and Markstein lengths extracted from propagating spherical flames. Since radiation and compression both exist in the experiments on ultra-lean methane/air mixtures reported in the literature, the measured laminar flame speeds and Markstein lengths are much lower than results from simulation and thus cannot be used for kinetic mechanism validation.     

The effect of hydrogen addition on premixed laminar acetylene–hydrogen–air and ethanol–hydrogen–air flames

In the present work, the laminar premixed acetylene–hydrogen–air and ethanol–hydrogen–air flames were investigated numerically. Laminar flame speeds, the adiabatic flame temperatures were obtained utilizing CHEMKIN PREMIX and EQUI codes, respectively. Sensitivity analysis was performed and flame structure was analyzed. The results show that for acetylene–hydrogen–air flames, combustion is promoted by H and O radicals. The highest flame speed (247 cm/s) was obtained in mixture with 95% H₂–5% C₂H₂ at λ = 1.0. The region between 0.95 < X_H2 < 1.0 was referred to as the acetylene-accelerating hydrogen combustion since the flame speed increases with increase the acetylene fraction in the mixture. Further increase in the acetylene fraction decreases the H radicals in the flame front. In ethanol–hydrogen–air mixtures, the mixture reactivity is determined by H, OH and O radicals. For X_H2 < 0.6, the flame speed in this regime increases linearly with increasing the hydrogen fraction. For X_H2 > 0.8, the hydrogen chemistry control the combustion and ethanol addition inhibits the reactivity and reduces linearly the laminar flame speed. For 0.6 < X_H2 < 0.8, the laminar flame speed increases exponentially with the increase of hydrogen fraction.     

Nonlinear effects in the extraction of laminar flame speeds from expanding spherical flames

Various factors affecting the determination of laminar flames speeds from outwardly propagating spherical flames in a constant-pressure combustion chamber were considered, with emphasis on the nonlinear variation of the stretched flame speed to the flame stretch rate, and the associated need to nonlinearly extrapolate the stretched flame speed to yield an accurate determination of the laminar flame speed and Markstein length. Experiments were conducted for lean and rich n-butane/air flames at initial pressure, demonstrating the complex and nonlinear nature of the dynamics of flame evolution, and the strong influences of the ignition transient and chamber confinement during the initial and final periods of the flame propagation, respectively. These experimental data were analyzed using the nonlinear relation between the stretched flame speed and stretch rate, yielding laminar flame speeds that agree well with data determined from alternate flame configurations. It is further suggested that the fidelity in the extraction of the laminar flame speed from expanding spherical flames can be facilitated by using small ignition energy and a large combustion chamber.     

An experimental and mechanistic study on the laminar flame speed,Markstein length and flame chemistry of the butanol isomers

Laminar flame speeds and Markstein lengths for n-butanol, s-butanol, i-butanol and t-butanol at pressures from 1 to 5 atm were experimentally measured in a heated, dual-chamber vessel. Results at all pressures show that n-butanol has the highest flame speeds, followed by s-butanol and i-butanol, and then t-butanol. Results further show that the reduced Markstein length measured for n-butanol as compared to other isomers is a flame thickness effect, and that all four isomers have similar Markstein numbers, which is the appropriate nondimensional parameter to quantify flame stretch. Computation and flame chemistry analysis were performed using two recently published kinetic models on butanol isomers by Sarathy et al. and Ranzi et al., respectively. Comparison shows the former model satisfactorily agrees with the present results while agreement of the latter is less satisfactory. Based on reaction path analysis the major differences of the two models on fuel cracking pathway were identified. It is concluded that the primary reason for the lowered flame speed of s-butanol, i-butanol and t-butanol is that they crack into more branched intermediate species which are relatively stable, such as iso-butene, iso-propenol and acetone. This indicates that the general rule that fuel branching reduces flame speed for hydrocarbons can also be applied to alcohols, and that the fundamental reason for this generality is that in alcohols CO has similar bond energy to the CC bond while OH has similar bond energy to the CH bond.     

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.     

Measurement of laminar burning velocity and Markstein length relative to fresh gases using a new postprocessing procedure: Application to laminar spherical flames for methane,ethanol and isooctane/air mixtures

The purpose of this study is to present a new tool for extracting the laminar burning velocity in the case of spherically outward expanding flames. This new procedure makes it possible to determine the laminar burning velocity directly based on the flame displacement speed and the global fresh gas velocity near the preheat zone of the flame front. It therefore presents a very interesting alternative to the standard method (commonly used in the literature), which is based on the flame front displacement and the ratio of unburned and burned gas densities. The influence of external flame stretching on the burning velocity can be characterized and the Markstein length relative to the unburned gases (i.e., fresh gases) can be deduced by using this new tool. Contrary to the standard procedure, the unstretched laminar burning velocity is determined directly without using the fuel mixture properties. The temporal evolution of the flame front is visualized by high-speed laser tomography and the algorithm, based on a tomographic image correlation method, makes it possible to accurately measure the fresh gas velocity near the preheat zone of the flame front. The measurements of laminar flame speeds are carried out in a high-pressure and high-temperature constant-volume vessel over a wide range of equivalence ratios for methane, ethanol, and isooctane/air mixtures. To validate the experimental facility and the postprocessing of the flame images, fresh gas velocities and unstretched laminar burning velocities, as well as Markstein lengths relative to burned and unburned gases, are presented and compared with experimental and numerical results of the literature for methane/air flames. New results concerning ethanol/air and isooctane/air flames are presented for various experimental conditions (373 K, equivalence ratios range 0.7–1.5, pressure range 0.1–5 MPa).     

Experimental and kinetic modeling study of PAH formation in methane coflow diffusion flames doped with n-butanol

In order to understand the interactions between butanol and hydrocarbon fuels in the PAH formation, experimental and kinetic modeling investigations were combined to study methane laminar coflow diffusion flames doped with two inlet mole fractions of n-butanol (1.95% and 3.90%) in this work. Mole fractions of flame species along the flame centerline were measured using synchrotron VUV photoionization mass spectrometry. A detailed kinetic model of n-butanol combustion, extended from a recent published n-butanol model, was provided in this work to reproduce the fuel decomposition and the formation of benzene and PAHs in the investigated flames. Numerical simulations were performed with laminarSMOKE code, a CFD code specifically conceived to handle large kinetic mechanisms. The simulation results were able to follow the observed effects of n-butanol addition from the experimental results. In particular, unsaturated hydrocarbons, especially C6–C16 aromatics, were predicted satisfactorily. The reaction flux analysis revealed that benzene precursors, especially C3 radicals, increase significantly with increasing inlet mole fraction of n-butanol. This enhances the formation of phenyl and benzyl radicals, which are important PAH precursors. Reactions of benzyl, phenyl radicals and benzene with C2–C3 species are the major formation pathways for indene and naphthalene. And PAHs with more carbon atoms are dominantly formed from naphthyl and indenyl radicals.     

Effect of preferential diffusion and flame stretch on flame structure and laminar burning velocity of syngas Bunsen flame using OH-PLIF

By using OH-PLIF technique, experiments were conducted for laminar Bunsen flame of premixed CO/H₂/air mixtures with equivalence ratio ranging from 0.5 to 1.8. Reynolds number was varied from 800 to 2200, X_H2 = H₂/(H₂+CO) in the mixture was varied from 20% to 100% to study the effects of both preferential diffusion and flame curvature on flame structures and laminar flame burning velocities. Results showed that the combined effects of preferential diffusion and curvature gave an interesting phenomenon of the flame OH radical distributions on high hydrogen content flames. Furthermore, with the increase of H₂ fraction in fuel mixture, the effects of both preferential diffusion and flame curvature were increased. Interpretation of flame stretch effect on laminar burning velocity is also provided in this paper.     

Radiation-induced uncertainty in laminar flame speed measured from propagating spherical flames

Laminar flame speeds measured using the propagating spherical flame method are inherently affected by radiation. Under certain conditions, a substantial uncertainty in laminar flame speed measurement is caused by radiation, which results in a great concern for kinetic mechanism validation and development. In this study, numerical simulations with detailed chemistry and different radiation models are conducted to examine the effects of radiation on spherical flame propagation. The emphasis is placed on quantifying the uncertainty and corrections associated with radiation in laminar flame speed measurements using propagating spherical flames. The radiation effects on flame speeds at normal and elevated temperatures and pressures are examined for different fuel/air mixtures including methane, propane, iso-octane, syngas, hydrogen, dimethyl ether, and n-heptane. The radiative effects are conservatively evaluated without considering radation reflection on the wall. It is found that radiation-induced uncertainty in laminar flame speeds is affected in the opposite ways by the initial temperature and pressure. An empirical correlation quantifying the uncertainty associated with radiation is obtained. This correlation is shown to work for different fuels at normal and elevated temperatures and pressures. Therefore, it can be directly used in spherical flame experiments measuring the laminar flame speed. Furthermore, a method to obtain the radiation-corrected flame speed (RCFS) is presented and it can be used for laminar flame speed measurement using the propagating spherical flame method.     

Computational diagnostics for n-heptane flames with chemical explosive mode analysis

Computational flame diagnostics (CFLDs) are systematic tools to extract important information from simulated flames, particularly when detailed chemical kinetic mechanisms are involved. The results of CFLD can be employed for various purposes, e.g. to simplify detailed chemical kinetics for more efficient flame simulations, and to explain flame behaviors associated with complex chemical kinetics. In the present study, the utility of a recently developed method of chemical explosive mode analysis (CEMA) for CFLD will be demonstrated with a variety of flames for n-heptane including auto-ignition, ignition and extinction in steady state perfectly stirred reactors (PSRs) and laminar premixed flames. CEMA was further utilized for analyses and visualization of a direct numerical simulation (DNS) dataset for a 2-D n-heptane–air flame under homogeneous charge compression ignition (HCCI) conditions. CEMA was found to be a versatile method for systematic detection of many critical flame features including ignition, extinction, premixed flame fronts, and flame stabilization mechanisms. The effects of cool flame chemistry for n-heptane on ignition, extinction and flame stability were also investigated with CEMA.