Aromatic formation pathways in non-premixed methane flames

A kinetic mechanism, previously developed and successfully applied to the prediction of the formation of benzene and aromatics in different flame conditions, was applied to assess the importance of the various benzene and aromatic formation pathways in non-premixed flames. Four sets of data were tested: the methane flame and the same flame doped with toluene, ethylbenzene, and tert-butylbenzene, as studied by Anderson and co-workers. The model predicts, with good accuracy, the growth of hydrocarbons and the formation of benzene and aromatic species. The modeling shows that in the undoped methane flame, benzene formation is controlled by propargyl radical combination. Acetylene addition to C4 radicals contributes a moderate amount, whereas toluene decomposition is insignificant. The predictions are almost unaffected by the fulvene pathway. Benzene is strongly perturbed by dopant addition to methane. Predictions agree quite well with benzene concentrations in the undoped flame and agree with the increase in benzene concentration when alkylbenzenes are added. Key reactions leading to the formation of naphthalene are the propargyl addition to benzyl radicals, and, to a lesser extent, the hydrogen-abstraction acetylene-addition mechanism. Cyclopentadienyl radical combination, which is the dominant route in premixed and partially premixed flames, is insignificant in these flame conditions.     

Numerical modeling for structure and NOx formation characteristics of oxygen-enriched syngas turbulent non-premixed jet flames

The present study numerically investigated the effect of oxygen enrichment on the precise structure and NO_x formation characteristics of turbulent syngas non-premixed flames. The turbulence-chemistry interactions were represented by a Lagrangian flamelet model. In context with the Lagrangian flamelet model, the NO concentration was obtained directly from the flamelet calculation based on full NO_x chemistry, with radiative heat loss being accounted for through the flamelet energy equation. Computations were performed for three different syngas compositions with a designated nitrogen dilution level. Numerical results indicated that, for the CO-rich composition with the lowest LHV yielding the highest scalar dissipation rate and shortest flight time, the flame structure was dominantly influenced by turbulence-chemistry interactions. On the other hand, with regard to the H₂-rich composition with the highest LHV yielding the lowest injection velocity and longest flight time, the flame structure was strongly influenced by radiative cooling. The peak NO level was remarkably elevated by increased oxygen level due to the elevated temperature of the oxygen-enriched flame. In the enhanced oxygen level (30%), the H₂-rich case produced the highest NO level due to a higher temperature and longer residence time within the hot flame zone, while the CO-rich case yielded the lowest NO level due to a lower temperature and shorter residence time. It was also found that, by enhancing the oxygen level, contributions of NNH and N₂O to total NO emission rapidly decreased while the contributions of the thermal NO path were progressively dominant for all cases.     

Analysis of turbulence and surface growth models on the estimation of soot level in ethylene non-premixed flames

Soot prediction in a combustion system has become a subject of attention, as many factors influence its accuracy. An accurate temperature prediction will likely yield better soot predictions, since the inception, growth and destruction of the soot are affected by the temperature. This paper reported the study on the influences of turbulence closure and surface growth models on the prediction of soot levels in turbulent flames. The results demonstrated that a substantial distinction was observed in terms of temperature predictions derived using the k-ε and the Reynolds stress models, for the two ethylene flames studied here amongst the four types of surface growth rate model investigated, the assumption of the soot surface growth rate proportional to the particle number density, but independent on the surface area of soot particles, f(A_s) = ρN_s, yields in closest agreement with the radial data. Without any adjustment to the constants in the surface growth term, other approaches where the surface growth directly proportional to the surface area and square root of surface area, f(A_s) = A_s and f(A_s) = √A_s, result in an under-prediction of soot volume fraction. These results suggest that predictions of soot volume fraction are sensitive to the modelling of surface growth.     

Autoignition of turbulent non-premixed flames investigated using direct numerical simulations

The autoignition of a laminar non-premixed flame placed in a field of homogeneous isotropic turbulence has been studied previously using single-step chemistry and/or simplified models for diffusion processes. The existence of a specific value of the mixture fraction, called “most-reactive,” and the importance of the scalar dissipation rate to predict the ignition location were demonstrated. The effect of the turbulence intensity on the ignition time was found to be non-monotonic. In this work, we wish to assess the influence of more realistic chemistry and transport models on ignition location and time. To do so, direct simulations are carried out using a detailed reaction scheme, multicomponent diffusion velocities and accurate thermodynamic properties. We observe that the turbulent non-premixed flame ignites always faster than the laminar one, even for the highest Reynolds numbers investigated. The scalar dissipation rate can still be used to predict the ignition site, as was observed in simple chemistry simulations. But the most-reactive conditions must of course be determined using the detailed modeling, and cannot any more be analytically predicted. The interest of repeating the direct simulations to get rid of the influence of random initial conditions is also demonstrated.     

On PAH formation in strained counterflow diffusion flames

The structural response of methane/air and methane-nitrogen/air counterflow diffusion flames to strain was investigated by measurements and computations. The numerical predictions were found to be in reasonably good agreement with the experiments. Different reaction pathways leading to PAH formation are examined computationally to obtain a deeper understanding of the process of soot precursor formation in strained diffusion flames. Both experimental and computational results indicate that the concentration of C₂H₂ and C₃H₃, as well as that of the PAH, leading candidates for soot precursor formation, diminish with increasing strain rates. The decrease of the PAH is caused by a depletion of the benzene precursors. In looking to find control parameters for strained reactive flows, it is suggested to image strain rates based on the CH₂O, respectively CHO, to C₂H₂ ratio.     

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.     

The effect of stretch on cellular formation in non-premixed opposed-flow tubular flames

Cellular formation in non-premixed flames is experimentally studied in an opposed-flow tubular burner. This burner allows independent variation of the global stretch rate and overall flame curvature. In opposed-flow flames formed by 21.7% hydrogen diluted in carbon dioxide versus air, cells are formed near extinction with a low fuel Lewis number and a low initial mixture strength. Using an intensified CCD camera, the flame chemiluminescence is imaged to study cellular formation from the onset of cells to near extinction conditions. The experimental onset of cellular instability is found to be at or at a slightly lower Damköhler number than the numerically determined extinction limit based on a two-point boundary value solution of the tubular flame. For fuel Lewis numbers less than unity, concave curvature towards the fuel retards combustion and weakens the flame and convex curvature towards the fuel promotes combustion and strengthens the flame. In the cell formation process, the locally concave flame cell midsection is weakened and the locally convex flame cell ends are strengthened. With increasing stretch rate, the flame breaks into cells and the cell formation process continues until near-circular cells are formed with no concave midsection. Further increase in the stretch rate leads to cell extinction. With increasing stretch rate, the flame thickness at the cell midsection decreases similar to a planar opposed-flow flame while the flame thickness at the cell edges is unchanged and can even increase due to the strengthening effect of convex curvature at the flame edges toward the low Lewis number fuel. The results show the existence of cellular flames well beyond the two-point boundary value extinction limit and the importance of local flame curvature in the formation of flame cells.     

Ignition of turbulent non-premixed flames

The initiation of turbulent non-premixed combustion of gaseous fuels through autoignition and through spark ignition is reviewed, motivated by the increasing relevance of these phenomena for new combustion technologies. The fundamentals of the associated turbulent-chemistry interactions are emphasized. Background information from corresponding laminar flow problems, relevant turbulent combustion modelling approaches, and the ignition of turbulent sprays are included. For both autoignition and spark ignition, examination of the reaction zones in mixture fraction space is revealing. We review experimental and numerical data on the stochastic nature of the emergence of autoignition kernels and of the creation of kernels and subsequent flame establishment following spark ignition, aiming to reveal the particular facet of the turbulence causing the stochasticity. In contrast to fully-fledged turbulent combustion where the effects of turbulence on the reaction are reasonably well-established, at least qualitatively, here the turbulence can cause trends that are not straightforward.     

Detailed chemical effects of ammonia as fuel additive in ethylene counterflow diffusion flames

Ammonia is considered one of the most competitive fuels due to its carbon neutrality. The chemical effects of NH₃ are distinguished by kinetic analysis via adding NH₃ as reactive NH₃ and fictitious inert NH₃. The flame temperature and the mole fraction profiles affected by the chemical effects of NH₃ addition for important species and soot are analyzed, with special emphasis on soot and its important precursor polycyclic aromatic hydrocarbons (PAHs). The results illustrate that NH₃ addition inhibits the production of A1-A4. The chemical effects of ammonia decrease the hydrogen abstraction–C₂H₂–addition (HACA) surface growth rate and PAH condensation rate, which further reduces soot volume fraction and average particle diameter D₆₃. The ammonia decomposition pathways interact with ethylene decomposition pathways via the four reactions: NH₃ + C₂H₅ = C₂H₆ + NH₂, HCN + C₂H₅ = C₂H₆ + CN, NH₂ + C₂H₄ = C₂H₃ + NH₃, and CH₂CH₂NH₂ = C₂H₄ + NH₂. The dilution and thermal effects of NH₃ are dominant effects on soot reduction, while the chemical effects further inhibit soot formation.     

PAH formation in one-dimensional premixed fuel-rich atmospheric pressure ethylbenzene and ethyl alcohol flames

This study addresses health-hazardous emissions from combustion of aromatic and oxygenated components of engine fuel blends. An investigation was conducted on the evolution of polycyclic aromatic hydrocarbons (PAH) and other pollutants (soot, CO, unburned light hydrocarbons) emitted from one-dimensional ethylbenzene and ethyl alcohol flames. The study of ethylbenzene combustion is also pertinent to that of waste polystyrene, as past work has indicated that ethylbenzene may be a surrogate for initial products of polystyrene pyrolysis. Both liquid fuels were prevaporized in nitrogen, mixed with oxygen and additional nitrogen, and upon ignition, premixed flat flames were stabilized over a burner. Temperature measurements and product sampling were conducted at various heights above the burner. Results showed that ethyl alcohol combustion generated small yields of PAH, even under the adverse fuel-rich conditions of this study (?=2.5). PAH mole fractions in the ethyl alcohol flame were 2-5 orders of magnitude lower than those in the ethylbenzene flame at the same location. PAH mole fractions often peaked in the postflame region and remained at relatively high levels thereafter. PAH mole fractions in premixed fuel-rich benzene, ethane, ethylene, and methane flames, published in the literature, were typically in between those found in the two flames of this study. Computations were conducted using a currently developed detailed kinetic model, allowing for the prediction of formation and depletion of major PAH and soot particles of different sizes. The computed chemical flame structures were compared to experimental data obtained in the present work. Predicted mole fractions were often close to the experimentally obtained values or, at worse, within the same order of magnitude for both fuels. Reaction pathways are discussed.     

Measurements of soot, OH, and PAH concentrations in turbulent ethylene/air jet flames

This paper presents results from an investigation of soot formation in turbulent, non-premixed, C₂H₄/air jet flames. Tests were conducted using a H₂-piloted burner with fuel issuing from a 2.18 mm i.d. tube into quiescent ambient air. A range of test conditions was studied using the initial jet velocity (16.2-94.1 m/s) as a parameter. Fuel-jet Reynolds numbers ranged from 4000 to 23,200. Planar laser-induced incandescence (LII) was employed to determine soot volume fractions, and laser-induced fluorescence (LIF) was used to measure relative hydroxyl radical (OH) concentrations and polycyclic aromatic hydrocarbons (PAHs) concentrations. Extensive information on the structure of the soot and OH fields was obtained from two-dimensional imaging experiments. Quantitative measurements were obtained by employing the LII and LIF techniques independently. Imaging results for soot, OH, and PAH show the existence of three soot formation/oxidation regions: a rapid soot growth region, in which OH and soot particles lie in distinctly different radial locations; a mixing-dominated region controlled by large-scale motion; and a soot-oxidation region in which the OH and soot fields overlap spatially, resulting in the rapid oxidation of soot particles. Detailed quantitative analyzes of soot volume fractions and OH and soot zone thicknesses were performed along with the temperature measurement using the N₂-CARS system. Measurements of OH and soot zone thicknesses show that the soot zone thickness increases linearly with axial distance in the soot formation region, whereas the OH zone thickness is nearly constant in this region. The OH zone thickness then rapidly increases with downstream distance and approximately doubles in the soot-oxidation region. Probability density functions also were obtained for soot volume fractions and OH concentrations. These probability density functions clearly define the spatial relationships among the OH, PAH concentrations, the soot formation, and oxidation processes.     

Structural study of non-premixed tubular hydrocarbon flames

Tubular non-premixed flames are formed by a uniquely designed opposed tubular burner. Structural measurements of hydrocarbon flames are conducted using the laser-induced Raman scattering technique. Temperature and major species concentrations are recorded for flames produced by 30% CH₄/N₂ and 15% C₃H₈/N₂ burning against air. Numerical simulations of these flames with detailed chemistry show good agreement between the measured and simulated results. By comparing the numerical results of the tubular curved flames to those of the opposed-jet planar flames, it is shown that flame curvature towards the fuel stream strongly effects the temperature (±80 K) of flames with low fuel Lewis number (15% H₂/N₂, Le_f = 0.41). The effect of curvature on flames with high (15% C₃H₈/N₂, Le_f = 1.51) and near-unity (30% CH₄/N₂, Le_f ≅ 1) fuel Lewis numbers is much less.     

Extinction and near-extinction instability of non-premixed tubular flames

Tubular non-premixed flames are formed by an opposed tubular burner, a new tool to study the effects of curvature on extinction and flame instability of non-premixed flames. Extinction of the opposed tubular flames generated by burning diluted H₂, CH₄ or C₃H₈ with air is investigated for both concave and convex curvature. To examine the effects of curvature on extinction, the critical fuel dilution ratios at extinction are measured at various stretch rates, initial mixture strengths and flame curvature for fuels diluted in N₂, He, Ar or CO₂. In addition, the onset conditions of the cellular instability are mapped as a function of stretch rates, initial mixture strengths, and flame curvature. For fuel mixtures with Lewis numbers much less than unity, such as H₂/N₂, concave flame curvature towards the fuel suppresses cellular instabilities.     

Computational investigation of non-premixed hydrogen-air laminar flames

Laminar diffusion hydrogen/air flames are numerically investigated. Detailed and global mechanisms are compared. NO formation is modelled by full nitrogen chemistry and the extended Zeldovich mechanism. A satisfactory agreement between the present predictions and the experiments of other authors is observed. Significance of different ingredients of mathematical modelling is analyzed. Minor roles of thermal diffusion and radiation, but a significant role of buoyancy is observed. It is observed that the full and quasi multi-component diffusion deliver the same results, whereas assuming Le = 1 to a remarkable difference. NO emissions logarithmically increase with increasing residence time. NO is the dominating nitrogen oxide. Its share increases with residence time, whereby NO₂ and N₂O show a reverse trend. It is observed that the NNH route plays a remarkable role in NO formation, where the share of the Zeldovich mechanism increases with residence time from about 20% to 85%, within the considered range.     

Behavior of hydrogen-enriched non-premixed swirled natural gas flames

The effects of hydrogen addition on a lean non-premixed natural gas swirl-stabilized flame were investigated. Fuel mixtures containing a volumetric fraction of hydrogen ranging from 0% up to 100% were burnt at ambient pressure with swirled air within a quartz chamber in a co-flow configuration. Tests were carried out keeping the sum of the volumetric fuels flow rate constant; thus, the fuel mixture mass flow rate, input thermal power and equivalence ratio decreased as hydrogen fraction was increased.     

A comparison of soot size and charge distributions from ethane, ethylene, acetylene, and benzene/ethylene premixed flames

The size and electrical charge distributions of soot particles generated in rich premixed flames are examined using a nano-differential mobility analyzer (nDMA). The aim is to investigate how these distributions vary with the choice of fuel, diluent, and flame gas velocity. The measured soot size distributions are typically bimodal. The dynamics of the upper size mode is qualitatively independent of the fuel. At increasing heights above the burner this mode increases in diameter and volume fraction, but decreases in number concentration, as expected from surface growth and coagulation. At about 6 mm above the burner a small fraction of the particles, <10%, acquire a bipolar charge. The charge is associated with the upper mode, where the fractions of positive and negative particles evolve to a Boltzmann distribution, whereas the lower mode remains electrically neutral. The existence of the lower mode depends sensitively on the choice of fuel, the flame gas velocity, and the diluent. Comparison to model calculations of flame structure reveals that as each of these are varied, the lower mode exhibits an inverse correlation with temperature, hydrogen atom, and pyrene concentrations.     

Large-scale turbulent structures in non-premixed,oxygen enriched flames

The objectives of the present study is to improve the fundamental knowledge of mixing and combustion at a 25 kW scale in full oxygen diffusion flames. In turbulent diffusion flames, there are strong interactions between the high temperature (high viscosity) layer formed by the burning surface and the turbulent structures generated at the dynamic shear layers. In addition, in confined flows, buoyancy effects and recirculation of large turbulent structures have to be taken into account. The study of the instantaneous phenomena of the reaction zone and of the flame tip provide valuable information about the turbulent structures and hence the mixing mechanisms in the different stages of a coaxial high temperature diffusion flame.