Flame Centerline Research Articles

Effects of H2O and CO2 diluted oxidizer on the structure and shape of laminar coflow syngas diffusion flames

The effects of up to 30% H2O and CO2 replacement of N2 in air on the structure and shape of laminar coflow syngas (COH2) diffusion flames were experimentally and numerically studied. Temperatures along the flame centerline were measured using a type-B thermocouple. The OH*-chemiluminescence and flame luminance were captured respectively by an intensified CCD camera and a CCD camera to determine the flame height and radii. The syngas diffusion flames were numerically modeled using detailed thermal and transport properties and the chemical reaction mechanism of Davis et al. (2005). Four pairs of artificial species were introduced in additional numerical calculations to isolate the chemical, thermal, transport, and radiative effects of H2O and CO2. The experimental and numerical results show that H2O and CO2 replacement of N2 in the oxidizer reduce the peak flame temperature, but they influence the flame centerline temperature distributions differently. The thermal and radiative effects of both H2O and CO2 addition decrease the flame temperatures. The chemical and transport effects of CO2 and H2O affect flame temperatures differently. H2O addition promotes the OH concentration though H+O2=O+OH and O+H2O=2OH, while CO2 addition decreases the OH concentration by suppressing those reactions. The higher concentrations of OH under H2O addition signify higher combustion intensity and hence lead to decreased flame height and radius. In contrast, the addition of CO2 suppresses flame temperature and the overall combustion process, resulting in increased flame height and radius.

Time-averaged probability density functions of soot nanoparticles along the centerline of a piloted turbulent diffusion flame using a scanning mobility particle sizer

In this study, we demonstrate the use of a scanning mobility particle sizer (SMPS) as an effective tool to measure the probability density functions (PDFs) of soot nanoparticles in turbulent flames. Time-averaged soot PDFs necessary for validating existing soot models are reported at intervals of ∆x/D = 5 along the centerline of turbulent, non-premixed, C2H4/N2 flames. The jet exit Reynolds numbers of the flames investigated were 10,000 and 20,000. A simplified burner geometry based on a published design was chosen to aid modelers. Soot was sampled directly from the flame using a sampling probe with a 0.5-mm diameter orifice and diluted with N2 by a two-stage dilution process. The overall dilution ratio was not evaluated. An SMPS system was used to analyze soot particle concentrations in the diluted samples. Sampling conditions were optimized over a wide range of dilution ratios to eliminate the effect of agglomeration in the sampling probe. Two differential mobility analyzers (DMAs) with different size ranges were used separately in the SMPS measurements to characterize the entire size range of particles. In both flames, the PDFs were found to be mono-modal in nature near the jet exit. Further downstream, the profiles were flatter with a fall-off at larger particle diameters. The geometric mean of the soot size distributions was less than 10nm for all cases and increased monotonically with axial distance in both flames.

Structural effects of biodiesel on soot formation in a laminar coflow diffusion flame

Structural effects of biodiesel in terms of unsaturation, i.e. C=C double bonds, its position and the presence of the ester moiety on soot formation are studied in this paper. Laminar coflow diffusion flames of 5-decene, 1-decene, n-decane, and a biodiesel surrogate consisting of a 50%/50% molar blend of n-decane and methyl-octanoate are selected for the investigation. It is observed that the presence of unsaturation promotes soot formation in the flames by increasing soot chemical surface growth species and soot inception species (aromatics) as it is evident from the larger primary particle size and number density of soot particles in the 1-decene, and 5-decene flames compared to the n-decane flame, respectively. More centrally located C=C double bond further speeds up soot inception as it promotes the formation of aromatic species. This is also evident from earlier detection of soot on the centerline and higher soot primary particle number density of the 5-decene flame compared to 1-decene and n-decane flames measured by the laser extinction and thermophoretic sampling methods, respectively. Effects of the ester moiety on soot formation are more on the concentrations of soot chemical surface growth species because the biodiesel surrogate flame has similar number density of primary particles compared to the n-decane flame but soot primary particles have smaller diameters.

Experimental and kinetic modeling study of laminar coflow diffusion methane flames doped with iso-butanol

In this work, laminar coflow diffusion flames fueled with methane/iso-butanol were investigated with synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Mole fractions of C2–C16 flame species, including small hydrocarbons, oxygenates and aromatics were measured along the flame centerline. The enhancement in the formation of aromatics was observed through the increment of the iso-butanol doping ratio (1.95% and 3.90% in two flames) in the inlet fuel gas mixture. A detailed kinetic model was developed from previous models to help understand the experimental findings. According to rate of production analysis in laminar coflow diffusion flames, unimolecular decomposition and H abstraction reactions provide equivalent contributions to the consumption of iso-butanol. Compared to the coflow flames doped with other butanol isomers with the identical condition, the water elimination reaction of iso-butanol has the lowest contribution to fuel consumption. Two reaction pathways provide major contribution to the formation of propargyl radical, i.e. iC4H9OH→iC3H7→C3H6→aC3H5→aC3H4→C3H3 and iC4H9OH→βC4H8OH→iC4H8→iC4H7→aC3H4→C3H3. Propargyl is concluded as the key precursor for aromatics, while phenyl and benzyl radicals are the main PAH precursors. Finally, based on the studies of laminar coflow flames fueled with butanol isomers, the sooting tendency of butanol isomers relates strictly to the efficiency of the propargyl formation pathways in fuel decomposition.

Soot measurements by two angle scattering and extinction in an N2-diluted ethylene/air counterflow diffusion flame from 2 to 5 atm

The soot formed in an N2-diluted ethylene/air counterflow diffusion flame at elevated pressure was investigated using two angle light scattering/extinction technique. To provide a well-controlled pressurized environment for the flame, a novel pressure vessel was built with the required optical access. The soot parameters were measured along the centerline of the counterflow flame. These properties included soot volume fraction (fv), primary particle diameter (dp), population averaged radius of gyration (Rg) and number density of primary particles (np). The Rayleigh–Debye–Gans theory for Fractal Aggregates (RDG-FA) was used to retrieve these properties from scattering and extinction measurements. Soot volume fraction was measured via light extinction from 2 to 5atm while maintaining the same global strain rate at all pressures. Scattered light from soot particles was measured at 45° and 135° and primary particle diameter was calculated using scattering/extinction ratio and the radius of gyration was determined from the dissymmetry ratio. Soot volume fraction, primary particle diameter and radius of gyration all increased with pressure while the number density of primary particles decreased with increasing pressure.

A comparison of the thermal and emission characteristics of co and counter swirl inverse diffusion flames

In this work the influence of co-swirl and counter-swirl burners on the inverse diffusion flame (IDF) characteristics has been studied using liquefied petroleum gas (LPG). The flame shape of the co and counter IDFs were compared. The momentum exchange between the air and fuel jets for swirled IDF burners effects on the produced flame length and stability. The analysis of the results showed that the co-swirling IDF is shorter and more stable than the counter swirling one. The influence of the air jet Reynolds number (Re) consequently the primary equivalence ratio (Φ) on the flame centerline temperature and exhaust gas emissions were examined. The results showed that for the co swirling IDF the centerline temperature profile is higher than the centerline temperature profile of the counter swirl IDF for all values of Φ used in this study. The CO and NOx emissions comparison were made for the two swirled burners. It was found that Re and swirl directions are two key factors for reducing CO emissions. This study indicates that the CO emission is decreased by using co-swirl IDF.

Effects of n-propylbenzene addition to n-dodecane on soot formation and aggregate structure in a laminar coflow diffusion flame

The effects of n-propylbenzene addition to n-dodecane on soot formation and aggregate structure in a laminar coflow diffusion flame is investigated. The goal is to gain insight on the influence of n-propylbenzene addition to n-dodecane on the underlying mechanisms of soot formation processes. The methane laminar coflow diffusion flame doped with pure n-dodecane establishes the base environment in this study. n-Propylbenzene is mixed into n-dodecane at three levels: 15%, 30% and 45% in mole fraction. The inlet total carbon flow rate is held constant for all of the flames. The combined laser extinction and two-angle elastic light scattering method is used to measure soot volume fraction, primary particle diameter and number density. As expected, soot volume fraction is shown to increase at all heights in the flames with increasing mole fraction of n-propylbenzene in the liquid fuel mixture. The differences in profiles of soot volume fraction suggest a non-linear relationship between the mole fraction of n-propylbenzene in the liquid fuel mixture and the increase in soot production. The relative importance of soot inception and surface growth affected by n-propylbenzene addition is shown to be different along the flame wing and centerline. Along the wing, the aromatic fuel molecular structure primarily influences the initial soot inception process at earlier times by providing a larger population of incipient soot particles for the subsequent surface growth. Along the centerline, the aromatic fuel chemistry effect is stronger, as it accelerates both the soot inception and the subsequent surface growth processes. The experimental data presented in this study is also useful for the validation of soot models of the liquid fuel surrogate components.

Modeling of pressure effects on flame structure and soot formation of n-heptane/air co-flow laminar flames by skeletal reaction mechanism

The conversion from chemical energy to thermal energy by the high-pressure combustion of hydrocarbon fuel/air is often accompanied by pollution emissions of PAHs and soot in aircraft gas turbines and diesel engines, and the measurement and analysis of PAHs and soot formations in the practical turbulent flame of engines are difficult. In the study, based on the simulation of the simple laminar co-flow diffusion flame of n-heptane/air by the developed skeletal reaction mechanism, the effects of pressure on flame structure and soot formation are investigated. The results indicate that flame height keeps constant at 0.7~3.0 MPa; the flame radius decreases with pressure as p−1/2; the maximum carbon conversion to soot (ηs,max) is proportional to pressure at 0.1~2.0 MPa; the maximum soot volume concentration (fv,max) increases with pressure as p2; the locations of fv,max and ηs,max along flame centerline are inconsistent, and fv,max and ηs,max occur respectively at the middle and lower parts of flame height; fv,max occurs in the region where the mixture fraction and temperature are respectively 0.08~0.09 and about 1200 K. The diffusion flame consists of three zones: fuel heating zone, fuel-rich reaction zone and oxidizer-rich reaction zone. n-C7H16 is firstly decomposed into small molecule gas (e.g., H2, CH4, C2H2,C2H4, C2H6, C3H4, C3H6, etc.) in the heating zone, and PAHs (C6H6, C8H8 and C10H8) and soot precursors (C2H2, C6H5, C6H6 and C2H4) are formed in fuel-rich reaction zone.

Sampling and dilution of nanoparticles at high temperature

ABSTRACTSampling and dilution of flame-generated, fractal-like ZrO2 aerosols is investigated by aerosol mass/mobility measurements and microscopy. Two broadly used sampler configurations, a straight-tube (ST) and a hole-in-a-tube (HiaT), at three different in-flow orientations and hole diameters are evaluated. The mobility size distributions, mass-mobility exponent, Dfm, prefactor, kfm, and average primary particle diameter are obtained at 10–60 cm height above the burner (HAB) of fuel-rich (hot) and fuel-lean (cold) spray flames by differential mobility analyzer (DMA) and aerosol particle mass (APM) measurements using a recent power law for fractal-like particles. The primary particle diameter, agglomerate size distributions, and corresponding standard deviations from aerosol measurements are compared to those by counting images of particles collected by thermophoretic sampling along the flame centerline. Once new particle formation is completed in the flame, both sampler configurations result in nearly ...

Two-dimensional flow effects on soot formation in laminar premixed flames

Experimental measurements on axisymmetric laminar premixed flames have been used extensively for chemical and soot model validation. Numerical simulations of these flames always rely on the assumption of one-dimensionality. However, the presumed one-dimensionality has not been justified in general, and may not be valid under all circumstances. In the current work, two-dimensional flow effects are investigated in four representative ethylene/air laminar premixed flames, which have been selected as validation targets for the International Sooting Flame workshop. These flames cover all typical experimental arrangements, namely stabilizing plate, steel plate with centered hole, and enclosed chamber. To assess the assumption of one-dimensionality, detailed numerical simulations with finite-rate chemistry are performed with the exact experimental set-ups. It is shown that flow entrainment and acceleration are significant for all four flames. Further, it is found that the flame centerlines cannot be approximated as one-dimensional, since the mass flow rates vary substantially along the centerlines. As a consequence, non-negligible differences are found between the soot profiles predicted in two-dimensional simulations and in simulations where one-dimensionality is assumed. Using data extracted from the two-dimensional simulations, a modified one-dimensional model is derived on the flame centerline to include two-dimensional effects. Results from the modified one-dimensional model are compared against detailed, two-dimensional simulation results and experimental measurements.

Experimental and numerical study of soot formation in laminar coflow diffusion flames of gasoline/ethanol blends

This paper reports the experimental and numerical results of soot formation in laminar coflow diffusion flames of vaporized gasoline/ethanol blends at atmospheric pressure to gain improved understanding of ethanol addition to gasoline on soot formation. Four gasoline/ethanol blends of different ethanol mole fractions in the fuel stream ranging from 0% up to 85%, E0, E20, E50, and E85, were investigated to quantify how soot loading varies with the amount of ethanol blending in the fuel. The laminar coflow diffusion flames were generated using a burner system designed for vaporized liquid fuels. The fuel stream was heavily diluted with nitrogen to lower the boiling points and to prevent the flames from smoking. The soot volume fraction distributions were measured using a 2D line-of-sight attenuation technique. These flames were also modeled numerically using the extensively validated CoFlame code and a recently developed reaction mechanism for gasoline surrogates including PAH formation. The results of experiment and numerical modeling agree quite well in terms of the levels of soot volume fraction and they both show a decrease in the soot loading as more ethanol is added in the fuel stream. The measured peak soot volume fraction occurs in the flame centerline region. The soot model used in this study is unable to capture this experimental feature and predicts the peak soot volume fraction in the co-annular region. Ethanol addition reduces the soot loading primarily by lowering soot inception and PAH condensation rates through decreasing the concentrations of aromatics. Additional measurements in gasoline surrogate/ethanol blend flames indicate that the gasoline surrogate model emulates well the sooting propensity of the real gasoline fuel.

Vibrational CARS thermometry and one-dimensional simulations in laminar H2/air counter-flow diffusion flames

Broadband N2 vibrational coherent anti-Stokes Raman scattering (VCARS) thermometry was performed along the nozzle centerline of counter-flow diffusion flames. Fuel mixtures consisted of varying percentages of H2 in N2 and the oxidizer was air. We investigated the influence of varying the percentage of H2 in the fuel stream on the temperature profiles along the nozzle centerline. Measurements were performed, with varying global strain rates, both near and far from flame extinction. One-dimensional simulations, performed using Combustion Simulation Lab (COSILAB) software, show that the temperature profiles are relatively insensitive to the nozzle exit centerline velocity; implying that the 1-D simulations provide an excellent approximation of the real temperature profiles along the nozzle centerline. Various chemical mechanisms were employed within the COSILAB framework for comparison against VCARS temperature measurements. Simulations were also performed to evaluate the influence of thermal diffusion and nitrogen chemistry on nozzle centerline temperature profiles.

Influence of fuel injection frequency on droplet dispersion in ethanol pulsed spray flames

The optimized use of ethanol in current and new vehicles requires detailed information on atomization, evaporation, mixing and combustion processes of this fuel. In this work, laser diagnostic tools are applied to analyze ethanol combustion in pulsed spray flames. Fuel droplet diameter and velocity distributions are measured by Phase Doppler Interferometry (PDI) across the flame. The evolution of droplets dispersion at early stages of the injection event is visualized in a diametral plane at exit of the injector by high repetition rate Mie scattering. The burner is formed by a swirler mixer and an automotive port fuel injector both positioned at the top exit of a vertical cylinder. Three injection frequencies (100 Hz, 250 Hz, and 400 Hz) are analyzed in depth. The increase in the injection frequency induced a transition from higher to lower density spray patterns impacting the combustion process of the droplets. The higher density spray presented droplet dispersion similar to a cloud in group combustion mode, promoting a mono-modal droplet diameter distribution across the flame centerline and produced an elongated flame with a visible reaction zone down to the injection plane of the burner. The lower density spray suggests droplets in the internal group combustion mode, being highly susceptible to swirling flow structures and promoting a bi-modal diameter distribution at the centerline, mainly produced by residual droplets with high axial velocity and preferential evaporation of small droplets. Furthermore, the lowest density spray led to a shorter flame presenting similarities to a lifted flame.

A multi-scale asymptotic scaling and regime analysis of flamelet equations including tangential diffusion effects for laminar and turbulent flames

Recently, the relevance of tangential diffusion effects has been identified for laminar flames. These effects are not considered in the classical flamelet equations. In the present work, flamelet equations including these effects are derived, and their relevance is investigated by a multi-scale asymptotic scaling analysis. The analysis yields characteristic ratios dependent on the local curvature of the mixture fraction field, the scalar dissipation rate, and the flame thickness, which indicate whether tangential diffusion effects become important. By comparing relevant scales, a regime diagram is developed and three different flamelet regimes are identified. In regime I, the classical flamelet equations are valid. In regime II, differential diffusion of species and temperature in flame-tangential direction becomes relevant. In regime III, additional transport along mixture fraction isosurfaces exhibits an influence on the flame structure. In the latter case it is not sufficient to condition species mass fractions and temperature on the mixture fraction alone since these quantities are not represented by a one-dimensional structure.The asymptotic scaling is verified against fully resolved numerical data of a laminar non-premixed methane–air flame and a turbulent lifted hydrogen jet flame. Budgets of different flamelet solutions of the laminar flame reveal that tangential diffusion effects are dominant over the classical flamelet terms near the flame centerline, while in regions away from it, standard flamelet terms are prevailing. For the turbulent flame, the results show that tangential diffusion effects are more localized and can exceed flame-normal transport, although, unsteadiness is more likely the key factor of the configuration.While the classical flamelet model is applicable for certain flames, this study shows that tangential diffusion effects may require consideration for general flame configurations.

Flame brush characteristics and burning velocities of premixed turbulent methane/air Bunsen flames

The flame brush characteristics and turbulent burning velocities of premixed turbulent methane/air flames stabilized on a Bunsen-type burner were studied. Particle image velocimetry and Rayleigh scattering techniques were used to measure the instantaneous velocity and temperature fields, respectively. Experiments were performed at various equivalence ratios and bulk flow velocities from 0.7 to 1.0, and 7.7 to 17.0m/s, respectively. The total turbulence intensity and turbulent integral length scale were controlled by the perforated plate mounted at different positions upstream of the burner exit. The normalized characteristic flame height and centerline flame brush thickness decreased with increasing equivalence ratio, total turbulence intensity, and longitudinal integral length scale, whereas they increased with increasing bulk flow velocity. The normalized horizontal flame brush thickness increased with increasing axial distance from the burner exit and increasing equivalence ratio. The non-dimensional leading edge and half-burning surface turbulent burning velocities increased with increasing non-dimensional turbulence intensity, and they decreased with increasing non-dimensional bulk flow velocity when other turbulence statistics were kept constant. Results show that the non-dimensional leading edge and half-burning surface turbulent burning velocities increased with increasing non-dimensional longitudinal integral length scale. Two correlations to represent the leading edge and half-burning surface turbulent burning velocities were presented as a function of the equivalence ratio, non-dimensional turbulence intensity, non-dimensional bulk flow velocity, and non-dimensional longitudinal integral length scale. Results show that the half-burning surface turbulent burning velocity normalized by the bulk flow velocity decreased as the normalized characteristic flame height increased.

Numerical study of the effects of pressure on soot formation in laminar coflow n-heptane/air diffusion flames between 1 and 10 atm

Laminar nitrogen-diluted n-heptane diffusion flames burning in coflow air were numerically simulated at pressures from 1 to 10atm. Numerical simulations were performed by using a detailed reaction mechanism containing 175 species and 1086 reactions with PAH formation up to pyrene and a sectional soot model. The soot model consists of inception as a result of the collision of two pyrene molecules, heterogeneous surface growth and oxidation following the hydrogen abstraction acetylene addition (HACA) mechanism, and soot particle coagulation and PAH surface condensation. Comparisons with available experimental data indicate that the numerical model reproduces successfully the influence of pressure on the flame structure and soot production. All the soot formation processes are enhanced with increasing pressure and HACA dominates the soot mass growth over the pressure range considered. PAH condensation is the most sensitive process to pressure whereas the inception and HACA processes exhibit similar pressure dependency. These trends are directly related to the pressure dependence of the molar concentrations of the species involved in soot formation. The increase in the molar concentrations of benzene and pyrene with pressure results from both physical and chemical effects. Propargyl recombination and interconversion of phenyl to benzene dominate the formation of benzene low in the flame over the pressure range considered, though the contributions of the addition of acetylene to n-butadienyl radical increases significantly with pressure in the flame centerline region.

Effect of the initial pressure on the characteristics of the flame propagation in hydrogen-propane-air mixtures

This paper is aimed at an experimental investigation on effects of initial pressure on flame propagation characteristics of binary fuels hydrogen-propane-air mixtures at room temperature. The experiments are performed in a square channel equipped with perforated orifice obstacles. Four initial pressures are examined. Based on pressure transducers along the channel, the flame velocity, maximum pressure of the front peak and characteristic distances are measured. Successive stages are observed as flame propagates: (i) a velocity increase at the beginning, (ii) a velocity equal to the sound speed of combustion products and (iii) a decrease of the velocity. When the initial pressure is more important, the flame velocity and the maximal pressure of the front peak are higher, which yields a shorter characteristic distance of flame propagation. By means of a Schlieren photography technique, the physical mechanisms of flame propagation are identified in its initial stage. The physical mechanisms such as flame surface area increase and combustion product expansion as well as delayed combustion between two adjacent plates are responsible for flame acceleration upon its initial stage. The oscillations of the centerline flame velocity are due to the constrained-expanded structure of flow in reactants ahead of flame when it crosses the plates.

A study of the effects of the ester moiety on soot formation and species concentrations in a laminar coflow diffusion flame of a surrogate for B100 biodiesel

The effects of the ester moiety on soot formation and species concentrations in a laminar coflow diffusion flame of a surrogate for a B100 biodiesel are investigated. The surrogate is a mixture of 50% n-decane/50% methyl-octanoate (molar) to represent methyl-oleate. The combustion chemistry and soot formation are solved using a mechanism with 288 species and 2073 reactions coupled with a sectional soot model, respectively. Soot volume fraction (fv) and temperature profiles are compared to the experimentally measured values for this biodiesel surrogate. In addition, the effects of the ester moiety on soot formation and flame chemistry are studied by numerically comparing the biodiesel surrogate flame with a pure n-decane flame. The model predicts both temperature and fv profiles with a good accuracy. Some discrepancies for fv on the flame centerline are observed between the model and the experiments; it is suggested that these discrepancies are because the model and the experiment cannot distinguish nascent transparent soot from mature soot and because the mechanism under-predicts PAH formation rates. Both n-decane and B100 surrogate flames have similar fv and temperature profiles when both flames have the same energy input. This suggests that the ester moiety does not have a major impact on soot formation. In addition, early production of CO and higher concentrations of some oxygenated species such as formaldehyde are observed in the predicted concentration contour plots of the B100 surrogate flame when they are compared to the n-decane flame. Reaction pathway analysis reveals that the higher peak concentrations of formaldehyde and the early production of CO from CH2CO and CH3CO2 that come directly from the ester moiety in the B100 surrogate are much more pronounced than other species in the B100 surrogate flame and are recognized as the main differentiating characteristics of the B100 surrogate flame from the n-decane flame.

Experimental Characterization of Methane Inverse Diffusion Flame

This article presents 10-kHz images of OH-PLIF simultaneously with 2-D PIV measurements in an inverse methane diffusion flame. Under a constant fuel flow rate, the central air jet Re was varied, leading to air to fuel velocity ratio, Vr, to vary from 8.3 to 66.5. Starting from Vr = 20.7, the flame is commonly characterized by three distinct zones. The length of the lower fuel entrainment region is inversely proportional to Vr. The flames investigated resemble a string shear layer confining this zone, and converging into the second distinct region, the flame neck zone. The third region is the rest of the flame, which spreads in a jet-like manner. The inverse diffusion flames exhibit varying degrees of partial premixing, depending upon on the velocity ratio Vr, and this region of partial premixing evolves into a well-mixed reaction zone along the flame centerline. The OH distribution correlated with the changes in the mean characteristics of the flow through reduction in the local Reynolds number due to heat release. The existence of a flame suppresses or laminarizes the turbulence at early axial locations and promotes fluctuations at the flame tip for flames with Vr < 49.8. In addition, the flame jet width can be correlated to the OH distribution. In upstream regions of the flames, the breaks in OH are counterbalanced by flame closures and are governed by edge flame propagation. These local extinctions were found to occur at locations where large flow structures were impinging on the flame and are associated with a locally higher strain rate or correlated to the local high strain rates at the flame hole edges without this flow impinging. Another contributor to re-ignition was found to be growing flame kernels. As the flames approach global blow-off, these kernels become the main mechanism for re-ignition further downstream of the flames. At low Vr, laminarization within the early regions of the flame provides an effective shield, preventing the jet flow from penetrating through the flame.

Experimental and kinetic modeling study of laminar coflow diffusion methane flames doped with 2-butanol

In order to understand the interactions between 2-butanol and hydrocarbon fuels in combustion chemistry, experimental and kinetic modeling investigations were performed to study laminar coflow diffusion methane flames doped with two inlet mole fractions of 2-butanol (1.95% and 3.90%) in this work. Mole fractions of flame species along the flame centerline, particularly unsaturated C2–C5 hydrocarbons, C6–C16 aromatics and some free radicals, were measured using synchrotron vacuum ultraviolet photoionization mass spectrometry. A detailed kinetic model was developed to simulate the fuel decomposition and the formation of benzene and PAHs in the investigated flames. The simulated results can reproduce the observed effects of 2-butanol addition. The reaction pathway analysis reveals that resonantly stabilized radicals, such as propargyl, cyclopentadienyl, phenyl, and benzyl radicals, are major precursors of indene and naphthalene. With the increasing inlet mole fraction of 2-butanol, the formation of these resonantly stabilized radicals increases significantly. PAHs with more carbon atoms, including acenaphthylene, phenanthrene, pyrene, and fluoranthene, are dominantly derived from indenyl and naphthyl radicals.