Premixed Methane Flames Research Articles

LOCAL KARLOVITZ NUMBERS AT EXTINCTION FOR VARIOUS FUELS IN COUNTERFLOW PREMIXED FLAMES

Extinction characteristics of stretched premixed flames have been numerically investigated by adopting a detailed chemistry in an axisymmetric counterflow for various fuels. The Karlovitz number, defined as the ratio of the characteristic one-dimensional reaction time for an unstretched premixed flame to the characteristic flow time of the inverse of flame stretch rate, exhibited large spread at extinction conditions. To test a universal extinction criterion that can be applied irrespective of fuel type, equivalence ratio, pressure, and initial temperature, a local Karlovitz number was defined as the ratio of the local reaction time to the characteristic flow time of stretched flames. The result showed that the local Karlovitz number based on the flame thickness at extinction can be reasonably approximated as 1.1, irrespective of equivalence ratio for the fuels tested including alkane (methane and propane), alkene (ethylene), alkyne (acetylene), alcohol (methanol), and hydrogen. By analyzing existing experimental data for premixed flames of methane and propane, the measured local Karlovitz numbers at extinction have also been found to have a near constant value. The local Karlovitz number was relatively insensitive to pressure variation up to 10 atm and initial temperature up to 1000 K. The present results implied that extinction occurs when the ratio of the characteristic flow time to the local characteristic reaction time exceeds a near constant value for the fuels tested irrespective of equivalence ratio, pressure, and initial temperature.

Investigation of NO production and flame structure in plasma enhanced premixed combustion

A pulsed plasma source is used to stabilize lean premixed methane flames. Nitric oxide (NO) production is measured using probe sampling and chemiluminescence analysis. While the discharge is a potential source of NO, it is observed that the flame also consumes some NO. The NO production in the flame is modeled by use of a modified opposed diffusion flame simulation (OPPDIF), augmented by the initial radical pool created by the discharge. The simulations are in qualitative and quantitative agreement with the measurements, despite the simplified modeled geometry. The structure of this plasma assisted premixed flame is discussed within the framework of two interacting premixed streams, as simulated by the OPPDIF configuration. Under certain conditions, we observe a cold pre-flame associated with the plasma-excited premixed stream, the products of which (OH radicals) have an unusually high vibrational temperature and low rotational temperature in electronically excited states, when compared to the OH characteristics found in a conventional lean premixed flames. While the role of the OH radicals in this cold pre-flame stream is not fully understood, we believe it plays an important role in igniting the surrounding combustible mixture, as confirmed by the simulations.

White in Time Scalar Advection Model as a Tool for Solving Joint Composition PDF Equations

A rapidly decorrelating velocity field model is used to derive stochastic partial differential equations (SPDE) allowing one to compute the modeled one-point joint probability density function of turbulent reactive scalars. Those SPDEs are shown to be hyperbolic advection/reaction equations. They are dealt with in a generalized sense, so that discontinuities in the scalar fields can be treated. The Eulerian Monte Carlo (EMC) method thus defined is coupled with a RANS solver and applied to the computation of a turbulent premixed methane flame over a backward facing step.

Transient response of premixed methane flames

The response of premixed methane–air flames to transient strain and local variations in equivalence ratio is studied during isolated interactions between a line–vortex pair and a V-flame. The temporal evolution of OH and CH is measured with planar laser-induced fluorescence for N 2-diluted flames with equivalence ratios ranging from 0.8 to 1.2. One-dimensional laminar flame calculations are used to simulate the flame response to unsteady strain and variations in reactant composition. When the reactant composition of the vortex pair and the V-flame are identical, the measurements and predictions show that the peak mole fractions of OH and CH decay monotonically in lean, stoichiometric, and rich flames. We also investigate the effects of a vortex pair with a leaner composition than the V-flame. In a stoichiometric flame, the leaner vortex enhances the decay of both OH and CH. In a rich flame, we observe an abrupt increase in OH-LIF signal and a disappearance of CH-LIF signal that are consistent with a previous experimental investigation. Our results indicate that the previously observed OH burst and CH breakage were caused by a difference in the equivalence ratios of the vortex pair and the main reactant flow. A numerical study shows that N 2 dilution enhances the response of premixed flames to unsteady strain and variations in stoichiometry. Reaction-path and sensitivity analyses indicate that the peak OH and CH mole fractions exhibit significant sensitivity to the main branching reaction, H + O 2 ↔ OH + O. The sensitivity of OH and CH to this and other reactions is enhanced by N 2 dilution. As a result, N 2-diluted flames provide a good test case for studying the reliability of chemical kinetic and transport models.

An experimental and numerical study on the adequacy of CH as a flame marker in premixed methane flames

The CH radical is frequently used as a flame marker because it is relatively short-lived and is present over a narrow region in flames. Discontinuities in the CH field are thus often interpreted as localized extinction of the flame. Recently, however, the adequacy of CH laser-induced fluorescence (LIF) as a flame marker was questioned by an experimental study of flame–vortex interactions in highly N 2-diluted premixed methane flames. We demonstrate both experimentally and numerically that anomalies in the transient response of CH in this earlier study were due to reactant composition variations in the vortex. In addition, we evaluate the adequacy of CH LIF as a flame marker over a much broader range of conditions. Previous numerical studies showed that heat release rate correlates reasonably well with peak [HCO] and the concentration product [OH][CH 2O], but poorly with [CH], in highly N 2-diluted premixed methane flames. Here, the correlation between heat release rate and CH is investigated both experimentally, by performing simultaneous measurements of CH, OH, and CH 2O LIF, and numerically. We consider undiluted and N 2-diluted premixed methane flames over a range of strain rates and stoichiometries. Results are reported for flames subjected to unsteady stretch and reactant composition variations. For all N 2-dilution levels considered, the peak CH LIF signal correlates poorly with heat release rate when the stoichiometry of the reactant mixture changes from rich to lean. However, when flames are subjected to stretch, the correlation between CH and heat release rate improves as the N 2-dilution level decreases. The correlation is reasonably good for undiluted flames with equivalence ratios of 0.8 < Φ < 1.2. This result is particularly encouraging, given the relevance of undiluted flames to practical applications, and it motivates further investigation of the parameter space for which difficulties may exist in using CH as a flame marker.

Measurements of flame orientation and scalar dissipation in turbulent partially premixed methane flames

Three turbulent flames were studied using a new experimental facility developed at Sandia National Laboratories. Line imaging of Raman and Rayleigh scattering and CO laser-induced fluorescence (LIF) yielded information on all major species, temperature, mixture fraction, and a 1D surrogate measure of scalar dissipation. Simultaneously, crossed planar OH LIF imaging provided information on the instantaneous flame orientation, allowing estimation of the full 3D (flame-normal) scalar dissipation rate. The three flames studied were methane–air piloted jet flames (Sandia flames C, D, and E), which cover a range in Reynolds number from 13,400 to 33,600. The statistics of the instantaneous flame orientation are examined in the different flames, with the purpose of studying the prevailing kinematics of isoscalar contours. The 1D and 3D results for scalar dissipation rate are examined in detail, both in the form of conditional averages and in the form of probability density functions. The effect of overall strain and Reynolds number on flame suppression and eventual extinction is also investigated, by examining the doubly conditional statistics of temperature in the form of S-shaped curves. This latter analysis reveals that double conditioning of temperature on both mixture fraction and scalar dissipation does not collapse the data from these flames onto the same curve at low scalar dissipation rates, as might be expected from simple flamelet concepts.

Two-photon LIF imaging of atomic oxygen in flames with picosecond excitation

Interference-free planar laser-induced fluorescence measurements of atomic oxygen are demonstrated in steady and unsteady laminar premixed methane flames. Two-dimensional LIF measurements of atomic oxygen present a challenge because of the relatively weak two-photon absorption cross-sections and the nonlinear dependence of the LIF signal on laser intensity. Previous two-photon LIF measurements of atomic oxygen in hydrocarbon flames using nanosecond laser excitation were complicated by significant interference from photolytically produced oxygen atoms. Recent results from line-imaging studies in our laboratory indicate that CO 2 is the dominant photolytic precursor. Here, we use picosecond excitation to image atomic oxygen with negligible photolytic interference. We demonstrate this capability in premixed CH 4/O 2/N 2 ( ϕ = 1.08 , X N 2 = 0.63 ) flames with an adiabatic flame temperature of 2480 K and an estimated peak O-atom mole fraction of 0.5%. Peak single-shot signal-to-noise ratios of 9 are achieved, and the estimated detection limit is approximately 80 ppm. Quantitative O-atom LIF measurements also require knowledge of the temperature-dependent quenching cross-sections, which are not currently available. The effects of collisional quenching on the O-atom LIF are predicted using two models for the temperature dependence of quenching cross-sections, and the results indicate that O-atom LIF signals can provide accurate measurements of O-atom profiles in flames. Measurements in an acoustically forced Bunsen flame illustrate the feasibility of using O-atom LIF to investigate flow–flame interactions.

Three-dimensional direct simulations and structure of expanding turbulent methane flames

Direct numerical simulations (DNS) are ideally suited to investigate in detail turbulent reacting flows in simple geometries. For an increasing number of applications, detailed models must be employed to describe the chemical processes with sufficient accuracy. Despite the huge cost of such simulations, recent progress has allowed the direct numerical simulation of turbulent premixed flames while employing complete reaction schemes. We briefly describe our own developments in this field and use the resulting DNS code to investigate more extensively the structure of premixed methane flames expanding in a three-dimensional turbulent velocity field, initially homogeneous and isotropic. This situation typifies, for example, the initial flame development after spark ignition in a gas turbine or an internal combustion engine. First investigation steps have been carried out at low turbulence levels on this same configuration in the past Symposium, and we build on top of these former results. Here, a considerably higher Reynolds number is considered, the simulation has been repeated twice in to limit the possibility of spurious, very specific results, and several complementary post-processing steps are carried out. Characteristic features concerning the observed combustion regime are presented. We then investigate in a quantitative manner the evolution of flame surface area, global stretch-rate, flame front curvature, flame thickness, and correlation between thickness and curvature. The possibility of obtaining reliable information on flame front curvature from two-dimensional slices is checked by comparison with the exact procedure.

PAH and Fullerene Formation from Low Pressure Combustion of Aromatic Hydrocarbon Fuel

A sooting flame was established in a reduced-pressure combustion chamber using the vitiated coflow burner with a premixed methane flame for the surrounding flame and a diffusion or premixed toluene flame for the center flame. Collected soot samples were analyzed using high performance liquid chromatography (HPLC) and gas chromatography to determine the fullerene content of soot and the species of PAHs. The results showed that the fullerene content of soot changed depending on the combustion conditions such as the chamber pressure, the center nozzle length and the equivalence ratio of the surrounding flame and the center flame. The species of PAHs also changed depending on the combustion condition. When the fullerene content of soot was relatively high, the fractions of Acenaphthylene, Fluorene, Phenanthrene, Fluoranthene and Pyrene were relatively high. The fact suggests that these PAH species have some relation to fullerene formation in flames.

Experimental study of a low pressure stoichiometric premixed methane, methane/ethane, methane/ethane/propane and synthetic natural gas flames

This study reports the structure analysis of four laminar stoichiometric premixed CH 4/O 2/Ar, CH 4/C 2H 6/O 2/Ar, CH 4/C 2H 6/C 3H 8/O 2/Ar and CH 4/C 2H 6/C 3H 8/ n-C 4H 10/ i-C 4H 10/ n-C 5H 12/ i-C 5H 12/ n-C 6H 14/O 2/Ar flames. The flames have been stabilized on a flat flame burner at low pressure (40 Torr). Experimental mole fraction profiles of stable and reactive species have been obtained by coupling molecular-beam/mass spectrometry (MB/MS) with gas chromatography/mass spectrometry (GC/MS) analyses. Temperature profiles were measured with a coated thermocouple in the sampling conditions. This work provides a detailed experimental data on the nature and concentrations of the stable and reactive species produced by oxidation of various representative natural gas mixtures. In addition to reactants and major products, the mole fractions of stable intermediate and reactive species have been measured. The experimental results show that ethane and propane, increase significantly intermediate species mole fractions especially olefins. On the other hand, the higher alkanes play a minor role in the oxidation of the natural gas. It appears therefore, that the combustion of methane/ethane/propane mixture represents appropriately natural gas combustion. A qualitative explanation of the experimental observations, based on the current literature mechanisms of alkanes combustion, is reported.

Source apportionment of polycyclic aromatic hydrocarbons in a coastal lagoon by molecular and isotopic characterisation

The origin of polycyclic aromatic hydrocarbon (PAH) contamination in sediments of a coastal lagoon of the Adriatic Sea (Pialassa, Baiona, Ravenna, Italy) has been investigated. Concentrations, molecular distributions and stable carbon isotopic ratios ( δ 13C) of PAHs extracted from core sediments and soils have been determined by high pressure liquid chromatography (HPLC) with fluorescence and diode array detection (DAD), by gas chromatography–mass spectrometry (GC–MS) and isotopic ratio mass spectrometry (GC–IRMS). The total concentrations of PAHs in core sediments are highly variable, ranging from 30 to 112,000 ng/g and exhibit depth profiles similar to those registered for mercury and polyvinyl chloride (PVC). PVC was manufactured in the nearby industry from acetylene, in turn produced by partial combustion/pyrolysis of methane, the process considered responsible for PAH emission into the lagoon. In accordance, the molecular pattern of PAHs found in sediments is similar to that reported for the interior of fuel rich premixed flames of methane, being characterised by the dominance of pyrene, cyclopenta[ cd]pyrene, benzo[ ghi]fluoranthene, fluoranthene and by the presence of several five-membered ring PAHs, including high levels of 4 H-cyclopenta[ def]phenanthrene and minor amounts of dicyclopentapyrenes. The extremely low δ 13C values of sedimentary PAHs (from −62‰ to −36‰) confirm that local biogenic methane is their major source. The distribution of the isotopic signatures suggests PAHs were introduced into the lagoon by direct discharge into the water body rather than by atmospheric fallout. The uncommon molecular and isotopic distribution of these PAHs is proposed as fingerprinting specific for pyrolysis of biogenic methane useful to source apportionment.

Experimental and artificial neural network modeling study on soot formation in premixed hydrocarbon flames ☆

The formation of soot in premixed flames of methane, ethane, propane, and butane was studied at three different equivalence ratios. Soot particle sizes, number densities, and volume fractions were determined using classical light scattering measurement techniques. The experimental data revealed that the soot properties were sensitive to the fuel type and combustion parameter equivalence ratio. Increase in equivalence ratio increased the amount of soot formed for each fuel. In addition, methane flames showed larger particle diameters at higher distances above the burner surface and propane, ethane, and butane flames came after the methane flames, respectively. Three-layer, feed-forward type artificial neural networks having seven input neurons, one output neuron, and five hidden neurons for soot particle diameter predictions and seven hidden neurons for volume fraction predictions were used to model the soot properties. The network could not be trained and tested with sufficient accuracy to predict the number density due to a large data range and greater uncertainty in determination of this parameter. The number of complete data set used in the model was 156. There was a good agreement between the experimental and predicted values, and neural networks performed better when predicting output parameters (i.e. soot particle diameters and volume fractions) within the limits of the training data.

Effects of dimethyl methylphosphonate on premixed methane flames

The impact of dimethyl methylphosphonate (DMMP) was studied in a premixed methane/oxygen/N 2-Ar flame in a flat flame burner slightly under atmospheric pressure at two different equivalence ratios: rich and slightly lean. CH 4, CO, CO 2, CH 2O, CH 3OH, C 2H 6, C 2H 4, and C 2H 2 profiles were obtained with a Fourier Transform Infrared (FTIR) spectrometer. Gas samples, analyzed in the FTIR, were extracted from the reaction zone using a quartz microprobe with choked flow at its orifice. Temperature profiles were obtained by measuring the probe flow rate through the choked orifice. Flame calculations were performed with two existing detailed chemical kinetic mechanisms for organophosphorus combustion. DMMP addition caused all profiles except that of CH 3OH to move further away from the burner surface, which can be interpreted as a consequence of a reduction in the adiabatic flame speed. Experimentally, the magnitude of the shift was 50% greater for the near-stoichiometric flame than for the rich flame. Experimental CH 3OH profiles were four to seven times higher in the doped flames than in the undoped ones. The magnitude of this effect is not predicted in the calculations, suggesting a need for further mechanism development. Otherwise, the two mechanisms are reasonably successful in predicting the effects of DMMP on the flame.

Laser-induced fluorescence detection of acetylene in low-pressure propane and methane flames

Laser-induced fluorescence is used to detect and record profiles of acetylene formed as an intermediate species in 10-Torr premixed propane and methane flames. In low-temperature regions of the flames, excitation spectra confirm acetylene as the spectral carrier. The spectra of acetylene overlap those of O2 and NO in terms of both excitation and detection wavelengths, however, acetylene can be detected with relatively little interference in the vicinity of 228 nm, using a detection wavelength of 260 nm. The fluorescence lifetime of acetylene in the flame conditions studied is approximately 20 ns, much shorter than the radiative lifetime, due to a high quenching rate for all the colliders investigated. This can be exploited in low-pressure flames to avoid interference from acetylene in monitoring nitric oxide. The acetylene mole fraction in propane flames reaches its peak value at nearly the same location as that of HCO, slightly closer to the burner than the peak CH mole fraction. The acetylene fluorescence signal is easily detected in propane flames over equivalence ratios from 0.6 to 1.2, although it increases under fuel-rich conditions. In methane flames, the acetylene signal is much weaker and is undetectable for fuel-lean conditions.

Simulations of premixed combustion in porous media

A numerical model for planar premixed flames of methane in ceramic porous media has been developed to improve the understanding of the structure of such flames. The model successfully reproduces experimental data for both single- and two-layer surface flames. The success is attributed to the detail given to the boundary conditions and the radiation modelling, which was done by solving the radiation transfer equation inside the porous medium without any simplifying models. Surface-stabilized flames yielded S L/S L0<1 (S L0 being the free-flame laminar burning velocity) and had energy balances where convection in the gas phase was balanced by heat transfer from the solid for most of the length of the burner before the reaction zone, while the heat release was mostly balanced by heat exchange with the solid. In contrast, submerged flames in foams with large pores yielded S L/S L0>1 and their energy balance was similar to that of a free flame, which implies that the burning velocity acceleration is due to the reactant preheat. The flame solutions were further analysed with concepts from the computational singular perturbation method to construct reduced mechanisms. For all types of combustion (surface or submerged), an almost identical ordering of chemistry timescales to free flames was found and previously developed reduced mechanisms for free flames were accurate also for the flames inside the porous medium. The results suggest that the thermal exchange between the two phases that is responsible for the flame behaviour remains decoupled from the fast part of the chemistry.

The effects of slight premixing on fuel decomposition and hydrocarbon growth in benzene-doped methane nonpremixed flames

Experimental measurements have been made in a series of non-premixed and slightly premixed benzene-doped methane flames to investigate the effect of fuel-side air on soot concentrations and to determine whether the self-reaction of cyclopentadienyl radical is a significant source of naphthalene under these conditions. The flames were generated by adding increasing amounts of primary air to the benzene-doped methane fuel mixture in a co-flowing axisymmetric non-premixed flame. The molar benzene/methane ratio was 2%, which was high enough for the benzene to dominate the production of aromatic hydrocarbons and soot. Measurements were made on the centerline of gas temperature with thermocouples, of soot volume fraction with laser-induced incandescence, and of major species and C1 to C13 hydrocarbon concentrations with online mass spectrometry. The maximum centerline soot volume fraction in the flames decreased strongly and monotonically as air was added to the fuel, with a reduction of one half from the non-premixed flame to a flame with a primary equivalence ratio of 15. This observation shows that even the small amounts of fuel-side air produced by ‘accidental’ mechanisms such as entrainment at the flame base can significantly impact soot concentrations. The hydrocarbon measurements showed that premixing caused a greater amount of benzene in the pyrolysis zone to be consumed through phenoxy formation, such that the concentrations of the phenoxy decomposition product cyclopentadienyl increased, whereas the concentrations of substituted benzenes such as phenylacetylene and toluene decreased. The naphthalene concentrations also decreased monotonically, which indicates that most of the naphthalene was formed from phenylacetylene by the HACA mechanism (or from benzyl by propargyl addition), and that negligible amounts of naphthalene were formed from cyclopentadienyl.

Inhibition of premixed methane flames by manganese and tin compounds

The first experimental measurements of the influence of manganese- and tin-containing compounds (MMT, TMT) on the burning velocity of methane/air flames are presented. Comparisons with Fe(CO) 5 and CF 3Br demonstrate that manganese and tin-containing compounds are effective inhibitors. The inhibition efficiency of MMT is about a factor of two less than that of iron pentacarbonyl, and that of TMT is about 26 times less effective, although TMT is still about twice as effective as CF 3Br. There exist conditions for which both MMT and TMT show a loss of effectiveness beyond that expected because of radical depletion, and the cause is believed to be particle formation. Kinetic models describing the inhibition mechanisms of manganese- and tin-containing compounds are suggested. Simulations of MMT- and TMT-inhibited flames show reasonable agreement with experimental data. The decomposition of the parent molecule for the tin and manganese species is found to have a small effect on the inhibition properties for the concentrations in this work. The inhibition effect of TMT is determined mostly by the rate of the association reaction H + SnO + M ↔ SnOH + M, and the catalytic recombination cycle is completed by the reactions SnOH + H ↔ SnO + H 2 and SnOH + OH ↔ SnO + H 2O. The inhibition mechanism by manganese-containing compounds includes the reactions: MnO + H 2O ↔ Mn(OH) 2; Mn(OH) 2 + H ↔ MnOH + H 2O, and MnOH + OH (or H) ↔ MnO + H 2O (or H 2), and the burning velocity is most sensitive to the rate of the reaction Mn(OH) 2 + H ↔ MnOH + H 2O.

A study of turbulent partially premixed flames based on simultaneous imaging of velocity field and oh radical

Turbulent partially premixed methane flames stabilized in a concentric flow conical nozzle burner have been investigated and presented in this work. The flames are classified in the thin reaction zones regime. The flames were investigated experimentally based on simultaneous measurements of the velocity field and OH-radical. A combined planar imaging velocimetry (PlV) and laser-induced predissociation fluorescence (LIPF) technique was applied. The mean, rms, and instantaneous structure of four flames are presented and discussed in this article. High stability of the investigated flames is partially due to the partial premixing of the jet and mainly to the flow divergence created by the conical nozzle. This creates a stabilization core with lower mean velocity at the center and lower rms of the velocity fluctuations at the flame brush. No significant recirculation could be observed within the stabilization core above the conical nozzle tip. The reaction zone structure in the present partially premixed flames is likely to show double reaction zones in all flames that support earlier theory and experiments.

Numerical simulation of premixed turbulent methane combustion

With adaptive-grid computational methodologies and judicious use of compressible and low Mach number combustion models, we are carrying out three-dimensional, time-dependent direct numerical simulations of a laboratory-scale turbulent premixed methane burner. In the laboratory experiment, turbulence is generated by a grid located in the throat of a 50mm diameter circular nozzle; swirl is be introduced by four tangential air jets spaced uniformly around the circumference of the nozzle just above the turbulence grid. A premixed methane flame is stabilized above the nozzle in the central core region where a velocity deficit is induced7the swirling flow. The time-dependent flow field inside the nozzle, from the turbulence grid and the high-speed jets, to the nozzle exit plane is simulated using an adaptive-grid embedded-boundary compressible Navier-Stokes solver. The compressible calculation then provides time-dependent boundary conditions for an adaptive low Mach number model of the swirl-stabilized premixed flame. The low Mach model incorporates detailed chemical kinetics and species transport using 20 species and 84 reactions. Laboratory diagnostics available for comparisons include characterizations of the flow field just down stream of the nozzle exit plane, and flame surface statistics, such as mean location, wrinkling and crossing frequencies.

Asymptotic structure of rich methane-air flames

The asymptotic structure of unstrained, laminar, fuel-rich, premixed methane flame is analyzed by using a reduced chemical-kinetic mechanism made up of three global steps. Analysis is carried out for values of equivalence ratio greater than 1.3. The flame structure is presumed to comprise three zones: an inert preheat zone, a thin reaction zone, and a post-flame zone. In contrast to previous asymptotic analyses of lean flames and moderately rich flames, where the reaction zone of these flames was presumed to be made up of two layers, for rich flames analyzed here all chemical reaction are presumed to take place in one layer. The structure of the reaction zone of rich flames is obtained by integrating two second order ordinary differential equations, one giving the consumption of fuel and the other the consumption of oxygen. For values of equivalence ratio greater than 1.3, burning velocities obtained from the asymptotic analysis are found to agree reasonably well with those obtained using a chemical-kinetic mechanism made up of elementary reactions.