Showing papers in "Combustion and Flame in 1998"

A Comprehensive Modeling Study of n-Heptane Oxidation

Abstract: A detailed chemical kinetic mechanism has been developed and used to study the oxidation of n-heptane in flow reactors, shock tubes, and rapid compression machines. Over the series of experiments numerically investigated, the initial pressure ranged from 1–42 atm, the temperature from 550–1700 K, the equivalence ratio from 0.3–1.5, and nitrogen-argon dilution from 70–99%. The combination of ignition delay time and species composition data provide for a stringent test of the chemical kinetic mechanism. The reactions are classed into various types, and the reaction rate constants are given together with an explanation of how the rate constants were obtained. Experimental results from the literature of ignition behind reflected shock waves and in a rapid compression machine were used to develop and validate the reaction mechanism at both low and high temperatures. Additionally, species composition data from a variable pressure flow reactor and a jet-stirred reactor were used to help complement and refine the low-temperature portions of the reaction mechanism. A sensitivity analysis was performed for each of the combustion environments. This analysis showed that the low-temperature chemistry is very sensitive to the formation of stable olefin species from hydroperoxy-alkyl radicals and to the chain-branching steps involving ketohydroperoxide molecules.

The measurement of laminar burning velocities and Markstein numbers for iso-octane-air and iso-octane-n-heptane-air mixtures at elevated temperatures and pressures in an explosion bomb

Abstract: Spherically expanding flames have been employed to measure flame speeds, from which have been derived corresponding laminar burning velocities at zero stretch rate. Two burning velocities are defined, one based upon the rate of propagation of the flame front, the other on the rate of formation of burned gas. To express the effects of flame stretch upon burning velocity, Markstein lengths and numbers for both strain and curvature also have been obtained from the same measurements of flame speed. The effects of the initial mixture temperature and pressure on these parameters also have been examined and data have been obtained for iso-octane–air mixtures at initial temperatures between 358 K and 450 K, at pressures between 1 and 10 bar, and equivalence ratios, φ, of 0.8 and 1.0. Burning velocities and Markstein numbers also are reported for a fuel comprised of 90% iso-octane and 10% n-heptane, with air, for the same range of pressures, temperatures, and equivalence ratios. An important observation is that, as the pressure increases, a cellular flame structure develops earlier during flame propagation. The reasons for this are discussed. As the flame surface becomes completely cellular there is an increase in flame speed and this continues as the flame propagates. The increase in the rate of flame propagation due to flame cellularity has been carefully charted. General expressions are presented for the increase in stretch-free burning velocity with initial temperature and its decrease with pressure. The measured burning velocities are compared with those of other researchers and reasons for the differences discussed.

Aromatic and Polycyclic Aromatic Hydrocarbon Formation in a Laminar Premixed n-Butane Flame

Abstract: Experimental and detailed chemical kinetic modeling work has been performed to investigate aromatic and polycyclic aromatic hydrocarbon (PAH) formation pathways in a premixed, rich, sooting, n-butane–oxygen–argon burner stabilized flame. An atmospheric pressure, laminar flat flame operated at an equivalence ratio of 2.6 was used to acquire experimental data for model validation. Gas composition analysis was conducted by an on-line gas chromatograph/mass spectrometer technique. Measurements were made in the main reaction and post-reaction zones for a number of low molecular weight species, aliphatics, aromatics, and polycyclic aromatic hydrocarbons (PAHs) ranging from two to five-fused aromatic rings. Reaction flux and sensitivity analysis were used to help identify the important reaction sequences leading to aromatic and PAH growth and destruction in the n-butane flame. Reaction flux analysis showed the propargyl recombination reaction was the dominant pathway to benzene formation. The consumption of propargyl by H atoms was shown to limit propargyl, benzene, and naphthalene formation in flames as exhibited by the large negative sensitivity coefficients. Naphthalene and phenanthrene production was shown to be plausibly formed through reactions involving resonantly stabilized cyclopentadienyl and indenyl radicals. Many of the low molecular weight aliphatics, combustion by-products, aromatics, branched aromatics, and PAHs were fairly well simulated by the model. Additional work is required to understand the formation mechanisms of phenyl acetylene, pyrene, and fluoranthene in the n-butane flame.

On the Adequacy of Certain Experimental Observables as Measurements of Flame Burning Rate

Abstract: This work presents detailed chemical kinetic computations and experimental measurements of a premixed stoichiometric N2-diluted methane-air flame in two-dimensional unsteady vortical flow, which are used to investigate the utility of several experimental observables as measurements of local burning and heat release rates. The computed mole fraction of HCO is found to have excellent correlation with flame heat release rate over the whole range of unsteady curvature and strain-rate investigated, for the flame under consideration. HCO planar laser induced fluorescence (PLIF) imaging is discussed and demonstrated in a V-flame experiment. On the other hand, we find the utility of peak dilatation rate as an indicator of heat release rate to be dependent on the unsteady strain-rate and flame curvature environment, and the associated modification in diffusional thermal fluxes within the flame. The integrated dilatation rate is found to be more robust under unsteady strain-rate, but still questionable in regions of high flame curvature. We also study the utility of a particular formulation for CO∗2 chemiluminescence, OH, and CH PLIF imaging, as well as OH∗, C∗2, and CH∗ chemiluminescence, as measurements of flame burning and heat release rates. We generally find these measures to be inferior to HCO. Experimental results suggest that CH, OH∗, C∗2, and CH∗ are not adequate indicators of local extinction; rather they provide signals of subtle shifts of hydrocarbon consumption among different chemical pathways. Moreover, numerical results suggest that both OH mole fraction and an existing CO∗2 chemiluminescence model do not correlate with burning or heat release rate variations in regions of high unsteady flame curvature. The present numerical investigation uses a single flame/vortex condition and a specific 46-step C(1) chemical mechanism. The conclusions reached herein may be generalized with further studies using more detailed mechanisms over ranges of stoichiometry, dilution, and flow time and spatial scales.

Kinetic modeling of hydrocarbon/nitric oxide interactions in a flow reactor

Abstract: The reduction of nitric oxide by reaction with C1 and C2 hydrocarbons under reducing conditions in a flow reactor has been analyzed in terms of a detailed chemical kinetic model. The experimental data were partly adopted from previous work and partly obtained in the present study; they cover the temperature range 800–1500 K and the reburn fuels CH4, C2H2, C2H4, C2H6, and natural gas. Modeling predictions indicate that, under the conditions investigated, HCCO + NO and CH3 + NO are the reactions most important in reducing NO. The HCCO + NO reaction is the dominant reaction when using natural gas or C2 hydrocarbons as reburn fuels. This reaction leads partly to HCNO, which is recycled to NO, and partly to HCN, which is converted to N2 or NO. When methane or natural gas are used as reburn fuel, the CH3 + NO reaction contributes significantly to remove NO. Modeling predictions are in reasonably good agreement with the experimental observations for the fuels investigated, even though the NO reduction potential is overpredicted for methane and underpredicted for ethane. Modeling predictions for NO are very sensitive to the formation of HCCO and to the product branching ratio for the HCCO + NO reaction. Furthermore, the present analysis indicates that more work is needed on critical steps in the hydrocarbon oxidation scheme.

A mixing model for turbulent reactive flows based on Euclidean minimum spanning trees

Abstract: In modeling turbulent reactive flows based on the transport equation for the joint probability density function (jpdf) of velocity and composition, the change in fluid composition due to convection and reaction is treated exactly, while molecular mixing has to be modeled. A new mixing model is proposed, which is local in composition space and which seeks to address problems encountered in flows with simultaneous mixing and reaction. In this model the change in particle composition is determined by particle interactions along the edges of a Euclidean minimum spanning tree (EMST) constructed in composition space. Results obtained for the model problem of passive scalars evolving under the influence of a mean scalar gradient in homogeneous turbulence are found to be in reasonable agreement with experimental findings of Sirivat and Warhaft (1983). The model is applied to the diffusion flame test model problem proposed by Norris and Pope (1991) and its performance is found to be superior to that of existing models. The essential feature of the new EMST mixing model, which accounts for its success in the diffusion flame test, is that mixing is modeled locally in composition space.

A Consistent Flamelet Formulation for Non-Premixed Combustion Considering Differential Diffusion Effects

Abstract: A flamelet formulation for non-premixed combustion that allows an exact description of differential diffusion has been developed. The main difference to previous formulations is the definition of a mixture fraction variable, which is not related directly to any combination of the reactive scalars, but defined from the solution of a conservation equation with an arbitrary diffusion coefficient and appropriate boundary conditions. Using this definition flamelet equations with the mixture fraction as the independent coordinate are derived without any assumptions about the Lewis numbers for chemical species. The formulation is shown to be exact if the scalar dissipation rate is prescribed as a function of the mixture fraction. Different approximations of the scalar dissipation rate that had been derived from analytic solutions for special cases are investigated by varying the diffusion coefficient of the mixture fraction transport equation. As examples, counterflow flames of hydrogen and n-heptane, which have much larger and much smaller diffusivities than oxygen and nitrogen, are considered. It is shown that the use of equal thermal and mixture fraction diffusivities yields a sufficiently well-described flame structure and is therefore recommended for the definition of the mixture fraction diffusion coefficient. Finally, the possibility of using constant species Lewis numbers has been examined. It has been found that once an appropriate set of Lewis numbers is determined, good results are achieved over wide ranges of the parameters, such as scalar dissipation rate, pressure, and oxidizer temperature.

A Kinetic Model of Carbon Burnout in Pulverized Coal Combustion

Abstract: The degree of carbon burnout is an important operating characteristic of full-scale suspension-fired coal combustion systems Prediction of carbon loss requires special char combustion kinetics valid through the very high conversions targeted in industry (typically >995%), and valid for a wide range of particle temperature histories occurring in full-scale furnaces The present paper presents high-temperature kinetic data for five coal chars in the form of time-resolved burning profiles that include the late stages of combustion The paper then describes the development and validation of the Carbon Burnout Kinetic Model (CBK), a coal-general kinetics package that is specifically designed to predict the total extent of carbon burnout and ultimate fly ash carbon content for prescribed temperature/oxygen histories typical of pulverized coal combustion systems The model combines the single-film treatment of char oxidation with quantitative descriptions of thermal annealing, statistical kinetics, statistical densities, and ash inhibition in the late stages of combustion In agreement with experimental observations, the CBK model predicts (1) low reactivities for unburned carbon residues extracted from commercial ash samples, (2) reactivity loss in the late stages of laboratory combustion, (3) the observed sensitivity of char reactivity to high-temperature heat treatment on second and subsecond time scales, and (4) the global reaction inhibition by mineral matter in the late stages of combustion observed in single-particle imaging studies The model ascribes these various char deactivation phenomena to the combined effects of thermal annealing, ash inhibition, and the preferential consumption of more reactive particles (statistical kinetics), the relative contributions of which vary greatly with combustion conditions

Collisional Quenching of CH(A), OH(A), and NO(A) in Low Pressure Hydrocarbon Flames

Abstract: Excited state lifetimes have been measured for the A-states of CH, OH, and NO in a number of low-pressure, premixed, laminar flow methane flames. From these lifetimes, collisional quenching rates were determined as a function of height above the burner and thus as a function of flame temperature and composition. The results were compared with values calculated using a model of the flame chemistry to predict collider mole fractions, together with parameterizations of quenching rate coefficients for each collider. Measured OH and NO quenching rates agree well with those calculated from these quenching rate coefficients and modeled flame composition data. This indicates that collisional quenching corrections for laser-induced fluorescence measurements can be calculated from knowledge of major species mole fractions and gas temperature. Predicted quenching rates for CH range from agreement with measured values to 27% higher than measured values. This discrepancies suggest insufficient knowledge of high temperature quenching by H2O and N2.

The evolution of soot precursor particles in a diffusion flame

Abstract: The chemical evolution of soot precursor particles on the centerline of the laminar ethene diffusion flame has been analyzed using laser microprobe mass spectrometry (LMMS) as they undergo the transition to carbonaceous aggregates. LMMS is a reliable microanalytical technique for the detection of intermediate and heavy polycyclic aromatic hydrocarbons (PAHs) in particulate material. The analyses show that many of the masses present within the precursor particles coincide with those predicted by Stein and Fahr (1985) to be most thermodynamically stable (stabilomers). The stabilomer PAHs that consist solely of six-membered rings, the benzenoid PAHs, prove to be the most important members of the stabilomer grid. Pericondensed PAHs as large 472 amu, which is attributed to the molecule C 38 H 16 with 12 hexagonal rings, are found to be constituents of the precursor particles. The PAH mass distribution diverges to the larger sizes with increasing height in the flame, and includes many of the species identified by others as gas-phase PAH constituents in hydrocarbon flames. Carbonization on the centerline of the flame occurs abruptly between 35 and 40 mm above the burner where the particle metamorphosis (from single precursor liquid-like particles to fused aggregates) and the decrease in hydrogen mole fraction (from 0.35 to 0.15) simultaneously occur. The presence of stabilomer PAHs reported by others in the particulate combustion product of a variety of fuels—aliphatic and aromatic gases, diesel fuel, crude oil, kerogen, carbon black feed stock, cigarette tobacco, and biomass—suggests that the stabilomer grid represents the common path for the growth of PAHs which contribute to the formation of carbonaceous soot in these diverse instances. This observation can account for the previously noted invariance of the soot product of combustion from diverse fuels and devices.

A Numerical Study of a Two-Dimensional H2-O2-Ar Detonation Using a Detailed Chemical Reaction Model

Abstract: Two-dimensional computations of the propagation of a detonation in a low-pressure, argon-diluted mixture of hydrogen and oxygen were performed using a detailed chemical reaction mechanism. Cellular structure developed after an initial perturbation was applied to a one-dimensional solution placed on a two-dimensional grid. The energy-release pattern in a detonation cell showed that, in addition to the primary release of energy behind the Mach stem, there is a secondary energy release that starts about two-thirds of the way through the cell. Reignition, which occurs as transverse waves collide, results in an explosion that spreads over a region and releases a considerable amount of energy. Resolution tests showed convergence of the detonation mode (number of triple points or transverse waves) reached at the end of the computations, as well as global and local energy release. The computations were performed on massively parallel Connection Machines for which new approaches were developed to maximize the speed and efficiency of integrations.

Measured and predicted properties of laminar premixed methane/air flames at various pressures

Abstract: Effects of positive flame stretch on the laminar burning velocities of methane/air flames were studied both experimentally and computationally, considering freely (outwardly) propagating spherical laminar premixed flames. Measurements based on motion picture shadowgraphs, and numerical simulations based on typical contemporary chemical reaction mechanisms, were used to find the sensitivities of the laminar burning velocities to flame stretch, characterized as Markstein numbers, and the fundamental laminar burning velocities of unstretched flames. Reactant conditions included methane/air mixtures having fuel-equivalence ratios of 0.60–1.35 and pressures of 0.5–4.0 atm, at normal temperatures. Both measured and predicted ratios of unstretched-to-stretched laminar burning velocities varied significantly from unity (in the range 0.6–2.3) even though present stretch levels did not approach quenching conditions. Absolute values of Markstein numbers increased with increasing pressure, while the transition from unstable to stable preferential-diffusion conditions with increasing fuel-equivalence ratio shifted from an equivalence ratio of 0.6 at 0.5 atm to 1.2 at 4.0 atm, suggesting increased unstable flame behavior due to preferential-diffusion effects at the elevated pressures of interest for many practical applications. Finally, predictions using two contemporary chemical reaction mechanisms were in reasonably good agreement with present measurements of both Markstein numbers and unstretched laminar burning velocities.

Dynamic Modeling of Soot Particle Coagulation and Aggregation: Implementation With the Method of Moments and Application to High-Pressure Laminar Premixed Flames

Abstract: The method of moments was extended to include a more complete description of particle coagulation, namely, the transition and continuum regimes as well as the formation and growth of fractal aggregates. The formalism preserves the numerical efficiency and the physical rigor of the method of moments by inclusion of just two additional moment equations and without prescribing a mathematical form to the particle size distribution function. The extended model was used to simulate soot formation in several 10 bar laminar premixed ethylene-air flames. The results demonstrate that the more complete formulation improves significantly model prediction of experimental data, thereby explaining the appearance of a catastrophic decrease in coagulation rates, which could not be rationalized within the classical theory of Brownian coagulation.

Instantaneous and Mean Compositional Structure of Bluff-Body Stabilized Nonpremixed Flames

Abstract: Turbulent nonpremixed flames stabilized on an axisymmetric bluff-body burner are studied with fuels ranging from simple H 2 /CO to complex H 2 /CH 4 and gaseous methanol. The fuel-jet velocity is varied to investigate the Damkohler number effects on gas emissions, localized extinction (LE) in the neck zone, and the structure of the recirculation zone dependency on the flow field. Simultaneous, single-point measurements of temperature, major species, OH, and NO are made using the Raman/Rayleigh/Laser induced fluorescence (LIF) technique. The data are collected at different axial and radial locations along the full length of most flames and are presented in the form of ensemble means, root-mean-square (rms) fluctuations, scatter plots, and probability density functions (PDF). It is found that up to three mixing layers may exist in the recirculation zone, one on the air side of the outer vortex, one between the inner and the outer vortices, and one between the fuel jet and the inner vortex. With increasing jet momentum flux, the average mixture in the outer vortex loses its strength and the stoichiometric contour shifts closer to the fuel jet. The decay rate of the mixture fraction on the centerline exhibits similar trends to the ordinary jet flame downstream of the recirculation zone whereas different trends are found inside the recirculation zone. The laminar flame computations with constant mass diffusivities and Lewis number (Le) = 1 are found to be better guides for the measured temperature and stable species mass fraction in the turbulent flames. The measured peak mass fractions of CO and H 2 are similar to those reported earlier for pilot-stabilized flames of similar fuels. Compared with laminar flame compositions with equal diffusivities and Le = 1.0, measured CO may be in superflamelet concentration. Hydroxyl radical and H 2 are found not to be in superflamelet levels contrary to earlier findings in piloted flames. The start of LE and the bimodality of the conditional PDF are consistent with those reported earlier for piloted flames of similar fuels.

Rapid Compression Machines: Heat Transfer and Suppression of Corner Vortex

Abstract: A method to suppress the piston corner vortex generated in compressing gas mixtures in rapid compression machines has been developed. A piston crevice was designed to swallow the thermal boundary layer along the wall, allowing better definition of core conditions in the reacting mixtures by confining the cold gases to the wall. Axisymmetric calculations of the flow and temperature fields show that the proposed design indeed suppresses the vortex formation, keeping the core reacting cases intact. A simple thermodynamic model based on the piston displacement history was formulated, incorporating the predicted heat transfer to the walls and mass transfer to the crevices. The model predictions agree very well with experimental pressure history under a range of initial pressures and types of different gases. The new experimental device and model allows the incorporation of complex chemical kinetics with early heat release without the need for experimental approximations for the heat transfer terms.

Computer-Aided Derivation of Gas-Phase Oxidation Mechanisms: Application to the Modeling of the Oxidation of n-Butane

Abstract: This paper describes a system that permits the computer-aided formulation of comprehensive primary mechanisms and simplified secondary mechanisms, coupled with the relevant thermochemical and kinetic data in the case of the gas-phase oxidation of alkanes and ethers. This system has been demonstrated by modeling the oxidation of n-butane at temperatures between 554 and 737 K, i.e., in the negative temperature coefficient regime, and at a higher temperature of 937 K. The system yields satisfactory agreement between the computed and the experimental values for the rates, the induction period and conversion, and also for the distribution of the products formed.

An experimental and numerical study of a laminar triple flame

Abstract: A lifted laminar axisymmetric diffusion flame is stabilized in the downstream region of a diluted methane jet that is surrounded by a lean methane-air co-flow and an outer co-flow of air. The flame shows a distinct triple flame structure in the stabilization region. It is investigated experimentally by PIV for the velocity field, OH-LIPF imaging, C2Hx-LIF imaging, and a 1D-Raman technique for major species concentrations, combined with a Rayleigh technique for temperature. This is complemented by numerical simulations solving the two-dimensional axisymmetric Navier-Stokes equations in the zero Mach number limit on an adapted mesh, coupled with balance equations for temperature and species. A simplified model for molecular transport properties was used with constant, but non-unity, Lewis numbers for all species. Chemistry is represented by a ten-step reduced mechanism for methane oxidation, which was derived starting from a 61-step elementary mechanism that includes the C1 and C2 chains. The agreement between the measured and the predicted flow field is very satisfactory. Owing to gas expansion, the velocity decreases immediately ahead of the flame and increases strongly at the flame front. Further downstream acceleration due to buoyancy is dominant and is predicted accurately. There is a good agreement between measurements and computations for flame shape and flame length. The measured OH-LIPF image and the computed OH concentrations indicate that OH is concentrated in the vicinity of stoichiometric mixture. The results from a newly developed C2Hx-LIF method are also supported by calculations. While these measurements were only qualitative, the temperature and mole fractions of the major species could be measured quantitatively with the combined Raman-/Rayleigh technique along a line and were found to agree well with the numerical predictions. It is found that the structure is a triple flame and is influenced essentially by two external parameters: heat exchange between the branches and heat loss at the curved flame front near the triple point.

Correlations for the Laminar-Burning Velocity of Methane/Diluent/Air Mixtures Obtained in Free-Fall Experiments

Abstract: The laminar-burning velocity of methane/air/diluent mixtures has been correlated for variations in unburnt gas temperature (within the range of 293 K to 454 K) and pressures (within the range of 0.5 to 10.4 bar), for as wide a range of mixtures as could be accommodated. The laminar-burning velocity measurements have been deduced from pressure time measurements made in near zero-gravity conditions in a spherical constant volume chamber. The correlations were obtained by a least squares fit of the data to an equation with 12 degrees of freedom. Carbon dioxide, nitrogen, and a 15% carbon dioxide/85% nitrogen mixture were diluents. The mixture of carbon dioxide and nitrogen was used to simulate the products of combustion, thus enabling measurements of the burning velocity to be made that correspond to when exhaust gas residuals (including those arising from exhaust gas recirculation) are present. The data presented here cover higher levels of diluents than previously reported, with up to 60% of either nitrogen or carbon dioxide as part of the fuel.

Vorticity generation and attenuation as vortices convect through a premixed flame

Abstract: A sequence of PIV images shows the time history of both the vorticity field and the velocity field as vortices of different strength convect through a premixed flame. The vortices represent individual eddies in turbulent flow; the goal is to understand how each eddy wrinkles the flame and how the flame also may alter the eddy. It is found that weak vortices are completely attenuated primarily due to volume expansion. Strong vortices do survive flame passage, but only if they can weaken the flame due to stretch effects. Intense flame-generated vorticity is measured which has a magnitude that exceeds that of the incident vortex in some cases. The flame-generated vorticity in the products induces a velocity field that tends to reduce the amplitude of flame wrinkling; thus it acts as an additional flame-stabilizing mechanism. This mechanism affects the wrinkling process and should be included in models. A new nondimensional vorticity enhancement parameter ( E ) is suggested as a way to estimate the effect of vortex size, strength, Reynolds number, and Froude number on vorticity attenuation and production. Measurements are made for E approximately equal to 0, −1, and −2, corresponding to no change in vorticity, total attenuation of the vortex, and flame-generated vorticity, respectively. Buoyancy forces are important in one case that is considered, but not in other cases. The results can be used to quantify the size of the small eddies that can be neglected in large eddy simulations; the role of small eddies is estimated in one example.

An investigation of the accuracy of manifold methods and splitting schemes in the computational implementation of combustion chemistry

Abstract: This paper is concerned with the efficient computational implementation of combustion chemistry, for use in PDF methods and other applications. A new method of coupling reactions and mixing processes based on manifold points with detailed chemistry is developed. Investigations are made of three different kinds of methods: the direct numerical integration of the coupled reaction and mixing equations; the direct numerical integration of the equations obtained by using operator-splitting to split reaction and mixing; and the new method—solving the split system based on manifold points with detailed chemical kinetics. Errors between the solutions are studied. It is found that chemical reactions have a significant influence on the accuracy of operator-splitting methods. The solution of the split systems based on manifold points provides an accurate representation for the solution of the full coupled equations. This means that tabulations can be made on manifolds with no simplification made to the chemistry.

Structure and Propagation of Methanol–Air Triple Flames

Abstract: The structure and propagation for a methanol (CH3OH)–air triple flame are studied using direct numerical simulations (DNS). The methanol (CH3OH)–air triple flame is found to burn with an asymmetric shape due to the different chemical and transport processes characterizing the mixture. The excess fuel, CH3OH, on the rich premixed flame branch is replaced by more stable fuels CO and H2 which burn at the diffusion flame. On the lean premixed flame side, a higher concentration of O2 leaks through to the diffusion flame. The general structure of the triple point features the contribution of both differential diffusion of radicals and heat. A mixture fraction–temperature phase plane description of the triple flame structure is proposed to highlight some interesting features in partially premixed combustion. The effects of differential diffusion at the triple point add to the contribution of hydrodynamic effects in the propagation of the triple flame. Differential diffusion effects are measured using two methods: a direct computation using diffusion velocities and an indirect computation based on the difference between the normalized mixture fractions of C and H. The mixture fraction approach does not clearly identify the effects of differential diffusion, in particular at the curved triple point, because of ambiguities in the contribution of carbon and hydrogen atoms’ carrying species.

Laser spark ignition and combustion characteristics of methane-air mixtures

Abstract: Ignition breakdown kernels of methane-air mixtures initiated by laser-induced sparks and by conventional electric sparks arc compared during initial stages. Experiments were conducted using a four-stroke (Otto-cycle) single-cylinder typical high-pressure combustion chamber. The piston is cycled in the cylinder by using an electric motor driven hydraulic ram. An cxcimer laser beam, either produced from krypton fluoride gas (λ = 248 nm) or argon fluoride gas (λ = 193 nm), or a Nd:YAG laser beam (λ = 1064 nm) is focused into a combustion chamber to initiate ignition. Conventional electric spark ignition is used as a basis for comparison between the two different ignition methods and the resultant early breakdown kernel characteristics. A streak camera is used to investigate and record the initial stages of kernel formation. Both a breakdown and a radial expansion wave of the ignition plasma are observed for certain laser ignition conditions of methane-air mixtures under typical internal combustion (IC) engine conditions. Results indicate that only certain wavelengths used for producing laser ignition produce a radial expansion wave. Laser ignition kernel size is calculated and laser-supported breakdown velocity is calculated by using Raizer's theory and is compared with measured results. Laser ignition results in a 4–6 ms decrease in the time for combustion to reach peak pressure than is obtained when using electric spark ignition in the same combustion chamber and under the same ignition conditions.

Subgrid-scale modeling for turbulent reacting flows

Abstract: The Large Eddy Simulation of non-premixed, turbulent, reacting flows is addressed. A new subgrid-scale chemistry model, previously proposed for incompressible, isothermal flows, is extended to the case of compressible combustion with multi-step, Arrhenius-rate reactions. The chemistry model predicts filtered chemical species concentrations and filtered reaction rates in a turbulent flow. It accounts for finite-rate chemistry by invoking the laminar flamelet approximation and employs an assumed form for the subgrid or “Large Eddy” Probability Density Function (LEPDF) of a mixture-fraction. It also uses an assumed counterflow form for the local scalar dissipation rate. Inputs to the chemistry model are the Favre-filtered mixture-fraction, its subgrid-scale variance, and filtered dissipation rate. The model is evaluated using (256) 3 point Direct Numerical Simulations of incompressible, nonisothermal decaying turbulence with a single-step reaction. Results indicate that, as the activation temperature is increased, the accuracy of the model degrades in an absolute sense but improves relative to an equilibrium chemistry assumption. Finally, it is also demonstrated that the assumed Beta distribution for the LEPDF yields reasonably accurate results for low (realistic) stoichiometric values of the mixture-fraction.

Modelling the Temporal Evolution of a Reduced Combustion Chemical System With an Artificial Neural Network

Abstract: The present work introduces a way of embedding a combustion chemical system in a neural network, in such a way that it can be used, with considerable CPU time and RAM memory savings, in fluid-flow-simulation codes. The system is composed of four neural networks, with three of them simulating the evolution of the reactive species and one providing density and temperature as a function of composition. The performance in terms of accuracy of the networks is assessed by comparison with the results of the direct integration of the thermochemical system for a large number of random samples. Error measurements are reported, and sample evolutions of the chemical system with both methods are compared. It can be summarized that the results of this exercise are satisfactory, and the CPU-time and memory savings encouraging.

Effects of pressure and nitrogen dilution on flame/stretch interactions of laminar premixed H2/O2/N2 flames

Abstract: Effects of positive flame stretch on the laminar burning velocities of H2/O2/O2 flames at normal temperatures and various pressures and nitrogen dilutions were studied both experimentally and computationally. Measurements and numerical simulations considered freely (outwardly)-propagating spherical laminar premixed flames at both stable and unstable preferential-diffusion conditions with fuel-equivalence ratios in the range 0.45–4.00, pressures in the range 0.35–4.00 atm, volumetric oxygen concentrations in the nonfuel gas in the range 0.125–0.210, and Karlovitz numbers in the range 0.0–0.6. For these conditions, both measured and predicted ratios of unstretched (plane flames) to stretched laminar burning velocities varied linearly with Karlovitz numbers, yielding Markstein numbers that were independent of Karlovitz numbers for a particular pressure and reactant mixture. Measured Markstein numbers were in the range −4 to 6, implying strong flame/stretch interactions. For hydrogen/air flames, the neutral preferential-diffusion condition shifted toward fuel-rich conditions with increasing pressure. Predictions of stretch-corrected laminar burning velocities and Markstein numbers, using typical contemporary chemical reaction mechanisms, were in reasonably good agreement with the measurements.

Determination of the Soot Volume Fraction in an Ethylene Diffusion Flame by Multiwavelength Analysis of Soot Radiation

Abstract: The features of the multiwavelength emission technique for the measurement of soot volume fraction and temperature in an ethylene diffusion flame have been investigated. For this purpose we have exploited the emission spectra from 300 nm to 800 nm and a mathematical Abel inversion procedure. To interpret the measurements, the quantity fv/Kabs = λ/(36πF(λ)), here called natural length for absorption, l abs, is modeled as a continuous function according to literature data. Soot temperature profiles were also obtained showing small variations in the investigated region of the flame. Extinction measurements on the same flame were compared, and good agreement was found between the two techniques. With respect to the more-common two-color emission technique, the use of a wide set of spectral data reduces the uncertainties due to the determination of the index of refraction of soot. Different sets of refractive index have been compared and the results are discussed.

Extinction of low-stretched diffusion flame in microgravity

Abstract: Extinction of counterflow diffusion flames of air and methane diluted with nitrogen is studied by drop tower experiments and numerical calculation using detailed chemistry and transport properties. Radiative heat loss from the flame zone is taken into consideration. Experimental results identified two kinds of extinction at the same fuel concentration, that is, in addition to the widely known stretch extinction, another type of extinction is observed when the stretch rate is sufficiently low. Consequently, plots of stretch rates versus fuel concentration limits exhibit a C-shaped extinction curve. Numerical calculation including radiative heat loss from the flame zone qualitatively agreed with the experimental results and indicated that the mechanism of counterflow diffusion flame extinction at low stretch rates was radiative heat loss.

Incorporation of parametric uncertainty into complex kinetic mechanisms: Application to hydrogen oxidation in supercritical water

Abstract: In this study, uncertainty analysis is applied to a supercritical water hydrogen oxidation mechanism to determine the effect of uncertainties in reaction rate constants and species thermochemistry on predicted species concentrations. Forward rate constants and species thermochemistry are assumed to be the sole contributors to uncertainty in the reaction model with all other model parameters and inputs treated as deterministic quantities. The analysis is conducted by treating the model parameters as random variables, assigning each a suitable probability density function, and propagating the parametric uncertainties through to the predicted species concentrations. Uncertainty propagation is performed using traditional Monte Carlo (MC) simulation and a new, more computationally efficient, probabilistic collocation method called the Deterministic Equivalent Modeling Method (DEMM). Both methods predict virtually identical probability distributions for the resulting species concentrations as a function of time, with DEMM requiring approximately two orders of magnitude less computation time than the corresponding MC simulation. The results of both analyses show that there is considerable uncertainty in all predicted species concentrations. The predicted H 2 and O 2 concentrations vary ± 70% from their median values. Similarly, the HO 2 concentration ranges of +90 to −70% of its median, while the H 2 O 2 concentration varies by + 180 to − 80%. In addition, the DEMM methodology identified two key model parameters, the standard-state heat of formation of HO 2 radical and the forward rate constant for H 2 O 2 dissociation, as the largest contributors to the uncertainty in the predicted hydrogen and oxygen species concentrations. The analyses further show that the change in model predictions due to the inclusion of real-gas effects, which are potentially important for SCWO process modeling, is small relative to the uncertainty introduced by the model parameters themselves.

Measured flame structure and kinetics in a fuel-rich ethylene flame

Abstract: Molecular-beam mass spectrometry (MBMS) has been used to map the structure of a fuel-rich C 2 H 4 /O 2 flame and infer new C 2 H 4 and C 2 H 3 kinetics. Axial profiles of concentration, area expansion ratio, and temperature were measured for a C 2 H 4 /O 2 /50% Ar (φ = 1.90) flame at 2.667 ± 0.001 kPa (20 Torr) and 62.5 cm/s burner velocity (300 K). Full concentration profiles were mapped for 42 radical and stable species. Elemental flux balances were within 12%, supporting the data’s accuracy and validity. Species flux-balance calculations were used to obtain net rates of reaction for the species. Rate constants were determined for H-abstraction from C 2 H 4 by H at 1850–2150 K, and their agreement with theory and previous lower-temperature data leads to recommendation of the ab initio /BAC-MP4 result: C 2 H 4 + H = C 2 H 3 + H 2 k = 4.49 × 10 7 × T 2.12 exp (−13,366/ RT ) in cm, mol, s, cal, K units. Data for abstraction by OH, combined with literature data, give: C 2 H 4 + OH = C 2 H 3 + H 2 O k = (5.53 ± 0.14) × 10 5 × T (2.310 ± 0.004) exp [−(2900 ± 60)/ RT ] for temperatures between 1400 and 1800 K. Rate constants for vinyl decomposition reaction C 2 H 3 = C 2 H 2 + H were analyzed, supporting the recent recommendations of Knyazev and Slagle [41] .

Inhibition effectiveness of halogenated compounds

Abstract: A numerical study of the inhibition efficiency of halogenated compounds was carried out for C 1 - 2 hydrocarbon-air laminar premixed flames. The inhibition efficiency of CF 3 Br, CF 3 I, CF 3 H, C 2 HF 5 , C 2 F 6 , and CF 4 additives was interpreted using an additive group method. In agreement with measurements, the calculated burning velocity decreased exponentially with increasing additive concentration over a wide concentration range. The inhibition parameter Φ proposed by Fristrom and Sawyer indicating inhibition efficiency was modified to take into account the exponential dependence of burning velocity on inhibitor concentration. The inhibition indices for halogen atoms and groups important in the inhibition process were determined for stoichiometric conditions. The physical and chemical effects of the additives were studied. With increasing additive concentration, the chemical influence of an inhibitor saturates and the physical influence increases. Therefore, use of a composite inhibitor composed of a mixture of an effective chemical inhibitor with a high heat capacity diluent may be beneficial. The contribution of physical and chemical components on inhibitor influence are estimated near entinction. A procedure for determination of a regeneration coefficient, which indicates an effective number of catalytic cycles involving inhibitor during the combustion process, is suggested. The regenation coefficient of HBr in stoichiometric methane-air flame with 1% CF 3 Br added is approximately 7.