Showing papers on "Laminar flame speed published in 1998"

Direct experimental determination of laminar flame speeds

Abstract: The stability of premixed flames at ultralow strain rates was assessed experimentally and numerically in the stagnation flow configuration. Results indicate that there are inherent limitations in establishing weakly strained planar flames, and that the accuracy of the laminar flame speeds obtained through linear extrapolations can, thus, be compromised. In view of these limitations, a new methodology is proposed for the direct experimental determination of laminar flame speeds. It includes the use of the stagnation flow configuration and large separation distances betwenn the nozzle and the stagnation plane, which allow for the establishment of Bunsen-type flames as the flow rate is reduced. The flow velocities are measured by using laser Doppler velocimetry. The proposed technique is based on the principle that whereas the planar, strained flames are positively stretched, the Bunsen flames are negatively stretched. Thus, by achieving a smooth, quasi-steady transition between planar and Bunsen flames, the flames pass through a near-zero strain-rate state. Real-time LDV measurements were obtained at numerous fixed spatial locations in the region within which transition occurs. The minimum velocity obtained in these measurements corresponds to the flame speed at the limit of near-zero stretch and is proposed as a representative value of the true laminar flame speed, SHo. Laminar flame speeds were obtained for atmospheric CH4/air, C2H6/air, and C3H8/air mixtures and for a wide range of equivalence ratios. The new Sno values were found to be systematically lower than the values that have been determined by using the traditional stagnation flow technique and linear extrapolations to zero strain rate.

Application of a flame-wrinkling les combustion model to a turbulent mixing layer

Abstract: The necessity for turbulent combustion modeling in the large-eddy simulation (LES) of premixed turbulent combustion is evident from the computational cost and the complexity of handling flame kinetics reaction mechanisms directly. In this paper, a new flame-wrinkling LES combustion model using conditional filtering is proposed. The model represents an alternative approach to the traditional flame-surface density based models in that the flame distribution is represented by a flame-wrinkle density function and that the effects of flame stretch and curvature are handled through a modeled transport equation for the perturbed laminar flame speed. For the purpose of validating the LES combustion model, LESs of isothermal and reacting shear layers formed at a rearward-facing step are carried out, and the results are compared with experimental data. For the isothermal case, the agreement between LES and the experimental data is excellent. For the reacting case, the evolution and topology of coherent structures is examined, and direct comparisons are made with time-averaged profiles of velocity and its fluctuations. temperature, and reaction products. Good agreement is obtained, to a large extent due to accurate modeling of the flame-wrinkle density but also to the novel treatment of the strain-rate effects on the laminar flame speed of the lean propane-air mixture.

Planar laser-induced fluorescence imaging of flame heat release rate

Abstract: Local heat release rate represents one of the most interesting experimental observables in the study of unsteady reacting flows. The direct measure of burning or heat release rate as a field variable is not possible. Numerous experimental investigations have relied on inferring this type of information as well as flame-front topology from indirect measures that are presumed to be correlated. A recent study has brought into question many of the commonly used flame-front marker and burning-rate diagnostics. This same study found that the concentration of formyl radical offers the best possibility for measuring flame burning rate. However, primarily due to low concentrations, the fluorescence signal level from formyl is too weak to employ this diagnostic for single-pulse measurements of turbulent-reacting flows. In this paper, we describe and demonstrate a new fluorescence-based reaction-front imaging diagnostic suitable for single-shot applications. The measurement is based on taking the pixel-by-pixel product of OH and CH2O planar laser-induced fluorescence (PLIF) images to yield an image closely related to a reaction rate. The spectroscopic and collisional processes affecting the measured signals are discussed, and the foundation of the diagnostic, as based on laminar and unsteady flame calculations, is presented. We report the results of applying this diagnostic to the study of a laminar premixed flame subject to an interaction with an isolated line-vortex pair.

Laminar flame speeds and oxidation kinetics of iso-octane-air and n-heptane-air flames

Abstract: Laminar flame speeds of iso-octane-air and n-heptane-air mixtures were determined experimentally over an extensive range of equivalence ratios at room temperature and atmospheric pressure, employing the counterflow twin flame configuration. Using both linear and nonlinear extrapolations, the effect of stretch was minimized by extrapolating the reference flame speed to vanishing stretch, with the nonlinearly extrapolated values being typically 2 cm/s smaller. The laminar flame speeds of iso-octane were found to be lower than those of n-heptane throughout the range of experimental equivalence ratios. Predictions, using a detailed kinetic model based on the works of Held et al, and of Curran, Pitz., and Westbrook, agreed quite well with experimental data, especially at lean equivalence ratios, while yielding somewhat lower values at stoichiometric to rich equivalence ratios. Since the n-heptane kinetics in the model were previously compared to flow-reactor data, the present model was also compared to an iso-octane oxidation flow-reactor experiment. The model accurately predicted the fuel decay profile as well as those of propene and iso-butene, which are the two major intermediates formed initially at flow-reactor conditions. The present analysis suggests that the major high-temperature reaction pathways proposed by Curran et al accurately describe the high-temperature oxidation of iso-octane and indicates that the development of a comprehensive model requires additional studies on the reaction kinetics of propene and iso-butene.

Method and apparatus for characterizing a combustion flame

Abstract: Characteristics of a flame within a turbine or burner are determined based upon ultraviolet, visible, and infrared measurements of the flame. The measurements include a measurement of the amplitute of frequency bands that are indicative of an efficient combustion process, such as those that increase when the flame temperature increases. The measurements also include of the amplitude of frequency bands that are indicative of an inefficient combustion process, such as those that do not vary, increase a relatively small amount, or decrease when the flame temperature increases. The temperature of the flame may therefore be determined accurately, to facilitate efficient operation of the turbine or burner while minimizing polluting emissions. A fiber structure, suitable for remote location of sensors and processing equipment passes energy for several spectra by providing a hollow core that passes infrared energy, in combination with a core of visible-transmissive material that passes visible or ultraviolet energy. Contaminants in the turbine or burner are detected, and a degree of contamination measured, by detection of energy levels for particular wavelengths associated with a respective contaminant.

Measurement of the resolved flame structure of turbulent premixed flames with constant reynolds number and varied stoichiometry

Abstract: Wire-stabilized premixed methane-air flames have been studied in a grid-generated homogeneous turbulent flow field in order to identify different burning regimes. The planar Rayleigh scattering technique was used with two parallel laser light sheets, which allows the detection of three-dimensional temperature gradients. For a detailed investigation of the flame structure and topology, the modification of the local temperature gradients at different progress variables c due to the turbulent motion was studied by varying the flame stoichiometry and thereby the Karlovitz number Ka while keeping the turbulent Reynolds number Ret constant at 87 or 134. Because of a nearly Gaussian shaped statistical distribution of the thermal gradients, the 50% median and the width of the distribution are suitable measures used to characterize the flame response. Compared with laminar unstrained calculations, especially very lean flames (

Effects of stretch on the local structure of preely propagating premixed low-turbulent flames with various lewis numbers

Abstract: An experimental investigation of the flame response to strain rate in the case of unsteady premixed low-turbulent flames is presented. In order to point out the fundamental aspects of the mutual interaction between combustion and turbulence, measurements of local flame properties (curvature, displacement speed) and tangential strain rate were performed under varying conditions of Lewis number and turbulence. Three different mixtures (methane/air, propane/air, and hydrogen/air) were successively spark ignited in a vertical wind tunnel. The expanding flame freely propagated in a grid-generated decaying turbulent flow. An advanced field imaging technique coupling high-speed laser tomography and cross-correlation particle image velocimetry (PIV) was used to measure the temporal evolution of local flame stretch exerted by the turbulent cold flow. Local flame curvature and local displacement speed were calculated from flame-front contours. Curvature probability density functions (PDFs) were negatively skewed, especially for nonunity Lewis numbers, and displacement speed distributions underlined the influence of local stretch and thermodiffusive effects on flame-speed variations. Tangential strain rate was determined by using the velocity field in the neighborhood of the flame front and appears to be independent of the Lewis numbers. A strong correlation between local flame curvature and tangential strain rate was demonstrated, underlining the cold flow effects on the local flame structure. The influences of turbulence and Lewis number were evaluated and compared with numerical simulations. Then, local flame stretch distributions were determined versus time, indicating that a significant proportion of the flame was under compression.

Diffusion Flames Based on a Laminar Spray Flame Library

Abstract: The present paper investigates the structure of turbulent spray diffusion flames by means of numerical simulations. The flamelet model for turbulent diffusion flames has recently been extended to turbulent spray diffusion flames. The model is suitable for considering detailed chemical reactions through use of a laminar flame library consisting of structures of laminar gas diffusion flamelets that are characterized by the mixture fraction and its scalar dissipation rate. The focus of the present paper is the implementation of laminar spray diffusion flames for use in turbulent flame computations. Since the structure of laminar spray flames is considerably different from that of their gaseous counterparts, new criteria need to be developed for the implementation of these structures. The present paper presents characteristics of laminar spray flames and their consideration in turbulent flame computations. Both the model predictions (using either laminar gas flames or laminar spray flames) are compar...

Interaction of a vortex with a flat flame formed between opposing jets of hydrogen and air

Abstract: Studies on individual vortex-flame interactions constitute important elements for the understanding of the turbulent-flame structure. Vortices having sufficiently high normal velocity can pass through the flame by extinguishing it locally. In several circumstances they deform the flame surface significantly before attaining extinction conditions. The development of curvature on the flame surface, especially in hydrogen flames, could lead to different quenching patterns. An experimental/numerical investigation is performed to explore possible quenching patterns in opposing-jet diffusion flames. A diluted hydrogen-nitrogen mixture is used as the fuel. Vortices are driven toward the flame surface with different velocities from the air side. The changes in the structure of the flame during its interaction with the incoming vortex are recorded by measuring instantaneous OH-concentration field using the laser-induced fluorescence (LIF) technique. A time-dependent CFDC code that incorporates 13 species and 74 reactions is used for the simulation of these vortex-flame interactions. Both the experiments and calculations have identified two types of quenching patterns: namely, point and annular. It is found that when an air-side vortex is forced toward the flame at a relatively high speed, then the flame at the stagnation line quenches, resulting in a well-known point-quenching pattern. On the other hand, when the vortex is forced at a moderate speed, the flame surface deforms significantly, and quenching develops in an annular ring away from the stagnation line, resulting in an unusual annular-quenching pattern. Detailed analyses performed just before the development of annular quenching and 1 ms later suggest that this unusual annular quenching did not result from the strain rate. Based on the understanding gained from previous investigations on curvature effects in coaxial hydrogen jet flames and the findings made in the present study, it is argued that such quenching develops as a result of the combined effect of preferential diffusion and flame curvature.

Turbulence, scalar transport, and reaction rates in flame-wall interaction

Abstract: Turbulent premixed flames are studied in a Couette channel flow using direct numerical simulation (DNS). The combustion is simulated by a single-step reaction with heat release, and the flow is fully developed turbulence at a Reynolds number of 504 based on channel half-width The flow configuration consists of a V-shaped flame held in place by a flame holder. One flame is far from the walls and remains adiabatic, while the other flame interacts with a wall and is nonadiabatic. The flame alters the turbulent velocity profile. Near-wall viscous sublayer scaling is found provided both the pressure gradient and the local, average wall shear stress are used. However, in the log layer, the standard scaling does not collapse the velocity profiles, indicating the importance of additional transport mechanisms caused by the flame. The turbulent length scale is reduced in the flame brush region. In the adiabatic flame, the reduction is to approximately the laminar flame thickness. In the nonadiabatic wall flame, the reduction is not as dramatic due to wall heat loss causing weaker reaction rates. The probability density function (PDF) of the scalar progress variable has a classical bi-modal shape in the adiabatic flame. Thus, in the adiabatic flame, the DNS results compare well with the standard Bray-Moss-Libby (BML) expressions for the turbulent scalar flux and flame surface density. However, in the nonadiabatic flame, which interacts with the wall, the scalar PDF is less bi-modal. In this case, the DNS results and the BML expression for the flame surface density does not correctly capture the heat loss effects of the wall. Modifications to the flame surface area are suggested and compare well with the DNS results.

Computational and experimental study of a forced, timevarying, axisymmetric, laminar diffusion flame

Abstract: Forced, time-varying flames are laminar systems that help bridge the gap between laminar and turbulent combustion. In this study, we investigate computationally and experimentally the structure of an acoustically forced, axisymmetric laminar methane-air diffusion flame, in which a cylindrical fuel jet is surrounded by a coflowing oxidizer jet. The flame is forced by imposing a sinusoidal modulation on the steady fuel flow rate. Rayleigh scattering and spontaneous Raman scattering of the fuel are used to generate the temperature profile. Particle image velocimetry (PIV) is used to measure the fuel tube exit velocity over a cycle of the forcing modulation. CH flame emission measurements have been done to predict the excitedstate CH (CH * ) levels. Computationally, we solve the transient equations for the conservation of total mass, momentum, energy, and species mass with detailed transport and finite-rate C 2 chemistry submodels to predict the pressure, velocity, temperature, and species concentrations as a function of the two independent spatial coordinates and time. The governing equations are written in primitive variables. Implicit finite differences are used to discretize the governing equations and the boundary conditions on a nonstaggered, noniumiform grid. Modified damped Newton's method nested with a Bi-CGSTAB iteration is utilized to solve the resulting system of equations. Results of the study include a detailed description of the fluid dynamic-thermochemical structure of the flame at a 20-Hz frequency. A comparison of experimentally determined and calculated temperature profiles and CH * levels agree well. Calculated mole fractions of species indicative of soot production (C 2 H 2 , CO) are compared against those levels in the corresponding steady flame and are observed to increase in peak concentration values and spatial extent. Analysis of acetylene production rates reveals additional significant production in the downstream region of the flame at certain times during the flame's cyclic history.

Nonlinear equation for a curved stationary flame and the flame velocity

Abstract: A nonlinear equation for a curved stationary flame subject to the Darrieus–Landau instability is obtained for an arbitrary ratio of the fuel density and the density of the burnt matter under the assumptions of a thin flame front and weak nonlinearity. On the basis of the nonlinear equation the velocity of a two-dimensional curved stationary flame is calculated. The obtained velocity is in a good agreement with the results of two-dimensional simulations of flame dynamics in tubes.

Investigating the effects of edge flames in liftoff in non-premixed turbulent combustion

Abstract: To help to understand and to model liftoff in non-premixed turbulent combustion, we have studied effects of unsteadiness at the edge of a diffusion flame stabilized by a triple flame. A model problem involving squeezing of the triple flame between two moving vortices has been solved using direct numerical simulation. Numbers representative of propagation and extinction properties of edge flames were chosen for this particular configuration. By varying those key numbers, the response of a triple flame submitted to vorticity and unsteady micromixing was quantified. The outcome is a diagram delineating conditions for upstream and downstream movement of a diffusion flame in such an environment. It is found that edges of diffusion flames can take advantage of vorticity and micromixing to propagate faster than laminar triple flames. This propagation was also observed for conditions at which the trailing diffusion flame is quenched. Moreover, local quenching of this trailing diffusion flame generates new ends of reaction zones, which may help to sustain combustion if they relax to triple flames. The properties of the relative velocity of the edge of the reaction zone are compared with the asymptotic solution obtained for a steady triple flame. This helps to discriminate between edge flames able to progress within a zone of intense unsteady mixing from those moving downstream with the flow: in particular, it is observed that a triple flame submitted to strong unsteady micromixing effects may lose its propagation properties and evolve into a “hot spot.”

Effect of the discrete nature of heat sources on flame propagation in particulate suspensions

Abstract: In some heterogeneous systems, such as dust clouds, the heat sources (i.e., particles) are separated by large spaces of a noncombustible, heat-conductive media. In spite of the clearly discrete nature of the heat sources in such systems, the source term in the energy conservation equation of the flame propagation models is commonly considered to be a continuous function of spatial coordinates. To analyze the validity of this assumption for dust flames, we have compared flame speeds predicted by two different flame models. One of the models is based on the common approximation of the heat sources by a continuos function of spatial coordinates, and the other considers the exact description of the coordinates of the burning particles. An analytical expression for the flame speed in the media with regularly distributed pointlike discrete heat sources (particles) has been obtained, and the parameter responsible for the relation between discrete and continuous models was identified. The flame speeds obtained from the continuous and discrete models coincide if the particle combustion time is much longer than the characteristic time of heat transfer between particles. In the opposite case, for the fast (compared with the time of heat transfer between particles) burning fuels, the flame speed in the discrete system is a weak function of the particle combustion rate and is an explicit function of the distance between particles. The effect of the discrete nature of the heat sources on flame propagation is illustrated by comparing flame speeds calculated both from continuous and discrete models in lean aluminum and zirconium particle-gassuspensions. Lower flame speeds and a weaker dependence of the speed on oxygen concentration are predicted by the discrete flame model. The possible effect of the nonuniform particle distribution in space on the flame propagation phenomena is also discussed in the present work.

The domain of influence of flame instabilities in turbulent premixed combustion

Abstract: Although a number of experiments and numerical simulations indicate persistence of flame instability effects in turbulent premixed flames, the exact domain of influence of these instabilities remains unknown. In this study, a simple estimate of that domain is obtained by comparing a characteristic flame stretch due to flame instabilities, K i , with a characteristic flame stretch due to turbulent eddies, K t . The resulting criterion, ( K t / K i )≤1, shows that instability effects are promoted by: small values of the ratio of turbulence intensity divided by the laminar flame speed ( u′/s L ): large values of the ratio of integral length scale divided by the laminar flame thickness ( l t / l F ) (given that ( l t / l F )>10): large values of the heat release factor τ: large positive values of the flame Richardson number Ri ( Ri measures buoyancy effects and is positive when the corresponding flow acceleration is directed from the fresh mixture to the burnt gas): small values (below one) of the flame Lewis number Le . Direct numerical simulations (DNS) are used to test the validity of the theoretical criterion. The numerical configuration corresponds to three-dimensional premixed flames propagating into a temporally decaying turbulent flow. The simulations are limited by DNS constraints to small length scale ratios, ( l t / l F )≤10; they use Le =1 and correspond to different values of ( u′/s L ), τ and Ri . Due to the turbulence decay, all simulated flames with τ≠0 and Ri ≥0 undergo a transition from turbulent to unstable flame surface dynamics. The DNS values of ( K t / K i ) at transition time are found to be of order one and are in good agreement with the theoretical predictions.

Two-dimensional direct numerical simulation of opposed-jet hydrogen-air diffusion flame

Abstract: Opposed-jet diffusion flame experiments are routinely analyzed with one-dimensional models obtained by assuming a specific form for the velocity field. In this study, two-dimensional simulations of the hydrogen-air laminar opposed-jet counterflow diffusion flame using detailed chemical kinetics and realistic transport were performed for parabolic and uniform inflow velocity profiles at the exits of the nozzles. Two-dimensional simulations allow for the detailed examination of the hydrodynamics and the assessment of the validity of the assumptions made in the traditional one-dimensional simulations. Using typical nozzle size and separation distance employed in experiments, we analyzed the effects of nozzle outflow boundary conditions, finite size, and finite separation distance on the structure of the strained laminar diffusion flame. We also analyzed the variations of the divergence of the velocity field (compressibility due to chemical reaction) and that of the hydrodynamic pressure. The two-dimensional simulation results show that the cost-effective one-dimensional model provides an accurate description of the flame structure even for low-strain hydrogen-air flame provided that the velocity profiles at the nozzle exits are uniform. Although in the one-dimensional model, the nozzle size to separation ratio is assumed to be large, our two-dimensional results show that a ratio of 1 is adequate. Finally, we observed that the velocity gradient (the axial derivative of the axial velocity component along the axis of symmetry) measured in experiments at a point just before the flame region is inadequate in describing the characteristic strain rate “seen” by the flame.

An experimental and numerical investigation of the structure of steady two-dimensional partially premixed methane-air flames

Abstract: Steady two-dimensional partially premixed slot-burner flames established by introducing a rich fuel-air mixture from the inner slot and air from the two outer slots are investigated. Numerical simulations are conducted using detailed chemistry, velocity measurements are made using particle image velocimetry, and images of the chemiluminescent reaction zones are obtained. Two reaction zones are evident: one in an inner rich-side premixedlike flame and the other in an outer lean-side non-premixed flame. Validation of the predictions involves a comparison of the (1) premixed and non-premixed flame heights, (2) the double-flame structure and (3) velocity vectors. The measured and predicted velocity vectors are in good agreement and show that the flame interface separates smaller velocity magnitudes on the reactant side from large values on the (partially) burned side. The outer flame temperature is higher than that of the inner premixed flame. A substantial amount of methane leaks past the inner flame and reacts in the outer non-remixed zone. The inner flame produces partially oxidized products such as H2 and CO, which provide the fuel for the non-premixed flame. The initiation reaction CH4+H⇔CH3+H2 proceeds strongly at the base of the flame where both the inner and outer flames are connected and at the tip of the inner flame, and it is weak along the sides of both inner and outer flames in accord with the chemiluminescent images. Carbon dioxide formation through the reaction CO+OH⇔CO2+H is more diffuse than methane consumption in the outer flame, because the availability of hydroxyl radicals in that region is limited through oxidizer transport.

Local flame propagation speeds along wrinkled, unsteady, stretched premixed flames

Abstract: The local displacement speed was measured along a premixed flame that is wrinkled, unsteady, stretched, and freely propagating The displacement speed is argued to be the most important and sensitive parameter that must be simulated correctly in numerical simulations of turbulent flames An axisymmetric flame wrinkle is created so that all components of the normal vector and stretch can be measured: particle image velocimetry and high-speed shadowgraph movies yield the difference between the interface velocity and the reactant velocity, which is the displacement speed This repeatable flame-vortex interaction problem provides useful test data for direct numberical simulation (DNS) models and flame stretch theory Displacement speed varies enormously (by a factor of 75) along the flame; the variation, is much larger than predicted by steady-state theory Furthermore, flame curvature plays a dominant role in determining displacement speed Observed trends do not agree with those predicted by the standard steady-state theory However, if a modified theory is based only on flame curvature and strain is ignored, the observed trends tend to agree with this modified theory in many regions of the flame, but not in all regions

Inherently unsteady flame spread to extinction over thick fuels in microgravity

Abstract: Results of an experiment for flame spread over thick PMMA in a quiescent, 50% O2 in N2, 1 atm, microgravity environment recently obtained aboard space shuttle mission STS 85 are described Previous experimental results indicate that the spread process is unsteady with the spread rate decreasing with time Although experiment time in the earlier experiments was insufficient to determine if steady spread is established or extinction occurs, computational modeling predicts extinction The sample length was extended over that of the earlier experiments to determine the ultimate fate of the flame Flame imaging shows that following ignition, the flame leading edge spreads at a continually decreasing rate for approximately 180 s, ceases to progress forward, and then retreats in the opposite direction for approximately an additional 360 s, at which time flame extinction occurs Computational modeling, including gas and fuel surface radiation, captures the observed behavior, which is predicted for all oxygen concentrations up to pure oxygen at 1 atm In the presence of a flow, a thin heated layer in the solid develops quickly with the heat transfer driving vaporization and steady spread, while in the quiescent environment, a heated layer of substantial thickness develops over time while the flame spreads, unsteadily, more slowly As a result, radiation is important, and the length-scale characteristic of the temperature field in the gas is decreased in comparison to the mass diffusion scale, which grows with time Ultimately, the mismatch in scales results in the flame being in a region to which oxygen is unable to diffuse at a sufficient rate, and the flame extinguishes Such self-extinction at microgravity has implications for fire safety considerations in spacecraft

Laser imaging of conditional velocities in premixed propane-air flames by simulataneous OH PLIF and PIV

Abstract: The turbulent scalar flux, p ¯ u ″ c ″ ˜ , of the reaction progress variable, c, is experimentally characterized by means of laser-imaging diagnostics. Two-dimensional measurements of the instantaneous velocity and hydroxyl concentration fields are obtained simultaneously by particle image velocimetry (PIV) and qualitative planar laser-induced fluorescence (PLIF), respectively. The combination of these two diagnostic techniques allows the determination of mean conditional velocities. The mean reactant and product velocities are used to infer the turbulent flux of reaction progress variable in turbulent premixed propane/air flames. Four flames having different ratios of root mean square (rms) velocity, u′, to laminar flame speed, SL, are considered. A transition from countergradient to gradient turbulent diffusion is found as u′/SL increases. Such a transition has been predicted by direct numerical simulations (DNSs) but has only been observed in one previous expreimental study of turbulent premixed methane/air flames. This work contributes to the experimental evidence of this transition from countergradient to gradient turbulent diffusion and addresses issues related to variations in Lewis number. The results indicate that there is little effect of Lewis number on the transition. It is important to understand this phenomenon so that it may be infcorporated into models of turbulent premixed combustion.