Showing papers on "Diffusion flame published in 1998"
TL;DR: In this article, 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.
664 citations
TL;DR: In this article, 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, where the change in particle composition is determined by particle interactions along the edges of a Euclidean minimum spanning tree (EMST).
407 citations
TL;DR: In this article, 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.
303 citations
01 Jan 1998
TL;DR: In this article, an unsteady flamelet model is applied in numerical simulations of a steady, turbulent, nitrogen-diluted hydrogen-air diffusion flame, and the results predicted by the model are in reasonable agreement to experimental data for temperature, major species mass fractions, OH, and NO mole fractions.
Abstract: The unsteady flamelet model is applied in numerical simulations of a steady, turbulent, nitrogen-diluted hydrogen-air diffusion flame. An unsteady flamelet is solved interactively with a CFD solver for the turbulent flow and the mixture fraction field. Transient effects occurring in steady jet diffusion flames are discussed in terms of the relevant timescales. It is shown that radiation can be neglected and that the flame structure is hardly influenced by transient effects for the present case. However, for predictions of slow processes, like the formation of NO, unsteady effects have to be considered. The results predicted by the model are in reasonable agreement to experimental data for temperature, major species mass fractions, OH, and NO mole fractions. On the contrary, the use of steady flamelet libraries yields good results for flame structure and even OH concentrations, but NO is overpredicted by an order of magnitude. However, reasonably well-predicted NO concentrations can also be obtained by solving an unsteady flamelet as a postprocessing mode.
290 citations
TL;DR: In this article, 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 were investigated.
256 citations
TL;DR: In this paper, a 2D LIF and Rayleigh imaging system was used to study the structures of OH, CH, NO, and temperature distributions in the flame and a comparison between two different CH detection schemes was presented.
Abstract: /H2/N2 diffusion flame. Important aspects of the measuring technique, such as accuracy, cross talk between different Raman bands, and the correction procedure for background from laser-induced fluorescence are discussed. In addition, a 2D LIF and Rayleigh imaging system were used to study the structures of OH, CH, NO, and temperature distributions in the flame. A comparison between two different CH detection schemes is presented. A main goal of the investigations was a detailed and accurate characterization of the investigated flame as well as the study of experimental techniques. Joint pdfs of the temperature and major species concentrations were determined at nearly 100 measuring locations covering the complete flame. Parts of the results are presented in the paper in order to discuss effects of differential diffusion, flame extinction, and interaction between flow field and chemistry. The measured data sets which are available on the Internet are well suited for testing and validating mathematical flame models.
216 citations
TL;DR: 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 coflow and an outer co-flow of air as discussed by the authors.
178 citations
01 Jan 1998
TL;DR: In this article, the results of two-dimensional numerical computations of turbulent methane flames using detailed and reduced chemistry are analyzed in the context of a new theory for premixed turbulent combustion for high turbulence intensity.
Abstract: Results of two-dimensional numerical computations of turbulent methane flames using detailed and reduced chemistry are analyzed in the context of a new theory for premixed turbulent combustion for high turbulence intensity. This theory defines the thin reaction zones regime, where the Kolmogorov scale is smaller than the preheat zone thickness, but larger than the reaction zone thickness. The two numerical computations considered in this paper fall clearly within this regime. A lean and a stoichiometric flame are considered. The former is characterized by a large ratio of the turbulence intensity to the laminar burning velocity and the latter by a smaller value of that ratio.
168 citations
TL;DR: The morphology of flame-generated silica and titania aggregates is characterized by small-angle X-ray scattering (SAXS) as mentioned in this paper, which is composed of ramified aggregates of nanoscale primary particles.
Abstract: The morphology of flame-generated silica and titania aggregates is characterized by small-angle X-ray scattering (SAXS). Nearly all these powders display mass-fractal morphologies, which are composed of ramified aggregates of nanoscale primary particles. Primary particle size, aggregate size, fractal dimension, and specific surface area are obtained from this analysis. The mass-fractal dimension varies from 2.5 to 1.6 for flame generated silica and titania aggregates in single and double diffusion flame reactors. However, titania powders made in a single diffusion flame reactor appear as nonaggregates and nonfractals. Silica powders synthesized with an imposed electric field in a laminar, premixed flame reactor are mass fractals with narrowly confined fractal dimensions from 1.5 to 1.9 regardless of aggregate size.
153 citations
TL;DR: Results are shown for a laminar ethene diffusion flame that demonstrate that concurring results are obtained for various laser irradiances, detection characteristics, and times of observation.
Abstract: A detailed analysis of various factors that influence the accuracy of time-resolved laser-induced incandescence for the determination of primary soot particles is given. As the technique relies on the measurement of the signal ratio at two detection times of the enhanced thermal radiation after an intense laser pulse, guidelines are presented for a suitable choice of detection times to minimize statistical uncertainty. An error analysis is presented for the issues of laser energy absorption, vaporization, heat conduction, and signal detection. Results are shown for a laminar ethene diffusion flame that demonstrate that concurring results are obtained for various laser irradiances, detection characteristics, and times of observation.
149 citations
01 Jan 1998
TL;DR: In this article, the results of previous combined numerical and experimental investigations of an axisymmetric laminar diffusion flame in which difference Raman spectroscopy, laser-induced fluorescence (LIF), and a multidimensional flame model were used to generate profiles of the temperature and major and minor species.
Abstract: In this study, we extend the results of previous combined numerical and experimental investigations of an axisymmetric laminar diffusion flame in which difference Raman spectroscopy, laser-induced fluorescence (LIF), and a multidimensional flame model were used to generate profiles of the temperature and major and minor species. A procedure is outlined by which the number densities of ground-state CH (X(sup 2)II) excited-state CH (A(sup 2)Delta, denoted CH*), and excited-state OH (A(sup 2)Sigma, denoted OH*) are measured and modeled. CH* and OH* number densities are deconvoluted from line-of-sight flame-emission measurements. Ground-state CH is measured using linear LIF. The computations are done with GRI Mech 2.11 as well as an alternate hydrocarbon mechanism. In both cases, additional reactions for the production and consumption of CH* and OH* are added from recent kinetic studies. Collisional quenching and spontaneous emission are responsible for the de-excitation of the excited-state radicals. As with our previous investigations, GRI Mech 2.11 continues to produce very good agreement with the overall flame length observed in the experiments, while significantly under predicting the flame lift-off height. The alternate kinetic scheme is much more accurate in predicting lift-off height but overpredicts the over-all flame length. Ground-state CH profiles predicted with GRI Mech 2.11 are in excellent agreement with the corresponding measurements, regarding both spatial distribution and absolute concentration (measured at 4 ppm) of the CH radical. Calculations of the excited-state species show reasonable agreement with the measurements as far as spatial distribution and overall characteristics are concerned. For OH*, the measured peak mole fraction, 1.3 x 10(exp -8), compared well with computed peaks, while the measured peak level for CH*, 2 x 10(exp -9), was severely underpredicted by both kinetic schemes, indicating that the formation and destruction kinetics associated with excited-state species in flames require further research.
TL;DR: In this paper, the structure and propagation of a methanol (CH3OH)-air triple flame were studied using direct numerical simulations (DNS) and a mixture fraction-temperature phase plane description of the triple flame structure was proposed to highlight some interesting features in partially premixed combustion.
TL;DR: In this article, the features of the multiwavelength emission technique for the measurement of soot volume fraction and temperature in an ethylene diffusion flame have been investigated, and good agreement was found between the two techniques.
TL;DR: In this article, the extinction of counterflow diffusion flames of air and methane diluted with nitrogen was studied by drop tower experiments and numerical calculation using detailed chemistry and transport properties, and the mechanism of extinction at low stretch rates was radiative heat loss from the flame zone.
TL;DR: In this article, a closed loop active control system was employed to suppress the thermoacoustic pressure oscillations and to reduce NO x emissions in an experimental low-emission swirl stabilized combustor, where the acoustic boundary conditions were modified to obtain combustion instability.
Abstract: Thermoacoustic instability was investigated and controlled in an experimental low-emission swirl stabilized combustor, in which the acoustic boundary conditions were modified to obtain combustion instability. Several axisymmetric and helical unstable modes were identified for fully premixed and partially premixed/diffusion combustion. These unstable modes were associated with flow instabilities related to the recirculation region on the combustor axis and shear layer instabilities at the sudden expansion (dump plane). The spatial locations of the intense combustion regions associated with the different unstable modes were visualized by phase locked images of OH chemiluminescence. The axisymmetric mode showed large variation of the heat release during one cycle, while the helical modes showed variations in the radial location of maximal heat release. A closed loop active control system was employed to suppress the thermoacoustic pressure oscillations and to reduce NO x emissions. Microphone and OH emission...
TL;DR: The branching ratio of the yield of HONO and NO was a function of the sampling spot at which the soot was collected within a diffusion flame and ranged from 23 to 2.2 with increasing distance from the flame base and thus age as mentioned in this paper.
Abstract: Uptake experiments at ambient temperature of NO 2 on ethylene. acetylene and toluene soot resulted in significant amounts of HONO. The initial uptake coefficient γ0 of NO 2 on ethylene soot decreased from 0.12 to 0.03 with increasing NO 2 concentration in the range 8x10 10 to 1.4x10 13 molecule s -1 . The HONO yields relative to NO 2 taken up were in the range 50 to 93% and were consistent with a mechanism in which either soot or hydrogen contained in it was the reducing agent. The branching ratio of the yield of HONO and NO was a function of the sampling spot at which the soot was collected within a diffusion flame and ranged from 23 to 2.2 with increasing distance from the flame base and thus age. The total amount of HONO generated in one uptake experiment was 1.2x10 16 molecules per mg of soot. Heating cycles bring back the ability of soot to generate HONO, but at diminishing yields.
TL;DR: In this article, the velocity field in the stabilization region of lifted, turbulent CH 4 -jet flames over a range of Reynolds numbers from 7000 to 19,500 was studied and it was shown that the average velocities at the flame base are considerably below the turbulent flame speeds derived from previous studies and show a dependence on the Reynolds number.
01 Jan 1998
TL;DR: In this article, the effect of additives on sooting laminar coaxial diffusion flames has been reexamined and it was shown that dilution effects of inerts should override the induced temperature effect postulated by Axelbaum and Law.
Abstract: Soot volume fractions and smoke heights of hydrocarbon fuels are postulated to be determined by the extent of incipient particle growth within coannular diffusion flames. The extent is determined by the distance between the incipient particle and the diffusion flame isotherms. With this description of hydrocarbon diffusion flames, the reported experimental results of Glassman, Gulder, and Santoro and their coworkers on the effect of additives on sooting laminar coaxial diffusion flames have been reexamined. The concept that dilution effects of inerts should override the induced temperature effect postulated by Axelbaum and Law is substantiated by the analysis developed, which shows that temperature effects cause a logarithmic variation in soot volume fraction, whereas dilution has a direct proportionality. Although original interpretation of reported results have led some of these investigators to conclude that certain additives such as O 2 and CS 2 could chemically inhibit soot formation, arguments are presented that the effects noted are due to the temperature field and the altering of the thermal diffusivity of the fuel jet. Gulder's important and significant results on the effect of H 2 inhibition on ethene's sooting tendency is explained, as are Santoro's results showing that propyl alcohol addition to an ethene fuel jet increases soot output. Considering the buoyancy induced fuel jet velocity permits one to predict that soot volume fraction (SVF) should vary as P 1,31 and that the volume fraction should be less than that encountered in microgravity. Fuel jet flames are predicted to be wider in microgravity and the SVF pressure dependency close to P 2 .
TL;DR: In this paper, the principal variable is the equivalence ratio, φ, while the swirl number and mean axial entry velocity remain constant, and the experiments show unstable combustion with low-frequency oscillations between the two states.
TL;DR: In this article, a comparison between laser-induced fluorescence and laser induced incandescence (LII) images and axial intensity profiles derived from these images of a laminar ethylene-air diffusion flame distinguish the polycyclic aromatic hydrocarbon (PAH) and soot containing regions along the axial flow streamline.
01 Jan 1998
TL;DR: In this article, a sooting, ethylene coflow diffusion flame has been studied both experimentally and computationally, where both probe (thermocouple and gas-sampling techniques) and optical diagnostic methods (Rayleigh scattering and laser-induced incandescence) are used to measure the temperature, gas species and soot volume fractions.
Abstract: A sooting, ethylene coflow diffusion flame has been studied both experimentally and computationally. The fuel is diluted with nitrogen and the flame is slightly fifted to minimize the effects of the burner. Both probe (thermocouple and gas-sampling techniques) and optical diagnostic methods (Rayleigh scattering and laser-induced incandescence) are used to measure the temperature, gas species, and soot volume fractions. A detailed soot growth model in which the equations for particle production are coupled to the flow and gaseous species conservation equations has been used to investigate soot formation in the flame. The two-dimensional system couples detailed transport and finite-rate chemistry in the gas phase with the aerosol equations in the sectional representation. The formulation includes detailed treatment of the transport, inception, surface growth, oxidation, and coalescence of soot particulates. Effects of thermal radiation and particle scrubbing of gas-phase growth and oxidation species are also included. Predictions and measurements of temperature, soot volume fractions, and selected species are compared over a range of heights and as a function of radius. The formation of benzene is primarily controlled by the recombination of propargyl radicals, and benzene production rates are found to limit the rate of inception, as well as the net rate of soot growth. The model predicted soot volume fractions well along the wings of the flame but underpredicted soot volume fractions by a factor of four along the centerline. Oxidation of particulates is dominated by reactions with hydroxyl radicals that attain levels approximately ten times higher than calculated equilibrium levels. Gas cooling effects due to radiative loss are shown to have a very significant effect on predicted temperatures.
Patent•
04 Dec 1998
TL;DR: In this article, the authors measured the amplitudes of frequency bands that are indicative of an efficient combustion process, such as those that increase when the flame temperature increases, or decrease when the temperature increases.
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.
TL;DR: Comparison of LII intensity profiles with soot volume fractions derived by light extinction validates LII for quantitative determination of f(v) upon calibration for laser fluences ranging from 0.09 to 0.73 J/cm(2).
Abstract: Assumptions of theoretical laser-induced incandescence (LII) models along with possible effects of high-intensity laser light on soot aggregates and the constituent primary particles are discussed in relation to selection of excitation laser fluence. Ex situ visualization of laser-heated soot by use of transmission electron microscopy reveals significant morphological changes (graphitization) induced by pulsed laser heating. Pulsed laser transmission measurements within a premixed laminar sooting flame suggest that soot vaporization occurs for laser fluences greater than 0.5 J/cm(2) at 1064 nm. Radial LII intensity profiles at different axial heights in a laminar ethylene gas jet diffusion flame reveal a wide range of signal levels depending on the laser fluence that is varied over an eight fold range. Results of double-pulse excitation experiments in which a second laser pulse heats in situ the same soot that was heated by a prior laser pulse are detailed. These two-pulse measurements suggest varying degrees of soot structural change for fluences below and above a vaporization threshold of 0.5 J/cm(2) at 1064 nm. Normalization of the radial-resolved LII signals based on integrated intensities, however, yields self-similar profiles. The self-similarity suggests robustness of LII for accurate relative measurement of soot volume fraction despite the morphological changes induced in the soot, variations in soot aggregate and primary particle size, and local gas temperature. Comparison of LII intensity profiles with soot volume fractions (f(v)) derived by light extinction validates LII for quantitative determination of f(v) upon calibration for laser fluences ranging from 0.09 to 0.73 J/cm(2).
TL;DR: In this paper, the soot volume fraction and polycyclic aromatic hydrocarbon (PAH) concentration are increased when a small amount of propane is added to ethylene diffusion flames, and the results of soot number density and particle size in low-temperature soot growth region also support continuous soot inception via PAH agglomeration.
01 Jan 1998
TL;DR: In this article, a common view of the stabilizing mechanism of methane diffusion flames, including jet and porous flat-plate burner flames, is presented, using a time-dependent, implicit, third-order accurate numerical model, including semidetailed chemical kinetics and buoyancy effects.
Abstract: A common view of the stabilizing mechanism of methane diffusion flames, including jet and porous flat-plate burner flames, is presented. Two-color particle-imaging velocimetry measured the velocity field in the stabilizing region of jet diffusion flames under near-lifting conditions. Computations using a time-dependent, implicit, third-order accurate numerical model, including semidetailed chemical kinetics and buoyancy effects, revealed the detailed structures of the vertical jet diffusion flames and flat-plate burner flames with different orientations of the plate surface and fuel injection. The numerical results are in a good agreement with the measurements in the flame base locations and surrounding velocity fields. In the calculations of both classes of flames, the highest reactivity spot (reaction kernel) with peak rates of heat release, oxygen consumption, and water vapor production, was formed in the relatively low-temperature (
01 Jan 1998
TL;DR: In this paper, the effects of partial premixing on the turbulent flame topology and the overall mean reaction rate were investigated in a staged combustion system with a first stage corresponding to a propagating premixed flame front followed by a second stage with multiple post-diffusion flames that burn the remaining excess fuel and excess oxidizer.
Abstract: Stratified spark-ignition engines that use direct fuel injection into the combustion chamber feature both small- and large-scale spatial variations in unburned mixture composition In these configurations, the spark-ignited turbulent flame propagates into a mixture with variable equivalence ratio Under lean-rich conditions, the reaction zone can be described as a staged combustion system with a first stage corresponding to a propagating premixed flame front followed by a second stage corresponding to multiple post-diffusion flames that burn the remaining excess fuel and excess oxidizer Direct numerical simulations (DNS) are used in the present study to bring basic information onto the effects of partial premixing both on the turbulent flame topology and the overall mean reaction rate and to determine if and how flamelet combustion models should be modified to account for these effects The numerical configuration corresponds to three-dimensional partially premixed flames propagating into a temporally decaying turbulent flow The simulations use different degrees of scalar inhomogeneity around mean stoichiometric conditions It is found that while partial premixing has a negligible contribution to the premixed flame surface wrinkling, it has a net negative impact on the overall mean premixed reaction rate This effect is related to the reduced values of mean flamelet mass burning rate per unit flame surface area observed in partially premixed configurations compared to perfectly premixed flames
TL;DR: In this paper, the authors investigated the structure of turbulent spray diffusion flames by means of numerical simulations and developed criteria for the implementation of laminar spray flames for use in turbulent flame computations.
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...
TL;DR: In this paper, a spatially varying straining flow was created using an opposed slot-jet burner with slightly nonparallel jet exits, and the most significant observation was that steady flame edges could be created where the flame would exist in the low-strain region but would be extinguished in the high-straining region.
TL;DR: Local rectangular refinement (LRR) solution-adaptive gridding method produces robust unstructured rectangular grids, utilizes novel multiple-scale finite-difference discretizations, and incorporates a damped modified Newton's method for simultaneously solving systems of governing elliptic PDEs as discussed by the authors.
Abstract: Within realistic combustion devices, physical quantities may change by an order of magnitude over an extremely thin flamefront, while remaining nearly unchanged throughout large areas nearby. Such behaviour dictates the use of adaptive numerical methods. The recently developed local rectangular refinement (LRR) solution-adaptive gridding method produces robust unstructured rectangular grids, utilizes novel multiple-scale finite-difference discretizations, and incorporates a damped modified Newton's method for simultaneously solving systems of governing elliptic PDEs. Here, the LRR method is applied to two axisymmetric laminar flames: a premixed Bunsen flame with one-step chemistry and a diffusion flame employing various complex chemical mechanisms. The Bunsen flame's position is highly dependent upon grid spacing, especially on coarse grids; it stabilizes only with adequate refinement. The diffusion flame results show excellent agreement with experimental data for flame structure, temperature and major sp...
01 Jan 1998
TL;DR: In this paper, a model for non-premixed turbulent combustion is described for subsonic methane jet flames and, on the basis of presented evidence, is based on combustion in strained premixed laminar flamelets.
Abstract: A model is described for non-premixed turbulent combustion. It is applied to subsonic methane jet flames and, on the basis of presented evidence, is based on combustion in strained premixed laminar flamelets. When the fuel discharges into the air, the shear-generated aerodynamic strain rate is initially sufficiently high to quench both diffusion and premixed flamelets. Farther downstream, the strain rate relaxes and premixed burning ensues. Data are drawn from models of premixed laminar flames and include heat-release rate-temperature profiles, Markstein numbers, and positive and negative stretch rates for flame quenching. A conditional probability density function (PDF) for reactedness is introduced and Reynolds stress, second-order closure is adopted. Predicted liftoff heights and blowoff velocities are in good agreement with available ineasurements, up to quite large diameters. Dimensionless correlations of these are presented as are detailed flow and combustion structures for the base of the flame. The contributions of different mixture fraction bands to the mean heat release are computed. The relative contribution of rich burning decreases with a decrease in pipe diameter and an increase in flow velocity. An increase in flow velocity results in leaner combustion and an increase in flame stretch rate that eventually produces extinction at blowoff. The implications of this for modeling are discussed.