Showing papers on "Diffusion flame published in 1986"
TL;DR: In this paper, the structure of a two-dimensional, axisymmetric, laminar methane-air diffusion flame with detailed transport and finite rate chemical kinetics is investigated, where a cylindrical fuel stream is surrounded by a coflowing oxidizer jet.
Abstract: We investigate computationally the structure of a two-dimensional, axisymmetric, laminar methane-air diffusion flame with detailed transport and finite rate chemical kinetics. We consider flames in which a cylindrical fuel stream is surrounded by a coflowing oxidizer jet. Unlike some models in which diffusion in the axial direction is neglected, we treat the fully elliptic problem. A discrete solution is obtained on a two-dimensional grid by combining a steady-state and a time-dependent solution method. A time- dependent approach is used to help obtain a converged numerical solution on an initial coarse grid using a flame sheet starting estimate. Grid points are then inserted adaptively and Newton’s method is used to complete the problem.
195 citations
TL;DR: In this paper, a theoretical analysis of diffusion flame extinction in the stagnation point region of a condensed fuel has been made including radiative heat loss from the fuel surface, and the extinction boundary consists of a blowoff and a radiative extinction branch.
187 citations
TL;DR: In this article, the extinction limits of counterflow diffusion flames burning methane and propane were investigated using activation energy asymptotic theories developed previously, and the theoretical predictions were compared with experimental results, and overall chemical kinetic rate parameters characterizing the gas phase oxidation of a diffusion flame were derived.
176 citations
TL;DR: In this paper, the major species production rates were measured in a small-scale two-layer environment (one meter diameter hood, 13 cm and 19 cm burners) using propane as a fuel.
119 citations
TL;DR: In this paper, a laminar diffusion flame with a single plane vortex and with a stretched line vortex is examined with the aim of determining the flame configuration and the augmentation to the reactant consumption rate resulting from the interaction.
Abstract: The time dependent interaction of a laminar diffusion flame with a single plane vortex and with a stretched line vortex is examined with the aim of determining the flame configuration and the augmentation to the reactant consumption rate resulting from the interaction. Elements
of the resulting curved flame sheets behave essentially as isolated flames until the neighboring flame sheets become so closely spaced that they interact and consume the intervening reactant. This process creates a core of combustion products with external isolated flame surfaces. The augmentation of the reactant consumption rate results both from the local straining of the flame in its own plane and from the overall increase in flame surface area. Three examples are treated in detail. The first is the plane problem in which an initially straight flame is distorted by a vortex. In the second, the situation is similar except that the problem is expanded to three dimensions
and the vortex line is being stretched along its own axis. Finally, the effects of the density change resulting from the heat release are examined.
105 citations
TL;DR: In this article, a flow visualization study of the near-nozzle regions of a cold jet, an attached flame, and a lifted flame of propane jet exiting a contoured nozzle with uniform velocity profile in quiescent air is presented.
Abstract: A flow visualization study of the near-nozzle regions of a cold jet, an attached flame, and a lifted flame of propane jet exiting a contoured nozzle with uniform velocity profile in quiescent air is presented. The observed flowfields and celerities of the coherent structures in the three cases are compared. The results show that com- bustion reactions occur away from both jet boundary and cores of coherent structures. The presence of flame retards the growth of the coherent structures and increases their celerity substantially. HE interaction of flames and flow structure in combus- tion systems has been the subject of several investiagations in the past. Many of these studies were concerned with pre- mixed flames. In recent years, however, considerable effort has been devoted to understand the flame/flow interaction in reacting plane shear layers1"3 and axisymmetric gas jets.4"7 Such studies are motivated by the necessity to understand the roles of coherent structures in the mixing rates.8"9 This infor- mation is needed to improve the burning characteristics of dif- fusion flames and to develop theoretical models for their prediction. A review of the uses of coherent structures was presented recently by Coles.10 An understanding of the detailed flow structure and its interaction with reaction zones is also crucial to unfold the stability mechanism of diffusion flames, including liftoff and blowout phenomena, which has been a topic of controversy in recent years.11'12 Here, we present some results from a flow visualization study in the near-nozzle region of propane gas jet flames where the flowfield is essen- tially an axisymmetric shear layer. The results not only con- firm the presence of large-scale structures in the flames but also reveal some characteristic features of their behavior in the presence of combustion. We focused our attention on the near-nozzle region in order to understand 1) the changes in flow structure of cold jets caused by the presence of a surrounding burner-attached flame, and 2) the liftoff and reattachment processes of flames.
88 citations
TL;DR: In this article, photographic methods are used to measure flame speeds in flowing mixtures of fuel props and air at atmospheric pressure, and the results show that flame propagation cannot occur in mixtures containing mean drop sizes larger than 300 to 400 microns, depending on the fuel type.
81 citations
TL;DR: In this article, the effect of small additions of oxygen to the fuel on formation of soot in methane-air diffusion flames was studied over a range of flow rates and of burner diameters.
Abstract: The effect of small additions of oxygen to the fuel on formation of soot in methane-air diffusion flames was studied over a range of flow rates and of burner diameters. The flames studied were shorter than those of previous studies, purely blue or blue and yellow without soot escape. Heights of various distinctive features were measured, and composition and temperature profiles were obtained; the distinctive features include.onset and termination of visible emission of radiation and deposition of material on a quartz filament inserted into the flame. The results indicate negligible influences of oxygen addition and thereby suggest that ions from the primary mechanism CH+Orarr;CHO++e- are unimportant in soot formation in these flames. A simplified one-step kinetic model accounting for buoyancy and momentum was developed and employed to obtain estimates of overall rate parameters for flame attributes related to soot formation.
77 citations
TL;DR: In this article, a theoretical model of a laminar diffusion flame at the leading edge of a fuel plate in a forced convective flow is presented and solved numerically to study the flame stabilization and blowoff phenomena.
Abstract: A theoretical model of a laminar diffusion flame at the leading edge of a fuel plate in a forced convective flow is presented and solved numerically to study the flame stabilization and blowoff phenomena. The system of governing equations consists of the two-dimensional Navier-Stokes momentum, energy and species equations with a one-step overall chemical reaction and second-order, finite rate Arrhenius kinetics. The computation is performed over a wide range of Damkohler numbers. For large Damkohler numbers, envelope flames are found to exist where the computed fuel evaporation rate, the flame stand-off distance and the velocity profiles show certain similitude. As the Damkohler number is lowered, a transition to open-tip flame takes place where the flame becomes stabilized on the sides of the fuel plate. Further decreasing of the Damkohler number pushes the diffusion flame downstream out of the leading edge region. In this paper, the flame structures of the envelope and the open-tip flames are p...
72 citations
TL;DR: In this paper, the soot particle inception rate in an atmospheric pressure premixed flat flame was measured and an aerosol dynamics model was developed to allow interpretation of the optical data, and an efficient technique was found to solve the differential equations.
59 citations
TL;DR: In this paper, high-speed photography of a windowed two-dimensional SFRJ combustor was used to study the combustion and flow properties of a boron-containing solid-fuel ramjet.
Abstract: Combustion phenomena of highly metallized, boron containing, solid fuels in solid-fuel ramjets (SFRJ) were studied by means of high-speed photography using a windowed two-dimensional SFRJ combustor. The ex- periments indicated the existence of a gas-phase diffusion flame of the volatile fuel ingredients within the bound- ary layer above the fuel surface. It was also revealed that material is often emitted from the surface in the form of large pieces arid segments. Flow impingement on the surface may cause surface heating and glowing by chemical reactions, which promote the high-speed ejection of hot particles and the emittance and disintegration of large glowing segments and pieces from the fuel surface layer into the gas stream. HERMOCHEMICAL evaluation of fuel candidates for advanced solid-fuel ramjet (SFRJ) systems reveals the high energy and energy density of some metals, especially boron and boron compounds.1 Compared with the commonly used hydrocarbon (HC) fuels, boron exhibits remarkable theoretical heat of combustion per unit mass (about 30% higher than HCs) and per unit volume (almost three times that of HC). As an ingredient of solid fuels for ramjets, metals, in- cluding elemental boron or boron compounds (e.g., boron carbide), are usually introduced as fine powders into a matrix of polymeric material. In spite of the promising potential, the practical use of solid-fuel formulations highly loaded with metal particles can present severe problems associated with complex burning phenomena, which have major effects on the energy genera- tion process within the combustor and may lead to poor combustion efficiencies and low motor performance. Experimental investigations and modeling of the combus- tion of nonmetallized fuels in SFRJ combustors have pro- vided a good description of the main combustion and flow characteristics.2'3 See Fig. la. The foremost region of the combustor is characterized by a separated recirculation flow zone generated by the inlet step used for flame stabilization. Downstream of the reattachmeht zone, often along most of the fuel grain length, a gas-phase diffusion flame, typically a narrow "flame sheet," is established within the turbulent boundary layer that develops over the condensed fuel sur- face.4 Fuel vapors diffuse from the decomposing fuel beneath the flame, while oxygen is transported to the flame from the core stream along the combustor centerline. Heat feedback from the flame to the condensed fuel determines the fuel regression rate and completes the combustion cycle. The situations encountered when using highly metallized fuels are somewhat different (Fig. Ib). Although the flow characteristics are similar to those existing in the combustor employing nonmetallized fuels, the combustion phenomena are not the same. Metal particles tend to accumulate and coalesce at the condensed fuel surface prior to their ejection into the gas stream. The result may be the formation of
TL;DR: In this article, a study of the structure and radiation properties of round turbulent carbon monoxide/air diffusion flames is described, where measurements of mean and fluctuating streamwise velocity and mean temperatures, concentrations of major gas species, spectral radiation intensities and radiant heat fluxes are made.
Abstract: A study of the structure and radiation properties of round turbulent carbon monoxide/air diffusion flames is described. Measurements of mean and fluctuating streamwise velocity and mean temperatures, concentrations of major gas species, spectral radiation intensities, and radiant heat fluxes were made. The measurements were used to evaluate predictions based on the laminar flamelet concept and narrow-band radiation models, both ignoring (using mean properties) and considering (using a stochastic method) the effects of turbulence/ra diation interactions. State relationships were found by correlating auxiliary measurements in laminar flames. Structure and radiation predictions were reasonably good for present test conditions. The effects of turbulence/ra diation interactions were small for these reactants (increasing spectal intensities ca. 10%) since radiation properties vary slowly with mixture fraction near stoichiometric conditions.
TL;DR: In this article, a model of laminar round diffusion flames is developed which quantitatively relates the maximum soot volume fraction and soot yield in the flame to the smoke point fuel flow rate.
TL;DR: In this article, the stability of a double-swirl burner was investigated in swirling air streams with both hydrogen and methane fuels, and it was found that the stability depended on the swirl intensity of both the fuel jet and the air stream.
TL;DR: In this paper, a theoretical study of an opposed jet spray diffusion flame is presented, where the fuel is introduced into the system in the form of liquid droplets suspended homogeneously in a nitrogen jet stream opposed to an air jet stream of the same hydrodynamic properties.
Abstract: Abstract— A theoretical study of an opposed jet spray diffusion flame is presented. The fuel is introduced into the system in the form of liquid droplets suspended homogeneously in a nitrogen jet stream opposed to an air jet stream of the same hydrodynamic properties. An initial monodisperse droplet size distribution is considered and it is assumed that the droplets follow the flow streamlines. Tambour's (1984) Eulerian "sectional" approach is employed to account for the downstream droplet size changes due to vaporization. Since the monodisperse distribution does not persist, a single "monosectional" equation for quasi-monodisperse sprays is used. The effects of (i) initial droplet and vapor mass fractions, (ii) initial droplet size, and (iii) droplet rate of vaporization, on flame location, temperature and species concentration distributions, for an n-decane fuel spray, are analyzed.
TL;DR: In this article, a two dimensional flame front visualisation technique based on Mie scattering from particles dispersed in the combusting mixture was developed, which was used in an I.C. engine simulator to study the freely propagating flame in premixed combustion.
Abstract: A two dimensional flame front visualisation technique, based on Mie scattering from particles dispersed in the combusting mixture, has been developed. The technique was used in an I.C. engine simulator to study the freely propagating flame in premixed combustion. It is shown that flame front structures can be resolved for scales as low as 2x10/sup -4/ m. These scales were observed at 1500 RPM where velocity fluctuations are known to be on the order of 6 m/s. For lean propane combustion. peninsulas and pockets of unburned mixture are observed in the postflame regions at 600 RPM. Higher turbulence levels increase the global flame front area by creating flame front corrugations of various length scales. Evidence of flame front wrinkles having sizes comparable to previously reported flame thickness in engines suggests that I.C. engine models should take into account the interaction between the velocity field and the detailed structure of the diffusive-reactive flame front zone.
TL;DR: In this article, the extinction characteristics of two interacting premixed flames in counterflow configuration were investigated, and conditions for the existence of superadiabatic flames within mixtures outside of the conventional flammability limit compositions were established, and practical implications of flame interaction for combustion in inhomogeneous mixtures were discussed.
Abstract: The framework of large activation energy asymptotics is used in an investigation of the extinction characteristics of two interacting premixed flames in counterflow configuration, analyzing the interactive combustion modes of two lean premixed flames, two rich premixed flames, and one of each type of flame separated by a diffusion flame. Regions corresponding to symbiotic combustion of two lean or two rich premixed flames exist in which either flame will be extinguished in the absence of the other. Conditions for the existence of superadiabatic flames within mixtures outside of the conventional flammability limit compositions are established, and practical implications of flame interaction for combustion in inhomogeneous mixtures are discussed.
TL;DR: In this paper, the conserved-scalar formalism, in conjunction with the laminar flamelet concept, was used to estimate flame structure and radiative heat fluxes.
Abstract: Recent data from large-scale turbulent natural gas/air diffusion flames (135-210 MW) were used to evaluate analysis of flame structure and radiation properties. The conserved-scalar formalism, in conjunction with the laminar flamelet concept, was used to estimate flame structure. The discrete-transfer method, in conjunction with a narrow-band radiation model, was used to predict radiative heat fluxes. The narrow-band model considered the nonluminous gas bands of water vapor, carbon dioxide, methane and carbon monoxide in the 1OOO-6000 nm wavelength range. Structure predictions were encouraging, with discrepancies for mean temperatures (ca 200 K in the hottest portions of the flames) comparable to experimental uncertainties, due to thermocouple errors, flame disturbances from ambient winds and lifting and external expansion effects near the injector. Radiative heat flux predictions were also reasonably good, e.g. predictions based on mean scalar properties were generally 15% lower than the measurements. The findings also suggest that continuum radiation from soot is negligible for these flames.
TL;DR: In this paper, the Froude number limit was investigated for gas well blow-out fire applications, and the authors derived the functional dependence of decreasing radiative fraction with increasing jet velocity.
Abstract: For water spray suppression of gas well blow-out fire applications, reasonably large scale (7 MW) subsonic methane diffusion flames have been investigated near the Froude number limit. Characteristics of this limit include constant flame height, increasing lift-off height, and decreasing radiative fraction. Flame blow-off has been observed with pipe sizes up to 30 mm diameter. Flame and lift-off heights, centerline temperatures, and incident radiative flux to nearby targets have been measured with and without water spray suppressant. Using a simple participating cylindrical flame model an effective gray absorption coefficient approaching 0.2 m−1 was determined at the blow-off limit. Away from the limit, this number rises rapidly as does flame temperature decrease with decreasing jet velocity, i.e., luminosity increases as temperature goes down. The derived shape of the functional dependence of decreasing radiative fraction with increasing jet Froude number in the limit is consistent with small scale liter...
TL;DR: In this paper, a monodisperse stream of methanol drops injected along the axis of a turbulent, methane-fueled diffusion flame burning in still air is studied experimentally and theoretically, in order to determine mean and fluctuating phase velocities, mean drop number flux, drop size distributions, and mean gas phase temperature.
Abstract: A monodisperse stream of methanol drops injected along the axis of a turbulent, methane-fueled diffusion flame burning in still air is studied experimentally and theoretically, in order to determine mean and fluctuating phase velocities, mean drop number flux, drop size distributions, and mean gas phase temperature. Measured values were compared with the predictions of two separated flow analyses, namely deterministic separated flow and stochastic separated flow. The stochastic analysis yielded the best agreement with measurements, due to its providing for the turbulent dispersion of drops.
TL;DR: In this article, a paraffin candle diffusion flame was collected on a quartz plate and exposed to oxygen atoms in a flow tube apparatus under carefully controlled conditions, and the soot adsorbed up to 25 % by weight oxygen.
TL;DR: In this paper, the conditional mean and rms values provided a data base for flame intermittency models, and correlations obtained by Effelsberg and Peters using a three-zone (laminar, turbulent, and superlayer) model indicate that the superlayer can contribute up to 60°7o of the pdf.
Abstract: Pulsed Raman scattering is used to determine zonal averages, intermittency, and conditional probability density functions (pdFs) for temperature, density, conserved scalar, and molecular composition in a turbulent, hydrogen jet diffusion flame. The conditional mean and rms values provide a data base for flame intermittency models. Both conventional and Favre-averaged turbulent zone pdf's of the conserved scalar are highly nonGaussian in the intermittent regions. Turbulent combustion models that assume a clipped Gaussian turbulent pdf in this region could give erroneous results for flame processes sensitive to pdf shapes such as NOX formation. Similar pdf shapes have been found in nonreacting wake flows and correlations obtained by Effelsberg and Peters using a three-zone (laminar, turbulent, and superlayer) model indicate that the superlayer can contribute up to 60°7o of the pdf.
TL;DR: In this article, the first four moments of conserved scalar probability density functions (p.d.s) measured by Raman scattering in an H2 turbulent jet diffusion flame are compared with those found by Pitts & Kashiwagi (1984) in a non-reacting CH4 jet.
Abstract: The first four moments of conserved scalar probability density functions (p.d.f.'s) measured by Raman scattering in an H2 turbulent jet diffusion flame are analysed and compared with those found by Pitts & Kashiwagi (1984) in a non-reacting CH4 jet. The measurements are in good agreement, indicating that heat release and combustion have little effect on p.d.f. shapes. However, the measured p.d.f.'s are not qualitatively similar to the simple forms often assumed in combustion modelling. A three-zone model by Effelsberg & Peters was used to separate the experimental p.d.f.'s into a delta function (non-turbulent zone), a Gaussian (turbulent zone) and the remainder (interface zone). The interface zone contributed as much as 90% of the total p.d.f. in both the H2 flame and the non-reacting CH4 jet. A physical interpretation for the existence of broad interface zones in reacting and non-reacting turbulent jet flows is suggested based upon large-scale structures.
Patent•
12 Sep 1986TL;DR: In this article, a device is presented for use in a combustion system to inhibit formation of oxides of nitrogen (NOx) by the combustion system thereby reducing NOx emissions from the combustion systems.
Abstract: A device is disclosed for use in a combustion system to inhibit formation of oxides of nitrogen (NOx) by the combustion system thereby reducing NOx emissions from the combustion system. The device is made of a material, such as stainless steel, which is positioned at the periphery of a combustion flame produced by a burner which is part of the combustion system, to temper the combustion flame by absorbing thermal energy from the combustion flame. The device sufficiently tempers the combustion flame to limit peak combustion flame temperatures and residence times at these peak combustion flame temperatures to levels which inhibit formation of oxides of nitrogen while allowing substantially complete combustion of the fuel supplied to the burner.
TL;DR: In this paper, Batchelor's theory of turbulent premixed flames is presented, which requires a knowledge only of the turbulent kinetic energy and its rate of dissipation to describe the turbulence, and only of laminar burning velocity to describe chemical system.
TL;DR: In this paper, the effect of fuel hydrocarbon structure on soot emissions using a carbon-14 isotope tracer technique was studied using a laminar wick diffusion flame generated radioactive soot from a #2 diesel fuel containing 14Chydrocarbons.
TL;DR: Instrumentation et discussion des resultats de mesure des temperatures dans une flamme de diffusion stabilisee par un corps a arete vive as mentioned in this paper, instrumentation and discussion.
Abstract: Instrumentation et discussion des resultats de mesure des temperatures dans une flamme de diffusion stabilisee par un corps a arete vive
TL;DR: In this paper, activation energy asymptotics are employed to investigate regimes of combustion of steady, planar, adiabatic deflagrations involving a four-step kinetic mechanism modeled after that of the ozone decomposition flame.
TL;DR: In this article, a utility burner for the production of laminar, homogenous diffusion flames, which is immune to flashbacks, is presented, because the fuel and oxidant mix on the surface of the burner rather than within the spray chamber, the flames cannot flashback.
Abstract: A general utility burner for the production of laminar, homogenous diffusion flames, which is immune to flashbacks, is presented. Because the fuel and oxidant mix on the surface of the burner rather than within the spray chamber, the flames cannot flashback. A wide variety of gas mixtures has been investigated, including oxygen, nitrous oxide, and nitric oxide as the oxidants. Any combination of fuel and oxidant can be safely burned to produced a stable, laminar, and audibly quiet flame. Flame temperatures can be varied over a wide range either by changing the fuel-oxidant ratio or by diluting the flame gases with an inert gas. In this manner, the optimum flame temperature and composition can be achieved. These burners are of general use in analytical emission, fluorescence, and photoacoustic spectrometry, as well as combustion diagnostics.
TL;DR: In this article, Favre averaged data for the validation of a new one-dimensional integral model of turbulent diffusion flame with large buoyancy effects have been carried out on the axis of a gas-fuelled burner at low Froude number.
Abstract: In order to provide confirmed experimental Favre averaged data for the validation of a new one-dimensional integral model of turbulent diffusion flame with large buoyancy effects, temperature and velocity fluctuation measurements have been carried out on the axis of a gas-fuelled burner at low Froude number. Temperature fluctuations have been measured by instantaneous compensation of the thermal inertia of fine wire thermocouples. Simultaneous measurements of temperature and velocity have pointed out that, on the burner axis, LDV results are not significantly biased toward hotter gas velocities, even if only the fuel is seeded. Results are used to calculate Favre averaged values of temperature and velocity, as well as velocity-temperature correlations.