Showing papers on "Diffusion flame published in 1989"

Soot formation in combustion processes

Abstract: Sooting tendencies of fuels are analyzed with respect to the type of experimental configuration used—per-mixed flames, normal and inverse co-annular, Wolfhard-Parker and counter-flowing diffusion flames, or shock tubes. The important effect of temperature in each type of experiment is examined. The relative tendency of various fuels to soot when consumed in pre-mixed and diffusion flames has been determined by the qualitative measurements of critical sooting equivalence ratios and smoke heights at various temperatures and quantitatively confirmed by measurements of chemical species, soot number density and volume fraction. Fuel structure has a significant effect on the sooting tendency of diffusion flames, but little influence in premixed flames. Irrespective of the fuel in diffusion flames soot inception occurs around 1400 K and is dependent somewhat on H atom diffusion. Particle burnout ceases at about 1300 K and is responsible for the smoke height. Incipient particle formation determines the volume fraction of soot and is controlled by the rate of formation of the first aromatic species containing one to two rings. The particle growth process is found to be the same regardless of flame type. A generalized chemical mechanism of soot formation irrespective of flame type or process is presented. The various routes in this mechanism support the relative sooting tendency of fuels found in both flame and shock tube experiments. The effect of oxygen added to various fuel in diffusion controlled experiments is analyzed.

Dynamics of stretched flames

Abstract: Recent advances in the understanding of the structure, propagation, and extinction of laminar flames under the influence of stretch, as manifested by the existence of flame curvature, flow nonuniformity, and flame motion, are reviewed. The emphasis is on premixed flames because of the richness and subtlety of the phenomena involved. The review distinguishes the influences of the tangential and normal velocity gradients on the flame response, both at the hydrodynamic scale and within the flame structure, and emphasizes the importance of the preferential diffusion nature of heat and mass transport, as well as the extent to which the flame can freely adjust its location in response to stretch in order to achieve complete reaction. It is then demonstrated that stretch has only minimal effect on an adiabatic, unrestrained, diffusionally-neutral flame with complete reaction in that the temperature, propagation rate, and thickness of the flame are invariant to stretch, and that stretch alone cannot extinguish such a flame. In the presence of preferential diffusion and/or when the flame movement is restrained, the response of the flame to stretch becomes more sensitive and extinction is also possible. The concept of flame stretch is applied to interpret such practical flame phenomena as flame stabilization and flame-front instability, determination of laminar flame speeds and flammability limits, concentration and temperature modifications in flame chemistry, and modeling of turbulent flames. The properties of stretched diffusion flames are then briefly discussed. The review closes with suggestions for further research.

Self-similar propagation of a free turbulent flame in mixed gas mixtures

Abstract: L. D. Landau believed that hydrodynamic instability of a flame, which he discovered theoretically [i], should in itself lead to the appearance of turbulent motion and to turbulent propagation of the combustion front already for Reynolds numbers Re = unR/v ~ 1 (u n is the normal rate of combustion and R is the radius of the visible front). Experiments show that a free spherical or cylindrical laminar flame from a weak source of ignition, in the absence of any significant perturbations (rigid wall, obstacles, buoyancy forces), does indeed become unstable, turbulent, and self-accelerating, but for Re e 103-105 .

Comparison of Soot Growth and Oxidation in Smoking and Non–Smoking Ethylene Diffusion Flames

Abstract: A probe sampling technique. which is based on the phenomenon of thermophoresis, is employed to study soot aggregate morphology within a soot emitting (smoking) coannular ethylene/air diffusion flame at atmospheric pressure. Primary particle diameters are obtained through electron microscope analysis of the soot aggregates collected from the flame environment. These observations, when combined with laser diagnostic and particle velocity measurements performed by others, result in the calculation of specific soot surface growth rates, soot surface areas and primary particle number densities along the path line exhibiting the maximum soot volume fraction throughout the growth region of the flame. The above quantities are compared with those measured in a similar non-smoking flame during a separate study. Specific soot oxidation rates are also obtained and compared for the aggregates transported in the upper portion of both flames. The transition from the non-smoking to the smoking behavior with incr...

Large-Scale Coherent Structures as Drivers of Combustion Instability

Abstract: The role of flow coherent structures as drivers of combustion instabilities in a dump combustor was studied. Results of nonreacting tests in air and water flows as well as combustion experiments in a diffusion flame and dump combustor are discussed to provide insight into the generation process of large-scale structures in the combustor flow and their interaction with the combustion process. It is shown that the flow structures, or vortices, are formed by interaction between the flow instabilities and the chamber acoustic resonance. When these vortices dominate the reacting flow, the combustion is confined initially to the circumference of their cores and further downstream proceeds into their core, leading to periodic heat release, which may result in the driving of high amplitude pressure oscillations. These oscillations are typical to the occurrence of combustion instabilities for certain operating conditions. The basic understanding of the interaction between flow dynamics and the combustion ...

The role of the recirculation vortex in improving fuel-air mixing within swirling flames

Abstract: An important example that shows how a flame interacts with a vortex is the case of a swirlstabilized flame. Such a flame essentially is a toroidal vortex into which a jet of fuel is injected along the torus centerline. Because of the central toroidal vortex, the overall fuel-air mixing rate within a swirl-stabilized flame is found to be a factor of five times greater than that of a simple jet flame, as evidenced by a fivefold shortening of flame length. Swirl flames are unique in that the fuel-air mixing rate can be varied by controlling the amount of swirl. Flow visualization shows how internal recirculation helps to enhance the mixing. The englufment mechanisms is similar to that of a simple jet flame but is enhanced because of two factors that increase the fuel-air contact area. First, the recirculation zone acts like a large eddy with a characteristic velocity and length scale that are much larger than those associated with eddies in a simple jet. The active role of the recirculation zone contradicts a previouslyheld concept that the recirculation zone is a passive obstacle and that its internal velocity is not important. Air is entrained into the toroidal vortex primarily in the downstream region of the vortex. A second reason why mixing is more intense within swirling flames than within jet flames is that there is impingement of opposed jets at the forward stagnation point. Thus pressure gradients, which are not present in simple jets, enhance the mixing rates. Flame length, which is a measure of overall fuel-air mixing, was found to have a different scaling in swirl flames than for simple jet flames. The physical reasons are explained by a scaling which was developed and which explains four trends in the flame length data. It is proposed to use a general, nondimensional circulation parameter that allows for general comparison of mixing efficiencies of swirl, bluff-body, and dump-combustor flames.

Turbulent reactive flows

Abstract: Structure: Diagnostics and Analysis.- Measurement of the Topology of Large-Scale Structures in Turbulent Reacting Flows.- Finite Chemical Kinetics Effects in a Subsonic Turbulent Hydrogen Flame.- Cars Study of Premixed Turbulent Combustion in a High Velocity Flow.- The Structure of Jet Diffusion Flames.- Instantaneous Radial Profiles of OH Fluorescence and Rayleigh Scattering Through a Turbulent H2- Air Diffusion Flame.- Flame Structure in Spark Ignited Engines, from Initiation to Free Propagation.- Comparison Between Two Highly Turbulent Flames Having Very Different Laminar Burning Velocities.- Structure of Turbulent Premixed Flames as Revealed by Spectral Analysis.- Turbulent Flow Field and Front Position Statistics in V-Shaped Premixed Flame With and Without Confinement.- Two-Component Velocity Probability Density Measurements During Premixed Combustion in a Spark Ignition Engine.- On the Accuracy of Laser Methods for Measuring Temperature and Species Concentration in Reacting Flows.- Structure of Flamelets in Turbulent Reacting Flows and Influences of Combustion on Turbulence Fields.- Flamelet Library for Turbulent Wrinkled Flames.- Diffusion Flame Attachment and Flame Spread Along Mixing Layers.- Length and Time Scales in Turbulent Combustion.- A Model for Reactions in Turbulent Jets: Effects of Reynolds, Schmidt, and Damkoehler Numbers.- A Fractal Description of Flamelets.- Some Results on the Structure of the Tempertature Field in Low Damkohler Number Reaction Zones.- Interaction of a Flame Front with Vortices : An Experiment.- Experimental Studies in Vortex Pair Motion Coincident with a Liquid Reaction.- On an Attempt to Measure the Decay of Concentration Fluctuations in a Quasi-Isotropic Grid by Use of the Fluorescence of the Solution.- Turbulent Reactive Flows of Liquids in Isothermal Stirred Tanks.- Structure and Predictive Schemes.- Turbulent Shear Layer Mixing With Fast Chemical Reactions.- The Use of Direct Numerical Simulation in the Study of Turbulent, Chemically-Reacting Flows.- Direct Numerical Simulation and Simple Closure Theory for a Chemical Reaction in Homogeneous Turbulence.- The Interaction Between Turbulence and Chemistry in Premixed Turbulent Flames.- On the Problem of Modelling Time or Length Scales in Turbulent Combustion.- Statistical Modelling of Turbulent Reactive Flows.- Coherent Flame Description of Turbulent Premixed Ducted Flames.- Turbulence-Combustion Interactions in a Reacting Shear Layer.- A PDF Method for Turbulent Recirculating Flows.- Dyanmics of Cold and Reacting Flows on Backward Facing Step Geometry.- PDF - Transport Equations for Chemically Reacting Flows.- Modelling the Effects of Combustion on a Premixed Turbulent Flow : A Review.- The Numerical Simulation of Compressible and Reactive Turbulent Structures.- Turbulent Multiphase Flows.- Lagrangian Simulation of Particle Dispersion.- Numerical Modelling of Devolatilization in Pulverised Coal Injection Inside a Hot Coflowing Air Flow.- Flame Stabilization in a Supersonic Combustor.- Mixing Problems in Supersonic Combustion.- Morphology of Flames Submitted to Pressure Waves.- Control of Turbulence in Combustion.- Progress Toward Shock Enhancement of Supersonic Combustion Processes.

Near-limit flame spread over a thin solid fuel in microgravity

Abstract: Diffusion flame spread over a thin solid fuel in quiescent and slowly moving atmospheres is studied in microgravity. The flame behavior is observed to depend strongly on the magnitude of the relative velocity between the flame and the atmosphere. In particular, a low velocity quenching limit is found to exist in low oxgen environments. Using both the microgravity results and previously published data at high opposed-flow velocities, the flame spread behavior is examined over a wide velocity range. A flammability map using molar oxygen percentages and characteristic relative velocities as coordinates is constructed. Trends of flame spread rate are determined and mechanisms for flame extinction are discussed.

Flowfield modelling of soot formation at elevated pressure

Abstract: A simplified two-equation model of soot formation is described which extends the laminar flamelet approach to combustion chemistry modelling and thereby admits application to turbulent non-premixed flames. The critical roles for mixture fraction and temperature are identified and extensive property maps have been generated experimentally in a laminar ethylene-air diffusion flame for purposes of model development and validation. Detailed measurements of mixture fraction by microprobe sampling and mass spectrometric analysis, temperature by fine wire thermocouple and soot volume fraction by laser extinction are reported. The problems of probe measurements in sooting regimes are briefly reviewed and a strategy based on transient thermocouple measurements, compensated for the effects of thermal inertia, is advocated. Measurements are reported up to pressures of 3 bar and compared with detailed model predictions. A model for soot formation, incorporating in an approximate manner the effects of nucleation, surface growth and agglomeration on particle number density and soot volume fraction, has been adapted to two-dimensional flame flow-field prediction. Encouraging agreement is achieved between the extensive data set and numerical simulation at atmospheric and elevated pressure.

The importance of time-dependent flame structures in stretched laminar flamelet models for turbulent jet diffusion flames

Abstract: The stretched laminar flamelet model provides a convenient mechanism for incorporating realistic chemical kinetics into calculations of turbulent nonpremixed flames. In the standard flamelet model, two scalars, a mixture fraction ξ and scalar dissipation η (a measure of flamelet stretch) suffice to specify the local instantaneous thermochemical state in the turbulent flow. One shortcoming of the flamelet approach is the implicit assumption that the reaction zone structure of laminar flamelets can respond on a time scale that is fast compared to the time scale of changes in mean scalar dissipation in the turbulent flow. analysis of the response of a one-dimensional unsteady strained diffusion layer to arbitrary time-dependent-strains is presented. This analysis suggests that laminar flames cannot approach the equilibrium (zero strain) structure as rapidly as the flamelet model implies far downstream in the jet; the flamelet model breaks down in the far jet. An ad hoc modification to the flamelet model is made to account for the limited response of the diffusive layer to time-varying strain rates. In a CO/H2/N2-air turbulent jet diffusion flame, the modified flamelet model yields a substantially slower approach to chemical equilibrium and improved agreement with experimental data compared to the standard flamelet model. This suggests that the final approach to equilibrium may be limited by the nonzero response time of flamelet chemical structure to the rapidly decaying strain rates encountered downstream in the turbulent jet. In the context of flamelet models, transient effects may be more important than previously assumed.

Scalar dissipation rates in turbulent jets and jet diffusion flames

Abstract: Laser Rayleigh scattering and laser Doppler measurements have been performed in round propane air jets and in the stabilization region of a lifted propane air diffusion flame. While the laser Doppler measurements provide the necessary information about the mean flow field the Rayleigh scattering measurements were performed in order to obtain new data about the turbulence structure of the scalar field. The mixture fraction and its spatial gradient was measured locally, such that the scalar dissipation rate and its probability density function (pdf) could be obtained. The pdf is compared with a lognormal distribution. It is found that its variance σ is remarkably constant within the entire jet and in the flame. The experimental results are compared with standard k-e-type model predictions.

Anomalous lewis number effects in tribrachial flames

Abstract: Tribrachial (three-armed) flames are generated when a premixed flame propagates into a nonuniform mixture, and consist of a fuel-rich branch joined to a fuel-lean branch, with a diffusion flame trailing from the junction (the stoichiometric point). They are important in the context of ‘laminar flamelet’ models of turbulent non-premixed flames. We provide a theoretical description of these flames, valid when the upstream concentration gradient is small. In addition, we assume that the Lewis number of the oxidizer is 1, but that of the fuel (Le f ) is greater than 1. Solutions are sought corresponding to steady, unbounded propagation into unburnt mixture, where we would expect—nominally—that all three arms of the flame trail behind the junction, so that to the left of some plane perpendicular to the oncoming flow there is everywhere no reaction. Instead, we find that the fuel-rich branch necessarily extends ahead of any plane, penetrating to upstream infinity. This corresponds to an anchored flame rather than one that is freely propagating, and appears to be a consequence of the stretch effects that are possible when Le f >1; a decrease in the mixture strength does not necessarily lead to a decrease in the local flame speed if flame curvature generates negative stretch. If we assume that extinction occurs at sufficiently low flame temperatures, the flame is bounded and our solution can be reinterpreted as a freely-propagating flame, albeit with unexpected shape.

Soot aerosol dynamics in a laminar ethylene diffusion flame

Abstract: A thermophoretic sampling technique is used to examine the soot morphological characteristics inside a coannular ethylene/air diffusion flame at atmospheric pressure. The data acquired with this sampling technique are combined with information obtained by others using optical diagnostics on an identical flame. The spatial variation of primary particle diameter, combined with particle velocity measurements performed by Santoro, allow the calculation of specific soot surface growth rate, primary particle number density, soot surface area and molecular flux of the growth species throughout the growth region. Specific soot burnout rates are also obtained throughout the oxidation region. A discussion about the spatial distribution of the soot aerosol processes is given. The particles are formed low in the flame (z≅5mm) on the fuel rich side of the flame sheet, close to the main reaction zone. They subsequently follow paths to the upper regions of the flame, growing through both agglomeration and surface growth reactions and finally they disappear entirely under the oxidative attack of various species. A bimodal integral solution of the aerosol dynamic equation that accounts for particle inception, free molecular coagulation and surface growth reactions is used to model the soot dynamics in the flame. This model is used to study the soot formation and growth processes throughout the lower and intermediate portions of the flame along a path line that coincides with the annular region. An estimate of the particle inception rate is obtained as a result of these calculations.

Coflowing turbulent jet diffusion flame blowout

Abstract: We present results from an experimental and theoretical investigation into the blowout mechanism in turbulent diffusion flames. The blowout stability limits of coflowing turbulent jet diffusion flames are formulated in terms of a recently proposed flame stabilization mechanism based on the large scale organization of entrainment and mixing observed in turbulent shear flows. In contrast to the linear similarity scaling of the more commonly studied simple turbulent jet flames, the nonlinear scaling of coflowing turbulent jets allows an essential element of this stabilization mechanism to be investigated. Results show that when the flame stability criterion is evaluated for the last large structure in the flame as is consistent with the underlying physical picture for this stabilization mechanism, a large reduction in the blowout limit is expected for even a small coflow velocity. This phenomenon is experimentally verified and good quantitative agreement is demonstrated with a set of measurements for the blowout limits of such coflowing turbulent jet flames.

Simulation of ceramic particle formation: Comparison with in-situ measurements

Abstract: Ceramic particle formation processes have been studied using SiO{sub 2} as a model compound. Silica particles have been synthesized in a counterpropagating diffusion flame reactor, in which in-situ measurements of particle size and number density have been made. In addition, the time-temperature history of the particle field has been calculated from a flame simulation. Numerical simulations using moment and sectional methods for particle formation have been applied and compared to the experimental measurements. The simulations for the particle formation assume a kinetically-constrained approach, allowing a simple representation of nucleation, surface growth and coagulation. The results suggest that, if the source rates are known well enough, particle formation of low vapor pressure species can be predicted. Both models do well in predicting the gross features of particle formation (number density and mean particle size), although the moment solution does a poor job of predicting the polydispersity effects during periods of high monomer generation rates.

Fuel shape effect on the determinitic properties of turbulent diffusion flames

Abstract: Measurements of flame height and temperature distribution above fuel surface were made on rectangular propane burners i,.p a huge calm eln~os~72.' Flame height is represented as a function of Q d; Q/ p CpT g AB , where A and B are the length of the shorter and m'1'onger ~ide 0 of a fuel respectively. Vertical distribution of temperature above the center of a fuel is compared with temperature profiles for square fuels and for line fuels. Finally a practical criterion for the approximation of rectangular fuels by square and line source is proposed. key words: turbulent flame, flame height, temperature, aspect ratio.

Measurements of Oxides of Nitrogen Emissions from Turbulent Propane Jet Diffusion Flames

Abstract: Measurements of global emission indices for NO and NO,were made in unconfined,vertical,propane jet diffusion flames. Initial fuel jet velocities were varied to provide a range of jet Reynolds numbers from 10000 to 60000. The measurements showed that scaling of NO, emission indices with velocity changes as the flame character changes from transitional-turbulent (Re ≈ 10000) to fully turbulent (Re > 20000). At the lower Re condition. the observed scaling approaches that reported in the literature for similar conditions. Arguments are presented to show that radiation losses from the flames may account for the observed scaling. Significant amounts ofN02 were found in the flame plumes. The variation of NO2/NOx, ratios with jet velocity and with radial position was consistent with the hypothesis that NO2is formed in the cooler post-flame regions. in agreement with the findings of others for CO and CO/H2 fueled diffusion flames.

The Influence of the Lewis Numbers of the Reactants on the Asymptotic Structure of Counterflow and Stagnant Diffusion Flames

Abstract: The asymptotic structure of counterflow and stagnant diffusion flames are analyzed in the limit for large values of the overall, nondimensional activation energy, Ta , characterizing the rate of the reaction and results are given for small values of the stoichiometric fuel to oxygen mass ratio. The chemical reaction between the fuel and the oxidizer is represented by a one-step, irreversible process. A new approach is developed to characterize the influence of the Lewis number of the fuel, LF and the Lewis number of the oxidizer, L0 , on the outer and inner structure of near equilibrium diffusion flames. Explicit algebraic formulas to predict the critical conditions of flame extinction are also given. For counterflow diffusion flames at fixed values of L0 , the flame moves significantly toward the oxidizer stream and the heat losses toward the oxidizer region of the flame increase significantly with decreasing values of LF . The value of the maximum flame temperature is relatively insensitive to ...

Dynamic light scattering and angular dissymmetry for the in situ measurement of silicon dioxide particle synthesis in flames.

Abstract: Particle size measurements have been made of silica formation in a counterflow diffusion flame reactor utilizing dynamic light scattering and angular dissymmetry methods. The results suggest that the techniques compare quite favorably in conditions of high signal to noise. However, the dynamic light scattering technique degrades rapidly as the signal strength declines, resulting in erroneously small particle diameters. As a general rule dynamic light scattering does not seem to possess the versatility and robustness of the classical techniques as a possible on-line diagnostic for process control. The drawbacks and limitations of the two techniques are also discussed.

Soot formation in diffusion flames of fuel/oxygen mixtures

Abstract: Sooting trends have been measured as a function of equivalence ratio of fuel/oxygen mixtures over the range of infinity (pure diffusion flames) to low values (double flames; an inner premixed flame and an outer diffusion flame combination). All interesting observations have been explained on the basis of changes due to oxygen addition in the fuel pyrolysis chemistry, flame structure, and interaction of double flames. This effect of oxygen addition to the fuel on soot formation has been studied in coflow and counterflow diffusion flames of ethene and propane by performing smoke, height, laser light extinction, temperature, and velocity measurements. In counterflow diffusion flames (CFDF's) established around a porous cylindrical burner, the equivalence ratio (φ) of the fuel/oxygen mixtures ranged from infinity to 2.60. Oxygen addition increased the, soot formed in ethene flames to anof about 6.0. Greater oxygen addition resulted in a sharp reduction of the soot loadings and no soot was detected in flames with an φ The smoke height measurements in coflow diffusion flames (CDF's) are in complete agreement with the CFDF results. Whereas oxygen addition to ethene decreased the sooting fuel flow rate substantially, an increase was measured for propane and isobutane. Moreover, CDF's with φ's less than about 3.6 for propane and 6.7 for ethene could not be stabilized due to flashback, and just before flashback propane flames were observed to soot copiously.

On the flame structure at the base of a pool fire

Abstract: A detailed mapping is presented of the temperature, CO, CO 2 and O 2 species concentrations, and monochromatic absorption coefficient at the base region of a kerosene pool fire. Comparative analysis of the isotherm and iso-concentration patterns indicates that the buoyant flame has an intermittent character at its base, contrary to its normally attributed persistent, or anchored, character. The temperature field shows a band of temperature maxima that approximately coincides with the luminous flame boundary. The temperature in this band, however, decreases toward the fire axis and is lower than that expected in a kerosene flame. This, in conjunction with the presence of high oxygen concentration in the fuel side of the temperature maxima band, indicates that the flame fluctuates around a mean position, and that as a consequence the measured temperatures are lower averaged values of the fluctuating temperature field. The CO and CO 2 iso-concentration fields show a region of elevated concentrations at the axis near the liquid surface that is displaced toward the fuel side with respect to the temperature maxima region, rather than coinciding with it. The lines of constant absorption coefficient, which can be associated with lines of constant soot concentration, also show a region of elevated values that approximately coincides in location with that of the CO and CO 2 concentrations. These results indicate the fluctuating character of the flame, and suggest the existence of a gas stagnation, or recirculation, zone at the axis near the liquid surface that permits the accumulation of combustion products in this region. From the observed characteristics of the flame structure it can be concluded that the flame at the pool fire base is better described as a fluctuating, laminar, diffusion flame, which later becomes a turbulent, intermittent one as it evolves upward along the fire plume.

Flame extinguishment properties of dry chemicals: Extinction concentrations for small diffusion pan fires

Abstract: An extensive experimental study has been made of the extinguishment of a smalln-heptane diffusion flame (14.7 cm diameter pan) by five common dry-chemical powders—potassium bicarbonate, sodium bicarbonate, potassium chloride, monoammonium phosphate, and Monnex. The fire extinguishing effectiveness (reciprocal of the observed minimum extinction concentration) of each dry-chemical substance, other than that due to chemical specificity, is fully explained on the basis of particle size and distribution. Literature correlations between effectiveness and particle surface area appear to be incidental and stem from the inherent relationship between particle size and surface area. An important finding of the study is a discontinuity in the extinguishing effectiveness of a powder as a function of particle size. This occurs for each substance at a unique particle diameter above which there is a dramatic five-to eight-fold decrease in effectiveness. For a given substance, all powders with particle sizes below the limiting value exhibit the same maximum effectiveness.

SiC-Based thin-filament pyrometry: Theory and thermal properties

Abstract: Blackbody radiation from a thin filament of β-SiC can be used to measure the spatial temperature distribution in a combusting flowfield. The technique, Thin-Filament Pyrometry, exhibits rapid temporal response and excellent spatial resolution because of the small diameter and low thermal conductivity of the filament. Factors which affect the accuracy, temporal response, and spatial resolution of the probe in a H 2 -N 2 jet diffusion flame are presented.

The Structure of Jet Diffusion Flames

Abstract: This paper presents the structural characteristics of free, round, jet diffusion flames as obtained using a new 2D laser sheet lighting visualization technique referred to as the RMS (Reactive Mie Scattering) method. The results of analyzing photographs and high speed movies of flames using the RMS method are discussed in terms of the visible flame structure. The fuel veloCity is varied from 0.16 to 17 m/s. The presence of large toroidal vortices formed outside the visible flame zone have been known for many Years but their importance in determining the dynamic structure of free jet diffusion flames has not been fully appreciated. The influence of the outer vortices on flame structure is prevalent for near laminar and transitional flames and diminishes for near turbulent flames. They are believed to result from a Kelvin-Helmholtz type instability formed by a buoyantly driven shear layer. They appear to be responsible for flame flicker defined by the separation of the flame tip or the oscillations of the flame surface and for determining the shape of the mean, rms, and pdf radial profiles of temperature. Vortex structures have also been observed inside the visible flame zone. In transitional flames established by a contoured nozzle, these structures are shown to be on the scale of the 10 mm diameter nozzle, toroidal, and coherent for a long distance downstream. However, they may have only a minimal impact on the mean temperature characteristics of transitional flames. Their impact on the visible flame structure of near turbulent flames is large. At high fuel velocities, coalescence of the large vortices appear to be correlated with the formation of small 3D vortices which are randomly distributed in size and space. Collisions of the small vortices with the visible flame front produce small localized flamelets which are responsible for the wrinkled appearance of the visible flame surface. The localized stretching of the flame surface is believed to invoke finite rate chemistry effects. Indeed, collisions are observed where the flame stretch is large enough to cause localized holes to form in the flame surface. This appears to occur when the radial velocities of the inner vortices are large. Holes formed near the lip of the jet are postulated to be one mechanism that induces flame lift-off.