Chaotic map models of soot fluctuations in turbulent diffusion flames

A lagrangian simulation of flamelet extinction and re-ignition in turbulent jet diffusion flames

A theoretical analysis of turbulent jet diffusion flames is developed in which the flame is regarded as an ensemble of laminar diffusion flamelets that are highly distorted. The flow inhomogeneities are considered to be sufficiently strong to produce local quenching events for flamelets as a consequence of excessive flame stretch. The condition for flamelet extinction is derived in terms of the instantaneous scalar dissipation rate, which is ascribed a log-normal distribution. Percolation theory for a random network of stoichiometri c sheets is used to predict quenching thresholds that define liftoff heights. Predictions are shown to be in reasonably satisfactory agreement with experimentally measured liftoff heights of methane jet diffusion flames, within experimental uncertainties. UEL issuing from a tube or duct into an oxidizing atmosphere forms a jet in which combustion may occur. The associated combustion process is the most classical example of a diffusion flame. At sufficiently high velocities of fuel flow (fundamentally, at sufficiently large Reynolds numbers) the entire diffusion flame is turbulent. The turbulent jet diffusion flame begins at the mouth of the duct for a range of values of the exit velocity. When a critical exit velocity is exceeded, the flame abruptly is detached from the duct and acquires a new configuration of stabilization in which combustion begins a number of duct diameters downstream. Flames in this state, stabilized in the mixing region, are termed lifted diffusion flames, and the critical exit velocity at which they appear is called the liftoff velocity. The liftoff height is the centerline distance from the duct exit to the plane of flame stabilization. A further increase in the exit velocity increases the liftoff height without significantly modifying the turbulent flame height (the centerline distance from the duct exit to the plane at which, on the average, combustion ceases). There is a second critical value of the exit velocity, called the blowoff velocity, beyond which the flame cannot be stabilized in the mixing region. The present study addresses questions of the structure of lifted turbulent diffusion flames at exit velocities between liftoff and blowoff values. Attention is focused especially on the calculation of liftoff heights. Liftoff characteristics for turbulent jet diffusion flames are of practical importance in connection with flame stabilization. Conditions for liftoff and blowoff must be known in developing rational designs of burners, e.g., in diffusion-flame combustors for power production or in flaring applications for the petroleum industry. They are also of interest in connection with extinguishment of certain fires that may occur in oil or gas rigs. The present work is directed toward developing an improved fundamental understanding of liftoff phenomena that may later prove useful for these applications.

Temperature measurement of H 2 gas was investigated experimentally in turbulent diffusion flame with co-flowing air stream with swirler. The air velocity changes from slow to high flow rate. Reynolds number is varying throughout the process from laminar to turbulent region. Results showed that the temperature decreases with increase the air flow rate and also temperature decreases as decrease in M.F.R. and equivalence ratio. Flame stability increases with increasing the hydrogen content in the hybrid fuel mixture.

Stretched laminar flamelet analysis of turbulents H 2 and CO/H 2/N 2 diffusion flames

The interaction between reaction and turbulent mixing strongly affects the structures of a turbulent diffusion flame, the characteristics of which are greatly affected by the combination of working conditions such as burner exit configurations, burner size, fuel and oxidant. This study discusses in detail the turbulent diffusion flame structure and its similarity using a laboratory-scale turbulent diffusion flame measured by laser Rayleigh scattering. It also discusses the factors affecting the similarity in flame structure and the turbulent diffusion flame length determined using its turbulent power spectral density. The -5/3 power law holds in the fuel jet and combustion regions but in the air entrainment regions, the -5/3 and -1 power laws coexist, and which shows that both turbulent and molecular thermal diffusions become important. The constancy of the turbulent diffusion flame length at a high Reynolds number is discussed with respects to the characteristics of flame structure.

Measurement of the dimensionless extinction coefficient of soot within laminar diffusion flames

Visual observations, high speed movie sequences and image processing techniques have been used to examine unconfined vertical lifted turbulent diffusion flames issuing from precessing jeta (PJ) nozzles. These techniques provide qualitative information about the dynamic motions in the flame and quantitative data on the size and number of flame structures, the celerity of those structures, flame dimensions, residence times and characteristic strain rates. The information is used to provide new insight into the flame stabilisation mechanism of, and combustion processes occurring in, a PJ flame and to enable a comparison with similar studies in free turbulent jet diffusion flames and pool fires. Large-scale structures are seen to form near to the base of the PJ flame and move downstream with a slow, nearly constant speed in a manner reminiscent of the puffing motions in a pool fire. The entrainment and mixing of these structures is such that the flame tip oscillates as the residual unburned mixture in each structure, becomes combustible and burns out rapidly as a single entity. It is proposed that the puffing motions in these unconfined flames are the result of a buoyant instability, which is analogous to that in a pool fire. However the buoyant puff structures are the result, not the cause, of the stabilisation process. The high stability of the PJ flame, previously reported, is deduced to be related to the rapid decay in jet velocity and to the increase in entrainment which occurs in and upstream from the stabilisation region. The frequency of puffing is found to be uncorrelatetagging.bd with the frequency of precession. Global residence times and characteristic rates of shear are also measured and found to be reduced relative to those in a simple jet flame. The implications of the reduced shear on models of NOx generation in jet flames is discussed.

Space- and time-resolved information, as well as macro- and microscopic visualizations, is essential for detailed study of mixing, reaction and heat transfer mechanisms in a turbulent diffusion flame. However, turbulent diffusion flame structures have been studied almost solely by time-resolved single-point measurement due to the restriction of diagnostics. The time-dependent spatial structures of turbulent diffusion flames have often been masked by the statistical analyses. In this study, time-dependent spatial mixing processes in turbulent diffusion flame are visualized by instantaneous flame luminosity and plane Mie scattering images by an ultrahigh-speed video system (Fastcam). It is found that the autosimilarity of the instantaneous turbulent diffusion flame structure has a fractal dimension defined by fractal analysis which appears corresponding to the fractal analysis of the combustion region. Macroscopic movements of the flame region are classified in terms of the instantaneous variations of flame shape.

A premixed flame assisted by the burning of a diffusion flame is used in gas-turbine combustors to reduce NOx emissions. A united model that can be applied to the premixed and diffusion flames is therefore required to simulate the combustion phenomena. This paper proposes such a united model based on the author’s premixed combustion model for reactive progress variable equation. The proposed model has the following features. 1) It includes the laminar flame speed and the gradient of the mixture fraction as parameters. When the gradient of the mixture fraction is close to zero, the model is also close to the previous premixed combustion model as an asymptotic form. 2) It considers the effects of pressure in the combustor, unburned gas temperature, and flame stretch on combustion based on the laminar flame speed. 3) It can be applied to all types of turbulence models like the k-ε model, large eddy simulations, and direct simulations in the case of wrinkled laminar flames. The effect of turbulence is considered through the turbulent eddy viscosity of all turbulence models. To verify the accuracy of the model, the opposed diffusion flame presented by Tsuji and Yamaoka was numerically simulated, as an example of a laminar diffusion flame. Further, a turbulent diffusion flame, which was assisted by the burning of a pilot jet, was demonstrated using the united combustion model as an example of the turbulent diffusion flame discussed by Barlow and Frank. The flame was known as Sandia Flame D. Model results were in good agreement with the experimental data and this agreement confirmed the proposed united model was able to accurately simulate both diffusion flames.

One-dimensional raman scattering for determination of multipoint joint scalar probability density functions in turbulent diffusion flames

The book Turbulent Combustion by Norbert Peters is a concise monograph on single-phase gaseous low Mach number turbulent combustion. It is compiled from the author's review papers on this topic plus some additional material. Norbert Peters characterizes turbulent combustion both by the way fuel and air are mixed and by the ratio of turbulent and chemical time scales. This approach leads naturally to detailed models, which are based on results of turbulence modelling and asymptotic flame theory. In both areas Norbert Peters has contributed significantly over the last two decades. The book has four sections. In chapter 1 he discusses briefly the state of the art of combustion models as they are used by different authors. Important turbulent and chemical scales are introduced, which are then used to introduce and explain the different combustion models. He distinguishes between premixed and non-premixed combustion and also between infinitely fast and finite rate chemistry. The current turbulent combustion models are described in order of their complexity and physical accuracy. He explains Eddy BreakUp and Eddy Dissipation Models, the fundamentals of the PDF transport equation, and the laminar flamelet concept applied to non-premixed and premixed turbulent combustion. Then, the Conditional Moment Closure, the Linear Eddy Model, and combustion models used in Large Eddy Simulations are described very briefly. Chapter 2 is devoted to premixed turbulent combustion. After introducing some characteristic dimensionless numbers Peters uses the level set approach and the flamelet concept to formulate a combustion model valid in the thin zone and corrugated reaction zone regimes. He shows parallels between this more fundamental model and standard models like the Bray-Moss-Libby model. He also presents models for the turbulent burning velocity, the Flame Surface Area Ratio, and discusses the effects of gas expansion. Very helpful for the reader's understanding is the presentation of three worked examples of a slot burner, a propagating spherical flame and an oscillating counter flow. Peters' model for premixed turbulent combustion is based on the equations for the mean and the variance of the $G$-equation, some closure relations as well as the flamelet equation for premixed combustion. A numerical example is used to discuss its accuracy. Non-premixed turbulent combustion is the subject matter of chapter 3. Peters uses the mixture fraction variable and asymptotic flame theory to explain the regimes of non-premixed turbulent combustion. Two worked examples of a counterflow diffusion flame and the one-dimensional unsteady laminar mixing layer help the reader to understand the theory. After discussing turbulent jet diffusion flames and introducing the flamelet equation he develops steady and unsteady flamelet models for non-premixed turbulent combustion. In particular, the Eulerian Particle Flamelet Model and the RIF (Representative Interactive Flamelet) Model are discussed. These models have been used to predict pollutant formation in a gas turbine and a direct injection Diesel engine, respectively. Finally, partially premixed combustion is discussed in chapter 4. Lifted turbulent diffusion flames are reviewed and the prediction of the lift-off height is identified as a key problem. This leads directly to the introduction of the concept of a triple flame. Different models for partially premixed combustion are then presented and the numerical simulation and the scaling of lift-off heights in turbulent jet flames are studied. There is no doubt that this book is well written and is an important contribution to combustion literature. Peters uses asymptotic theory and scale separation to develop combustion models from first principles. Also, the book contains a comprehensive review of the current literature on turbulent combustion. It is clearly a `must have' for experienced combustion modellers and experimentalists. The book has not been written for non-experts and beginners; these readers would probably have liked more worked examples and exercises, a list of symbols, some material on the mathematical techniques of asymptotic analysis, and a more detailed discussion of the standard combustion models that are often used in the literature. Also, the numerical aspects of turbulent combustion modelling are not discussed in the book. Nevertheless, the book will be a useful source for advanced courses on combustion. Markus Kraft

This paper presents the results of an experimental and numerical study of the radiation and temperataure fields of turbulent jet diffusion flames in a reduced gravity environment. Tests were conducted for non-premixed, nitrogen-diluted propane flames burning in quiescent air in the NASA Glenn 5.18 Second Zero Gravity Facility. Flame radiation from horizontal, wedge-shaped slices of the flame were measured. The radiation data are compared with those obtained from corresponding normal gravity tests. With the aid of a k-c numerical model, the effects of flame radiation on the thermal field is demonstrated and qualitative agreement is obtained between the predictions and measurements. In addition, power spectra of the measured flame radiation are used to assess the turbulent structure of the flame. INTRODUCTION The first investigations on hydrocarbon turbulent diffusion flames in microgravity were initiated recently13. These investigations have been conducted for flames with moderate injection (cold jet) Reynolds numbers under 10000. Scaling relationships for measured flame heights were obtained3 and found to be similar to the relationships for corresponding flames in normal gravity, in particular, the scaling of flame height with fuel dilution and nozzle diameter. However, the heights of the microgravity flames are about twice the normal gravity flame heights. The difference in flame heights of the microgravity and normal gravity flames indicates that the Froude number of the investigated normal gravity flames is not high enough for these flames to be momentum-controlled and buoyancy plays an important role in their behavior. For example, buoyant acceleration counteracts the centerline velocity decay that would otherwise occur due to jet spreading and momentum diffusion. That such flow differences yield different flame characteristics may be explained by noting that the primary time scales in diffusion flames have components of flow time and chemical time. Flow time scales are related to flow processes uch as mixing of fuel and oxidizer prior to reaction whereas chemical time scales are related to the times for the various reactions occurring in the flame. In most cases, flow times are several orders of magnitude larger than chemical times, with the result that flow processes control characteristics of diffusion flames such as size and shape. Hence, factors such as buoyancy that modify the flow patterns in these flames are expected to have significant influence on flame behavior. Flame radiation is an important quantity that is impacted by buoyancy as has been shown in previous studies by the authors4 and also by Urban et a15. It was found that jet diffision flames burning under microgravity conditions have significantly higher radiative loss (about five times higher) compared to their normal gravity counterparts because of larger flame size in microgravity and larger convective heat loss fraction from the flame in normal gravity. These studies, however, were confined to laminar flames. l Principal Researcher. Associate Fellow, AIAA. + Senior Scientist. Senior Member AIAA. “Copyright

A technique for obtaining simultaneous quantitative images of the hydroxyl radical, OH, temperature, mixture fraction, and scalar dissipation rates in turbulent diffusion flames is described. Mixture fraction is obtained from images of Rayleigh and fuel Raman scattering. We quantified the OH laser-induced fluorescence (LIF) images using detailed calibration and a correction for quenching and population distribution effects based on the simultaneous mixture fraction and temperature images. This correction was derived from calculations of laminar counterflow diffusion flames for identical fuel mixtures. These laminar flame computations are further used to estimate the errors in the measured OH concentrations. The technique is applied to piloted, nonpremixed flames over a range of jet velocities. The measured mixture fraction, temperature, and OH concentrations are in good agreement with those obtained earlier in similar flames using the single-point Raman/Rayleigh/LIF technique.

LES/PDF based modeling of soot–turbulence interactions in turbulent flames

Time-series analysis and measurements of intermendiate-species concentration spectra in turbulent non-premixed flames