Liftoff characteristics of turbulent jet diffusion flames

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

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

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.

Experimental studies of bluff-body stabilized LPG diffusion flames

In this paper, unsteady process of ignition and combustion of turbulent plane-jet diffusion flame of ethylene/air at varied fuel/air ratios is numerically simulated with Large Eddy Simulation (LES) and a reduced kinetic mechanism of ethylene. The kinetic mechanism consisting of 25species and 131steps is reduced from a 71species/395steps detailed mechanism via the method of error-propagation-based directed relation graph (DRGEP) and sensitivity analysis. The LES results of height of flame lift-up and averaged temperature profiles at different downstream locations are compared with the DNS result of Yoo (2011) and satisfactory agreements are found. Unsteady processes of ignition and combustion of ethylene plane-jet diffusion flame are simulated with varied fuel injection velocities. Dynamic evolutions of temperature field as well as CH2 and OH radicals are obtained, which are found to be strongly related to turbulence eddies caused by jet/air mixing layer. The present numerical study shows that LES method with reduced mechanism of hydrocarbon fuels can effectively simulate temporal and spatial evolution of ignition and combustion process. © 2015, AIAA American Institute of Aeronautics and Astronautics. All rights reserved.

A lagrangian model for predicting turbulent diffusion flames with chemical kinetic effects

The blowout limits of a co-flowing turbulent methane jet diffusion flame with addition of diluent in either jet fuel or surrounding air stream is studied both analytically and experimentally. Helium, nitrogen, and carbon dioxide were employed as the diluents. Experiments indicated that an addition of diluents to the jet fuel or surrounding air stream decreased the stability limit of the jet diffusion flames. The strongest effect was observed with carbon dioxide as the diluent followed by nitrogen and then by helium. A model of extinction based on recognized criterion of the mixing time scale to characteristic combustion time scale ratio using experimentally derived correlations is proposed. It is capable of predicting the large reduction of the jet blowout velocity due to a relatively small increase in the co-flow stream velocity along with an increase in the concentration of diluent in either the jet fuel or surrounding air stream. Experiments were carried out to validate the model. The predicted blowout velocities of turbulent jet diffusion flames obtained using this model are in good agreement with the corresponding experimental data.

A dielectric barrier discharge (DBD) actuator is used to stabilize the base of a lifted ethane turbulent jet diffusion flame by modifying the coflow velocity field. The velocity field and flame base are measured by particle image velocimetry and unfiltered flame chemiluminescence imaging, respectively. An axisymmetric DBD actuator, integrated onto the nozzle body and driven by 8 kHz, 10-12-kV peak-to-peak sinusoidal voltage, generates the directional flow that opposes the coflow that surrounds the jet nozzle. This flow induces a separation bubble that retards and diverts the surrounding fluid further away from the fuel jet. The turbulent jet diffusion flame liftoff height is found to be significantly reduced by this plasma actuation.

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 numerical study on flame stability at the transition point of jet diffusion flames

In this paper measurements and scaling laws for NOx-emissions from large flames with up to 9 MW of thermal heat release are presented. These represent much larger flames than reported earlier in the literature. Measurements of oxides of nitrogen have been performed in small vertical laboratory flames (<0.7 MW) with nozzle exit Reynolds and Froude numbers ranging from 13,000 to 110,000 and 17 to 100,000, respectively. This has been achieved by varying the exit gas velocity between 2.2 and 63 m/s and the exit nozzle diameter between 4 and 28 mm. Both axial and radial profiles have been measured above the visible flame region. The measuring equipment includes continuous gas analyzers and water cooled quartz/stainless steel suction probes. Experiments on medium size vertical propane jet diffusion flames have also been performed. The experimental setup was similar to that of the smaller laboratory flames. A nozzle with an exit diameter of 17 mm was used, and an exit Mach number close to 0.9, resulting in a visible flame height of approximately 7.3 m. The maximum heat release was approximately 9 MW. Scaling laws for the global NOx emission index as a function of the nozzle exit velocity, the nozzle outlet diameter and the Froude number are presented. The results show a 0.2 power law dependence on the exit velocity and a 0.4 power law dependence on the nozzle exit diameter. The results further show that the NOx emission index is strongly dependent upon the Froude number. These formulas can be employed for engineering purposes and provide guidance for calculating NOx emissions from turbulent propane jet diffusion flames.

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

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