Lean Premixed Turbulent Flame Front Structure and Implications for Modeling

2010 ◽  
Author(s):  
Frank T. C. Yuen ◽  
O¨mer L. Gu¨lder
Keyword(s):  
Flame Front ◽  
Turbulent Flame ◽  
Turbulent Flames ◽  
Experimental Conditions ◽  
Flame Thickness ◽  
Premixed Turbulent Combustion ◽  
Wide Range ◽  
Significant Difference ◽  
Lean Premixed ◽  
Front Curvature

Premixed turbulent flames of methane-air and propane-air stabilized on a Bunsen type burner were studied to investigate the structure of the flame front at a wide range of turbulence intensities covering the range of interest in lean premixed combustors. The flame front data were obtained using planar Rayleigh imaging, and particle image velocimetry was used to measure instantaneous velocity field for the experimental conditions studied. The fuel-air equivalence ratio range was from lean 0.6 to stoichiometric for methane flames, and from 0.7 to stoichiometric for propane flames. The non-dimensional turbulent rms velocity, u′/SL, covered a range from 3 to 24. Flame front thickness and flame front curvature statistics were obtained from 2D measurements. Flame front thickness increased slightly with increasing non-dimensional turbulence rms velocity in both methane and propane flames, although the flame thickening was more prominent in propane flames. There was not any significant difference in flame thickening whether the flame thickness is evaluated at progress variable 0.5, corresponding to the reaction zone, or 0.3, corresponding to the preheat zone. Variations of front curvature and flame thickness are presented for different premixed combustion regimes and implications of these findings for modelling premixed turbulent combustion are discussed.

The Effect of Lewis Number on Instantaneous Flamelet Speed and Position Statistics in Counter-Flow Flames With Increasing Turbulence

2017 ◽  
Author(s):  
Sean D. Salusbury ◽  
Ehsan Abbasi-Atibeh ◽  
Jeffrey M. Bergthorson
Keyword(s):  
Flame Front ◽  
Turbulence Intensity ◽  
High Speed ◽  
Rayleigh Scattering ◽  
Turbulent Flame ◽  
Lewis Number ◽  
Turbulent Flames ◽  
Flame Speed ◽  
Laminar Flames ◽  
Counter Flow

Differential diffusion effects in premixed combustion are studied in a counter-flow flame experiment for fuel-lean flames of three fuels with different Lewis numbers: methane, propane, and hydrogen. Previous studies of stretched laminar flames show that a maximum reference flame speed is observed for mixtures with Le ≳ 1 at lower flame-stretch values than at extinction, while the reference flame speed for Le ≪ 1 increases until extinction occurs when the flame is constrained by the stagnation point. In this work, counter-flow flame experiments are performed for these same mixtures, building upon the laminar results by using variable high-blockage turbulence-generating plates to generate turbulence intensities from the near-laminar u′/SLo=1 to the maximum u′/SLo achievable for each mixture, on the order of u′/SLo=10. Local, instantaneous reference flamelet speeds within the turbulent flame are extracted from high-speed PIV measurements. Instantaneous flame front positions are measured by Rayleigh scattering. The probability-density functions (PDFs) of instantaneous reference flamelet speeds for the Le ≳ 1 mixtures illustrate that the flamelet speeds are increasing with increasing turbulence intensity. However, at the highest turbulence intensities measured in these experiments, the probability seems to drop off at a velocity that matches experimentally-measured maximum reference flame speeds in previous work. In contrast, in the Le ≪ 1 turbulent flames, the most-probable instantaneous reference flamelet speed increases with increasing turbulence intensity and can, significantly, exceed the maximum reference flame speed measured in counter-flow laminar flames at extinction, with the PDF remaining near symmetric for the highest turbulence intensities. These results are reinforced by instantaneous flame position measurements. Flame-front location PDFs show the most probable flame location is linked both to the bulk flow velocity and to the instantaneous velocity PDFs. Furthermore, hydrogen flame-location PDFs are recognizably skewed upstream as u′/SLo increases, indicating a tendency for the Le ≪ 1 flame brush to propagate farther into the unburned reactants against a steepening average velocity gradient.

Flame Characteristics and Turbulent Flame Speeds of Turbulent, High-Pressure, Lean Premixed Methane/Air Flames

2005 ◽  
Author(s):  
P. Griebel ◽  
R. Bombach ◽  
A. Inauen ◽  
R. Scha¨ren ◽  
S. Schenker ◽  
...  
Keyword(s):  
Flame Front ◽  
Gas Turbines ◽  
Equivalence Ratio ◽  
Turbulent Flame ◽  
Flame Speed ◽  
Operating Conditions ◽  
Turbulent Flame Speed ◽  
Front Position ◽  
Preheating Temperature ◽  
Lean Premixed

The present experimental study focuses on flame characteristics and turbulent flame speeds of lean premixed flames typical for stationary gas turbines. Measurements were performed in a generic combustor at a preheating temperature of 673 K, pressures up to 14.4 bars (absolute), a bulk velocity of 40 m/s, and an equivalence ratio in the range of 0.43–0.56. Turbulence intensities and integral length scales were measured in an isothermal flow field with Particle Image Velocimetry (PIV). The turbulence intensity (u′) and the integral length scale (LT) at the combustor inlet were varied using turbulence grids with different blockage ratios and different hole diameters. The position, shape, and fluctuation of the flame front were characterized by a statistical analysis of Planar Laser Induced Fluorescence images of the OH radical (OH-PLIF). Turbulent flame speeds were calculated and their dependence on operating conditions (p, φ) and turbulence quantities (u′, LT) are discussed and compared to correlations from literature. No influence of pressure on the most probable flame front position or on the turbulent flame speed was observed. As expected, the equivalence ratio had a strong influence on the most probable flame front position, the spatial flame front fluctuation, and the turbulent flame speed. Decreasing the equivalence ratio results in a shift of the flame front position farther downstream due to the lower fuel concentration and the lower adiabatic flame temperature and subsequently lower turbulent flame speed. Flames operated at leaner equivalence ratios show a broader spatial fluctuation as the lean blow-out limit is approached and therefore are more susceptible to flow disturbances. In addition, because of a lower turbulent flame speed these flames stabilize farther downstream in a region with higher velocity fluctuations. This increases the fluctuation of the flame front. Flames with higher turbulence quantities (u′, LT) in the vicinity of the combustor inlet exhibited a shorter length and a higher calculated flame speed. An enhanced turbulent heat and mass transport from the recirculation zone to the flame root location due to an intensified mixing which might increase the preheating temperature or the radical concentration is believed to be the reason for that.

Role of Darrieus–Landau instability in propagation of expanding turbulent flames

2018 ◽  
Vol 850 ◽  
pp. 784-802 ◽  
Author(s):  
Sheng Yang ◽  
Abhishek Saha ◽  
Zirui Liu ◽  
Chung K. Law
Keyword(s):  
Flame Front ◽  
Flame Propagation ◽  
High Pressures ◽  
Turbulent Flame ◽  
Turbulent Flames ◽  
Scaling Analysis ◽  
Turbulent Eddies ◽  
Front Instability ◽  
Cellular Flame

In this paper we study the essential role of Darrieus–Landau (DL), hydrodynamic, cellular flame-front instability in the propagation of expanding turbulent flames. First, we analyse and compare the characteristic time scales of flame wrinkling under the simultaneous actions of DL instability and turbulent eddies, based on which three turbulent flame propagation regimes are identified, namely, instability dominated, instability–turbulence interaction and turbulence dominated regimes. We then perform experiments over an extensive range of conditions, including high pressures, to promote and manipulate the DL instability. The results clearly demonstrate the increase in the acceleration exponent of the turbulent flame propagation as these three regimes are traversed from the weakest to the strongest, which are respectively similar to those of the laminar cellularly unstable flame and the turbulent flame without flame-front instability, and thus validating the scaling analysis. Finally, based on the scaling analysis and the experimental results, we propose a modification of the conventional turbulent flame regime diagram to account for the effects of DL instability.

Effects of molecular diffusion and of thermal expansion on the structure and dynamics of premixed flames in turbulent flows of large scale and low intensity

1982 ◽  
Vol 116 ◽  
pp. 251-282 ◽  
Author(s):  
P. Clavin ◽  
F. A. Williams
Keyword(s):  
Thermal Expansion ◽  
Flame Front ◽  
Turbulent Flows ◽  
Thermal Model ◽  
Laminar Flame ◽  
Premixed Flames ◽  
Turbulent Flames ◽  
Flame Thickness ◽  
The Mean ◽  
Diffusivity Ratio

To study effects of flow inhomogeneities on the dynamics of laminar flamelets in turbulent flames, with account taken of influences of the gas expansion produced by heat release, a previously developed theory of premixed flames in turbulent flows, that was based on a diffusive-thermal model in which thermal expansion was neglected, and that applied to turbulence having scales large compared with the laminar flame-thickness, is extended by eliminating the hypothesis of negligible expansion and by adding the postulate of weak-intensity turbulence. The consideration of thermal expansion motivates the formal introduction of multiple-scale methods, which should be useful in subsequent investigations. Although the hydrodynamic-instability mechanism of Landau is not considered, no restriction is imposed on the density change across the flame front, and the additional transverse convection correspondingly induced by the tilted front is described. By allowing the heat-to-reactant diffusivity ratio to differ slightly from unity, clarification is achieved of effects of phenomena such as flame stretch and the flame-relaxation mechanism traceable to transverse diffusive processes associated with flame-front curvature. By carrying the analysis to second order in the ratio of the laminar flame thickness to the turbulence scale, an equation for evolution of the flame front is derived, containing influences of transverse convection, flame relaxation and stretch. This equation explains anomalies recently observed at low frequencies in experimental data on power spectra of velocity fluctuations in turbulent flames. It also shows that, concerning the diffusive-stability properties of the laminar flame, the density change across the flame thickness produces a shift of the stability limits from those obtained in the purely diffusive-thermal model. At this second order, the turbulent correction to the flame speed involves only the mean area increase produced by wrinkling. The analysis is carried to the fourth order to demonstrate the mean-stretch and mean-curvature effects on the flame speed that occur if the diffusivity ratio differs from unity.

Numerical Modeling of Stationary But Developing Premixed Turbulent Flames

2006 ◽  
Author(s):  
Pratap Sathiah ◽  
Andrei N. Lipatnikov
Keyword(s):  
Bluff Body ◽  
Burning Velocity ◽  
Rectangular Channel ◽  
Turbulent Flame ◽  
Flame Stabilization ◽  
Turbulent Flames ◽  
Turbulent Diffusivity ◽  
Flame Thickness ◽  
Premixed Turbulent Flames ◽  
Premixed Turbulent Flame

A typical stationary premixed turbulent flame is the developing flame, as indicated by the growth of mean flame thickness with distance from flame-stabilization point. The goal of this work is to assess the importance of modeling flame development for RANS simulations of confined stationary premixed turbulent flames. For this purpose, submodels for developing turbulent diffusivity and developing turbulent burning velocity, which were early suggested by our group (FSC model) and validated for expanding spherical flames [4], have been incorporated into the so-called Zimont model of premixed turbulent combustion and have been implemented into the CFD package Fluent 6.2. The code has been run to simulate a stationary premixed turbulent flame stabilized behind a triangular bluff body in a rectangular channel using both the original and extended models. Results of these simulations show that the mean temperature and velocity fields in the flame are markedly affected by the development of turbulent diffusivity and burning velocity.

scholarly journals Extraction of passion fruit seed oil and microencapsulation in poly(ε-caprolactone).

2014 ◽  
Vol 5 (2) ◽  
pp. 1 ◽  
Author(s):  
Jéssica Cintia Barbieri ◽  
Fernanda Vitória Leimann
Keyword(s):  
Seed Oil ◽  
Unsaturated Fatty Acids ◽  
Average Diameter ◽  
Passion Fruit ◽  
Experimental Conditions ◽  
Stirring Rate ◽  
Wide Range ◽  
Significant Difference ◽  
Emulsification Solvent Evaporation ◽  
Fruit Oil

<p><em>The processing of passion fruit juice generates a large amount of by-products, which are seeds and shells, which are usually discarded becoming an industrial waste problem. It represents significant waste amounts, turning it a scientific and technological interesting resource to add value. The passion fruit seed oil has a wide range of application in industry, but it presents a high content of unsaturated fatty acids, being susceptible to oxidative rancidity. An alternative to avoid the exposure of the oil to atmospheric air, that leads to its oxidation, and increase its stability is its microencapsulation. In this work the passion fruits seed oil was extracted with hexane and the resulting oil was microencapsulated by emulsification solvent evaporation with poly(&epsilon;-caprolactone) (PCL). The influence of the experimental conditions, stirring rate and oil amount, was evaluated on the final average diameter (Dp) and size distribution (Polydispersion index, PDI) of the microcapsules by Students T-test with 95% of confidence. The interaction of the polymeric shell with the encapsulated oil was evaluated by Spectroscopy Fourier Transform Infrared Spectroscopy (FTIR). The yield of oil extraction obtained was equal to 29.5%. Microcapsules with 137.4 &micro;m were obtained when 500 RPM of stirring rate was used. A significant difference was observer in Dp between the blank microparticles (no oil) and microcapsules at 500 RPM. When the stirring rate was increased to 1,000 RPM any significant difference was observed in the Dp nor in PDI to microcapsules. It was possible to observe interactions between the polymeric shell and the passion fruit oil by FTIR spectra.</em></p><p><em>DOI: 10.14685/rebrapa.v5i2.151</em></p>

A Modeling Approach for Hydrogen-Doped Lean Premixed Turbulent Combustion

2006 ◽  
Author(s):  
Siva P. R. Muppala ◽  
Miltiadis V. Papalexandris
Keyword(s):  
Turbulent Combustion ◽  
Turbulent Flame ◽  
Turbulent Flames ◽  
Flame Speed ◽  
Hydrocarbon Mixtures ◽  
Empirical Constant ◽  
Premixed Turbulent Combustion ◽  
Turbulent Flame Speed ◽  
Preferential Diffusion ◽  
Model Predictions

In this study, we investigate some preliminary reaction model predictions analytically in comparison with experimental premixed turbulent combustion data from four different flame configurations, which include i) high-jet enveloped, ii) expanding spherical, iii) Bunsen-like, and iv) wide-angled diffuser flames. The special intent of the present work is to evaluate the workability range of the model to hydrogen and hydrogen-doped hydrocarbon mixtures, emphasizing on the significance of preferential diffusion, PD, and Le effects in premixed turbulent flames. This is carried out in two phases: first, involving pure hydrocarbon and pure hydrogen mixtures from two independent measured data, and second, with the blended mixtures from two other data sets. For this purpose, a novel reaction closure embedded with explicit high-pressure and exponential Lewis number terms developed in the context of hydrocarbon mixtures is used. These comparative studies based on the global quantity, turbulent flame speed, indicate that the model predictions are encouraging yielding proper quantification along with reasonable characterization of all the four different flames, over a broad range of turbulence, fuel-types and for varied equivalence ratios. However, with each flame involved the model demands tuning of the (empirical) constant to allow for either or both of these effects, or for the influence of the burner geometry. This provisional stand remains largely insufficient. Therefore, a submodel for chemical time scale from the leading point analysis based on the critically curved laminar flames employed in earlier studies for expanding spherical flames is introduced here. By combining the submodel and the reaction closure, the dependence of turbulent flame speed on physicochemical properties of the burning mixtures including the strong dependence of preferential diffusion and/or Le effects can be determined.

Influence of Pressure on Markstein Number Effect in Turbulent Flame Front Propagation

2013 ◽  
Author(s):  
Vlade Vukadinovic ◽  
Peter Habisreuther ◽  
Nikolaos Zarzalis ◽  
Rainer Suntz
Keyword(s):  
Flame Front ◽  
Burning Velocity ◽  
Turbulent Flame ◽  
Initial Pressure ◽  
Front Propagation ◽  
Mie Scattering ◽  
Turbulent Flames ◽  
Laser Method ◽  
Turbulent Burning Velocity ◽  
Optical Laser

In gas turbine operation a turbulent flame is employed. Thus, better understanding of the turbulent flame propagation is the key for further optimisation of turbine combustors and reduction of the environmental footprint. As turbulent flames are exposed to stretch, the effect of flame-stretch interaction must be better understood especially at higher pressures. In present study, turbulent burning velocity of two mixtures, hydrogen/air and propane/air, with negative and positive Ma, respectively are experimentally investigated in fan-stirred explosion vessel. For the investigation an optical laser method is employed based on the Mie-scattering of the laser light by smoke particles. Within this study the influence of initial parameters as initial pressure and turbulence intensity on the flame front propagation is investigated by giving special attention on influence of Ma variation. The experiments were performed at three different pressures 1, 2, 4 bar. The RMS fluctuation velocity was varied in the range of 0–2.77 m/s. The observed results are compared and discussed in detail.

Investigation of Lean Premixed Swirl-Stabilized Hydrogen Burner With Axial Air Injection Using OH-PLIF Imaging

2015 ◽  
Vol 137 (11) ◽  
Author(s):  
Thoralf G. Reichel ◽  
Katharina Goeckeler ◽  
Oliver Paschereit
Keyword(s):  
Flow Field ◽  
Flame Front ◽  
Gas Turbines ◽  
Equivalence Ratio ◽  
Air Injection ◽  
Turbulent Flame Speed ◽  
Air Jet ◽  
Wide Range ◽  
Lean Premixed ◽  
Inlet Conditions

In the context of lean premixed combustion, the prevention of upstream flame propagation in the premixing zone, referred to as flashback (FB), is a crucial challenge related to the application of hydrogen as a fuel for gas turbines. The location of flame anchoring and its impact on FB tendencies in a technically premixed, swirl-stabilized hydrogen burner are investigated experimentally at atmospheric pressure conditions using planar laser-induced fluorescence of hydroxyl radicals (OH-PLIF). The inlet conditions are systematically varied with respect to equivalence ratio (ϕ=0.2−1.0), bulk air velocity u0 = 30–90 m/s, and burner preheat temperature ranging from 300 K to 700 K. The burner is mounted in an atmospheric combustion test rig, firing at a power of up to 220 kW into a 105 mm diameter quartz cylinder, which provides optical access to the flame region. The experiments were performed using an in-house burner design that previously proved to be highly resistant against FB occurrence by applying the axial air injection strategy. Axial air injection constitutes a nonswirling air jet on the central axis of the radial swirl generator. While a high rate of axial air injection yields excellent FB resistance, reduced rates of air injection are utilized to trigger FB, which allowed to investigate the near FB flame behavior. Results show that both, fuel momentum of hydrogen and axial air injection, alter the isothermal flow field as they cause a downstream shift of vortex breakdown and, thus, the axial flame front location. Such a shift is proven beneficial for FB resistance from the recorded FB limits. This effect was quantified by applying an edge detection algorithm to the OH-PLIF images, in order to extract the location of maximum flame front probability xF. By these means, it was revealed that for hydrogen xF is shifted downstream with increasing equivalence ratio due to the added momentum of the fuel flow, superseding any parallel augmentation in the turbulent flame speed. The parameter xF is identified to be governed by J, the momentum ratio between fuel and air flow, over a wide range of inlet conditions. These results contribute to the understanding of the sensitivity of FB to changes in the flow field, stemming from geometry changes or specific fuel properties.

Modeling Approach for Lean Blowout Phenomenon

2007 ◽  
Author(s):  
Yu. G. Kutsenko ◽  
S. F. Onegin ◽  
L. Y. Gomzikov ◽  
A. Belokon’ ◽  
V. Zakharov
Keyword(s):  
Gas Turbine ◽  
Turbulent Flame ◽  
Combustion Process ◽  
Turbine Engine ◽  
Lean Blowout ◽  
Turbulent Flame Speed ◽  
Wide Range ◽  
Lean Premixed ◽  
Lean Fuel ◽  
Operational Modes

Modern concepts for reducing thermal NO emissions require the use of very lean fuel/air mixtures. Therefore a problem of lean quench should be solved during design process of gas turbine combustor and it’s operational development. Since maintenance of flame stability for wide range of gas turbine engine operational modes is essential, therefore there is a great demand for models which are able to predict lean blow out limits of turbulent, premixed and partially premixed, aerodynamically stabilized flames. In this paper a model describing flame destabilization process is presented. This model takes into account various physical processes, which lead to flame destabilization. The model is based on equation for reaction progress variable. An expression of source term of this equation contains turbulent flame speed, which is calculated with the use of Zimont’s formula modification, proposed by authors. The results of simulation were compared with test results for our lean premixed combustor. Fuel mass flow rate of pilot zone was decreased during test until heat release of pilot flame front became insufficient and couldn’t support a combustion process in a lean premixed zone. Our simulation with modified model allows to get prediction of lean blowout limit.

Sign in / Sign up

Export Citation Format

Share Document