Flamelet Studies of Reduced and Detailed Kinetic Mechanisms for Methane/Air Diffusion Flames

2000 ◽  
Author(s):  
Foluso Ladeinde ◽  
Xiaodan Cai ◽  
Balu Sekar
Keyword(s):  
Dissipation Rate ◽  
Diffusion Flames ◽  
Mixture Fraction ◽  
Intermediate Species ◽  
Flamelet Model ◽  
Mass Fractions ◽  
Kinetic Mechanisms ◽  
Moderate Effect ◽  
Extinction Strain Rate ◽  
Air Diffusion

We adopt a steady-state flamelet model in this paper to study the performance of reduced and detailed kinetic mechanisms for methane/air diffusion flames. Through the numerical calculations, we investigate the sensitivity of the main and intermediate species mass fractions to the mixture fraction dissipation rate, χ. Our results seem to suggest a weak to moderate effect of χ on the calculated species mass fraction. It has also been shown in this paper that the current flamelet calculations fail to predict the extinction strain rate.

A Skewed Two-Dimensional Probability Density Function for Methane-Air Turbulent Diffusion Flames

2010 ◽  
Author(s):  
Mohsen Abou-Ellail ◽  
Ryo S. Amano ◽  
Samer Elhaw ◽  
Karam Beshay ◽  
Hatem Kayed
Keyword(s):  
Mathematical Model ◽  
Probability Density Function ◽  
Probability Density ◽  
Density Function ◽  
Dissipation Rate ◽  
Diffusion Flames ◽  
Mixture Fraction ◽  
Two Dimensional ◽  
Flamelet Model ◽  
The Mean

The present paper describes a mathematical model for turbulent methane-air jet diffusion flames. The mathematical model solves density-weighted governing equations for momentum, mass continuity, turbulent kinetic energy and its dissipation rate. The combustion model solves density-weighted transport equations for the mixture fraction “f”, its variance “g” and its skewness “s”. These variables are used to compute one part of the probability density function (PDF) in mixture fraction domain. The second part of the PDF is computed from the numerical solutions of the mixture fraction dissipation rate “χ” and its variance χ˜″2. The resulting two-dimensional PDF is defined in the mixture-fraction-scalar-dissipation-rate 2D space. The flamelet combustion sub-model is used to compute the mean flame temperature, density and species mass fractions. The flamelet model provides instantaneous state relationships for the stretched flamelets up to the extinction limit. The mean flame properties are computed through the integration of the stretched flamelet state relationships over the two-dimensional PDF. The present 2D probability density function model can predict rim-attached flames as well as unstable lifted flames. This is because the flamelet model provides information on the flame instability arising from the stretching effects of highspeed flowing gases. The new two-dimensional probability density function is used to predict the flame properties of a free jet methane-air flame for which experimental data exists.

Mixing Models for Large-Eddy Simulation of Nonpremixed Turbulent Combustion

2000 ◽  
Vol 123 (2) ◽  
pp. 341-346 ◽  
Author(s):  
S. M. deBruynKops ◽  
J. J. Riley
Keyword(s):  
Turbulent Combustion ◽  
Dissipation Rate ◽  
Mixture Fraction ◽  
Reaction Rates ◽  
Large Eddy Simulations ◽  
Scalar Dissipation ◽  
Mixing Models ◽  
Mixing Rate ◽  
Flamelet Model ◽  
Large Eddy

The application of mixture fraction based models to large-eddy simulations (LES) of nonpremixed turbulent combustion requires information about mixing at length scales not resolved on the LES grid. For instance, the large-eddy laminar flamelet model (LELFM) takes the subgrid-scale variance and the filtered dissipation rate of the mixture fraction as inputs. Since chemical reaction rates in nonpremixed turbulence are largely governed by the mixing rate, accurate mixing models are required if mixture fraction methods are to be successfully used to predict species concentrations in large-eddy simulations. In this paper, several models for the SGS scalar variance and the filtered scalar dissipation rate are systematically evaluated a priori using benchmark data from a DNS in homogeneous, isotropic, isothermal turbulence. The mixing models are also evaluated a posteriori by applying them to actual LES data of the same flow. Predictions from the models that depend on an assumed form for the scalar energy spectrum are very good for the flow considered, and are better than those from models that rely on other assumptions.

scholarly journals Structure and Radiation Properties of Luminous Turbulent Acetylene/Air Diffusion Flames

1988 ◽  
Vol 110 (1) ◽  
pp. 173-181 ◽  
Author(s):  
J. P. Gore ◽  
G. M. Faeth
Keyword(s):  
Radiative Heat ◽  
Diffusion Flames ◽  
Mixture Fraction ◽  
Heat Fluxes ◽  
Band Model ◽  
Spectral Radiation ◽  
Radiative Heat Loss ◽  
Radiation Properties ◽  
Air Diffusion ◽  
Laminar Flamelet

An experimental and theoretical study of the structure and radiation properties of luminous, round, turbulent acetylene/air diffusion flames is described. Measurements were made of mean and fluctuating velocities, mean concentrations, laser extinction (514 and 632.8 nm), spectral radiation intensities (1200–5500 nm), and radiative heat fluxes. The measurements were used to evaluate structure predictions based on the laminar flamelet concept, and radiation predictions based on a narrow-band model both ignoring and considering turbulence/radiation interactions. State relationships needed for the laminar flamelet concept were found from auxiliary measurements in laminar flames. Predictions were encouraging; however, quantitative accuracy was inferior to earlier findings for luminous flames. This is attributed to the large radiative heat loss fractions of acetylene/air flames (approaching 60 percent of the heat release rate); coupled structure and radiation analysis should be considered for improved results. The findings suggest significant turbulence/radiation interactions (increasing spectral intensities 40–100 percent from estimates based on mean properties); and that soot volume fractions may approximate universal fractions of mixture fraction in turbulent acetylene/air diffusion flames.

A General Semicausal Stochastic Model for Turbulence/Radiation Interactions in Flames

1997 ◽  
Vol 119 (3) ◽  
pp. 509-516 ◽  
Author(s):  
S. H. Chan ◽  
X. C. Pan
Keyword(s):  
Causal Model ◽  
Diffusion Flames ◽  
Mixture Fraction ◽  
Reynolds Numbers ◽  
Time Space ◽  
Proposed Model ◽  
Geometric Space ◽  
Mean And Variance ◽  
Air Diffusion ◽  
Spectral Intensities

This paper presents a general two-dimensional non-stationary semicausal model for the simulation of mixture fraction, which improves our previous causal model. The proposed model includes not only the pre-correlation predictors (both in time space and geometric space) as well as the cross-correlation predictors, as in the causal model, but also post-correlation predictors. The latter makes possible the consideration of interactions of a scalar, such as mixture fraction, at a physical location with that of all its adjacent locations. It has also been shown that the complicated second-and higher-order correlation predictors can be neglected in the semicausal simulation of mixture fraction. To show the validity of the model, the stochastic mean and variance of the spectral intensities at different wavelengths were predicted and compared with detailed experimental data for turbulent carbon monoxide/hydrogen/air diffusion flames having different Reynolds numbers. These comparisons showed excellent agreement with existing data and the improvement over the prior causal model.

scholarly journals A combustion model sensitivity study for CH4/H2 bluff-body stabilized flame

2007 ◽  
Vol 221 (11) ◽  
pp. 1377-1390 ◽  
Author(s):  
M Hossain ◽  
W Malalasekera
Keyword(s):  
Bluff Body ◽  
Mixture Fraction ◽  
Sensitivity Study ◽  
Combustion Model ◽  
Flamelet Model ◽  
Combustion Models ◽  
Mass Fractions ◽  
Experimental Values ◽  
Constituent Mass ◽  
Laminar Flamelet

The objective of the current work is to assess the performance of different combustion models in predicting turbulent non-premixed combustion in conjunction with the k-∊ turbulence model. The laminar flamelet, equilibrium chemistry, constrained equilibrium chemistry, and flame sheet models are applied to simulate combustion in a CH4/H2 bluff-body flame experimentally studied by the University of Sydney. The computational results are compared to experimental values of mixture fraction, temperature, and constituent mass fractions. The comparison shows that the laminar flamelet model performs better than other combustion models and mimics most of the significant features of the bluff-body flame.

Development of a Species-Based Extended Coherent Flamelet Model (SB-ECFM) for Gasoline Direct Injection Engine (GDI) Simulations

2018 ◽  
Author(s):  
O. Colin ◽  
S. Chevillard ◽  
J. Bohbot ◽  
P. K. Senecal ◽  
E. Pomraning ◽  
...  
Keyword(s):  
Reference Model ◽  
Mixture Fraction ◽  
Direct Injection ◽  
Gasoline Direct Injection ◽  
Current Work ◽  
Flamelet Model ◽  
Progress Variable ◽  
Auto Ignition ◽  
Mass Fractions ◽  
The Difference

The current work presents a recent development of the Extended Coherent Flamelet Model (ECFM) for 3D combustion modeling in spark-ignited gasoline engines. The reference-based ECFM model, originally published in 2003, computes the conditional unburned and burned gas species mass fractions from both real species and species tracers. This current work is motivated by two limitations of the reference-based model. First, the difference between convection of species tracers and convection of real species leads to small discrepancies between the two, due to high velocity gradients during gas exchange. This can lead to inaccurate estimation of the progress variable and consequently to negative conditional mass fractions in the burned gases after ignition. Second, the reference-based ECFM model assumes implicitly that the unburned and burned states correspond to the same mixture fraction. This assumption is valid for low stratification cases, but it can lead to substantial conditioning errors for highly stratified systems like gasoline direct injection (GDI) engines. To address these shortcomings, a new species-based ECFM (SB-ECFM) implementation is presented. In this species-based model, the unburned and burned gas states are entirely defined by the transported species in each zone. It is shown that SB-ECFM more reliably defines conditional quantities and the progress variable. This enhancement allows the use of a second-order central scheme in space when using full decoupling of auto-ignition and premixed flame progress variables as proposed in Robert et al., Proc. Comb. Inst, 2015, while the reference model is limited to the first-order upwind scheme in this case. Finally, simulations of a GDI engine are presented at different loads and rpm conditions. It is shown that, with the higher order scheme, SB-ECFM demonstrates very good agreement with measured pressure.

Three-Stream Flamelet Model for Industrial Applications

2010 ◽  
Vol 132 (6) ◽  
Author(s):  
Dirk Riechelmann ◽  
Masahiro Uchida
Keyword(s):  
Turbulent Combustion ◽  
Diffusion Flames ◽  
Mixture Fraction ◽  
Industrial Applications ◽  
Exhaust Gas ◽  
Water Steam ◽  
Flamelet Model ◽  
Jet Diffusion Flames ◽  
Key Points ◽  
Lift Off

Efficient turbulent combustion models are typically designed for the numerical simulation of two-stream problems, namely, the combustion of fuel in air. There are applications, however, where large amounts of a diluent such as water steam or recirculated exhaust gas is supplied to the combustor independent of fuel and air supplies. In such cases, classical approaches become quite time-consuming. In the present paper, a new three-stream flamelet model is presented, which is essentially an extension of the two-stream flamelet model for diffusion flames. Key points of the approach are the introduction of a second mixture fraction variable and the efficient establishment of the flamelet library. After presentation of the theory, the applicability of the new model is demonstrated by comparison with experimental results for the lift-off height of jet diffusion flames.

Derivation and Evaluation of a Multi-regime Spray Flamelet Model

2015 ◽  
Vol 229 (4) ◽  
Author(s):  
Hernan Olguin ◽  
Eva Gutheil
Keyword(s):  
Dissipation Rate ◽  
Mixture Fraction ◽  
Lewis Number ◽  
Scalar Dissipation ◽  
Scalar Dissipation Rate ◽  
Flamelet Model ◽  
Spray Flames ◽  
Special Cases ◽  
Turbulent Spray ◽  
Combustion Regimes

AbstractThe formulation of a comprehensive flamelet model to consider detailed chemical reaction mechanisms in the simulation of turbulent spray flames is a very challenging task due to the inherent multi-regime structure of spray flames. Non-premixed, premixed, and evaporation-controlled combustion regimes may be found in a single spray flame. Recently, attempts have been made to extend classical single regime flamelet models to more complex situations, where at least two combustion regimes coexist. The objective of this work is to develop a framework in which two-regime flamelet models can be described and combined in order to advance the development of a comprehensive flamelet model for turbulent spray flames. For this purpose, a set of spray flamelet equations in terms of the mixture fraction and a reaction progress variable is derived, which includes the evaporation, characterizing the spray flames, and which describes all combustion regimes appearing in spray flames. The two-regime and single regime flamelet equations available in the literature are retrieved from these multi-dimensional spray flamelet equations as special cases. The derived set of spray flamelet equations is then used to evaluate structures of laminar ethanol/air spray flames in the counterflow configuration in order to determine the significance of different combustion regimes. The present study concerns spray flames with no pre-vaporized liquid in the oxidizing gas phase, and it is found that only non-premixed and evaporation-controlled combustion regimes exist, so that premixed effects may be neglected. Moreover, an exact transport equation for the scalar dissipation rate is derived, which explicitly takes spray evaporation and detailed transport into account. This equation is then used to evaluate assumptions commonly adopted in the literature. The results show that the spatial variation of the mean molecular weight of the mixture may be neglected in the formulation of the mixture fraction, but it may be significant for its scalar dissipation rate. The assumption of unity Lewis number may lead to non-physical values of the scalar dissipation rate of the mixture fraction, whereas the use of a mass-averaged diffusion coefficient of the mixture is a good approximation for the spray flames under investigation.

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