Flamelet Model of NOx in a Diffusion Flame Combustor

2000 ◽  
Vol 123 (4) ◽  
pp. 774-778 ◽  
Author(s):  
D. V. Volkov ◽  
A. A. Belokin ◽  
D. A. Lyubimov ◽  
V. M. Zakharov ◽  
G. Opdyke,
Keyword(s):  
Diffusion Flame ◽  
Reverse Flow ◽  
Mixture Fraction ◽  
Formation Rate ◽  
Navier Stokes ◽  
Scalar Dissipation ◽  
Nox Formation ◽  
Flamelet Model ◽  
Good Potential ◽  
Detailed Kinetics

This paper describes a model used for the prediction of the formation of nitrogen oxides in modifications of an industrial diffusion flame, natural gas fueled can combustor. The flowfield inside the modified combustors is calculated using a Navier-Stokes solver. A fast chemistry assumption is used for modeling the heat release. Calculated turbulence parameters are then used for the calculation of the NOx formation rate in the post-processing mode with the aid of a flamelet model. The flamelet model permits the use of detailed kinetics with only minimal computational expense. The dependence of the NOx formation rate on the mixture fraction and scalar dissipation is calculated separately for each given condition. The validation of the model predictions is based on field test data taken earlier on several low NOx modifications recently applied to an industrial, reverse flow can type combustor. The reduced level of NOx emissions was achieved in these modifications by changes in the air distribution within the combustor liner. A comparison of the predicted and measured NOx emission levels shows good potential of the flamelet model.

Flamelet Model of NOx in a Diffusion Flame Combustor

2000 ◽  
Author(s):  
Dmitry V. Volkov ◽  
Alexandr A. Belokon ◽  
Dmitry A. Lyubimov ◽  
Vladimir M. Zakharov ◽  
George Opdyke
Keyword(s):  
Diffusion Flame ◽  
Reverse Flow ◽  
Mixture Fraction ◽  
Formation Rate ◽  
Navier Stokes ◽  
Scalar Dissipation ◽  
Nox Formation ◽  
Flamelet Model ◽  
Good Potential ◽  
Detailed Kinetics

This paper describes a model used for the prediction of the formation of nitrogen oxides in modifications of an industrial diffusion flame, natural gas fueled can combustor. The flow field inside the modified combustors is calculated using a Navier-Stokes solver. A fast chemistry assumption is used for modeling the heat release. Calculated turbulence parameters are then used for the calculation of the NOx formation rate in the post-processing mode with the aid of a flamelet model. The flamelet model permits the use of detailed kinetics with only minimal computational expense. The dependence of the NOx formation rate on the mixture fraction and scalar dissipation is calculated separately for each given condition. The validation of the model predictions is based on field test data taken earlier on several low NOx modifications recently applied to an industrial, reverse flow can type combustor. The reduced level of NOx emissions was achieved in these modifications by changes in the air distribution within the combustor liner. A comparison of the predicted and measured NOx emission levels shows good potential of the flamelet model.

Numerical Analysis of NOx Formation in a Diffusion Flame Combustor Based on a Flamelet Model

2001 ◽  
Author(s):  
Dmitry V. Volkov ◽  
Alexandr A. Belokon ◽  
Dmitry A. Lyubimov ◽  
Vladimir M. Zakharov ◽  
George Opdyke
Keyword(s):  
Diffusion Flame ◽  
Turbulent Mixing ◽  
Flow Distribution ◽  
Nox Emission ◽  
Scalar Dissipation ◽  
3D Flow ◽  
Nox Formation ◽  
Flamelet Model ◽  
Primary Zone ◽  
Consumption Rates

Laminar flamelet models have demonstrated good quality predictions of NOx emission from diffusion flame type combustors. In this paper, the NOx formation process is analyzed by using a flamelet model and 3D flow calculations to take a virtual look inside a combustor. The main phenomena affecting NOx emission are turbulent mixing and the turbulence-chemistry interaction. Local scalar dissipation is the main parameter responsible for the turbulence-chemistry interaction within the flamelet model. At the same time, scalar dissipation is also related to the mixing process. On one hand, higher values of scalar dissipation correspond to higher fuel consumption rates, which decrease the volume of the high temperature zones. On the other hand, higher values of scalar dissipation lead to higher NOx formation rates. Unfortunately, scalar dissipation is not commonly used by combustion engineers because of the difficulty of the clear physical interpretation of this variable and its relationship with the usual parameters. In this paper, the influence of several design features, such as primary zone equivalence ratio and air flow distribution along the liner, is studied relative to scalar dissipation distributions in the combustion zones and to NOx formation. A real industrial diffusion flame combustor is used as an example, and the results can provide a better understanding of real combustor processes. The NOx prediction results are in reasonable agreement with test data.

Finite Element Volume Analysis of Propane Preheated Air Flame Passing Through a Minichannel

2014 ◽  
Author(s):  
Masoud Darbandi ◽  
Majid Ghafourizadeh ◽  
Gerry E. Schneider
Keyword(s):  
Finite Element ◽  
Radiation Effects ◽  
Mixture Fraction ◽  
Flame Structure ◽  
Current Method ◽  
Reacting Flow ◽  
Wall Functions ◽  
Flamelet Model ◽  
Combustion Modeling ◽  
Detailed Kinetics

A hybrid finite-element-volume FEV method is extended to simulate turbulent non-premixed propane air preheated flame in a minichannel. We use a detailed kinetics scheme, i.e. GRI mechanism 3.0, and the flamelet model to perform the combustion modeling. The turbulence-chemistry interaction is taken into account in this flamelet modeling using presumed shape probability density functions PDFs. Considering an upwind-biased physics for the current reacting flow, we implement the physical influence upwinding scheme PIS to estimate the cell-face mixture fraction variance in this study. To close the turbulence closure, we employ the two-equation standard κ-ε turbulence model incorporated with suitable wall functions. Supposing an optically thin limit, it needs to take into account radiation effects of the most important radiating species in the current modeling. Despite facing with so many flame instabilities in such small size configuration, the current method performs suitably with proper convergence, and the encountered instabilities are damped out automatically. Comparing with the experimental measurements, the current extended method accurately predicts the flame structure in the minichannel configuration.

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.

Mathematical Imaging of Piloted Diffusion Methane-Air Flames Under Anisotropic Scalar Dissipation Rates

2005 ◽  
Author(s):  
Mohsen M. Abou-Ellail ◽  
Karam R. Beshay ◽  
David R. Halka
Keyword(s):  
Diffusion Flame ◽  
Dissipation Rate ◽  
Mixture Fraction ◽  
Flame Temperature ◽  
Kinetic Mechanism ◽  
Chemical Kinetic ◽  
Scalar Dissipation ◽  
Elementary Reactions ◽  
Chemical Kinetic Mechanism ◽  
Limiting Value

The present work is a numerical simulation of the, piloted, non-premixed, methane–air flame structure in a new mathematical imaging domain. This imaging space has the mixture fraction of diffusion flame Z1 and mixture fraction of pilot flame Z2 as independent coordinates to replace the usual physical space coordinates. The predications are based on the solution of two–dimensional set of transformed second order partial differential conservation equations describing the mass fractions of O2, CH4, CO2, CO, H2O, H2 and sensible enthalpy of the combustion products which are rigorously derived and solved numerically. A three–step chemical kinetic mechanism is adopted. This was deduced in a systematic way from a detailed chemical kinetic mechanism by Peters (1985). The rates for the three reaction steps are related to the rates of the elementary reactions of the full reaction mechanism. The interaction of the pilot flame with the non-premixed flame and the resulting modifications to the structure and chemical kinetics of the flame are studied numerically for different values of the scalar dissipation rate tensor. The dissipation rate tensor represents the flame stretching along Z1, the main mixture fraction, and in the perpendicular direction, along Z2, the pilot mixture fraction. The computed flame temperature contours are plotted in the Z1-Z2 plane for fixed values of the dissipation rate along Z1 and Z2.These temperature contours show that the flame will become unstable when the dissipate rates along Z1 and Z2 increase, simultaneously, to the limiting value for complete flame extinction of 45 s−1. However, the diffusion flame will extinguish for dissipate rates less than 20 1/s, if unpiloted. It is also noticed that the flame will remain stable if the dissipation rate along Z2 is increased to the limiting value, while the dissipation rate, along Z2, remains constant at a value less than 30 s−1.

Numerical Analysis of Spray Dispersion, Evaporation and Combustion in a Single Gas Turbine Combustor

2008 ◽  
Author(s):  
M. Chrigui ◽  
A. Sadiki ◽  
J. Janicka
Keyword(s):  
Reynolds Stress ◽  
Mixture Fraction ◽  
Measurement Techniques ◽  
Reynolds Stress Model ◽  
Scalar Dissipation ◽  
Stress Model ◽  
Vapor Source ◽  
Flamelet Model ◽  
Equilibrium Evaporation ◽  
Non Equilibrium

Spray dispersion, evaporation and combustion have been numerically studied in a complex industrial configuration, which consists in a single annular combustor that was experimentally measured by Rolls-Royce-Deutschland Company. Simulations have been achieved using the Eulerian-Lagrangian approach. The computations of the continuous phase have been performed by means of RANS simulations. Though the k-ε as well as the Reynolds Stress model (Jones-Musonge) have been used for turbulence modeling. The 3D-computations have been performed in a fully two-way coupling. The effects of turbulence on droplets distribution are accounted for using the Markov sequence dispersion model. The equilibrium as well as the non-equilibrium evaporation model have been applied. In order to account for the combustion, the diffusion flame model is chosen. It relies on the computation of the mixture fraction that has been affected by the presence of vapor source terms. For the interaction of the turbulence with the chemistry, the mixture fraction variance has also been solved. For that purpose a presumed beta-PDF function has been considered. The equilibrium and the flamelet chemistry approaches have been used for the generation of the chemistry tables. The performed simulations have also been compared to commercial CFD-codes. From there one observes, that the obtained results using the mentioned sub-models combination agree most favorably with experimental measurements. One noted that the Reynolds Stress model provided smoother temperature distribution compared to k-ε. The flamelet model has been performed using three different scalar dissipation rates. One observes that differences are mainly located at the nozzle exit, where the scalar dissipation rate has got the highest value. Although the comparison between the numerical results and the experimental data was possible only at the combustor exit, due to the limitation on the measurement techniques, one can reiterate that the combination of the following sub-models: thermodynamically consistent model for the turbulence modulation, Langmuir-Knudsen non-equilibrium model for the evaporation, Reynolds Stress Model for the turbulence and flamelet model for the chemistry establish a reliable complete model that seems to allows a better description of reactive multi-phase flow studied in the frame of this work.

Aerodynamic control of a diffusion flame to optimize materials' transition in a rotary cement kiln

2020 ◽  
Vol 21 (4) ◽  
pp. 414
Author(s):  
Mohamed Nial ◽  
Larbi Loukarfi ◽  
Hassane Naji
Keyword(s):  
Diffusion Flame ◽  
Reverse Flow ◽  
Combustion Efficiency ◽  
Navier Stokes ◽  
Cement Kiln ◽  
Ansys Fluent ◽  
Rotary Cement Kiln ◽  
Finite Volume Approach ◽  
Secondary Air ◽  
Aerodynamic Modelling

The aim of this work is to deepen the understanding of the aerodynamics of a diffusion flame in a rotary cement kiln. The kiln is a rotary with a cylindrical shaped, long and equipped with a burner, and it is the seat of a diffusion flame with an axisymmetric turbulent jet. The kiln has a capacity of 8,000 Nm3 to 13,000 Nm3 of natural gas and primary air at T = 25 °C which interacts with a secondary hot air volume at T = 800 °C. The aerodynamic modelling of the furnace is achieved using the turbulence model RNG k–ε, which is able to handle the turbulence and capture the vortex shedding process. The Ansys/Fluent code, based on the finite volume approach to solve the Reynolds averaged Navier-Stokes (RANS), was used in this study. The interactions between turbulence and diffusion flame were handled by the PDF (Probability Density Function) approach. The numerical simulations have been validated by experiments from the kiln considered. Based on the findings obtained, it is concluded that the recirculation zone seems of paramount importance when combustion is taken into account because the reverse flow improves the flame stability and affects the combustion efficiency. In addition, limiting the secondary air flow through the furnace is major to improve combustion and avoid disturbing the advancement of the material along the kiln.

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.

scholarly journals Modelling of Conditional Scalar Dissipation Rate in Turbulent Premixed Combustion

Computation ◽  
2021 ◽  
Vol 9 (3) ◽  
pp. 26 ◽  
Author(s):  
Shokri Amzin ◽  
Mariusz Domagała
Keyword(s):  
Dissipation Rate ◽  
Premixed Flames ◽  
Moment Closure ◽  
Navier Stokes ◽  
Scalar Dissipation ◽  
Flame Surface ◽  
Scalar Dissipation Rate ◽  
Turbulent Premixed Flames ◽  
Conditional Moment ◽  
Turbulent Premixed

In turbulent premixed flames, for the mixing at a molecular level of reactants and products on the flame surface, it is crucial to sustain the combustion. This mixing phenomenon is featured by the scalar dissipation rate, which may be broadly defined as the rate of micro-mixing at small scales. This term, which appears in many turbulent combustion methods, includes the Conditional Moment Closure (CMC) and the Probability Density Function (PDF), requires an accurate model. In this study, a mathematical closure for the conditional mean scalar dissipation rate, <Nc|ζ>, in Reynolds, Averaged Navier–Stokes (RANS) context is proposed and tested against two different Direct Numerical Simulation (DNS) databases having different thermochemical and turbulence conditions. These databases consist of lean turbulent premixed V-flames of the CH4-air mixture and stoichiometric turbulent premixed flames of H2-air. The mathematical model has successfully predicted the peak and the typical profile of <Nc|ζ> with the sample space ζ and its prediction was consistent with an earlier study.

Analysis of the Mixing and Emission Characteristics in a Model Combustor

2018 ◽  
Author(s):  
Shan Li ◽  
Shanshan Zhang ◽  
Lingyun Hou ◽  
Zhuyin Ren
Keyword(s):  
Gas Turbines ◽  
Schmidt Number ◽  
Numerical Study ◽  
Flame Temperature ◽  
Scalar Dissipation ◽  
Mixing Process ◽  
Nox Formation ◽  
Turbulent Schmidt Number ◽  
Wide Range ◽  
Air Mixing

Modern gas turbines in power systems employ lean premixed combustion to lower flame temperature and thus achieve low NOx emissions. The fuel/air mixing process and its impacts on emissions are of paramount importance to combustor performance. In this study, the mixing process in a methane-fired model combustor was studied through an integrated experimental and numerical study. The experimental results show that at the dump location, the time-averaged fuel/air unmixedness is less than 10% over a wide range of testing conditions, demonstrating the good mixing performance of the specific premixer on the time-averaged level. A study of the effects of turbulent Schmidt number on the unmixedness prediction shows that for the complex flow field involved, it is challenging for Reynolds-Averaged Navier-Stokes (RANS) simulations with constant turbulent Schmidt number to accurately predict the mixing process throughout the combustor. Further analysis reveals that the production and scalar dissipation are the key physical processes controlling the fuel/air mixing. Finally, the NOx formation in this model combustor was analyzed and modelled through a flamelet-based approach, in which NOx formation is characterized through flame-front NOx and its post-flame formation rate obtained from one-dimensional laminar premixed flames. The effect of fuel/air unmixedness on NOx formation is accounted for through the presumed probability density functions (PDF) of mixture fraction. Results show that the measured NOx in the model combustor are bounded by the model predictions with the fuel/air unmixedness being 3% and 5% of the maximum unmixedness. In the context of RANS, the accuracy in NOx prediction depends on the unmixedness prediction which is sensitive to turbulent Schmidt number.

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