Analysis of Radiation Reabsorption Effects on Flame Characteristics and NOx Emission in Laminar Flames

2010 ◽  
Author(s):  
Jingfu Wang ◽  
Guoqiang Li
Keyword(s):  
Flame Temperature ◽  
Laminar Flames ◽  
Radiative Properties ◽  
Band Model ◽  
Nox Formation ◽  
Flame Thickness ◽  
Radiation Heat Loss ◽  
Extinction Limit ◽  
Radiation Extinction ◽  
Radiation Reabsorption

The radiation reabsorption effects on NOx formation and flame characteristics in CH4/Air laminar flames were numerically investigated by using full chemistry mechanism and detailed transport properties. The radiative gases were treated as non-gray gas and their spectral radiative properties were evaluated by means of the statistical narrow-band model. The radiative heat transfer equation was solved by the discrete ordinate method. It was found that the reabsorption of emitting radiation leads to substantially wider flame thickness and higher flame temperature than those calculated by using the optically thin model, and the radiation reabsorption effect on the “radiation extinction limit” becomes more important. The results show that the level of NOx is predicted to be highest in the adiabatic flames, that is, flames without radiation heat loss, and that the level of NOx is predicted to be lowest in the flames by the optically thin model. In the flames by the SNB model, the predicted amount of NOx lies between these two levels. The calculated results also show that the radiation reabsorption effect on NOx formation grows stronger as the stretch rate decreases, particularly when CO2, a strong absorber, is added to the unburned gas mixture. In this study, the effectiveness and validity of the optically thin radiation model for calculating NOx formation in laminar flames was also investigated in comparison with the SNB model.

On the extinction limit and flammability limit of non-adiabatic stretched methane–air premixed flames

1997 ◽  
Vol 342 ◽  
pp. 315-334 ◽  
Author(s):  
YIGUANG JU ◽  
HONGSHENG GUO ◽  
KAORU MARUTA ◽  
FENGSHAN LIU
Keyword(s):  
Equivalence Ratio ◽  
Flame Temperature ◽  
Lewis Number ◽  
Flammability Limit ◽  
Radiative Heat Loss ◽  
Stretch Rate ◽  
Stable Branch ◽  
Extinction Limit ◽  
Radiation Extinction ◽  
Normal Flame

Extinction limits and the lean flammability limit of non-adiabatic stretched premixed methane–air flames are investigated numerically with detailed chemistry and two different Planck mean absorption coefficient models. Attention is paid to the combined effect of radiative heat loss and stretch at low stretch rate. It is found that for a mixture at an equivalence ratio lower than the standard lean flammability limit, a moderate stretch can strengthen the combustion and allow burning. The flame is extinguished at a high stretch rate due to stretch and is quenched at a low stretch rate due to radiation loss. A O-shaped curve of flame temperature versus stretch rate with two distinct extinction limits, a radiation extinction limit and a stretch extinction limit respectively on the left- and right-hand sides, is obtained. A C-shaped curve showing the flammability limit of the stretched methane–air flame is obtained by plotting these two extinction limits in the mixture strength coordinate. A good agreement is shown on comparing the predicted results with the experimental data. For equivalence ratio larger than a critical value, it is found that the O-shaped temperature curve opens up in the middle of the stable branch, so that the stable branch divides into two stable flame branches; a weak flame branch and a normal flame branch. The weak flame can survive between the radiation extinction limit and the opening point (jump limit) while the normal flame branch can survive from its stretch extinction limit to zero stretch rate. Finally, a G-shaped curve showing both extinction limits and jump limits of stretched methane–air flames is presented. It is found that the critical equivalence ratio for opening up corresponds to the standard flammability limit measured in microgravity. Furthermore, the results show that the flammability limit (inferior limit) of the stretched methane–air flame is lower than the standard flammability limit because flames are strengthened by a moderate stretch at Lewis number less than unity.

Study on the Effect of Adiabatic Flame Temperature on NOx Formation Using Ethanol Gasoline Blend in SI Engine

2013 ◽  
Vol 781-784 ◽  
pp. 2471-2475 ◽  
Author(s):  
B. M. Masum ◽  
M.A. Kalam ◽  
H.H. Masjuki ◽  
S. M. Palash
Keyword(s):  
Equivalence Ratio ◽  
Gasoline Engine ◽  
Flame Temperature ◽  
Inlet Temperature ◽  
Si Engine ◽  
Nox Formation ◽  
Adiabatic Flame Temperature ◽  
No Formation ◽  
Blended Fuel ◽  
Active Research

Active research and development on using ethanol fuel in gasoline engine had been done for few decades since ethanol served as a potential of infinite fuel supply. This paper discussed analytically and provides data on the effects of compression ratio, equivalence ratio, inlet temperature, inlet pressure and ethanol blend in cylinder adiabatic flame temperature (AFT) and nitrogen oxide (NO) formation of a gasoline engine. Olikara and Borman routines were used to calculate the equilibrium products of combustion for ethanol gasoline blended fuel. The equilibrium values of each species were used to predict AFT and the NO formation of combustion chamber. The result shows that both adiabatic flame temperature and NO formation are lower for ethanol-gasoline blend than gasoline fuel.

Analysis of Water–Fuel Ratio Variation in a Gas Turbine With a Wet-Compressor System by Change in Fuel Composition

2017 ◽  
Vol 140 (5) ◽  
Author(s):  
Yonatan Cadavid ◽  
Andres Amell ◽  
Juan Alzate ◽  
Gerjan Bermejo ◽  
Gustavo A. Ebratt
Keyword(s):  
Gas Turbine ◽  
Burning Velocity ◽  
Thermal Power ◽  
Flame Temperature ◽  
Fuel Composition ◽  
Nox Formation ◽  
Gaseous Hydrocarbon ◽  
Power Generators ◽  
Fuel Ratio ◽  
Combustion Properties

The wet compressor (WC) has become a reliable way to reduce gas emissions and increase gas turbine efficiency. However, fuel source diversification in the short and medium terms presents a challenge for gas turbine operators to know how the WC will respond to changes in fuel composition. For this study, we assessed the operational data of two thermal power generators, with outputs of 610 MW and 300 MW, in Colombia. The purpose was to determine the maximum amount of water that can be added into a gas turbine with a WC system, as well as how the NOx/CO emissions vary due to changes in fuel composition. The combustion properties of different gaseous hydrocarbon mixtures at wet conditions did not vary significantly from each other—except for the laminar burning velocity. It was found that the fuel/air equivalence ratio in the turbine reduced with lower CH4 content in the fuel. Less water can be added to the turbine with leaner combustion; the water/fuel ratio was decreased over the range of 1.4–0.4 for the studied case. The limit is mainly due to a reduction in flame temperature and major risk of lean blowout (LBO) or dynamic instabilities. A hybrid reaction mechanism was created from GRI-MECH 3.0 and NGIII to model hydrocarbons up to C5 with NOx formation. The model was validated with experimental results published previously in literature. Finally, the effect of atmospheric water in the premixed combustion was analyzed and explained.

Co Emission Modeling in a Heavy Duty Annular Combustor Operating with Natural Gas

2021 ◽  
Author(s):  
Roberto Meloni ◽  
Stefano Gori ◽  
Antonio Andreini ◽  
Pier Carlo Nassini
Keyword(s):  
Turbulent Flame ◽  
Flame Temperature ◽  
Production Region ◽  
Turbulent Flame Speed ◽  
Eddy Simulation ◽  
Extinction Limit ◽  
Flamelet Generated Manifold ◽  
Annular Combustor ◽  
Definition Of ◽  
Co Emission

Abstract The present paper summarizes the development of a Large-Eddy Simulation (LES) based approach for the prediction of CO emission in an industrial gas turbine combustor. Since the operating point of the modern combustors is really close to the extinction limit, the availability of a tool able to detect the onset of high-CO production can be useful for the proper definition of the combustion chamber air split or to introduce design improvements for the premixer itself. The accurate prediction of CO cannot rely on the flamelet assumption, representing the fundament of the modern combustion models. Consequently, in this work, the Extended Turbulent Flame Speed Closure (ETFSC) of the standard Flamelet Generated Manifold (FGM) model is employed to consider the effect of the heat loss and the strain rate on the flame brush. Moreover, a customized CO-Damköhler number is introduced to de-couple the in-flame CO production region from the post-flame contribution where the oxidation takes place. A fully premixed burner working at representative values of pressure and flame temperature of an annular combustor is selected for the validation phase of the process. The comparison against the experimental data shows that the process is not only able to capture the trend but also to predict CO in a quantitative manner. In particular, the interaction between the flame and the air fluxes at some critical sections of the combustor, leading the CO emission from the equilibrium value to the super-equilibrium, has been correctly reproduced.

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.

Numerical Simulation of Combustion in a Cylindrical Porous Medium

2002 ◽  
Author(s):  
A. A. Mohamad
Keyword(s):  
Heat Transfer ◽  
Porous Medium ◽  
Flame Temperature ◽  
Axial Flow ◽  
Nox Formation ◽  
Porous Burner ◽  
Flat Type ◽  
Wide Range ◽  
Stability Limits ◽  
Cylindrical Burner

Convectional free flame combustion causes the temperature rise in the vicinity of the flame to be very steep, resulting in high temperatures, consequently NOx formation enhances. The fact is that the thermal conductivity of gases are very low, i.e., poor thermal conductors. Combustion in porous media elevates this problem by enhancing heat conduction and thermal radiation from the flame zone, which reduces the flame temperature and NOx formation. Also, heat transfer from the free flame to a load is mainly by convection, while heat transfer is by convection and radiation from combustion zone in porous medium to a load. Moreover, it is easy to stabilize the flame in a porous medium, where the thermophysical properties of the porous medium can engineered for specific application. Most of the work is done on flat type porous burner, where the axial flow of gaseous fuel air mixture forces through a layer of porous medium. In this report a concept of cylindrical porous burner is introduced, where the fuel air mixture is forced to flow radially. Mathematical models and simulation results are introduced for both burners, axial and radial flow burners. Preliminary results of the comparison between the thermal performances between the mentioned burners are discussed. The results revealed that the cylindrical burner has superiority over the convectional flat burner. The cylindrical burner has a wide range stability limits and may produce less NOx than the flat type burners.

Band Models for Radiative Transfer in Non-LTE Diatomic Molecules of CO2-N2 Plasmas

2010 ◽  
Author(s):  
Se´bastien Depraz ◽  
Philippe Rivie`re ◽  
Marie-Yvonne Perrin ◽  
Anouar Soufiani
Keyword(s):  
Least Square ◽  
Radiative Properties ◽  
Model Parameters ◽  
Band Model ◽  
Electronic Systems ◽  
General Description ◽  
Line Profiles ◽  
Equilibrium Conditions ◽  
Thermodynamic State ◽  
Narrow Band Model

A statistical narrow-band model is developed for the optically non-thin electronic systems of carbonaceous molecules in CO2-N2 plasmas and its accuracy is studied under equilibrium and non-equilibrium conditions. Line by line calculations are used to produce curves of growth of transmissivities from which band model parameters are calculated by least-square adjustments. The model is shown to provide quite accurate description of radiative properties and radiative intensities for Doppler, Lorentz, and Voigt line profiles, and for both local thermodynamic equilibrium and a multi-temperature description of the gas mixture thermodynamic state. The model is also suitable for a more general description of the gas thermodynamic state where the electronic state populations are arbitrary.

NOx Emissions Modeling and Uncertainty From Exhaust-Gas-Diluted Flames

2015 ◽  
Vol 138 (5) ◽  
Author(s):  
Antonio C. A. Lipardi ◽  
Jeffrey M. Bergthorson ◽  
Gilles Bourque
Keyword(s):  
Nox Reduction ◽  
Flame Temperature ◽  
No Production ◽  
Exhaust Gas ◽  
Oxides Of Nitrogen ◽  
Nox Formation ◽  
Kinetic And Thermodynamic Parameters ◽  
Emissions Modeling ◽  
Combustion Processes ◽  
Gas Recirculation

Oxides of nitrogen (NOx) are pollutants emitted by combustion processes during power generation and transportation that are subject to increasingly stringent regulations due to their impact on human health and the environment. One NOx reduction technology being investigated for gas-turbine engines is exhaust-gas recirculation (EGR), either through external exhaust-gas recycling or staged combustion. In this study, the effects of different percentages of EGR on NOx production will be investigated for methane–air and propane–air flames at a selected adiabatic flame temperature of 1800 K. The variability and uncertainty of the results obtained by the gri-mech 3.0 (GRI), San-Diego 2005 (SD), and the CSE thermochemical mechanisms are assessed. It was found that key parameters associated with postflame NO emissions can vary up to 192% for peak CH values, 35% for thermal NO production rate, and 81% for flame speed, depending on the mechanism used for the simulation. A linear uncertainty analysis, including both kinetic and thermodynamic parameters, demonstrates that simulated postflame nitric oxide levels have uncertainties on the order of ±50–60%. The high variability of model predictions, and their relatively high associated uncertainties, motivates future experiments of NOx formation in exhaust-gas-diluted flames under engine-relevant conditions to improve and validate combustion and NOx design tools.

Radiation extinction limit of counterflow premixed lean methane-air flames

1997 ◽  
Vol 109 (4) ◽  
pp. 639-646 ◽  
Author(s):  
Hongsheng Guo ◽  
Yiguang Ju ◽  
Kaoru Maruta ◽  
Takashi Niioka ◽  
Fengshan Liu
Keyword(s):  
Extinction Limit ◽  
Radiation Extinction
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