Combustion Chemical Kinetics Research Articles

Skeletal Methane–Air Reaction Mechanism for Large Eddy Simulation of Turbulent Microwave-Assisted Combustion

Irradiating a flame via microwave radiation is a plasma-assisted combustion (PAC) technology that can be used to modify the combustion chemical kinetics in order to improve flame stability and to delay lean blow-out. One practical implication is that combustion engines may be able to operate with leaner fuel mixtures and have an improved fuel flexibility capability including biofuels. Furthermore, this technology may assist in reducing thermoacoustic instabilities, which is a phenomenon that may severely damage the engine and increase NOX production. To further understand microwave-assisted combustion, a skeletal kinetic reaction mechanism for methane–air combustion is developed and presented. The mechanism is detailed enough to take into account relevant features, but sufficiently small to be implemented in large eddy simulations (LES) of turbulent combustion. The mechanism consists of a proposed skeletal methane–air reaction mechanism accompanied by subsets for ozone, singlet oxygen, chemionization, and...

Investigations of microwave stimulation of a turbulent low-swirl flame

Irradiating a flame by microwave radiation is one of several plasma-assisted combustion (PAC) technologies that can be used to modify the combustion chemical kinetics in order to improve flame-stability and to delay lean blow-out. One practical implication is that engines may be able to operate with leaner fuel mixtures and have an improved fuel flexibility capability including biofuels. In addition, this technology may assist in reducing thermoacoustic instabilities that may severely damage the engine and increase emission production. To examine microwave-assisted combustion a combined experimental and computational study of microwave-assisted combustion is performed for a lean, turbulent, swirl-stabilized, stratified flame at atmospheric conditions. The objectives are to demonstrate that the technology increases both the laminar and turbulent flame speeds, and modifies the chemical kinetics, enhancing the flame-stability at lean mixtures. The study combines experimental investigations using hydroxyl (OH) and formaldehyde (CH2O) Planar Laser-Induced Fluorescence (PLIF) and numerical simulations using finite rate chemistry Large Eddy Simulations (LES). The reaction mechanism is based on a methane (CH4)–air skeletal mechanism expanded with sub-mechanisms for ozone, singlet oxygen, chemionization, electron impact dissociation, ionization and attachment. The experimental and computational results show similar trends, and are used to demonstrate and explain some significant aspects of microwave-enhanced combustion. Both simulation and experimental studies are performed close to lean blow off conditions. In the simulations, the flame is gradually subjected to increasing reduced electric field strengths, resulting in a wider flame that stabilizes nearer to the burner nozzle. Experiments are performed at two equivalence ratios, where the leaner case absorbs up to more than 5% of the total flame power. Data from experiments reveal trends similar to simulated results with increased microwave absorption.

Modeling Ignition of a Heptane Isomer: Improved Thermodynamics, Reaction Pathways, Kinetics, and Rate Rule Optimizations for 2-Methylhexane.

Accurate chemical kinetic combustion models of lightly branched alkanes (e.g., 2-methylalkanes) are important to investigate the combustion behavior of real fuels. Improving the fidelity of existing kinetic models is a necessity, as new experiments and advanced theories show inaccuracies in certain portions of the models. This study focuses on updating thermodynamic data and the kinetic reaction mechanism for a gasoline surrogate component, 2-methylhexane, based on recently published thermodynamic group values and rate rules derived from quantum calculations and experiments. Alternative pathways for the isomerization of peroxy-alkylhydroperoxide (OOQOOH) radicals are also investigated. The effects of these updates are compared against new high-pressure shock tube and rapid compression machine ignition delay measurements. It is shown that rate constant modifications are required to improve agreement between kinetic modeling simulations and experimental data. We further demonstrate the ability to optimize the kinetic model using both manual and automated techniques for rate parameter tunings to improve agreement with the measured ignition delay time data. Finally, additional low temperature chain branching reaction pathways are shown to improve the model's performance. The present approach to model development provides better performance across extended operating conditions while also strengthening the fundamental basis of the model.

Surrogate Definition and Chemical Kinetic Modeling for Two Different Jet Aviation Fuels

For emulation of the chemical kinetic combustion phenomena and physical properties of S-8 POSF 4734 and Jet-A POSF 4658, two surrogate fuels were formulated by directly matching their molecular structure and functional groups. The same functional groups, CH3, CH2, CH, C, and phenyl, were chosen to formulate the S-8 and Jet-A surrogates with n-dodecane/2,5-dimethylhexane (0.581/0.419 mole fraction) and n-dodecane/2,5-dimethylhexane/toluene (0.509/0.219/0.272 mole fraction), respectively. The numerical results using the surrogate fuels were compared with the experimental data and the results predicted by other surrogate fuel formulation methods. The results show that the present method can formulate surrogate mixtures of both jet fuels and Fischer–Tropsch real fuels and reproduce the combustion characteristics in homogeneous ignition and the flow reactor oxidation process. The idea presented here could be extended to other real fuels with the appropriate choice of surrogate fuel components.

Numerical optimisation for model evaluation in combustion kinetics

Numerical optimisation related to the estimation of kinetic parameters and model evaluation is playing an increasing role in combustion as well as in other areas of applied energy research. The present work aims at presenting the current probability-based approaches along applications to real problems of combustion chemical kinetics. The main methods related to model and parameter evaluation have been explicated. An in-house program for the systematic adjustment of kinetic parameters to experimental measurements has been described and numerically validated. The GRI (Gas research institute) mechanism (version 3.0) has been shown to initially lead to results which are greatly at variance with experimental data concerning the combustion of CH3 and C2H6. A thorough optimisation of all parameters has been performed with respect to these profiles. A considerable improvement could be reached and the new predictions appear to be compatible with the measurement uncertainties. It was also found that neither GRI 3.0 nor three other reaction mechanisms considered during the present work should be employed (without prior far-reaching optimisation) for numerical simulations of combustors and engines where CH3 and C2H6 play an important role. Overall, this study illustrates the link between optimisation methods and model evaluation in the field of combustion chemical kinetics.

Evaluated Kinetics of the Reactions of H and CH3 with n-Alkanes: Experiments with n-Butane and a Combustion Model Reaction Network Analysis.

Presented is a combined experimental and modeling study of the kinetics of the reactions of H and CH3 with n-butane, a representative aliphatic fuel. Abstraction of H from n-alkane fuels creates alkyl radicals that rapidly decompose at high temperatures to alkenes and daughter radicals. In combustion and pyrolysis, the branching ratio for attack on primary and secondary hydrogens is a key determinant of the initial olefin and radical pool, and results propagate through the chemistry of ignition, combustion, and byproduct formation. Experiments to determine relative and absolute rate constants for attack of H and CH3 have been carried out in a shock tube between 859 and 1136 K for methyl radicals and 890 to 1146 K for H atoms. Pressures ranged from 140 to 410 kPa. Appropriate precursors are used to thermally generate H and CH3 in separate experiments under dilute and well-defined conditions. A mathematical design algorithm has been applied to select the optimum experimental conditions. In conjunction with postshock product analyses, a network analysis based on the detailed chemical kinetic combustion model JetSurf 2 has been applied. Polynomial chaos expansion techniques and Monte Carlo methods are used to analyze the data and assess uncertainties. The present results provide the first experimental measurements of the branching ratios for attack of H and CH3 on primary and secondary hydrogens at temperatures near 1000 K. Results from the literature are reviewed and combined with the present data to generate evaluated rate expressions for attack on n-butane covering 300 to 2000 K for H atoms and 400 to 2000 K for methyl radicals. Values for generic n-alkanes and related hydrocarbons are also recommended. The present experiments and network analysis further demonstrate that the C-H bond scission channels in butyl radicals are an order of magnitude less important than currently indicated by JetSurf 2. Updated rate expressions for butyl radical fragmentation reactions are provided.

Plasma assisted combustion: Effects of O3 on large scale turbulent combustion studied with laser diagnostics and Large Eddy Simulations

In plasma-assisted combustion, electric energy is added to the flame where the electric energy will be transferred to kinetic energy of the free electrons that, in turn, will modify the combustion chemical kinetics. In order to increase the understanding of this complex process, the influence of one of the products of the altered chemical kinetics, ozone (O3), has been isolated and studied. This paper reports on studies using a low-swirl methane (CH4) air flame at lean conditions with different concentrations of O3 enrichment. The experimental flame diagnostics include Planar Laser Induced Fluorescence (PLIF) imaging of hydroxyl (OH) and formaldehyde (CH2O). The experiments are also modeled using Large Eddy Simulations (LES) with a reaction model based on a skeletal CH4-air reaction mechanism combined with an O3 sub-mechanism to include the presence of O3 in the flame. This reaction mechanism is based on fundamental considerations including reactions between O3 and all other species involved. The experiments reveal an increase in CH2O in the low-swirl flame as small amounts of O3 is supplied to the CH4-air stream upstream of the flame. This increase is well predicted by the LES computations and the relative radical concentration shift is in good agreement with experimental data. Simulations also reveal that the O3 enrichment increase the laminar flame speed, su, with ∼10% and the extinction strain-rate, σext, with ∼20%, for 0.57% (by volume) O3. The increase in σext enables the O3 seeded flame to burn under more turbulent conditions than would be possible without O3 enrichment. Sensitivity analysis indicates that the increase in σext due to O3 enrichment is primarily due to the accelerated chain-branching reactions H2+O⇔OH+H, H2O+O⇔OH+OH and H+O2⇔OH+O. Furthermore, the increase in CH2O observed in both experiments and simulations suggest a significant acceleration of the chain-propagation reaction CH3+O⇔CH2O+H.

New Application of Microcombustion: Fuel Reactivity Measurement and Research Tool for Combustion Chemical Kinetics

New Application of Microcombustion: Fuel Reactivity Measurement and Research Tool for Combustion Chemical Kinetics

The combustion kinetics of a synthetic paraffinic jet aviation fuel and a fundamentally formulated, experimentally validated surrogate fuel

A surrogate fuel is formulated in an a priori manner through a combustion property matching technique to emulate the gas phase chemical kinetic combustion phenomena of S-8 POSF 4734, an alternative aviation fuel derived from natural gas via the Fischer–Tropsch process. A fundamental concept is described which identifies n-dodecane and iso-octane as being appropriate surrogate fuel components for the non-aromatic synthetic fuels. The performance of the formulated 51.9/48.1mole % n-dodecane/iso-octane mixture as a surrogate for the target real fuel is evaluated by the measurement of a series of combustion phenomena exhibited by both fuels including:(1)The oxidative reactivity of stoichiometric mixtures of each fuel in O2/N2 at 12.5atm and 500–1050K, for a residence time of 1.8s at a fixed carbon content of 0.3% using a variable pressure flow reactor.(2)The autoignition behavior of stoichiometric mixtures of each fuel in air at compressed conditions of 667–1223K and ∼20atm by the reflected shock technique.(3)The strained extinction limits of diffusion flames of each fuel at 1atm.The performance of available kinetic models for n-dodecane/iso-octane mixtures is evaluated by analysis of their computations of this experimental data. Furthermore, the impact of oxidation kinetics unique to the mono methylated alkanes which are the dominant molecular structure in synthetic fuels is examined by an experimental study involving the formulation of an n-decane/iso-octane mixture as a surrogate fuel for 2-methyl heptane, a proposed model molecule for such real fuel components.

A comprehensive chemical kinetic combustion model for the four butanol isomers

Alcohols, such as butanol, are a class of molecules that have been proposed as a bio-derived alternative or blending agent for conventional petroleum derived fuels. The structural isomer in traditional “bio-butanol” fuel is 1-butanol, but newer conversion technologies produce iso-butanol and 2-butanol as fuels. Biological pathways to higher molecular weight alcohols have also been identified. In order to better understand the combustion chemistry of linear and branched alcohols, this study presents a comprehensive chemical kinetic model for all the four isomers of butanol (e.g., 1-, 2-, iso- and tert-butanol). The proposed model includes detailed high-temperature and low-temperature reaction pathways with reaction rates assigned to describe the unique oxidation features of linear and branched alcohols. Experimental validation targets for the model include low pressure premixed flat flame species profiles obtained using molecular beam mass spectrometry (MBMS), premixed laminar flame velocity, rapid compression machine and shock tube ignition delay, and jet-stirred reactor species profiles. The agreement with these various data sets spanning a wide range of temperatures and pressures is reasonably good. The validated chemical kinetic model is used to elucidate the dominant reaction pathways at the various pressures and temperatures studied. At low-temperature conditions, the reaction of 1-hydroxybutyl with O2 was important in controlling the reactivity of the system, and for correctly predicting C4 aldehyde profiles in low pressure premixed flames and jet-stirred reactors. Enol–keto isomerization reactions assisted by radicals and formic acid were also found to be important in converting enols to aldehydes and ketones under certain conditions. Structural features of the four different butanol isomers leading to differences in the combustion properties of each isomer are thoroughly discussed.

A Transport Equation Residual Model Incorporating Refined G-Equation and Detailed Chemical Kinetics Combustion Models

A Transport Equation Residual Model Incorporating Refined G-Equation and Detailed Chemical Kinetics Combustion Models

Calculation of Multidimensional Flames Using Large Chemical Kinetics

A time-dependent, two-dimensional, detailed-chemistry computational fluid dynamics model, known as UNICORN (unsteady ignition and combustion using reactions), is used for solving complex flame problems. The unique features incorporated in UNICORN for handling extremely large chemical kinetics with ease and efficiency are discussed. A submixture concept that is used for evaluating transport properties is described. This concept increases the computational speed by a factor of five for a 208-species mechanism and is expected to have even higher efficiency with larger mechanisms. An implicit treatment for certain reaction-rate terms applied during the solution of species-conservation equations is described. Moving the reaction-rate source terms to the left-hand side of the partial differential equations eases the stiffness problem that is typically associated with combustion chemical kinetics. Computational speeds are further improved in UNICORN by completely integrating the chemical-kinetics mechanisms with the solution algorithm. A software-generated computational fluid dynamics approach is used to avoid the tedious and near-impossible task of manually integrating a large chemical-kinetics mechanism into a computational fluid dynamics code. Several calculations demonstrating the abilities of the UNICORN code are presented. Chemical-kinetics mechanisms up to 366 species and 3700 reaction steps are incorporated, and simulations for unsteady multidimensional flames are performed on personal computers. Making use of the robustness and efficiency of the UNICORN code, detailed chemical mechanisms developed for JP-8 fuel are tested for their accuracy, and a parametric study on the role of parent species of a surrogate mixture in predicting flame extinguishment is performed. Ease of changing chemical kinetics in the UNICORN code is demonstrated through the investigation of effects of additives in JP-8 fuel.

Speciation and Chemical Evolution of Nitrogen Oxides in Aircraft Exhaust near Airports

Measurements of nitrogen oxides from a variety of commercial aircraft engines as part of the JETS-APEX2 and APEX3 campaigns show that NOx (NOx [triple bond] NO + NO2) is emitted primarily in the form of NO2 at idle thrust and NO at high thrust. A chemical kinetics combustion model reproduces the observed NO2 and NOx trends with engine power and sheds light on the relevant chemical mechanisms. Experimental evidence is presented of rapid conversion of NO to NO2 in the exhaust plume from engines at low thrust. The rapid conversion and the high NO2/NOx emission ratios observed are unrelated to ozone chemistry. NO2 emissions from a CFM56-3B1 engine account for approximately 25% of the NOx emitted below 3000 feet (916 m) and 50% of NOx emitted below 500 feet (153 m) during a standard ICAO (International Civil Aviation Organization) landing-takeoff cycle. Nitrous acid (HONO) accounts for 0.5% to 7% of NOy emissions from aircraft exhaust depending on thrust and engine type. Implications for photochemistry near airports resulting from aircraft emissions are discussed.

Experimental and computational studies of oxidizer and fuel side addition of ethanol to opposed flow air/ethylene flames

Results of computations based on a detailed chemical kinetic combustion mechanism and results of experiments are compared to understand the influence of ethanol vapor addition upon soot formation and OH radical concentration in opposed flow ethylene/air diffusion flames. For this work, ethanol vapor was added to either the fuel or the oxidizer gases. Experiment and calculations are in qualitative agreement, and both show differing concentrations of soot, soot precursors, and OH depending on whether the ethanol is added to the fuel or oxidizer gases. An explanation for the observed differences for oxidizer or fuel side ethanol addition to opposed flow ethylene/air diffusion flames is proposed, based on an analysis of the chemical kinetic mechanism used in the computations.

Calculation of a Confined Axisymmetric Laminar Diffusion Flame Using a Local Grid Refinement Technique

Abstract In this paper a laminar diffusion flame is predicted using a local grid refinement technique and non-staggered meshes. A single coarse grid covers the whole domain and local grid refinement is carried out in the regions of high gradients without changing the basic grid structure. The Navier-Stokes, the energy and the chemical species transport equations are solved and a finite rate chemical kinetics combustion model is employed. Transport equations for the species concentration are solved using a pseudo time splitting technique. The predictions are in good agreement with the experimental measurements for a confined laminar methane flame. The numerical method proved to be robust and accurate.

Numerical study of ignition within hydrogen-air supersonic boundary layers

Development of scramjet engines requires a better knowledge of the coupling between supersonic flow and combustion chemical kinetics. In this paper we consider a stationary uniform, laminar supersonic flow of a hydrogen-air mixture on a flat plate. Conditions of the flow at the outer edge of the boundary layer are chosen such that the chemical time is «infinitely» large when compared to the transit time over the computational domain. Ignition is triggered inside the boundary layer by viscous dissipation effects and/or wall temperature. This problem is solved numerically by a finite difference technique coupled with a chemical kinetics special solver and using classical boundary-layer assumptions, with two different types of boundary conditions: 1) adiabatic wall, and 2) constant temperature wall