Charles K. Westbrook

Simplified Reaction Mechanisms for the Oxidation of Hydrocarbon Fuels in Flames

Abstract Simplified reaction mechanisms for the oxidation of hydrocarbon fuels have been examined using a numerical laminar flame model. The types of mechanisms studied include one and two global reaction steps as well as quasi-global mechanisms. Reaction rate parameters were varied in order to provide the best agreement between computed and experimentally observed flame speeds in selected mixtures of fuel and air. The influences of the various reaction rate parameters on the laminar flame properties have been identified, and a simple procedure to determine the best values for the reaction rate parameters is demonstrated. Fuels studied include n-paraffins from methane to n-decane, some methyl-substituted n-paraffins, acetylene, and representative olefin, alcohol and aromatic hydrocarbons. Results show that the often-employed choice of simultaneous first order fuel and oxidizer dependence for global rate expressions cannot yield the correct dependence of flame speed on equivalence ratio or pressure and can...

A comprehensive modeling study of n-heptane oxidation

A detailed chemical kinetic mechanism has been developed and used to study the oxidation of n-heptane in flow reactors, shock tubes, and rapid compression machines. Over the series of experiments numerically investigated, the initial pressure ranged from 1–42 atm, the temperature from 550–1700 K, the equivalence ratio from 0.3–1.5, and nitrogen-argon dilution from 70–99%. The combination of ignition delay time and species composition data provide for a stringent test of the chemical kinetic mechanism. The reactions are classed into various types, and the reaction rate constants are given together with an explanation of how the rate constants were obtained. Experimental results from the literature of ignition behind reflected shock waves and in a rapid compression machine were used to develop and validate the reaction mechanism at both low and high temperatures. Additionally, species composition data from a variable pressure flow reactor and a jet-stirred reactor were used to help complement and refine the low-temperature portions of the reaction mechanism. A sensitivity analysis was performed for each of the combustion environments. This analysis showed that the low-temperature chemistry is very sensitive to the formation of stable olefin species from hydroperoxy-alkyl radicals and to the chain-branching steps involving ketohydroperoxide molecules.

Chemical kinetic modeling of hydrocarbon combustion

Abstract Chemical kinetic modeling of high temperature hydrocarbon oxidation in combustion is reviewed. First, reaction mechanisms for specific fuels are discussed, with emphasis on the hierarchical structure of reaction mechanisms for complex fuels. The concept of a comprehensive mechanism is developed, requiring model validation by comparison with data from a wide range of experimental regimes. Fuels of increasing complexity from hydrogen to n -butane are described in detail, and further extensions of the general approach to other fuels are discussed. Kinetic modification to fuel oxidation kinetics is considered, including both inhibition and promotion of combustion. Simplified kinetic models are then described by comparing their features with those of detailed kinetic models. Finally, application of kinetic models to study real combustions systems are presented, beginning with purely kinetic-thermodynamic applications, in which transport effects such as diffusion of heat and mass can be neglected, such as shock tubes, detonations, plug flow reactors, and stirred reactors. Laminar flames and the coupling between diffusive transport and chemical kinetics are then described, together with applications of laminar flame models to practical combustion problems.

A Comprehensive Modeling Study of iso-Octane Oxidation

A detailed chemical kinetic mechanism has been developed and used to study the oxidation of iso-octane in a jet-stirred reactor, flow reactors, shock tubes and in a motored engine. Over the series of experiments investigated, the initial pressure ranged from 1 to 45 atm, the temperature from 550 K to 1700 K, the equivalence ratio from 0.3 to 1.5, with nitrogen-argon dilution from 70% to 99%. This range of physical conditions, together with the measurements of ignition delay time and concentrations, provide a broad-ranging test of the chemical kinetic mechanism. This mechanism was based on our previous modeling of alkane combustion and, in particular, on our study of the oxidation of n-heptane. Experimental results of ignition behind reflected shock waves were used to develop and validate the predictive capability of the reaction mechanism at both low and high temperatures. Moreover, species’ concentrations from flow reactors and a jet-stirred reactor were used to help complement and refine the low and intermediate temperature portions of the reaction mechanism, leading to good predictions of intermediate products in most cases. In addition, a sensitivity analysis was performed for each of the combustion environments in an attempt to identify the most important reactions under the relevant conditions of study.

CHEMICAL KINETICS OF HYDROCARBON IGNITION IN PRACTICAL COMBUSTION SYSTEMS

Chemical kinetic factors of hydrocarbon oxidation are examined in a variety of ignition problems. Ignition is related to the presence of a dominant chain-branching reaction mechanism that can drive a chemical system to completion in a very short period of time. Ignition in laboratory environments is studied for problems including shock tubes and rapid compression machines. Modeling of the laboratory systems is used to develop kinetic models that can be used to analyze ignition in practical systems. Two major chain-branching regimes are identified, one consisting of high temperature ignition with a chain branching reaction mechanism based on the reaction between atomic hydrogen with molecular oxygen, and the second based on an intermediate temperature thermal decomposition of hydrogen peroxide. Kinetic models are then used to describe ignition in practical combustion environments, including detonations and pulse combustors for high temperature ignition and engine knock and diesel ignition for intermediate temperature ignition. The final example of ignition in a practical environment is homogeneous charge, compression ignition (HCCI), which is shown to be a problem dominated by the kinetics of intermediate temperature hydrocarbon ignition. Model results show why high hydrocarbon and CO emissions are inevitable in HCCI combustion. The conclusion of this study is that the kinetics of hydrocarbon ignition are actually quite simple, since only one or two elementary reactions are dominant. However, many combustion factors can influence these two major reactions, and these are the features that vary from one practical system to another.

Aromatic and polycyclic aromatic hydrocarbon formation in a laminar premixed n-butane flame

Experimental and detailed chemical kinetic modeling work has been performed to investigate aromatic and polycyclic aromatic hydrocarbon (PAH) formation pathways in a premixed, rich, sooting, n-butane–oxygen–argon burner stabilized flame. An atmospheric pressure, laminar flat flame operated at an equivalence ratio of 2.6 was used to acquire experimental data for model validation. Gas composition analysis was conducted by an on-line gas chromatograph/mass spectrometer technique. Measurements were made in the main reaction and post-reaction zones for a number of low molecular weight species, aliphatics, aromatics, and polycyclic aromatic hydrocarbons (PAHs) ranging from two to five-fused aromatic rings. Reaction flux and sensitivity analysis were used to help identify the important reaction sequences leading to aromatic and PAH growth and destruction in the n-butane flame. Reaction flux analysis showed the propargyl recombination reaction was the dominant pathway to benzene formation. The consumption of propargyl by H atoms was shown to limit propargyl, benzene, and naphthalene formation in flames as exhibited by the large negative sensitivity coefficients. Naphthalene and phenanthrene production was shown to be plausibly formed through reactions involving resonantly stabilized cyclopentadienyl and indenyl radicals. Many of the low molecular weight aliphatics, combustion by-products, aromatics, branched aromatics, and PAHs were fairly well simulated by the model. Additional work is required to understand the formation mechanisms of phenyl acetylene, pyrene, and fluoranthene in the n-butane flame.

Biofuel Combustion Chemistry: From Ethanol to Biodiesel

Biofuels, such as bio-ethanol, bio-butanol, and biodiesel, are of increasing interest as alternatives to petroleum-based transportation fuels because they offer the long-term promise of fuel-source regenerability and reduced climatic impact. Current discussions emphasize the processes to make such alternative fuels and fuel additives, the compatibility of these substances with current fuel-delivery infrastructure and engine performance, and the competition between biofuel and food production. However, the combustion chemistry of the compounds that constitute typical biofuels, including alcohols, ethers, and esters, has not received similar public attention. Herein we highlight some characteristic aspects of the chemical pathways in the combustion of prototypical representatives of potential biofuels. The discussion focuses on the decomposition and oxidation mechanisms and the formation of undesired, harmful, or toxic emissions, with an emphasis on transportation fuels. New insights into the vastly diverse and complex chemical reaction networks of biofuel combustion are enabled by recent experimental investigations and complementary combustion modeling. Understanding key elements of this chemistry is an important step towards the intelligent selection of next-generation alternative fuels.

Diesel combustion: an integrated view combining laser diagnostics, chemical kinetics, and empirical validation

This paper proposes a structure for the diesel combustion process based on a combination of previously published and new results. Processes are analyzed with proven chemical kinetic models and validated with data from production-like direct injection diesel engines. The analysis provides new insight into the ignition and particulate formation processes, which combined with laser diagnostics, delineates the two-stage nature of combustion in diesel engines. Data are presented to quantify events occurring during the ignition and initial combustion processes that form soot precursors. A framework is also proposed for understanding the heat release and emission formation processes.

Modeling of Aromatic and Polycyclic Aromatic Hydrocarbon Formation in Premixed Methane and Ethane Flames

Detailed chemical kinetic modeling has been performed to investigate aromatic and polyaromatic hydrocarbon formation pathways in rich, sooting, methane and ethane premixed flames. An atmospheric pressure, laminar flat flame operated at an equivalence ratio of 2.5 was used to acquire experimental data for model validation. Gas composition analysis was conducted by an on-line gas chromatograph / mass spectrometer technique. Measurements were made in the flame and post-flame zone for a number of low molecular weight species, aliphatics, aromatics, and polycyclic aromatic hydrocarbons (PAHs) ranging from two to five-aromatic fused rings. The modeling results show the key reaction sequences leading to aromatic and polycyclic aromatic hydrocarbon formation primarily involve the combination of resonantly stabilized radicals. In particular, propargyl and I-methylallenyl combination reactions lead to benzene and methyl substituted benzene formation, while polycyclic aromatics are formed from cyclopentadienyl and f...

A WIDE RANGE MODELING STUDY OF DIMETHYL ETHER OXIDATION

A detailed chemical kinetic model has been used to study dimethyl ether (DME) oxidation over a wide range of conditions. Experimental results obtained in a jet-stirred reactor (JSR) at I and 10 atm, 0.2 < 0 < 2.5, and 800 < T < 1300 K were modeled, in addition to those generated in a shock tube at 13 and 40 bar, 0 = 1.0 and 650 :5 T :5 1300 K. The JSR results are particularly valuable as they include concentration profiles of reactants, intermediates and products pertinent to the oxidation of DME. These data test the Idnetic model severely, as it must be able to predict the correct distribution and concentrations of intermediate and final products formed in the oxidation process. Additionally, the shock tube results are very useful, as they were taken at low temperatures and at high pressures, and thus undergo negative temperature dependence (NTC) behavior. This behavior is characteristic of the oxidation of saturated hydrocarbon fuels, (e.g. the primary reference fuels, n-heptane and iso- octane) under similar conditions. The numerical model consists of 78 chemical species and 336 chemical reactions. The thermodynamic properties of unknown species pertaining to DME oxidation were calculated using THERM.