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.

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Chemical kinetics of hydrocarbon ignition in practical combustion systems

We propose a H2–CO kinetic model which incorporates the recent thermodynamic, kinetic, and species transport updates relevant to high-temperature H2 and CO oxidation. Attention has been placed on obtaining a comprehensive and kinetically accurate model able to predict a wide variety of H2–CO combustion data. The model was subject to systematic optimization and validation tests against reliable H2–CO combustion data, from global combustion properties (shock-tube ignition delays, laminar flame speeds, and extinction strain rates) to detailed species profiles during H2 and CO oxidation in flow reactor and in laminar premixed flames.

An optimized kinetic model of H2/CO combustion

Kinetic modeling of hydrocarbon/nitric oxide interactions in a flow reactor

Implementation of the NCN pathway of prompt-NO formation in the detailed reaction mechanism

Profile measurements of the H2/O2 reaction have been obtained using a variable pressure flow reactor over pressure and temperature ranges of 0.3–15.7 atm and 850–1040 K, respectively. These data span the explosion limit behavior of the system and place significant emphasis on HO2 and H2O2 kinetics. The explosion limits of dilute H2/O2/N2 mixtures extend to higher pressures and temperatures than those previously observed for undiluted H2/O2 mixtures. In addition, the explosion limit data exhibit a marked transition to an extended second limit which runs parallel to the second limit criteria calculated by assuming HO2 formation to be terminating. The experimental data and modeling results show that the extended second limit remains an important boundary in H2/O2 kinetics. Near this limit, small increases in pressure can result in more than a two order of magnitude reduction in reaction rate. At conditions above the extended second limit, the reaction is characterized by an overall activation energy much higher than in the chain explosive regime.



The overall data set, consisting primarily of experimentally measured profiles of H2, O2, H2O, and temperature, further expand the data base used for comprehensive mechanism development for the H2/O2 and CO/H2O/O2 systems. Several rate constants recommended in an earlier reaction mechanism have been modified using recently published rate constant data for H + O2 (+ N2) = HO2 (+ N2), HO2 + OH = H2O + O2, and HO2 + HO2 = H2O2 + O2. When these new rate constants are incorporated into the reaction mechanism, model predictions are in very good agreement with the experimental data. ©1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 113–125, 1999

Flow reactor studies and kinetic modeling of the H2/O2 reaction

A methodology for developing gas-phase reaction mechanisms for nitramine propellants is discussed. Examples of fundamental kinetic experiments and detailed modeling calculations are given that emphasize a hierarchical construction procedure for both the types of experiments necessary for collecting kinetic data and the specific chemical submodels that need to be studied. In particular, three kinetic submodels of increasing complexity are described with selected results from flow reactor experiments. With kinetic data generated from these experiments and others, chemical submodels are developed and validated by comparison of model predictions with experimental measurements using reaction flux and sensitivity analyses to guide the process. Collectively, the submodels, validated over the entire range of experimentally studied conditions, are assembled to form the gasphase nitramine combustion mechanism. Because of the nature of the developmental procedure, e.g., the necessity for different types of experiments, ranging from "microscopic" measurements of isolated reaction rate constants to "macroscopic" studies on kinetic mechanisms in flames, the process is inherently iterative. As an example of this iterative procedure, numerical results are presented on the ignition behavior of hexahydro-l,3,5-trinitro1,3,5-triazine (RDX) after updating a previously developed model with recent results from submodel validation experiments. These results show changes in the pathways of many secondary species involved in RDX decomposition.

Development of Gas-Phase Reaction Mechanisms for Nitramine Combustion

High pressure studies of moist carbon monoxide/nitrous oxide kinetics

Reaction experiments on mixtures of N2O/H2O/N2 were performed in a variable pressure flow reactor over temperature, pressure, and residence time ranges of 1103–1173 K, 1.5–10.5 atm, and 0.2–0.8 s, respectively. Mixtures of approximately 1% N2O in N2 were studied with the addition of varying amounts of water vapor, from background to 3580 ppm. Experimentally measured profiles of N2O, O2, NO, NO2, H2O, and temperature were compared with predictions from detailed kinetic modeling calculations to assess the validity of a reaction mechanism developed from currently available literature thermochemical and rate constant parameters. Sensitivity and reaction flux analyses were performed to determine key elementary reaction path processes and rates.



Reaction rate constants for the uni-molecular reaction, N2O N2 + O, were determined at various pressures in order to match overall experimental and numerical decomposition rates of N2O. The numerical model included a newly determined rate constant for N2O + OH HO2 + N2 with an upper limit of 5.66 × 108 cm3 mol−1 sec−1 at 1123 K. This is considerably smaller than presently reported in the literature. The experimentally observed rate of N2O decomposition was found to be slightly dependent on added water concentration. The rate constant determined for the elementary decomposition is strongly dependent on the choice of rate constants for the N2O + O N2 + O2 and N2O + O NO + NO reactions. In the absence of accurate data at the temperatures of this work, and based on these and other experiments in this laboratory, we presently recommend rate constants from the review of Baulch et al. The basis for this recommendation is discussed, including the impact on the rate constants derived for elementary nitrous oxide decomposition. The uncertainties in the rate constants as reported here are ±30%.



The present mechanism was applied to previously reported high-pressure shock tube data and yields a high-pressure limit rate constant a factor of three larger than previously reported at these temperatures. The following expressions for the elementary decomposition reaction are recommended: k = 9.13 × 1014 exp (−57, 690/RT) cm3 mol−1 s−1 and k∞ = 7.91 × 1010 exp(−56020/RT) s−1. Simple Lindemann fits utilizing these parameters reproduce the pressure dependent rate constants measured here within ±25%. © 1995 John Wiley & Sons, Inc.

Mark T. Allen

Papers

Development of Gas-Phase Reaction Mechanisms for Nitramine Combustion

High pressure studies of moist carbon monoxide/nitrous oxide kinetics

The decomposition of nitrous oxide at 1.5 ⩽ P ⩽ 10.5 atm and 1103 ⩽ T ⩽ 1173 K

Hydrogen/nitrous oxide kinetics—Implications of the NxHy species