Henry J. Curran

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.

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.

The reaction kinetics of dimethyl ether. I: High‐temperature pyrolysis and oxidation in flow reactors

Dimethyl ether reaction kinetics at high temperature were studied in two different flow reactors under highly dilute conditions. Pyrolysis of dimethyl ether was studied in a variable-pressure flow reactor at 2.5 atm and 1118 K. Studies were also conducted in an atmospheric pressure flow reactor at about 1085 K. These experiments included trace-oxygen-assisted pyrolysis, as well as full oxidation experiments, with the equivalence ratio (ϕ) varying from 0.32 ≤ ϕ ≤ 3.4. On-line, continuous, extractive sampling in conjunction with Fourier Transform Infra-Red, Non-Dispersive Infra-Red (for CO and CO2) and electrochemical (for O2) analyses were performed to quantify species at specific locations along the axis of the turbulent flow reactors. Species concentrations were correlated against residence time in the reactor and species evolution profiles were compared to the predictions of a previously published detailed kinetic mechanism. Some changes were made to the model in order to improve agreement with the present experimental data. However, the revised model continues to reproduce previously reported high-temperature jet-stirred reactor and shock tube results.

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.

The reaction kinetics of dimethyl ether. II: Low‐temperature oxidation in flow reactors

Dimethyl ether oxidation has been studied in a variable-pressure flow reactor over an initial reactor temperature range of 550–850 K, in the pressure range 12–18 atm, at equivalence ratios of 0.7 ≤ ϕ ≤ 4.2, with nitrogen diluent of approximately 98.5%. On-line extraction sampling in conjunction with FTIR, NDIR (for CO and CO2), and electrochemical (for O2) analyses were performed to quantify species at specific locations along the axis of the turbulent flow reactor. Product species concentrations were correlated against residence time (at constant inlet temperature) and against temperature (at fixed mean residence time) in the reactor. Formic acid was observed as a major intermediate of dimethyl ether oxidation at low temperatures. The experimental species-evolution profiles were compared to the predictions of a previously published detailed kinetic mechanism [1]. This mechanism did not predict the formation of formic acid. In the current study we have included chemistry leading to formic acid formation (and oxidation). This new chemistry is discussed and is able to reproduce the experimental observations with good accuracy. In addition, this model is able to reproduce low-temperature kinetic data obtained in a jet-stirred reactor [2] and the shock-tube results of Pfahl et al. [3]

Detailed Chemical Kinetic Mechanisms for Combustion of Oxygenated Fuels

Thermodynamic properties and detailed chemical kinetic models have been developed for the combustion of two oxygenates: methyl butanoate, a model compound for biodiesel fuels, and methyl formate, a related simpler molecule. Bond additivity methods and rules for estimating kinetic parameters were adopted from hydrocarbon combustion and extended. The resulting mechanisms have been tested against the limited combustion data available in the literature, which was obtained at low temperature, subatmospheric conditions in closed vessels, using pressure measurements as the main diagnostic. Some qualitative agreement was obtained, but the experimental data consistently indicated lower overall reactivities than the model, differing by factors of 10 to 50. This discrepancy, which occurs for species with wellestablished kinetic mechanisms as well as for methyl esters, is tentatively ascribed to the presence of wall reactions in the experiments. The model predicts a region of weak or negative dependence of overall reaction rate on temperature for each methyl ester. Examination of the reaction fluxes provides an explanation of this behavior, involving a temperature-dependent competition between chain-propagating unimolecular decomposition processes and chain-branching processes, similar to that accepted for hydrocarbons. There is an urgent need to obtain more complete experimental data under well-characterized conditions for thorough testing of the model.

Extinction and Autoignition of n-Heptane in Counterflow Configuration

A study is performed to elucidate the mechanisms of extinction and autoignition of n-heptane in strained laminar flows under nonpremixed conditions. A previously developed detailed mechanism made UP of 2540 reversible elementary reactions among 557 species is the starting point for the study. The detailed mechanism was previously used to calculate ignition delay times in homogeneous reactors, and concentration histories of a number of species in plug-flow and jet-stirred reactors. An intermediate mechanism made up of 1282 reversible elementary reactions among 282 species and a short mechanism made up of 770 reversible elementary reactions among 160 species are assembled from this detailed mechanism. Ignition delay times in an isochoric homogeneous reactor calculated using the intermediate and the short mechanism are found to agree well with those calculated using the detailed mechanism. The intermediate and the short mechanism are used to calculate extinction and autoignition of n-heptane in strained laminar flows. Steady laminar flow of two counter flowing Streams toward a stagnation plane is considered. One stream made up of prevaporized n-heptane and nitrogen is injected from the fuel boundary and the other stream made up of air and nitrogen is injected from the oxidizer boundary. Critical conditions of extinction and autoignition given by the strain rate, temperature and concentrations of the reactants at the boundaries, are calculated. The results are found to agree well with experiments. Sensitivity analysis is carried out to evaluate the influence of various elementary reactions on autoignition. At all values of the strain rate investigated here, high temperature chemical processes are found to control autoignition. In general, the influence of low temperature chemistry is found to increase with decreasing strain. A key finding of the present study is that strain has more influence on low temperature chemistry than the temperature of the reactants.

Oxidation of automotive primary reference fuels at elevated pressures

Automotive engine knock limits the maximum operating compression ratio and ultimate thermodynamic efficiency of spark-ignition (SI) engines. In compression-ignition (CI) or diesel cycle engines, the premixed burn phase, which occurs shortly after injection, determines the time it takes for autoignition to occur. In order to improve engine efficiency and to recommend more efficient, cleaner-burning alternative fuels, they must understand the chemical kinetic processes that lead to autoignition in both SI and CI engines. These engines burn large molecular-weight blended fuels, a class to which the primary reference fuels (PRF) n-heptane and iso-octane belong. In this study, experiments were performed under engine like conditions in a high-pressure flow reactor using both the pure PRF fuels and their mixtures in the temperature range 550-880 K and 12.5 atm pressure. These experiments not only provide information on the reactivity of each fuel but also identify the major intermediate products formed during the oxidation process. A detailed chemical kinetic mechanism is used to simulate these experiments, and comparisons of experimentally measured and model predicted profiles for O{sub 2}, CO, CO{sub 2}, H{sub 2}O and temperature rise are presented. Intermediates identified in the flow reactor are compared with those present in the computations, and the kinetic pathways leading to their formation are discussed. In addition, autoignition delay times measured in a shock tube over the temperature range 690-1220 K and at 40 atm pressure were simulated. Good agreement between experiment and simulation was obtained for both the pure fuels and their mixtures. Finally, quantitative values of major intermediates measured in the exhaust gas of a cooperative fuels research engine operating under motored engine conditions are presented together with those predicted by the detailed model.

Laminar Flame Speed and Ignition Delay Time Data for the Kinetic Modeling of Hydrogen and Syngas Fuel Blends

Laminar flame speeds and ignition delay times have been measured for hydrogen and various compositions of H2/CO (syngas) at elevated pressures and elevated temperatures. Two constant-volume cylindrical vessels were used to visualize the spherical growth of the flame through the use of a schlieren optical setup to measure the laminar flame speed of the mixture. Hydrogen experiments were performed at initial pressures up to 10atm and initial temperatures up to 443K. A syngas composition of 50/50 by volume was chosen to demonstrate the effect of carbon monoxide on H2-O2 chemical kinetics at standard temperature and pressures up to 10atm. All atmospheric mixtures were diluted with standard air, while all elevated-pressure experiments were diluted with a He:O2 ratio of 7:1 to minimize instabilities. The laminar flame speed measurements of hydrogen and syngas are compared to available literature data over a wide range of equivalence ratios, where good agreement can be seen with several data sets. Additionally, an improved chemical kinetics model is shown for all conditions within the current study. The model and the data presented herein agree well, which demonstrates the continual, improved accuracy of the chemical kinetics model. A high-pressure shock tube was used to measure ignition delay times for several baseline compositions of syngas at three pressures across a wide range of temperatures. The compositions of syngas (H2/CO) by volume presented in this study included 80/20, 50/50, 40/60, 20/80, and 10/90, all of which are compared to previously published ignition delay times from a hydrogen-oxygen mixture to demonstrate the effect of carbon monoxide addition. Generally, an increase in carbon monoxide increases the ignition delay time, but there does seem to be a pressure dependency. At low temperatures and pressures higher than about 12atm, the ignition delay times appear to be indistinguishable with an increase in carbon monoxide. However, at high temperatures the relative composition of H2 and CO has a strong influence on ignition delay times. Model agreement is good across the range of the study, particularly at the elevated pressures. [DOI: 10.1115/1.4007737]

Detailed chemical kinetic modeling of diesel combustion with oxygenated fuels

The influence of oxygenated hydrocarbons as additives to diesel fuels on ignition, NOx emissions and soot production has been examined using a detailed chemical kinetic reaction mechanism. N-heptane was used as a representative diesel fuel, and methanol, ethanol, dimethyl ether and dimethoxymethane were used as oxygenated fuel additives. It was found that addition of oxygenated hydrocarbons reduced NOx levels and reduced the production of soot precursors. When the overall oxygen content in the fuel reached approximately 25% by mass, production of soot precursors fell effectively to zero, in agreement with experimental studies. The kinetic factors responsible for these observations are discussed.