L.R. Sochet

A rapid compression machine investigation of oxidation and auto-ignition of n-heptane: measurements and modeling

n-Heptane oxidation and auto-ignition in a rapid compression machine is studied in the low and intermediate temperature regimes at high pressures. Experimental ignition delay times and some phenomenological aspects related to knock in engines are presented, providing additional information at lower temperatures on previously published delays from shock tube experiments. The products of oxidation are identified and time profiles are measured during a two-stage ignition process. Eight C7 heterocycles, heptenes, lower 1-alkenes, aldehydes, and carbon monoxide are the main species. Their origin is discussed in relation to the isomerization and decomposition of heptylperoxy radicals. The high selectivity observed in the formation of lower 1-alkenes is explained by the scission of the β CC bond of the β-hydroperoxyheptyl radicals weakened by the presence of oxygen atoms. Numerical simulation of the experiments with Warnatzs comprehensive chemical mechanism gives satisfactory results for cool flame and total ignition delays, but fails to reproduce the detailed chemistry before auto-ignition.

Oxidation and combustion of low alkylbenzenes at high pressure: comparative reactivity and auto-ignition

Abstract The auto-ignition features of 11 alkylbenzenes in a rapid compression machine have been compared for stoichiometric mixtures in the lower temperature region (600–900 K), and at compressed pressures up to 25 bar, by following pressure traces and light emission. They are classified in two groups. Toluene, m -xylene, p -xylene, and 1,3,5-trimethylbenzene ignite only above 900 K and 16 bar. o -Xylene, ethylbenzene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, n -propylbenzene, 2-ethyltoluene, and n -butylbenzene ignite at much lower temperature and pressure. The second group shows a complex phenomenology similar to alkanes and alkenes when submitted to adapted conditions of reactant concentrations. Ignition in two steps and negative temperature dependence of ignition delays are observed in favorable cases. Some of them show a low-temperature luminescence. Ignition features of o -xylene and n -butylbenzene are similar, in spite of their dissimilar molecular structure. The higher degree of reactivity of the second group is ascribed to the close proximity and/or length of their alkyl chains.

Experimental and Modeling Study of Oxidation and Autoignition of Butane at High Pressure

Oxidation and autoignition of stoichiometric, lean (ϕ = 0.8), and rich (ϕ = 1.2) butane-“air” mixtures are studied in a rapid compression machine between 700–900 K and 9–11 bar. Information is obtained concerning cool flames and ignition delays. Product profiles for selected major and minor species are measured during a two-stage ignition process. The presence of C4 heterocycles may be connected to isomerization and decomposition of butylperoxy radicals. The experimental results are compared with numerical predictions of an homogeneous adiabatic model based on the Pitz-Westbrook comprehensive chemical mechanism of 1990. The experimental and predicted delays are in the same order of magnitude. A relatively good agreement is found for the major species profiles. Improvement of the mechanism is needed to account for the minor products. The different paths of OH formation are discussed.

High pressure auto-ignition and oxidation mechanisms of o-xylene, o-ethyltoluene, and n-butylbenzene between 600 and 900 K

Abstract A complex phenomenology of auto-ignition, similar to n -alkanes and n -alkenes, has been revealed between 600–900 K and at pressures above 14 bar by studying in a rapid compression machine stoichiometric mixtures of o -xylene ( o -methyltoluene), o -ethyltoluene, or n -butylbenzene in oxygen with lower concentrations than in air. Extensive chemical analyses of the reacting mixtures before ignition were performed to elucidate the mechanisms of reaction. The classical low temperature scheme, modified for the reactivities of benzylic-type hydrogen atoms and radicals, is valid. It appears that the addition of molecular oxygen to benzylic-type radicals leads to a double peroxidation and low temperature branching only when the transfer of hydrogen in the isomerization step occurs either from an ortho-alkyl group, or from another carbon atom of the same alkyl chain. The products observed are shown to be consistent with the proposed mechanism. The same complex pattern of auto-ignition is found, not only for o -xylene, o -ethyltoluene, and n -butylbenzene, but also for 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, n -propylbenzene, and ethylbenzene. When easily transferable hydrogen atoms are not available for selective radicals such as peroxy radicals, branching occurs through completely different pathways, which require higher temperatures and pressures. Then, the pattern of auto-ignition is much simpler, as already observed for toluene, m -xylene, p -xylene, and 1,3,5-trimethylbenzene [1] .

Autoignition Delays of a Series of Linear and Branched Chain Alkanes in the Intermediate Range of Temperature

A set of ignition data of linear and branched chain alkanes (n-butane, n-pentane, neopentane, n-heptane, and isooctane) measured in an original rapid compression machine is provided. It allows a comparison of the ignition conditions of pressure, temperature and equivalence ratio for these hydrocarbons. Detailed mechanisms from different research groups based on a similar generic scheme of hydrocarbon oxidation are tested against the experimental ignition delays. The differences between experimental and modeling results are discussed.

Detailed analysis of low-pressure premixed flames of CH4 + O2 + N2: a study of prompt-NO

Abstract A detailed experimental study of low-pressure premixed CH 4 /O 2 /N 2 flames has been undertaken for equivalence ratios of 0.8–1.2, to provide an experimental data base for testing chemical mechanisms of hydrocarbon combustion and their ability to predict NO formation. The experimental procedure involved microprobe sampling and gas chromatographic analysis (GC), together with laser-induced fluorescence (LIF). The major and intermediate stable species were determined using GC. Concentrations of OH, CH, and NO were measured by one-photon LIF; those of CO, H, and O by a two-photon excitation scheme. All concentrations, except that of CH, were measured absolutely using an appropriate calibration method. Temperature was measured using the LIF excitation technique on the OH radical. Predictions from three chemical kinetic models, based on the Miller and Bowman (MB) and Gas Research Institute (GRI) mechanisms, are compared with the experimental results. In the case of major and reactive species, the experimental results are well reproduced by the modeling. However some discrepancies are observed for the C 2 hydrocarbon intermediates. The measured concentrations of CH and NO vary with equivalence ratio as predicted by the MBGRI 1.2 mechanism (the MB scheme for forming NO has been added to the GRI 1.2 one for the oxidation of CH 4 ). Under our experimental conditions the kinetic analysis shows a preponderance of prompt-NO formation. Trends in the evolution of CH with equivalence ratio are well predicted by the GRI 2.11 mechanism, but important disagreements are pointed out for predictions of NO. Important discrepancies are also observed in the amounts of CH and NO with the MB mechanism. These discrepancies are developed and could be directly linked to uncertainties in the reactions of CH and H 2 .

Temperature Distribution Induced by Pre-lgnition Reactions in a Rapid Compression Machine

Abstract The temperature after compression of reactive and unreactive gas mixtures are measured at different locations and times in a rapid compression machine by a thin wire thermocouple. Validation is done by laser Rayleigh scattering. Inspection or the temperature fields of unreactive gas and during the delays in three different patterns of spontaneous ignition of isooctane-“air” mixtures shows that the gas temperature is homogeneous for some limes after top dead centre. Then heat losses to the walls and gas recirculation induce temperature inhomogeneities. They are partially levelled as hot ignition is approached. The gas temperature before hot ignition is nearly the same and constant in time in both two-stage or one-stage ignitions. The levelling effect is attributed to the complex chemical kinetics of low temperature alkane oxidation in the range of the negative temperature coefficient of the global reaction rate.

The low-temperature autoignition of alkylaromatics: Experimental study and modeling of the oxidation of n-butylbenzene

The low-temperature oxidation of n -butylbenzene, an intermediate structure between alkanes and short-chain alkylaromatics, was studied between 640 and 840 K by rapid compression and by modeling. Delay times of one- and two-stage autoignitions were measured, and intermediate species after the cool flame were analyzed. First, a detailed mechanism for n -butane was developed with existing material. Then, an n -butylbenzene mechanism was built by taking into account the change of reactivity due to the introduction of the aromatic nucleus. Both mechanisms have been validated by simulations of the delays and the product concentrations. Finally, the n -butylbenzene mechanism was used to analyze the main low-temperature reaction pathways. The comparative calculation of the concentrations of alkyl, alkylperoxy, and hydroperoxyalkyl radicals in the cool flame of n -butane and n -butylbenzene illustrates the effects of the aromatic nucleus on the first steps of oxidation. A study of the competitive channels to the main molecular intermediate species shows that the internal transfer of a benzylic hydrogen to the peroxy sites is a major event in the development of reactions leading to branching and ignition. This can explain a previous observation that alkylaromatics with two oitho -alkyl groups or a long single lateral chain have the possibility of an internal transfer of a benzylic hydrogen and manifest a greater low-temperature reactivity than aromatics that have neither oitho -alkyl groups nor a long lateral chain.

Autoignition of butane: A burner and a rapid compression machine study

The autoignition of butane has been studied by two techniques. At a relatively low pressure (1.8 bar), a two-stage flame has been fully described by following the stable-species and peroxy-radical evolution. At higher pressures, studies have been conducted in a rapid-compression machine in order to investigate the evolution of the autoignition delay times with temperature. These experimental results are compared with predictions obtained from a butane-oxidation model based on a complete mechanism of 133 species and 689 reversible reactions as proposed and tested at low pressure by Pitz, Wilk, Westbrook and Cernansky. In a reduced version (45 species, 272 reversible reactions) the model agrees with the measured major species produced in the second stage of a burner-stabilized two-stage flame. In its complete version, it also predicts the negative temperature coefficient observed at high pressure in the rapid-compression machine. However to account for the ignition delay, it was necessary to modify the rate constants associated with the low-temperature mechanism as suggested by Pitz, Leppard and Westbrook.

Experimental and Numerical Analysis of a Low Pressure Stoichiometric Methanol-Air Flame

Abstract A stoichiometric methanol-air flame stabilized at low pressure was investigated by coupling gas chromatography and electron spin resonance in order to verify a postulated chemical mechanism for methanol combustion. To interpret experimental investigation two codes, based on the CHEMKIN code package, are used: CALFLA, a code calculating the net reaction rates and PREM1X, the flame modeling code from SANDIA. The experimental and modeled results are compared in terms of species mole fraction profiles and, more significantly, in terms of net reaction rate profiles. Furthermore, ihe formation and consumption rates of different species, as well as a first order sensitivity analysis are used in order to describe the importance of the different reactions throughout the flame. The experimental results are in good agreement with the computed values analyzed in terms of the Dove-Warnatz mechanism.