Mole Fraction Time-histories Research Articles

New insights into the effect of molecular structure on stable intermediate formation during the pyrolysis of normal and branched alkanes − II: Impact of carbon number and degree of branching

Transitioning to scalable and sustainable fuels produced from renewable feedstocks will be essential for decarbonizing the transportation sector. As these fuels typically encompass a wide range of molecular classes, it is important to understand the impact of varying molecular structure on their overall combustion behavior. To that end, experiments were conducted to investigate the thermal decomposition of ten neat hydrocarbons with varying carbon numbers and degree of branching in a shock tube at engine-relevant temperatures. A multi-wavelength, laser absorption spectroscopy-based approach was used to measure key stable intermediates formed during the pyrolysis process. The experiments revealed clear trends in intermediate formation and distribution, influenced by the molecular structure of each fuel. Additional experiments were conducted with a 2% composite fuel (consisting of a 50% n-heptane/50% iso-octane mixture)/Ar mixture to assess the impact of fuel blending on the pyrolysis process. A companion paper (Boddapati et al., 2024) delineates the experimental strategy and provides comprehensive mole fraction time histories of the major products, along with thorough analysis of carbon atom recovery. This paper establishes empirical trends in the measured intermediate yields with varying carbon number and degree of branching, and provides a discussion on the chemical phenomena governing the thermal decomposition of these fuels. In addition, this paper also lays the foundation for developing compact, robust chemical kinetic models for conventional and alternative fuels based on compositional information obtained through infrared spectra. The insights gained from these studies have the potential to guide sustainable fuel design and contribute to modeling the combustion chemistry of next-generation fuels.

New insights into the effect of molecular structure on stable intermediate formation during the pyrolysis of normal and branched alkanes – I: Multi-species time history measurements

Decarbonization of energy systems that drive the transportation industry is imperative for effectively mitigating global greenhouse gas emissions. In this pursuit, the transition towards scalable and sustainable fuels will play a key role. These fuels are predominantly composed of a diverse array of hydrocarbons spanning various molecular classes. To better understand the complex chemistry of such fuels, we studied the impact of molecular structure on the distribution of pyrolysis products of C6-C12 hydrocarbons, encompassing both normal and branched alkanes, in a shock tube at combustion-relevant temperatures. An extensive suite of laser absorption spectroscopy-based diagnostics was used to simultaneously measure the time histories of key intermediates (methane, ethylene, and larger alkenes) and fuel during the pyrolysis of ten neat hydrocarbons: n-hexane, n-heptane, n-octane, n-decane, n-dodecane, 2-methylhexane, 2-methylheptane, 2-methylnonane, triptane (or 2,2,3-trimethylbutane), and iso-octane (or 2,2,4-trimethylpentane). These experiments were conducted behind reflected shock waves using 2% Fuel/Argon test mixtures at a nominal pressure of 2 atm over the temperature range of 1100–1500 K. Conducting these experiments under similar test conditions facilitated the observation of clear trends in the distribution of intermediates formed during the pyrolysis of these fuels. The current paper delineates the experimental strategy and presents the measured mole fraction time histories of the stable intermediates. A companion paper [1] delves into the observed trends and effects of molecular structure, in particular, the impact of increasing carbon number and the impact of varying degree of branching on the yields of these pyrolysis products. We also investigate the impact of fuel blending and provide insights into the role of inter-coupling on the distribution of stable intermediates. The authors anticipate that the trends and findings presented in these two papers will guide the design of sustainable fuels and play a pivotal role in modeling the combustion chemistry of the next generation of fuels. To the best of our knowledge, this work presents the first systematic, detailed pyrolysis study of several transportation-relevant hydrocarbon fuels, involving simultaneous measurements of multiple species in a shock tube using laser absorption spectroscopy.

Understanding the impact of molecular structure on the formation of stable intermediates during the pyrolysis of monoalkylated cyclohexanes in a shock tube

Alkylcyclohexanes are common constituents of conventional jet fuels and are expected to be a vital molecular subclass in next-generation, sustainable aviation fuels (SAFs). In an effort to advance the current understanding of the combustion chemistry of these species at engine-relevant conditions, we measured the time-resolved evolution of key stable intermediates during the pyrolysis of cyclohexane (CH) and four monoalkylated cyclohexanes: methylcyclohexane (MCH), ethylcyclohexane (ECH), propylcyclohexane (PCH), and butylcyclohexane (BCH), using laser absorption spectroscopy (LAS) in a shock tube. These experiments were conducted using 2 % Fuel/Argon test mixtures at a nominal pressure of 2 atm over the temperature range 1150–1530 K. Simultaneous tracking of the mole fraction time histories of methane, ethylene, and larger alkenes (>C2) provided new, valuable insight into the high-temperature reactivity of the five fuels studied. Studying all five fuels under similar test conditions enabled us to observe clear trends in the yields of different pyrolysis products with varying molecular structure, specifically increasing the number of carbon atoms in the alkyl chain. The fuel structure effects investigated in this work are instrumental in characterizing the pyrolysis product distribution, and thereby the overall combustion behavior of cyclohexane derivatives with longer (>C4) alkyl chains. We believe the multi-wavelength speciation data presented in this work can significantly contribute towards the development of robust, simplified, and fuel-specific kinetic models for monoalkylated cyclohexanes.

Experimental and chemical kinetic modeling study of high-temperature oxidation of diisopropyl methylphosphonate (DIMP) - A sarin simulant

Chemical warfare (CW) agent simulants are used in laboratory experiments to study the combustion characteristics of CW agents due to their high toxicity. A crucial CW agent simulant with a chemical structure similar to the deadly nerve agent Sarin (GB) is diisopropyl methylphosphonate (DIMP), an organophosphate compound (OPC). In this study, the high-temperature oxidation of DIMP is investigated in a shock tube at temperatures between 1440 K and 1710 K and a pressure of 1–2 atm. The carbon monoxide mole fraction time histories near 4.9 µm were obtained using laser absorption spectroscopy. The rate parameters for DIMP's H-abstraction reactions with O, and OH radicals were determined using molecular simulations. The rates of reactions involving smaller phosphorous species were also calculated at the CBS-QB3 level. These reactions along with the isopropanol sub-mechanism from the literature were added to the LLNL model to obtain an improved chemical kinetic mechanism for DIMP. Since isopropanol was a major intermediate in DIMP decomposition, validations were conducted with CO time histories during the oxidation of isopropanol. The new model predicted CO during isopropanol oxidation reasonably well. Both the LLNL model and the model developed in this work could predict CO time histories during DIMP oxidation satisfactorily. To comprehend the CO formation pathways and sensitive reactions during DIMP oxidation, reaction path analysis and sensitivity analysis were also carried out. The reaction mechanism developed here will help in the design, development, and optimization of efficient, effective and secure CW destruction techniques.

Butyl Acetate Pyrolysis and Combustion Chemistry: Mechanism Generation and Shock Tube Experiments.

The combustion and pyrolysis behaviors of light esters and fatty acid methyl esters have been widely studied due to their relevance as biofuel and fuel additives. However, a knowledge gap exists for midsize alkyl acetates, especially ones with long alkoxyl groups. Butyl acetate, in particular, is a promising biofuel with its economic and robust production possibilities and ability to enhance blendstock performance and reduce soot formation. However, it is little studied from both experimental and modeling aspects. This work created detailed oxidation mechanisms for the four butyl acetate isomers (normal-, sec-, tert-, and iso-butyl acetate) at temperatures varying from 650 to 2000 K and pressures up to 100 atm using the Reaction Mechanism Generator. About 60% of species in each model have thermochemical parameters from published data or in-house quantum calculations, including fuel molecules and intermediate combustion products. Kinetics of essential primary reactions, retro-ene and hydrogen atom abstraction by OH or HO2, governing the fuel oxidation pathways, were also calculated quantum-mechanically. Simulation of the developed mechanisms indicates that the majority of the fuel will decompose into acetic acid and relevant butenes at elevated temperatures, making their ignition behaviors similar to butenes. The adaptability of the developed models to high-temperature pyrolysis systems was tested against newly collected high-pressure shock experiments; the simulated CO mole fraction time histories have a reasonable agreement with the laser measurement in the shock tube. This work reveals the high-temperature oxidation chemistry of butyl acetates and demonstrates the validity of predictive models for biofuel chemistry established on accurate thermochemical and kinetic parameters.

Exploring low-temperature oxidation chemistry of 2- and 3-pentanone

The low-temperature oxidation chemistry of 2- and 3-pentanone was investigated over a wide range of conditions using a jet-stirred reactor (JSR) and a rapid compression machine (RCM). The JSR oxidation experiment was performed at the pressure of 93.3 kPa over a temperature range of 600-1000 K. Detailed speciation information was obtained using synchrotron vacuum ultraviolet photoionization mass spectrometry. Ignition delay times (IDTs) of 2- and 3-pentanone were measured in an RCM from 640 to 820 K at pressures of 15 and 25 bar and an equivalence ratio of 1.0. The two C5 ketones showed NTC behavior and two-stage ignition phenomena. Mole fraction time histories of intermediate species during the two-stage ignition process of both ketones were obtained using a fast-sampling system coupled with gas chromatography. There are distinct differences between 2- and 3-pentanone in species concentration profiles and IDTs. A kinetic mechanism for the low-temperature oxidation of 2- and 3-pentanone was developed, which can satisfactorily predict all available measurements. The reaction path analyses indicate that the intramolecular hydrogen migration reaction of ROO radicals tends to produce resonance-stabilized QOOH radical structures. The secondary oxygen addition reaction of resonance-stabilized QOOH radicals thus is the most important source of OH radicals in the low-temperature oxidation of ketone fuels. The intramolecular hydrogen migration reactions are slowed down in the presence of the carbonyl functional group, which makes the low-temperature reactivity of the two C5 ketones lower than that of n-pentane. The position of the carbonyl functional group affects the species pools during the oxidation of the two ketones to a great extent. Larger production of CH4, C3H6, CH3COCH3, and C2H5CHO were observed in 2-pentanone oxidation, while the production of CH3CHO was favored during 3-pentanone oxidation both in the JSR and RCM experiments. The different lengths of the carbon chain on both sides of the carbonyl group in 2- and 3-pentanone resulted in the difference in the species distribution.

A mid-IR laser diagnostic for HCN detection

Hydrogen cyanide (HCN) is a major source of prompt-NOx formation especially in fuel-bound nitrogen systems. To date, there is still a significant disagreement between experimental data and theoretical predictions of the rate coefficients of combustion reactions involving HCN as a prompt-NOx precursor. Accurate modeling of NOx formation would greatly benefit from a diagnostic capable of performing high-fidelity measurements of HCN formation/consumption time-histories. In this study, a laser diagnostic is developed for sensitive and selective HCN sensing by probing its most intense absorption feature in the mid-infrared (MIR). The diagnostic is based on difference-frequency generation (DFG) between a CO2 gas laser and an external-cavity quantum cascade laser in a nonlinear orientation-patterned gallium arsenide crystal which results in a DFG laser tunable over 11.56 − 15 µm. HCN measurements were carried out at the peak of the Q-branch of its strong ν2 vibrational band near 14 µm. Pressure dependence of the absorption cross-section was investigated at room temperature over the pressure range of 0.07 − 1.07 bar. Temperature-dependent absorption cross-section measurements were conducted behind reflected shock waves over the temperature range of 850 − 3000 K. The diagnostic was demonstrated in reactive experiments in a shock tube where HCN mole fraction time-histories were measured during the thermal decomposition of isoxazole (C3H3NO) and the first-order rate coefficients of C3H3NO → HCN + CH2CO reaction were determined.

High-temperature pyrolysis experiments and chemical kinetics of diisopropyl methylphosphonate (DIMP), a simulant for Sarin

Due to the high toxicity of chemical warfare (CW) agents, laboratory experiments are carried out using CW surrogates. Diisopropyl Methylphosphonate (DIMP), an organophosphate compound (OPC), is a crucial CW surrogate that has a chemical structure similar to the deadly nerve agent Sarin (GB). In this work, high-temperature pyrolysis of DIMP is conducted in a shock tube within a temperature range of 1400 - 1800 K at near 1 atm pressure. Laser absorption spectroscopy was used to obtain carbon monoxide mole fraction time-histories during high-temperature pyrolysis for DIMP. In order to predict the CO mole fraction time-histories during DIMP pyrolysis, a reaction mechanism was developed using the Lawrence Livermore National Lab (LLNL)’s OPC mechanism. Several important reactions of DIMP and its important intermediates during pyrolysis were studied at the CBS-QB3 level of theory, and corresponding reaction rates were determined using transition state theory. The reactions related to methyl phosphonic acid (MPA) and methyl(oxo)phosphoniumolate (MOPO) were added from literature and by analogies with similar compounds. Thermochemical properties of crucial species involved in DIMP pyrolysis were calculated using the CBS-QB3 composite method and were updated in the model. A reaction mechanism was compiled for DIMP pyrolysis using these rates. Two versions of this mechanism (‘Model A’ and ‘Model B’) were generated to understand the significance of new reactions in predicting CO mole fraction time histories. Significant improvement in the prediction of CO was observed compared to the LLNL model with either of these mechanisms. A reaction path analysis was conducted to understand the formation pathways of CO during DIMP pyrolysis. Finally, sensitivity analysis was performed to get insights into important reactions leading to CO formation. It was found that the H-abstraction from MOPO by methyl radical plays an important role in CO formation.

Shock-Tube Laser Absorption Measurements of CO and H2O during Iso-Octane Combustion

The oxidation of iso-octane was studied in two different shock tubes by recording mole fraction time histories of CO and H2O at various equivalence ratios (0.5, 1.0, and 2.0) at around 1.5 atm, bet...

Spectroscopic inference of alkane, alkene, and aromatic formation during high-temperature JP8, JP5, and Jet-A pyrolysis

The speciation of jet-fuel pyrolysis plays an important role in the development of predictive models for jet-fuel combustion. In this work, a multi-wavelength speciation technique was applied to a large dataset of shock-tube laser-absorption measurements of jet-fuel pyrolysis. A novel absorbance model was developed to interpret the measurements at common conditions, and pyrolysis product time-histories and early-time yields are reported for three jet fuels at five high-temperature conditions. Specifically, methane, ethylene, and lumped vinyl-group time-histories, in addition to aromatics (benzene and toluene) yields, were quantified using eight wavelengths over conditions 1040–1480 K, 1–3 atm, for jet fuels JP8, Jet-A, and JP5. Results are presented at 1150 K, 1200 K, 1250 K, 1300 K, and 1350 K. Additional relationships between vinyl-group-containing species, namely propene, 1-butene, and 1,3-butadiene, are explored. The mole fraction time-histories exhibit good agreement with the literature and extend the speciation of these jet fuels to the limits of current spectroscopic techniques.

Simultaneous measurements of acetylene and soot during the pyrolysis of ethylene and benzene in a shock tube

Acetylene is one of the most important precursors of soot and contributes to soot growth by the hydrogen-abstraction acetylene-addition (HACA) mechanism. In this work, we undertake time-resolved simultaneous measurements of acetylene and soot behind reflected shock waves at temperatures of 1600–2200K and pressures of 3–5bar. Acetylene mole fraction time-histories are measured from the absorption of a quantum-cascade laser operating around 13.6µm. The soot volume fraction, particle size and number densities are calculated from the extinction and scattering of a cw Nd:Yag laser at 532nm. Acetylene and soot are generated from the pyrolysis of 1% benzene in argon, 2.35% ethylene in argon, and binary mixtures of ethylene with propane/methane in argon. We note that acetylene time-histories exhibit a two-stage growth during the pyrolysis of benzene, which can be correlated to the initial rapid increase of soot volume fraction and a later plateauing. In comparison to ethylene pyrolysis, the pyrolysis of benzene results in larger values of the soot volume fraction, particle diameter and number density. We compare the measured data against the values simulated using the method-of-moments routine in Chemkin-Pro and a detailed PAH mechanism based on KM2 [1] and AramcoMech 1.3 [2]. Large discrepancies are observed between the measured and predicted values of the soot parameters. The data obtained from our experiments may assist future validation and development of soot mechanisms.

Intermediate species measurement during iso-butanol auto-ignition

This work presents the time histories of intermediate species during the auto-ignition of iso-butanol at high pressure and intermediate temperature conditions obtained using a rapid compression machine and recently developed fast sampling system. Iso-butanol ignition delays were acquired for iso-butanol/O2 mixture with an inert/O2 ratio of 7.26, equivalence ratio of 0.4, in the temperature range of 840–950 K and at pressure of 25 bar. Fast sampling and gas chromatography were used to acquire and quantify the intermediate species during the ignition delay of the same mixture at P = 25.3 bar and T = 905 K. The ignition delay times and quantitative measurements of the mole fraction time histories of methane, ethene, propene, iso-butene, iso-butyraldehyde, iso-butanol, and carbon monoxide were compared with predictions from the detailed mechanisms developed by Sarathy et al., Merchant et al., and Cai et al. It is shown that while the Sarathy mechanism well predicts the overall ignition delay time, it overpredicts ethene by a factor of 6–10, underpredicts iso-butene by a factor of 2, and overpredicts iso-butyraldehyde by a factor of 2.Reaction path and sensitivity analyses were carried out to identify the reactions responsible for the observed inadequacy. The rates of iso-butanol hydrogen atom abstraction by OH radical and the beta-scission reactions of hydroxybutyl radicals were updated based on recently published quantum calculation results. Significant improvements were achieved in predicting ignition delay at high pressures (25 and 30 bar) and the species concentrations of ethene and iso-butene. However, the updated mechanism still overpredicts iso-butyraldehyde concentrations. Also, the updated mechanism degrades the prediction in ignition delay at lower pressure (15 bar) compared to the original mechanism developed by Sarathy et al.

Laser absorption diagnostic for measuring acetylene concentrations in shock tubes

A fixed-wavelength direct absorption laser diagnostic for high-temperature measurements of acetylene concentration was developed. The diagnostic, based on a tunable continuous wave distributed feedback diode laser, was optimized primarily for studying chemical kinetics behind reflected shock waves. The center wavelength (3335.55cm−1) of the tunable diagnostic was typically set at the peak of the 3300cm−1 absorption band of acetylene at high temperatures. The absorption spectrum of acetylene diluted in argon was characterized using scanned-wavelength direct absorption measurements from 1070 to 1720K and 0.8 to 4.0atm. Line fitting of the measured absorption spectra was not possible due to the large number of transitions overlapped by pressure broadening that contribute to the spectrum. Instead, empirical fits for the peak absorption coefficient and its corresponding wavelength as a function of temperature and pressure were generated. Furthermore, in order to allow for characterization of interference absorption in kinetic studies, empirical fits for the acetylene absorption coefficient in the region around the primary absorption feature were developed. Absorption coefficient measurements of propyne and 1-butyne, which may be the primary interference candidates, reveal that their absorption coefficients are constant in the wavelength range of interest, and are much smaller than those of acetylene. Therefore, the acetylene concentration in the presence of these interfering species can be inferred using two-color techniques. The utility of the acetylene diagnostic was demonstrated by measuring acetylene mole fraction time-histories during the pyrolysis of propene and 1-butene.

Shock Tube Measurements of the Rate Constant for the Reaction Ethanol + OH

The overall rate constant for the reaction ethanol + OH → products was determined experimentally from 900 to 1270 K behind reflected shock waves. Ethan(18)ol was utilized for these measurements in order to avoid the recycling of OH radicals following H-atom abstraction at the β-site of ethanol. Similar experiments were also performed with unlabeled ethan(16)ol in order to infer the rate constant that excludes reactivity at the β-site. The two data sets were used to directly infer the branching ratio for the reaction at the β-site. Experimental data in the current study and in previous low-temperature studies for the overall rate constant are best fit by the expression koverall = 5.07 × 10(5) T[K](2.31) exp(608/T[K]) cm(3) mol(-1) s(-1), valid from 300 to 1300 K. Measurements indicate that the branching ratio of the β-site is between 20 and 25% at the conditions studied. Pseudo-first-order reaction conditions were generated using tert-butylhydroperoxide (TBHP) as a fast source of (16)OH with ethanol in excess. (16)OH mole fraction time-histories were measured using narrow-line width laser absorption near 307 nm. Measurements were performed at the linecenter of the R22(5.5) transition in the A-X(0,0) band of (16)OH that does not overlap with any absorption features of (18)OH, thus producing a measurement of the (16)OH mole fraction that is insensitive to the presence of (18)OH.

Shock Tube Measurements of the tert-Butanol + OH Reaction Rate and the tert-C4H8OH Radical β-Scission Branching Ratio Using Isotopic Labeling

The overall rate constant for the reaction tert-butanol + OH → products was determined experimentally behind reflected shock waves by using (18)O-substituted tert-butanol (tert-butan(18)ol) and tert-butyl hydroperoxide (TBHP) as a fast source of (16)OH. The data were acquired from 900 to 1200 K near 1.1 atm and are best fit by the Arrhenius expression 1.24 × 10(-10) exp(-2501/T [K]) cm(3) molecule(-1) s(-1). The products of the title reaction include the tert-C4H8OH radical that is known to have two major β-scission decomposition channels, one of which produces OH radicals. Experiments with the isotopically labeled tert-butan(18)ol also lead to an experimental determination of the branching ratio for the β-scission pathways of the tert-C4H8OH radical by comparing the measured pseudo-first-order decay rate of (16)OH in the presence of excess tert-butan(16)ol with the respective decay rate of (16)OH in the presence of excess tert-butan(18)ol. The two decay rates of (16)OH as a result of reactions with the two forms of tert-butanol differ by approximately a factor of 5 due to the absence of (16)OH-producing pathways in experiments with tert-butan(18)ol. This indicates that 80% of the (16)OH molecules that react with tert-butan(16)ol will reproduce another (16)OH molecule through β-scission of the resulting tert-C4H8(16)OH radical. (16)OH mole fraction time histories were measured using narrow-line-width laser absorption near 307 nm. Measurements were performed at the line center of the R22(5.5) transition in the A-X(0,0) band of (16)OH, a transition that does not overlap with any absorption features of (18)OH, hence yielding a measurement of (16)OH mole fraction that is insensitive to any production of (18)OH.

Methane and ethylene time-history measurements in n-butane and n-heptane pyrolysis behind reflected shock waves

CH4 and C2H4 concentration time-histories were measured behind reflected shock waves during the pyrolysis of two n-alkanes: n-butane and n-heptane. Experiments were conducted at temperatures of 1200–1600K and at pressures near 1.5atm, with fuel concentrations of 1% in Ar. A mid-IR scanned-wavelength laser absorption diagnostic with a difference frequency generation (DFG) laser near 3.43μm was used to measure CH4 concentration time-histories. C2H4 was measured using a fixed-wavelength absorption scheme at 10.532μm with a CO2 laser. The mechanism of Wang et al. with a constant volume gasdynamic model was used to calculate temperature and pressure profiles and the mole fraction time-histories of CH4 and C2H4. The measured CH4 and C2H4 time-histories in n-butane pyrolysis were compared to simulations based on the comprehensive n-alkane mechanism by Wang et al. and the detailed n-butane mechanism by Marinov et al. Based on these comparisons, the n-butane decomposition rates measured by Oehlschlaeger et al. were incorporated into the Wang et al. mechanism and two additional butane abstraction reaction rate constant adjustments were also made. The measured CH4 and C2H4 time-histories during n-heptane pyrolysis were also compared to simulations based on the mechanisms by Wang et al. and Curran et al. The overall n-heptane decomposition rate measured by Davidson et al. was incorporated into the Wang et al. mechanism, and the two methyl abstraction reactions from n-heptane were adjusted, and the H-abstraction reaction rate from ethylene was updated. Using these modified mechanisms the agreement between simulation and experimental time-histories of CH4 and C2H4 were both significantly improved for n-butane and n-heptane pyrolysis.

On the Chemical Kinetics of n-Butanol: Ignition and Speciation Studies

Direct measurements of intermediates of ignition are challenging experimental objectives, yet such measurements are critical for understanding fuel decomposition and oxidation pathways. This work presents experimental results, obtained using the University of Michigan Rapid Compression Facility, of ignition delay times and intermediates formed during the ignition of n-butanol. Ignition delay times for stoichiometric n-butanol/O(2) mixtures with an inert/O(2) ratio of 5.64 were measured over a temperature range of 920-1040 K and a pressure range of 2.86-3.35 atm and were compared to those predicted by the recent reaction mechanism developed by Black et al. (Combust. Flame 2010, 157, 363-373). There is excellent agreement between the experimental results and model predictions for ignition delay time, within 20% over the entire temperature range tested. Further, high-speed gas sampling and gas chromatography techniques were used to acquire and analyze gas samples of intermediate species during the ignition delay of stoichiometric n-butanol/O(2) (χ(n-but) = 0.025, χ(O(2)) = 0.147, χ(N(2)) = 0.541, χ(Ar) = 0.288) mixtures at P = 3.25 atm and T = 975 K. Quantitative measurements of mole fraction time histories of methane, carbon monoxide, ethene, propene, acetaldehyde, n-butyraldehyde, 1-butene and n-butanol were compared with model predictions using the Black et al. mechanism. In general, the predicted trends for species concentrations are consistent with measurements. Sensitivity analyses and rate of production analyses were used to identify reactions important for predicting ignition delay time and the intermediate species time histories. Modifications to the mechanism by Black et al. were explored based on recent contributions to the literature on the rate constant for the key reaction, n-butanol+OH. The results improve the model agreement with some species; however, the comparison also indicates some reaction pathways, particularly those important to ethene formation and removal, are not well captured.

Speciation studies of methyl butanoate ignition

The current work presents the results of an experimental study of the intermediates formed during ignition of methyl butanoate (C 5H 10O 2) and air mixtures. A rapid-sampling system and the University of Michigan rapid compression facility were used to acquire gas samples at conditions of P = 10.2 atm and T = 985 K using mixtures of χ mb = 0.96%, χ O 2 = 20.79%, χ N 2 = 52.89%, and χ Ar = 25.25% (mole fraction, percent basis); corresponding to ϕ = 0.30 and an inert gas to O 2 molar ratio of 3.76. The samples were analyzed using gas chromatography. Quantitative measurements of mole fraction time-histories of methane, ethane, propane, ethene, propene, and 1-butene are compared with model predictions based on a reaction mechanism developed in previous work. The methane and ethene time-histories are in excellent agreement (within ∼20%), while propene and ethane are underpredicted by the model. Sensitivity analysis shows ignition is controlled primarily by competition between H 2O 2 and HO 2 kinetics at these conditions. Reaction path analysis shows the methyl butanoate fuel consumption is dominated by H-atom abstraction by OH.

A rapid compression facility study of OH time histories during iso-octane ignition

Iso-octane ignition delay times ( τ ign ) and hydroxyl (OH) radical mole fraction ( χ OH ) time histories were measured under conditions relevant to homogeneous charge compression ignition engine operating regimes using the University of Michigan rapid compression facility. Absolute quantitative OH mole fraction time histories were obtained using differential narrow-line laser absorption of the R 1(5) line of the A 2 Σ + ← X 2 Π i ( 0 , 0 ) band of the OH spectrum ( ν 0 = 32606.56 cm −1 ). Ignition delay times were determined using pressure and OH data. Diluted iso-octane/argon/nitrogen/oxygen mixtures were used with fuel/oxygen equivalence ratios from ϕ = 0.25 to 0.6 for τ ign measurements and from ϕ = 0.25 to 0.35 for χ OH measurements. The pressures and temperatures after compression ranged from 8.5 to 15 atm and from 945 to 1020 K, respectively, for the combined τ ign and χ OH data. The maximum mole fraction of OH during ignition and the plateau value of OH after ignition are compared with model predictions using different iso-octane oxidation mechanisms. Sensitivity and rate of production analyses for OH identify reactions important in iso-octane ignition under these lean, intermediate-temperature conditions. The OH time histories show significant sensitivity to the OH + OH + M = H 2O 2 + M, CH 3 + HO 2 = CH 3O + OH, and CH 3 + HO 2 = CH 4 + O 2 reactions, which have rate coefficients with relatively high uncertainties. Improved predictions of the OH time histories can be achieved by modifying the rate coefficient for these reactions. The enthalpy of formation used for OH also has a significant effect on the predicted ignition delay times.