5 Search Results

In this work, we studied the oxidation of neo-pentane by combining experiments, theoretical calculations, and mechanistic developments to elucidate the impact of the 3rd O₂ addition reaction network on ignition delay time predictions. The experiments are based on photoionization mass spectrometry in jet-stirred and time-resolved flow reactors allowing for sensitive detection of the keto-hydroperoxide (KHP) and keto-dihydroperoxide (KDHP) intermediates. With neo-pentane exhibiting a unique symmetric molecular structure, which consequently results only in single KHP and KDHP isomers, theoretical calculations of ionization and fragment appearance energies and of absolute photoionization cross sections enabled the unambiguous identification and quantification of the KHPmore » intermediate. Its temperature and time-resolved profiles together with calculated and experimentally observed KHP-to-KDHP signal ratios were compared to simulation results based on a newly developed mechanism that describes the 3rd O₂ addition reaction network. A satisfactory agreement has been observed between the experimental data points and the simulation results, thus adding confidence to the model's overall performance. Finally, this mechanism was used to predict ignition delay times reported previously in shock tube and rapid compression machine experiments (J. Bugler et al., Combust. Flame 163 (2016) 138–156). While the model accurately reproduces the experimental data, simulations with and without the 3rd O₂ addition reaction network included reveal only a negligible effect on the predicted ignition delay times at 10 and 20 atm. According to model calculations, low temperatures and high pressures promote the importance of the 3rd O2 addition reactions.« less

The reaction networks responsible for aromatics formation in counterflow flames of the C₃H₄ isomers allene and propyne are identified through a combined experimental and modeling study. Mole fraction profiles of near-atmospheric pressure (933 mbar) diffusion flames fueled by the C₃H₄ isomers are analyzed by means of a newly assembled, chemically detailed kinetic mechanism. The experiment consists of a counterflow burner system that is coupled to a high-resolution time-of-flight molecular-beam mass spectrometer with single-photon ionization via synchrotron-generated vacuum-ultraviolet photons. Flame-sampled, mass-specific photoionization efficiency curves are used to identify the presence of aliphatically substituted aromatic species in addition to the commonly consideredmore » pericondensed ring structures. The new mechanism describes the formation and growth of aromatics through repetitive sequences of radical–radical and radical–molecule reactions that include C_{16 intermediates. Higher concentrations of aromatic species are observed in the allene flame and the new mechanism captures the observed experimental trends very accurately. The results indicate the importance of the aliphatically substituted aromatics and of ring-enlargement reactions for the growth reactions. According to the model simulations, radical+radical recombination and PAH-radical+molecule reactions play an important role in PAH growth.}« less

Highly branched iso-alkanes are an important class of hydrocarbons found in conventional petroleum-derived and alternative renewable fuels used for combustion applications. Recognizing that chemical kinetics for most of these iso-alkanes, especially at low-to-intermediate temperatures, has not been well studied, an experimental and modeling investigation of two selected iso-alkanes, iso-octane (2,2,4-trimethylpentane, iC8) and iso-dodecane (2,2,4,6,6-pentamethylheptane, iC12), is conducted to understand the fuel molecular structure effect on their autoignition characteristics. Using a rapid compression machine (RCM), the ignition responses of iC8 and iC12 at varying pressures, temperatures, and equivalence ratios are characterized and compared. The newly-acquired experimental ignition delay times have beenmore » compared with the literature RCM and shock tube data, demonstrating the complementary nature of the current dataset. Further comparison of the experimental pressure traces and ignition delay times illustrates the reactivity crossover between iC8 and iC12. Namely, there exists a temperature window in the negative temperature coefficient regime within which iC12 is less reactive than iC8, but iC12 becomes more reactive outside this temperature window. Furthermore, a chemical kinetic model of iso-alkanes including both iC8 and iC12 is developed. Simulated results using this model are then compared to the experimental data obtained in this study and available in the literature, showing its ability to predict the experimental trends. Chemical kinetic analyses have also been conducted to identify the important reaction pathways controlling autoignition at varying conditions, and to elucidate the underlying mechanism leading to different reactivity trends between iC8 and iC12.« less

A counterflow configuration was used to measure thermal and species structure in water-vapor diluted nonpremixed methane–air flames. The motivation is to understand the chemical and thermal effects that water has when it is introduced as a diluent into the fuel side. Here, this work is relevant to combustion processes where water is incorporated naturally in the fuel; e.g., methane hydrates, and when water is added intentionally for emission reduction such as in flares and H₂O/fuel emulsions combustion. Experimental data are compared to 1-D computations. The agreement is generally very good, but the one dimensional counterflow diffusion model overpredicts flame temperaturemore » and major radical, OH, concentration very near extinction in highly diluted H₂O–methane/air diffusion flames. Changes in flame position, flame width, and peak temperature with the addition of water were measured. Flame temperatures were measured with thin filament pyrometry. OH-PLIF is used to characterize the flame reaction zone with water dilution; the OH distribution, flame position and thickness from the OH-PLIF images were measured. The results show that the OH intensity and reaction zone thickness decreases with the increase in water. Predictions and experiments demonstrate that water mainly acts thermally to lower the flame temperature until extinction. The OH maximum intensity shifts towards the air side of the counterflow burner with water addition. OH is also measured with CO₂ dilution of the fuel stream, and the results are compared with H₂O addition, including comparisons with the OH molar peak predictions obtained using the GRI 3.0 mechanism and the CHEMKIN Pro one dimensional counterflow model. The study indicates that water’s chemical effects are to change the production and depletion of OH, H and O radicals, especially near extinction. Chemical kinetics simulation of the flame demonstrates good agreement in OH and flame temperatures over a wide range of dilution away from extinction, particularly for CO₂. Lastly, an over prediction of the water carrying capacity near extinction is found for highly water-diluted flames.« less

Our study investigates the autoignition behavior of two gasoline surrogates doped with an alkyl nitrate cetane enhancer, 2-ethyl-hexyl nitrate (2EHN) to better understand dopant interactions with the fuels, including influences of accelerating kinetic pathways and enhanced exothermicity. A primary reference fuel (PRF) blend of n-heptane/iso-octane, and a toluene reference fuel (TRF) blend of n-heptane/iso-octane/toluene are used where the aromatic fraction of the latter is set to 20% (liquid volume), while the content of n-heptane is adjusted so that the overall reactivity of the undoped fuels is similar, e.g., Anti-Knock Index (AKI) of similar to 91, Cetane Number (CN) similar tomore » 25. Doping levels of 0.1, 1.0 and 3.0% (liquid volume basis) are used where tests are conducted within a rapid compression machine (RCM) at a compressed pressure of 21 bar, covering temperatures from 675 to 1025 K with stoichiometric fuel-oxygen ratios at O-2 = 11.4%. At the experimental conditions, it is found that the doping effectiveness of 2EHN is fairly similar between the two fuels, though 2EHN is more effective in the aromatic blend at the lowest temperatures, while it is slightly more effective in the non-aromatic blend at intermediate temperatures. Furthermore, kinetic modeling of the experiments indicates that although some of the reactivity trends can be captured using a detailed model, the extents of predicted Cetane Number enhancement by 2EHN are too large, while differences in fuel interactions for the two fuels result in excessive stimulation of the non-aromatic blend. Sensitivity analysis using the kinetic model indicates that the CH₂O and CH₃O₂ chemistry are very sensitive to the dopant at all conditions. The rate of 2EHN decomposition is only important at low temperatures where its decomposition rate is slow due to the high activation energy of the reaction. At higher temperatures, dopant-derived 3-heptyl radicals are predicted to play an important role stimulating ignition. Finally, nitrogen chemistry is important through the 'NO - NO₂ loop' where this can generate substantial amounts of OH. But, at the highest doping levels the formation of methyl and ethyl nitrite, and nitric acid significantly competes with this so that less OH is generated and this constrains the reactivity enhancement of 2EHN.« less