Search

Abstract: Pyrolysis experiments on n-heptane, 1-heptene and 1,6-heptadiene have been performed using the UIC High-Pressure Shock Tube (HPST) at pressures relevant to diesel combustion systems. The experimental pressures for these experiments ranged from 25 to 50atm and temperatures varied from 1000 to 1350K with reaction times ranging from 1 to 3ms. Dilute reagent mixtures ∼100ppm were prepared in bulk argon and shock heated to study the stable intermediates. The experimental data has been used to develop and validate an updated kinetic model for the pyrolysis of saturated and unsaturated C7 hydrocarbons. The experimental results and their implication on increased NO emissions from biodiesel blends will also be discussed. [Copyright &y& Elsevier]

Abstract: The high pressure oxidation of dilute CO mixtures doped with 150–200ppm of H2 has been studied behind reflected shock waves in the UIC high pressure single pulse shock tube. The experiments were performed over the temperature range from 1000 to 1500K and pressures spanning 21–500bars for stoichiometric (Φ =1) and fuel lean (Φ =0.5) oxidation. Stable species sampled from the shock tube were analyzed by standard GC, GC/MS techniques. The experimental data obtained in this work were simulated using a detailed model for H2/CO combustion that was validated against a variety of experimental observables/targets that span a wide range of conditions. These simulations have shown that within experimental error the model is able to capture the experimental trends for the lower pressure data sets (average nominal pressures of 24 and 43bars). However the model under predicts the CO and O2 decay and subsequent CO2 formation for the higher pressure data sets (average nominal pressures of 256 and 450bars). The current elevated pressure data sets span a previously unmapped regime and have served to probe HO2 radical reactions which appear to be among the most sensitive reactions in the model under these conditions. With updated rate parameters for a key HO2 radical reaction OH+HO2 =H2O+O2, the model is able to reconcile the elevated pressure data sets thereby extending its capability to an extreme range of conditions. [Copyright &y& Elsevier]

An examination has been conducted of the oxidation of iso-butylene and an iso-butylene/n-octane mixture at 1080 K and one atmosphere. It was found that the addition of iso-butylene to n-octane results in a delay in the formation of CO and CO2 even ifenough additional O2 is added to maintain a n overall equivalence ratio of one. Analysis of the oxidation of iso-butylene alone suggests that the inhibiting effect of iso-butylene addition results from a reduction in radical concentrations due to the abstraction reactions of the radicals with the iso-butylene to yield unreactive methyl radicaland allene. [ABSTRACT FROM PUBLISHER]

An experimental study has heen conducted in a flow reactor on the oxidation of n-ocltane and iso-octane at 1080 K, one atmosphere pressure and equivalence ratio equal to one. It was found that (despite similar ratios of initial fuel decay) the conversion of the reaction intermediates to CO and COa is much slower during the oxidation of iso-octane than n-octane. Iso-octane produced primarily iso-butylene and propylene as intermediate hydrocarbons whereas n-octane oxidation resulted primarily in the formation or ethne. Both the nature of the intermediates formed from each fuel and the relative rates of oxidation of the intermediates were shown to be related to the number of primary, secondary and tertiary H atoms present in the initial fuel. The basic chemistry revealed by the flow reactor experiments is suggestive of new insights into the chemical phenomena pertinent to knock in spark ignition engines. [ABSTRACT FROM PUBLISHER]

Fuel ignition quality, measured in the form of Derived Cetane Number (DCN), is an important part of integrating fuels, including sustainable aviation fuels, in compression ignition engines. DCN has been correlated with simulated and/or real spectroscopic measurements as well as other physical and chemical properties, but rarely have these correlations developed into a pathway to application. One application of the correlations is the use of miniaturized onboard fuel sensors that could assist, by using predicted DCN, in real-time feedforward engine control. To aid in the application of developing such DCN fuel sensors, Raman spectra coupled with chemometrics and a selection of influential spectral features were investigated. In this study, the Raman spectra were obtained from a database that included jet fuels, jet fuel mixtures, pure hydrocarbon components, and their weighted mixtures. The resulting Raman spectral database from the experimental measurements included spectra of components that span a wide range of DCNs and covered all the expected chemical functional groups present in a standard jet fuel. Chemometric models were developed to associate Raman spectra with DCN in subsets of the spectral range to aid in sensor miniaturization. The models were tested on jet fuels such as National Jet Fuel Combustion Program fuels designated A-1, A-2, and A-3 along with mixtures of jet fuels that spanned a wide range of DCN, simulating fuels that could represent real-world scenarios. An Artificial Neural Network (ANN) model trained on the fingerprint region (500 cm -1 - 1800 cm -1 ) of the Raman spectra was able to capture the non-linearity of the association between the Raman spectra and DCN with a test R 2 score of 0.926, a test MSE of 3.61, and a test MPE of 3.41. Around 97 % of the unseen test samples were predicted within 10 % of the DCN measured with an Ignition Quality Tester. One hundred features of the fingerprint region influencing DCN predictions in the optimal ANN model were extracted using a Global Surrogate (GS) model. A reduced ANN model trained on only these one hundred features performed slightly better with a test R 2 score of 0.935, test MSE of 3.19, test MPE of 3.20 and with the entire set of unseen test samples predicted within 10 % of the measured DCN. For assessing applicability of real-time and online DCN sensing, the Raman spectrometer was integrated with a flow cell capable of allowing measurements of DCN in flowing fuel samples and included the optimal ANN model of the fingerprint region and the 100-feature GS-ANN model on a Raspberry Pi computer. A number of unseen F-24/alcohol-to-jet fuel mixtures composed of unknown volumes were tested using the flow cell for DCN, and all of these samples were predicted within 10 % of the measured DCN., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

The high pressure and temperature oxidation of methyl trans-2-nonenoate, methyl trans-3-nonenoate, 1-octene, and trans-2-octene are investigated experimentally to probe the influence of the double bond position on the chemical kinetics of long esters and alkenes. Single pulse shock tube experiments are performed in the ranges p = 3.8-6.2 MPa and T = 850-1500 K, with an average reaction time of 2 ms. Gas chromatographic measurements indicate increased reactivity for trans-2-octene compared to 1-octene, whereas both methyl nonenoate isomers have reactivities similar to that of 1-octene. A difference in the yield of stable intermediates is observed for the octenes when compared to the methyl nonenoates. Chemical kinetic models are developed with the aid of the Reaction Mechanism Generator to interpret the experimental results. The models are created using two different base chemistry submodels to investigate the influence of the foundational chemistry (i.e., C0-C4), whereas Monte Carlo simulations are performed to examine the quality of agreement with the experimental results. Significant uncertainties are found in the chemistry of unsaturated esters with the double bonds located close to the ester groups. This work highlights the importance of the foundational chemistry in predictive chemical kinetics of biodiesel combustion at engine relevant conditions.

The recombination and disproportionation of allyl radicals has been studied in a single pulse shock tube with gas chromatographic measurements at 1-10 bar, 650-1300 K, and 1.4-2 ms reaction times. 1,5-Hexadiene and allyl iodide were used as precursors. Simulation of the results using derived rate expressions from a complementary diaphragmless shock tube/laser schlieren densitometry study provided excellent agreement with precursor consumption and formation of all major stable intermediates. No significant pressure dependence was observed at the present conditions. It was found that under the conditions of these experiments, reactions of allyl radicals in the cooling wave had to be accounted for to accurately simulate the experimental results, and this unusual situation is discussed. In the allyl iodide experiments, higher amounts of allene, propene, and benzene were found at lower temperatures than expected. Possible mechanisms are discussed and suggest that iodine containing species are responsible for the low temperature formation of allene, propene, and benzene.

Recent theoretical investigations of the radical/π-bond addition between single-ring aromatic hydrocarbons highlight the importance of this category of reactions for the formation of PAH intermediates and soot. The present investigation extends the theory of the radical/π-bond addition reactions to the o-benzyne + cyclic C(5) hydrocarbons systems. The calculations, performed using the uB3LYP/6-311+G(d,p) method, have addressed the possible role of the reaction between o-benzyne and cyclopentadiene in the formation of indene through the fragmentation of the bicyclo intermediate benzonorbornadiene. The complex potential energy surface for the reaction between o-benzyne and cyclopentadienyl radical was also investigated. In this case, the formation of the bicyclo benzonorbornadienyl radical and its subsequent fragmentation to indenyl radical and acetylene is not the main reaction pathway, although it could be relevant at relatively high temperatures. At lower temperatures, the isomerization reactions, which lead to the formation of a variety of multiring compounds, are dominant.

The experimental investigations performed in the 1960s on the o-benzyne + benzene reaction as well as the more recent studies on reactions involving π-electrons highlight the importance of π-bonding for different combustion processes related to PAH's and soot formation. In the present investigation radical/π-bond addition reactions between single-ring aromatic compounds have been proposed and computationally investigated as possible pathways for the formation of two-ring fused compounds, such as naphthalene, which serve as precursors to soot formation. The computationally generated optimized structures for the stationary points were obtained with uB3LYP/6-311+G(d,p) calculations, while the energies of the optimized complexes were refined using the uCCSD(T) method and the cc-pVDZ basis set. The computations have addressed the relevance of a number of radical/π-bond addition reactions including the singlet benzene + o-benzyne reaction, which leads to formation of naphthalene and acetylene through fragmentation of the benzobicyclo[2,2,2]octatriene intermediate. For this reaction, the high-pressure limit rate constants for the individual elementary reactions involved in the overall process were evaluated using transition state theory analysis. Other radical/π-bond addition reactions studied were between benzene and triplet o-benzyne, between benzene and phenyl radical, and between phenyl radicals, for all of which potential energy surfaces were produced. On the basis of the results of these reaction studies, it was found necessary to propose and subsequently confirm additional, alternative pathways for the formation of the types of PAH compounds found in combustion systems. The potential energy surface for one reaction in particular, the phenyl + phenyl addition, is shown to contain a low-energy channel leading to formation of naphthalene that is energetically comparable to the other examined conventional pathways leading to formation of biphenyl compounds. This channel is the first evidence of a reaction which involves an aromatic radical adding to the nonradical π-bond site of another aromatic radical which leads directly to a fused ring structure.

The mutual sensitization of the oxidation of NO and a CH(4)-C(2)H(6) (10 : 1) simulated natural gas (NG) blend was studied under fuel lean conditions (Phi = 0.5) at 50 atm and 1000-1500 K in the UIC high pressure shock tube (HPST). New experimental results were also obtained for the mutual sensitization of methane and the NG blend in the CNRS jet stirred reactor (JSR) at 10 atm. A detailed chemical kinetic model was assembled to describe the observed changes in reactivity in the CH(4) and NG blends, with and without NO, in the HPST and the JSR. The data and the validated model (tested against a variety of targets) show a reduced difference of reactivity between methane and NG blends in the presence of NO at characteristic reaction times for the JSR (250-1000 micros). However the HPST data and subsequent simulations using the validated model have revealed that at higher pressures and in the millisecond time scale regime representative of the HPST experiments (and practical combustion devices) there still persists a significant difference in reactivity between methane and NG blends in the presence of NO. The experimental data, the model development and validations and its predictions and utility as a tool to probe the NO-hydrocarbon sensitization effects under practical combustion conditions is discussed.

1,2,4,5-Hexatetraene (1245HT) is, according to theory, a key intermediate to benzene from propargyl radicals in a variety of flames; however, it has only been experimentally observed once in previous studies of the C3H3 + C3H3 reaction. To determine if it is indeed an intermediate to benzene formation, 1245HT was synthesized, via a Grignard reaction, and pyrolysized in a single-pulse shock tube at two nominal pressures of 22 and 40 bar over a temperature range from 540 to 1180 K. At temperatures T < 700 K, 1245HT converts efficiently to 3,4-dimethylenecyclobutene (34DMCB) with a rate constant of k = 10(10.16) x exp(-23.4 kcal/RT), which is in good agreement with the one calculated by Miller and Klippenstein. At higher temperatures, various C6H6 isomers were generated, which is consistent with theory and earlier experimental studies. Thus, the current work strongly supports the theory that 1245HT plays a bridging role in forming benzene from propargyl radicals. RRKM modeling of the current data set has also been carried out with the Miller-Klippenstein potential. It was found that the theory gives reasonably good predictions of the experimental observations of 1245HT, 1,5-hexadiyne (15HD), and 34DMCB in the current study and in our earlier studies of 15HD pyrolysis and propargyl recombination; however, there is considerable discrepancy between experiment and theory for the isomerization route of 1,2-hexadien-5-yne (12HD5Y) --> 2-ethynyl-1,3-butadiene (2E13BD) --> fulvene.

The self-reaction of propargyl (C3H3) radicals has been widely suggested as one of the key routes forming benzene in a variety of aliphatic flames. Currently, in the majority of aromatic models, the C3H3 + C3H3 submechanism often contains one or two C6H6 isomers and a few global reaction steps, which do not adequately represent the actual recombination chemistry. Recent experimental and theoretical studies on the direct propargyl recombination and subsequent C6H6 isomerization have provided sufficient information to revisit and revise the C3H3 + C3H3 reaction submechanism. In the present work, a semidetailed kinetic model consisting of seven isomeric C6H6 species and 14 reaction steps was constructed based on the most recent potential energy surface for this system. The trial model was subjected to systemic optimization by use of a recently developed physically bounded Gauss-Newton (PGN) method against detailed species profiles of direct propargyl recombination and 1,5-hexadiyne (15HD) isomerization obtained from experiments at high temperatures in a shock tube and at low temperatures in a flow reactor, which were all measured at very high pressure (shock tube) or atmospheric (flow reactor) conditions. Predictions of the optimized model were in excellent agreement with all experimental measurements. The optimized C3H3 + C3H3 reaction subset was also tested for flame modeling. Two different aromatic chemistry models that incorporate benzene formation from propargyl radicals as a single step reaction were modified to include the complete submechanism for propargyl recombination. The updated models predict significant percentages of three isomeric species [2-ethynyl-1,3-butadiene (2E13BD), fulvene, and benzene] in premixed fuel-rich acetylene and ethylene flames, reflecting the observed flame structures.

We have investigated the isomeric C6H6 product distributions of the self-reaction of propargyl (C3H3) radicals at two nominal pressures of 25 and 50 bar over the temperature range 720-1350 K. Experiments were performed using propargyl iodide as the radical precursor in a high-pressure single-pulse shock tube with a residence time of 1.6-2.0 ms. The relative yields of the C6H6 products are strongly temperature dependent, and the main products are 1,5-hexadiyne (15HD), 1,2-hexadiene-5-yne (12HD5Y), 3,4-dimethylenecyclobutene (34DMCB), 2-ethynyl-1,3-butadiene (2E13BD), fulvene, and benzene, with the minor products being cis- and trans-1,3-hexadiene-5-yne (13HD5Y). 1,2,4,5-Hexatetraene (1245HT) was observed below 750 K but the concentrations were too low to be quantified. The experimentally determined entry branching ratios are: 44% 15HD, 38% 12HD5Y, and 18% 1245HT, which is efficiently converted to 34DMCB. Following the initial recombination step, various C6H6 isomers are formed by thermal rearrangement. The experimentally observed concentrations for the C6H6 species are in good agreement with earlier experiments on 15HD thermal rearrangement.

The design and validation are described of an optical probe for measurements of parts per million OH concentrations in actual combustion environments (1100 K, 1 atm).