Shock Tube and Kinetic Modeling Study of Cyclopentane and Methylcyclopentane

Ignition delay times for 1% cyclopentane/O₂ and 0.833% methylcyclopentane/O₂ mixtures diluted by argon were measured behind reflected shock waves at pressures of 1.1 and 10 atm, with equivalence ratios of 0.577, 1.0, and 2.0, and in the temperature range from 1150 to 1850 K. Submechanisms for cyclopentane and methylcyclopentane were developed and added to the JetSurF2.0 mechanism for the kinetic interpretation of cyclopentane and methylcyclopentane oxidation chemistry at the high temperature region. Simulations with the model exhibit fairly good agreements with the measured ignition delay times of both cyclopentane and methylcyclopentane under all tested conditions. Cyclopentane shows longer ignition delay time than methylcyclopentane, especially for the fuel-lean mixture. Reaction pathways and sensitivity analyses were conducted to get insights into the oxidation process of cyclopentane and methylcyclopentane. Then, three factors are given for the effect of a cyclic ring and substitution of a methyl group. Substitution of a methyl group weakens the C–C bond to motivate fuel unimolecular decomposition. The shape of the cyclic ring determines the chain alkyl radicals, affecting regeneration and accumulation of H radical. The presence of a methyl group also leads to different alkyl radicals

Experimental and Kinetic Study on Ignition Delay Times of Di‑<i>n</i>‑butyl Ether at High Temperatures

Ignition delay times of di-<i>n</i>-butyl ether (DBE)/oxygen mixtures diluted with argon were measured behind reflected shock waves for the pressures between 1.2 and 4 bar, the temperatures between 1100 and 1570 K, and the equivalence ratios of 0.5, 1.0, and 1.5. A recently developed DBE model was employed to simulate the autoignition process of the homogeneous mixture. Comparisons between the measured and calculated ignition delay times indicate that the model yields fairly good agreement under all test conditions. Results show that the ignition delay time increases with the decrease of the pressure and the increase of the dilution ratio. The ignition delay time demonstrates a strong negative dependence upon the equivalence ratio at high temperatures, and the difference among the ignition delay times tends to decrease when the temperature is decreased. Sensitivity analysis reveals the importance of H-abstraction reactions and decomposition of α fuel radicals in the ignition process of DBE. Reaction pathway analysis confirms that the consumption of DBE is dominated by the H-abstraction reactions at lower temperatures, and when the temperature is increased, the unimolecular decomposition reactions become more important. Comparisons of ignition delay times as well as fuel consumption and radical growth history of DBE to dimethyl ether (DME) and diethyl ether (DEE) for given equivalence ratios indicate that DBE has the strongest overall reactivity, although the reactant concentration of DBE is the lowest

Laminar Flame Speeds and Kinetic Modeling of <i>n</i>‑Pentanol and Its Isomers

A comprehensive experimental and computational study was conducted on the laminar combustion characteristics and chemical kinetics of four pentanol isomer–air mixtures (<i>n</i>-pentanol, 3-methyl-1-butanol, 2-methyl-1-butanol, and 2-methyl-2-butanol). Experiments were performed at the equivalence ratios ranging from 0.6 to 1.8, three initial temperatures (393, 433, and 473 K), and four pressures (0.1, 0.25, 0.5, and 0.75 MPa) using outwardly propagating flames. Results show that the laminar flame speeds of the four pentanol isomers decrease in the order of <i>n</i>-pentanol, 2-methyl-1-butanol, 3-methyl-1-butanol, and 2-methyl-2-butanol. The most significant differences among the isomers are observed around the stoichiometric mixture. Simulations on the laminar flame speeds of <i>n</i>-pentanol and 3-methyl-1-butanol were respectively performed using the model of Heufer et al. and Sarathy et al. Comparisons between the simulations and experimental data show the <i>n</i>-pentanol model yields satisfactory agreement with the data at most conditions but slight overpredictions at rich mixtures and atmospheric pressure and the 3-methyl-1-butanol model yields close agreement with the data at all conditions. For 2-methyl-1-butanol, a model based on the model proposed by Tang et al. was developed and validated against the data of laminar flame speed as well as ignition delay times. Sensitivity analysis indicates that the laminar flame speeds of the isomer–air flames (<i>n</i>-pentanol, 3-methyl-1-butanol, and 2-methyl-1-butanol) are mainly sensitive to small molecule reactions involving H₂–O₂ and C₁–C₃ species but not to fuel-specific reactions. The concentrations of H₂–O₂ and C₁–C₃ intermediates are responsible for the laminar flame speed difference among the isomers. Additionally, butene isomers working as the important intermediates occupy different fractions in various pentanol isomer flames, confirming the differences on the chemical structure and the reaction pathways of the isomers

Laminar Flame Speeds and Kinetic Modeling of <i>n</i>‑Pentanol and Its Isomers

A comprehensive experimental and computational study was conducted on the laminar combustion characteristics and chemical kinetics of four pentanol isomer–air mixtures (<i>n</i>-pentanol, 3-methyl-1-butanol, 2-methyl-1-butanol, and 2-methyl-2-butanol). Experiments were performed at the equivalence ratios ranging from 0.6 to 1.8, three initial temperatures (393, 433, and 473 K), and four pressures (0.1, 0.25, 0.5, and 0.75 MPa) using outwardly propagating flames. Results show that the laminar flame speeds of the four pentanol isomers decrease in the order of <i>n</i>-pentanol, 2-methyl-1-butanol, 3-methyl-1-butanol, and 2-methyl-2-butanol. The most significant differences among the isomers are observed around the stoichiometric mixture. Simulations on the laminar flame speeds of <i>n</i>-pentanol and 3-methyl-1-butanol were respectively performed using the model of Heufer et al. and Sarathy et al. Comparisons between the simulations and experimental data show the <i>n</i>-pentanol model yields satisfactory agreement with the data at most conditions but slight overpredictions at rich mixtures and atmospheric pressure and the 3-methyl-1-butanol model yields close agreement with the data at all conditions. For 2-methyl-1-butanol, a model based on the model proposed by Tang et al. was developed and validated against the data of laminar flame speed as well as ignition delay times. Sensitivity analysis indicates that the laminar flame speeds of the isomer–air flames (<i>n</i>-pentanol, 3-methyl-1-butanol, and 2-methyl-1-butanol) are mainly sensitive to small molecule reactions involving H₂–O₂ and C₁–C₃ species but not to fuel-specific reactions. The concentrations of H₂–O₂ and C₁–C₃ intermediates are responsible for the laminar flame speed difference among the isomers. Additionally, butene isomers working as the important intermediates occupy different fractions in various pentanol isomer flames, confirming the differences on the chemical structure and the reaction pathways of the isomers

Shock Tube Measurements and Kinetic Investigation on the Ignition Delay Times of Methane/Dimethyl Ether Mixtures

In this work, the ignition delay times of stoichiometric methane/dimethyl ether (DME) were measured behind the reflected shock waves over a wide range of conditions: temperatures between 1134 and 2105 K, pressures of 1, 5, and 10 bar, a DME blending ratio from 0 to 100% (M100 to M0), and an argon concentration of 95%. The present shock tube facility was validated by comparing the measured ignition delay times of DME with literature values and was used for measurement of the subsequent methane/DME ignition delay times. The ignition delay times of all mixtures exhibit a negative pressure dependence. For a given temperature, the ignition delay time of methane/DME decreases remarkably with the presence of only 1% DME. As the DME blending ratio increases, the ignition delay times are correspondingly decreased; however, the ignition promotion effect of DME is decreased. The calculated ignition delay times of methane/DME mixtures using two recently developed kinetic mechanisms are compared with those of measurements. The NUI C4 mechanism yields good prediction for the ignition delay time of methane. With an increase of the DME blending ratio, the performance of this model becomes moderated. Zhao’s DME model yields good prediction for all of the mixtures studied in this work; thus, it was selected for analyzing the ignition kinetics of methane/DME fuel blends, through which the nonlinear effect of DME addition in promoting ignition is interpreted

Kinetics of Hydrogen Abstraction and Addition Reactions of 3‑Hexene by ȮH Radicals

Rate coefficients of H atom abstraction and H atom addition reactions of 3-hexene by the hydroxyl radicals were determined using both conventional transition-state theory and canonical variational transition-state theory, with the potential energy surface (PES) evaluated at the CCSD­(T)/CBS//BHandHLYP/6-311G­(d,p) level and quantum mechanical effect corrected by the compounded methods including one-dimensional Wigner method, multidimensional zero-curvature tunneling method, and small-curvature tunneling method. Results reveal that accounting for approximate 70% of the overall H atom abstractions occur in the allylic site via both direct and indirect channels. The indirect channel containing two van der Waals prereactive complexes exhibits two times larger rate coefficient relative to the direct one. The OH addition reaction also contains two van der Waals complexes, and its submerged barrier results in a negative temperature coefficient behavior at low temperatures. In contrast, The OH addition pathway dominates only at temperatures below 450 K whereas the H atom abstraction reactions dominate overwhelmingly at temperature over 1000 K. All of the rate coefficients calculated with an uncertainty of a factor of 5 were fitted in a quasi-Arrhenius formula. Analyses on the PES, minimum reaction path and activation free Gibbs energy were also performed in <i>this study</i>

Experimental and Kinetic Study on the Ignition Delay Times of 2,5-Dimethylfuran and the Comparison to 2‑Methylfuran and Furan

The ignition phenomena of 2,5-dimethylfuran (DMF), 2-methylfuran (MF), and furan are systematically investigated. Ignition delay times are measured over the temperature range of 1150–2010 K and pressures of 1.2, 4, and 16 bar for lean, stoichiometric, and rich DMF/Ar/O₂ mixtures. For comparison, similar measurements for stoichiometric MF/O₂/Ar and furan/O₂/Ar are also conducted. Through a multi-regression analysis, the measured ignition delay times of DMF are fitted empirically in an Arrhenius-like form as a function of experimental parameters. It is observed that, when the fuel concentration, pressure, temperature, and equivalence ratio are kept constant, furan has the longest ignition delay times, while the reactivity of DMF and MF strongly depends upon the temperature. The experimental ignition delay times are compared to model predictions of Somers et al. and Liu et al. Both models could give qualitative agreement with DMF and MF ignition data, while the model of Somers et al. provides better quantitative agreement. Modifications have been made to the model of Somers et al. to agree better with present ignition delay times and other sets of furan data. Further reaction pathway and sensitivity analysis are carried out to understand their combustion kinetics

Experimental and Kinetic Modeling Study on <i>trans</i>-3-Hexene Ignition behind Reflected Shock Waves

<i>trans</i>-3-Hexene ignition delay times were measured behind reflected shock waves for fuel-lean (Φ = 0.5), stoichiometric (Φ = 1.0), and fuel-rich (Φ = 1.5) mixtures between 1080 and 1640 K, at pressures between 1.2 and 10 atm. Two fuel concentrations (1000 and 5000 ppm <i>trans</i>-3-hexene) diluted in argon were examined, and the ignition delay times were obtained by following OH* radical chemiluminescence emission. The experimental results satisfied the Arrhenius equation, and the influences of pressure, equivalence ratio, fuel concentration, and dilution gas on <i>t</i><i>rans</i>-3-hexene ignition behavior were discussed. The Lawrence Livermore National Laboratory (LLNL) model overestimates the low-temperature reactivity and underestimates the pressure-dependence at high-temperature. Improvements have been made to the LLNL model, and the modified mechanism offers better predictions for the ignition delay times of this work as well as the shock tube, rapid compression machine, and jet stirred reactor experimental data from the literature. Reaction pathway and sensitivity analysis were performed to gain insight into the <i>trans</i>-3-hexene oxidation chemistry

Laminar Flame Characteristics of <i>iso</i>-Octane/<i>n</i>‑Butanol Blend–Air Mixtures at Elevated Temperatures

In this work, laminar flame speeds and Markstein lengths of <i>iso</i>-octane/<i>n</i>-butanol–air mixtures were measured via the outwardly expanding spherical flame method and high-speed schlieren photography over a wide range of equivalence ratios and blending ratios of <i>n</i>-butanol at elevated initial temperatures. Results show that laminar flame speeds of fuel blends slightly increase with increasing blending ratio of <i>n</i>-butanol. The effect of blending ratio on laminar flame speed of fuel blends was mechanistically interpreted through examining the thermodynamic and diffusive property, as well as the overall oxidation kinetics. Measurements on the burned gas Markstein length show a generalized behavior. There exists a minimum absolute Markstein length at the critical equivalence ratio ϕ*, under which the stretch effect on the flame propagating speed is minimized. At equivalence ratios less than ϕ*, Markstein length is decreased with increasing blending ratio of <i>n</i>-butanol, indicating that the addition of <i>n</i>-butanol reduces the diffusional thermal stability of the blend; while at equivalence ratios larger than ϕ*, Markstein length increases with increasing blending ratio of <i>n</i>-butanol. This experimental observation on the Markstein length is consistent with the theoretical investigation. A correlation of laminar flame speed of <i>n</i>-butanol/iso-octane blend as a function of equivalence ratio, temperature, and blending ratio of <i>n</i>-butanol is given on the basis of experimental data

Numerical Study toward Fast Intermediate Species Sampling with High Accuracy in a Rapid Compression Machine

Rapid compression machines (RCMs), as prominent zero-dimensional homogeneous reactors, have been widely applied in autoignition chemistry investigations. However, species sampling in these reactors is challenged by quantitative accuracy and the limited time allowed for transient sampling. In this study, the uncertainty of quantitative species sampling during the autoignition of a typical fuel/oxidizer mixture was numerically evaluated. Results show that the sampled species profiles tend to lag behind the “true” values due to the dilution of gases outside of the “core” region. When sampling at a longer ignition delay time, increasing the sample duration over 2 ms has very limited improvement on the sampling accuracy, and adopting a 10 mm probe tube is long enough to minimize the errors caused by dilution. In the reaction chamber where nonideal gas disturbance occurs, the sampled gases can be seriously diluted, leading to large deviations in the results. Concentration uncertainties vary among species, while uncertainties in the normalized time are consistent, which can be limited to 6% with proper piston design and sampling setups