Comparative Study on the H-Abstraction Reactions of Isopropyl Acetate and Propyl Acetate by HO2 and OH Radicals.

Isopropyl acetate (IPA) and propyl acetate (PA) are recognized as promising biofuels suitable for applications as fuel additives and biodiesel models. The H-abstraction reactions with radicals stand out as the fundamental initiating reactions in the combustion kinetic models for IPA and PA. In the present work, the kinetic calculations of IPA and PA plus HO2 and OH radicals were investigated at M06-2X/cc-pVTZ//G4, M08-HX/maug-cc-pVTZ, and CCSD(T)/jul-cc-pVTZ levels. The thermodynamic calculations were obtained based on the G4 and CBS-APNO methods. Rate coefficients were calculated using both transition state theory and canonical variational transition state theory with tunneling correction at the temperature range of 250-2000 K. The total rate constants for the IPA + OH system were fitted as follows: k = 0.4674 × T3.927 exp(2128/T) (cm3 mol-1 s-1), and for the PA + OH system, the total rate constants were determined using the following equation: k = 0.0161 × T4.373 exp(2220/T) (cm3 mol-1 s-1). The rate coefficients of IPA + OH reactions determined based on the M08-HX/maug-cc-pVTZ level effectively replicate the experimental data, while H-abstraction rate coefficients of PA + OH by the CCSD(T)/jul-cc-pVTZ method accurately reproduce the experimental data. Refining the H-abstraction rate coefficients in the kinetic mechanism of PA, as proposed by Dayma et al. [Proc. Combust. Inst. 37 (2019) 429-436], has been achieved through incorporating the present calculated data, leading to the development of a revised mechanism. The validation of the updated mechanism against jet-stirred reactor data is presented, showcasing its effective performance in predicting JSR data.

[Research on the Time-Histories of Exited Radicals during the Combustion of Methylcyclohexane by Emission Spectroscopy].

By using three monochromator! detecting systems, the light emissions of excited-state OH*, CH* and C* radicals during the transient combustion of methylcyclohexane at high temperatures behind the reflected shock wave have been measured. The dependence of the time-history and the relative intensity of excited radicals on the temperature have been obtained. The reflected shock temperatures are 1 200-1 700 K, the shock pressure is 1.5 atm, the mole fraction of methylcyclohexane is 0.1% and the equivalence ratio is 1.0. At the beginning of the combustion process, these three radicals were produced at the same time. The durations of these radicals became shorter when the temperature increases. At the same ignition temperature, the durations of CH* and OH* are longer than that of C2*. The C2* signal disappears below 1 400 K. The emission intensities of OH* and CH* are not sensitive to the temperature at T < 1 400 K. However, at high temperature (T > 1 400 K), the peak intensity of CH* increases rapidly as temperature increases, while C2* and OH* increase slowly. Current results were compared to the simulation results of corresponding chemical reaction mechanism. The obtained time-history of OH* radical matches well with the prediction of mechanism at low temperatures, but shows difference at high temperatures. The time-history of CH* radical matches well between experimental and simulated results at high temperatures, however, the simulated durations of CH* are longer than the experimental results at low temperatures. Current work provides experimental data to validate and optimize corresponding chemical reaction mechanism containing excited-state species.

[Measurement of Soot Yield from the Pyrolysis of Toluene at High Temperatures by Laser Extinction Method].

The measurement system for the detection of soot production as high-temperature pyrolysis of hydrocarbon fuels behind the reflected shock wave was established. By using the laser extinction method, the soot yields of toluene/argon mixtures were measured at high temperatures. The mole fractions of toluene were 0.25% and 0.5% while the pressures were approximate 2 and 4 atm. The temperatures ranged from 1 630 to 2 273 K. The dependence of soot yield on the temperature, pressure and fuel concentration was obtained. With the changes of temperature, the soot yield is a Gauss distribution. The soot yield increases as the pressure or fuel concentration increases. The maximum of soot yield was as high as 55%. The peak temperature of soot yield was not changed dramatically with the pressure. However, it changed from 1 852 to 1 921 K as the concentration of toluene increase from 0.25% to 0.5%. Moreover, we compared the soot yield between toluene and methylcyclohexane at pressure of 4 atm with fuel concertation of 0.5%. During the pyrolysis of methylcyclohexane, the peak temperature of soot yield was 2 045 K, which is about 135 K higher than that of toluene. However, the maximum soot yield of methylcyclohexane is only 1/8 of toluene. This work provides experimental reference for the research of soot particle emission in the engines and the mechanism of soot formation.