Xueliang Yang

Plasma Assisted Low Temperature Combustion

This paper presents recent kinetic and flame studies in plasma assisted low temperature combustion. First, the kinetic pathways of plasma chemistry to enhance low temperature fuel oxidation are discussed. The impacts of plasma chemistry on fuel oxidation pathways at low temperature conditions, substantially enhancing ignition and flame stabilization, are analyzed base on the ignition and extinction S-curve. Secondly, plasma assisted low temperature ignition, direct ignition to flame transition, diffusion cool flames, and premixed cool flames are demonstrated experimentally by using dimethyl ether and n-heptane as fuels. The results show that non-equilibrium plasma is an effective way to accelerate low temperature ignition and fuel oxidation, thus enabling the establishment of stable cool flames at atmospheric pressure. Finally, the experiments from both a non-equilibrium plasma reactor and a photolysis reactor are discussed, in which the direct measurements of intermediate species during the low temperature oxidations of methane/methanol and ethylene are performed, allowing the investigation of modified kinetic pathways by plasma-combustion chemistry interactions. Finally, the validity of kinetic mechanisms for plasma assisted low temperature combustion is investigated. Technical challenges for future research in plasma assisted low temperature combustion are then summarized.

Ab Initio Reaction Kinetics of CH3OĊ(═O) and ĊH2OC(═O)H Radicals

The dissociation and isomerization kinetics of the methyl ester combustion intermediates methoxycarbonyl radical (CH3OĊ(═O)) and (formyloxy)methyl radical (ĊH2OC(═O)H) are investigated theoretically using high-level ab initio methods and Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) theory. Geometries obtained at the hybrid density functional theory (DFT) and coupled cluster singles and doubles with perturbative triples correction (CCSD(T)) levels of theory are found to be similar. We employ high-level ab initio wave function methods to refine the potential energy surface: CCSD(T), multireference singles and doubles configuration interaction (MRSDCI) with the Davidson-Silver (DS) correction, and multireference averaged coupled-pair functional (MRACPF2) theory. MRSDCI+DS and MRACPF2 capture the multiconfigurational character of transition states (TSs) and predict lower barrier heights than CCSD(T). The temperature- and pressure-dependent rate coefficients are computed using RRKM/ME theory in the temperature range 300-2500 K and a pressure range of 0.01 atm to the high-pressure limit, which are then fitted to modified Arrhenius expressions. Dissociation of CH3OĊ(═O) to ĊH3 and CO2 is predicted to be much faster than dissociating to CH3Ȯ and CO, consistent with its greater exothermicity. Isomerization between CH3OĊ(═O) and ĊH2OC(═O)H is predicted to be the slowest among the studied reactions and rarely happens even at high temperature and high pressure, suggesting the decomposition pathways of the two radicals are not strongly coupled. The predicted rate coefficients and branching fractions at finite pressures differ significantly from the corresponding high-pressure-limit results, especially at relatively high temperatures. Finally, because it is one of the most important CH3Ȯ removal mechanisms under atmospheric conditions, the reaction kinetics of CH3Ȯ + CO was also studied along the PES of CH3OĊ(═O); the resulting kinetics predictions are in remarkable agreement with experiments.

Ab Initio Unimolecular Reaction Kinetics of CH2C(═O)OCH3 and CH3C(═O)OCH2 Radicals

The unimolecular dissociation and isomerization kinetics of the methyl ester combustion intermediates methoxycarbonylmethyl (CH2C(═O)OCH3) and acetyloxylmethyl (CH3C(═O)OCH2) are theoretically investigated using high-level ab initio methods and the Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) theory. Potential energy surfaces (PESs) are obtained using coupled cluster singles and doubles with perturbative triples correction (CCSD(T)), multireference singles and doubles configuration interaction (MRSDCI) with the Davidson-Silver (DS) correction, and multireference averaged coupled pair functional (MRACPF2) theory. The transition states exhibit high T1 diagnostics in coupled cluster calculations, suggesting the need for a multireference correlated wave function treatment. MRSDCI+DS and MRACPF2 capture their multiconfigurational character well, yielding lower barrier heights than CCSD(T) for these reactions. The rate coefficients are computed using the RRKM/ME theory over a 500-2500 K temperature range and at a pressure range of 0.01 atm to the high-pressure limit. The temperature- and pressure-dependent rate coefficients are given in modified Arrhenius expressions. The β-scission of CH2C(═O)OCH3 is predicted to have a much higher barrier than the corresponding isomerization reaction and the β-scission of CH3C(═O)OCH2. Consequently, the rate coefficients for β-scission of CH2C(═O)OCH3 are the smallest among the three reactions and the isomerization followed by decomposition to CH3C(═O) and HCHO is the dominant reaction pathway for CH2C(═O)OCH3. Both radicals CH2C(═O)OCH3 and CH3C(═O)OCH2 are predicted to mainly decompose to CH3C(═O) + HCHO rather than to the bimolecular product CH2C(═O) + CH3O. A newly developed MA combustion mechanism, using our theoretical rate coefficients for the MA-related reactions, predicts combustion properties in good agreement with available experimental data.

HP-Mech: A High Pressure Kinetic Mechanism for C2 Flames with Exhaust Gas Dilution

This work represents continued development of a high pressure combustion chemistry mechanism (HP-Mech) for C2 mixtures with exhaust gas recirculation (EGR) at Princeton University. New burning velocity measurements were performed for ethane flames using the expanding spherical flame method at pressures from 1 to 20 atm with carbon dioxide and water vapor dilution. By holding the adiabatic flame temperature constant with dilution, the chemical effects of these diluents are investigated. These new measurements, as well as previous work with acetylene and ethylene, are used to improve and validate HP-Mech for C2 fuels.

Towards Simultaneous Measurement of OH and HO 2 in Combustion Using Faraday Rotation Spectroscopy

Preliminary results from the development of a dual wavelength Faraday rotation spectrometer for simultaneous quantification of HO2 and OH in combustion research are presented.

In-situ diagnostics of HOx Radicals in Low- and Intermediate-Temperature Oxidation of Dimethyl Ether

Simultaneous in-situ quantification of HO2 and OH in combustion of dimethyl ether using a Faraday rotation spectroscopy is presented. The experiments were performed in an atmospheric flow reactor over a temperature range of 500-1150K.