Formation pathways of HO2 and OH changing as a function of temperature in photolytically initiated oxidation of dimethyl ether

The time resolved product formation in oxidation of dimethyl ether (DME) has been studied between 298-625 K and 20-90 torr total pressure. Near-infrared frequency modulation spectroscopy (FMS) with Herriott type multi pass optics and UV absorption spectroscopy (UV) were conducted in the same cell. The reaction was initiated by pulsed photolysis in a mixture of Cl₂, O₂, and DME via CH₃OCH₂ radical formation. The reaction process was investigated through FMS measurement of HO₂ and OH, and UV measurement of CH₃OCH₂O₂. The yields of HO₂ and OH are obtained by comparison with reference mixtures, Cl₂, O₂, and CH₃OH for HO₂, and Cl₂, O₂, CH₃OH, and NO for OH, which convert 100% of initial Cl to HO₂ and OH. The CH₃OCH₂O₂ yield is also obtained. It was found that the HO₂ yield increases sharply over 500 K mainly with a longer time constant than that of R + O₂ reaction, while a prompt component exists throughout the temperature range at a few percent yield. OH was found to be produced promptly at a yield considerably larger than that known for the simplest alkanes. The CH₃OCH₂O₂ profile has a prompt rise followed by a gradual decay whose rate is consistent with the slow HO₂ formation. The species profiles were successfully predicted with a model constructed by modifying the existing one to suit the reduced pressure condition. After modification, it was inferred that the HO₂ formation over 500 K is secondary from HCHO + OH and HCO + O₂ and a part of HCO is formed directly from the O₂ adduct, whereas the HO₂ formation below 500 K is governed by CH₃OCH₂O₂ chemistry. The HCO forming pathway via isomerization-decomposition of the O₂ adduct, which was not included in the former models, was supported by our quantum-chemical calculations.     

Ignition of dimethyl ether/air mixtures by hot particles: Impact of low temperature chemical reactions

Understanding and characterizing ignition of flammable mixtures by hot particles is important for assessing and reducing the risk of accidental ignition and explosion in industry and aviation. Recently, many studies have been conducted for ignition of gaseous mixtures by hot particles. However, the effects of low-temperature chemistry (LTC) on ignition by hot particles received little attention. LTC plays an important role in the ignition of most hydrocarbon fuels and may induce cool flames. The present study aims to numerically assess the effects of LTC on ignition by the hot particles. We consider the transient ignition processes induced by a hot spherical particle in quiescent and flowing stoichiometric dimethyl ether/air mixtures. 1D and 2D simulations, respectively, are conducted for the ignition process by hot-particles in quiescent and flowing mixtures. A detailed kinetic model including both LTC and high-temperature chemistry (HTC) is used in simulations. The results exhibit a premixed cool flame to be first initiated by the hot particle. Then a double-flame structure with both premixed cool and hot flames is observed at certain conditions. At zero or low inlet flow velocities, the hot flame catches up and merges with the leading cool flame. At high inlet flow velocities, the hot flame cannot be initiated due to the short residence time and large convective loss of heat and radicals. Comparing the results with and without considering LTC confirms that LTC accelerates substantially ignition via HTC in a certain range of hot particle temperatures. The mechanism of ignition promotion by LTC is interpreted by analyzing the radical pool produced by the LTC and HTC surrounding the hot particle. Moreover, the influence of inlet flow velocity on ignition by hot particles is assessed. Non-monotonic change of ignition delay time with flow velocity is observed and discussed.     

Stark and Frequency Measurements in the FIR Spectrum of H2O2

The submillimeter-wave spectrum of H₂O₂has been recorded by means of a tunable FIR spectrometer. Stark measurements have been performed on three selected transitions in then= 0 state, namely, the 2₂₀–1₁₀(τ = 4 ← 2) at 1 272 297 MHz, the 8₂₆–7₁₆(τ = 1 ← 3) at 882 451 MHz, and the 9₂₈–8₁₈(τ = 1 ← 3) at 962 933 MHz. Accurate values of the transition dipole moments of the molecule have been derived by considering the interaction between the levels involved in the transition and the close near-resonant levels. About 40 new lines, belonging to the^rQ₄and^rQ₅subbranches of the rotational transitions between the lowest torsional states (τ = 1, 2, 3, 4n= 0), have been measured in the 2.8 and 3.4 THz spectral regions and analyzed together with the previously measured millimeter- and submillimeter-wave, as well as IR, transitions.     

Measurements and calculations of CO-H2O line-widths at high temperature

Diode-laser measurements of H₂O broadening of the CO P4 line in the 400–900 K range are presented. First comparisons between experimental CO-H₂O line-widths and the results of a recent theoretical model are also presented.     

Measurements of burning velocities of dimethyl ether and air premixed flames at elevated pressures

Laminar burning velocities of dimethyl ether (DME) and air premixed flames at elevated pressures up to 10 atm were measured by using a newly developed pressure-release type spherical bomb. The measurement system was validated using laminar burning velocities of methane–air flames. A comparison with the previous experimental data shows an excellent agreement and demonstrates the accuracy and reliability of the present experimental system. The measured flame speeds of DME–air flames were compared with the previous experimental data and the predictions using the full and reduced mechanisms. At atmospheric pressure, the measured laminar burning velocities of DME–air flames are in reasonable agreement with the previous data from spherical bomb method, but are much lower than both predictions and the experimental data of the PIV based counterflow flame measurements. The laminar burning velocities of DME–air flames at 2, 6, and 10 atm were also measured. It was found that flame speed decreases considerably with the increase of pressure. Moreover, the measured flame speeds are also lower than the predictions at high pressures. In addition, experiments showed that at high pressures the rich DME–air flames are strongly affected by the hydrodynamic and thermal-diffusive instabilities. Markstein lengths and the overall reaction order at different equivalence ratios were extracted from the flame speed data at elevated pressures. Sensitivity analysis showed that reactions involving methyl and formyl radicals play an important role in DME–air flame propagation and suggested that systematic modification of the reactions rates associated with methyl and formyl formations are necessary to reduce the discrepancies between predictions and measurements.     

Shock tube measurements of ignition delay times and OH time-histories in dimethyl ether oxidation

Ignition delay times and OH concentration time-histories were measured in DME/O₂/Ar mixtures behind reflected shock waves. Initial reflected shock conditions covered temperatures (T₅) from 1175 to 1900 K, pressures (P₅) from 1.6 to 6.6 bar, and equivalence ratios (?) from 0.5 to 3.0. Ignition delay times were measured by collecting OH^∗ emission near 307 nm, while OH time-histories were measured using laser absorption of the R₁(5) line of the A-X(0,0) transition at 306.7 nm. The ignition delay times extended the available experimental database of DME to a greater range of equivalence ratios and pressures. Measured ignition delay times were compared to simulations based on DME oxidation mechanisms by Fischer et al. [7] and Zhao et al. [9]. Both mechanisms predict the magnitude of ignition delay times well. OH time-histories were also compared to simulations based on both mechanisms. Despite predicting ignition delay times well, neither mechanism agrees with the measured OH time-histories. OH Sensitivity analysis was applied and the reactions DME ↔ CH₃O + CH₃ and H + O₂ ↔ OH + O were found to be most important. Previous measurements of DME ↔ CH₃O + CH₃ are not available above 1220 K, so the rate was directly measured in this work using the OH diagnostic. The rate expression k[1/s] =  1.61 × 10⁷⁹T^−18.4 exp(−58600/T), valid at pressures near 1.5 bar, was inferred based on previous pyrolysis measurements and the current study. This rate accurately describes a broad range of experimental work at temperatures from 680 to 1750 K, but is most accurate near the temperature range of the study, 1350-1750 K. When this rate is used in both the Fischer et al. and Zhao et al. mechanisms, agreement between measured OH and the model predictions is significantly improved at all temperatures.     

A numerical study of the autoignition of dimethyl ether with temperature inhomogeneities

The autoignition of dimethyl ether (DME) with temperature inhomogeneities is investigated by one-dimensional numerical simulations with detailed chemistry at high pressure and a constant volume. The primary purpose of the study is to provide an understanding of the autoignition of DME in a simplified configuration that is relevant to homogeneous charge compression ignition (HCCI) engines. The ignition structure and the negative temperature coefficient (NTC) behaviour are characterised in a homogeneous domain and one-dimensional domains with thermal stratification, at different initial mean temperatures and length scales. The thermal stratification is shown to strongly affect the spatial structure and temporal progress of ignition. The importance of diffusion and conduction on the ignition progress is assessed. It is shown that the effects of molecular diffusion decay relative to those of chemical reaction as the length-scale increases. This is to be expected, however the present study shows that these characteristics also depend on the mean temperature due to NTC behaviour. For the range of conditions studied here, which encompass a range of stratification length scales expected in HCCI engines, the effects of molecular transport are found to be small compared with chemical reaction effects for mean temperatures within the NTC regime. This is in contrast to previous work with fuels with single-stage ignition behaviour where practically realisable temperature gradients can lead to molecular transport effects becoming important. In addition, thermal stratification is demonstrated to result in significant reductions of the pressure-rise rate (PRR), even for the present fuel with two-stage ignition and NTC behaviour. The reduction of PRR is however strongly dependent on the mean initial temperature. The stratification length-scale is also shown to have an important influence on the pressure oscillations, with large-amplitude oscillations possible for larger length scales typical of integral scales in HCCI engines.     

Exploring the low-temperature oxidation chemistry of 1-butene and i-butene triggered by dimethyl ether

In order to better understand the low-temperature oxidation chemistry of alkenes, 1-butene and i-butene oxidation experiments triggered by dimethyl ether (DME) were conducted in a jet-stirred reactor at 790 Torr, 500–725 K and the equivalence ratio of 0.35. Low-temperature oxidation intermediates involved in alcoholic radical chemistry and allylic radical chemistry were detected by using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). To better interpret the experimental data, a kinetic model was proposed based on our low-temperature oxidation model of DME and comprehensive oxidation models of 1-butene and i-butene in literature. Based on present experimental results and modeling analysis, alcoholic radical chemistry initiated by OH addition is mainly responsible for the low-temperature chain propagation of butenes, since the Waddington mechanism plays a dominant role compared with the chain-branching pathways through the second O₂ addition. Allylic radical+HO₂ reactions producing alkenyl hydroperoxides and fuel+O₂ serve as the major chain-branching and chain-termination pathways, respectively, and they are competitive in the negative temperature coefficient (NTC) region. In contrast, chain-branching pathways originating from allylic radical+O₂ and alkyl-like radical+O₂ reactions have little contribution to the OH formation. Comparison with the simulation results of butane/DME mixtures demonstrates that butenes can largely inhibit the reactivity of DME at low temperatures due to its reduced low-temperature chain-branching process. However, in the NTC region, butenes may not be good OH absorbents since the allylic radicals can convert HO₂ to OH and consequently enhance the oxidation reactivity.     

Experimental investigation of partially premixed,highly-diluted dimethyl ether flames at low temperatures

Atmospheric-pressure highly-diluted laminar dimethyl ether (DME)–oxygen flames with temperatures below 1500 K were stabilized for the first time on a specially designed burner which allows preheating of the gas streams of fuel and oxidizer. With regard to the partially premixed structure of these flames which contain up to 90% argon in the unburnt gases, molecular-beam mass spectrometry (MBMS) with electron ionization (EI) was used to investigate chemical species profiles of reactants, intermediates, and products at a series of lateral positions and as function of distance from the burner. The flame structure reveals a near one-dimensional behavior at the flame front and beyond, towards the burnt gas. In a systematic approach, combustion parameters including stoichiometry, dilution, and gas preheating temperature were varied. The partial premixing effects upon the flame were revealed by comparing the distribution of flame species in a full two-dimensional concentration field above the burner, which is a starting point to model such flames in further studies. Formaldehyde and the methyl radical as two prominent species in the combustion process of DME were used to discuss characteristics of both high- and low-temperature kinetics.     

Mass spectrometric investigation of the low-temperature dimethyl ether oxidation in an atmospheric pressure laminar flow reactor

Low-temperature combustion is a major strategy today to reduce both soot and NO_x emission. Kinetic reaction models for low-temperature combustion which are validated against a wide range of experimental data are necessary e.g. for control purposes or as a basis for subsequent model reduction.In this study, the low-temperature oxidation of dimethyl ether in a highly diluted gas mixture was investigated experimentally in an atmospheric laminar flow reactor. The respective gas composition was analyzed by a time-of-flight mass spectrometer. This technique allows detection of all species simultaneously within the investigated temperature regime. Stoichiometries of ? = 0.8, 1.0, and 1.2 were studied with high temperature resolution in the range of 400–1200 K, and quantitative species mole fraction profiles have been determined.This wide temperature range comprises the different kinetic regimes occurring during the DME oxidation, which have been clearly resolved. The distinct negative temperature coefficient (NTC) region of the system was observed and extensive speciation is available. Special attention is given to species only occurring in the low-temperature region including formic acid and methyl formate.     

A kinetics and dynamics study on the auto-ignition of dimethyl ether at low temperatures and low pressures

Though the combustion chemistry of dimethyl ether (DME) has been widely investigated over the past decades, there remains a dearth of ignition data that examines the low-temperature, low-pressure chemistry of DME. In this study, DME/‘air’ mixtures at various equivalence ratios from lean (0.5) to extremely rich (5.0) were ignited behind reflected shock waves at a fixed pressure (3.0 atm) over the temperature range 625–1200 K. The ignition behavior is different from that at high-pressures, with a repeatable ignition delay time fall-off feature observed experimentally in the temperature transition zone from the negative temperature coefficient (NTC) regime to the high-temperature regime. This could not be reproduced using available kinetic mechanisms as conventionally homogeneous ignition simulations. The fall-off behavior shows strong equivalence ratio dependence and disappears completely at an equivalence ratio of 5.0. A local ignition kernel postulate was implemented numerically to quantifiably examine the inhomogeneous premature ignition. At low temperature, no pre-ignition occurs in the mixture. A conspicuous discrepancy was observed between the measurements and constrained UV simulations at temperatures beyond the NTC regime. A third O₂ addition reaction sub-set was incorporated into AramcoMech 3.0, together with related species thermochemistry calculated using the G3/G4/CBS-APNO compound method, to explore the low-temperature deviation. The new reaction class does not influence the model predictions in IDTs, but the updated thermochemistry does. Sensitivity analyses indicate that the decomposition of hydroperoxy-methylformate plays a critical role in improving the low-temperature oxidation mechanism of DME but unfortunately, the thermal rate coefficient has never been previously investigated. Further experimental and theoretical endeavors are required to attain holistic quantitative chemical kinetics based on our understanding of the low-temperature chemistry of DME.     

Measurements and interpretation of the low temperature dielectric relaxation in glasses

In order to test a phenomenological model of dielectric relaxation in glasses using asymmetric double-well potentials low-frequency dielectric measurements have been performed on multicomponent glasses at temperatures between 1 mK and 300K.     

Low temperature ESR in Ni(BrO3)26H2O2

ESR experiments on Ni(BrO₃)₂6H₂O were performed down to liquid helium temperatures, and the g-value, the magnitude and sign of the single-ion anistotropy parameter, and the strength of the Ni²⁺Ni²⁺ exchange interaction, were determined. The present data support a recent interpretation of the susceptibility data for this salt that assumes tetrahedral antiferromagnetism at very low temperatures.     

Raman study of (NH4)2CuCl4·2H2O—I: Dynamics and structure in low temperature phase

We report the results of a Raman scattering study of (NH₄)₂CuCl₄2H₂O at 300, 205 and 100°K, in order to elucidate the dynamics and phase transition in this double salt. A group theoretical calculation of the symmetry vectors, in the high temperature phase (D_4h¹⁴), is made and the various modes are identified. The deuterated compound (ND₄)₂CuCl₄·2D₂O has also been investigated to help in identifying the modes involving motion of the ammonium ions and water molecules. Through a careful analysis of the spectra at 100°K, the space group in the low temperature phase has been established as D_2d³. The important consequence of this result is that this leads to parallel spatial ordering of ammonium tetrahedra in this compound in the low temperature phase.     

Steric effect in O+/H2 and H+/H2O collisions

The positive water and hydronium ions are of interest in a variety of chemical and biological applications. Here we study the steric effect in charge transfer collisions, i.e. the spatial dependence of single electron capture, in collisions mediated by these ions. In particular, the steric effect is demonstrated in the O⁺(²D)/H₂ and H⁺/H₂O charge transfer collisions in the energy range of 100 eV/amu to 10 keV/amu.     

傅里叶变换红外光谱法测量大气中CO2和H2O的稳定同位素

光谱技术的发展使得连续测量环境大气中的稳定同位素成为可能。描述了应用傅里叶变换红外(FTIR)光谱技术测量环境大气中稳定同位素的方法。为了验证该方法对环境大气中的稳定同位素进行连续测量的可行性,在七天的外场实验中,应用开放光程FTIR系统直接测量环境大气中CO2的稳定同位素12 CO2,13 CO2和H2O的稳定同位素H216 O和HD16 O,并得到大气中碳同位素比值δ13 C和氘同位素比值δD。对同位素比值δ13 C和δD,系统的测量精度分别约为1.08‰和1.32‰。采用Keeling图方法,在不同的时间尺度上对CO2和H2O的同位素数据进行分析,得到了水汽地表蒸散的氘同位素特征δET。外场实验的结果证明了开放光程FTIR系统长期测量环境大气中稳定同位素的潜力。     

Theoretical study on collision dynamics of H+ + H2O at low energies

Scattering dynamics of a water molecule in collision with proton is studied based on a time-dependent density functional theory and coupled with the molecular dynamics method, in which the electrons are described by quantum mechanics and the nuclei are described by classical mechanics. Four different incident directions at 46 eV are chosen in order to investigate the orientations effect, and the energy-dependent effect in low energy region is explored under impact energies 27, 36 and 46 eV. Reaction channels, scattering angles and energy loss of protons are calculated. The differences between those characteristics are unobvious in large impact parameters, which are irrespective of the incident orientations due to weak projectile-target interaction. In small impact parameters, the results strongly depend on the collision energy and orientation.     

Use of Zeeman Atomic Absorption Flame Spectrometry for Measurements in C2H2/Air and C2H2/N2O Flames

The Hitachi Zeeman atomic absorption spectrometer system is used to evaluate the influence of observation height and C₂H₂ flow rate upon the atomic absorption sensitivity (slope of calibration curve) and upon the atomic absorption signals for 6 elements in an C₂H₂/Air flame and for 3 elements in an C₂H₂/N₂O flame. Fuel-rich conditions result in greater absorption signals and sensitivities in all cases even though there is a significant temperature drop. Optimal observation heights for each case are evaluated. Greater linearity of analytical calibration curves occurs for fuel-rich conditions under Zeeman background correction than under no background correction. The Zeeman atomic absorption flame spectrometer should find more use in the future.