Ignition and combustion of isopropyl nitrate

The ignition delay times for mixtures of isopropyl nitrate (IPN) with air and argon are measured in a rapid-injection reactor at a pressure of 1 atm and in a shock tube at 2–3 atm. It is shown that the ignition delay time τ of mixtures in which heat is largely released due to oxidation by the oxygen contained in the IPN molecule is determined by the unimolecular decomposition of IPN over the entire temperature range covered (500–730 K). For mixtures in which heat is mainly produced by oxidation reactions involving air oxygen, the ignition delay time at high temperatures is controlled by secondary reactions of oxidation of the hydrocarbon moiety of the IPN molecule, leading to an increase in τ by more than an order of magnitude. Liquid IPN burns in a nitrogen atmosphere only at pressures above 40 atm, at a linear rate of ~4 mm/s. The measured flame temperatures are in close agreement with the respective values calculated using a thermodynamic code.     

Temperature-dependent gas-surface chemical kinetic model for methane ignition catalyzed by in situ generated palladium nanoparticles

A temperature-dependent gas-surface kinetic model for methane oxidation over palladium is proposed. Thermodynamic data for the surface species (O, H, OH, H₂O, and CO) are derived from statistical mechanic analysis using literature heats of desorption and vibrational frequencies. The rate parameters in the model also satisfy thermo-kinetic constraints. The hydrogen oxidation submodel is validated against literature stagnation flow reactor experiments at 1300 K and 13 Pa. The current model is further tested against catalytic methane ignition in a laminar flow reactor at atmospheric pressure, and with time-resolved measurements of the size distribution of palladium nanoparticles generated in situ from an aerosol containing palladium acetate. The improved gas-surface model predicts closely the experimental data. The role of palladium nanoparticles in enhancing methane ignition is attributed to heat release due to catalytic methane oxidation over distributed nanoparticle surfaces, leading to a temperature rise and thus an accelerated gas-phase chain-branching process.     

Influence of the structure of the Ti-49.8 at % Ni alloy on its thermal expansion during the martensitic phase transformation

The temperature dependence of thermal expansion of the Ti-49.8 at % Ni alloy has been measured after rolling at temperatures of 470, 570, 670, 770, and 870 K. The maximum dilatation jump during the martensitic phase transition has been observed for the samples rolled at 570 and 670 K. A fragmented structure, in which the phase transformations are hampered, is formed during the low-temperature rolling. An increase in the rolling temperature to 770–870 K leads to the return processes and dynamic recrystallization of the material; as a result, the slope of dilatation curves changes and the range of phase transitions narrows.     

Low-temperature ignition of methane-air mixtures under the action of nonequilibrium plasma

A plasma-chemical kinetic mechanism of the low-temperature (600 < T < 1000 K) oxidation/combustion of methane under conditions of nonequilibrium plasma over a wide pressure range (P = 0.1?100 atm) is developed and verified. The mechanism is comprised of three types of elementary processes: chemical reaction of neutral atoms and molecules, primary plasma-chemical processes involving electrons, and secondary plasma-chemical processes involving atomic and molecular ions and excited species. Application of the developed mechanism to describing the plasma-assisted oxidation of methane shows that this mechanism can describe the experimental results qualitatively and quantitatively.     

The oxidation and co reduction kinetics of a platinum surface

A thermogravimetric technique was used to study the oxidation and CO reduction kinetics of Pt as a function of temperature, T. The measurements were performed on Pt powder (particle sizes 0.8–2.5 μm) at 1 atm pressure. After exposure to 1 atm of oxygen at 600 K for 1 h, the total uptake of oxygen by the powder amounted to less than one oxygen atom per Pt surface atom and followed a logarithmic growth law. For 400 < T < 600 K, the logarithmic rate constant, K₀, could be described by an Arrhenius law with an apparent activation energy of 2.6 kcal/mole. Above 600 K, K₀ slowly decreased, an effect believed to be associated with the dissociation of the oxide between 650 and 850 K. Oxidation isotherms were calculated using the low pressure oxygen sticking coefficient data of Hopster et al. The calculated and measured oxidation isotherms were found to be in remarkable agreement. The CO reduction data were more difficult to analyze but showed that the reduction rate had a stronger temperature dependence (~11 kcal/mole) than the oxidation rate. For427 < T<487 K, the general time scale of the reduction process was 10–50 min. Typical durations of the oxidation and CO reduction processes are consistent with the periods observed in studies of the oscillatory rate of CO oxidation over a Pt catalyst.     

利用OH自由基特征发射谱测量正庚烷的点火延迟时间

在化学激波管中利用反射激波进行点火,采用OH自由基在306.4nm处特征发射谱线强度的急剧变化标志燃料的着火,由光谱单色仪、光电倍增管、压力传感器和示波器组成测量系统,测量了正庚烷/氧气的点火延迟时间,点火压力(1.0±0.1)和(0.75±0.05)atm,点火温度1 170～1 730K,当量比1.0,得到了在此实验条件下正庚烷/氧气点火延迟时间随温度变化的关系式。研究结果表明正庚烷/氧气点火延迟时间随温度的增加呈指数减小,点火压力为0.75atm时,随着点火温度的增加,点火延迟时间的变化率要小于1.0atm条件时。实验结果为建立正庚烷燃烧反应动力学模型,验证正庚烷燃烧反应机理提供了实验依据。     

Effect of preparation conditions on the microstructural characteristics and optical properties of oxidized zinc films

Thin films of zinc (Zn) were deposited onto glass substrates (maintained at room temperature) by thermal evaporation under vacuum. The metallic zinc films were submitted to thermal oxidation in air at 670 K and 770 K, respectively, for 5–90 min, in order to obtain zinc oxide (ZnO) thin films. X-ray diffraction patterns revealed that the ZnO thin films were polycrystalline and had a wurtzite (hexagonal) structure. The morphology of the prepared ZnO thin films was investigated using atomic force microscopy and scanning electron microscopy techniques. Transmission spectra were recorded in the spectral domain from 300 nm to 1400 nm. The optical energy bandgap calculated from the absorption spectra (supposing allowed direct transitions) was in the range 3.05–3.30 eV.     

Application of an aerosol shock tube to the measurement of diesel ignition delay times

Shock tube ignition delay times were measured for DF-2 diesel/21% O₂/argon mixtures at pressures from 2.3 to 8.0 atm, equivalence ratios from 0.3 to 1.35, and temperatures from 900 to 1300 K using a new experimental flow facility, an aerosol shock tube. The aerosol shock tube combines conventional shock tube methodology with aerosol loading of fuel-oxidizer mixtures. Significant efforts have been made to ensure that the aerosol mixtures were spatially uniform, that the incident shock wave was well-behaved, and that the post-shock conditions and mixture fractions were accurately determined. The nebulizer-generated, narrow, micron-sized aerosol size distribution permitted rapid evaporation of the fuel mixture and enabled separation of the diesel fuel evaporation and diffusion processes that occurred behind the incident shock wave from the chemical ignition processes that occurred behind the higher temperature and pressure reflected shock wave. This rapid evaporation technique enables the study of a wide range of low-vapor-pressure practical fuels and fuel surrogates without the complication of fuel cracking that can occur with heated experimental facilities. These diesel ignition delay measurements extend the temperature and pressure range of earlier flow reactor studies, provide evidence for NTC behavior in diesel fuel ignition delay times at lower temperatures, and provide an accurate data base for the development and comparison of kinetic mechanisms for diesel fuel and surrogate mixtures. Representative comparisons with several single-component diesel surrogate models are also given.     

Heat release of carbon particle formation from hydrogen-free precursors behind shock waves

The process of heat release during carbon particle formation and growth after pyrolysis of carbon suboxide C₃O₂ behind shock waves was investigated. For this goal, temperature and optical density of gas-particle mixtures initially consisting of 3% C₃O₂ + 5% CO₂ in Ar were measured as a function of time. The temperature was determined by two-channel emission-absorption spectroscopy at λ = 2.7 ± 0.4 μm, corresponding to the CO₂ (1,0,1) vibrational band. In the range of initial temperatures behind the shock waves from 1600 up to 2200 K a significant heating of the mixture during particle formation and growth was observed that increased towards higher temperatures. The analysis of the obtained data in combination with previous results about the temperature dependence of the particle size shows a decrease of the heat release of condensation from ∼200 kJ/mol per atom for particles containing ∼1000 atoms to ∼50 kJ/mol per atom for particle containing ∼10⁶ atoms.     

Autoignition of propane–air mixtures behind reflected shock waves

Ignition times and autoignition modes for propane–air mixtures have been studied behind reflected shock waves. Experiments were performed over temperatures between 1000 and 1750 K, pressures between 2 and 20 atm, and equivalence ratios of = 0.5, 1.0, and 2.0. Ignition delay times were determined using pressure measurements, C₂ emission profiles, and luminosity measurements in the visible spectrum (380–680 nm). Empirical correlations for ignition time for low temperature (1000–1300 K) and high temperature (1300–1800 K) ranges have been deduced from the experimental data. Different autoignition modes of the mixture (strong, transient, and weak) were identified by comparing velocities of reflected shock wave at different distances from the reflecting wall.     

Experiments and modeling of ignition delay times, flame structure and intermediate species of EHN-doped stoichiometric n-heptane/air combustion

The combustion of stoichiometric Ethyl-hexyl-nitrate (EHN)-doped n-heptane/oxygen/argon and (EHN)-doped n-heptane/air mixtures, respectively, was investigated in a low-pressure burner with a molecular-beam mass spectrometer and ignition delay-time (τ_ign) measurements were performed in a high-pressure shock tube. The experiments with the low-pressure flame were used for the determination of the flame structure including concentration profiles of reactants, products and important intermediates in the flame. The shock tube experiments provided τ_ign for a temperature range of 690 K ? T ? 1275 K at a pressure of 40 ± 2 bar for stoichiometric and lean mixtures under engine relevant conditions. A chemical mechanism for n-heptane/EHN mixtures was developed from an automatically generated mechanism for n-heptane by manually adding reactions to describe the influence of EHN. This mechanism was validated against the shock-tube data for various temperatures, levels of EHN-doping and equivalence ratios by homogeneous reactor calculations.The ignition delay times predicted by the model agree well with the shock tube results for a large range of temperatures, equivalence ratios and EHN concentrations. The influence of EHN onto ignition delay was largest in the low-temperature regime (770-1000 K).Numerical analysis suggests that the prevalent reason for the ignition-enhancing effect of EHN is the formation of highly reactive heptyl radicals by thermal decomposition of EHN. Due to this comparatively simple and generic mechanism, EHN is expected to have a similar ignition-enhancing effect also for other hydrocarbon fuels.     

Decomposition of nitromethane in shock waves: The primary stage and the kinetics of decomposition at pressures of about 40 atm

The kinetics of nitromethane (NM) decomposition and the observed rate constant of the process were measured behind reflected shock wave using absorption spectroscopy at λ = 230 nm, temperatures of 1060 to 1350 K, pressure of ∼40 atm, and initial reactant concentration within 30–100 ppm. It was observed that, at the initial stage, nitromethane decomposes exponentially, without autoacceleration. The results of numerical simulations with the help of three most known kinetic schemes of nitromethane decomposition proved to be in close to agreement with our experimental data over the entire temperature range covered. It was demonstrated that the measured rate constant is identical to the rate constant of the dissociation CH₃NO₂ → CH₃ + NO₂. The temperature dependence of k ₁ was approximated by the Arrhenius formula k ₁ = 2.57 × 10¹⁴ exp(−52.85/RT) s⁻¹ (activation energy in kcal/mol), which suggests that the nitromethane dissociation proceeds in the falloff pressure region.     

The resistive-heating characterization of laser heating system and LaB6 characterization of X-ray diffraction of beamline 12.2.2 at advanced light source

X-ray diffraction from LaB₆ standards document a precision of 478 ppm in lattice-parameter determinations for beamline 12.2.2 at Lawrence Berkeley National Laboratory′s Advanced Light Source, a facility for characterizing materials at high pressures and temperatures using laser- and resistance-heated diamond cells. Melting of Ni, Mo, Pt and W, resistively heated at 1 atm pressure in Ar, provides a validation of the beamline spectroradiometric system that is used to determine sample temperatures. The known melting temperatures, which range from 1665 to 3860 K for these metals, are all reproduced to within ±80 K.     

Theoretical investigation of the possibility of using multicomponent (N2-O2-CH4-Y2O) clathrate hydrates for methane recovery from mine gas

Phase equilibria in a multicomponent ice-gas mixture-hydrate system are investigated for a mixture of nitrogen, oxygen, and methane, depending on the gas phase composition, pressure, and temperature. Equilibrium compositions of the formed hydrates are found, depending on the gas mixture composition. Calculations show that with increasing concentration ofmethane in gas phase the pressure of hydrate formation gradually decreases. It is shown that this pressure at considerable methane concentrations approximately corresponds to partial pressure of methane in the gas phase. Conditions of hydrate formation are calculated in the range of temperatures from 258 to 273 K, at pressures from 1 to 350 atm. The obtained results are in agreement with the known experimental data for hydrates of pure gases-nitrogen, oxygen, and methane.     

Ignition behavior of pure and blended methyl octanoate,n-nonane,and methylcyclohexane

Excited-state species profiles and ignition delay times were obtained under dilute conditions (99% Ar) using a heated shock tube for methyl octanoate (C₉H₁₈O₂), n-nonane (n-C₉H₂₀), and methylcyclohexane (MCH) over a broad range of temperature and equivalence ratio (? = 0.5, 1.0, 2.0) at pressures near 1 and 10 atm. Measurements were then extended to include two ternary blends of the fuels using 5% and 20% (vol.) of the methyl ester under stoichiometric conditions. Using three independently validated chemical kinetics mechanisms, a model was compiled to assess the influence of methyl ester concentration on ignition delay times of the ternary blends. Under near-atmospheric pressure, ignition delay times were of the following order for the pure fuels: methyl octanoate < n-nonane < methylcyclohexane. Experimental results indicate that the ignition behavior of the higher-order methyl ester approaches that of the higher-order linear alkane with increased pressure regardless of equivalence ratio. Methyl octanoate also displayed significantly lower pressure dependence relative to the linear alkane and the cycloalkane species. Both of these results are supported by model calculations. Blending of methyl octanoate with n-nonane and methylcyclohexane impacted ignition delay time results more strongly at 1.5 atm, yet had only a small effect near 10 atm for temperatures above 1400 K. The study provides the first shock-tube data for a ternary blend of a linear alkane, a cycloalkane, and a methyl ester. Ignition delay time measurements for the C₉:0 methyl ester were also measured for the first time.     

Electrical conductivity of MgCO3 at high pressures and high temperatures

The electrical conductivity of polycrystalline magnesite (MgCO₃) was measured at 3-6 GPa at high temperatures using complex impedance spectroscopy in a multi-anvil high-pressure apparatus. The electrical conductivity increased with increasing pressure. The activation enthalpy calculated in the temperature range 650-1000 K also increased with increasing pressure. The effect of pressure was interpreted as being the activation volume in the Arrhenius equation, and the fitted data gave an activation energy and volume of 1.76±0.03 eV and −3.95±0.78 cm³/mole, respectively. The negative activation volume and relatively large activation energy observed in this study suggests that the hopping of large polarons is the dominant mechanism for the electrical conductivity over the pressure and temperature range investigated.     

Transitioning model potentials to real systems

The parameters of two pair potentials that describe argon over its entire liquid phase at a fixed pressure were optimized through a novel application of constant temperature and pressure molecular dynamics (NPT-MD) and Monte Carlo (NPT-MC) computer simulations. The forms of these potentials were those of a modified Lennard-Jones potential and a Lennard-Jones potential. The optimized potential determined using NPT-MD simulations reproduces experimental densities, internal energies and enthalpies with an error less than 1% over most of the liquid range and yields self-diffusion coefficients that are in excellent agreement with experiment. The results using the potential determined by NPT-MC simulations are in almost as good agreement with deviations from experiment of no more than 5.89% for temperatures up to vaporization. Additionally, molar volumes predicted using this potential at pressures in the range 100–600 atm and over temperatures in the range 100–140K were within 0.83% of experimental values. These results show that, when properly parametrized, Lennard-Jones-like potentials can describe a system well over a large temperature range. Further, the method introduced is easy to implement and is independent of the form of the interaction potential used.     

Experimental study and modeling of shock tube ignition delay times for hydrogen-oxygen-argon mixtures at low temperatures

Recent literature has indicated that experimental shock tube ignition delay times for hydrogen combustion at low-temperature conditions may deviate significantly from those predicted by current detailed kinetic models. The source of this difference is uncertain. In the current study, the effects of shock tube facility-dependent gasdynamics and localized pre-ignition energy release are explored by measuring and simulating hydrogen-oxygen ignition delay times. Shock tube hydrogen-oxygen ignition delay time data were taken behind reflected shock waves at temperatures between 908 to 1118 K and pressures between 3.0 and 3.7 atm for two test mixtures: 4% H₂, 2% O₂, balance Ar, and 15% H₂, 18% O₂, balance Ar. The experimental ignition delay times at temperatures below 980 K are found to be shorter than those predicted by current mechanisms when the normal idealized constant volume (V) and internal energy (E) assumptions are employed. However, if non-ideal effects associated with facility performance and energy release are included in the modeling (using CHEMSHOCK, a new model which couples the experimental pressure trace with the constant V, E assumptions), the predicted ignition times more closely follow the experimental data. Applying the new CHEMSHOCK model to current experimental data allows refinement of the reaction rate for H + O₂ + Ar ↔ HO₂ + Ar, a key reaction in determining the hydrogen-oxygen ignition delay time in the low-temperature region.     

A comparative reactivity study of 1-alkene fuels from ethylene to 1-heptene

A comparative reactivity study of 1-alkene fuels from ethylene to 1-heptene has been performed using ignition delay time (IDT) measurements from both a high-pressure shock tube and a rapid compression machine, at an equivalence ratio of 1.0 in ‘air’, at a pressure of 30 atm in the temperature range of 600–1300 K. At low temperatures (< 950 K), the results show that 1-alkenes with longer carbon chains show higher fuel reactivity, with 1-pentene being the first fuel to show negative temperature coefficient (NTC) behavior followed by 1-hexene and 1-heptene. At high temperatures (> 950 K), the experimental results show that all of the fuels except propene show very similar fuel reactivity, with the IDTs of propene being approximately four times longer than for all of the other 1-alkenes. To analyze the experimental results, a chemistry mechanism has been developed using consistent rate constants for these alkenes. At 650 K, flux analyses show that hydroxyl radicals add to the double bond, followed by addition to molecular oxygen producing hydroxy?alkylperoxy radicals, which can proceed via the Waddington mechanism or alternate internal H-atom isomerizations in chain branching similar to those for alkanes. We have found that the major chain propagation reaction pathways that compete with chain branching pathyways mainly produce hydroxyl rather than hydroperoxyl radicals, which explains the less pronounced NTC behavior for larger 1-alkenes compared to their corresponding alkanes. At 1200 K, flux analyses show that the accumulation of hydroperoxyl radicals is important for the auto-ignition of 1-alkenes from propene to 1-heptene. The rate of production of hydroperoxyl radicals for 1-alkenes from 1-butene to 1-heptene is higher than that for propene, which is due to the longer carbon chain facilitating hydroperoxyl radical formation via more efficient reaction pathways. This is the major reason that propene presents lower fuel reactivity than the other 1-alkenes at high temperatures.     

Ammonia–methane interaction in jet-stirred and flow reactors: An experimental and kinetic modeling study

The influence of the addition of ammonia on the oxidation of methane was investigated both experimentally and numerically. Experiments were carried out at atmospheric pressure, using a fused silica jet-stirred reactor, and a recrystallized alumina tubular reactor designed on purpose to reach temperatures as high as ～2000 K. A temperature range of 600–1200 K was investigated in the jet-stirred reactor at a residence time of 1.5 s, while experiments in the flow reactor were carried out between 1200 and 2000 K, for a fixed residence time of about 25 ms in the reactive zone. A methane/ammonia mixture, diluted in helium, was used in both reactors with equivalence ratios varied between 0.5 and 2 in the first reactor, while stoichiometric conditions were investigated in the second one. The measurements indicate that CH₄ reactivity was promoted by NH₃ addition below 1200 K, but not so much influenced above. These results were interpreted and explained using a comprehensive kinetic model, previously validated over a wider range of operating conditions. The mechanism allowed to shed light on the underlying causes of the anticipated methane reactivity at low temperature, and of the major role played by NO_x in it. This effect was shown to become less significant at higher temperatures, where the reactivity is mainly governed by H-abstractions on both fuels.