An experimental and modeling study of the low- and high-temperature oxidation of cyclohexane

The experimental study of the oxidation of cyclohexane has been performed in a jet-stirred reactor at temperatures ranging from 500 to 1100 K (low- and intermediate temperature zones including the negative temperature-coefficient area), at a residence time of 2 s and for dilute mixtures with equivalence ratios of 0.5, 1, and 2. Experiments were carried out at quasi-atmospheric pressure (1.07 bar). The fuel and reaction product mole fractions were measured using online gas chromatography. A total of 34 reaction products have been detected and quantified in this study. Typical reaction products formed in the low-temperature oxidation of cyclohexane include cyclic ethers (1,2-epoxycyclohexane and 1,4-epoxycyclohexane), 5-hexenal (formed from the rapid decomposition of 1,3-epoxycyclohexane), cyclohexanone, and cyclohexene, as well as benzene and phenol. Cyclohexane displays high low-temperature reactivity with well-marked negative temperature-coefficient (NTC) behavior at equivalence ratios 0.5 and 1. The fuel-rich system (? = 2) is much less reactive in the same region and exhibits no NTC. To the best of our knowledge, this is the first jet-stirred reactor study to report NTC in cyclohexane oxidation. Laminar burning velocities were also measured by the heated burner method at initial gas temperatures of 298, 358, and 398 K and at 1 atm. The laminar burning velocity values peak at ? = 1.1 and are measured as 40 and 63.1 cm/s for T_i = 298 and 398 K, respectively. An updated detailed chemical kinetic model including low-temperature pathways was used to simulate the present (jet-stirred reactor and laminar burning velocity) and literature experimental (laminar burning velocity, rapid compression machine, and shock tube ignition delay times) data. Reasonable agreement is observed with most of the products observed in our reactor, as well as the literature experimental data considered in this paper.     

High-pressure low-temperature ignition behavior of syngas mixtures

Ignition properties of simulated syngas mixtures were systematically investigated at high-pressure low-temperature conditions relevant to gas turbine combustor operation using the University of Michigan Rapid Compression Facility. Pressure time history measurements and high-speed imaging of the ignition process in this facility were used to determine auto-ignition delay times and observe and characterize ignition behaviors. The simulated syngas mixtures were composed of H₂ and CO with a molar ratio of 0.7, for equivalence ratios (φ) of 0.1 and 0.5, near air dilution (i.e. molar O₂ to inert gas ratio of 1:3.76), with N₂ as the primary diluent gas. The pressures and temperatures after compression ranged from 3–15 atm and 870–1150 K respectively. The comprehensive results of the present work combined with those from previous shocktube studies in the literature clearly illustrate the existence of both homogeneous and inhomogeneous auto-ignition behaviors at these conditions. Analysis of patterns in the ignition behaviors revealed a dependence on temperature, pressure, and equivalence ratio with distinct thermodynamic regions in which the ignition behavior is consistent and repeatable. Predicted locations of the strong ignition limit made using a criterion which compares laminar flame speed to a thermal gradient driven front propagation speed have excellent agreement with the experimental findings for each φ and an assumed gradient of 5 K/mm. Experimental validation of this unique and powerful criterion means that it can be used for a priori prediction of the strong ignition limit using basic computational simulations. The validity of this criterion is fundamentally important, quantitatively describing the roles of chemical kinetics, thermo-physical properties, and device dependent thermal characteristics in determining auto-ignition behavior. Additionally, a comparison of the measured auto-ignition delay times to predictions made using zero-dimensional homogeneous reactor modeling revealed that agreement was dependent on φ, with excellent agreement for φ = 0.1 and large discrepancies for φ = 0.5. These results indicate that while inhomogeneous ignition phenomena are not entirely avoidable by reducing equivalence ratio, the subsequent effects on the accuracy of typical auto-ignition delay time predictions may be reduced or eliminated.     

Experiments and modeling of the autoignition of methylcyclohexane at high pressure

New experimental data are collected for methyl-cyclohexane (MCH) autoignition in a heated rapid compression machine (RCM). Three mixtures of MCH/O₂/N₂/Ar at equivalence ratios of ? = 0.5, 1.0, and 1.5 are studied and the ignition delays are measured at compressed pressure of 50 bar and for compressed temperatures in the range of 690–900 K. By keeping the fuel mole fraction in the mixture constant, the order of reactivity, in terms of inverse ignition delay, is measured to be ? = 0.5 > ? = 1.0 > ? = 1.5, demonstrating the dependence of the ignition delay on oxygen concentration. In addition, an existing model for the combustion of MCH is updated with new reaction rates and pathways, including substantial updates to the low-temperature chemistry. The new model shows good agreement with the overall ignition delays measured in this study, as well as the ignition delays measured previously in the literature using RCMs and shock tubes. This model therefore represents a strong improvement compared to the previous version, which uniformly over-predicted the ignition delays. Chemical kinetic analyses of the updated mechanism are also conducted to help understand the fuel decomposition pathways and the reactions controlling the ignition. Combined, these results and analyses suggest that further investigation of several of the low-temperature fuel decomposition pathways is required.     

A high pressure ignition delay time study of 2-methylfuran and tetrahydrofuran in shock tubes

Ignition delay time studies for tetrahydrofuran (THF) and 2-methylfuran (2MF) as well as optical investigations of combustion for 2MF have been carried out using two shock tubes. The experiments with undiluted THF/air mixtures were performed at 20 and 40 bar in a high pressure shock tube (HPST) at an equivalence ratio of Ф = 1 covering an overall temperature range of 780–1100 K and 691–1006 K, respectively. Undiluted 2MF/air mixtures (Ф = 1) were also investigated in the HPST at 40 bar in the temperature range of 820–1215 K. The experimental data of 2MF obtained at 40 bar were supported with kinetic simulations of existing models from literature. Additionally, sensitivity analyses of 2MF at several temperatures were performed for finding out the most sensitive reactions. Schlieren imaging was employed in a rectangular shock tube (RST) utilizing a high speed video camera through which the ignition process was captured for a stoichiometric 2MF/O₂/Ar mixture at pressures of about 10 bar and in the temperature range of 871–1098 K.     

An experimental and modeling study of 2-methyl-1-butanol oxidation in a jet-stirred reactor

In an effort to understand the oxidation chemistry of new generation biofuels, oxidation of a pentanol isomer (2-methyl-1-butanol) was investigated experimentally in a jet-stirred reactor (JSR) at a pressure of 10 atm, equivalence ratios of 0.5, 1, 2 and 4 and in a temperature range of 700–1200 K. Concentration profiles of the stable species were measured using GC and FTIR. A detailed chemical kinetic mechanism including oxidation of various hydrocarbon and oxygenated fuels was extended to include the oxidation chemistry of 2-methyl-1-butanol, the resulting mechanism was used to simulate the present experiments. In addition to the present data, recent experimental data such as ignition delay times measured in a shock tube and laminar flame speeds were also simulated with this mechanism and satisfactory results were obtained. Reaction path and sensitivity analyses were performed in order to interpret the results.     

The transition of heterogeneous–homogeneous ignitions of dispersed coal particle streams

This work is assessing a study of the collective ignition behaviors of dispersed coal particle streams, with ambience temperature from 1200 K to 1800 K and oxygen mole fractions in the range of 10–30%. The dispersed coal particles of 65–74 μm are injected into an optical Hencken flat-flame burner by a novel de-agglomeration feeder. Three kinds of pulverized coals from different ranks, Hulunbel lignite, high-ash-fusion bituminous and low-ash-fusion bituminous, are considered. The normalized visible light signal intensity, deleting the background noise, is established to characterize the ignition delay of coal particle streams. Firstly, the prevalent transition from heterogeneous ignition to hetero–homogeneous ignition due to ambience temperature is observed. The pure homogeneous ignition rarely occurs, with an exception under high temperature and low oxygen for high-volatile coal. By comparing time scales between pyrolysis and heating processes, the competition of the volatile evolution and heterogeneous surface reaction are discussed. Then, the effects of ambience temperature, oxygen mole fraction and coal rank on the characteristic ignition delay are examined. Finally, the transient mode is developed, which not only well interprets the observed ignition transition phenomena, but also approximately predicts a variation of heterogeneous ignition time as a function of oxygen fraction.     

Kinetic interactions between hydrogen and carbon monoxide oxidation over platinum

The heterogeneous chemistry coupling of H₂ and CO over platinum was investigated experimentally and numerically for H₂/CO/O₂/N₂ mixtures with overall lean equivalence ratios φ = 0.13–0.26, H₂:CO molar ratios 1:5–3:1, and a pressure of 5 bar. Experiments were performed in an optically accessible channel-flow reactor at surface temperatures 510–827 K and involved in situ Raman measurements of major gas-phase species concentrations and thermocouple measurements of surface temperatures. Emphasis was placed on the low temperature range 510–600 K, whereby H₂ inhibited the CO oxidation, and which was of particular relevance to gas turbine idling and part-load operation. Comparisons of measurements with 2-D simulations attested the aptness of the employed kinetic scheme, not only for H₂/CO fuel mixtures but also for pure CO. Measured and predicted transition temperatures below which H₂ inhibited CO oxidation agreed well with each other, showing a main dependence on the overall equivalence ratio (550 ± 5 K at φ = 0.13 and 600 ± 5 K at φ = 0.26) and a weaker dependence on the H₂:CO ratio. Furthermore, this inhibition was non-monotonically dependent on the H₂:CO ratio, becoming higher at a value of 1:1. The inhibiting kinetic effect of H₂ was an outcome of the competition between H₂ and CO/O₂ for surface adsorption and, most importantly, of the competition between the adsorbed H(s) and CO(s) for surface-deficient O(s). Finally, transient simulations in practical catalytic channels revealed the interplay between kinetic and thermal effects. While at φ = 0.13 the H₂/CO reactive mixture exothermicity was insufficient to overtake the kinetic inhibition, at φ = 0.26 catalytic ignition could still be achieved at temperatures well-below the transition temperature. The effect of H₂:CO molar ratio on the light-off times was quite strong, suggesting care when designing syngas catalytic rectors with varying compositions.     

Experiments and modeling of propane combustion with vitiation

The chemical species composition of a vitiated oxidizer stream can significantly affect the combustion processes that occur in many propulsion and power generation systems. Experiments were performed to investigate the chemical kinetic effects of vitiation on ignition and flame propagation of hydrocarbon fuels using propane. Atmospheric-pressure flow reactor experiments were performed to investigate the effect of NO_x on propane ignition delay time at varying O₂ levels (14–21 mol%) and varying equivalence ratios (0.5–1.5) with reactor temperatures of 875 K and 917 K. Laminar flame speed measurements were obtained using a Bunsen burner facility to investigate the effect of CO₂ dilution on flame propagation at an inlet temperature of 650 K. Experimental and modeling results show that small amounts of NO can significantly reduce the ignition delay time of propane in the low- and intermediate-temperature regimes. For example, 755 ppmv NOx in the vitiated stream reduced the ignition delay time of a stoichiometric propane/air mixture by 75% at 875 K. Chemical kinetic modeling shows that H-atom abstraction reaction of the fuel molecule by NO₂ plays a critical role in promoting ignition in conjunction with reactions between NO and less reactive radicals such as HO₂ and CH₃O₂ at low and intermediate temperatures. Experimental results show that the presence of 10 mol% CO₂ in the vitiated air reduces the peak laminar flame speed by up to a factor of two. Chemical kinetic effects of CO₂ contribute to the reduction in flame speed by suppressing the formation of OH radicals in addition to the lower flame temperature caused by dilution. Overall, the detailed chemical kinetic mechanism developed in the current work predicts the chemical kinetic effects of vitiated species, namely NO_x and CO₂, on propane combustion reasonably well. Moreover, the reaction kinetic scheme also predicts the negative temperature coefficient (NTC) behavior of propane during low-temperature oxidation.     

Comparison of the performance of several recent hydrogen combustion mechanisms

A large set of experimental data was accumulated for hydrogen combustion: ignition measurements in shock tubes (770 data points in 53 datasets) and rapid compression machines (229/20), concentration–time profiles in flow reactors (389/17), outlet concentrations in jet-stirred reactors (152/9) and flame velocity measurements (631/73) covering wide ranges of temperature, pressure and equivalence ratio. The performance of 19 recently published hydrogen combustion mechanisms was tested against these experimental data, and the dependence of accuracy on the types of experiment and the experimental conditions was investigated. The best mechanism for the reproduction of ignition delay times and flame velocities is Kéromnès-2013, while jet-stirred reactor (JSR) experiments and flow reactor profiles are reproduced best by GRI3.0-1999 and Starik-2009, respectively. According to the reproduction of all experimental data, the Kéromnès-2013 mechanism is currently the best, but the mechanisms NUIG-NGM-2010, ÓConaire-2004, Konnov-2008 and Li-2007 have similarly good overall performances. Several clear trends were found when the performance of the best mechanisms was investigated in various categories of experimental data. Low-temperature ignition delay times measured in shock tubes (below 1000 K) and in RCMs (below 960 K) could not be well-predicted. The accuracy of the reproduction of an ignition delay time did not change significantly with pressure and equivalence ratio. Measured H₂ and O₂ concentrations in JSRs could be better reproduced than the corresponding H₂O profiles. Large differences were found between the mechanisms in their capability to predict flow reactor data. The reproduction of the measured laminar flame velocities improved with increasing pressure and total diluent concentration, and with decreasing equivalence ratio. Reproduction of the flame velocities measured using the flame cone method, the outwardly propagating spherical flame method, the counterflow twin-flame technique, and the heat flux burner method improved in this order. Flame cone method data were especially poorly reproduced. The investigation of the correlation of the simulation results revealed similarities of mechanisms that were published by the same research groups. Also, simulation results calculated by the best-performing mechanisms are more strongly correlated with each other than those of the weakly performing ones, indicating a convergence of mechanism development. An analysis of sensitivity coefficients was carried out to identify reactions and ranges of conditions that require more attention in future development of hydrogen combustion models. The influence of poorly reproduced experiments on the overall performance was also investigated.     

The effects of hydrogen addition on Fenimore NO formation in low-pressure,fuel-rich-premixed,burner-stabilized CH4/O2/N2 flames

We investigate the effects of hydrogen addition on Fenimore NO formation in fuel-rich, low-pressure burner-stabilized CH₄/O₂/N₂ flames. Towards this end, axial profiles of temperature and mole fractions of CH and NO are measured using laser-induced fluorescence (LIF). The experiments are performed at equivalent ratios of 1.3 and 1.5, using 0.25 mole fraction of hydrogen in the fuel, while varying the mass flux through the burner. The results are compared with those reported previously for burner-stabilized CH₄/O₂/N₂ flames. The increased burning velocity caused by hydrogen addition is seen to result in a lower flame temperature as compared to methane flame stabilized at the same mass flux. This increase in burner stabilization upon hydrogen addition results in significantly lower CH mole fractions at φ = 1.3, but appears to have little effect on the CH profile at φ = 1.5. In addition, the results show that not only the maximum flame temperature is reduced upon hydrogen addition, but the local gas temperature in the region of the CH profile is lowered as well. The measured NO mole fractions are seen to decrease substantially for both equivalence ratios. Analysis of the factors responsible for Fenimore NO formation shows the reduction in temperature in the flame front to be the major factor in the decrease in NO mole fraction, with a significant contribution from the decrease in CH mole fraction at φ = 1.3. At φ = 1.5, the results suggest that the lower flame temperature upon hydrogen addition further retards the conversion of residual fixed-nitrogen species to NO under these rich conditions as compared to the equivalent methane flames.     

Effects of hydrogen preconversion on the homogeneous ignition of fuel-lean H2/O2/N2/CO2 mixtures over platinum at moderate pressures

The impact of fractional hydrogen preconversion on the subsequent homogeneous ignition characteristics of fuel-lean (equivalence ratio φ = 0.30) H₂/O₂/N₂/CO₂ mixtures over platinum was investigated experimentally and numerically at pressures of 1, 5 and 8 bar. Experiments were performed in an optically accessible channel-flow reactor and involved Raman measurements of major species over the catalyst boundary layer and planar laser induced fluorescence (LIF) of the OH radical. Simulations were carried out with a 2-D elliptic code that included detailed hetero-/homogeneous chemistry. The predictions reproduced the LIF-measured onset of homogeneous ignition and the Raman-measured transport-limited catalytic hydrogen consumption. For 0% preconversion and wall temperatures in the range 900 K ? T_w ? 1100 K, homogeneous ignition was largely suppressed for p ? 5 bar due to the combined effects of intrinsic gas-phase hydrogen kinetics and the competition between the catalytic and gas-phase pathways for fuel consumption. A moderate increase of preconversion to 30% restored homogeneous combustion for p ? 5 bar, despite the fact that the water formed due to the upstream preconversion inhibited homogeneous ignition. The catalytically-produced water inhibited gas-phase combustion, particularly at higher pressures, and this kinetic inhibition was exacerbated by the diffusional imbalance of hydrogen that led to over-stoichiometric amounts of water in the near-wall hot ignitable regions. Radical adsorption/desorption reactions hindered the onset of homogeneous ignition and this effect was more pronounced at 1 bar. On the other hand, over the post-ignition reactor length, radical adsorption/desorption reactions significantly suppressed gas-phase combustion at 5 and 8 bar while their impact at 1 bar was weaker. By increasing hydrogen preconversion, the attained superadiabatic surface temperatures could be effectively suppressed. An inverse catalytically stabilized thermal combustion (CST) concept has been proposed, with gas-phase ignition achieved in an upstream porous burner via radiative and heat conduction feedback from a follow-up catalytic reactor. This arrangement moderated the superadiabatic surface temperatures and required modest or no preheat of the incoming mixture.     

Combustion characteristics of mixture of anode off gas and LNG in reformer

Fuel processing system which converts hydrocarbon fuel into hydrogen rich gas (by stream reforming, partial oxidation, auto-thermal reforming) needs high temperature environment (600-1000 °C). Generally, anode off gas or mixture of anode off gas and LNG are used as input gas for a fuel reformer. In order to constitute efficient and low emission burner system for fuel reformer, it is necessary to elucidate the combustion and emission characteristics of fuel reformer burner. In this study, lean flat flame using the ceramic porous burner was analyzed numerically and experimentally. Burning velocity of anode off gas calculated by CHEMKIN simulation was 51.8 cm, which was faster than that of LNG having 40.63 cm/s at the stoichiometric ratio because of high composition of hydrogen in anode off gas. As composition of LNG in mixture of anode off gas + LNG is increased, the burning velocity decreases and in the other hand the adiabatic temperature increases. CO, NO_x were measured below 50 ppm in operating load range of the reformer. Blue flame pattern was found as stable flame region for design of fuel reformer and anode off gas flame was maintained in blue flame pattern at equivalence ratio 0.55-0.62 under 1-5 kW power range.     

Experimental and kinetic study on ignition delay times of DME/H2/O2/Ar mixtures

Ignition delay times of dimethyl ether (DME)/hydrogen/oxygen/argon mixtures (hydrogen blending ratio ranging from 0% to 100%) were measured behind reflected shock waves at pressures of 1.2–10 atm, temperature range of 900–1700 K, and for the lean (? = 0.5), stoichiometric (? = 1.0) and rich (? = 2.0) mixtures. For more understanding the effect of initial parameters, correlations of ignition delay times for the lean mixtures were obtained on the basis of the measured data (X_H2 ? 95%) through multiple linear regression. Ignition delay times of the DME/H₂ mixtures demonstrate three ignition regimes. For X_H2 ? 80%, the ignition is dominated by the DME chemistry and ignition delay times show a typical Arrhenius dependence on temperature and pressure. For 80% ? X_H2 ? 98%, the ignition is dominated by the combined chemistries of DME and hydrogen, and ignition delay times at higher pressures give higher ignition activation energy. However, for X_H2 ? 98%, the transition in activation energy for the mixture was found as decreasing the temperature, indicating that the ignition is dominated by the hydrogen chemistry. Simulations were made using two available models and different results were presented. Thus, sensitivity analysis was performed to illustrate the causes of different simulation results of the two models. Subsequently, chemically interpreting on the effect of hydrogen blending ratio on ignition delay times was made using small radical mole fraction and reaction pathway analysis. Finally, high-pressure simulations were performed, serving as a starting point for the future work.     

An experimental and kinetic modeling study of n-propanol and i-propanol ignition at high temperatures

Ignition delay times of n- and i-propanol mixtures in argon-diluted oxygen were measured behind reflected shocks. Experimental conditions are: temperatures from 1100 and 1500 K, pressures from 1.2 to 16.0 atm, fuel concentrations of 0.5%, 0.75%, 1.0%, and equivalence ratios of 0.5, 1.0 and 2.0. A detailed kinetic model consisting of 238 species and 1448 reactions was developed to simulate the ignition of the two propanol isomers, with the computed ignition delay times agreeing well with the present measured results as well as the literature data at other conditions. Further validation of the kinetic mechanism was conducted by comparing the simulated results with measured JSR data and laminar flame speeds, and reasonable agreements were achieved for all test conditions. Moreover, reaction pathway analysis indicated that n-propanol mainly produces ethenol, ethene and propene, while i-propanol primarily produces acetone and propene. Finally, sensitivity analysis demonstrated that some fuel-species reactions can be found in the most important reactions for both propanols, and these are mainly the H-abstraction reactions.     

An experimental and kinetic modeling study of the autoignition of α-methylnaphthalene/air and α-methylnaphthalene/n-decane/air mixtures at elevated pressures

The autoignition of α-methylnaphthalene (AMN), the bicyclic aromatic reference compound for the cetane number (CN), and AMN/n-decane blends, potential diesel surrogate mixtures, was studied at elevated pressures for fuel/air mixtures in a heated high-pressure shock tube. Additionally, a comprehensive kinetic mechanism was developed to describe the oxidation of AMN and AMN/n-decane blends. Ignition delay times were measured in reflected shock experiments for Φ = 0.5, 1.0, and 1.5 AMN/air mixtures (CN = 0) for 1032-1445 K and 8-45 bar and for Φ = 1.0 30%-molar AMN/70%-molar n-decane/air (CN = 58) and 70%-molar AMN/30%-molar n-decane/air mixtures (CN = 28) for 848-1349 K and 14-62 bar. Kinetic simulations, based on the comprehensive AMN/n-decane mechanism, are in good agreement with measured ignition times, illustrating the emerging capability of comprehensive mechanisms for describing high molecular weight transportation fuels. Sensitivity and reaction flux analysis indicate the importance of reactions involving resonance stabilized phenylbenzyl radicals, the formation of which by H-atom abstractions with OH radicals has an important inhibiting effect on ignition.     

Numerical study of the effects of non-equilibrium plasma on the ignition delay of a methane–air mixture using detailed ion chemical kinetics

Researches on non-equilibrium plasmas in ignition and combustion processes have drawn attention of many scientists, because a non-equilibrium plasma-assisted approach provides a useful method to ignite a combustible mixture and stabilize the combustion process. The ignition delay times of methane–air mixtures have been investigated experimentally and numerically; however, the influence of non-equilibrium plasma on the ignition of argon-free methane–air mixtures has seen relatively little discussion. Here, we investigate the ignition delay time of methane–air mixtures via numerical analysis using detailed chemical kinetics. Discharge process and following ignition process are simulated separately, because of significant differences in their time scales and mechanisms. Data on the concentration of atoms and radicals produced in the discharge processes were used as the initial input data to determine the subsequent ignition process because they play an important role in the subsequent ignition process. We focus on the effects of the strength of the reduced electric field, the discharge duration, and the initial temperature on the ignition delay time for zero-dimensional and axisymmetric one-dimensional models. The simulation results showed that the reduced electric field was important in promoting chemical reactions for both the one-dimensional model and the zero-dimensional model; for a constant reduced electric field, longer discharge durations provided more energy to excite the nitrogen, leading to a larger mole fraction of excited nitrogen species during discharge; the gaps between ignition delay times for E/N = 0 and E/N ? 50 Td were very small at high initial temperatures; however they became very large at low initial temperatures.     

In situ plasma activated low temperature chemistry and the S-curve transition in DME/oxygen/helium mixture

The effect of non-equilibrium plasma activated low temperature chemistry (PA-LTC) on the ignition and extinction of Dimethyl Ether (DME)/O₂/He diffusion flames has been investigated experimentally in a counterflow burner with in situ nanosecond pulsed discharge at 72 Torr. A uniform discharge is generated between the burner nozzles by placing porous metal electrodes at the nozzle exits. The ignition and extinction characteristics of DME/O₂/He are studied by employing OH and CH₂O Planar Laser Induced Fluorescence (PLIF) techniques at constant strain rates and O₂ mole fractions on the oxidizer side with varying the DME mole fractions. Contrary to the conventional understanding, strong low temperature reactivity during ignition process is observed for DME with non-equilibrium plasma activation even at 72 Torr and flow residence time of a few milliseconds. The OH PLIF shows strong OH signal at and after ignition, whereas extremely low OH signal before ignition. However, the CH₂O PLIF experiments demonstrate that, with the increase of DME mole fraction on the fuel side, the CH₂O PLIF signal intensity increases significantly before ignition and decreased rapidly after ignition. The low OH number density and high CH₂O number density before DME ignition clearly demonstrates the existence of PA-LTC at low pressure. Moreover, at higher O₂ mole fraction and discharge repetition frequency, the in situ discharge significantly modifies the characteristics of ignition and extinction, thus creating a new monotonically and fully stretched ignition S-curve without an extinction limit. Compared to our previous study of methane, the existence of strong low temperature reactivity in DME oxidation makes ignitions occur at much lower fuel mole fractions, thus accelerating the transition of ignition curve from conventional S-curve to the fully stretched S-curve. The transition from the conventional S-curve to the new stretched ignition curve at high plasma repetition rate indicates that the plasma could dramatically change the chemical kinetic pathways of DME oxidation by activating the low temperature chemistry even at low pressure. The chemical kinetic model for the plasma–flame interaction has been also developed based on the assumption of constant electric field strength in the bulk plasma region. Both experiments and modeling results reveal that the PA-LTC has a much shorter timescale comparing with that of thermally activated low temperature chemistry owing to the rapid radical production by plasma. The reaction pathways analysis shows that atomic O generated by the discharge is critical to controlling the population of radical pool.     

Gas phase chemistry in catalytic combustion of methane/air mixtures over platinum at pressures of 1 to 16 bar

The gas-phase combustion of fuel-lean methane/air premixtures over platinum was investigated experimentally and numerically in a laminar channel-flow catalytic reactor at pressures 1 bar?p?16 bar. In situ, spatially resolved one-dimensional Raman and planar laser induced fluorescence (LIF) measurements over the catalyst boundary layer were used to assess the concentrations of major species and of the OH radical, respectively. Comparisons between measured and predicted homogeneous (gaseous) ignition distances have led to the assessment of the validity of various elementary gas-phase reaction mechanisms. At low temperatures (900 K?T?1400 K) and fuel-to-air equivalence ratios (0.05?φ?0.50) typical to catalytic combustion systems, there were substantial differences in the performance of the gaseous reaction mechanisms originating from the relative contribution of the low- and the high-temperature oxidation routes of methane. Sensitivity analysis has identified the significance of the chain-branching reaction CHO + M = CO + H + M on homogeneous ignition, particularly at lower pressures. It was additionally shown that C2 chemistry could not be neglected even at the very fuel-lean conditions pertinent to catalytic combustion systems. A gas-phase reaction mechanism validated at 6 bar?p?16 bar has been extended to 1 bar?p?16 bar, thus encompassing all catalytic combustion applications. A reduced gas-phase mechanism was further derived, which when used in conjunction with a reduced heterogeneous (catalytic) scheme reproduced the key catalytic and gaseous combustion characteristics of the full hetero/homogeneous reaction schemes.     

Ignition properties of methane/hydrogen mixtures in a rapid compression machine

We investigate changes in the combustion behavior of methane, the primary component of natural gas, upon hydrogen addition by characterizing the autoignition behavior of methane/hydrogen mixtures in a rapid compression machine (RCM). Ignition delay times were measured under stoichiometric conditions at pressures between 15 and 70 bar, and temperatures between 950 and 1060 K; the hydrogen fraction in the fuel varied between 0 and 1. The ignition delay times in methane/hydrogen mixtures are well correlated with the ignition delay times of the pure fuels by using a simple mixing relation reported in the literature. Simulations of the ignition delay times using various chemical mechanism are also reported. The mechanism given by Petersen et al. shows excellent agreement with the measurements for all mixtures studied. Initial results on fuel–lean mixtures show a modest effect of equivalence ratio on the delay times.     

Ignition properties of n-butane and iso-butane in a rapid compression machine

Autoignition delay times of n-butane and iso-butane have been measured in a Rapid Compression Machine in the temperature range 660–1010 K, at pressures varying from 14 to 36 bar and at equivalence ratios φ = 1.0 and φ = 0.5. Both butane isomers exhibit a negative-temperature-coefficient (NTC) region and, at low temperatures, two-stage ignition. At temperatures below ～900 K, the delay times for iso-butane are longer than those for the normal isomer, while above this temperature both butanes give essentially the same results. At temperatures above ～720 K the delay times of the lean mixtures are twice those for stoichiometric compositions; at T < 720 K, the equivalence ratio is seen to have little influence on the ignition behavior. Increasing the pressure from 15 bar to 30 bar decreases the amplitude of the NTC region, and reduces the ignition delay time for both isomers by roughly a factor of 3. In the region in which two-stage ignition is observed, 680–825 K, the duration of the first ignition stage decreases sharply in the range 680–770 K, but is essentially flat above 770 K. Good quantitative agreement is found between the measurements and calculations for n-butane using a comprehensive model for butane ignition, including both delay times in the two-stage region, with substantial differences being observed for iso-butane, particularly in the NTC region.