Simulation of transient turbulent methane jet ignition and combustion under engine-relevant conditions using conditional source-term estimation with detailed chemistry

The ignition and combustion processes of transient turbulent methane jets under high-pressure and moderate temperature conditions were simulated using a computationally efficient combustion model. Closure for the mean chemical source-terms was obtained with Conditional Source-term Estimation (CSE) using first conditional moment closure in conjunction with a detailed chemical kinetic mechanism, which was reduced to a Trajectory-Generated Low-Dimensional Manifold (TGLDM). The accuracy of the manifold was first validated against the direct integral method by comparing the predicted reactive scalar profiles in three methane–air reaction systems: a laminar premixed flame, a laminar flamelet and a perfectly stirred reactor. Detailed CFD simulations incorporating the CSE-TGLDM model were able to provide reasonably good predictions of the experimental ignition delay and initial ignition kernel locations of the methane jets reported in the literature with relatively low computational cost. Nitrogen oxides formed in the methane jet flame were found to be underpredicted by the model by as much as a factor of 2. The discrepancy may be attributable to the inability of the simulation to account for the effects of the rarefaction wave in the shock-tube experiments.     

Oxidation of H2/CO2 mixtures and effect of hydrogen initial concentration on the combustion of CH4 and CH4/CO2 mixtures: Experiments and modeling

An experimental investigation of the oxidation of hydrogen diluted by nitrogen in presence of CO₂ was performed in a fused silica jet-stirred reactor (JSR) over the temperature range 800-1050 K, from fuel-lean to fuel-rich conditions and at atmospheric pressure. The mean residence time was kept constant in the experiments: 120 ms at 1 atm and 250 ms at 10 atm. The effect of variable initial concentrations of hydrogen on the combustion of methane and methane/carbon dioxide mixtures diluted by nitrogen was also experimentally studied. Concentration profiles for O₂, H₂, H₂O, CO, CO₂, CH₂O, CH₄, C₂H₆, C₂H₄, and C₂H₂ were measured by sonic probe sampling followed by chemical analyses (FT-IR, gas chromatography). A detailed chemical kinetic modeling of the present experiments and of the literature data (flame speed and ignition delays) was performed using a recently proposed kinetic scheme showing good agreement between the data and this modeling, and providing further validation of the kinetic model (128 species and 924 reversible reactions). Sensitivity and reaction paths analyses were used to delineate the important reactions influencing the kinetic of oxidation of the fuels in absence and in presence of additives (CO₂ and H₂). The kinetic reaction scheme proposed helps understanding the inhibiting effect of CO₂ on the oxidation of hydrogen and methane and should be useful for gas turbine modeling.     

Experimental and semi-detailed kinetic modeling study of decalin oxidation and pyrolysis over a wide range of conditions

Decalin is the simplest polycyclic alkane (polynaphtenic hydrocarbon) found in liquid fuels (jet fuels, Diesel). In order to better understand the combustion characteristics of decalin, this study provides new experimental data for its oxidation in a jet-stirred reactor. For the first time, stable species concentration profiles were measured in a jet-stirred reactor at a constant mean residence time of 0.1 s and 0.5 s at respectively 1 and 10 atm, over a range of equivalence ratios (? = 0.5–1.5) and temperatures (750–1350 K). The oxidation of decalin under these experimental conditions was modeled using a semi-detailed chemical kinetic reaction mechanism (11,000 reactions involving 360 species) derived from a previously proposed scheme for the ignition of the same fuel in a shock-tube. The proposed mechanism that includes both low- and high-temperature chemistry shows reasonably good agreement with the present experimental data set. It can also represent well decalin pyrolysis and oxidation data available in the literature. Reaction path analyses and sensitivity analyses were conducted to interpret the results.     

Isomer-specific speciation behaviors probed from premixed flames fueled by acetone and propanal

Chemical structures of low-pressure premixed flames respectively fueled by two C₃ carbonyl isomers, acetone and propanal, at different equivalence ratios (1.0 and 1.5) were experimentally investigated in this work. Detailed speciation information was obtained by employing molecular-beam mass spectrometry with tunable synchrotron photoionization. A detailed kinetic model including the chemistry of acetone and propanal was developed and tested with the current flame speciation measurements. By combining experimental observations and modeling interpretations, comparisons were made regarding fuel-specific reaction pathways and the resulting different species pools. Some fuel-specific intermediates were detected and quantified in this work, such as ketene in acetone flames and methylketene in propanal flames. Particularly, the quantitative speciation measurements of ketene, an important primary intermediate of acetone, were satisfactorily predicted by the current model, which included an updated ketene sub-mechanism. Major efforts in this work were devoted to gaining some insights into the effects of the carbonyl position in fuel molecules on the speciation behaviors under premixed flame conditions. Carbonyl functionalities in the two C₃ carbonyl compounds are tightly bonded and preferably preserved in CO. Due to the different position of the CO bond in the two isomers, the oxidation of propanal leads to abundant ethyl as a chain carrier, while the acetone consumption easily results in a significant amount of methyl, an inhibitor on the fuel reactivity. As a result, higher reactivity of propanal was observed. More importantly, the different fuel consumption patterns also influence the speciation behaviors. Specifically, the larger concentration of benzene precursors such as allyl, was observed in the propanal flames. Besides, typical oxygenated emissions formaldehyde and acetaldehyde had more remarkable concentrations in acetone and propanal flames, respectively.     

汽油低温氧化特性及替代物模型研究

本文利用射流搅拌反应器对国产汽油以及汽油替代物模型中的重要组分异辛烷开展了低温氧化特性实验研究,并通过对国产实际汽油进行组分测量,获得了不同组分类型的摩尔分数.结合实际汽油的组分结构和物化特性,选取了五种代表性组分来构建合适的汽油替代物模型并发展了动力学机理,利用该机理分别对单组分燃料和实际汽油的低温氧化特性进行了模拟...     

Ab initio evaluation of primary cyclo-hexane oxidation reaction rates

Naphthenes are chemical species that are always present in liquid hydrocarbon fuels and their pyrolysis and oxidation can play an important role in real liquid fuel combustion. In spite of its practical relevance, the chemical kinetics of naphthene pyrolysis and oxidation is not yet thoroughly investigated and there is not a general agreement on the role and rate of several elementary reactions involved. In this paper, the kinetics of the pyrolysis and oxidation of a simple naphthene, namely cyclo-hexane, has been investigated through detailed kinetic modeling. Ab initio calculations were performed to estimate the kinetic parameters of some primary reactions following the oxygen attack to the cyclo-hexane radical. In fact, due to the complex behavior induced by the ring structure of cyclo-hexane, such data were difficult to determine through thermo-chemical methods. Density functional theory (B3LYP/6-31g(d, p)) was adopted to determine structure and vibrational frequencies of transition states and reaction intermediates, while energies were evaluated using the G2MP2 approach. The kinetic parameters of the investigated primary reactions were then introduced in a general detailed kinetic model consisting of elementary reactions whose kinetic constants were taken from the literature. The so obtained kinetic model was used to simulate ignition delay times and species concentrations measured in various experiments reported in the literature. The agreement between experimental data and theoretical predictions shows the validity of the chosen approach and supports the correctness of the proposed kinetic model.     

Modelling of aromatics and soot formation from large fuel molecules

There is a need for prediction models of soot particles and polycyclic aromatic hydrocarbons (PAHs) formation in parametric conditions prevailing in automotive engines: large fuel molecules and high pressure. A detailed kinetic mechanism able to predict the formation of benzene and PAHs up to four rings from C₂ fuels, recently complemented by consumption reactions of decane, was extended in this work to heptane and iso-octane oxidation. Species concentrations measured in rich, premixed flat flames and in a jet stirred reactor (JSR) were used to check the ability of the mechanism to accurately predict the formation of C₂ and C₃ intermediates and benzene at pressures ranging from 0.1 to 2.0 MPa. Pathways analyses show that propargyl recombination is the only significant route to benzene in rich heptane and iso-octane flames. When included as the first step of a soot particle formation model, the gas-phase kinetic mechanism predicts very accurately the final soot volume fraction measured in a rich decane flame at 0.1 MPa and in rich ethylene flames at 1.0 and 2.0 MPa.     

On the combustion and sooting behavior of standard and hydro-treated jet fuels: An experimental and modeling study on the compositional effects

In a context of growing level of environmental awareness, emission from aviation are the subject of increasing scrutiny. This situation poses important challenges because, due to safety, practical and economic factors, aero-transportation technologies are not likely to undergo rapid paradigm shifts. An area where important innovations are being introduced is fuel technology: fuels from alternative processes, potentially from renewable sources, offer the opportunity of limiting the carbon footprint of transportation, moreover, a better control on fuel quality can contribute to reducing emissions.Hydro-treating of oil based fuels can reduce their sulfur and aromatic content promoting a cleaner combustion. In order to better understand the impact of hydro-treating on emissions of PAHs and soot from jet fuels, new speciation data covering oxidation intermediates and soot precursors were measured in a flow reactor for a standard jet fuel and its hydro-treated counterpart. Using a detailed kinetic mechanism and complex surrogate blends mimicking the composition of the real fuels, the speciation data from the flow reactor were simulated. Additionally, soot formation trends were calculated and compared with previously published data. Using the kinetic model, which is based on mechanistic principles, it was possible to separate the relative contribution of different processes and, for the fuel blends of interest, the role played by specific components in the PAHs and soot formation. The results obtained provide useful information towards more effective fuel formulation strategies and fuel blends modeling.     

Kinetics of ultrasound-assisted lipase-catalyzed glycerolysis of olive oil in solvent-free system

This work reports experimental kinetic data of solvent-free glycerolysis of olive oil using a commercial immobilized lipase (Novozym 435) under the influence of ultrasound irradiation. The experiments were performed in a mechanically stirred reactor under ultrasound irradiation, evaluating the effects of temperature (50-70 °C), enzyme concentration (2.5-10 wt%) and glycerol to oil molar ratio (0.8:1-3:1). Results show that ultrasound-assisted lipase-catalyzed glycerolysis might be a potential alternative route to conventional methods, as high contents of reaction products, especially monoglycerides, were achieved at mild irradiation power supply (∼130 W) and temperature, in a relatively short reaction time (2 h) and low enzyme content (7.5 wt%). To completeness, two simplified kinetic modeling approaches, based on the ordered-sequential bi bi mechanism and reaction stoichiometry, were employed to represent the experimental data, thus allowing a better understanding of the reaction kinetics.     

环己烷在射流搅拌反应器中的低温氧化动力学初探:实验和动力学模型研究

We report the investigation on the low-temperature oxidation of cyclohexane in a jet-stirred reactor over 500-742 K. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used for identifying and quantifying the oxidation species. Major products, cyclic olefins, and oxygenated products including reactive hydroperoxides and high oxygen compounds were detected. Compared with n-alkanes, a narrow low-temperature window (~80 K) was observed in the low-temperature oxidation of cyclohexane. Besides, a kinetic model for cyclohexane oxidation was developed based on the CNRS model[Combust. Flame 160, 2319 (2013)], which can better capture the experimental results than previous models. Based on the modeling analysis, the 1,5-H shift dominates the crucial isomerization steps of the first and second O₂ addition products in the low-temperature chain branching process of cyclohexane. The negative temperature coefficient behavior of cyclohexane oxidation results from the reduced chain branching due to the competition from chain inhibition and propagation reactions, i.e. the reaction between cyclohexyl radical and O₂ and the decomposition of cyclohexylperoxy radical, both producing cyclohexene and HO₂ radical, as well as the decomposition of cyclohexylhydroperoxy radical producing hex-5-en-1-al and OH radical.     

正丁烷燃料中低温氧化的骨架反应机理

本文基于Healy等人建立的正丁烷详细反应机理(230个组分,1328个反应),采用直接关系图法,反应路径分析以及敏感性分析相结合的方法,构建了一个包含83个组分,397个反应的中低温反应动力学骨架模型。路径分析发现,在低温反应中,正丁烷氧化着火主要受链传播反应中的放热循环控制。而在中温反应中,正丁烷及其下游产物正丁基的裂解反应变得重要,大分子裂解后的小分子氧化加快反应进程。本文骨架模型在温度范围550~1050 K、压力范围0.1~3MPa、当量比范围0.5~2.0条件下对着火延迟时间、层流火焰速度、温度以及重要组分浓度分布的预测均与详细机理保持很好的一致性,同时与文献中快压机、定容燃烧弹和搅拌射流反应器的实验结果也吻合较好。     

New insights into the oxidation chemistry of pyrrole,an N-containing biomass tar component

Pyrrole, the smallest molecule with a nitrogen atom in the heterocycle ring, is an important tar component from coal and nitrogen-rich biomass devolatilization. Understanding the combustion chemistry of pyrrole can help to elucidate the pollutant formation chemistry from fuel nitrogen, thus enabling cleaner biomass energy utilization technologies. Experimental measurements were performed in a jet stirred reactor coupled with time of flight molecular beam mass spectrometry using synchrotron vacuum ultraviolet beam as photon ionization source, and gas chromatography-mass spectrometry to provide comprehensive measurements of 31 species including nine C₄ and C₅ N-containing compounds. Based on the evidence from the experiments and aiming to improve the kinetic model performance, possible formation routes are proposed with OH addition as the entrance reaction. Reaction rate coefficients for the OH addition channel as well as those for key H-atom abstraction reactions (H, OH, CH_3, and HO₂) were calculated by quantum chemical methods and updated in the model. The updated model can qualitatively predict the identified C₄ N-containing species and perform reasonably well for a large set of experimental data considered for validation, overall improving the performance of the previous model. The influence of the investigated reactions on the predictions of fuel reactivity and pollutant formation motivates further investigations of N-containing fuel chemistry.     

Large-eddy simulation of MILD combustion using partially stirred reactor approach

Subgrid-scale (SGS) parameterization and method for calculating filtered reaction rate are critical components of an accurate large-eddy simulation (LES) of turbulent flames. In this study, we integrate gradient-type structural SGS models with a partially stirred reactor approach by using detailed chemical kinetics to simulate a turbulent methane/hydrogen jet flame under moderate or intense low-oxygen dilution (MILD) conditions. The study examines two oxygen dilution levels. The framework is assessed through a systematic and comprehensive comparison of temperature, and mass fractions of major and minor species with experimental data and other reference simulation results. Overall, the statistics of the combustion field show excellent agreement with measurements at different axial locations, and a significant improvement compared to some previous simulations. It suggests that the proposed nonlinear LES framework is able to accurately model MILD combustion with reasonable computational cost.     

Experimental study of incomplete oxidation of methane in a ring channel

Chemical transformations at incomplete methane oxidation in the air medium were studied experimentally at reaction activation on the wall of an annular microchannel. Methane was oxidized incompletely on a rhodium catalyst deposited on an inner wall of the channel. Concentrations of the products of chemical transformations were measured in the outlet gas mixture for different reactor temperatures and stay times. We have determined the range of channel wall temperatures and stay times of the mixture corresponding to an increase in the portion of hydrogen and carbon dioxide indicating transition from predominant methane combustion to cascade chemical reactions with activation of steam and carbon dioxide methane conversions. It is shown that the kinetic model of chemical transformations of methane in the air medium depends significantly on the temperature of channel walls and stay time of the mixture. The effect of outer diffusion deceleration on the rate of chemical transformations at incomplete methane oxidation under the strained conditions is determined. The work was financially supported by the Russian Foundation for Basic Research (Grant No. 05-08-65526).     

Experimental and kinetic modeling studies of low- to moderate-temperature oxidation of 2-furfuryl alcohol in a jet-stirred reactor

2-Furfuryl alcohol, a promising platform chemical, and alternative fuels or additives, is produced from the hydrogenation of furfural. However, the low- to moderate-temperature oxidation study of 2-furfuryl alcohol is scarce up to now. The present study first performed the oxidation experiments in a jet-stirred reactor at the equivalence ratios of 0.5, 1.0, and 2.0. The oxidation species were identified and measured by the synchrotron vacuum ultraviolet photoionization mass spectrometry. No negative-temperature-coefficient behavior was observed. A detailed kinetic model was developed to better understand the consumption of 2-furfuryl alcohol and the formation of oxidation products. The present model can well reproduce the experimental results. Based on the rate of production analysis, H-abstraction reactions on hydroxymethyl group forming hydroxyl(2-furyl)methyl radical are the dominant pathways. Besides, the formation and consumption of main products, such as furfural, furan, acrolein, acetaldehyde, etc., are also discussed. Fuel radicals control the formation of furfural. Furan has multiple formation sources, such as the H-addition reactions on 2-furfuryl alcohol, furfural, and 2-hydroxyfuran. Acrolein is produced by the H-addition reaction on 2-hydroxyfuran and OH-addition reactions on furan. Also, the present work compares and discusses peak concentration for major oxidation products of 2-furfuryl alcohol and 2-methyl furan under the same simulated condition. 2-Furfuryl alcohol oxidation produces a higher concentration of furfural, furan, and acrolein than that of 2-methyl furan. OH on hydroxymethyl group promotes the H-abstraction reactions on the side chain and inhibits the OH-addition reaction on the furan ring.     

Acetaldehyde oxidation at elevated pressure

A detailed chemical kinetic model for oxidation of CH₃CHO at intermediate to high temperature and elevated pressure has been developed and evaluated by comparing predictions to novel high-pressure flow reactor experiments as well as shock tube ignition delay measurements and jet-stirred reactor data from literature. The flow reactor experiments were conducted with a slightly lean CH₃CHO/O₂ mixture highly diluted in N₂ at 600–900 K and pressures of 25 and 100 bar. At the highest pressure, the oxidation of CH₃CHO was in the NTC regime, controlled to a large extent by the thermal stability and reactions of peroxide species such as HO₂, CH₃OO, and CH₃C(O)OO. Model predictions were generally in good agreement with the experimental data, even though the predicted temperature for onset of reaction was overpredicted at 100 bar. This discrepancy was attributed mainly to uncertainties in the CH₃C(O)OO reaction subset. Predictions of ignition delays in shock tubes and species profiles in JSR experiments were also satisfactory. At temperatures above the NTC regime, acetaldehyde ignition and oxidation is affected mainly by the competition between dissociation of CH₃CHO and reaction with the radical pool, and by reactions in the methane subset.     

A chemical kinetic study of n-butanol oxidation at elevated pressure in a jet stirred reactor

Biofuels are attractive alternatives to petroleum derived transportation fuels. n-Butanol, or biobutanol, is one alternative biofuel that can replace gasoline and diesel in transportation applications. Similar to ethanol, n-butanol can be produced via the fermentation of sugars, starches, and lignocelluloses obtained from agricultural feedstocks. n-Butanol has several advantages over ethanol, but the detailed combustion characteristics are not well understood. This paper studies the oxidation of n-butanol in a jet stirred reactor at 10 atm and a range of equivalence ratios. The profiles for CO, CO₂, H₂O, H₂, C₁-C₄ hydrocarbons, and C₁-C₄ oxygenated compounds are presented herein. High levels of carbon monoxide, carbon dioxide, water, hydrogen, methane, formaldehyde, ethylene, and propene are detected. The experimental data are used to validate a novel detailed chemical kinetic mechanism for n-butanol oxidation. The proposed mechanism well predicts the concentration of major product species at all temperatures and equivalence ratios studied. Insights into the prediction of other species are presented herein. The proposed mechanism indicates that n-butanol consumption is dominated by H-atom abstraction from the α, β, and γ carbon atoms. A sensitivity analysis is also presented to show the effects of reaction kinetics on the concentration of several poorly predicted species.     

Experimental and modelling study of gasoline surrogate mixtures oxidation in jet stirred reactor and shock tube

The oxidation of several mixtures of surrogate for gasoline was studied using a jet stirred reactor and a shock tube. One representative of each classes constituting gasoline was selected: iso-octane, toluene, 1-hexene and ethyl tert-butyl ether (ETBE). The experiments were carried out in the 800-1880 K temperature range, for two different initial pressures (0.2 and 1 MPa), with an initial fuel molar fraction of 0.001. The equivalence ratio varied from 0.5 to 1.5. Each hydrocarbon sub-mechanism was validated using shock tube data. The full mechanism describing the surrogate fuel oxidation is constituted of the sub-mechanisms for each fuel components and by adding interaction reactions between different hydrocarbon fragments. Good agreement between the experimental results and the computations was observed under JSR and shock tube conditions.     

A study of the Soret effect in laser-induced chemical vapor deposition

This paper presents an investigation of the modeling of the process of pyrolytic laser-induced chemical vapor deposition (LCVD) applied to study the Soret effect. LCVD is a thermally activated process characterized by strongly coupled mass and energy transport phenomena, together with chemical reactions, which are difficult to investigate experimentally. A physical and numerical model based on a commercial computational fluid dynamics package is developed and used to simulate a reactor operating at conditions of room temperature and pressure. The proposed numerical methodology will allow us to assess and analyze the effect of various factors controlling the process, and in particular the Soret effect. This numerical model is validated by comparison with the measured growth rate of the fiber. While several studies have proposed simulations of the LCVD process, this is among the first attempts at including the Soret effect in the numerical modeling at the micro-scale level. It is expected that the fundamental insights thus obtained will guide experimental investigations which can be applied to establish reactor design and process control guidelines.