Low-temperature Reaction Pathways Research Articles

Real Gas Effects in High-Pressure Ignition of n-Dodecane/Air Mixtures.

This work studies the real gas effects on the autoignition of hydrocarbon fuels under high pressures, using normal dodecane (n-dodecane) as the representative fuel and the Redlich-Kwong equation of state (EoS) as the real gas description. It is demonstrated that the real gas description yields a shorter ignition delay time (IDT) compared with the ideal gas description, especially in low-temperature regimes which could encompass the negative temperature coefficient (NTC) phenomena and has a stronger dependence on the molecular volume than the attractive potential. The study further shows that high pressure facilitates low-temperature reaction pathways, where the compressibility factors of key reactants contribute to real gas effects. Moreover, the results suggest that accounting for real gas behavior leads to an increase in the formation of polycyclic aromatic hydrocarbons (PAHs), which, in turn, promotes soot generation.

Understanding the Nonlinear Reactivity Promoting Effect of n-Heptane Addition on the Binary Mixture From Low to Intermediate Temperature: A Case of Methane/n-Heptane Mixtures

Abstract To understand the effect of n-heptane (NC7H16) addition on the auto-ignition of methane (CH4) at low to intermediate temperatures, the ignition delay times (IDTs) of stoichiometric CH4/NC7H16 blends with varying NC7H16 concentrations were measured at temperatures from 600 to 1000 K, pressures of 20 and 40 bar. Detailed chemical kinetic mechanisms were validated against the newly measured IDTs. Adding NC7H16 in the binary mixture shows a nonlinear promoting effect on the IDTs: micro-addition of NC7H16 can significantly reduce the IDTs of the binary mixture when the NC7H16 is lower than 20%. However, the decrease of the IDTs becomes much slower when further increasing the NC7H16 addition. Affected by the negative temperature coefficient (NTC) behavior of NC7H16, this nonlinear effect is particularly notable at around 795 K, the low boundary of the NTC region. To reveal the nonlinear reactivity-promoting effect of NC7H16 addition on the binary mixture, reaction flux, ignition sensitivity, rate of production of the key radicals along heat production analyses were conducted. Apart from contributing more O˙H production through the low-temperature chain-branching reaction pathways of NC7H16, adding NC7H16 also promotes the pre-ignition heat release of the binary mixture. The heat release raises the system temperature and further promotes the mixture ignition, enhancing the nonlinear effect at low temperatures.

Low-temperature oxidation of n-butanol in a jet-stirred reactor: Detailed species measurements and modeling studies

n-Butanol is a new generation of liquid biofuel. A detailed understanding of the low-temperature combustion chemistry of n-butanol is instructive for its practical application in advanced low-temperature engines. The high-temperature combustion chemistry of n-butanol has been extensively studied, but its low-temperature combustion chemistry remains underexplored, especially the detailed species distribution. In this work, ozone addition was used to enhance the low-temperature oxidation reaction activity to achieve the oxidation of n-butanol in a jet-stirred reactor (JSR) at atmospheric pressure and in the temperature range of 400–800 K. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used for detailed measurements of species distribution. The identification of some intermediate species contributed to elucidating and verifying the low-temperature oxidation reaction pathways of n-butanol, such as butanal, C4 hydroxyalkyl hydroperoxides, C4 hydroxyl cyclic ether, and C4 keto-hydroperoxide. Furthermore, the formation of several elusive intermediates was also identified (e.g., C2H4O2, C3H6O2, C4H6On (n = 1∼3), and C4H8O2), highlighting the necessity of further kinetic studies on the low-temperature oxidation of n-butanol. Based on detailed experimental measurements, the n-butanol model was further developed to improve the model predictions by updating the rate constants for the H-atom abstraction reactions of n-butanol by the ȮH radical, adding the detailed butanal sub-mechanism, etc. Especially, the premature formation of CO2 in the present experiment was satisfactorily explained and predicted by the addition of a generally accepted reaction class (i.e., RȮ2 + RȮ2 = RȮ + RȮ + O2 of butanal). Overall, this updated model satisfactorily predicts both the ignition delay time reported in the literature and the species distribution obtained in this work.

Low-temperature oxidation characteristics and reaction pathways of crude oil within tight shale during air injection

The investigation of low-temperature oxidation (LTO) of crude oil within tight shale holds significant importance due to its implications for subsequent oxidation reactions and enhanced oil recovery in the process of air injection. In this study, the tight shale sample underwent oxidation at various LTO temperatures, followed by an analysis of the resulting gas composition. Furthermore, the oxidized oil was separated from the tight shale and subjected to characterization using electron paramagnetic resonance, nuclear magnetic resonance, and negative ion electrospray Fourier transform-ion cyclotron resonance mass spectrometry techniques. The primary focus was on examining the distinct LTO reaction pathways observable across different temperature ranges. The findings demonstrated a correlation between LTO temperature and the concentration of free radicals, which predominantly resided on aromatic hydrocarbons, alkanes, and oxygen atoms. Additionally, the proton count of polycyclic aromatic hydrocarbons exhibited a continuous increase from 83 to 350 °C, suggesting intensified aromatization and condensation reactions involving aliphatic and aromatic compounds. With rising LTO temperature, the molecular structure of O2 compounds underwent significant transformations, characterized by increased condensation degree and a decrease in low carbon number molecular structures, while higher equivalent double bonds and carbon number molecular structures became more prevalent. The influence of cycle path 1 diminished at temperatures ranging from 83 to 150 °C and 250 to 350 °C, whereas the significance of cycle paths 2 and 3 increased, resulting in an overall escalation of the oxidation rate with temperature elevation. It was observed that the shale oil LTO process exhibited a negative temperature coefficient within the temperature range of 150-250 °C, emphasizing the criticality of overcoming the energy barrier in this region to achieve stable combustion. This comprehensive investigation provides valuable insights into the mechanisms underlying LTO in crude oil confined within tight shale.

Ignition enhancement and NOx formation of NH3/air mixtures by non-equilibrium plasma discharge

This work computationally studies plasma assisted low temperature NH3/air ignition and NOx formation in a repetitively-pulsed nanosecond discharge at atmospheric pressure by using an experimentally-validated plasma-combustion kinetic model. The results show that plasma discharge significantly enhances low temperature NH3 ignition. Compared with thermal ignition, the ignition delay time is shortened by 2–5 orders of magnitude due to the kinetic enhancement of excited species and radicals. The radicals (NH2, NH, H and O) produced through electron impact dissociation and quenching of electronically excited N2*, O(1D) and N(2D) promote OH production and further accelerate NH3 oxidation. The results show that there exists a non-monotonic dependence of ignition delay time on the reduced electric field strength. The optimum ignition enhancement is achieved at 250 Td at which the production of electronically excited species and radicals is most efficient. The vibrationally excited species produced at lower electric fields (<100 Td) are less effective in enhancing ignition because they only induce gas heating through the fast vibrational-translational relaxation by NH3 and H2O. At a higher electric field, although the efficient production of NH2, NH, H, O and OH by plasma creates new low temperature reaction pathways in enhancing low temperature NH3 ignition, the ignition is inhibited through NH + NO = N2O + H and chain-termination reaction NH2 + HO2 = NH3 + O2. The ignition delay times at different equivalence ratios show that the ignition enhancement by plasma is more effective at fuel-lean conditions due to the faster generation of N2(B), O(1D) and O from air, leading to accelerated NH3 oxidation via O(1D) + NH3→ NH2 + OH, NH3 + O = NH2 + OH and NH2 + O = NH + OH. The sensitivity analysis shows that the reactions involving O and O(1D) production are more effective on NH3 ignition enhancement than the fuel dissociation by electrons. Moreover, the ignition is also enhanced by NOx formation in plasma via reactions NH2 + NO = NNH + OH and NO + HO2 = NO2 + OH. This work advances the understanding of non-equilibrium excitation and NOx formation by plasma discharge on low temperature NH3 ignition.

Ozone-initiated low-temperature oxidation of two propanol isomers: revealing the effects of Ö/ȮH atoms/radicals

The low-temperature oxidation of propanol isomers (n-propanol and iso-propanol) initiated by ozone addition was carried out in a jet-stirred reactor at an equivalence ratio of 0.5, in a temperature range of 400–800 K and at atmospheric pressure. Synchrotron vacuum ultraviolet photoionization mass spectrometry was used to follow the temperature dependence of key intermediates, including aldehydes, ketones, hydroperoxides, alcohols, alkanes, and alkenes, to explore their low-temperature oxidation reaction pathways. n-Propanol and iso-propanol presented different intermediate distributions during low-temperature oxidation, due to the different position of the hydroxyl group in the molecular structure. The elucidation of the distribution of these intermediate species contributed to the understanding of the low-temperature oxidation reaction network of n-propanol and iso-propanol. Furthermore, a detailed kinetic model of propanol isomers was tentatively developed by systematically updating the sub-mechanism of β-hydroxypropyl radical and by adding the four reaction channels for CO2 formation. These four reactions involve the reactions of the Ö/ȮH atom/radical with formaldehyde/acetaldehyde. The rate of production analysis reveals the crucial roles of Ö/ȮH atoms/radicals on the whole reaction process, highlighting the controlling effects of reactions directly related to Ö/ȮH atoms/radicals. Especially, the reaction between HȮ2 radicals and ozone dominates ȮH radical formation at lower temperatures of ca. 400–475 K, while Waddington reactions, the cyclization reactions of hydroperoxyl hydroxyalkyl radicals and the H-atom abstraction reactions of fuels/intermediates are dominantly responsible for the formation of ȮH radicals at elevated temperatures of ca. > 500 K.

Effects of hydrogen addition on laminar dilute n-dodecane spray flames

With the increasing demand for carbon–neutral propulsion technology, hydrogen addition appears to be a viable short-to-medium-term solution. This study investigates the dynamics of overall lean, laminar dilute n-dodecane spray flames in premixed hydrogen/air mixture by simulating one-dimensional dilute spray flames over a wide range of hydrogen addition and n-dodecane droplet diameter. The characteristic residence time and evaporation time are analyzed to reveal the pre-evaporation mode, and examine the distinct evaporation characteristics throughout the flames. The flame structure and flame propagation speed are then examined, with the impact of subtle fuel chemistry coupling at low-temperature-chemistry regime being revealed. At atmospheric pressure conditions, the flame speed monotonically increases with hydrogen addition due to the enhanced reactivity. However, it has a nonmonotonic trend with initial droplet diameter due to the subtle coupling of evaporation and chemical kinetics. More interestingly, at engine-relevant conditions, the flame propagation speeds firstly decrease and then increase with hydrogen addition, this is because small hydrogen addition inhibits the low-temperature reaction pathway that could offset the effects of enhanced high-temperature reactivity on flame propagation. In addition, the flame propagation speeds are less sensitive to the droplet diameter unless the droplets are sufficiently large and the long evaporation process reduces the effective local equivalence ratio of the overall fuel-lean mixtures.

A hierarchical reduction method with selective targets for combustion modeling of large Hydrocarbons: A case study of 1,3,5-Trimethylbenzene skeletal mechanism

A novel hierarchical method of skeletal reduction for detailed hydrocarbon reaction mechanisms is proposed and applied to generate a skeletal mechanism of 1,3,5-trimethylbenzene (135TMB) which is relevant to practical fuels and their surrogates. The proposed reduction method is based on the hierarchical sensitivity analysis (HSA) implemented in a top-to-bottom manner in terms of carbon levels with selective reduction targets, such as autoignition delay time, laminar flame speed, flame extinction, speciation, etc. These reduction targets can be selected based on the desired combustion phenomena to be simulated and are only validated at their corresponding carbon levels where the respective chemistries are dominating. In contrast to the conventional global sensitivity analysis (GSA) in which species and reactions are randomly evaluated, HSA evaluates species and reactions following the oxidation sequence of fuel molecule, and therefore is able to generate skeletal mechanisms as compact as possible with significantly higher efficiency than GSA. In addition, quasi-steady-state approximation and reaction lumping are integrated in the current reduction process. Low-temperature reaction pathway is also optimized with reaction rates tuned using a particle swarm algorithm to better predict species fractions in the low-to-intermediate temperature regime. As such, based on the detailed 135TMB mechanism of Diévart et al. (2013) with 450 species and 4569 reactions, a skeletal mechanism with 46 species and 197 reactions has been successfully generated including both high- and low-temperature reaction pathways. Extensive validations of the resulting skeletal mechanism are performed against the detailed mechanism predictions and the literature experimental data over a wide range of conditions, and good agreements are observed. These results not only show the accuracy and reliability of the present skeletal 135TMB mechanism but also demonstrate the effectiveness and reduction efficiency of the proposed hierarchical reduction method.

Observation of two different cool flame regimes of diethyl ether in a counterflow burner

This short communication reports, for the first time, the existence of two different self-sustaining cool flame regimes of diethyl ether (DEE) in a diffusion counterflow burner: a weaker autoignition-assisted cool flame near the fuel burner and a normal diffusion cool flame near the stagnation plane. The results show that the normal diffusion cool flame extinction limit increases monotonically with the fuel mole fraction, while the autoignition-assisted cool flame approaches a plateau and can exist at a fuel mole fraction below the normal diffusion cool flame. It is shown that both flame regimes are governed by the same low-temperature chain-branching reaction pathway of DEE. By using in situ laser diagnostics, entrainment of unburned fuel stream to oxygen stream at the outer edge of the fuel burner is identified as the governing physical mechanism causing a partially premixed self-sustained hollow cool flame structure. The results reveal that when a fuel with high low-temperature reactivity, two different cool flame regimes can be observed in a counterflow flame experiment. Future studies with high-reactivity fuels in a counterflow burner must ensure to distinguish between the two self-sustaining cool flame regimes. Moreover, the existence of these different cool flame regimes needs to be examined so that they would not trigger an uncontrolled combustion phasing in advanced engines fed with high low-temperature reactivity fuels.

Effect of NO2 addition on flame dynamics and chemical kinetics of near-limit n-dodecane cool and warm diffusion flames

The present study is a continuation of the previous study by Zhou et al. (2021), in which kinetic effects of NO addition on n-dodecane cool and warm diffusion flames are investigated via counterflow flames and a skeletal n-dodecane/NOx model is developed by performing jet-stirred reactor experiments. Here, effects of NO2 addition on the flame dynamics and kinetics of n-dodecane diffusion flames are studied. The focus is on understanding the kinetic effects of NO2 on cool flame extinction limits and the transitions between cool, warm, and hot flames and elucidating the differentiated effects of NO and NO2 on low- and intermediate-temperature chemistry of n-dodecane. NO2 also plays different roles in cool and warm flames. Specifically, NO2 addition reduces the cool flame temperature and strain rates at cool flame extinction, indicating that NO2 inhibits the low-temperature chemistry of n-dodecane. NO2 addition increases the warm flame temperature, delays the warm flames extinction to cool flames, and promotes the warm flames re-ignition to hot flames, demonstrating NO2 enhances the intermediate-temperature oxidation of n-dodecane. However, reaction pathways and sensitivity analyses reveal that reactions responsible for NO2 and NO affecting the low- and intermediate-temperature oxidation of n-dodecane diffusion flames are differentiated. For cool flames, reaction R + NO2 <=> RO + NO for NO2 addition and reaction RO2 + NO <=> RO + NO2 for NO addition are the major corresponding reactions, respectively, which compete with the low-temperature chain-branching reaction pathway. For warm flames, the kinetic effects of NO on the intermediate-temperature oxidation are due to NO converting the relative inactive HO2 to reactive OH radicals via the reaction NO + HO2 <=> OH + NO2, which simultaneously accelerates radical production of QOOH => QO + OH to the increase of warm flame temperature, while the effect of NO2 is mainly induced by the conservation of NO2 to NO. Moreover, comparisons of the effects of NO and NO2 addition on cool flame extinction limits are performed, and results show that the kinetic effects of NO2 addition are weaker than those of NO addition since the effects of NO2 are partially contributed by NO from the conservation of NO2.

Methanol oxidation up to 100 atm in a supercritical pressure jet-stirred reactor

Methanol (CH3OH) has attracted considerable attention as a renewable fuel or fuel additive with low greenhouse gas emissions. Methanol oxidation was studied using a recently developed supercritical pressure jet-stirred reactor (SP-JSR) at pressures of 10 and 100 atm, at temperatures from 550 to 950 K, and at equivalence ratios of 0.1, 1.0, and 9.0 in experiments and simulations. The experimental results show that the onset temperature of CH3OH oxidation at 100 atm is around 700 K, which is more than 100 K lower than the onset at 10 atm and this trend cannot be predicted by the existing kinetics models. Furthermore, a negative temperature coefficient (NTC) behavior was clearly observed at 100 atm at fuel rich conditions for methanol for the first time. To understand the observed temperature shift in the reactivity and the NTC effect, we updated some key elementary reaction rates of relevance to high pressure CH3OH oxidation from the literature and added some new low-temperature reaction pathways such as CH2O + HO2 = HOCH2O2 (RO2), RO2 + RO2 = HOCH2O (RO) + HOCH2O (RO) + O2, and CH3OH + RO2 = CH2OH + HOCH2O2H (ROOH). Although the model with these updates improves the prediction somewhat for the experimental data at 100 atm and reproduces well high-temperature ignition delay times and laminar flame speed data in the literature, discrepancies still exist for some aspects of the 100 atm low-temperature oxidation data. In addition, it was found that the pressure-dependent HO2 chemistry shifts to lower temperature as the pressure increases such that the NTC effect at fuel-lean conditions is suppressed. Therefore, as shown in the experiments, the NTC phenomenon was only observed at the fuel-rich condition where fuel radicals are abundant and the HO2 chemistry at high pressure is weakened by the lack of oxygen resulting in comparatively little HO2 formation.

Experimental and kinetic studies of extinction limits of counterflow cool and hot diffusion flames of ammonia/n-dodecane

The present study focuses on the effects of ammonia on the extinction limits of n-dodecane cool and hot diffusion flames and the kinetic interactions between ammonia and n-dodecane at low and high temperatures. The extinction limits of cool and hot diffusion flames for ammonia/n-dodecane blends are measured in an atmospheric counterflow burner. The results show that the extinction limits of both cool and hot flames decrease with the increase of the ammonia blend ratios and an obvious non-linear tendency can be found for the hot flames. Moreover, the laminar flame speeds and ignition delay times of ammonia/n-dodecane blends are also measured and a detailed ammonia/n-dodecane model is developed and validated with the measured data. With the assistance of kinetic modeling, the physical and chemical effects of ammonia on the extinction limits of ammonia/n-dodecane hot and cool diffusion flames are examined. It is found that the physical effects of ammonia suppress the extinction limits of both cool and hot diffusion flames, relative to the nitrogen dilution. Chemical effects of ammonia play a leading role and increase the extinction limits of hot diffusion flames but decrease the extinction limits of cool diffusion flames, indicating that the ammonia promotes the high-temperature oxidation of n-dodecane but inhibits the low-temperature oxidation of n-dodecane. Kinetic analyses reveal that ammonia addition inhibits the low-temperature chemistry of n-dodecane via NH3 + OH <=> NH2 + H2O (NH3 directly involves the reactions slowing down the n-dodecane H-abstraction reaction) and NO + RO2 <=> RO + NO2 (NO produced from NH3 and NO competes with low-temperature chain-branching reaction pathways for RO2). Moreover, different major reaction pathways of ammonia oxidation at low and high temperature are summarized: NH3 → NH2 → H2NO → HNO → NO is responsible for the ammonia consumption at low temperature, while NH3 → NH2 → NH → N → NO is the one at high temperature.

An experimental and modeling study on the low-temperature oxidation of methylcyclopentane in a jet-stirred reactor

Cycloalkanes are important compounds of conventional petroleum-based fuel. Methylcyclopentane (MCP) is a representative five-membered ring cycloalkane, but studies on its oxidation behavior are still limited. Therefore, a comprehensive experimental and kinetic study on MCP low-temperature oxidation was conducted. The experiments were carried out in a jet-stirred reactor (JSR) at the equivalence ratios of 0.5, 1.0 and 2.0, in the temperature range T = 500–1100 K and at atmospheric pressure. Mole fractions of MCP, oxygen, CO, CO2 and other intermediates during fuel oxidation were measured using a gas chromatograph, at the residence time of 2 s and initial fuel mole fraction of 0.5%. Further, a detailed MCP oxidation kinetic model including both high- and low-temperature reaction pathways was developed based on the models reported in the literature, and the updated model was validated against the mole fraction profiles of the oxidation species measured in this study, and the ignition delay times in the literature. To improve the model performance, the rate constants of beta-scission reactions of methylcyclopentyl radicals were calculated at CCSD(T)/CBS//M06-2X/6–311++G(d,p) level of theory by solving the Rice-Ramsperger-Kassel-Marcus/Master Equation. The results show that our updated model has a satisfactory agreement with the oxidation species mole fraction profiles in current study and the ignition delay times in the literature. Reaction pathway analysis indicates that the reactivity difference at different equivalence ratios is attributed to the branching ratios of beta-scission reactions of methylcyclopentyl radicals.

Autoignition behavior of methanol/diesel mixtures: Experiments and kinetic modeling

Methanol is an attractive oxygenate increasingly used as primary fuel for dual-fuel combustion technology yielding beneficial thermal-efficiency and emissions in modern engines. As such, it is significant to fundamentally understand the autoignition behavior of methanol/diesel mixtures. This study measured the ignition delay times (IDTs) of methanol/diesel blends with different mixing ratios (30%, 50%, 70% methanol by mol.) on a heated shock tube and a heated rapid compression machine at temperatures of 650−1450 K, pressures of 6−20 bar, and equivalence ratios of 0.5, 1.0 and 2.0. The typical two-stage ignition characteristics with the negative temperature coefficient response were observed for dual-fuel mixtures. In general, both the total and first-stage IDTs decrease with the increment of pressure and equivalence ratio as well as diesel proportion in mixtures. The simulation results performed with a published detailed mechanism in conjunction with a tri-component diesel surrogate demonstrate generally good agreement with the experimental data at all test conditions. Moreover, a crossover of IDTs occurs at a higher temperature (~1500 K) for varying equivalence ratios, and experiment and simulation both exhibit a non-linear mixing effect of methanol addition on diesel ignition. Interestingly, simulation results clearly suggest a crossover (~940 K) for mixtures with varying methanol content at an intermediate-temperature, where the IDT of the mixture with a lower methanol ratio becomes longer as the temperature is higher than that of the crossover. Furthermore, brute-force sensitivity analyses assisted with reaction path analysis were conducted to gain deeper insights into the autoignition chemistry of dual-fuel mixtures, especially for the chemical interaction between both fuels during the low-temperature oxidation process. It is found that methanol hardly generates •OH, whereas •OH is mainly produced by the low-temperature reaction pathways of diesel. Thus, •OH is the bridge for both fuels during the ignition process. At the high methanol ratio, the H-abstraction of diesel is mainly via •HO2 while CH3OH consumes a large percentage of •OH. Consequently, the competition between methanol and diesel for •OH radicals inhibits the overall reactivity of reaction network. In addition, the original data reported here lay a foundation for the development and validation of more accurate and robust dual-fuel kinetic schemes.

Kinetic Study of Two-Stage Low-Temperature Heat Release in iso-Octane Autoignition.

Recognizing that low-temperature ignition of alkanes is usually associated with one heat release peak, we report herein that, for iso-octane under certain ranges of initial temperatures and pressures, two separate heat release peaks were observed through computational simulations using several kinetic mechanisms. The inherent chemical reason for this phenomenon is discussed using reaction channel analysis and is identified to result from the competition between R + O2 → RO2 and the β scission reactions. By further utilizing sensitivity and path flux analyses, an isomeric effect is identified in that the different isomeric structures produced through the H-abstraction reactions can lead to differences in the subsequent low-temperature reaction pathways.

Exploring low temperature oxidation of 1-butene in jet-stirred reactors

1-butene is an important intermediate in combustion of various hydrocarbon fuels and oxygenated biofuels (e.g., butanol). Understanding its oxidation chemistry can help improve ignition and combustion process in advanced engines and provide better emission control. This work addresses a discrepancy between experiments and simulations in 1-butene oxidation at low temperatures, wherein simulations with AramcoMech 3.0 model show greater fuel reactivity than experiments. To further explore 1-butene low temperature reaction pathways from 550 to 910 K, experiments were conducted in three jet-stirred reactors: two coupled to time-of-flight molecular beam mass spectrometers with synchrotron vacuum ultraviolet radiation as the photoionization source, and one coupled to gas chromatography mass spectrometer. Isomeric structure identification, comprehensive species datasets, and reactor cross examinations are provided by the combination of three experiments. The identified isomer-resolved species provide evidence of various 1-butene low temperature reaction pathways. For example, the identification of propanal confirms the Waddington reaction pathway. The kinetic model over-predicts fuel reactivity in the low temperature regime (550–700 K). Updating the rate coefficients of key reactions in the Waddington pathways, e.g., forward and reverse isomerization of hydroxyl-butyl-peroxide to butoxyl-peroxide and Waddington decomposition of butoxyl-peroxide reduces the discrepancies. The role of rate constant updates in each step of the Waddington pathway is evaluated and discussed to provide directions for future model development.

Attached and lifted flame stabilization in a linear array of swirl injectors

Effective flame anchoring maximizes the low-emissions operating window of gas-turbine systems featuring closely spaced, multiple lean flames on single injector units. The complex flame-flow interactions inherent to such designs are not well understood, in particular, at part-load gas turbine conditions. In the present work, large eddy simulation (LES) was used to investigate flames in an optically accessible, confined, linear array of five industrial swirl injector elements. Well-anchored, partially attached and fully lifted flames were observed and three cases corresponding to these were selected for modeling. The dynamically thickened flame (DTF) model was used with a newly developed reduced chemical kinetics scheme that contains low temperature reaction pathways. Chemical explosive mode analysis (CEMA) shows that the flames exhibit thin ignition zones near their base, while a more distributed region of strongly positive modes occur downstream when a transition to partial blow-off takes place. Extinctions are frequent in this region and create broken reaction zones which seem to exhibit both autoignition as well as flamelet characteristics. Further decrease in reactivity leads to a fully lifted flame. It is clear that part-load conditions are best investigated by models that are combustion regime independent.

On the reversible deactivation of cobalt ferrite spinel nanoparticles applied in selective 2-propanol oxidation

CoFe2O4 nanoparticles (NPs) were synthesized by using a colloidal one-pot synthesis method based on the decomposition of metal acetylacetonates in the presence of oleyl amine. The characterization by X-ray diffraction, transmission electron microscopy and N2physisorption revealed non-porous spinel phase CoFe2O4 NPs with an average particle size of 4 nm. The unsupported metal oxide NPs were applied in the selective oxidation of 2-propanol in a continuously operated fixed-bed reactor under quasi steady-state conditions using a heating rate of 0.5 k min−1. 2-Propanol was found to be oxidatively dehydrogenated over CoFe2O4 yielding acetone and H2O with high selectivity. Only to a minor extent dehydration to propene and total oxidation to CO2 was observed at higher temperatures. The detected low-temperature reaction pathway with maxima at 430 and 510 K was inhibited after the initial 2-propanol oxidation up to 573 K, but an oxidative treatment in O2 or N2O atmosphere led to full regeneration. No correlation between the desorbing amount or the surface oxygen species investigated by O2 temperature-programmed desorption experiments and the low-temperature activity was observed. The amounts of evolving CO2 during the TPO experiments indicate deactivation due to formation of carbonaceous species. Inhibition experiments with pre-adsorbed reaction intermediates and infrared spectroscopy identified acetate species as reversible poison, whereas carbonates are rather spectators. In addition, carbon deposition was detected by X-ray photoelectron spectroscopy, which also revealed a minor influence of cobalt reduction during the deactivation process as confirmed by X-ray absorption spectroscopy studies.

A new skeletal mechanism for diesel-n-butanol blends combustion in engine

A new skeletal mechanism consisting of 116 species and 433 reactions for diesel-n-butanol blends combustion in engine is proposed in this study. Starting from a detailed chemical kinetics considering high-temperature and low-temperature reaction pathways of n-butanol, a reduced reaction mechanism was built using directed relation graph with error propagation and sensitivity analysis (DRGEPSA), computational singular perturbation (CSP) and reaction pathway analysis (RPA) methods. Then, it was further combined with a reduced diesel surrogate fuel chemical reaction mechanism of n-heptane/toluene with the NOx and soot formation kinetics embedded. After optimizing the reaction rate constants of selected reactions in the coupled mechanism based on sensitivity analysis, the final n-heptane/toluene-n-butanol skeletal mechanism for diesel-n-butanol blend oxidation was generated and validated against ignition delay time, laminar flame speed and premixed flame species profiles, and a good prediction was presented. In addition, the current mechanism was coupled into the KIVA3V2-CANTERA to simulate the combustion and emissions characteristics of an engine fueled with B00, B20 and B40 (0 vol%, 20 vol% and 40 vol% n-butanol blended with diesel). It is indicated that the in-cylinder pressure, heat release rate and NOx and soot emission of diesel/n-butanol blend can be well reproduced by the newly developed skeletal mechanism with a reliable accuracy.

Modeling pre-spark heat release and low temperature chemistry of iso-octane in a boosted spark-ignition engine

Recent trends among automotive manufacturers towards downsized, boosted engines make it imperative to understand specific fuel chemistry interactions encountered in this new operating regime. At these elevated pressure conditions a phenomenon called pre-spark heat release has recently been discovered, and is characterized by kinetically controlled heat release before spark, with resultant changes in end-gas thermodynamic state and composition. These reactions typically occur in the end-gas during normal operation, but are obscured by the deflagration heat release and therefore cannot be easily studied. A 2-zone spark-ignition engine model was utilized to determine whether chemical kinetic mechanisms are able to predict this phenomenon, and whether they accurately capture end-gas thermodynamic history. Experimental engine data at a range of boosted operating conditions demonstrating pre-spark heat release were compared with simulations using mechanisms representing the latest developments in gasoline kinetic modeling. The results demonstrated significant discrepancies between mechanisms, and between experimental and simulated results in terms of low-temperature heat release magnitude, end-gas thermodynamic state, and autoignition propensity. The results highlight shortcomings in low-temperature reaction pathways, and indicate the necessity of simultaneously matching first-stage ignition delay and heat release magnitude, in addition to second-stage ignition delay, in order to accurately predict end-gas thermodynamics and autoignition.