Including Ignition Delay Times Research Articles

Uncertainty quantification and data consistency analysis for the development of hydrogen and syngas oxidation model

The exponential growth of contemporary chemical kinetics data coupled with the potential inherent high uncertainty poses significant challenges for developing chemical kinetic models. To obtain a high-quality dataset for the optimization of H2 and syngas oxidation model, this study developed a framework for data consistency analysis and uncertainty analysis of chemical kinetic data. The framework combines simulations based on solving conservation equations, data-driven metaheuristic algorithms, and statistical analysis. Over 2000 experimental data points, including ignition delay times measured in shock tubes, laminar flame speeds, and concentration profiles measured in perfectly stirred reactors and plug flow reactors, were collected and used to construct the experimental dataset. Seventeen parameters of a detailed H2 and syngas oxidation model were selected to build the reaction rate constant (RRC) dataset. The initially estimated experimental uncertainties and RRC uncertainty factors were updated by slack variables in the iterative algorithm. Some high-uncertainty data were identified and excluded from the optimization dataset. The final experimental dataset achieves data consistency, and the feasible parameter set of RRC shows good prediction capability. The experimental data with high uncertainties were highlighted, and the uncertainty sources were analyzed.

Development of a reduced primary reference fuel – oxymethylene dimethyl ether (PRF-OMEx) mechanism for diesel engine applications

Oxymethylene dimethyl ethers (OMEx), having a chemical formula of CH3O-(CH2O)x-CH3 where x varies from 1 to 5, have been widely considered as a promising fuel to partially replace diesel in CI engines in terms of reducing soot emissions. This work is focused on developing a reduced primary reference fuel (PRF)-OMEx chemical mechanism to better describe the combustion and emission characteristics of gasoline/diesel blends with OMEx. The novelty of this work lies in the fact that the OMEx part of the mechanism is represented not only by OME3 as done in most studies found in literature, but also with other OME chain lengths that is, OME2-4 which are considered to be optimum and better represent the commercial OMEx blends. For this purpose, a detailed OMEx mechanism is reduced by applying different reduction techniques considering a wide range of operating conditions including pressure, temperatures, equivalence ratios and fuel compositions. The result is merged with an already validated PRF mechanism to form a reduced PRF-OMEx mechanism consisting of 213 species and 840 reactions. The newly formed mechanism is validated against a wide set of experimental data including ignition delay times, laminar flame speeds and species concentration profiles. Furthermore, a rigorous set of numerical simulations for various diesel-OMEx blends in a compression ignition engine are carried out at two different operating points to validate the developed mechanism. Simulation results highlight that the developed mechanism not only replicates the experimental behavior in terms of in-cylinder pressure and heat release rate but exhibits a better combustion phasing closer to experimental data when compared with other mechanisms where only OME3 is utilized to represent OMEx. Overall, the developed PRF-OMEx mechanism proves to be realistic and suitable for application in engine combustion simulations involving gasoline/diesel and OMEx blends.

The combustion chemistry of ammonia and ammonia/hydrogen mixtures: A comprehensive chemical kinetic modeling study

Ammonia (NH3) is relatively less reactive compared to hydrocarbon fuels. Therefore, ammonia mixtures blended with hydrogen (H2) have been shown to be a promising fuel for internal combustion engines. In this study, a detailed NH3/H2 chemical kinetic model is developed over a wide range of engine-relevant conditions and comprehensively validated to describe the combustion of NH3/H2 mixtures using available experimental literature data, including ignition delay times, laminar flame speeds and species concentration profiles. The new model captures very well the combustion properties of pure NH3 and NH3/H2 mixtures at most conditions. By performing sensitivity and reaction path flux analyses the key reactions controlling fuel reactivity at high-temperature (≥ 1500 K) and low-to-intermediate temperature (1000 ≤ T ≤ 1500 K) regimes are identified. Moreover, the formation and consumption pathways of nitrogen oxides (NOx) in NH3/H2 combustion at different conditions have also been investigated, which are found to be highly coupled to the underlying chemical reactions that dictate fuel reactivity. The kinetic data for the important reactions and species thermochemistry data used in our model are rigorously evaluated and are discussed in detail.

Simplified Reaction Mechanisms for the Oxidation of Jet Fuel

ABSTRACT Jet fuel is a complex mixture comprising thousands of components, which poses challenges for conducting kinetic simulations on its oxidation process. To address this issue, the HyChem (hybrid chemistry) approach has recently been proposed as an effective method for modeling the oxidation of real multicomponent fuels. Nevertheless, the simplest HyChem approach still involves 31 species and more than 100 reactions, presenting a significant challenge for computational fluid dynamics (CFD) simulations of real multicomponent fuels. In order to enhance the computational efficiency and affordability of numerical simulations for reactive flow, this study developed three simplified mechanisms, namely S66 (28 species and 66 reactions), S8 (12 species and 8 reactions), and S2 (5 species and 2 reactions), which depict the high-temperature (T = 1000～1470 K), high-pressure (P > 1.0 atm), and fuel-lean (φ < 0.7) oxidation of real multicomponent jet fuels inspired by the HyChem methodology and single-step overall reaction. The accuracy of the mechanisms was validated using experimental data, including ignition delay times and laminar flame speed from the literature. Notably, the CPU time required for the simplest HyChem model is longer than 20 times that of the S2 mechanism under identical conditions in the CFD simulation example. This is due to the highly simplified approach of the S2 mechanism. Consequently, the S2 mechanism is expected to be applicable in combustion kinetic simulations in complex environments such as combustion chambers because of its high computational efficiency and relative accuracy.

Development of reduced and optimized mechanism for ammonia/ hydrogen mixture based on genetic algorithm

As carbon-free and renewable fuels, ammonia and hydrogen are promising fuels to be utilized in combustion devices. For this, the genetic algorithm was employed to reduce and optimize the mechanism for ammonia/hydrogen mixture, which was aimed to develop a compact and reliable skeletal mechanism for real engine simulation with ammonia as fuel. During the progress of GA reduction, the extracted sub-mechanisms was constrained high fidelity with original mechanism on predicting the combustion characteristics through the definition of penalty functions. Besides, the number of reactions and computer calculation time of the sub-mechanism were simultaneously tracked to reduce the chemical stiffness. The resulting mechanism contained 29 species and 63 reactions and was streamlined to be coupled into complex CFD calculation. The reduced mechanism was adjusted to better capture fundamental combustion data and the mechanism optimization was finished automatically through GA operation. To verify the reliability of the optimized mechanism, the experimental data including ignition delay times, laminar flame speed, JSR species concentration, premixed flame structure and HCCI engine in-cylinder pressure at various conditions was utilized to evaluate the performance of the models. As a result, the optimized mechanism was capable to reproduce those measurements and was credible for engine combustion simulation.

Testing of NH3/H2 and NH3/syngas combustion mechanisms using a large amount of experimental data

A possible solution to improve the combustion properties of ammonia is to blend it with other fuels. Two of the most usually used co-fuels are hydrogen and syngas (H2/CO). To investigate the chemistry of the co-combustion with these fuels, a large amount of indirect experimental data for the combustion of neat NH3, and NH3/H2 and NH3/syngas fuel mixtures were collected from the literature including ignition delay times measured in shock tubes, concentration measurements in jet stirred and flow reactors, and laminar burning velocity measurements. Altogether, 4898 data points (in 472 data series) were recorded which cover wide ranges of equivalence ratio, temperature, and pressure. These experimental data are available in data files in the ReSpecTh site (http://respecth.hu). The performances of 18 recently published detailed reaction mechanisms were quantitatively assessed using the collected experiments. There are significant differences between the performances of the models, and the performance of a mechanism may also vary significantly with the different types of experiments. The best-performing mechanisms are POLIMI-2020, Han-2020, and KAUST-2021 for NH3/H2 fuel mixtures, and Shrestha-2021, Mei-2021, and Mei-2020 for NH3/syngas systems. The results indicate that further mechanism development is needed to reproduce the measurements more accurately. Local sensitivity analysis was carried out on the kinetic and thermodynamic parameters of the best-performing mechanisms. Even though the investigated models have different parameter sets, the most important reactions and thermodynamic properties are similar. The most important reactions are not the same for the different types of experiments but most of them include the NH3, NH2, and/or NNH species. Among the thermodynamic parameters, model outputs are most sensitive to the data of NH3 and NH2.

Experimental and Updated Kinetic Modeling Study of Neopentane Low Temperature Oxidation.

Neopentane is an ideal fuel model to study low-temperature oxidation chemistry. The significant discrepancies between experimental data and simulations using the existing neopentane models indicate that an updated study of neopentane oxidation is needed. In this work, neopentane oxidation experiments are carried out using two jet-stirred reactors (JSRs) at 1 atm, at a residence time of 3 s, and at three different equivalence ratios of 0.5, 0.9, and 1.62. Two different analytical methods (synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography) were used to investigate the species distributions. Numerous oxidation intermediates were detected and quantified, including acetone, 3,3-dimethyloxetane, methacrolein, isobutene, 2-methylpropanal, isobutyric acid, and peroxides, which are valuable for validating the kinetic model describing neopentane oxidation. In the model development, the pressure dependencies of the rate constants for the reaction classes Q̇OOH + O2 and Q̇OOH decompositions are considered. This addition improves the prediction of the low-temperature oxidation reactivity of neopentane. Another focus of model development is to improve the prediction of carboxylic acids formed during the low-temperature oxidation of neopentane. The detection and identification of isobutyric acid indicates the existence of the Korcek mechanism during neopentane oxidation. Regarding the formation of acetic acid, the reaction channels are considered to be initiated from the reactions of ȮH radical addition to acetaldehyde/acetone. This updated kinetic model is validated extensively against the experimental data in this work and various experimental data available in the literature, including ignition delay times (IDTs) from both shock tubes (STs) and rapid compression machines (RCMs) and JSR speciation data at high temperatures.

A detailed kinetic model for aromatics formation from small hydrocarbon and gasoline surrogate fuel combustion

This work develops a detailed chemical kinetic model for Polycyclic Aromatic Hydrocarbon (PAH) chemistry that builds on a chemical model for gasoline surrogates by Cai et al. [Proc. Combust. Inst. 37 (2019) 639–647]. Informed by ab-initio and experimental studies, the model development emphasizes the prediction of soot precursors starting from C3H4 isomers up to the size of acepyrene. The proposed model was validated against experimental measurements from 79 publications, including ignition delay times, laminar burning velocities, and speciation data for several fuels in various conditions. A validation against measured peak mole fractions from 31 counterflow flames demonstrates an average prediction error of aromatic species up to the size of acenaphthalene below a factor of three. Additionally, the effects of experimental uncertainty are quantitatively discussed by computing confidence intervals for the true model prediction error. The formation of two-ring aromatic species is often still considered poorly understood. Accordingly, special attention is given to the formation of indene and naphthalene. Reaction flux analyses based on rigorously defined species selection criteria reveal that indene formation pathways are initiated by reactions of phenyl and benzyl and described by the reaction kinetics from theoretical calculations. In contrast, naphthalene formation is dominated by more uncertain pathways relying on estimated rates that are initiated by reactions of benzene and fulvenallenyl. Five-member rings on the periphery of PAH species and soot particles play a crucial role in recent experimental and theoretical studies. Therefore, this work investigates acenaphthene and other C12H8 isomers. The most abundant C12H8 isomers are acenaphthene, 1-ethynylnaphthalene, and 2-ethynylnaphthalene, and their respective concentrations are predicted to be similar in many flames. Furthermore, reaction flux analyses show that their formation is dominated by HACA routes starting from naphthalene.

Construction of a skeletal multi-component diesel surrogate model by integrating chemical lumping and genetic algorithm

Towards high-fidelity combustion chemistry modeling of real diesel fuel, this study aims to construct a skeletal multi-component kinetic model with higher accuracy and heavy hydrocarbons (n-hexadecane, iso-cetane, n-octadecane, α-methylnaphthalene, decalin), which is feasible for engine computational fluid dynamics (CFD) applications. The developed skeletal model with 220 species and 1109 reactions is composed of lumped fuel-specific sub-mechanisms and a skeletal C0–C3 core mechanism derived from the well-validated detailed NUIGMech 1.1 mechanism. Chemical lumping integrated with skeletal reduction methods and reaction pathway analysis is used to develop compact fuel-molecule sub-models. Ignition delay and species concentration are then taken as targets simultaneously to optimize the original model automatically using genetic algorithm (GA) within given parameter ranges. Eventually, the optimized kinetic model is validated against various experimental results of pure components, their mixtures and real diesel fuels, including ignition delay times in both shock tubes and rapid compression machines, species concentration profiles in jet-stirred reactors, and laminar flame speeds in the counterflow configurations as well as actual engine data. In addition, it should be noted that due to the absence of test data on pure n-octadecane, the sub-model of n-octadecane is not directly validated in this work, but compared with measurements of real diesel. Overall, the simulation results are in good agreement with experiments under a wide range of engine-like conditions, suggesting that the present skeletal multi-component mechanism can be applied to mimic autoignition and oxidation of actual diesel. As such, this will provide a more accurate and robust combustion model for the development and optimization of advanced engines with high efficiency and ultra-low emissions.

A comprehensive experimental and kinetic modeling study of dimethoxymethane combustion

Dimethoxymethane (DMM, CH3OCH2OCH3), the simplest member in the class of polyoxymethylene dimethyl ethers (PODE), is regarded as a promising fuel substitute for compression ignition engines. To better understand its combustion characteristics, a comprehensive experimental and kinetic modeling study on the combustion of DMM was conducted. Ignition delay times (IDTs) of DMM/O2/Ar mixtures were measured in a shock tube at pressures from 1.0 to 10 atm, for temperatures from 1050 to 1450 K, and equivalence ratios of 0.5, 1.0 and 2.0. A predominantly ab initio derived detailed kinetic model of DMM with 121 species and 646 reactions was developed based on AramcoMech2.0 with an updated sub-mechanism of methyl formate (MF, CH3OCHO). CO bond fissions occurred in CH2O and CH3O moieties were demonstrated to be the dominating reaction pathways in DMM high temperature chemistry rather than the competing non-radical decomposition channels. Flux and sensitivity analyses indicated that the two CO bond fissions have a comparatively promoting effect on reactivity, while the DMM = CH3OCH2O + CH3 reaction was the dominating channel at high temperatures. The proposed model was also validated against literature experimental data, including ignition delay times, jet stirred reactor species concentrations, laminar pre-mixed flame speciation, laminar burning velocities and plug-flow reactor speciation. The good performance of the proposed model for reproducing these data revealed its ability to predict DMM combustion over a wide range of conditions. Major reaction pathways of DMM could also apply to larger PODE compounds.

Construction of reduced oxidation mechanisms of polyoxymethylene dimethyl ethers (PODE1–6) with consistent structure using decoupling methodology and reaction rate rule

Due to high oxygen content and lack of CC bond, polyoxymethylene dimethyl ethers (PODEn) have become desired fuel additives to simultaneously reduce nitrogen oxides (NOx) and soot emissions. However, there is a vacancy of reduced reaction mechanisms to describe the long-chain PODEn (n>3) combustion for engine simulations. This study aims at the construction of reduced mechanisms for the series of PODE1–6 with a consistent structure based on the decoupling methodology and reaction rate rule. First, the reduced mechanisms of PODE1–2 are constructed using the reaction class-based global sensitivity analysis and the decoupling methodology. In the reduced mechanisms of PODE1–2, the skeletal fuel-related sub-mechanism is integrated with a detailed H2/CO/C1 sub-mechanism and a reliable reduced C2–C3 sub-mechanism. Then, due to the similar molecular structure, the skeletal fuel-specific sub-mechanisms of the long-chain PODE3–6 are constructed based on the benchmark fuel of PODE2. The kinetic parameters of the reactions in the skeletal fuel-specific sub-mechanism of PODE3–6 are obtained using the modified linear lumping method and the reaction rate rule assumption. Through the approach proposed in this study, the reduced mechanisms of long-chain PODEn can be effectively constructed based on the chemistry of short-chain PODEn, especially for PODE5–6 with no available detailed mechanisms so far. The final reduced mechanism for the complete series of PODE1–6 only consists of 92 species and 389 reactions. To validate the predictions of the reduced mechanism, extensive comparisons are carried out including ignition delay times in shock tubes, major species concentrations in jet-stirred reactors, and laminar flame speeds and flame species concentrations in premixed laminar flames. Moreover, the measurements of homogeneous charge compression ignition (HCCI) engines fueled with PODE2–4 and PODE3–6 are used for the validations of in-cylinder pressure, heat release rate, and emissions. The overall performance of the reduced mechanism of PODE1–6 is satisfactory under wide operating conditions. This present approach can also be extended to build compact-size reduced mechanisms for the fuels with similar molecular structures, even though no detailed mechanism is available.

Hydrogen addition effect on NO formation in methane/air lean-premixed flames at elevated pressure

The present study investigates freely propagating methane/hydrogen lean-premixed laminar flames at elevated pressures to understand the hydrogen addition effect of natural gas on the NO formation under the conditions of industrial gas turbine combustors. The detailed chemical kinetic model which was used in the previous study on the NO formation in high pressure methane/air premixed flames was adopted for the present study to analyze NO formation of methane/hydrogen premixed flames. The present mechanism shows good agreement with experimental data for methane/hydrogen mixtures, including ignition delay times, laminar burning velocities, and NO concentration in premixed flames. Hydrogen addition to methane/air mixtures with maintaining methane content leads to the increase of NO concentration in laminar premixed flames due to the higher flame temperature. Methane/hydrogen/argon/air premixed flames are simulated to avoid the flame temperature effect on NO formation over a pressure range of 1–20atm and equivalence ratio of 0.55. Kinetic analyses shows that the N2O mechanism is important on NO formation for lean flames between the reaction zone and postflame region, and thermal NO is dominant in the postflame zone. The hydrogen addition leads to the increase of NO formation from prompt NO and NNH mechanisms, while NO formation from thermal and N2O mechanisms are decreased. Additionally, the NO formation in the postflame zone has positive pressure dependencies for thermal NO with an exponent of 0.5. Sensitivity analysis results identify that the initiation reaction step for the thermal NO and the N2O mechanism related reactions are sensitive to NO formation near the reaction zone.

Development of a skeletal mechanism for four-component biodiesel surrogate fuel with PAH

A four-component skeletal mechanism consisting of 1,4-hexadiene, methyl decanoate, methyl trans-3-hexenoate, n-hexadecane was developed to represent biodiesel fuel from various sources. The methyl decanoate and n-hexadecane were selected to represent saturated fatty acid methyl esters, and trans-3-hexenoate and 1,4-hexadiene were applied to adjust degree of unsaturation. A skeletal mechanism for this four-component biodiesel surrogate fuel with PAH was first formulated based on decoupling methodology, which contained a detailed C1 mechanism, a reduced C2–C3 mechanism, PAH mechanism and sub-mechanisms of four surrogate fuels, including 314 reactions and 98 species. After that, the skeletal mechanism was widely verified against various fundamental combustion experiments for each pure component and their mixtures. Furthermore, the experimental results of biodiesel soot volume fractions from biodiesel spray combustion in a constant-volume combustion vessel were used to verify the accuracy of the mechanism, and the skeletal mechanism was also coupled into the CFD-software to simulate the combustion characteristics of a diesel engine. Results showed that the calculated results agreed well with the experimental data including ignition delay times (IDTs), primary species concentrations, laminar flame speed, soot prediction and in-cylinder pressure of the engine. Overall, the developed compact skeletal mechanism was suitable for the combustion simulation of biodiesel fuel.

Chemical kinetics study of 1,3-butadiene + HȮ2; implications for combustion modeling and simulation

Detailed reaction kinetics for the system of 1,3-butadiene + hydroperoxyl radical (HȮ2) and its application in combustion modeling have been investigated in this work. Zero-point energy (ZPE) corrected potential energy surfaces (PESs) for the title reactions have been obtained using high-level ab initio electronic structure methods of ROCCSD(T)/CBS//BHandHLYP/6–311++G(d,p), and the energies relative to the reactants for all stationary points were used in the kinetic calculations. Pressure- and temperature-dependent rate constants for each individual reaction pathway have been calculated by solving the Rice-Ramsperger-Kassel-Marcus/Master Equation (RRKM/ME) implemented in the Master Equation System Solver (MESS) program. The bimolecular pathways forming vinyl oxirane, (CYOC)CC, and hydroxyl radical (ȮH) from the reactants are found to be dominant in the temperature range of 800–2000 K below 1 atm, and keep competitive when temperature is higher than 1000 K at 100 atm. At elevated pressures, the stabilizations of the two entrance channels forming radicals of 1-hydroperoxy-3-buten-2-yl (C4H61–3OOH4) and 3-butenyl-2-peroxy (C4H71-OO3) are more important than the formation of bimolecular products (CYOC)CC + ȮH at temperatures below 1000 K. Temperature-dependent thermochemical properties for important species on the PESs of 1,3-butadiene + HȮ2 have also been calculated. The kinetics and thermochemistry data for the title reaction system calculated in this work have been incorporated into AramcoMech 3.0 mechanism and their influence on the performance of AramcoMech 3.0 in simulating the combustion properties of 1,3-butadiene, including ignition delay times and species profiles against temperature or time, has been investigated. This is a first-time study on the reaction kinetics between HȮ2 radical and species including two double bonds.

Development of a skeletal surrogate mechanism for emulating combustion characteristics of diesel from direct coal liquefaction

A surrogate describing the oxidation characteristics of Diesel from direct coal liquefaction (DDCL) was proposed in this work. n-dodecane and decalin were chosen as representative species of alkane and cycloalkane hydrocarbons in DDCL according to compositional analysis results. A proportion of 24.6% n-dodecane and 75.4% decalin by mole was determined by emulating real DDCL properties including Cetane number (CN), Hydrogen-Carbon (H/C) ratio, lower heating value (LHV) and density. Sub-mechanisms of n-dodecane and decalin were constructed on top of a detailed C0C6 core mechanism and then sensitivity analysis was used to eliminate unimportant species and reactions. The final version of the skeletal DDCL surrogate mechanism consists of 131 species and 461 reactions. The skeletal mechanism was extensively validated against experimental data including ignition delay times, species concentration profiles and laminar flame speeds for each pure component. Furthermore, diesel engine experiments fueled with DDCL were conducted and the corresponding 3-D CFD simulation results show that the present skeletal mechanism gives reliable performance for combustion behavior as well as NOx and CO emissions predictions, indicating the proposed mechanism can be applied for modeling DDCL in practical engine applications.

A Methane Mechanism for Oxy-Fuel Combustion: Extinction Experiments, Model Validation, and Kinetic Analysis

While fuel combustion in oxygen-enriched environments provides a number of significant advantages, such as reduced nitrogen oxide emissions and high carbon dioxide purity for carbon sequestration, it is characterized by different physico-chemical oxidation behavior than combustion in air. Compared to nitrogen, carbon dioxide has different specific heat and effective Lewis number, and is chemically more active. Therefore, chemical mechanisms developed for the oxidation of fuel/air mixtures can fail to predict targets of interest for oxy-combustion accurately. In this study, a chemical mechanism of methane, which has been previously validated with data from experiments using air, is evaluated in terms of its prediction accuracy at oxy-conditions by comparing against available literature data. The validation takes various combustion properties into account, including ignition delay times, laminar burning velocities, and extinction strain rates, and covers a wide range of experimental conditions with respect to temperature, pressure, equivalence ratio, and carbon dioxide concentration. As additional targets, extinction strain rates of non-premixed oxy-methane flames are determined in a counterflow burner at conditions, where literature data have not yet been reported. The extensive validation demonstrates that the mechanism is able to describe oxy-methane combustion with reasonable prediction accuracy. For further insights into the underlying kinetics of diffusion flames of methane in oxy-atmosphere compared to its oxidation in air, reaction path and sensitivity analyses are performed using the validated mechanism. Notable differences between both combustion regimes are observed in the branching ratios of H-abstraction reactions by OH and H radicals and in the consumption channels of singlet methylene, which is a key species in the formation of polycyclic aromatic hydrocarbons.

Experimental and modeling study of the low to high temperature oxidation of the linear pentanone isomers: 2-pentanone and 3-pentanone

Short-carbon-chain ketones are known for their high octane numbers, their knock resistance, and their low soot emission. However, studies on the combustion behavior of ketones, particularly in the low temperature regime, are sparse, and small ketones as acetone and butanone show limited low temperature chemistry (LTC). Therefore, saturated linear five-carbon ketones are good candidates to better understand the LTC of ketones and the effect of the functional carbonyl group. In this study, the ignition delay times of stoichiometric non-diluted 2- and 3-pentanone-oxygen-inert gas mixtures were measured in a rapid compression machine at pressures between 20 and 40 bar and temperatures ranging between 650 and 950 K. Furthermore, the mole fraction profiles of fuel, oxygen, intermediate species, and products were measured in a laminar flow reactor coupled with molecular-beam mass spectrometry at 0.97 bar, an equivalence ratio of 0.8 and temperatures ranging from 800 to 1050 K, to achieve a better understanding of the oxidation process of linear pentanones. A consistent detailed kinetic model was developed and validated against the newly measured experimental data for 2-pentanone and 3-pentanone to provide insight into the oxidation mechanism of pentanones. The model shows also good agreement with available data from the literature, including ignition delay times measured in a shock tube, speciation in a flat flame burner and laminar burning velocities.

Development of a practical reaction model of polycyclic aromatic hydrocarbon (PAH) formation and oxidation for diesel surrogate fuel

A practical reaction model for the formation and oxidation of polycyclic aromatic hydrocarbons (PAHs) up to coronene (A7) was developed focusing on two diesel surrogate fuels, i.e., a single-component surrogate including n-heptane and a multi-component surrogate including iso-octane, n-decane, methylcyclohexane, and toluene, for engine simulations. The important PAH relevant reactions are identified by using the global sensitivity analysis of the up-to-date detailed mechanisms. In the reduced PAH reaction model, the up-to-date reaction channels involving PAH formation were considered, and the variations and characteristics of the formation pathways of benzene and large PAHs for large alkanes, aromatics, and cycloalkanes with the fuel molecular structure were captured with the present under wide operating conditions. By integrating the PAH reaction model with a reduced oxidation mechanism of the two diesel surrogates, the final chemical mechanism of diesel oxidation consists of 137 species and 447 reactions. The chemical mechanism was extensively validated against the fundamental experimental data including ignition delay times in shock tubes and species profiles in premixed laminar flames, diffusion flames, and jet-stirred reactors (JSR). The performance of the mechanism for the predictions of PAHs from the oxidation of individual component, their mixtures, and diesel fuel was comprehensively evaluated. The computational results indicate that the effect of fuel structure on the PAH formation is satisfactorily reproduced by the present mechanism. Based on pathway analysis in the premixed flame, the contribution of the components in the diesel surrogate for the PAH formation behavior is understood.

Development of a skeletal mechanism for tri-component diesel surrogate fuel: N-hexadecane/iso-cetane/1-methylnaphthalene

A tri-component skeletal mechanism consisting of n-hexadecane, iso-cetane and 1-methylnaphthalene was developed. These surrogate components were selected because they can accurately reflect not only the physical and chemical properties of practical diesel fuel but also the combustion and emission characteristics of diesel engines. In addition, the carbon number of fuel surrogate is much closer to that of the real diesel fuel, what’s more, n-hexadecane and iso-cetane are important reference compounds for the cetane number (CN) rating. A skeletal mechanism for this tri-component diesel surrogate fuel was formulated based on decoupling methodology, which contained: a detailed H2/CO/C1 mechanism, a reduced mechanism of C2-C3, and the sub-mechanisms of n-hexadecane, iso-cetane and 1-methylnaphthalene, including 234 reactions and 83 species. After that, the skeletal mechanism was widely verified against various fundamental combustion experiments for each pure component and their mixtures. Furthermore, the experimental results of real diesel fuel in both fundamental burners and homogeneous charge compression ignition (HCCI) engine were also used to validate the skeletal mechanism. Results showed that the calculated results agreed well with the experimental data including ignition delay times (IDTs), primary species concentrations, laminar flame speed and in-cylinder pressure of HCCI engine. Overall, the developed compact skeletal mechanism is suitable for the combustion simulation of diesel and each component.

A detailed chemical mechanism for low to high temperature oxidation of n-butylcyclohexane and its validation

A good knowledge of the reaction kinetics of n-butylcyclohexane, a widely used surrogate component for jet fuels and diesel, is crucial for understanding the combustion chemistry of practical fuels. This paper proposes a detailed kinetic model consisting of 1802 species and 7246 reactions to describe from low to high temperature chemistry of n-butylcyclohexane. The autoignition and oxidation of this fuel were investigated over a wide range of conditions. Ignition delay times were measured at 10, 15 and 20 bar for equivalence ratios of 1 and 1.5 under highly diluted conditions in a heated rapid compression machine. Oxidation experiments were performed in a flow reactor for equivalence ratios of 1 and 1.5 over the temperature range of 650–1075 K at 1 atm. Several oxidation products were identified and their mole fraction profiles were measured as a function of temperature. The model was evaluated with respect to the new experimental data. Additionally, the model was compared to experimental data in the literature, including ignition delay times in both rapid compression machine and shock tube, speciation profiles in jet-stirred reactor. Good agreement was achieved with all the data. Reaction flux analyses and sensitivity analyses were performed in order to provide insight into the combustion kinetics of n-butylcyclohexane. The present model can be used to construct the kinetic models for surrogates of jet fuels or diesel, as well as to work as the starting mechanism for the development of kinetic models of larger alkylcyclohexane and bicyclic cyclohexane fuels.