Skeletal mechanism generation with CSP and validation for premixed n-heptane flames

An automated procedure has been previously developed to generate simplified skeletal reaction mechanisms for the combustion of n-heptane/air mixtures at equivalence ratios between 0.5 and 2.0 and different pressures. The algorithm is based on a Computational Singular Perturbation (CSP)-generated database of importance indices computed from homogeneous n-heptane/air ignition solutions. In this paper, we examine the accuracy of these simplified mechanisms when they are used for modeling laminar n-heptane/air premixed flames. The objective is to evaluate the accuracy of the simplified models when transport processes lead to local mixture compositions that are not necessarily part of the comprehensive homogeneous ignition databases. The detailed mechanism was developed by Curran et al. and involves 560 species and 2538 reactions. The smallest skeletal mechanism considered consists of 66 species and 326 reactions. We show that these skeletal mechanisms yield good agreement with the detailed model for premixed n-heptane flames, over a wide range of equivalence ratios and pressures, for global flame properties. They also exhibit good accuracy in predicting certain elements of internal flame structure, especially the profiles of temperature and major chemical species. On the other hand, we find larger errors in the concentrations of many minor/radical species, particularly in the region where low-temperature chemistry plays a significant role. We also observe that the low-temperature chemistry of n-heptane can play an important role at very lean or very rich mixtures, reaching these limits first at high pressure. This has implications to numerical simulations of non-premixed flames where these lean and rich regions occur naturally.     

Systematic analysis and reduction of combustion mechanisms for ignition of multi-component kerosene surrogate

Currently, most detailed chemical kinetic mechanisms for combustion are still not comprehensive enough and update of key reaction rate is still required to improve the combustion mechanisms. The development of systematic mechanism reduction methods have made significant progress, and have greatly facilitated analysis of the reaction mechanisms and identification of important species and key reactions. In the present work, time-integrated element flux analysis is employed to analyze a skeletal combustion mechanism of a tri-component kerosene surrogate mixture, consisting of n-decane, n-propylcyclohexane, and n-propylbenzene. The results of element flux analysis indicate that major reaction pathways for each component in the surrogate model are captured by the skeletal mechanism compared with the detailed mechanism. After that, sensitivity analysis (SA) and chemical explosive mode analysis (CEMA) are conducted to identify the dominant ignition chemistry. The SA and CEMA results demonstrate that the ignition of n-decane and n-propylcyclohexane is sensitive only to the oxidation chemistry of H₂/CO and C1–C4 small hydrocarbons, while the ignition of n-propylbenzene is very sensitive to the initial reactions of n-propylbenzene and related aromatic intermediates. This demonstrates that the hierarchic structure should be maintained in the reduction of detailed mechanism of substituted aromatic fuels. The skeletal mechanism is further reduced by combining the computational singular perturbation (CSP) method and quasi steady state approximation (QSSA). A 34-species global reduced mechanism is obtained and validated over a wide range of parameters for ignition.     

Methyl concentration time-histories during iso-octane and n-heptane oxidation and pyrolysis

Methyl radical concentration time-histories were measured during the oxidation and pyrolysis of iso-octane and n-heptane behind reflected shock waves. Initial reflected shock conditions covered temperatures of 1100-1560 K, pressures of 1.6-2.0 atm and initial fuel concentrations of 100-500 ppm. Methyl radicals were detected using cw UV laser absorption near 216 nm; three wavelengths were used to compensate for time- and wavelength-dependent interference absorption. Methyl time-histories were compared to the predictions of several current oxidation models. While some agreement was found between modeling and measurement in the early rise, peak and plateau values of methyl, and in the ignition time, none of the current mechanisms accurately recover all of these features. Sensitivity analysis of the ignition times for both iso-octane and n-heptane showed a strong dependence on the reaction C₃H₅ + H = C₃H₄ + H₂, and a recommended rate was found for this reaction. Sensitivity analysis of the initial rate of CH₃ production during pyrolysis indicated that for both iso-octane and n-heptane, reaction rates for the initial decomposition channels are well isolated, and overall values for these rates were obtained. The present concentration time-history data provide strong constraints on the reaction mechanisms of both iso-octane and n-heptane oxidation, and in conjunction with OH concentration time-histories and ignition delay times, recently measured in our laboratory, should provide a self-consistent set of kinetic targets for the validation and refinement of iso-octane and n-heptane reaction mechanisms.     

Autoignition and combustion of hydrocarbon-hydrogen-air homogeneous and heterogeneous ternary mixtures

A numerical simulation of the ignition and combustion of hydrocarbon-hydrogen-air homogeneous and heterogeneous (gas-drop) ternary mixtures for three hydrocarbon fuels (n-heptane, n-decane, and n-dodecane) is for the first time performed. The simulation is carried out based on a fully validated detailed kinetic mechanism of the oxidation of n-dodecane, which includes the mechanisms of the oxidation of n-decane, n-heptane, and hydrogen as constituent parts. It is demonstrated that the addition of hydrogen to a homogeneous or heterogeneous hydrocarbon-air mixture increases the total ignition delay time at temperatures below 1050 K, i.e., hydrogen acts as an ignition inhibitor. At low temperatures, even ternary mixtures with a very high hydrogen concentration show multistage ignition, with the temperature dependence of the ignition delay time exhibiting a negative temperature coefficient region. Conversely, the addition of hydrogen to homogeneous and heterogeneous hydrocarbon-air mixtures at temperatures above 1050 K reduces the total ignition delay time, i.e., hydrogen acts as an autoignition promoter. These effects should be kept in mind when discussing the prospects for the practical use of hydrogen-containing fuel mixtures, as well as in solving the problems of fire and explosion safety.     

An experimental and modeling study of the shock tube ignition of a mixture of n-heptane and n-propylbenzene as a surrogate for a large alkyl benzene

Alkyl aromatics are an important chemical class in gasoline, jet and diesel fuels. In the present work, an n-propylbenzene and n-heptane mixture is studied as a possible surrogate for large alkyl benzenes contained in diesel fuels. To evaluate it as a surrogate, ignition delay times have been measured in a heated high pressure shock tube (HPST) for a mixture of 57% n-propylbenzene/43% n-heptane in air (≈21% O₂, ≈79% N₂) at equivalence ratios of 0.29, 0.49, 0.98 and 1.95 and compressed pressures of 1, 10 and 30 atm over a temperature range of 1000–1600 K. The effects of reflected-shock pressure and equivalence ratio on ignition delay time were determined and common trends highlighted. A combined n-propylbenzene and n-heptane reaction mechanism was assembled and simulations of the shock tube experiments were carried out. The simulation results showed very good agreement with the experimental data for ignition delay times. Sensitivity and reaction pathway analyses have been performed to reveal the important reactions responsible for fuel oxidation under the shock tube conditions studied. It was found that at 1000 K, the main consumption pathways for n-propylbenzene are abstraction reactions on the alkyl chain, with particular selectivity to the allylic site. In comparison at 1500 K, the unimolecular decomposition of the fuel is the main consumption pathway.     

Low- and high-temperature study of n-heptane combustion chemistry

n-Heptane has been used extensively in various fundamental combustion experiments as a prototypical hydrocarbon fuel. While the formation of polycyclic aromatic hydrocarbon (PAH) in n-heptane combustion has been studied preferably in premixed flames, this study aims to investigate the combustion chemistry of n-heptane in less-studied diffusion flame and highly rich high-temperature homogeneous oxidation configurations by using a counterflow burner and a flow reactor, respectively. This work addresses the formation of higher-molecular species in the mass range up to about 160 u in both configurations. Samples are analyzed by time-of-flight (TOF) molecular beam mass spectrometry (MBMS) using electron-impact (EI) and single-photon ionization (PI). Highly resolved speciation data are reported. Laminar flow reactor experiments cover a wide temperature range. Especially the measurements at low temperatures provide speciation data of large oxygenates produced in the low-temperature oxidation of n-heptane, which are scarce in the literature. Important precursor molecules for PAH and soot formation, such as C₉H₈, C₁₀H₈, C₁₁H_10, and C₁₂H₈, are formed during the high-temperature combustion process in the counterflow flame, while oxygenated growth species are observed under low-temperature conditions, even at the fuel-rich equivalence ratio of ?=4.00.Numerical modeling for both conditions is performed by using a newly developed kinetic model of n-heptane, which includes the n-heptane and PAH formation chemistry with state-of-the-art kinetic knowledge. Good agreement between model predictions and experimental data of counterflow flame and flow reactor is observed for the major species and some intermediates of n-heptane oxidation. While the concentrations of benzene and toluene measured in the counterflow burner are well-reproduced, the numerical results for flow reactor data are not satisfactory. Differences are found between the formation pathways of fulvene, from whose isomerization benzene is produced in diffusion flame and flow reactor.     

The application of the QSSA via reaction lumping for the reduction of complex hydrocarbon oxidation mechanisms

The quasi-steady-state approximation (QSSA) has been widely applied for the purposes of chemical kinetic model reduction. Although it is essentially a low-order approximation, it can be shown to lead to significant reductions in the number of fast variables within a mechanism without significant loss of accuracy for model predictions. Due to the couplings between QSSA expressions, the species are commonly solved for using numerical inner iteration techniques. Therefore, although the stiffness of the model system can be reduced, there is a computational overhead in solving the often nonlinear QSSA equations. Greater computational savings can be made where QSS species can be removed from the chemical model via explicit analytical expressions. In many cases these expressions are equivalent to reaction lumping. Where such reaction lumping can be achieved, a reduced mechanism in standard kinetic form can be developed, which contains new lumped reaction rate coefficients, but leads to the removal of QSS species. This paper describes such an approach for mechanisms describing the oxidation of the hydrocarbon fuels n-heptane and cyclohexane, and shows that significant reductions in both species and reactions can be achieved, leading to substantial computational speed-ups. The resulting schemes clearly demonstrate the main atomic flux patterns within the oxidation process. Patterns related to the time-scales of hydrocarbon radical species within alkane oxidation mechanisms are discussed, as well as the potential significance of non-QSS radicals in determining ignition behaviour.     

2D computations of FREI with cool flames for n-heptane/air mixture

Two-dimensional axisymmetric numerical simulation reproduced flames with repetitive extinction and ignition (FREI) in a micro flow reactor with a controlled temperature profile with a stoichiometric n-heptane/air mixture, which have been observed in the experiment. The ignition of hot flame occurred from consumption reactions of CO that was remained in the previous cycle of FREI. Between extinction and ignition locations of hot flames, several other heat release rate peaks related to cool and blue flames were observed for the first time. After the extinction of the hot flame, cool flame by the low-temperature oxidation of n-heptane appeared first and was stabilized in a low wall temperature region. In the downstream of the stable cool flame, a blue flame by the consumption reactions of cool flame products of CH₂O and H₂O₂ appeared. After that, the hot flame ignition occurred from the remaining CO in the downstream of the blue flame. Then after the next hot flame ignition, the blue flame was swept away by the propagating hot flame. Soon before the hot flame merged with the stable cool flame, the hot flame propagation was intensified by the cool flame. After the hot flame merged with the stable cool flame, the hot flame reacted with the incoming fresh mixture of n-C₇H₁₆ and O₂.     

Autoignition of gasoline surrogate mixtures at intermediate temperatures and high pressures: Experimental and numerical approaches

Ignition-delay times were measured in shock-heated gases for a surrogate gasoline fuel comprised of ethanol/iso-octane/n-heptane/toluene at a composition of 40%/37.8%/10.2%/12% by liquid volume with a calculated octane number of 98.8. The experiments were carried out in stoichiometric mixtures in air behind reflected shock waves in a heated high-pressure shock tube. Initial reflected shock conditions were as follows: Temperatures of 690-1200 K, and pressures of 10, 30 and 50 bar, respectively. Ignition delay times were determined from CH^∗ chemiluminescence at 431.5 nm measured at a sidewall location. The experimental results are compared to simulated ignition delay times based on detailed chemical kinetic mechanisms. The main mechanism is based on the primary reference fuels (PRF) model, and sub-mechanisms were incorporated to account for the effect of ethanol and/or toluene. The simulations are also compared to experimental ignition-delay data from the literature for ethanol/iso-octane/n-heptane (20%/62%/18% by liquid volume) and iso-octane/n-heptane/toluene (69%/17%/14% by liquid volume) surrogate fuels. The relative behavior of the ignition delay times of the different surrogates was well predicted, but the simulations overestimate the ignition delay, mostly at low temperatures.     

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

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

Mechanisms of the oxidation and combustion of normal paraffin hydrocarbons: Transition from C1–C10 to C11–C16

Recently, detailed kinetic mechanisms of the oxidation and combustion of higher hydrocarbons, composed of hundreds of components and thousands of elementary reactions, have been proposed. Despite the undoubtful advantages of such detailed mechanisms, their application to simulations of turbulent combustion and gas dynamic phenomena is difficult because of their complexity. At the same time, to some extent limited, they cannot be considered exhaustive. This work applies previously proposed algorithm for constructing an optimal mechanism of the high- and low-temperature oxidation and combustion of normal paraffin hydrocarbons, which takes into account the main processes determining the reaction rate and the formation of key intermediates and final products. The mechanism has the status of a nonempirical detailed mechanism, since all the constituent elementary reactions have a kinetic substantiation. The mechanism has two specific features: (1) it does not include reactions of so-called double oxygen addition (first to the peroxide radical, and then to its isomeric form), i.e., the first addition turns out to be sufficient; (2) it does not include isomeric compounds and their derivatives as intermediates, since this oxidation pathway is slower than the oxidation of molecules and radicals with normal structure. Application of the algorithm makes it possible to compile a compact mechanism, which is important for modeling chemical processes involving paraffin hydrocarbons C _n with large n. Previously, based on this algorithm, compact mechanisms of the oxidation and combustion of propane, n-butane, n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane have been constructed. In this work, we constructed a nonempirical detailed mechanism of the oxidation and combustion of hydrocarbons from n-undecane to n-hexadecane. The most important feature of the new mechanism is its staged nature, which manifests itself through the emergence of cool and blue flames during low-temperature autoignition. The calculation results are compared with experimental data.     

A dynamic adaptive chemistry scheme for reactive flow computations

An on-the-fly kinetic mechanism reduction scheme, referred to as dynamic adaptive chemistry (DAC), has been developed to incorporate detailed chemical kinetics into reactive flow computations with high efficiency and accuracy. The procedure entails reducing a detailed mechanism to locally and instantaneously accurate sub-mechanisms at each hydrodynamic time step of the calculation, and consequently no a priori information regarding simulation conditions is needed. The reduction utilizes an extended version of the directed relation graph (DRG) method in which the edges are weighted by a value that measures the dependence of the tail species (vertex) on the head species. An R-value is then defined at each vertex as the maximum of the products of these weights along all paths to that vertex from an initiating species. Active species are identified by their R-values exceeding a threshold value, ε_R, using a modified breadth-first search (BFS) that starts from a pre-defined set of initiating species. Chemical kinetics equations are then formulated with respect to the active species, with the inactive species considered only as third body collision partners. The DAC method is implemented into CHEMKIN and tested by simulating homogeneous charge compression ignition (HCCI) combustion using detailed and pre-reduced n-heptane mechanisms (578 species and 178 species, respectively) as the full mechanisms. The DAC scheme reproduces with high accuracy the pressure curves and species mass fractions obtained using the full mechanisms. The on-the-fly mechanism reduction scheme introduces minimal computational overhead and achieves more than 30-fold time reduction in calculations using the 578-species mechanism.     

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.     

Direct ignition and S-curve transition by in situ nano-second pulsed discharge in methane/oxygen/helium counterflow flame

A well-defined plasma assisted combustion system with novel in situ discharge in a counterflow diffusion flame was developed to study the direct coupling kinetic effect of non-equilibrium plasma on flame ignition and extinction. A uniform discharge was generated between the burner nozzles by placing porous metal electrodes at the nozzle exits. The ignition and extinction characteristics of CH₄/O₂/He diffusion flames were investigated by measuring excited OH¹ and OH PLIF, at constant strain rates and O₂ mole fraction on the oxidizer side while changing the fuel mole fraction. It was found that ignition and extinction occurred with an abrupt change of OH¹ emission intensity at lower O₂ mole fraction, indicating the existence of the conventional ignition-extinction S-curve. However, at a higher O₂ mole fraction, it was found that the in situ discharge could significantly modify the characteristics of ignition and extinction and create a new monotonic and fully stretched ignition S-curve. The transition from the conventional S-curves to a new stretched ignition curve indicated clearly that the active species generated by the plasma could change the chemical kinetic pathways of fuel oxidation at low temperature, thus resulting in the transition of flame stabilization mechanism from extinction-controlled to ignition-controlled regimes. The temperature and OH radical distributions were measured experimentally by the Rayleigh scattering technique and PLIF technique, respectively, and were compared with modeling. The results showed that the local maximum temperature in the reaction zone, where the ignition occurred, could be as low as 900 K. The chemical kinetic model for the plasma–flame interaction has been developed based on the assumption of constant electric field strength in the bulk plasma region. The reaction pathways analysis further revealed that atomic oxygen generated by the discharge was critical to controlling the radical production and promoting the chain branching effect in the reaction zone for low temperature ignition enhancement.     

Comparative study on ethyl butanoate reactivity – Experimental investigation and kinetic modeling of the C6 ethyl ester

Ethyl butanoate is a representative for oxygenated hydrocarbons as they are discussed as future liquid fuels from sustainable production pathways. An in-depth understanding of the influence of oxygen on the reactivity of those fuel candidates is mandatory for the molecular design and their application in internal combustion engines. Towards this goal, ignition delay times for ethyl butanoate were measured at conditions relevant to internal combustion engines using a shock tube and a rapid compression machine. These experiments were conducted for stoichiometric mixtures with air-like conditions at pressures of 20, 30 and 40 bar and a total temperature range of 680–1260 K. A negative temperature coefficient regime was found where the ignition delay times increased with increasing temperatures for all covered pressures. To further understand the kinetics of ethyl butanoate and the influence of the ester functional group, a detailed kinetic mechanism was developed and validated against the measured ignition delay times. A good agreement between the measured data and the prediction by the newly developed mechanism was achieved. The findings of this work were then used to compare ethyl butanoate to di-ethyl carbonate, methyl pentanoate and n-heptane, which also show a seven-heavy-atom-membered main chain and have all been kinetically studied before. The differences between the molecular structures and their effect on the kinetic pathways was discussed to extract information for future fuel design. It was found that especially the inhibting effect of oxgen atoms on six-membered internal H-atom migration reactions has a significant impact on the fuel’s reactivity.     

Development of a reduced biodiesel surrogate model for compression ignition engine modeling

Methylbutanoate (MB), a C₄ methyl ester, represents the simplest surrogate that captures the chemical effects of the ester moiety in biodiesel and biodiesel surrogates. An updated chemical kinetic model has been developed to characterize the ignition and flame characteristics of MB. The mechanistic elements within this model that relate to the MB and smaller ester/oxygenate sub-mechanisms are drawn from the prototypical Fisher et al. model and from more recent theory and modeling efforts. The MB model development which is based on an iterative procedure involving global sensitivity analyses to identify elementary reactions that govern ignition and subsequent high level ab initio based theoretical updates to these reaction rates are presented. The MB model makes reasonable predictions of ignition delays and laminar flame speeds.The C₅–C₇ submechanisms from the LLNL n-heptane (NH) model were merged with the present MB model to obtain a detailed chemical kinetics model for a surrogate blend representing biodiesel. The detailed MB-NH model (661 species) was reduced using graph based techniques. The robust reduction techniques employed result in a reduced model (145 species) that is in good agreement with the detailed model over a wide range of conditions. 3-D compression ignition (CI) engine simulations utilizing this reduced chemistry model for MB-NH blends as a surrogate for biodiesel show good agreement with the experimental data suggesting the utility of this model for predictions of combustion and emission characteristics of biodiesel in realistic CI engine simulations.     

Mechanism of the oxidation and combustion of normal paraffin hydrocarbons: Transition from C<Subscript>1</Subscript>–C<Subscript>6</Subscript> to C<Subscript>7</Subscript>H<Subscript>16</Subscript>

A previously proposed algorithm for constructing an optimal mechanism of the high- and low-temperature oxidation and combustion of normal paraffin hydrocarbons was used, which includes the major processes that determine the rate of reaction and the formation of the main intermediate and final products. The mechanism has the status of a nonempirical detailed mechanism, since all the constituent elementary reactions have a kinetic substantiation. The mechanism has two specific features: it included no reactions of so-called double addition of oxygen and no isomeric compounds and derivatives thereof as intermediate species. Realization of this algorithm leads to fairly compact models, a circumstance important for studies of chemical processes involving paraffin hydrocarbons C _n with large n. Previously, based on this algorithm, compact mechanisms of oxidation and combustion of propane, n-butane, n-pentane, and n-hexane were constructed. In this paper, we develop a nonempirical detailed mechanism of oxidation and combustion of n-heptane. The most important feature of the new mechanism is its ability to predict the staging of the process in the form of cool and blue flames at low autoignition temperatures. A comparison of the simulation results with the available experimental data is conducted.     

Experimental counterflow ignition temperatures and reaction mechanisms of 1,3-butadiene

The ignition temperatures of nitrogen-diluted 1,3-butadiene by heated air in counterflow were experimentally determined for pressures up to 5 atmospheres and pressure-weighted strain rates from 100 to 250 s⁻¹. The experimental data were compared with computational results using the mechanism of Laskin et al. [A. Laskin, H. Wang and C.K. Law, Int. J. Chem. Kinet. 32 (10) (2000) 589-614], showing that while the overall prediction is approximately within the experimental uncertainty, the mechanism over-predicts ignition temperature by about 25-40 K, with the differences becoming larger at high pressure/low temperature region. Sensitivity analyses for the near-ignition states were performed for both reactions and diffusion, which identified the importance of H₂/CO chain reactions, three 1,3-butadiene reaction pathways, and the binary diffusion between 1,3-butadiene and N₂ on ignition. The detailed mechanism, consisting of 94 species and 614 reactions, was then simplified to a skeletal mechanism consisting of 46 species and 297 reactions by using a new reduction algorithm combining directed relation graph and sensitivity analysis. The skeletal mechanism was further simplified to a 30-step reduced mechanism by using computational singular perturbation and quasi-steady-state assumptions. Both the skeletal and reduced mechanisms mimic the performance of the detailed mechanism with good accuracy in both homogeneous and heterogeneous systems.     

Effect of pressure on soot formation in the pyrolysis of <Emphasis Type="Italic">n</Emphasis>-hexane and the oxidation of fuel-rich mixtures of <Emphasis Type="Italic">n</Emphasis>-heptane behind reflected shock waves

The results of detailed kinetic simulations of the formation of soot particles in the pyrolysis of n-hexane–argon mixtures and in the oxidation of fuel-rich (φ = 5) n-heptane–oxygen–argon mixtures behind reflected shock waves at pressures of 20–100 bar and a constant concentration of carbon atoms or a constant fraction of argon in the initial mixture within the framework of a modified reaction mechanism are reported. The choice of n-hexane and n-heptane for examining the effect of pressure on the process of soot formation was motivated by the availability for these hydrocarbons of experimental measurements in reflected shock waves at high pressures (up to ~100 bar). The temperature dependences of the yield of soot particles formed in the pyrolysis of n-hexane are found to be very weakly dependent on pressure and slightly shifting to lower temperatures with increasing pressure. In general, pressure produces a very weak effect on the soot formation in the pyrolysis of n-hexane. The effect of pressure and concentration of carbon atoms in the initial mixture on the process of soot formation during the oxidation of fuel-rich n-heptane mixtures behind reflected shock waves is studied. The results of our kinetic simulations show that, for both the pyrolysis of n-hexane and the oxidation of fuel-rich n-heptane–oxygen mixtures, the influence of pressure on the process of soot formation is negligible. By contrast, the concentration of carbon atoms in the initial reaction mixture produces a much more pronounced effect.     

Detailed and simplified kinetic models of n-dodecane oxidation: The role of fuel cracking in aliphatic hydrocarbon combustion

A detailed kinetic model is proposed for the combustion of normal alkanes up to n-dodecane above 850 K. The model was validated against experimental data, including fuel pyrolysis in plug flow and jet-stirred reactors, laminar flame speeds, and ignition delay times behind reflected shock waves, with n-dodecane being the emphasis. Analysis of the computational results reveal that for a wide range of combustion conditions, the kinetics of fuel cracking to form smaller molecular fragments is fast and may be decoupled from the oxidation kinetics of the fragments. Subsequently, a simplified model containing a minimal set of 4 species and 20 reaction steps was developed to predict the fuel pyrolysis rate and product distribution. Combined with the base C₁-C₄ model, the simplified model predicts fuel pyrolysis rate and product distribution, laminar flame speeds, and ignition delays as close as the detailed reaction model.