A skeletal mechanism for flame inhibition by trimethylphosphate

On the basis of a multi-step kinetic mechanism for flame inhibition by organophosphorus compounds including more than 200 reactions, a skeletal mechanism for flame inhibition by trimethylphosphate was developed. The mechanism consists of 22 irreversible elementary reactions, involving nine phosphorus-containing species. Selection of the crucial steps was performed by analysing P-element fluxes from species to species and by calculating net reaction rates of phosphorus-involving reactions versus the flames zone. The developed mechanism was validated by comparing the modelling results with the measured and simulated (using the starting initial mechanism) speed and the chemical structure of H₂/O₂, CH₄/O₂ and syngas/air flames doped with trimethylphosphate. The mechanism was shown to satisfactorily predict the speed of H₂/O₂/N₂ flames with various dilution ratios, CH₄/air and syngas/air flames doped with trimethylphosphate. The skeletal mechanism was also shown to satisfactorily predict the spatial variation of H and OH radicals and the final phosphorus-containing products of the inhibitor combustion. Further reduction of the skeletal mechanism without modification of the rate constants recommended in the starting mechanism was shown to result in noticeable disagreement of the flame speed and structure.     

Issues arising in the construction of QSSA mechanisms: the case of reduced n-heptane/air models for premixed flames

A model reduction methodology, based on the quasi steady-state approximation (QSSA), is employed for the construction of reduced mechanisms in the case of an n-heptane/air premixed flame. Several issues related to the construction of these reduced mechanisms are discussed; such as the influence of the size of the starting skeletal mechanism, the stiffness reduction, and the truncation/simplification of (i) the expressions of the global rates and (ii) the steady-state relations. The starting point for the reduction is two skeletal mechanisms that involve 177/768 and 66/326 species/reactions, respectively [J. Prager, H.N. Najm, M. Valorani, and D.A. Goussis, Skeletal mechanism generation with CSP and validation for premixed n-heptane flames, Proc. Combust. Inst. 32 (2009), pp. 509–517] and which were derived from the detailed mechanism of Curran et al. [H.J. Curran, P. Gaffuri, W.J. Pitz, and C.K. Westbrook, A comprehensive modeling study of iso-octane oxidation, Combust. Flame 129 (2002), pp. 253–280], which involves 561/2538 species/reactions. From these two skeletal mechanisms, a number of reduced mechanisms of various sizes are produced and analysed. The validity of the reduced mechanism with the minimum size is demonstrated by considering its accuracy regarding the mass fractions of major and minor species, the temperature, and the flame speed, over a wide range of equivalence ratios and pressures.     

正丁烷燃料中低温氧化的骨架反应机理

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

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.     

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.     

Evaluation of the reaction rate constants for the gas-phase Al-CH4–air combustion chemistry

The most likely reaction pathways and reaction products in the Al-CH₄-O₂-N₂ system are investigated using density functional theory and ab initio calculations. The B3LYP functional with extended 6–311+G(3df,2p) basis set as well as the CBS-QB3 composite method are mainly utilised. Theoretical analysis of corresponding reaction rate constants is also performed with the use of simple theoretical models. A critical overview of current knowledge on combustion-relevant reactions with aluminium compounds is given. On the basis of critical comparison of available experimental kinetic data with theoretical calculations, the approximations for rate constants for 44 reversible elementary reactions involving Al-containing species are recommended for use in combustion issues.     

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.     

Mechanisms of the oxidation and combustion of normal alkanes: Passage from C1-C4 to C2H5

A previously proposed algorithm was applied to constructing an optimal n-pentane oxidation mechanism capable of predicting the reaction rate and the formation of the main intermediate and final products. The mechanism can be considered a nonempirical detailed mechanism, since the constituent elementary reactions are kinetically validated. The mechanism is based on two assumptions: it ignores reactions involving double addition of oxygen (first, to a peroxy radical and then to its isomeric form) and isomer compounds and derivatives thereof. For low-temperature autoignition, the mechanism reproduces the stage character of the oxidation of n-pentane, more specifically, the emergence of cool and blue flames. The calculation results were compared with the published experimental data.     

Skeletal mechanism generation and analysis for n-heptane with CSP

We use a procedure based on the decomposition into fast and slow dynamical components offered by the Computational Singular Perturbation (CSP) method to generate automatically skeletal kinetic mechanisms for the simplification of the kinetics of n-heptane oxidation. The detailed mechanism of the n-heptane oxidation here considered has been proposed by Curran et al. and involves 561 species and 2538 reactions. After carrying out a critical assessment of important aspects of this procedure, we show that the comprehensive skeletal kinetic mechanisms so generated are able to reproduce the main features of n-heptane ignition at various initial pressures and temperatures and equivalence ratios. A by-product of the algorithm that generates the skeletal mechanisms is the identification of the network of important species and reactions at a given state of the kinetic system. The analysis of this network is carried out by resorting to a visual representation of the pathways at selected time instants of the ignition process. Visual inspection of the pathways enables the identification and comparison of the relevant kinetic processes as obtained at different ignition regimes. The graphs are generated by interfacing the model reduction procedure with the open-source package graphviz.     

Effects of temperature and equivalence ratio on the ignition of n-heptane fuel spray in turbulent flow

Direct numerical simulations were performed to study the autoignition process of n-heptane fuel spray in a turbulent field. For the solution of the carrier gas fluid, the Eulerian method is employed, while for the fuel droplets, the Lagrangian method is used. Droplets are initialized at random locations in a two-dimensional isotropic turbulent field. A chemistry mechanism for n-heptane with 44 species and 112 reactions was adopted to describe the chemical reactions. Three cases with the same initial global equivalence ratio (0.5) and different initial gas phase temperatures (1100, 1200, and 1300 K) were simulated. In addition, two cases with initial global equivalence ratios of 1.0 and 1.5 and initial temperature 1300 K were simulated to examine the effect of equivalence ratio. Evolution of temperature, species mass fraction, reaction rate, and the joint PDF of temperature and equivalence ratio are presented. Effects of the initial gas temperature and equivalence ratio on vaporization and ignition are discussed. A correlation was derived relating ignition delay times to temperature and equivalence ratio. It was confirmed that with the increase of initial temperature, the autoignition occurs earlier. With the increase of the initial equivalence ratio, however, autoignition occurs later due to a larger decrease in gas phase temperature caused by fuel droplet evaporation. The results obtained in this study are expected to be constructive in understanding fuel spray combustion, such as that in homogeneous charge compression ignition systems.     

Simulation of the autoignition and combustion of <Emphasis Type="Italic">n</Emphasis>-heptane droplets using a detailed kinetic mechanism

The autoignition and combustion of n-heptane droplets are simulated using a detailed kinetic mechanism. A mathematical model, based on first principles, contains no adjustable parameters. The burning rate constants for the combustion of droplets are calculated over a wide range of pressures, temperature, fuel-to-oxidizer equivalence ratios of the gas-droplet suspension, and droplet diameters. The calculated and measured delay times of autoignition of droplets are compared. The calculation results agree well with the available experimental data. The detonability of gas-droplet suspensions with partial pre-evaporation of fuel is estimated.     

A reduced mechanism for biodiesel surrogates with low temperature chemistry for compression ignition engine applications

Biodiesel is a promising alternative fuel for compression ignition (CI) engines. It is a renewable energy source that can be used in these engines without significant alteration in design. The detailed chemical kinetics of biodiesel is however highly complex. In the present study, a skeletal mechanism with 123 species and 394 reactions for a tri-component biodiesel surrogate, which consists of methyl decanoate, methyl 9-decanoate and n-heptane was developed for simulations of 3-D turbulent spray combustion under engine-like conditions. The reduction was based on an improved directed relation graph (DRG) method that is particularly suitable for mechanisms with many isomers, followed by isomer lumping and DRG-aided sensitivity analysis (DRGASA). The reduction was performed for pressures from 1 to 100 atm and equivalence ratios from 0.5 to 2 for both extinction and ignition applications. The initial temperatures for ignition were from 700 to 1800 K. The wide parameter range ensures the applicability of the skeletal mechanism under engine-like conditions. As such the skeletal mechanism is applicable for ignition at both low and high temperatures. Compared with the detailed mechanism that consists of 3299 species and 10806 reactions, the skeletal mechanism features a significant reduction in size while still retaining good accuracy and comprehensiveness. The validations of ignition delay time, flame lift-off length and important species profiles were also performed in 3-D engine simulations and compared with the experimental data from Sandia National Laboratories under CI engine conditions.     

The mechanisms of oxidation and combustion of normal alkane hydrocarbons: The transition from C1–C3 to C4H10

The known detailed mechanisms of oxidation of the higher hydrocarbons include hundreds of particles and thousands of reactions. In spite of their merits, the use of such mechanisms for solving applied problems of the gas dynamics of combustion is impeded at present because of great computational expenditures. We suggest a compact kinetic mechanism of the oxidation of n-butane including the main processes and intermediate and final reaction products. The mechanism can be classified as a nonempirical detailed mechanism, because all its elementary reactions are kinetically substantiated. The mechanism does not contain reactions of the double addition of oxygen and intermediate species in the form of isomeric compounds and their derivatives. The calculation results are compared with the experimental data on the oxidation, self-ignition, and combustion of n-butane.     

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.     

Criteria for selection of components for surrogates of natural gas and transportation fuels

The present paper addressed the production of soot precursors, acetylene, benzene and higher aromatics, by the paraffinic (n-, iso-, and cyclo-) and aromatic components in fuels. To this end, a normal heptane mechanism compiled from sub-models in the literature was extended to large normal-, iso-, and cyclo-paraffins by assigning generic rates to reactions involving paraffins, olefins, and alkyl radicals in the same reaction class. Lumping was used to develop other semi-detailed sub-models. The resulting mechanism for components of complex fuels (named the Utah Surrogate Mechanism) includes detailed sub-models of n-butane, n-hexane, n-heptane, n-decane, n-dodecane, n-tetradecane and n-hexadecane, and semi-detailed sub-models of i-butane, i-pentane, n-pentane, 2,4-dimethyl pentane, i-octane, 2,2,3,3-tetramethyl butane, cyclohexane, methyl cyclohexane, tetralin, 2-methyl 1-butene, 3-methyl 2-pentene and aromatics. Generic rates of reaction classes were found adequate to generate reaction mechanisms of large paraffinic components. The predicted maximum concentrations of the fuel, oxidizer, and inert species, major products and important combustion intermediates, which include critical radicals and soot precursors, were in good agreement with the experimental data of three premixed flames of composite fuels under various conditions. The relative importance in benzene formation of each component in the kerosene surrogate was found to follow the trend aromatics > cyclo-paraffins > iso-paraffins > normal-paraffins. In contrast, acetylene formation is not that sensitive to the fuel chemical structure. Therefore, in formulation of surrogate fuels, attention should be focused on selecting components that will yield benzene concentrations comparable to those produced by the fuel, with the assurance that the acetylene concentration will also be well approximated.     

Analysis of first stage ignition delay times of dimethyl ether in a laminar flow reactor

The combustion chemistry of the first stage ignition and chemistry/flow interactions are studied for dimethyl ether (DME) with a mathematical analysis of two systems: a plug flow reactor study is used to reduce the reaction chemistry systematically. A skeletal reaction mechanism for the low temperature chemistry of DME until the onset of ignition is derived on the basis of the detailed DME mechanism of the Lawrence Livermore National Laboratory – see Curran, Fischer and Dryer, Int. J. Chem. Kinetics, Vol. 32 (2000). It is shown that reasonably good results for ignition delay times can be reached using a simple system of three ordinary differential equations and that the resulting analytical solution depends only on two reaction rates and the initial fuel concentration. The stepwise reduction of the system based on assumptions yields an understanding on why these reactions are so important. Furthermore, the validation of the assumptions yields insight into the influence of the fuel and the oxygen concentration on the temperature during the induction phase. To investigate the influence of chemistry/flow interactions, a 2D model with a laminar Hagen–Poiseuille flow and 2D-polynomial profiles for the radial species concentration is considered. For the 2D model, it is found that only the diffusion coefficients and the reactor radius need to be taken into consideration additionally to describe the system sufficiently. Also, the coupling of flow and chemistry is clarified in the mathematical analysis. The insight obtained from the comparison of the 2D model and the plug flow model is used to establish an average velocity for the conversion of ignition locations to ignition delay times in a laminar flow reactor. Finally, the 2D analytical solution is compared against new experimental data, obtained in such a laminar flow reactor for an undiluted DME/air mixture with an equivalence ratio of φ = 0.835 and a temperature range of 555 to 585 K at atmospheric pressure.     

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

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