A comprehensive experimental and modeling study of iso-pentanol combustion

Biofuels are considered as potentially attractive alternative fuels that can reduce greenhouse gas and pollutant emissions. iso-Pentanol is one of several next-generation biofuels that can be used as an alternative fuel in combustion engines. In the present study, new experimental data for iso-pentanol in shock tube, rapid compression machine, jet stirred reactor, and counterflow diffusion flame are presented. Shock tube ignition delay times were measured for iso-pentanol/air mixtures at three equivalence ratios, temperatures ranging from 819 to 1252 K, and at nominal pressures near 40 and 60 bar. Jet stirred reactor experiments are reported at 5 atm and five equivalence ratios. Rapid compression machine ignition delay data was obtained near 40 bar, for three equivalence ratios, and temperatures below 800 K. Laminar flame speed data and non-premixed extinction strain rates were obtained using the counterflow configuration. A detailed chemical kinetic model for iso-pentanol oxidation was developed including high- and low-temperature chemistry for a better understanding of the combustion characteristics of higher alcohols. First, bond dissociation energies were calculated using ab initio methods, and the proposed rate constants were based on a previously presented model for butanol isomers and n-pentanol. The model was validated against new and existing experimental data at pressures of 1–60 atm, temperatures of 650–1500 K, equivalence ratios of 0.25–4.0, and covering both premixed and non-premixed environments. The method of direct relation graph (DRG) with expert knowledge (DRGX) was employed to eliminate unimportant species and reactions in the detailed mechanism, and the resulting skeletal mechanism was used to predict non-premixed flames. In addition, reaction path and temperature A-factor sensitivity analyses were conducted for identifying key reactions at various combustion conditions.     

Numerical investigation on ignition intensification of n-butane with tert-butyl hydroperoxide (TBHP) addition

Aimed to intensify the ignition and combustion process of n-butane fuel in micro internal combustion (IC) engines, ignition delay characteristics of n-butane/air mixtures with tert-butyl hydroperoxide (TBHP) addition ratios below 10% were investigated numerically by CHEMKIN-PRO software. Results show that ignition delay times of n-butane can be shortened nonlinearly with TBHP addition at initial temperatures of 650 K to 1000 K, namely, the reduction rate of ignition delay times rises slowly as the TBHP addition ratio increases. In addition, the negative temperature coefficient (NTC) behavior can be weakened significantly with TBHP addition at 650 K to 1000 K. Especially, the ignition delay time of n-butane can be reduced about 32 times with 10% TBHP addition at 750 K. However, the ignition intensification effect of TBHP addition is slight when the initial temperature is above 1000 K. The acting mechanism of TBHP addition was investigated in detail by the rate of production and consumption (ROP), sensitivity, and reaction pathway analysis. The ROP analysis shows that the released and quickly consumed OH radicals with TBHP addition play an important role in promoting n-butane ignition at the lower initial temperature. However, the relatively slow consumption rate of OH radicals at higher temperature weakens the intensification effect. Furthermore, the sensitivity analysis and reaction pathway analysis indicate that the dominated elemental reactions and reaction pathway vary significantly with TBHP addition at lower initial temperature.     

微型内燃机工况下C1–C4烷烃着火延迟数值模拟

燃料着火延迟时间对采用蓄热自着火方式的微型内燃机非常重要。利用Chemkin-Pro软件,分别对甲烷、乙烷、丙烷和正丁烷空气混合气在微型内燃机运行工况下进行着火延迟时间模拟计算,探究初始温度（500 K ~ 1 000 K）、压力（1~ 10atm）和当量比（0.6 ~ 1.2）对着火延迟时间的影响。同时分析了微型内燃机扫气不尽的尾气残留组分（N2、CO2和H2O）对正丁烷着火延迟时间的影响。结果表明：在四种燃料中,正丁烷的低温着火延迟特性最佳,是一种适合于采用蓄热自着火方式的微型内燃机燃料;初始温度、压力的提高和当量比的增大有利于燃料着火延迟时间的缩短;尾气残留使得燃料着火延迟时间变长,着火延迟特性变差,尾气各组分的热效应和基元反应对燃料着火延迟有着不同的影响机制。     

Low temperature n-butane oxidation skeletal mechanism, based on multilevel approach

In order to reconcile an increasingly large deviation (order of magnitude) of the ignition delay time at decreasing initial temperature, computed using the prior art kinetic schemes, with the available experimental values, a new skeletal mechanism (54 species, 94 reactions) for low-temperature (500-800 K) ignition of n- butane in air based on ab initio calculations is developed. The skeletal mechanism obtained accurately reproduces n-butane combustion kinetics for the practically important ranges of pressure, temperature and fuel-air equivalence ratio, especially in the low-temperature range. The elaborated first principal skeletal chemical kinetic mechanism of n-butane oxidation was validated against available experimental results for normal and elevated initial pressure (1-15 atm) using the Chemical Work Bench code. A good agreement with experiments was shown.     

Skeletal mechanism generation for high-temperature oxidation of kerosene surrogates

A skeletal mechanism with 106 species and 382 reactions is developed from a detailed kerosene combustion chemical kinetic mechanism that includes 209 species and 1673 reversible reactions for a tricomponent surrogate mixtures, consisting of n-decane, n-propylcyclohexane, and n-propylbenzene. The directed relation graph (DRG) method for skeletal mechanism reduction is applied as the first step for reduction of mechanisms with large numbers of species. A revised DRG approach (Z.Y. Luo, T.F. Lu, et al., Energy Fuels 24 (2010) 6283–6293) is compared with the original one and it is shown to be more stable than the original DRG method. The simplified iterative screening and structure analysis method (ISSA) is used subsequently based on the reduced mechanism generated by the DRG method to remove redundant species and reactions simultaneously. A minimal skeletal mechanism from the detailed mechanism for kerosene combustion is thus obtained. It is found that the discrete local reaction rates rather than the time-averaged reaction rates should be adopted when using the ISSA method for mechanism reduction due to large temperature difference and different reaction pathways in combustion at different simulation conditions. Although reduced in size by a factor of 2 for species and a factor of 4 for reactions, the skeletal mechanism exhibits high accuracy for high-temperature applications in predicting global combustion parameters, such as ignition delay, detailed profiles of species concentrations, and laminar flame speed. Furthermore, numerical simulation results of different mixture compositions are also comparable with those based on the detailed mechanism, indicating that the major reaction pathways of each component are captured by the reduced mechanism and the hierarchical structure of the detailed mechanism is maintained.     

n-Butane: Ignition delay measurements at high pressure and detailed chemical kinetic simulations

Ignition delay time measurements were recorded at equivalence ratios of 0.3, 0.5, 1, and 2 for n-butane at pressures of approximately 1, 10, 20, 30 and 45 atm at temperatures from 690 to 1430 K in both a rapid compression machine and in a shock tube. A detailed chemical kinetic model consisting of 1328 reactions involving 230 species was constructed and used to validate the delay times. Moreover, this mechanism has been used to simulate previously published ignition delay times at atmospheric and higher pressure. Arrhenius-type ignition delay correlations were developed for temperatures greater than 1025 K which relate ignition delay time to temperature and concentration of the mixture. Furthermore, a detailed sensitivity analysis and a reaction pathway analysis were performed to give further insight to the chemistry at various conditions. When compared to existing data from the literature, the model performs quite well, and in several instances the conditions of earlier experiments were duplicated in the laboratory with overall good agreement. To the authors’ knowledge, the present paper presents the most comprehensive set of ignition delay time experiments and kinetic model validation for n-butane oxidation in air.     

Pyrolysis and oxidation of decalin at elevated pressures: A shock-tube study

Ignition delay times and ethylene concentration time-histories were measured behind reflected shock waves during decalin oxidation and pyrolysis. Ignition delay measurements were conducted for gas-phase decalin/air mixtures over temperatures of 769–1202 K, pressures of 11.7–51.2 atm, and equivalence ratios of 0.5, 1.0, and 2.0. Negative-temperature-coefficient (NTC) behavior of decalin autoignition was observed, for the first time, at temperatures below 920 K. Current ignition delay data are in good agreement with past shock tube data in terms of pressure dependence but not equivalence ratio dependence. Ethylene mole fraction and fuel absorbance time-histories were acquired using laser absorption at 10.6 and 3.39 μm during decalin pyrolysis for mixtures of 2200–3586 ppm decalin/argon at pressures of 18.2–20.2 atm and temperatures of 1197–1511 K. Detailed comparisons of these ignition delay and species time-history data with predictions based on currently available decalin reaction mechanisms are presented, and preliminary suggestions for the adjustment of some key rate parameters are made.     

Autoignition of surrogate biodiesel fuel (B30) at high pressures: Experimental and modeling kinetic study

Ignition delay times of surrogate biodiesel fuels were measured in a high-pressure shock tube over a wide range of experimental conditions (pressures of 20 and 40 bar, equivalence ratios in the range 0.5–1.5, and temperatures ranging from 700 to 1200 K). A detailed chemical kinetic mechanism developed for the oxidation of a biodiesel fuel and a B30 biodiesel surrogate (49% n-decane, 21% 1-methylnaphthalene, and 30% methyloctanoate in mol%) was used to simulate the present experiments. Cross reactions between radicals from the three fuel components and reactions of methylnaphthalene oxidation recently proposed in the literature were introduced into the model in order to improve ignition delay time predictions at low temperatures. The new scheme (7865 reversible reactions and 1975 species) yields improved model predictions of concentration profiles measured earlier in a jet-stirred reactor, and also represents fairly well the present experimental data over the entire range of conditions of this study. Sensitivity analyses and reaction path analyses were used to rationalize the results.     

Low-temperature speciation and chemical kinetic studies of n-heptane

Although there have been many ignition studies of n-heptane—a primary reference fuel—few studies have provided detailed insights into the low-temperature chemistry of n-heptane through direct measurements of intermediate species formed during ignition. Such measurements provide understanding of reaction pathways that form toxic air pollutants and greenhouse gas emissions while also providing key metrics essential to the development of chemical kinetic mechanisms. This paper presents new ignition and speciation data taken at high pressure (9 atm), low temperatures (660–710 K), and a dilution of inert gases-to-molecular oxygen of 5.64 (mole basis). The detailed time-histories of 17 species, including large alkenes, aldehydes, carbon monoxide, and n-heptane were quantified using gas chromatography. A detailed chemical kinetic mechanism developed previously for oxidation of n-heptane reproduced experimentally observed ignition delay times reasonably well, but predicted levels of some important intermediate chemical species that were significantly different from measured values. Results from recent theoretical studies of low temperature hydrocarbon oxidation reaction rates were used to upgrade the chemical kinetic mechanism for n-heptane, leading to much better agreement between experimental and computed intermediate species concentrations. The implications of these results to many other hydrocarbon fuel oxidation mechanisms in the literature are discussed.     

Chemical kinetic reaction mechanism for the combustion of propane

A detailed chemical kinetic reaction mechanism for the combustion of propane is presented and discussed. The mechanism consists of 27 chemical species and 83 elementary chemical reactions. Ignition and combustion data as determined in shock tube studies were used to evaluate the mechanism. Numerical simulation of the shock tube experiments showed that the kinetic behavior predicted by the mechanism for stoichiometric mixtures is in good agreement with the experimental results over the entire temperature range examined (1150–2600K). Sensitivity and theoretical studies carried out using the mechanism revealed that hydrocarbon reactions which are involved in the formation of the HO₂ radical and the H₂O₂ molecule are very important in the mechanism and that the observed nonlinear behavior of ignition delay time with decreasing temperature can be interpreted in terms of the increased importance of the HO₂ and H₂O₂ reactions at the lower temperatures.     

正庚烷均质压燃燃烧简化动力学模型

在详细化学反应动力学研究的基础上，通过对正庚烷均质压燃燃烧各阶段反应途径分析和敏感性分析，构建了一个新的包括35种物质和41个基元反应的正庚烷均质压燃简化动力学模型．在发动机模拟方面，对此模型进行有效性分析，结果表明，在一定的边界条件范围内，简化动力学模型在着火时刻、缸内温度和压力方面都与详细动力学模型吻合较好，简化动力学模型适用于模拟HCCI部分燃烧边界条件．     

正辛醇/正丁醚骨架机理的构建及验证

采用定向关联图误差传播敏感度分析法(DRGEPSA)、同分异构体合并和峰值浓度分析法对正丁醚和正辛醇的详细机理进行简化,并将简化后的正丁醚和正辛醇机理合并,获得了包含117个组分和601个基元反应的骨架机理.利用正丁醚和正辛醇详细机理的着火延迟、正丁醚层流火焰速度的试验数据和正辛醇在射流搅拌反应器(JSR)中组分摩尔浓度的试验数据对获得的骨架机理进行验证.最后分析了正丁醚和正辛醇燃料在低温及高温下的化学反应路径,结果表明:该骨架机理的着火延迟时间、层流火焰速度及JSR中组分摩尔浓度与试验数据和详细机理较吻合,可较好地重现正丁醚和正辛醇的燃烧特性.     

A two-step chemical scheme for kerosene-air premixed flames

A reduced two-step scheme (called 2S_KERO_BFER) for kerosene-air premixed flames is presented in the context of Large Eddy Simulation of reacting turbulent flows in industrial applications. The chemical mechanism is composed of two reactions corresponding to the fuel oxidation into CO and H₂O, and the CO − CO₂ equilibrium. To ensure the validity of the scheme for rich combustion, the pre-exponential constants of the two reactions are tabulated versus the local equivalence ratio. The fuel and oxidizer exponents are chosen to guarantee the correct dependence of laminar flame speed with pressure. Due to a lack of experimental results, the detailed mechanism of Dagaut composed of 209 species and 1673 reactions, and the skeletal mechanism of Luche composed of 91 species and 991 reactions have been used to validate the reduced scheme. Computations of one-dimensional laminar flames have been performed with the 2S_KERO_BFER scheme using the CANTERA and COSILAB softwares for a wide range of pressure ([1; 12] atm), fresh gas temperature ([300; 700] K), and equivalence ratio ([0.6; 2.0]). Results show that the flame speed is correctly predicted for the whole range of parameters, showing a maximum for stoichiometric flames, a decrease for rich combustion and a satisfactory pressure dependence. The burnt gas temperature and the dilution by Exhaust Gas Recirculation are also well reproduced. Moreover, the results for ignition delay time are in good agreement with the experiments.     

A comprehensive modeling study of iso-octane oxidation

A detailed chemical kinetic mechanism has been developed and used to study the oxidation of iso-octane in a jet-stirred reactor, flow reactors, shock tubes and in a motored engine. Over the series of experiments investigated, the initial pressure ranged from 1 to 45 atm, the temperature from 550 K to 1700 K, the equivalence ratio from 0.3 to 1.5, with nitrogen-argon dilution from 70% to 99%. This range of physical conditions, together with the measurements of ignition delay time and concentrations, provide a broad-ranging test of the chemical kinetic mechanism. This mechanism was based on our previous modeling of alkane combustion and, in particular, on our study of the oxidation of n-heptane. Experimental results of ignition behind reflected shock waves were used to develop and validate the predictive capability of the reaction mechanism at both low and high temperatures. Moreover, species’ concentrations from flow reactors and a jet-stirred reactor were used to help complement and refine the low and intermediate temperature portions of the reaction mechanism, leading to good predictions of intermediate products in most cases. In addition, a sensitivity analysis was performed for each of the combustion environments in an attempt to identify the most important reactions under the relevant conditions of study.     

Development of reduced and optimized reaction mechanisms based on genetic algorithms and element flux analysis

The present paper introduces an approach for the automatic development of reduced reaction mechanisms for hydrocarbon combustion. An iterative reduction procedure is adopted with the aim of gradually reducing the number of species involved in the mechanism, while still maintaining its predictiveness in terms of not only ignition delay times, but also the time evolution of important species. In particular, a global error function is defined taking into account a set of 18 ignition delay calculations at different, engine-relevant, initial mixture compositions, temperatures and pressures. The choice of the species to be deleted is performed exploiting the element flux analysis method, first introduced by Revel et al.; when a global error function of the reduced mechanism exceeds the required accuracy, the collision frequencies and activation energies of selected reactions are corrected by means of a GA-based code. The procedure is repeated until the lowest number of species at the required global error tolerance is achieved. The methodology is applied to a detailed mechanism of ethanol combustion consisting of 58 species and 383 reactions to produce an optimal reduced mechanism of 33 species and 155 reactions.     

Experimental and modeling study on ignition delay of ammonia/methane fuels

Ammonia mixed with methane is a potential clean fuel for engine applications toward a low carbon economy. Studies are scarce on ignition phenomenon for ammonia/methane fuels in literature. In the present study, the ignition characteristics for ammonia–methane–air mixtures have been investigated by both experimental measurements and numerical simulations. Ignition processes of a 60%ammonia/40%methane (mol%) fuel blend were investigated with shock-tube experiments. Measurements of the ignition delay times were performed behind reflected shock waves for such fuel/air mixtures with different equivalence ratios of 0.5, 1, and 2, at pressures around 2 and 5 atm within the temperature range of 1369 to 1804 K. Experimental results were then compared with numerical prediction results employing detailed kinetic mechanism, which showed satisfactory agreement within most of the range of the temperatures, equivalence ratios, and pressures investigated. Within the temperature range of 1300 to 1900 K, pressure range of 1 to 10 atm, equivalence ratio range of 0.5 to 2, and methane proportion range of 0% to 50% in fuel blends, the impacts of temperature, pressure, equivalence ratio, and methane additive were simulated on the ignition delay times of the fuel blends based upon the numerical model. It was found that the improvement of ammonia/methane ignition is significant with the increase of temperature, pressure, and methane additive while it is relatively not sensitive to equivalence ratio within the studied conditions. This suggests a promising potential of such fuel blends in real engine application. In addition to the calculations, reaction sensitivity analyses were also performed to have a deep insight into the observed differences between ammonia/methane/air ignition delay times with variation of conditions.     

Shock tube measurements of ignition delay times for the butanol isomers

Ignition delay times of the four isomers of butanol were measured behind reflected shock waves over a range of experimental conditions: 1050–1600 K, 1.5–43 atm, and equivalence ratios of 1.0 and 0.5 in mixtures containing 4% O₂ diluted in argon. Additional data were also collected at 1.0–1.5 atm in order to replicate conditions used by previous researchers. Good agreement is seen with past work for 1-butanol ignition delay times, though our measured data for the other isomers were shorter than those found in some previous studies, especially at high temperatures. At most conditions, the ignition delay time increases for each isomer in the following order: 1-butanol, 2-butanol and i-butanol nearly equal, and t-butanol. In addition, t-butanol has a higher activation energy than the other three isomers. In a separate series of high-pressure experiments, ignition delay times of 1-butanol in stoichiometric air were measured at temperatures as low as 800 K. At temperatures below 1000 K, pre-ignition pressure rises as well as significant rollover of ignition delay times were observed. Modeling of all collected data using several different chemical kinetic mechanisms shows partial agreement with the experimental data depending on the mechanism, isomer, and conditions. Only the mechanism developed by Vranckx et al. [1] partially explains the rollover and pre-ignition observed in stoichiometric experiments in air.     

Shock tube study on ignition delay of multi-component syngas mixtures – Effect of equivalence ratio

Ignition delays were measured in a shock tube for syngas mixtures with argon as diluent at equivalence ratios of 0.3, 1.0 and 1.5, pressures of 0.2, 1.0 and 2.0 MPa and temperatures from 870 to 1350 K. Results show that the influences of equivalence ratio on the ignition of syngas mixtures exhibit different tendency at different temperatures and pressures. At low pressure, the ignition delay increases with an increase in equivalence ratio at tested temperature. At high pressures, however, an opposite behavior is presented, that is, increasing equivalence ratio inhibits the ignition at high temperature and vice versa at intermediate temperature. The affecting degree of equivalence ratio on ignition delay is different for each mixture at given condition, especially for the syngas with high CO concentration. Sensitivity analyses demonstrate that reaction H + O₂ = O + OH (R1) dominates the syngas oxidation under all conditions. With the increase of CO mole fraction, reactions CO + OH = CO₂ + H (R27) and CO + HO₂ = CO₂ + OH (R29) become more important in the syngas ignition kinetics. With the increase of pressure, the reactions related to HO₂ and H₂O₂ play the dominate role. The opposite influence of equivalence ratio on ignition delay at high- and intermediate-temperatures is chemically interpreted through kinetic analyses.     

Mechanism reduction for multicomponent surrogates: A case study using toluene reference fuels

Strategies and recommendations for performing skeletal reductions of multicomponent surrogate fuels are presented, through the generation and validation of skeletal mechanisms for a three-component toluene reference fuel. Using the directed relation graph with error propagation and sensitivity analysis method followed by a further unimportant reaction elimination stage, skeletal mechanisms valid over comprehensive and high-temperature ranges of conditions were developed at varying levels of detail. These skeletal mechanisms were generated based on autoignition simulations, and validation using ignition delay predictions showed good agreement with the detailed mechanism in the target range of conditions. When validated using phenomena other than autoignition, such as perfectly stirred reactor and laminar flame propagation, tight error control or more restrictions on the reduction during the sensitivity analysis stage were needed to ensure good agreement. In addition, tight error limits were needed for close prediction of ignition delay when varying the mixture composition away from that used for the reduction. In homogeneous compression-ignition engine simulations, the skeletal mechanisms closely matched the point of ignition and accurately predicted species profiles for lean to stoichiometric conditions. Furthermore, the efficacy of generating a multicomponent skeletal mechanism was compared to combining skeletal mechanisms produced separately for neat fuel components; using the same error limits, the latter resulted in a larger skeletal mechanism size that also lacked important cross reactions between fuel components. Based on the present results, general guidelines for reducing detailed mechanisms for multicomponent fuels are discussed.     

Selecting the optimum quasi-steady-state species for reduced chemical kinetic mechanisms using a genetic algorithm

A genetic optimization algorithm has been applied to the selection of quasi-steady-state (QSS) species in reduced chemical kinetic mechanisms. The algorithm seeks to minimize the error between reduced and detailed chemistry for simple reactor calculations approximating conditions of interest for a computational fluid dynamics simulation. The genetic algorithm does not guarantee that the global optimum will be found, but much greater accuracy can be obtained than by choosing QSS species through a simple kinetic criterion or by human trial and error. The algorithm is demonstrated for methane–air combustion over a range of temperatures and stoichiometries and for homogeneous charge compression ignition engine combustion. The results are in excellent agreement with those predicted by the baseline mechanism. A factor of two reduction in the number of species was obtained for a skeletal mechanism that had already been greatly reduced from the parent detailed mechanism.