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.     

Isobutane ignition delay time measurements at high pressure and detailed chemical kinetic simulations

Rapid compression machine and shock-tube ignition experiments were performed for real fuel/air isobutane mixtures at equivalence ratios of 0.3, 0.5, 1, and 2. The wide range of experimental conditions included temperatures from 590 to 1567 K at pressures of approximately 1, 10, 20, and 30 atm. These data represent the most comprehensive set of experiments currently available for isobutane oxidation and further accentuate the complementary attributes of the two techniques toward high-pressure oxidation experiments over a wide range of temperatures. The experimental results were used to validate a detailed chemical kinetic model composed of 1328 reactions involving 230 species. This mechanism has been successfully used to simulate previously published ignition delay times as well. A thorough sensitivity analysis was performed to gain further insight to the chemical processes occurring at various conditions. Additionally, useful ignition delay time correlations were developed for temperatures greater than 1025 K. Comparisons are also made with available isobutane data from the literature, as well as with 100% n-butane and 50-50% n-butane-isobutane mixtures in air that were presented by the authors in recent studies. In general, the kinetic model shows excellent agreement with the data over the wide range of conditions of the present study.     

Effects of buffer gas composition on autoignition

This work quantifies the chemical kinetic and thermal effects of buffer gas composition on autoignition of three fuels at conditions relevant to engines, combustors, and experimental facilities used to study ignition kinetics. Computational simulations of autoignition of iso-octane, n-heptane, and of n-butanol were used to characterize the effects of buffer gas composition on ignition delay time and heat release rate. Stoichiometric mixtures, ? = 1.0, and a temperature range of 600–1100 K were considered. Iso-octane and n-heptane were studied at initial pressures of 9.0 atm and 60.0 atm, and n-butanol was studied at initial pressures of 3.2 atm and 60.0 atm. Two dilution levels of buffer gas to O₂ of 3.76:1 (mole basis) and 5.64:1 were considered (∼21% and ∼15% O₂ respectively, mole basis). The fuels and simulation conditions were selected based on the relevance to engine operating conditions and previously published ignition studies. The buffer gases considered were argon, nitrogen, water, and carbon dioxide. Simulation results predicted changes of greater than a factor of 2 in ignition delay time and heat release rate as a function of buffer gas composition in the negative temperature coefficient (NTC) region for n-heptane and iso-octane. Outside the NTC region, the predicted effects of changes in buffer gas composition were small (<20%); however, experimental data for n-heptane indicate larger effects of buffer gas composition on ignition delay time at higher temperatures (>a factor of 2). The heat release rates were also sensitive to buffer gas composition, with carbon dioxide exhibiting relatively high levels of early and late heat release relative to the other buffer gases. Sensitivity analysis of the third-body collision efficiencies for the buffer gases showed the effects of uncertainties in the third body collision efficiencies on ignition delay time and heat release rate. The results highlight the significance of buffer gas composition on low-temperature combustion chemistry, particularly via H₂O₂ and HO₂ decomposition and recombination reactions.     

Autoignition of n-decane under elevated pressure and low-to-intermediate temperature conditions

An experimental investigation of the autoignition for various n-decane/oxidizer mixtures is conducted using a rapid compression machine, in the range of equivalence ratios of ?=0.5-2.2, dilution molar ratios of N₂/(O₂ + N₂) = 0.79-0.95, compressed gas pressures of PC=7-30 bar, and compressed gas temperatures of TC=635-770 K. The current experiments span a temperature range not fully investigated in previous autoignition studies on n-decane. Two-stage ignition, characteristic of large hydrocarbons, is observed over the entire range of conditions investigated, as demonstrated in the plots of raw experimental pressure traces. In addition, experimental results reveal the sensitivity of the first-stage and total ignition delays to variations in fuel and oxygen mole fractions, pressure, and temperature. Predictability of two kinetic mechanisms is compared against the present data. Discrepancies are noted and discussed, which are of direct relevance for further improvement of kinetic models of n-decane at conditions of elevated pressures and low-to-intermediate temperatures.     

Study on combustion and ignition characteristics of natural gas components in a micro flow reactor with a controlled temperature profile

Combustion and ignition characteristics of natural gas components such as methane, ethane, propane and n-butane were investigated experimentally and computationally using a micro flow reactor with a controlled temperature profile. Special attention was paid to weak flames which were observed in a low flow velocity region. The observed weak flame responses for the above fuels were successfully simulated by one-dimensional computations with a detailed kinetic model for natural gas. Since the position of the weak flame indicates the ignition characteristics as well as the reactivity of each fuel, the experimental and computational results were compared with research octane number (RON) which is a general index for ignition characteristics of ordinary fuels. At 1 atm, ethane showed the highest reactivity among these fuels, although RON of ethane (115) is between those of methane (120) and propane (112). Since the pressure conditions are different between the present experiment and the general RON test, weak flame responses to the pressure were investigated computationally for these fuels. The order of the fuel reactivity by the reactor agreed with that by RON test when the pressure was higher than 4 atm. Reaction path analysis was carried out to clarify the reasons of the highest reactivity of ethane at 1 atm among the employed fuels in this study. The analysis revealed that C₂H₅ + O₂ ⇔ C₂H₄ + HO₂ is a key reaction and promotes ethane oxidation at 1 atm. The effect of the pressure on the fuel oxidation process in the present reactor was also clarified by the analysis. In addition, weak flame responses to various mixing ratios of methane/n-butane blends were investigated experimentally and computationally. The results indicated a significant effect of n-butane addition in the blends on combustion and ignition characteristics of the blended fuels.     

A skeletal n-butane mechanism with integrated simplification method

A new skeletal mechanism of n-butane is developed for describing its ignition and combustion characteristics applicable over a wide range of conditions: initial temperature 690–1430 K, pressure 1–30 atm, and equivalence ratio 0.5–2.0. Starting with a detailed chemical reaction kinetic model of 230 species and 1328 reactions (Healy et al., Combust. Flame, 2010), the directed relation graph method is applied as the first step to derive a semi-detailed mechanism with 134 species. Then, the reaction path analysis in conjunction with temperature sensitivity analysis is used to remove the redundant species and reaction paths simultaneously under the condition of low-temperature and moderate-to-high temperatures, respectively. Finally, a skeletal n-butane mechanism consisting of 86 species and 373 reactions can be obtained. Mechanism validation indicates that the new developed skeletal mechanism is in good agreement with the detailed mechanism in predicting the global ignition and combustion characteristics. The new skeletal mechanism is further validated using extensive available literature data including rapid pressure machine ignition delay time, shock-tube ignition delay time, laminar flame speed, and jet-stirred reaction oxidation, covering a large range of temperatures, pressures, and equivalence ratios. The comparison results demonstrate that a satisfactory agreement between predictions and experimental measurements is achieved.     

An experimental and kinetic modeling study of the autoignition of α-methylnaphthalene/air and α-methylnaphthalene/n-decane/air mixtures at elevated pressures

The autoignition of α-methylnaphthalene (AMN), the bicyclic aromatic reference compound for the cetane number (CN), and AMN/n-decane blends, potential diesel surrogate mixtures, was studied at elevated pressures for fuel/air mixtures in a heated high-pressure shock tube. Additionally, a comprehensive kinetic mechanism was developed to describe the oxidation of AMN and AMN/n-decane blends. Ignition delay times were measured in reflected shock experiments for Φ = 0.5, 1.0, and 1.5 AMN/air mixtures (CN = 0) for 1032-1445 K and 8-45 bar and for Φ = 1.0 30%-molar AMN/70%-molar n-decane/air (CN = 58) and 70%-molar AMN/30%-molar n-decane/air mixtures (CN = 28) for 848-1349 K and 14-62 bar. Kinetic simulations, based on the comprehensive AMN/n-decane mechanism, are in good agreement with measured ignition times, illustrating the emerging capability of comprehensive mechanisms for describing high molecular weight transportation fuels. Sensitivity and reaction flux analysis indicate the importance of reactions involving resonance stabilized phenylbenzyl radicals, the formation of which by H-atom abstractions with OH radicals has an important inhibiting effect on ignition.     

Experiments and modeling of the autoignition of methylcyclohexane at high pressure

New experimental data are collected for methyl-cyclohexane (MCH) autoignition in a heated rapid compression machine (RCM). Three mixtures of MCH/O₂/N₂/Ar at equivalence ratios of ? = 0.5, 1.0, and 1.5 are studied and the ignition delays are measured at compressed pressure of 50 bar and for compressed temperatures in the range of 690–900 K. By keeping the fuel mole fraction in the mixture constant, the order of reactivity, in terms of inverse ignition delay, is measured to be ? = 0.5 > ? = 1.0 > ? = 1.5, demonstrating the dependence of the ignition delay on oxygen concentration. In addition, an existing model for the combustion of MCH is updated with new reaction rates and pathways, including substantial updates to the low-temperature chemistry. The new model shows good agreement with the overall ignition delays measured in this study, as well as the ignition delays measured previously in the literature using RCMs and shock tubes. This model therefore represents a strong improvement compared to the previous version, which uniformly over-predicted the ignition delays. Chemical kinetic analyses of the updated mechanism are also conducted to help understand the fuel decomposition pathways and the reactions controlling the ignition. Combined, these results and analyses suggest that further investigation of several of the low-temperature fuel decomposition pathways is required.     

Experimental and kinetic modeling study of pyrolysis and oxidation of n-decane

The pyrolysis of n-decane was investigated in a flow reactor at 5, 30, 150 and 760 Torr, and the oxidation of n-decane at equivalence ratios of 0.7, 1.0 and 1.8 was studied in laminar premixed flames at 30 Torr. In both experiments, synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used to identify combustion species and measure their mole fraction profiles. A new detailed kinetic model of n-decane with 234 species and 1452 reactions was developed for applications in intermediate and high temperature regions, and was validated against the experimental results in the present work. The model was also validated against previous experimental data on n-decane combustion, including species profiles in pyrolysis and oxidation in high pressure shock tube and atmospheric pressure flow reactor, jet stirred reactor oxidation, atmospheric pressure laminar premixed flame, counterflow diffusion flame and global combustion parameters such as laminar flame speeds and ignition delay times. In general, the performance of the present model in reproducing these experimental data is reasonably good. Sensitivity analysis and rate of production analysis were conducted to understand the decomposition processes of n-decane.     

A detailed experimental study of n-propylcyclohexane autoignition in lean conditions

The autoignition chemistry of lean n-propylcyclohexane/“air” mixtures (? = 0.3, 0.4, 0.5) was investigated in a rapid compression machine at compressed gas temperatures ranging from 620 to 930 K and pressures ranging from 0.45 to 1.34 MPa. Cool flame and ignition delay times were measured. Cool flame delay times were found to follow an Arrhenius behavior, and a correlation including pressure and equivalence ratio dependences was deduced. The present ignition delay data were compared with recent experimental results and simulations from the available thermokinetic models in the literature. Negative temperature coefficient zones were observed when plotting ignition delay times versus compressed gas temperature. The oxidation products were identified and quantified during the ignition delay period. Formation pathways for the C₉ bicyclic ethers and conjugate alkenes are proposed. The experimental data provide an extensive database to test detailed thermokinetic oxidation models.     

Shock tube determination of ignition delay times in full-blend and surrogate fuel mixtures

Autoignition characteristics of n-heptane/air, gasoline/air, and ternary surrogate/air mixtures were studied behind reflected shock waves in a high-pressure, low-temperature regime similar to that found in homogeneous charge compression ignition (HCCI) engine cycles. The range of experiments covered combustion of fuel in air for lean, stoichiometric, and rich mixtures (Φ=0.5, 1.0, 2.0), two pressure ranges (15-25 and 45-60 atm), temperatures from 850 to 1280 K, and exhaust gas recirculation (EGR) loadings of (0, 20, and 30%). The ignition delay time measurements in n-heptane are in good agreement with the shock tube study of Fieweger et al. (Proc. Combust. Inst. 25 (1994) 1579-1585) and support the observation of a pronounced, low-temperature, NTC region. Strong agreement was seen between ignition delay time measurements for RD387 gasoline and surrogate (63% iso-octane/20% toluene/17% n-heptane by liquid volume) over the full range of experimental conditions studied. Ignition delay time measurements under fuel-lean (Φ=0.5) mixture conditions were longer than with Φ=1.0 mixtures at both the low- (15-25 atm) and high- (45-60 atm) pressure conditions. Ignition delay times in fuel-rich (Φ=2.0) mixtures for both gasoline and surrogate were indistinguishable in the low-pressure (15-25 atm) range, but were clearly shorter at high-pressures (45-60 atm). EGR loading affected the ignition delay times similarly for both gasoline and surrogate, with clear trends indicating an increase in ignition delay time with increased EGR loading. This data set should provide benchmark targets for detailed mechanism validation and refinement under HCCI conditions.     

Experimental study and chemical analysis of n-heptane homogeneous charge compression ignition combustion with port injection of reaction inhibitors

The control of ignition timing in the homogeneous charge compression ignition (HCCI) of n-heptane by port injection of reaction inhibitors was studied in a single-cylinder engine. Four suppression additives, methanol, ethanol, isopropanol, and methyl tert-butyl ether (MTBE), were used in the experiments. The effectiveness of inhibition of HCCI combustion with various additives was compared under the same equivalence ratio of total fuel and partial equivalence ratio of n-heptane. The experimental results show that the suppression effectiveness increases in the order MTBE < isopropanol ? ethanol < methanol. But ethanol is the best additive when the operating ranges, indicated thermal efficiency, and emissions are considered. For ethanol/n-heptane HCCI combustion, partial combustion may be observed when the mole ratio of ethanol to that of total fuel is larger than 0.20; misfires occur when the mole ratio of ethanol to that of total fuel larger than 0.25. Moreover, CO emissions strongly depend on the maximum combustion temperature, while HC emissions are mainly dominated by the mole ratio of ethanol to that of total fuel. To obtain chemical mechanistic informations relevant to the ignition behavior, detailed chemical kinetic analysis was conducted. The simulated results also confirmed the retarding of the ignition timing by ethanol addition. In addition, it can be found from the simulation that HCHO, CO, and C₂H₅OH could not be oxidized completely and are maintained at high levels if the partial combustion or misfire occurs (for example, for leaner fuel/air mixture).     

Heavy-alkane oxidation kinetic-mechanism reduction using dominant dynamic variables,self similarity and chemistry tabulation

A model of local and full or partial self similarity is developed for situations in which a phenomenon exhibits a dominant variable, with the goal of applying the model to obtain reduced oxidation kinetics from detailed kinetics for n-heptane, iso-octane, n-decane and n-dodecane. Upon appropriate normalization, it is shown that the state vector for all four alkanes indeed obeys local full self similarity with respect to the dominant variable which is here a normalized temperature. Further, the vector of species mass fractions is partitioned into major species which are those of interest to calculate, and thus for which equations are solved, and minor species which are those of no interest to calculate and are therefore modeled. The goal of the chemical kinetic reduction is to provide a model which expresses the influence of the minor species on the major species. The identification of major species with the light species, and of the minor species with the heavy species leads to partitioning the energetics into computed and modeled parts. This partition of the species set is shown to lead to local full self similarity of the reaction rates between the modeled and calculated species; the local full self similarity also prevails for the energy of the modeled species and for the average heat capacity at constant volume of the heavy species. A methodology is developed to take advantage of this self similarity by considering the initial condition as a point in the three-dimensional space of the initial pressure, initial temperature and equivalence ratio, choosing eight points surrounding the initial condition in this space, developing the self similarity graphs at these eight points using the LLNL detailed mechanism in conjunction with CHEMKIN II, and calculating at each time step the modeled contributions at the surrounded point by interpolating from those known at the eight points. Once the modeled contributions are known, the conservation equations for the species and the energy, coupled with a real-gas equation of state, are solved. With a focus on the high-pressure conditions in automotive engines, extensive results are shown for the four alkanes over a wide range of initial temperatures (650–1000 K) and equivalence ratios (0.35–3.00) at 20 bar and 40 bar. The results consist of timewise profiles of the temperature and species, allowing the calculation of the ignition time and the equilibrium or maximum temperature. Comparisons between the reduced mechanism and the detailed mechanism show excellent to very good agreement for all alkanes when only 20 progress-variable light species are used in the reduced mechanism; the 20 species are the same for all fuels, and for n-decane and n-dodecane this represents a reduction in the species progress variables by factor of more than 100. As an example, calculations that excellently duplicate the elemental mechanism are also shown for n-dodecane using only 15 or 6 progress-variable light species, indicating the potential for further progress-variable reduction beyond the 20 species.     

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.     

Autoignited laminar lifted flames of methane, ethylene, ethane, and n-butane jets in coflow air with elevated temperature

The autoignition characteristics of laminar lifted flames of methane, ethylene, ethane, and n-butane fuels have been investigated experimentally in coflow air with elevated temperature over 800 K. The lifted flames were categorized into three regimes depending on the initial temperature and fuel mole fraction: (1) non-autoignited lifted flame, (2) autoignited lifted flame with tribrachial (or triple) edge, and (3) autoignited lifted flame with mild combustion.For the non-autoignited lifted flames at relatively low temperature, the existence of lifted flame depended on the Schmidt number of fuel, such that only the fuels with Sc > 1 exhibited stationary lifted flames. The balance mechanism between the propagation speed of tribrachial flame and local flow velocity stabilized the lifted flames. At relatively high initial temperatures, either autoignited lifted flames having tribrachial edge or autoignited lifted flames with mild combustion existed regardless of the Schmidt number of fuel. The adiabatic ignition delay time played a crucial role for the stabilization of autoignited flames. Especially, heat loss during the ignition process should be accounted for, such that the characteristic convection time, defined by the autoignition height divided by jet velocity was correlated well with the square of the adiabatic ignition delay time for the critical autoignition conditions. The liftoff height was also correlated well with the square of the adiabatic ignition delay time.     

Self-ignition of a lean mixture of n-pentane and air over a wide range of pressures

The shock-tube technique is used to measure the ignition delay time of a lean (?=0.5) mixture of n-pentane and air in a wide range of temperatures from 867 to 1534 K and pressures from 11 to 530 atm. The previously developed detailed kinetic model of ignition of hydrocarbons [Kinet. Catal. (2005), in press] is used to interpret the experimental data. The kinetic model includes mechanisms of ignition at high and low temperatures and a mechanism of ignition in the range of intermediate (1000-1200 K) temperatures. Each of these mechanisms is analyzed. The effect of the mixture pressure on the ignition at a temperature of 1000-1100 K is demonstrated.     

Chemical kinetics modeling of n-nonane oxidation in oxygen/argon using excited-state species time histories

Chemical reactions of ground-state species strongly govern the formation of excited-state species, including OH^* and CH^*, which are commonly used to determine ignition delay times of fuels. With well-characterized chemiluminescence rates embedded in chemical kinetics mechanisms, time histories of excited-state species can aid in identifying influential ground-state reactions which are important to processes such as ignition delay time. Placing emphasis on the high-temperature regime, improvements were made to a detailed chemical kinetics mechanism of n-nonane oxidation developed previously by the authors. Using characteristic features of OH^* time histories measured in shock-tube experiments as a metric, detailed model analyses were performed over a broad range of conditions: T > 1100 K, 1.5 < P (atm) < 10.5, ? = 0.5, 1.0, 2.0. OH^* time history measurements, particularly under fuel-rich conditions (? = 2.0), displayed a two-peak behavior, with the first peak occurring within the first 5–10 μs after reflected-shock passage, and the second, wider peak corresponding to main oxidation and ignition. In the initial version of the kinetics mechanism, the two peaks at rich conditions were predicted to merge, blurring the main ignition process prior to the second peak. The work herein presents modifications to the initial chemical kinetics mechanism which led to improved agreement between measurements and model-based predictions, with emphasis on the fuel-rich condition. To this end, the predicted shapes of the OH^* time histories were crucial to matching the two-peak behavior detected in the experiments. A first-order resistance–capacitance (RC) model of the experimental time response of the optical setup was developed and shown to reproduce the measured time dependence and peak behavior that are vital for matching the OH^* behavior near time-zero. The RC model processes the kinetics predictions in a way that allows the kinetics model predictions to directly correspond to the true conditions in the experiment. In moving towards improved agreement in OH^*-profile predictions for all conditions, improvements in the kinetics mechanism were also realized at the two leaner equivalence ratios (? = 1.0 and 0.5), both in terms of OH^* profile shape and ignition delay times. Model calculations of oxidation processes indicate that reactions leading to the first OH^* peak originate from fuel homolysis. The resulting (alkyl) radicals lead to the formation of methyl which then, through a series of H-abstraction reactions, leads to the production of the methylidyne radical (CH) that reacts with molecular oxygen to form OH^*. The oxidation processes near time-zero terminate, in part, due to methyl depletion by methylene forming C₂H₄ + H₂. In addition to the insight gained on n-nonane ignition and oxidation chemistry, the present study highlights the utility of correctly interpreted OH^* measurements for inference of kinetic information other than ignition delay times.     

An experimental and modeling study of the low- and high-temperature oxidation of cyclohexane

The experimental study of the oxidation of cyclohexane has been performed in a jet-stirred reactor at temperatures ranging from 500 to 1100 K (low- and intermediate temperature zones including the negative temperature-coefficient area), at a residence time of 2 s and for dilute mixtures with equivalence ratios of 0.5, 1, and 2. Experiments were carried out at quasi-atmospheric pressure (1.07 bar). The fuel and reaction product mole fractions were measured using online gas chromatography. A total of 34 reaction products have been detected and quantified in this study. Typical reaction products formed in the low-temperature oxidation of cyclohexane include cyclic ethers (1,2-epoxycyclohexane and 1,4-epoxycyclohexane), 5-hexenal (formed from the rapid decomposition of 1,3-epoxycyclohexane), cyclohexanone, and cyclohexene, as well as benzene and phenol. Cyclohexane displays high low-temperature reactivity with well-marked negative temperature-coefficient (NTC) behavior at equivalence ratios 0.5 and 1. The fuel-rich system (? = 2) is much less reactive in the same region and exhibits no NTC. To the best of our knowledge, this is the first jet-stirred reactor study to report NTC in cyclohexane oxidation. Laminar burning velocities were also measured by the heated burner method at initial gas temperatures of 298, 358, and 398 K and at 1 atm. The laminar burning velocity values peak at ? = 1.1 and are measured as 40 and 63.1 cm/s for T_i = 298 and 398 K, respectively. An updated detailed chemical kinetic model including low-temperature pathways was used to simulate the present (jet-stirred reactor and laminar burning velocity) and literature experimental (laminar burning velocity, rapid compression machine, and shock tube ignition delay times) data. Reasonable agreement is observed with most of the products observed in our reactor, as well as the literature experimental data considered in this paper.     

Role of peroxy chemistry in the high-pressure ignition of n-butanol – Experiments and detailed kinetic modelling

Despite considerable interest in butanol as a potential biofuel candidate, its ignition behaviour at elevated pressures still remains largely unexplored. The present study investigates the oxidation of n-butanol in air at pressures near 80 bar. Ignition delays were determined experimentally in the temperature range of 795–1200 K between 61 and 92 bar. The time of ignition was determined by recording pressure and CH-emission time histories throughout the course of the experiments. The results display the first evidence of the influence of negative temperature coefficient (NTC) behaviour which was not observed in earlier ignition studies. The high-pressure measurements show that NTC behaviour is enhanced as pressures are increased. The experimental results were modelled using an improved chemical kinetic mechanism which includes a simplified sub-mechanism for butyl-peroxy formation and isomerisation reactions currently incompletely accounted for in n-butanol kinetic models. The detailed mechanism validated with the high-pressure ignition results for realistic engine in-cylinder conditions can have significant impact on future advanced low-temperature combustion engines.     

An experimental and modeling study of surrogate mixtures of n-propyl- and n-butylbenzene in n-heptane to simulate n-decylbenzene ignition

This paper presents experimental data for the oxidation of two surrogates for the large alkylbenzene class of compounds contained in diesel fuels, namely n-decylbenzene. A 57:43 molar% mixture of n-propylbenzene:n  -heptane in air (21% _O2${\mathrm{O}}_2$, 79% _N2${\mathrm{N}}_2$) was used in addition to a 64:36 molar% mixture of n-butylbenzene:36% n-heptane in air. These mixtures were designed to contain a similar carbon/hydrogen ratio, molecular weight and aromatic/alkane ratio when compared to n-decylbenzene. Nominal equivalence ratios of 0.3, 0.5, 1.0 and 2.0 were used. Ignition times were measured at 1 atm in the shock tube and at pressures of 10, 30 and 50 atm in both the shock tube and in the rapid compression machine. The temperature range studied was from approximately 650–1700 K. The effects of reflected shock pressure and equivalence ratio on ignition delay time were determined and common trends highlighted. It was noted that both mixtures showed similar reactivity throughout the temperature range studied. A reaction mechanism published previously was used to simulate this data. Overall the reaction mechanism captures the experimental data reasonably successfully with a variation of approximately a factor of 2 for mixtures at 10 atm and fuel-rich and stoichiometric conditions.