Experimental study of the oxidation of methyl oleate in a jet-stirred reactor

The experimental study of the oxidation of a blend containing n-decane and a large unsaturated ester, methyl oleate, was performed in a jet-stirred reactor over a wide range of temperature covering both low and high temperature regions (550-1100 K), at a residence time of 1.5 s, at quasi atmospheric pressure with high dilution in helium (n-decane and methyl oleate inlet mole fractions of 1.48 × 10⁻³ and 5.2 × 10⁻⁴) and under stoichiometric conditions.The formation of numerous reaction products was observed. At low and intermediate temperatures, the oxidation of the blend led to the formation of species containing oxygen atoms like cyclic ethers, aldehydes and ketones deriving from n-decane and methyl oleate. At higher temperature, these species were not formed anymore and the presence of unsaturated species was observed. Because of the presence of the double bond in the middle of the alkyl chain of methyl oleate, the formation of some specific products was observed. These species are dienes and esters with two double bonds produced from the decomposition paths of methyl oleate and some species obtained from the addition of H-atoms, OH and HO₂ radicals to the double bond.Experimental results were compared with former results of the oxidation of a blend of n-decane and methyl palmitate performed under similar conditions. This comparison allowed highlighting the similarities and the differences in the reactivity and in the distribution of the reaction products for the oxidation of large saturated and unsaturated esters.     

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.     

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.     

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.     

Skeletal mechanism generation for surrogate fuels using directed relation graph with error propagation and sensitivity analysis

A novel implementation for the skeletal reduction of large detailed reaction mechanisms using the directed relation graph with error propagation and sensitivity analysis (DRGEPSA) is developed and presented with examples for three hydrocarbon components, n-heptane, iso-octane, and n-decane, relevant to surrogate fuel development. DRGEPSA integrates two previously developed methods, directed relation graph-aided sensitivity analysis (DRGASA) and directed relation graph with error propagation (DRGEP), by first applying DRGEP to efficiently remove many unimportant species prior to sensitivity analysis to further remove unimportant species, producing an optimally small skeletal mechanism for a given error limit. It is illustrated that the combination of the DRGEP and DRGASA methods allows the DRGEPSA approach to overcome the weaknesses of each, specifically that DRGEP cannot identify all unimportant species and that DRGASA shields unimportant species from removal. Skeletal mechanisms for n-heptane and iso-octane generated using the DRGEP, DRGASA, and DRGEPSA methods are presented and compared to illustrate the improvement of DRGEPSA. From a detailed reaction mechanism for n-alkanes covering n-octane to n-hexadecane with 2115 species and 8157 reactions, two skeletal mechanisms for n-decane generated using DRGEPSA, one covering a comprehensive range of temperature, pressure, and equivalence ratio conditions for autoignition and the other limited to high temperatures, are presented and validated. The comprehensive skeletal mechanism consists of 202 species and 846 reactions and the high-temperature skeletal mechanism consists of 51 species and 256 reactions. Both mechanisms are further demonstrated to well reproduce the results of the detailed mechanism in perfectly-stirred reactor and laminar flame simulations over a wide range of conditions. The comprehensive and high-temperature n-decane skeletal mechanisms are included as supplementary material with this article.     

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.     

Multi-species time-history measurements during n-heptane oxidation behind reflected shock waves

Concentration time-histories were measured behind reflected shock waves during n-heptane oxidation for five species: n-heptane, C₂H₄, OH, CO₂, and H₂O. Experiments were conducted at temperatures of 1300-1600 K and a pressure of 2 atm using a mixture of 300 ppm n-heptane and 3300 ppm oxygen (? = 1) in argon. n-Heptane and ethylene were monitored using IR gas laser absorption at 3.39 and 10.53 μm, respectively; OH was monitored using UV laser absorption at 306.5 nm; and CO₂ and H₂O were monitored using tunable IR diode laser absorption at 2.7 and 2.5 μm, respectively. These time-histories provide kinetic targets to test and refine large reaction mechanisms for n-heptane and demonstrate the potential of this type of data for validation of large reaction mechanisms. Comparisons are made with the predictions of several recently developed reaction mechanisms.     

Modeling of the oxidation of methyl esters—Validation for methyl hexanoate, methyl heptanoate, and methyl decanoate in a jet-stirred reactor

The modeling of the oxidation of methyl esters was investigated and the specific chemistry, which is due to the presence of the ester group in this class of molecules, is described. New reactions and rate parameters were defined and included in the software EXGAS for the automatic generation of kinetic mechanisms. Models generated with EXGAS were successfully validated against data from the literature (oxidation of methyl hexanoate and methyl heptanoate in a jet-stirred reactor) and a new set of experimental results for methyl decanoate. The oxidation of this last species was investigated in a jet-stirred reactor at temperatures from 500 to 1100 K, including the negative temperature coefficient region, under stoichiometric conditions, at a pressure of 1.06 bar and for a residence time of 1.5 s: more than 30 reaction products, including olefins, unsaturated esters, and cyclic ethers, were quantified and successfully simulated. Flow rate analysis showed that reactions pathways for the oxidation of methyl esters in the low-temperature range are similar to that of alkanes.     

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.     

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.     

Detailed chemical kinetic mechanism for the oxidation of biodiesel fuels blend surrogate

Detailed chemical kinetic mechanisms were developed and used to study the oxidation of two large unsaturated esters: methyl-5-decenoate and methyl-9-decenoate. These models were built from a previous methyl decanoate mechanism and were compared with rapeseed oil methyl esters oxidation experiments in a jet-stirred reactor. A comparative study of the reactivity of these three oxygenated compounds was performed and the differences in the distribution of the products of the reaction were highlighted showing the influence of the presence and the position of a double bond in the chain. Blend surrogates, containing methyl decanoate, methyl-5-decenoate, methyl-9-decenoate and n-alkanes, were tested against rapeseed oil methyl esters and methyl palmitate/n-decane experiments. These surrogate models are realistic kinetic tools allowing the study of the combustion of biodiesel fuels in diesel and homogeneous charge compression ignition engines.     

Temperature dependence of Cu2ZnSn(SexS1−x)4 monograin solar cells

The temperature dependence of open-circuit voltage (V_oc), short-circuit current (I_sc), fill factor (FF), and relative efficiency of monograin Cu₂ZnSn(Se_xS_1−x)₄ solar cell was measured. The light intensity was varied from 2.2 to 100 mW/cm² and temperatures were in the range of T = 175-300 K. With a light intensity of 100 mW/cm²dV_oc/dT was determined to be −1.91 mV/K and the dominating recombination process at temperatures close to room temperature was found to be related to the recombination in the space-charge region. The solar cell relative efficiency decreases with temperature by 0.013%/K. Our results show that the diode ideality factor n does not show remarkable temperature dependence and slightly increases from n = 1.85 to n = 2.05 in the temperature range between 175 and 300 K.     

The high-temperature autoignition of biodiesels and biodiesel components

Ignition delay time measurements are reported for two reference fatty-acid methyl ester biodiesel fuels, derived from methanol-based transesterification of soybean oil and animal fats, and four primary constituents of all methyl ester biodiesels: methyl palmitate, methyl stearate, methyl oleate, and methyl linoleate. Experiments were carried out behind reflected shock waves for gaseous fuel/air mixtures at temperatures ranging from 900 to 1350 K and at pressures around 10 and 20 atm. Ignition delay times were determined by monitoring pressure and ultraviolet chemiluminescence from electronically-excited OH radicals. The results show similarity in ignition delay times for all methyl ester fuels considered, irrespective of the variations in organic structure, at the high-temperature conditions studied and also similarity in high-temperature ignition delay times for methyl esters and n-alkanes. Comparisons with recent kinetic model efforts are encouraging, showing deviations of at most a factor of two and in many cases significantly less.     

A shock tube study of methyl decanoate autoignition at elevated pressures

A shock tube study of the autoignition of methyl decanoate, a candidate surrogate for biodiesel fuels containing large methyl esters, has been carried out. Ignition delay times were measured in reflected-shock-heated gases by monitoring electronically-excited OH chemiluminescence and pressure. Methyl decanoate/air mixtures were studied at equivalence ratios of 0.5, 1.0, and 1.5, at temperatures from 653 to 1336 K, and for pressures around 15–16 atm. The experimental results illustrate negative-temperature-coefficient behavior characteristic of alkanes, with ignition delay times very similar at high temperatures and somewhat longer at low temperatures than those for n-decane. Experimental results are compared to the kinetic modeling predictions of Herbinet et al. [Combust. Flame 154 (2008) 507–528] with remarkable agreement. Both reaction flux analysis and the comparison of experimental methyl decanoate and n-decane ignition delay times illustrate the importance of the long alkyl chain in controlling methyl decanoate overall reactivity and the subtle role the methyl ester group has on inhibiting low-temperature reactivity.     

Laminar flame propagation of atmospheric iso-cetane/air and decalin/air mixtures

Laminar flame speeds of iso-cetane/air and decalin/air mixtures were measured in the counterflow configuration at atmospheric pressure and an elevated unburned mixture temperature of 443 K. Axial flow velocities were measured along the stagnation streamline using the digital particle image velocimetry. The laminar flame speeds were determined by determining the variation of a reference flame speed as a function of strain rate and computationally assisted non-linear extrapolations. The data are the first to be reported in the literature, and they were modeled using a recently developed kinetic model that includes 187 species and 6086 elementary reactions. In general, the computed results were found to be in close agreement with the data. In order to get insight into kinetic effects on flame propagation, detailed sensitivity and reaction path analyses were performed using the computed flame structures. The results revealed that at the same equivalence ratio, laminar flame speeds of iso-cetane/air mixtures are lower than those of n-hexadecane/air mixtures. Additionally, it was found that the laminar flame speeds of iso-cetane/air and decalin/air mixtures are sensitive largely to C₀–C₄ kinetic subset, and that the lower reactivity of iso-cetane compared to n-hexadecane could be attributed to the higher production of relatively stable intermediates.     

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.     

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.     

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.     

Nonpremixed ignition of C7–C16 normal alkanes in stagnating liquid pool

The nonpremixed ignition temperatures of n-decane, n-dodecane, and n-hexadecane were measured in a liquid pool by heated stagnating oxidizing flow at atmospheric pressure. Together with previous results on n-heptane, it is shown that, for the C₇–C₁₆n-alkanes, the nonpremixed ignition temperature increases monotonically with increasing carbon number, and as such is contrary to the behavior of homogeneous ignition delays. Numerical simulation of the ignition events for n-heptane, n-decane and n-dodecane, employing a recently developed high temperature kinetic model, showed good agreement with the experimental results both qualitatively and quantitatively. Sensitivity and computational analyses indicate that the reason for the higher ignition temperature with increasing fuel molecular size is mostly due to their progressively reduced diffusivity, which leads to correspondingly reduced fuel concentration in the ignition kernel.