Negative Temperature Coefficient Region Research Articles

The autoignition of Heptamethylnonane at moderate-to-high temperatures and elevated pressures: Shock tube study and improved chemical kinetic model

Iso-cetane (2,2,4,4,6,8,8-Heptamethylnonane) is used as a reference fuel to define the cetane number rating of diesel fuel and as an important component for diesel and jet fuel surrogates. This experimental study presented the ignition delay measurement of iso-cetane in a heated shock tube at a temperature range of 838–1617 K, pressures of 10, 15, and 20 bar and over equivalence ratios of Φ = 0.5–2.0 with fuel/air mixture and at Φ = 1.5 with diluted fuel mixture. The experimental conditions were designed to study the effect of pressure, equivalence ratio, and oxygen concentration variation on ignition delay times. The ignition delay time was found to be sensitive to all the investigated factors. The experimental data obtained were in good agreement with both low and high-temperature data reported in the previous studies. The comparison with experimental data of iso-alkanes of various chain lengths showed an impact of chain length on reactivity noticeably at intermediate and low temperatures. The onset of the negative temperature coefficient region is also found to be influenced by an increase in the chain length. The mechanism of Yu et al. was revised by incorporating new rate rules. The kinetic modeling predictions of the updated model showed a noticeable improvement in the mechanism performance except under diluted conditions. The reaction flux and sensitivity analyses were conducted to further discuss the mechanism performance and overall kinetic behavior of the fuel.

Three-stage auto-ignition of n-heptane and methyl-cyclohexane mixtures at lean conditions in a flat piston rapid compression machine

One approach to enhancing the thermal efficiency of combustion systems is to burn fuels at ultra-lean conditions (equivalence ratio below 0.5). It has been recently reported that the auto-ignition of some hydrocarbon fuels, under specific temperature, pressure, and mixture conditions, releases heat in three distinctive stages. The three auto-ignition stages can be divided as a first low-temperature auto-ignition stage with conventional low temperature, and a high-temperature stage separated into two sub-stages. This study presents ignition delay time measurements of n-heptane and methyl-cyclohexane (MCH) mixtures in a flat piston rapid compression machine (RCM) under ultra-lean conditions. It provides experimental evidence of three-stage auto-ignition. This phenomenon of delayed high-temperature heat release is seldom reported in the literature and this is the first time to be reported for these types of fuels. The experiments cover two binary n-heptane/MCH mixtures of 15/85 and 70/30 by volume, pressures of 11 bar and 16 bar, temperature range of 700 to 900 K, and equivalence ratio of 0.4. The RCM optical access was utilized for high-speed chemiluminescence imaging. Detailed chemical kinetic simulations in a homogenous batch reactor with variable volume were conducted to further interrogate the three-stage auto-ignition phenomenon. Chemiluminescence shows that three-stage auto-ignition occurs in the adiabatically compressed end-gas, which indicates that this phenomenon is chemically-driven and is not induced by a thermal stratification in the RCM experiments. The model predicts the features of three-stage auto-ignition, which were experimentally observed at temperatures approximately below 750 K. As expected, significant discrepancies are observed in the ignition delays of experiment and simulation in the negative temperature coefficient (NTC) region. The simulation of the n-heptane/MCH 70/30 mixture shows better agreement with experiments in the Positive Temperature Coefficient (PTC) region compared to the 15/85 mixture.

A study on the mechanism reduction and evaluation of biodiesel with the change of mechanism reduction factors

This study aims to confirm the effect of changing the various factors with the directed relation graph error propagation–based methods and to adopt a new approach of the mechanism evaluation in the reduction of biodiesel mechanism. The factors considered in this study were a threshold value, target species, ambient conditions, and the evaluation formula consists of the reduction rates and the maximum error rate of ignition delay to objectively compare the skeletal mechanisms generated under different conditions. For a threshold value, the automatic mechanism reduction process was used to select the appropriate threshold value by applying the relative tolerance and absolute tolerance; so relative tolerance and absolute tolerance represent the factor of the threshold value. Also, the seven steps of mechanism reduction process consist of directed relation graph error propagation, directed relation graph error propagation with sensitivity analysis, peak concentration analysis, full species sensitivity analysis, and A-factor modification. As a result of the mechanism reduction, different relative tolerance and absolute tolerance values should be applied to each step to select the appropriate threshold value. For target species, considering polycyclic aromatic hydrocarbon species as target species shows higher efficiency of mechanism reduction. Also, considering the negative temperature coefficient region as ambient conditions helps the mechanism be reduced efficiently than a wide range of ambient conditions. Finally, the reduced mechanism which had 247 species and 1129 reactions was generated, and the maximum error rate of ignition delay was about 30%. For the applicability of three-dimensional computational fluid dynamics and verification of the reduced mechanism, the compression ignition engine simulation was performed. As a result of three-dimensional computational fluid dynamics, the predicted cylinder pressure, rate of heat release, indicated mean effective pressure, and power were similar to the experimental results. However, the results of carbon monoxide and nitrogen oxide emissions did not match the experimental results.

Effect of Ozone Addition on the Cool Flame and Negative Temperature Coefficient Regions of Propane-Oxygen Mixtures.

In this study, the effects of ozone addition on the cool flame and NTC (negative temperature coefficient) regions of stoichiometric C3H8/O2 mixtures are computationally studied through the explosion limit profiles. The results show that with minute quantities of ozone addition (the mole fraction of ozone is 0.1%), the cool flame area is enlarged to the low-temperature region. Further increases in the mole fraction of ozone gradually weaken the NTC behavior, and a monotonic explosion limit is eventually achieved. The sensitivity analysis of the main reactions involving ozone reveals that the explosion limit is mainly controlled by the ozone unimolecular decomposition reaction O3 (+M) = O2 + O (+M). However, as its reverse reaction is a third-body reaction, this reaction will lose its effect on the explosion limit in the high-pressure region. On the contrary, the reaction O3 + HO2 = OH + O2 + O2 has a significant effect on the explosion limit in the high-pressure and low-temperature region, as the concentration of HO2 increases through the rapid third-body reaction H + O2 + M = HO2 + M.

Experimental study on the oxidation behaviors of Wolfcamp light crude oil and its saturate, aromatic and resin fractions using accelerated rate calorimetry tests

The air injection process (AIP) has long been applied to enhance crude oil recovery. Oxidation of crude oil is essential for the success of air injection projects. However, the lack of accurate descriptions of the crude oil oxidation has led to incorrect predictions of crude oil oxidation behavior. Compared with the conventional method to characterize crude oil by density and viscosity, saturate, aromatic, resin, and asphaltene (SARA) analysis has proved to be robust for studying crude oil oxidation. Also, accelerated rate calorimetry (ARC) has proved to be suitable for evaluating adiabatic self-heating behavior. This study combined the SARA analysis with ARC tests to determine the details of crude oil oxidation. This paper first reviews the laboratory techniques for studying crude oil oxidation. Then it presents ARC tests on Wolfcamp light crude oil and its saturate, aromatic, and resin fractions. Due to its extremely low concentration in Wolfcamp oil, the asphaltene fraction was not tested during this study. Compared to 50 bar tests, all tested fractions showed enhanced LTO exotherm during 100 bar tests. Also, from the ARC temperature and pressure curves, we identified two endothermic processes that might cause a negative temperature coefficient region. One endothermic process showed decreasing trend in pressure while the other process showed increase in pressure. Finally, evidence of natural antioxidants was found after testing mixtures of saturates with other fractions. Those chain-breaking antioxidants can decelerate the fast-oxidation behavior of saturates. This paper provides details of the crude oil oxidation process derived from experiments.

Fischer-Tropsch coal-to-liquid fuel negative temperature coefficient region (NTC) and low-temperature heat release (LTHR) in a constant volume combustion chamber (CVCC)

The purpose of the study was to investigate the correlation between thermal characteristics of neat Iso-Paraffinic Kerosene (IPK) in relation to Low Temperature Heat Release (LTHR), ignition delay, and combustion delay within the Negative Temperature Coefficient region (NTC), utilizing a Box-Behnken design matrix. There were 15 trials of 3 control input parameters which were strategic manipulations of ASTM standard D7668-14a. The standard parameters are: 1000 bar for injection pressure, a combustion chamber temperature of 595.5 °C, and an injection pulse width of 2.5 ms. It was observed that, at ASTM standard, the Derived Cetane Number of neat Ultra-Low Sulfur Diesel (ULSD) and IPK was 47.38 and 25.9, respectively. Results show that the LTHR duration and energy release were reduced when two parameters were simultaneously increased in ULSD operation. Meanwhile, for both fuels, the duration and energy release in LTHR has increased when two parameters were simultaneously reduced. It was found that the lower set wall temperature resulted in increased NTC region and LTHR increased to over 10% of the total energy release. The maximum combustion pressure for both ULSD and IPK where increased by 17.2% and 16.1% respectively from the standard due to the increase in injection pressure and pulse width.

Experimental and kinetic modeling investigation on ethylcyclohexane low-temperature oxidation in a jet-stirred reactor

In this work, the oxidation of ethylcyclohexane was studied in a jet-stirred reactor at 780 Torr, 480–780 K and equivalence ratios of 0.5, 1.0 and 2.0. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used for the detection of oxidation products, including a series of reactive intermediates such as cycloalkylhydroperoxides, keto-hydroperoxides, alkenal-hydroperoxides and highly oxygenated molecules. Quantum chemistry calculations were performed to obtain ionization energies for the identification of some important intermediates and energy barriers of several important pathways. On the other hand, this work presents the first efforts on developing a low-temperature oxidation model of ethylcyclohexane. The present model can reasonably capture the low-temperature oxidation reactivities and negative temperature coefficient behaviors observed in both present and previous experimental work of ethylcyclohexane oxidation. Modeling analyses were performed to provide insight into the low-temperature oxidation chemistry of ethylcyclohexane. The two-stage O2 addition mechanism is concluded to dominate the chain-branching process in the low-temperature region. The concerted elimination reactions of cycloalkylperoxy radical and “formally direct” chemically activated reactions of cycloalkyl+O2 result in cycloalkenes and HO2 formation at the negative temperature coefficient region and serve as main chain-termination pathways. Compared with smaller cycloalkanes like cyclohexane and methylcyclohexane, the ethyl sidechain structure in ethylcyclohexane reduces the energy barriers of cycloalkylperoxy radical isomerization and facilitates the formation of keto-hydroperoxides which leads to more pronounced low-temperature oxidation reactivity of ethylcyclohexane.

Nitromethane pyrolysis and oxidation in a jet-stirred reactor: Experimental measurements, kinetic model validation and interpretation

A jet-stirred reactor was used to investigate the nitromethane pyrolysis and oxidation characteristics over the temperature range of 675–1250 K at atmospheric pressure. Mole fraction profiles of reactants, intermediate hydrocarbon species and products were identified using gas chromatography and gas chromatography-mass spectrometer. Results show that the fuel begins to be destructed at around 725 K and completely vanishes at 900 K for all experimental conditions. Pyrolysis of nitromethane as a function of temperature shows three distinct regions for H2, CO and C2H4 species. In addition, a negative temperature coefficient (NTC) behavior of CO2 species for nitromethane oxidation was observed, which has never been reported previously. The NTC region of CO2 species extends with the increase of equivalence ratio. These measured data were then used to validate several recently developed models of nitromethane. Results show that the model of Mathieu et al. (Mathieu et al. Fuel 182(2016):597) predicts the measured data well at low temperature (below 900 K) for all conditions. However, at intermediate temperature Mathieu model predicted intermediated species and main products deviate noticeably from measured data. Detailed reaction pathway analysis was conducted to direct future model refinement. Results show that the deviation between model predictions and measurements at intermediate temperature may be attributed to the reactions that involve the H and OH radicals. Further comparisons of the sensitive reactions for H and OH formations from different literatures were conducted, and large deviations in rate constants among these reactions suggest the directions of future model refinement.

Investigation into pressure dependence of flame speed for fuels with low and high octane sensitivity through blending ethanol

Spark assistance for homogeneous charge compression ignition (HCCI) can control combustion phasing, improve thermal efficiency, and reduce emissions in gasoline engines. As the characteristics of flame propagation determine the control authority of ignition timing, it is important and necessary to investigate pressure dependence of flame speed in the lean-premixed mixture relative to engine operating conditions. Experimental study in an optical rapid compression machine (RCM) and simulation work were carried out using two fuels comprising n-heptane/iso-octane/ethanol with varied octane sensitivity (S). The effective pressure ranged from 10 to 35 bar, temperature from 715 to 860 K, and equivalence ratios between 0.3 and 0.7 to cover the region of lean flammability limits of low and high S fuels with ethanol blended. Based on pressure profiles, flame speed extracted from images, and sensitivity analysis of flame speed, the dependence of flame speed on the effective pressure in low and high S fuels was discovered and the fundamental mechanism behind this phenomena became to be understood in the negative temperature coefficient (NTC) and non-NTC regions, respectively. In the studied temperature conditions, the flame speed of high S fuel has stronger dependence on the pressure than that of low S fuel does. In the NTC region, this phenomenon is attributed to the dependence of H radical concentration on pressure in the unburned mixture and flame structure. In the non-NTC region, promoting effect of dominant reactions varied with pressure can significantly influence pressure dependence of flame speed. Although quite limited data of laminar burning velocity for studied fuels were obtained in high pressures (>15 bar), the trend of flame speed's dependence on pressure was well predicted by two models with different but well-accepted core mechanisms, showing consistent results with the experimental ones in the RCM.

Structures and propagation speeds of autoignition-assisted premixed n-heptane/air cool and warm flames at elevated temperatures and pressures

The laminar flame speeds and structures of near-limit autoignition-assisted cool and warm n-heptane/air flames at different ignition Damkӧhler numbers which are the ratios between flow residence time and the ignition delay time, elevated temperatures and pressures are studied computationally and analytically over a broad range of equivalence ratios. The primary objective of this work is to understand the effects of the ignition Damkӧhler number, mixture temperature, equivalence ratio, and pressure on the dynamics and structures of cool and warm flame propagation near the flammability limit. Different transitions among near-limit cool, warm, and hot flames are examined. The results show that both cool and warm flame structures and propagation speeds change dramatically with the increase of the ignition Damkӧhler number. Moreover, the dependence of normalized cool and warm flame speeds on the ignition Damkӧhler number is affected by the equivalence ratio, pressure, and flame regimes. Furthermore, for equivalence ratios within the hot flame flammability limits, the results show that there exist two flame speeds, one for the hot flame and the other for the cool flame. It is shown that the cool flame speed has a non-monotonic dependence on the initial mixture temperature due to the negative temperature coefficient (NTC) effect. However, the warm flame speed has a much weaker NTC effect and the hot flame speed only increases monotonically with the increase of the initial temperature. The results also reveal that the lean cool flame speed can be much higher than the hot flame speed near the NTC region. Finally, a simple analytical model for predicting the flame speed of autoignition assisted flames is developed. The model implies that the reduced activation energy of autoignition strongly affects the flame speed dependence on the ignition Damkӧhler number. The present results significantly advanced the understanding of the near-limit low temperature flame dynamics.

Shock Tube and Kinetic Study on the Effects of CO2 on Dimethyl Ether Autoignition at High Pressures

First-stage and overall ignition delay times of dimethyl ether (DME) were measured in a high-pressure shock tube for various mixtures containing high amounts of CO2. The data are available for DME/air mixtures diluted with 40% CO2 as well as for the mixture of DME and oxidizer containing 20.5% O2 and 79.5% CO2. As a reference, data were also collected for mixtures containing the corresponding amounts of N2. Measurements were conducted for pressures of 15, 35, and 50 bar and a range of temperatures (744–1316 K). For the DME/air mixture diluted with CO2, the equivalence ratio was varied in the range of 0.5–2.0. The results demonstrate that CO2 dilution has a strong effect on ignition delay times in the NTC (negative temperature coefficient) region. The kinetic study that was conducted showed that this phenomenon can be attributed to a thermal effect resulting from the high heat capacity of CO2. In low and high temperature ranges, the effect of CO2 is less pronounced. Additionally, a chemical effect was identified. At high temperatures this effect can be attributed mostly to the influence of CO2 on third-body reactions and leads to slight acceleration of ignition. Various chemical-kinetic models available were evaluated with respect to their accuracy in prediction of ignition delay times for mixtures containing DME and large amounts of CO2.

Experimental and theoretical investigation on the possible half-metallic behaviour of equiatomic quaternary Heusler alloys: CoRuMnGe and CoRuVZ (Z = Al, Ga)

In this report, structural, electronic, magnetic and transport properties of quaternary Heusler alloys CoRuMnGe and CoRuVZ (Z = Al, Ga) are investigated. All the three alloys are found to crystallize in cubic structure. CoRuMnGe exhibits L21 structure whereas, the other two alloys have B2-type disorder. For CoRuMnGe and CoRuVGa, the experimental magnetic moments are in close agreement with the theory as well as those predicted by the Slater-Pauling rule, while for CoRuVAl, a relatively large deviation is seen. The reduction in the moment in case of CoRuVAl possibly arises due to the anti-site disorder between Co and Ru sites as well as V and Al sites. Among these alloys, CoRuMnGe has the highest TC of 560 K. Resistivity variation with temperature reflects the half-metallic nature in CoRuMnGe alloy. CoRuVAl shows metallic character in both paramagnetic and ferromagnetic states, whereas the temperature dependence of resistivity for CoRuVGa is quite unusual. In the last system, ρ vs. T curve shows an anomaly in the form of a maximum and a region of negative temperature coefficient of resistivity (TCR) in the magnetically ordered state. The ab initio calculations predict nearly half-metallic ferromagnetic state with high spin polarization of 91, 89 and 93% for CoRuMnGe, CoRuVAl and CoRuVGa respectively. In the case of CoRuMnGe, the XRD analysis reveals that the Co and Ru sites are equally probable. Hence, to investigate the electronic properties of the experimentally observed structure, the Co-Ru swap disordered structures of CoRuMnGe alloy are also simulated and it is found that the disordered structures retain half-metallic nature, high spin polarization with almost same magnetic moment as in the ideal structure. Nearly half-metallic character, high TC and high spin polarization make CoRuMnGe alloy promising for room temperature spintronic applications.

Insights into the Effects of Mechanism Reduction on the Performance of n-Decane and Its Ability to Act as a Single-Component Surrogate for Jet Fuels

In this study, a detailed chemical reaction mechanism of n-decane containing 1034 species and 4268 reactions has been reduced at three different reduction levels to study the effects of subsequent mechanism reductions on the performance of n-decane. The detailed and reduced mechanisms were then used to validate ignition delays, laminar flame speeds, flame species, and species in a jet-stirred reactor. The one-half reduced mechanism performed nearly the same as the detailed mechanism in most of the cases. The one-fourth and one-eighth reduced mechanisms performed fairly well as compared to the detailed mechanism in some cases. The differences were further elaborated by sensitivity analyses of ignition delays and laminar flame speeds at different conditions followed by reaction pathway analysis of the detailed and one-eighth reduced mechanisms. These analyses indicated the absence or presence of certain reaction classes in reduced mechanisms that shaped the particular behavior of the mechanisms. To evaluate the capability of n-decane as a single-component surrogate for jet fuels, the experimental data of real-life jet fuels were used to validate the ignition delays and laminar flame speeds using the reaction mechanisms. Among all of the tested fuels, the ignition delay of jet A was reproduced fairly well by the detailed mechanism followed by jet S8 and jet RP-3 fuels with a noticeable discrepancy in the negative temperature coefficient (NTC) region. The one-eight reduced mechanism performed well in the NTC region. The laminar flame speeds of jet A and jet S8 fuels were predicted quite well by the detailed mechanism with the one-fourth reduced mechanism performing well at fuel-lean conditions. This strengthened the capability of n-decane as a single-component surrogate for jet fuels.

Auto-ignition characteristics of end-gas in a rapid compression machine under super-knock conditions

Spark ignition induced super-knock was generated in a rapid compression machine using stoichiometric iso-octane/O2/N2 mixture. In addition to the pressure traces, the combustion processes were also recorded using high-speed photography, from which the characteristics of the end-gas auto-ignition were analyzed. The effect of negative temperature coefficient (NTC) on the end-gas auto-ignition was investigated using detailed and reduced reaction mechanisms. The end-gas auto-ignition of diluted methanol/O2/Ar mixture without NTC behavior was also tested for comparison. The results showed that the end-gas auto-ignition of iso-octane/O2/N2 mixture exhibited a two-stage ignition process. During the second ignition stage of the end-gas, two auto-ignition events with very short time interval were sequentially observed in the end-gas region. The first auto-ignition event generated a weak shock wave, and the second one initiated detonation. Both the two auto-ignition events occurred near the wall but at different sites. Due to the heat loss to the wall, the near-wall region is generally considered to be colder than the adiabatic core region, and thus the near-wall auto-ignition of end-gas was usually considered as a result of fuel's NTC behavior. However, in this study the chemical kinetic calculation showed that the evolution of the end-gas almost bypassed the NTC region in the ignition delay-temperature diagram. Furthermore, for the methanol/O2/Ar mixture the end-gas auto-ignition also started at the very near-wall region. Considering that methanol fuel does not has an NTC behavior, the near-wall auto-ignition of methanol/O2/Ar mixture should be a result of other factors than NTC. Therefore, it was concluded that NTC might not play a dominant role in the near-wall auto-ignition of the end-gas.

High-pressure pyrolysis and oxidation of DME and DME/CH4

The pyrolysis and oxidation of dimethyl ether (DME) and its mixture with methane were investigated at high pressure (50 and 100 bar) and intermediate temperature (450–900 K). Mixtures highly diluted in nitrogen with different fuel–air equivalence ratios (Φ=∞, 20, 1, 0.06) were studied in a laminar flow reactor. At 50 bar, the DME pyrolysis started at 825 K and the major products were CH4, CH2O, and CO. For the DME oxidation at 50 bar, the onset temperature of reaction was 525 K, independent of fuel–air equivalence ratio. The DME oxidation was characterized by a negative temperature coefficient (NTC) zone which was found sensitive to changes in the mixture stoichiometry but always occurring at temperatures of 575–625 K. The oxidation of methane doped by DME was studied in the flow reactor at 100 bar. The fuel–air equivalence ratio (Φ) was varied from 0.06 to 20, and the DME to CH4 ratio changed over 1.8–3.6%. Addition of DME had a considerable promoting effect on methane ignition as the onset of reaction shifted to lower temperatures by 25–150 K. A detailed chemical kinetic model was developed by adding a DME reaction subset to a model developed in previous high-pressure work. The model was evaluated against the present data as well as data from literature. Additional work is required to reconcile experimental and theoretical work on reactions on the CH3OCH2OO PES with ignition delay measurements in the NTC region for DME.

Investigating auto-ignition behavior of n-heptane/iso-octane/ethanol mixtures for gasoline surrogates through rapid compression machine measurement and chemical kinetics analysis

Overall ignition delay time (OID) is used as indicator to characterize reactivity and anti-knock quality of fuel. In this work, both experimental measurement and modeling work have been conducted to evaluate OID properties for six ternary blends comprising n-heptane/iso-octane/ethanol. Experimental measurement was performed in a rapid compression machine, while modeling work was based on the gasoline surrogate mechanism. The decoupling study on research octane number (RON)/motor octane number (MON) and octane sensitivity (S) has been carried out by adjusting fuel composition. The thermal conditions of experiments cover low-to-medium temperatures from 640 K to 740 K and pressures from 9.5 bar to 21 bar. Oxygen is utilized as the oxidizer while nitrogen and argon are regarded as the buffer gas to adjust thermal conditions. Combined with chemical kinetics analysis, negative temperature coefficient (NTC) behavior in the experiment is entirely attributed to iso-octane and the timing of the first peak HO2 radical concentration has more significant influence on NTC behavior than peak value itself. Octane number and/or S cannot fully revealed fuel anti-knock quality, which is substantially influence by the property of fuels and thermal conditions. At low temperature (640 K), fuels with higher S exhibit better anti-knock performance. However, this advantage will disappear when temperature falls into NTC region. For engine operating in this area, high RON but low S fuels are recommended. Ethanol extends the beneficial area of high S contents due to its low temperature chemical inertness. The low temperature boundary of NTC for iso-octane can be regarded as the demarcation line to determine whether high S contents should be utilized, which is an important reference for fuel-engine co-optima research.

Benchmarking dual-level MS-Tor and DLPNO-CCSD(T) methods for H-abstraction from methyl pentanoate by an OH radical.

Methyl pentanoate (MP) was recently proposed as a potential biodiesel surrogate due to its negative temperature coefficient region. To provide a basis for constructing an accurate mechanism, chemical kinetics of H-abstraction from MP by an OH radical are investigated theoretically at 200-2000 K. M06-2X/cc-pVTZ is applied for geometry optimizations and frequency calculations. Given the long alkyl-chain in MP, the multi-structural torsional anharmonicity is characterized by using the dual-level MS-T method due to its relatively low computational cost and established accuracy. In particular, AM1 and M06-2X/cc-pVTZ are adopted as low-level and high-level methods in dual-level MS-T, respectively. The results of dual-level MS-T are further used to benchmark against the full high-level method (M06-2X/cc-pVTZ), leading to an uncertainty of less than 30% in the high temperature range. For the single-point energy calculations, the lower computational cost DLPNO-CCSD(T) method is first used to benchmark against the gold-standard method CCSD(T) for small methyl ester (C2-C4) + OH reaction systems, yielding an overestimation of less than 1.1 kcal mol-1 for barrier height; it is then used to refine the electronic energies for the present reaction system MP + OH. Phase-space theory and conventional transition state theory are used to calculate the H-abstraction rate constants. After compensating the uncertainty of barrier height, the calculated phenomenological H-abstraction rate constants agree well with the experimental data at 263-372 K. Branching ratio analysis indicates that the β-site H abstraction is the dominant channel at 200-1200 K due to its lowest barrier height.

Shock tube study of normal heptane first-stage ignition near 3.5 atm

Shock tube ignition delay times and species time history measurements for Primary Reference Fuels (PRFs) such as normal heptane provide targets for the validation of combustion models, which in turn are used to develop more fuel-efficient engines that have smaller environmental footprints. However, a review of the literature has revealed that most of the shock tube ignition delay time and species measurement data for normal heptane have been obtained at elevated pressures, rather than at relatively low pressures where many other important experimental techniques such as jet-stirred reactors and flow reactors can provide corroborating results. One central problem preventing previous shock tube studies from examining first-stage ignition at these lower pressures was that ignition times were too long under these conditions to be measured within the available shock tube test times. To address this issue, recent advances in shock tube techniques for achieving long uniform test times have been applied in order to measure low-pressure first-stage ignition times of normal heptane together with normal heptane fuel time-history records at times up to about 30 ms. These measurements were performed in the Negative Temperature Coefficient (NTC) region (T=664−792 K) in lean mixtures (21%O2/Ar, equivalence ratio ϕ=0.5) at pressures of roughly P=3.5 atm using both the conventional and Constrained Reaction Volume (CRV) shock tube filling strategies. The data have been used to evaluate the performance of several combustion models, have been compared with other higher-pressure shock tube first-stage ignition times in n-heptane/20–21%O2 mixtures found in the literature, and have been fit using a two-zone Arrhenius model for first-stage ignition delay times. The results showed that some combustion models yield ignition delay time predictions that differ from the experimental results by as much as an order of magnitude, that under these conditions n-heptane first-stage ignition times scale by roughly P−0.71 but are insensitive to ϕ, and that the fraction of fuel remaining after first-stage ignition increases with increasing initial experimental temperature.

Experimental and modeling study of the mutual oxidation of N-pentane and nitrogen dioxide at low and high temperatures in a jet stirred reactor

The mutual oxidation of n-pentane and NO2 at 500–1000 K has been studied at equivalence ratios of 0.5 and 1.33 by using an atmospheric-pressure jet stirred reactor (JSR). N-pentane, O2, NO, NO2, CO, CO2, CH2O, C2H4, and CH3CHO are simultaneously quantified, in-situ by using an electron-impact molecular beam mass spectrometer (EI-MBMS), a micro-gas chromatograph (μ-GC), and a mid-IR dual-modulation faraday rotation spectrometer (DM-FRS). Both fuel lean and rich experiments show that, in 550–650 K, NO2 addition inhibits low temperature oxidation. With an increase of temperature to the negative temperature coefficient (NTC) region (650–750 K), NO2 addition weakens the NTC behavior. In 750–1000 K, high temperature oxidation is accelerated with NO2 addition and shifted to lower temperature. Two kinetic models, a newly developed RMG n-pentane/NOx model and Zhao's n-pentane/NOx model (Zhao et al., 2018, Submitted) were validated against experimental data. Both models were able to capture the temperature-dependent NO2 sensitization characteristics successfully. The results show that although NO2 addition in n-pentane has similar effects to NO at many conditions due to fast NO and NO2 interconversion at higher temperature, it affects low temperature oxidation somewhat differently. When NO2/NO interconversion is slow, NO2 is relatively inert while NO can strongly promote or inhibit oxidation.

Autoignition of DME/C2H6 Mixtures Under High-Pressure and Low-Temperature Conditions

ABSTRACTThe impact of ethane (C2H6) addition to ignition delay of dimethyl ether (DME) under high-temperature conditions is opposite to that of other small molecule alkanes, but the trend is still unclear under low-temperature conditions. Thus, ignition delays of DME/C2H6 mixtures (C2H6 blending ratio ranging from 0% to 70%) were measured at temperatures of 624–913 K, pressures of 9–35 bar, equivalence ratios of 0.5–1 and dilution ratios of 4 and 5.5 using a rapid compression machine. Chemical kinetic simulations were further carried out using the NUI Aramco Mech 2.0, and a good agreement between experimental and simulation results was demonstrated. Results show that DME/C2H6 mixtures exhibit two-stage ignition and negative temperature coefficient (NTC) characteristics. The ignition delays DME/C2H6 mixtures decrease with increasing pressure, decreasing dilution ratio and increasing equivalence ratio, and the ignition-promoting effects become more prominent in the NTC region compared to the lower temperature region. Unlike the addition of C2H6 to DME at high temperatures, it appears anomalous behavior with other small molecule alkanes, and the addition of C2H6 at low temperatures nonlinearly increases the ignition delay especially at lower pressures, which is consistent with that of other small molecule alkanes. Kinetic analysis indicates that the addition of C2H6 to DME reduces the low-temperature chain-branching routes and more fuel molecule undergoes chain propagation ones. In the DME/C2H6 binary mixtures, the oxidation of DME is inhibited while that of C2H6 is promoted and even exhibits two-stage characteristics, which is due to their competition for OH radicals dominantly produced by low-temperature chain-branching of DME. On the whole, the consumption of overall fuel is inhibited with C2H6 addition, leading to less OH radicals and heat release accumulated during the low-temperature oxidation and thus longer ignition delays.