Development Of Kinetic Mechanisms Research Articles

Radical Stereochemistry: Accounting for Diastereomers in Kinetic Mechanism Development.

Recent work in combustion and atmospheric chemistry has revealed cases in which diastereomers must be distinguished to accurately model a reacting flow. This paper presents an open-source framework for introducing such stereoisomer resolution into a kinetic mechanism. We detail our definitions and algorithms for labeling and enumerating the stereoisomers of a molecule and then generalize our system to describe the transition state (TS) of a reaction. This allows for the stereospecific enumeration of reactants and products while accounting for "fleeting" stereochemistry that is unique to the TS. We also present the AutoMech Chemical Identifier (AMChI), an InChI-like string identifier that accounts for stereocenters omitted by InChI. This identifier is extended to describe the TSs of reactions, providing a universal lookup key for specific reaction channels. The final piece of our methodology is an analytic formula to remove redundancy from a stereoresolved mechanism when its enantiomers exist as a racemic mixture, making it as compact as possible while fully accounting for the differences between diastereomers. In applying our methodology to two subsets of the NUIGMech1.1 mechanism, we find that our approach reduces the extra species added for large-fuel oxidation from 2231 (133%, full expansion) to 694 (41%, nonredundant expansion). We also find that for pyrolysis more than a quarter of the species in the expanded mechanism cannot be properly described by an InChI string, requiring an AMChI string to communicate their identity. Finally, we find that roughly one-quarter of the large-fuel oxidation reactions and one-third of the pyrolysis reactions include fleeting TS stereochemistry, which may have relevant effects on their kinetics.

Experimental and Modeling Study on the Ignition Kinetics of Nitromethane behind Reflected Shock Waves.

Nitromethane (NM) is the simplest nitroalkane fuel and has demonstrated potential usage as propellant and fuel additive. Thus, understanding the combustion characteristics and chemistry of NM is critical to the development of hierarchical detailed kinetic models of nitro-containing energetic materials. Herein, to further investigate the ignition kinetics of NM and supplement the experimental database for kinetic mechanism development, an experimental and kinetic modeling analysis of the ignition delay times (IDTs) of NM behind reflected shock waves at high fuel concentrations is reported against previous studies. Specifically, the IDTs of NM are measured via a high-pressure shock tube within the temperature from 900 to 1150 K at pressures of 5 and 10 bar and equivalence ratios of 0.5, 1.0, and 2.0. Brute-force sensitivity analysis and chemical explosive mode analysis in combination with reaction path analysis are employed to reveal the fundamental ignition kinetics of NM. Finally, a skeletal mechanism for NM is derived via the combination of directed relation graph-based methods, which demonstrates good prediction accuracy of NM ignition and flame speeds. The present work should be valuable for understanding the combustion chemistry of NM and the development of the fundamental reaction mechanism of nitroalkane fuels.

An investigation on the laminar flame propagation and auto-ignition characteristics of a coal-derived rocket kerosene-Part I: Experimental study

The energy structure of coal-rich and oil-poor in China, makes the conversion of coal to liquid fuels an attractive option to reduce petroleum dependence. In this study, the combustion characteristics of a newly developed coal-derived rocket kerosene were investigated to provide fundamental parameters for evaluating the feasibility and compatibility of the new fuel in potential applications. The laminar flame propagation properties of the fuel were studied at pressures of 1, 2 and 5 bar over a temperature range of 423–483 K for equivalence ratio of 0.7–1.8 in a constant volume combustion bomb, and the auto-ignition characteristics of the target fuel were then experimentally studied in a heated shock tube. Flame propagation process was studied based on the recorded flame morphology firstly. The unsteady transition from spark assisted ignition kernel propagation to normal flame was analyzed and was found to become less conspicuous in flames with stronger reaction intensity (higher initial pressure or temperature) and smaller Lewis number (higher equivalence ratio). The Markstein length was extracted by extrapolating the flame trajectories and used to investigate the flame instability. The results shows that the increase of pressure, equivalence ratio enhance the flame instability, but the impact of initial temperature shows no prominent pattern. The laminar burning velocity and ignition delay times were reported over a wide range to strengthen the understanding of combustion characteristics of the new fuel. A kinetic model was proposed to simulate the combustion chemistry of the fuel and kinetic analysis was performed to identify the key reactions driving the flame propagation and auto-ignition. The present study is hoped to provide insights into the evaluation of fuel application in the future and database support for further combustion reaction kinetic mechanism development.

Kinetic Mechanism Development for the Direct Reduction of Single Hematite Pellets in H2/CO Atmospheres

Increasing interest in the direct reduction process of iron ore has revived investigations on gaseous and heterogeneous iron oxides’ kinetics. Despite extensive studies on the reduction of iron oxides with pure hydrogen or syngas, the development of a generic reduction mechanism for iron oxides is still lacking. The conventional shrinking core model hardly distinguishes between transport and reaction processes, leading to biases or even errors in the kinetic models. In the present study, a porous solid model, which solves mass balances of the individual gas‐phase species and solid ones assuming spherical symmetry, is used for developing a heterogeneous kinetic mechanism accounting for different iron oxidation stages (Fe2O3, Fe3O4, FeO). It also accounts for carbon accumulation and cementite formation to model the carburization phenomena when using carbon‐containing reducing agents like syngas. The proposed generic mechanism successfully reproduces 49 experimental data sets from the literature for single iron oxide pellets’ isothermal reduction with syngas of varying hydrogen content up to pure hydrogen without any adjustment of its parameters. The model predictions are in excellent agreement with the experimental data. Finally, the carburization kinetics are explained, and the factors (e.g., temperature, hydrogen content) affecting the carbon deposition are analyzed.

A comparative single-pulse shock tube experiment and kinetic modeling study on pyrolysis of cyclohexane, methylcyclohexane and ethylcyclohexane

The pyrolysis of cyclohexane, methylcyclohexane, and ethylcyclohexane have been studied behind reflected shock waves at pressures of 5 and10 bar and at temperatures of 930–1550 K for 0.05% fuel diluted by Argon. A single-pulse shock tube (SPST) is used to perform the pyrolysis experiments at reaction times varying from 1.65 to 1.74 ms. Major products are obtained and quantified using gas chromatography analysis. A flame ionization detector and a thermal conductivity detector are used for species identification and quantification. Kinetic modeling has been performed using several detailed and lumped chemical kinetic mechanisms. Differences in modeling results among the kinetic models are described. Reaction path analysis and sensitivity analysis are performed to determine the important reactions controlling fuel pyrolysis and their influence on the predicted concentrations of reactant and product species profiles. The present work provides new fundamental knowledge in understating pyrolysis characteristics of cyclohexane compounds and additional data set for detailed kinetic mechanism development.

Species quantification during n-heptane autoignition using filtered natural emission of species

Quantitative species concentration measurements taken during hydrocarbon oxidation are critical for the development of kinetic mechanisms. Spectroscopic methodologies are often employed to take such measurements due to their unobtrusive nature. A new methodology for species concentration measurements, called the filtered natural emission of species (FNES), has been proposed as it provides greater spatial resolution and a broader range of viable test conditions than other spectroscopic methods, such as laser absorption spectroscopy (LAS). To validate these claims, the concentrations of CO, CO2 and H2O were measured during the oxidation of a lean, n-heptane mixture in a rapid compression machine using FNES. The mixture was tested at an average pressure of 22 bar in the temperature range between 710 and 784 K. The measured data was compared with results of 0-D and 3-D numerical models using multiple kinetic mechanisms and available data found in literature. The empirical data of this work agreed well with the peak concentration measurements reported in the literature; however, the FNES methodology was able to provide insight into combustion processes when inhomogeneities caused by flow or boundary layer effects existed due to its unique field-of-view. As such, elevated CO concentrations were observed after the peak concentration when using the FNES methodology. This observation was not reported in literature nor captured by the 0-D numerical models; however, the 3-D model was able to explain the phenomenon as the result of the temperature field near the wall. 3-D simulations are recommended for the benefit of future works. In addition, the measured species concentrations displayed a great sensitivity to temperature, which was a function of the detailed kinetic mechanisms. Different kinetic mechanisms were found to significantly alter the results of the measured concentrations. Kinetic mechanisms must be chosen with care for accurate modeling of temperature for use of species concentration measurements.

Ignition Studies in a Motored Engine for Kinetic Mechanism Development and Validation

Ignition Studies in a Motored Engine for Kinetic Mechanism Development and Validation

Mechanisms of Advanced Oxidation Processes, the Principle of Detailed Balancing, and Specifics of the UV/Chloramine Process.

Advanced oxidation processes tend to have very complex reaction mechanisms, and models containing over 150 steps have been developed to describe the chemistry. Without the aid of automation, it is extremely difficult to avoid the development of kinetic mechanisms that violate the principle of detailed balancing. Here, we apply DETBAL, a computer application, to systematically identify many violations of the principle of detailed balancing in a model proposed for the UV/chloramine process. We then show that these violations can also be found in dozens of other proposed models for advanced oxidation processes. Suggested repairs to these violations are provided. These repairs lead to no significant changes in the model predictions because the illegal loops include steps that are unnecessary under the conditions modeled. The model omits certain steps that do have significant effects on the model predictions.

An experimental and kinetic modeling study of the oxidation of hexane isomers: Developing consistent reaction rate rules for alkanes

Alkanes are key components in gasoline, jet and diesel fuels and considerably influence the combustion behavior of these fuels because of their wide range of reactivity. An improved understanding of their combustion behavior and the development of chemical kinetic models that can accurately simulate their combustion behavior are important for the development of next-generation internal-combustion and gas-turbine engines. The current work provides improved insight into oxidation mechanisms of a representative family of hydrocarbon fuels, specifically the hexane isomers: n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane. These isomers provide carbon “skeletons” ranging from straight-chained to highly-branched and provide a framework for the subsequent development of kinetic mechanisms for larger alkanes. New ignition delay times for the four branched hexane isomers were measured in a high-pressure shock tube and in a rapid compression machine, both at stoichiometric conditions (φ = 1), p = 15 bar and XO2 = 21% over temperatures from 600 to1300 K. These data were combined with previously published measurements under the same conditions for the remaining n-hexane isomer to provide a complete body of experimental data for kinetic modeling analysis. In addition, very recent experimental measurements of individual intermediate chemical species concentrations from all five hexane isomers in a jet-stirred reactor are also included and provide another family of data for further assessment of hexane isomer reactivity. Different reactivities were observed for each hexane isomer in each experimental facility, resulting from differences in their molecular structures. Consistent reaction rate rules have been applied to develop a combined detailed chemical kinetic model for all five hexane isomers. Kinetic model validation studies are reported to show that the current model reproduces well the ignition delay times of all five alkane isomers, as well as their variations in reactivity over a wide range of temperatures and other operating conditions. Equally important, these results show that it is not necessary to have a separate, different kinetic model for each isomer of a family of alkane fuels and that a single, coherent, integrated set of reaction rate classes and rules is sufficient to accurately describe combustion rates of combustion of straight-chain n-alkanes and branched-chain alkane fuels. This suggests strongly that a single set of reaction classes and rate rules should be sufficient to describe combustion kinetics of alkane fuels of any size and degree of branching.

Single-cylinder engine evaluation of a multi-component Diesel surrogate fuel at partially-premixed and low-temperature combustion modes

To investigate and develop efficient and clean combustion systems, advanced CFD modeling tools need accurate kinetic models capable of predicting the chemical and physical processes that take place in the combustion chamber during and after fuel injection and air mixing. Given the complex composition of market Diesel fuels, simpler surrogate fuels composed of a limited number of pure substances are used to model the physical, chemical, and combustion properties of Diesel fuel. The surrogates must closely reproduce the market Diesel fuel properties and duplicate the real-world engine combustion and emissions behavior. Previous work created a multi-component surrogate fuel that consisted of normal-hexadecane/2,2,4,4,6,8,8-heptamethylnonane/decahydronaphthalene/1-methylnaphthalene. The surrogate fuel properties closely matched the market Diesel fuel. The surrogate fuel was then evaluated under conventional Diesel engine combustion conditions and obtained excellent results replicating the real-world combustion and emissions of the market Diesel fuel. In this work, the surrogate fuel was investigated using a contemporary Diesel engine, operating at a low-load where partially-premixed and low temperature combustion conditions were achieved. For comparison and validation, the original market Diesel fuel was also tested under the same operating conditions. The experimental results from EGR and combustion phasing sweeps under partially-premixed and low temperature combustion showed the surrogate fuel closely duplicated the ignition delay, low temperature and high temperature heat release of the market Diesel fuel. The exhaust CO, NOx and HC emissions along with the particle size distributions were also closely matched. This investigation demonstrates that the four-component surrogate accurately represents the market Diesel fuel under low load, premixed and low temperature combustion regimes. Therefore, the surrogate fuel formulation should be suitable for continued work such as kinetic mechanism development, 3-dimensional spray and combustion simulation, and further exploratory research under premixed and low temperature combustion conditions.

Perspective on Mechanism Development and Structure‐Activity Relationships for Gas‐Phase Atmospheric Chemistry

ABSTRACTThis perspective gives our views on general aspects and future directions of gas‐phase atmospheric chemical kinetic mechanism development, emphasizing on the work needed for the sustainable development of chemically detailed mechanisms that reflect current kinetic, mechanistic, and theoretical knowledge. Current and future mechanism development efforts and research needs are discussed, including software‐aided autogeneration and maintenance of kinetic models as a future‐proof approach for atmospheric model development. There is an overarching need for the evaluation and extension of structure‐activity relationships (SARs) that predict the properties and reactions of the many multifunctionalized compounds in the atmosphere that are at the core of detailed mechanisms, but for which no direct chemical data are available. Here, we discuss the experimental and theoretical data needed to support the development of mechanisms and SARs, the types of SARs relevant to atmospheric chemistry, the current status and limitations of SARs for various types of atmospheric reactions, the status of thermochemical estimates needed for mechanism development, and our outlook for the future. The authors have recently formed a SAR evaluation working group to address these issues.

Single-cylinder engine evaluation of a multi-component diesel surrogate fuel at a part-load operating condition with conventional combustion

Simple, yet fully-representative surrogate fuels are needed to model market Diesel fuels. The surrogates must closely mimic market Diesel fuel properties and match the engine combustion and emissions behavior. To this end, the combustion and emissions performance of a market Diesel fuel and a four-component surrogate fuel were investigated using a single-cylinder, light-duty Diesel engine with contemporary combustion and fuel injection technology. The surrogate fuel, which was developed in previous work, was composed of normal-hexadecane/2,2,4,4,6,8,8-heptamethylnonane/decahydronaphthalene/1-methylnaphthalene with a volume fraction formulation of 0.37/0.33/0.18/0.12. Fuel test results showed the physical, chemical and combustion properties of the market Diesel fuel were closely matched by the surrogate. The fuels were evaluated by engine tests conducted at 1500 r/min and 9 bar IMEP. At this operating condition, the Diesel combustion process demonstrated low-temperature heat release, premixed and diffusion combustion regions. Test results from EGR and combustion phasing sweeps showed the engine combustion behavior, exhaust CO, HC, NOx, smoke and particle size distributions from the market Diesel fuel were very closely matched by the surrogate fuel. It can be concluded that under the engine conditions evaluated by this work, the four-component surrogate fuel accurately represents the market Diesel fuel. Thus, for conventional Diesel combustion conditions, the surrogate should prove useful for future applications such as kinetic mechanism development, fuel spray and combustion simulation, and further experimental investigations.

A detailed reaction mechanism for oxidative coupling of methane over Mn/Na2WO4/SiO2 catalyst for non-isothermal conditions

Direct production of ethylene from natural gas or other methane sources has proven to be challenging from catalytic, process design and economic perspectives, with no reports of a commercially viable process to date. Understanding of the underlying non-ideal homogeneous and heterogeneous reactions involved could lead to success, enabling utilization of vast natural gas resources as a replacement for petrochemicals. In this regards, an improved reaction mechanism and computational model, validated experimentally, for oxidative coupling of methane (OCM) over a Mn/Na2WO4/SiO2 catalyst is developed, taking into account the non-isothermal behavior exhibited by the gas and surface reactions. The reaction mechanism involves detailed gas-phase and surface elementary steps to describe the OCM chemistry and employing a reduced model for computational efficiency. The model uses laboratory-scale packed-bed reactor experiments for kinetic mechanism development. The modeling and simulation is particularly focused on a one-dimensional packed-bed reactor. The kinetic model is validated over a wide range of experimental conditions. The model-predicted results and their comparisons with the experimental data are presented wide temperature ranges and methane-to-oxygen feed ratios under adiabatic reactor conditions. The kinetic model is first of its kind in which non-isothermal OCM kinetic behavior is captured using detailed gas-phase and heterogeneous reaction kinetics.

Oxidation of substituted aromatic hydrocarbons in the tropospheric aqueous phase: kinetic mechanism development and modelling

Monocyclic aromatic compounds are ubiquitous in the polluted troposphere and contribute to the formation of tropospheric ozone and anthropogenic secondary organic aerosol, including brown carbon. Currently available physico-chemical data including aqueous-phase kinetic and mechanistic data, as well as phase-transfer parameters have been compiled and reviewed, to construct a novel aqueous-phase oxidation mechanism for monocyclic aromatic compounds. The performed chemical mechanism development results in a comprehensive aqueous-phase oxidation mechanism (addressed as CAPRAM-AM1.0), which includes 292 processes considering the oxidation of different aromatic compounds. Detailed numerical simulations with the air parcel model SPACCIM are carried out for different urban environmental and seasonal conditions. Results show that the aqueous-phase chemistry of aromatic compounds, particularly in clouds, increases the organic aerosol mass by up to 10% in total. The absolute contribution to aqSOA in summertime is modelled to be 260 ng m-3 and 1.2 μg m-3 under moderate and strongly polluted conditions, respectively. Aqueous-phase oxidations of aromatic compounds are important not only for the degradation, but also for the formation of nitrated aromatic compounds. In-cloud chemistry contributes up to 54% to the nitrocatechol oxidation and up to 37% to its formation under polluted tropospheric conditions. Besides, nitrated aromatic compounds contribute up to 5.4 μg m-3 to modelled brown carbon concentration in cloud droplets and 140 ng m-3 in aerosol particles. Further, the model simulations indicate that besides OH radical oxidations, aromatic compounds with two hydroxyl groups are also strongly oxidised by O3 and HO2. O3 contributes with 49% to 68% and HO2 with 19% to 22% to the aqueous-phase oxidation of catechol under moderate and strong polluted environmental conditions studied.

An updated experimental and kinetic modeling study of n-heptane oxidation

This work presents an updated experimental and kinetic modeling study of n-heptane oxidation. In the experiments, ignition delay times of stoichiometric n-heptane/air mixtures have been measured in two different high-pressure shock tubes in the temperature range of 726–1412K and at elevated pressures (15, 20 and 38bar). Meanwhile, concentration versus time profiles of species have been measured in a jet-stirred reactor at atmospheric pressure, in the temperature range of 500–1100K at φ=0.25, 2.0 and 4.0. These experimental results are consistent with those from the literature at similar conditions and extend the current data base describing n-heptane oxidation.Based on our experimental observations and previous modeling work, a detailed kinetic model has been developed to describe n-heptane oxidation. This kinetic model has adopted reaction rate rules consistent with those recently developed for the pentane isomers and for n-hexane. The model has been validated against data sets from both the current work and the literature using ignition delay times, speciation profiles measured in a jet-stirred reactor and laminar flame speeds over a wide range of conditions. Good agreement is observed between the model predictions and the experimental data. The model has also been compared with several recently published kinetic models of n-heptane and shows an overall better performance. This model may contribute to the development of kinetic mechanisms of other fuels, as n-heptane is a widely used primary reference fuel. Since the sub-mechanisms of n-pentane, n-hexane and n-heptane have adopted consistent reaction rate rules, the model is more likely to accurately simulate the oxidation of mixtures of these fuels. In addition, the successful implementation of these rate rules have indicated the possibility of their application for the development of mechanisms for larger hydrocarbon fuels, which are of great significance for practical combustion devices.

Recent progress and challenges in fundamental combustion research

More than 80% of world energy is converted by combustion. Development of efficient next generation advanced engines by using alternative fuels and operating at extreme conditions is one of the most important solutions to increase energy sustainability. To realize the advanced engine design, the challenges in combustion research are therefore to advance fundamental understanding of combustion chemistry and dynamics from molecule scales to engine scales and to develop quantitatively predictive tools and innovative combustion technologies. This review will present the recent progresses and technical challenges in fundamental combustion research in seven areas including advanced engine concepts using low temperature fuel chemistry, new combustion phenomena in extreme conditions, alternative and surrogate fuels, multi-scale modeling, high pressure combustion kinetics, experimental methods and advanced combustion diagnostics Firstly, new engine concepts such as the Homogeneous Charge Compression Ignition (HCCI), Reactivity Controlled Compression Ignition (RCCI), and pressure gain combustion will be introduced. The impact of low temperature combustion chemistry of fuels on combustion in advanced engines will be demonstrated. This is followed by the discussions of the needs of fundamental combustion research for new engine technologies. Secondly, combustion phenomena and flame regimes involving new combustion concepts such as fuel and thermal stratifications, plasma assisted combustion, and cool flames at extreme conditions will be analyzed. Thirdly, alternative fuels and methodologies to formulate surrogate fuel mixtures to model the target combustion properties of real fuels will be presented. A new concept of radical index and transport weighted enthalpy will be introduced to rank the fuel reactivity and to assess the impact of molecular structure on combustion properties The success and limitations of the current surrogate fuel models will be discussed by using jet fuels and biodiesels as examples. Fourthly, the difficulty of modeling large kinetic mechanism of real fuel will be discussed The multi-time scale (MTS) method and the correlated dynamic adaptive chemistry (CO-DAC) method for kinetic model reduction and computationally efficient modeling will be compared and analyzed. Fifthly, the progress and challenges of high pressure combustion kinetics for hydrogen and larger hydrocarbons will be discussed. The important pressuredependent reaction pathways and key intermediate species at high pressure will be analyzed. Fundamental experimental methods for combustion and their uncertainties in acquiring combustion properties for the validation of kinetic mechanism will be discussed. Finally, recent progress in diagnostics of HO2, H2O2, RO2, ketohydroperoxide, and other key intermediate species for high pressure kinetic mechanism development will be summarized. Conclusions and opportunities of future combustion research will be made.

Development of Kinetic Mechanism for the Esterification of Acrylic Acid with Hexanol Catalyzed by Ion-Exchange Resin

The heterogeneous esterification reaction of acrylic acid with hexanol over three different cation-exchange resins, Amberlyst 131, Amberlyst 15, and Dowex 50Wx-400, was investigated. Surface area, pore volume, average pore diameter, and acid capacity analyses were carried out for these three catalysts. Amberlyst 131 gave the highest conversion, and therefore, further experiments were performed catalyzed by Amberlyst 131. The absence of external and internal mass transfer resistances was investigated for the esterification of acrylic acid with hexanol. Experiments showed that the reaction is controlled by chemical step rather than external and internal mass transfer steps. The effects of temperature, catalyst loading, and alcohol to acid molar ratio on the conversion of acrylic acid were determined. The activation energy and kinetic and adsorption constants were determined according to the Langmuir-Hinshelwood-Hougen-Watson mechanism.

Optimization of Reduced Kinetic Models for Reactive Flow Simulations

A robust optimization scheme, known as rkmGen, for reaction rate parameter estimation has been developed for the generation of reduced kinetics models of practical interest for reactive flow simulations. It employs a stochastic optimization algorithm known as simulated annealing (SA), and is implemented in C++ and coupled with Cantera, a chemical kinetics software package, to automate the reduced kinetic mechanism generation process. Reaction rate parameters in reduced order models can be estimated by optimizing against target data generated from a detailed model or by experiment. Target data may be of several different kinds: ignition delay time, blow-out time, laminar flame speed, species time-history profiles, and species reactivity profiles. The software allows for simultaneous optimization against multiple target data sets over a wide range of temperatures, pressures, and equivalence ratios. In this paper, a detailed description of the optimization strategy used for the reaction parameter estimation is provided. To illustrate the performance of the software for reduced kinetic mechanism development, a number of test cases for various fuels were used: one-step, three-step, and four-step global reduced kinetic models for ethylene, Jet-A and methane, respectively, and a 50 step semiglobal reduced kinetic model for methane. The 50 step semiglobal reduced kinetic model was implemented in the Star*CCM+ commercial CFD code to simulate Sandia Flame D using laminar flamelet libraries and compared with the experimental data. Simulations were also performed with the GRI3.0 mechanism for comparisons.

Ab Initio Kinetics for Thermal Decomposition of CH3N•NH2, cis-CH3NHN•H, trans-CH3NHN•H, and C•H2NNH2 Radicals

The thermal decomposition of the CH(3)N(•)NH(2), cis-CH(3)NHN(•)H, trans-CH(3)NHN(•)H, and C(•)H(2)NNH(2) radicals, which are the four radical products from the H-abstraction reactions of monomethylhydrazine, were theoretically studied by using ab initio Rice-Ramsperger-Kassel-Marcus (RRKM) transition-state theory and master equation analysis. Various decomposition pathways were identified by using either the QCISD(T)/cc-pV∞Z//CASPT2/aug-cc-pVTZ or the QCISD(T)/cc-pV∞Z//B3LYP/6-311++G(d,p) quantum chemistry methods. The results reveal that the β-scission of NH(2) to form methyleneimine is the predominant channel for the decomposition of the C(•)H(2)NNH(2) radical due to its small energy barrier of 13.8 kcal mol(-1). The high pressure limit rate coefficient for the reaction is fitted by 3.88 × 10(19)T(-1.672) exp(-9665.13/T) s(-1). In addition, the pressure dependent rate coefficients exhibit slight temperature dependence at temperatures of 1000-2500 K. The cis-CH(3)NHN(•)H and trans-CH(3)NHN(•)H radicals are the two distinct spatial isomers with an energy barrier of 26 kcal mol(-1) for their isomerization. The β-scission of CH(3) from the cis-CH(3)NHN(•)H radical to form trans-diazene has an energy barrier of 35.2 kcal mol(-1), and the β-scission of CH(3) from the trans-CH(3)NHN(•)H radical to form cis-diazene has an energy barrier of 39.8 kcal mol(-1). The CH(3)N(•)NH(2) radical undergoes the β-scission of methyl hydrogen and amine hydrogen to form CH(2)═NNH(2), trans-CH(3)N═NH, and cis-CH(3)N═NH products, with the energy barriers of 42.8, 46.0, and 50.2 kcal mol(-1), respectively. The dissociation and isomerization rate coefficients for the reactions were calculated via the E/J resolved RRKM theory and multiple-well master equation analysis at temperatures of 300-2500 K and pressures of 0.01-100 atm. The calculated rate coefficients associated with updated thermochemical property data are essential components in the development of kinetic mechanisms for the pyrolysis and oxidation of MMH and its derivatives.

Jet fuel ignition delay times: Shock tube experiments over wide conditions and surrogate model predictions

Ignition delay times were measured for gas-phase jet fuel (Jet-A and JP-8) in air behind reflected shock waves in a heated high-pressure shock tube. Initial reflected shock conditions were as follows: temperatures of 715–1229 K, pressures of 17–51 atm, equivalence ratios of 0.5 and 1, and oxygen concentrations of 10 and 21% in synthetic air. Ignition delay times were measured using sidewall pressure and OH* emission at 306 nm. Longer ignition delay times at low temperatures (715–850 K) were accessed by utilizing driver-gas tailoring methods. Also presented is a review of previous ignition delay time measurements of kerosene-based fuels and recent work on surrogate fuel and kinetic mechanism development. To our knowledge, we report the first gas-phase shock tube ignition delay time data for JP-8, and our measurements for Jet-A are for a broader range of conditions than previously available. Our results have very low scatter and are in excellent agreement with the limited previous shock tube data for Jet-A. Although JP-8 and Jet-A have slightly different compositions, their ignition delay times are very similar. A simple 1 / P dependence was found for ignition delay times from 874 to 1220 K for the pressure range studied for both fuels. Ignition delay time variations with equivalence ratio and oxygen concentration were also investigated. The new experimental results were compared with predictions of several kinetic mechanisms, using different jet fuel surrogate mixtures.