Predicted Ignition Delay Times Research Articles

A combustion mechanism reduction method based on entropy production analysis in fuel auto-ignition and laminar flames

This study provides a chemical mechanism reduction strategy based on entropy production analyses in both auto-ignition and laminar flames, which enhances the predictive accuracy for laminar burning velocities. In addition to chemical reactions, other irreversible sources causing entropy generation, such as mass diffusion and heat conduction, are considered in the modified approach. Specifically, initial skeletal mechanisms are first generated based on important reactions that contribute to entropy production in auto-ignition processes. Mechanism patches are then constructed to include important species and reactions, which contribute to entropy production from mass diffusion and heat conduction in laminar premixed flames beyond the pre-defined thresholds, respectively. Finally, the initial skeletal mechanisms and mechanism patches are combined to establish the final skeletal mechanisms. In this way, two final skeletal mechanisms for n-dodecane, consisting of 162 species and 2276 reactions, and 160 species and 1916 reactions, respectively, are developed from the detailed POLIMI mechanism with 451 species and 17,848 reactions. The two final skeletal mechanisms are proven to accurately predict laminar burning velocities and entropy production in n-dodecane flames with insignificant variations in the simulation results compared to the detailed mechanism, while their accuracy in predicting ignition delay times relies on the initial skeletal mechanisms. Specifically, the reduced mechanism with 160 species and 1916 reactions exhibits less satisfactory performance in predicting ignition delay compared to that with 162 species and 2276 reactions, indicating that a lower threshold is required to generate the initial skeletal mechanism through entropy production analysis of homogeneous auto-ignition processes. Additionally, compared with the reduced mechanisms with similar sizes obtained with other mechanism reduction strategies, the two final skeletal mechanisms accurately capture the characteristics of laminar burning velocities and ignition delay times, with similar calculation time required.

Ignition delay times of C1–C7 natural gas blends in the intermediate and high temperature regimes: Experiment and correlation

This study presents new ignition delay time data for two multi-component natural gas (NG) blends composed of C1–C7n-alkanes with methane as the major component. New experimental data were recorded using a high-pressure shock tube (HPST) at reflected shock pressures (p5) of 10–30 bar, at temperatures (T5) in the range 770–1480 K, and at equivalence ratios (φ) of 0.5–1.5 in ‘air’. The current results together with published rapid compression machine (RCM) measurements show that higher concentrations of larger molecular weight n-alkanes in the NG blends increase fuel reactivity by more than an order of magnitude with mixtures that have ∼18.75% of C3–C5 components compared to mixtures that have ∼44.4% of C3–C7 components at temperatures below 1000 K. On the contrary, at higher temperatures the effect of increasing reactivity is reduced. NUIGMech1.2 is used to simulate the conditions studied and shows good agreement with the experimental ignition delay time data. A correlation equation is developed through regression analyses using NUIGMech1.2 to predict ignition delay times for a wide range of C1–C7 NG mixtures, in the pressure range 10–50 bar, at temperatures in the range 950–2000 K, and at φ = 0.3–3.0 in air. The proposed correlation expression that employs a traditional Arrhenius form is successfully validated against the new experimental data as well as previously published HPST experimental data.

Development of a detailed kinetic model for high-pressure ethanol oxidation in gas phase and supercritical water

The present study aims to develop a detailed chemical kinetic model for high-pressure ethanol oxidation in the gas phase and supercritical water (SCW). Kinetic parameters for key elementary reactions involving HO2 and CH3OO radicals were updated, deriving from a careful analysis of ignition delay data or analogy. The model performance is validated comprehensively by comparison to a wide variety of recent experimental data from both gas-phase and SCW conditions. The model predicted ignition delay times and speciation measurements (15–60 bar) in the gas phase fairly well. Additionally, the model showed satisfactory performance in reproducing the species concentration profiles at SCW conditions. A comparative kinetic analysis was further conducted to investigate the characteristic differences and similarities of ethanol oxidation at the two conditions. The results indicated SCW lowered the onset temperature for ethanol ignition and facilitated the ethanol oxidation rate, but did not introduce additional oxidation pathways.

Modelling of acetaldehyde and acetic acid combustion

Despite the beneficial impact of biofuels on most regulated pollutants and carbon dioxide emissions, their combustion results in the generation of undesired pollutants, such as acetaldehyde and acetic acid. To better understand the chemistry of these species, detailed chemical kinetic models deriving from two alternative strategies for mechanism generation were developed and validated against available data. The first model represents a semi-lumped mechanism comprising 89 species and 366 reactions, whereas the latter is automatically generated to aggregate elemental steps based on a rate-based algorithm, and it contains 541 species and 27,334 reactions. Under the studied conditions, the two kinetic models fairly predicted ignition delay times and laminar burning velocity data of acetic acid and acetaldehyde. Few discrepancies were observed for ignition delay time at temperatures lower than 1300 K. However, the overall agreement between experimental measurements and numerical estimations allowed for the use of the two kinetic models to unravel the chemistry of the investigated species. Highlights Identification of key primary reactions for acetic acid and acetaldehyde Integration of an existing kinetic mechanism with selected reactions Development of a detailed kinetic mechanism through an automated algorithm Comparison of experimental and numerical data for overall reactivity Analysis of the chemistry of acetic acid and acetaldehyde

Accurate determination of homogeneous ignition of single solid fuel particles enabled by machine learning

This work presents an experimental investigation of solid fuel combustion in a laminar flow reactor with high-speed laser diagnostics. The main focus lays on the development of image evaluation approaches for accurate determination of single-particle ignition based on optical measurement data. Homogeneous ignition of individual bituminous coal particles is visualized by simultaneous planar laser-induced fluorescence of OH radicals (OH-LIF) and diffuse backlight-illumination (DBI) techniques at 10kHz. Two coal particle sizes of 90–125μm and 160–200μm are investigated in conventional air and oxy-fuel conditions with increasing oxygen concentrations. A comprehensive experimental data set is established, containing in total 1518 single-particle events and high-quality ground truth labels of ignition delay times. The previously proposed structure and signal (SAS) analysis is evaluated. Errors and uncertainties are evaluated in a sensitivity analysis with variation in the threshold selection, implying inherent restrictions of conventional image processing methods. An improved blob detection approach is applied, enabling feature detection at various scales and higher accuracy in predicting ignition delay times. Moreover, two deep learning architectures, namely residual network (ResNet) and feature pyramid network (FPN) are implemented. Both networks are capable of detecting ignition with significantly higher accuracy and precision than other algorithms in this study. Influences of training data and depth of networks on the prediction performance of a trained model are examined. The current study shows that the hierarchical feature extraction of the convolutions networks clearly facilitates data evaluation for high-speed optical measurements and could be transferred to other solid fuel experiments with similar boundary conditions.

Experimental Ignition Delay Time Measurements and Chemical Kinetics Modeling of Hydrogen/Ammonia/Natural Gas Fuels

Abstract In recent years, hydrogen-carrying compounds have accrued interest as an alternative to traditional fossil fuels due to their function as zero-emission fuels. As such, there is interest in investigating hydrogen-carrying compounds to improve understanding of the fuels' characteristics for use in high pressure systems. In the current study, the oxidation of ammonia/natural gas/hydrogen mixtures was carried out to study carbon monoxide (CO) formation profiles as well as the ignition delay times (IDTs) behind reflected shock waves in order to refine chemical kinetic models. Experiments were carried out in the University of Central Florida's shock tube facility by utilizing chemiluminescence to obtain OH* emission and laser absorption spectroscopy to obtain CO profiles over a temperature range between 1200 K and 1800 K with an average pressure of 2.2 atm. Experimental mixtures included both neat and combination natural gas/hydrogen with ammonia addition, with all mixtures except one having an equivalence ratio of 1. Results were then compared with the GRI 3.0 mechanism, as well as the newly developed UCF 2022 mechanism utilizing chemkin-pro software. In general, both models were able to capture the trend in auto-ignition delay times and CO time histories for natural gas and ammonia mixtures. However, for ammonia–hydrogen mixtures, GRI 3.0 failed to predict ignition delay times, whereas the UCF 2022 mechanism was able to capture the IDTs within the uncertainty limits of the experiments. A sensitivity analysis was conducted for different mixtures to understand the important reactions at the experimental conditions. Finally, a reaction pathway analysis was carried out to understand important ammonia decomposition pathways in the presence of hydrogen and natural gas.

From electronic structure to model application of key reactions for gasoline/alcohol combustion: Hydrogen-atom abstraction by CH3OȮ radicals

Hydrogen atom abstraction by methyl peroxy (CH3OȮ) radicals can play an important role in gasoline/ethanol interacting chemistry for fuels that produce high concentrations of methyl radicals. Detailed kinetic reactions for hydrogen atom abstraction by CH3OȮ radicals from the components of FGF-LLNL (a gasoline surrogate) including cyclopentane, toluene, 1-hexene, n-heptane, and isooctane have been systematically studied in this work. The M06–2X/6–311++G(d,p) level of theory was used to obtain the optimized structure and vibrational frequency for all stationary points and the low-frequency torsional modes. The 1-D hindered rotor treatment for low-frequency torsional modes was treated at M06–2X/6–31G level of theory. The UCCSD(T)-F12a/cc-pVDZ-F12 and QCISD(T)/CBS level of theory were used to calculate single point energies for all species. High pressure limiting rate constants for all hydrogen atom abstraction channels were performed using conventional transition state theory with unsymmetric tunneling corrections. Individual rate constants are reported in the temperature range from 298.15 to 2000 K. Our computed results show that the abstraction of allylic hydrogen atoms from 1-hexene is the fastest at low temperatures. When the temperature increases, the hydrogen atom abstraction reaction channel at the primary alkyl site gradually becomes dominant. Thermodynamics properties for all stable species and high-pressure limiting rate constants for each reaction pathway obtained in this work were incorporated into the latest gasoline surrogate/ethanol model to investigate the influence of the rate constants calculated here on model predicted ignition delay times.

Effects of ambient pressure and nozzle diameter on ignition characteristics in diesel spray combustion

Numerical simulations are performed to investigate the effects of ambient density (ρam) and nozzle diameter (Dnoz) on the ignition characteristic of diesel spray combustion under engine-like conditions. A total of nine cases which consist of different ρam of 14.8, 30.0, and 58.5 kg/m3 and different Dnoz of 100, 180, and 363μm are considered. The results show that the predicted ignition delay times are in good agreement with measurements. The current results show that the mixture at the spray central region becomes more fuel-rich as Dnoz increases. This leads to a shift in the high-temperature ignition location from the spray tip towards the spray periphery as Dnoz increases at ρam of 14.8 kg/m3. At higher ρam of 30.0 and 58.5 kg/m3, the ignition locations for all Dnoz cases occur at the spray periphery due to shorter ignition timing and the overly fuel-rich spray central region. The numerical results show that the first ignition location during the high-temperature ignition occurs at the fuel-rich region at ρam⩽30.0 kg/m3 across different Dnoz. At ρam=58.5 kg/m3, the ignition occurs at the fuel-lean region for the 100 and 180μm cases, but at the fuel-rich region for the 363μm nozzle case. This distinctive difference in the result at 58.5 kg/m3 is likely due to the relatively longer ignition delay time in the 363μm nozzle case. Furthermore, the longer ignition delay time as Dnoz increases can be related to the higher local scalar dissipation rate in the large nozzle case.

Experimental investigation of auto-ignition of ethylene-nitrous oxide propellants in rapid compression machine

Considering the available experimental investigations of auto-ignition behaviors of N2O-C2 hydrocarbons propellants are limited in high-temperature regime, in the work the ignition delay times for N2O-C2H4 propellants have been measured under the conditions of TC = 885–940 K, PC = 2.5–4.3 MPa, φ = 1.05 and 1.35 using the rapid compression machine experiments. Then the relational expression between ignition delay time and condition factors is derived as lgτig=11.781000/Tc-0.26Pc-0.83φ-9.41 by linear fitting. The expression can be used to predict ignition delay times of N2O-C2H4 propellants in practice. Moreover, the calculated ignition delay times by three kinetic models are compared with experimental data. The results show that these models can well predict ignition behaviors at high temperature regime, but fail at low-temperature regime. Through temperature sensitivity analysis, at high-temperature regime the N2O decomposition reaction has the highest sensitivity coefficient and dominated the overall reaction. While at low-temperature regime, the oxidation reactions of N2O and hydrocarbons become more important steps. For better prediction of low-temperature ignition behaviors, the low-temperature oxidation mechanism of N2O and hydrocarbons needs further development, and the kinetic parameters of the highly sensitive reactions should be improved.

Investigation of Chemical Kinetic Model for Hypergolic Propellant of Monomethylhydrazine and Nitrogen Tetroxide

Abstract Hypergolic bipropellant of monomethylhydrazine (MMH) and nitrogen tetroxide (NTO) is extensively used in spacecraft propulsion applications and rocket engines. But studies on the chemical kinetic mechanism of MMH/NTO are limited. So, in this study by integrating the submechanisms of MMH decomposition, NTO thermal decomposition, MMH/NTO and intermediates, and small hydrocarbons, the comprehensive chemical mechanism of MMH/NTO bipropellant is proposed. The present chemical mechanism consists of 72 species and 406 elementary reactions. In two respects of ignition delay times and combustion flame temperatures, the present model has been validated against the theoretical calculation results and also compared with other kinetic models in the literature. The validations show that the predicted ignition delay times by the present kinetic model are highly consistent with the theoretical data and well describe the pressure-dependent characteristic. For combustion flame temperature, the present model also exhibits better predictions to the theoretical calculation results, which are also the same as the predictions by the MMH-RFNA model. Furthermore, the influences of initial temperature, chamber pressure, and NTO/HHM mass ratio (O/F) on the ignition delay time and combustion flame temperature are investigated. The auto-ignition behavior of MMH/NTO propellant is sensitive to initial temperature and chamber pressure, and the combustion flame temperature is more sensitive to the O/F. This study provides a detail chemical kinetics model for further mechanism simplification and combustion numerical simulation.

An extended methodology for automated calculations of non-Boltzmann kinetic sequences: H + C2H2 + X and combustion impact

It is generally assumed in phenomenological kinetic models that bimolecular reactions only occur between species whose rovibrational energy follows a Boltzmann (thermal) distribution. That is, any complexes initially formed in non-Boltzmann distributions are assumed to be thermalized by energy-transferring collisions prior to bimolecular reactions. Given the high mole fractions of reactive species, X, in combustion environments, reactive collisions of the complexes with X often occur on the same timescale as energy-transferring collisions – yielding sequences proceeding through non-Boltzmann intermediates across multiple potential energy surfaces. Recent studies have shown that such non-Boltzmann kinetic sequences can have substantial impact on the global reactivity in combustion systems. Simulations of these non-Boltzmann reaction sequences, which can be described in phenomenological kinetic models via chemically termolecular reactions, require that rovibrational excitation from one potential energy surface be carried over to the next. This paper presents an extended theoretical and computational methodology that couples multiple master equations and derives rate constants for phenomenological reactions describing the conversion of thermal reactants to thermal products for use in phenomenological kinetic schemes. The methodology is then implemented using in-house scripts for non-Boltzmann sequences involving C2H3* + X (with X = O2, H, and OH) where C2H3* is formed via H + C2H2 association – which were identified as having strong potential for influencing combustion predictions in a recent study. The results reveal that non-Boltzmann reaction sequences for X = O2 (the primary focus of this paper) significantly alters the total conversion rate from H + C2H2 to products and product branching fractions from those of thermal sequential pathways. Furthermore, the present results demonstrate that non-Boltzmann reaction sequences have significant impact – as high as an order of magnitude – on predicted ignition delay times. Similarly, they yield significantly different dependence of ignition delay times with temperature and O2 mole fraction – yielding signatures that are likely observable experimentally.

Detailed simulations of the DLR auto-igniting pulsed jet experiment

Numerical simulations of an auto-igniting pulsed jet in a vitiated co-flow experiment by DLR (German Aerospace Center) are conducted by highly-resolved large-eddy simulations using direct chemistry with an augmented reduced mechanism. The experiments consist of two operation modes: continuous injection used for code-verification and pulsed injection utilized for fundamental investigation of auto-ignition dynamics. Initially, reference one-dimensional self-igniting counter-flow flames are investigated. Then, a grid convergence study has been performed. It is shown that even a coarser grid would be sufficient to describe the ignition chemistry since the ignition kernel appears at low velocities and fuel-lean conditions in zones of low scalar dissipation rates. For the statistically steady jet, numerical predictions are in a very good agreement with the experiments, giving confidence in the applied models. For the pulsed jet, all of the predicted ignition delay times and locations are in the range of the experimental observations. Time-resolved statistics reveal that thermochemical properties of the gas in a pulsed jet achieve states that are impossible to reproduce in laminar conditions. For further analysis, hydroxyl and formaldehyde are chosen as a marker for the established flame and for the ignition, respectively. In laminar conditions, these two species are perfectly correlated. However, the unsteady dynamics of the pulsed jet invalidates the correlation between the minor species chemistry prior to ignition. This yields the discrepancy in the auto-ignition delay time and the location of the ignition kernel between different pulses, as the thermochemical state needed for the ignition occurs in a random manner.

The impact of the third O2 addition reaction network on ignition delay times of neo-pentane

We studied the oxidation of neo-pentane by combining experiments, theoretical calculations, and mechanistic developments to elucidate the impact of the 3rd O2 addition reaction network on ignition delay time predictions. The experiments are based on photoionization mass spectrometry in jet-stirred and time-resolved flow reactors allowing for sensitive detection of the keto-hydroperoxide (KHP) and keto-dihydroperoxide (KDHP) intermediates. With neo-pentane exhibiting a unique symmetric molecular structure, which consequently results only in single KHP and KDHP isomers, theoretical calculations of ionization and fragment appearance energies and of absolute photoionization cross sections enabled the unambiguous identification and quantification of the KHP intermediate. Its temperature and time-resolved profiles together with calculated and experimentally observed KHP-to-KDHP signal ratios were compared to simulation results based on a newly developed mechanism that describes the 3rd O2 addition reaction network. A satisfactory agreement has been observed between the experimental data points and the simulation results, thus adding confidence to the model's overall performance. Finally, this mechanism was used to predict ignition delay times reported previously in shock tube and rapid compression machine experiments (J. Bugler et al., Combust. Flame 163 (2016) 138–156). While the model accurately reproduces the experimental data, simulations with and without the 3rd O2 addition reaction network included reveal only a negligible effect on the predicted ignition delay times at 10 and 20 atm. According to model calculations, low temperatures and high pressures promote the importance of the 3rd O2 addition reactions.

Reduced kinetic model for the combustion of n-cetane and heptamethylnonane based on a primary reference fuel reduced kinetic model

A reduced kinetic model for the combustion of n-heptane, i-octane, n-cetane and heptamethylnonane was developed based on a prior model designed for use with a primary reference fuel consisting of n-heptane and i-octane. The present model, which can be easily employed in conjunction with a conventional computational fluid dynamics code, contains 59 chemical species and 96 reactions. Predicted ignition delay times under high pressure and temperature conditions were generated using this new kinetic model and compared with those obtained from full kinetic models. The results indicate that the general trends exhibited by the ignition delay times as temperature and pressure are varied are accurately predicted with this reduced model. The present model was also combined with a commercial computational fluid dynamics code and used to simulate the ignition of a diesel spray at high pressure and temperature. Finally, the effects of the cetane number of the fuel on the ignition process were investigated.

Development of a highly compact and robust chemical reaction mechanism for the oxidation of tetrahydrofurans under engine relevant conditions

This work presents a compact and robust chemical reaction mechanism for modeling the combustion of saturated furans including tetrahydrofuran, 2-methyltetrahydrofuran and 2-buthyltetrahydrofuran under engine relevant conditions. A decoupling method is adopted to construct the mechanism. The oxidation reaction for the small species is described by a mature and detailed H2/CO/C1 sub-mechanism, based on which the skeletal sub-mechanisms for tetrahydrofuran, 2-methyltetrahydrofuran and 2-buthyltetrahydrofuran are incorporated via a compact yet robust C2-C3 sub-mechanism. The sub-mechanisms for the three tetrahydrofuranic fuel components are selected from the detailed chemical mechanisms in the literature via a series of species rate of production analysis, sensitivity analysis, isomer lumping and reaction lumping. The Arrhenius pre-exponential A factors for these selected reaction pathways are then optimized via a single objective genetic algorithm. The resulting mechanism is rather compact consisting of only 56 species among 183 reactions. The performance of the developed mechanism for predicting the combustion chemistry for the three fuel components has been evaluated against the experimental measurements in the literature. Reasonable agreement between the predicted ignition delay times, speciation profiles and laminar flame speeds with the experimental data is achieved for all the cases considered, indicating the high accuracy and robustness of the developed mechanism.

A multi-component reaction mechanism of n-butanol, n-octanol, and di-n-buthylether for engine combustion

The purpose of this study is to develop a multi-component skeletal mechanism including renewable fuels of n-butanol, n-octanol, and DnBE (di-n-buthylether) for engine combustion. Mechanism reductions using the directed relation graph, directed relation graph with error propagation and sensitivity analysis, peak concentration analysis, and isomer lumping methods were first carried out on the detailed reaction mechanisms for each type of the fuels. Then, the obtained single-fuel skeletal mechanisms were combined to construct the multi-component skeletal mechanism. Finally, the developed reaction mechanism, which consisting of 117 species and 610 elementary reactions, was obtained. To validate the fuel model, calculations on ignition delay times for each component of the skeletal mechanism, species concentrations in JSR for n-octanol, laminar flame speeds for DnBE, and 3-D diesel engine combustion fueled with n-octanol and DnBE were carried out. Results demonstrated that the predicted ignition delay times can well match the data given by detailed mechanisms and experiments under the conditions covering a wide range of temperatures, pressures, and equivalence ratios. Also, the concentration profiles of important species in a jet stirred reactor were well reproduced by the skeletal mechanism with equivalence ratios of 0.5, 1.0, and 2.0. The laminar flame speeds of DnBE were also in good agreement with experimental data at 1 atm with a wide range of equivalence ratios. For diesel engine simulations fueled with n-octanol and DnBE, with different engine speeds and loads, both in-cylinder pressure traces and heat release rate profiles can be well represented by the present skeletal reaction mechanism.

Pathway analysis of skeletal kinetic mechanisms for small alcohol fuels at engine conditions

Seven skeletal reaction mechanisms are presented to highlight key chemistry for mechanism reduction of small alcohol fuels at engine conditions. The mechanisms predict ignition delay times, laminar burning velocity, and extinction strain rate, within 5% of their corresponding complex mechanisms used as a start point for reduction. The mechanisms were developed for equivalence ratio range of 0.7–1.4, pressure range of 1–40 atm, and low to high temperature ignition (700–1700 K), using air mixed with methanol, ethanol, or n-propanol The study points out the importance of tailoring the reduced mechanism to the target computational fluid dynamic simulation case. Moreover, important reaction pathways when constructing LES suitable mechanisms are highlighted, and a further need to study the reduced low temperature ignition chemistry in order to meet reasonable sizes for computational fluid dynamic simulations are identified.

A reduced chemical kinetic reaction mechanism for kerosene-air combustion

Development of a new reduced chemical kinetic reaction mechanism for kerosene-air combustion is presented. The new mechanism uses a modular based development technique and is a further development on previously presented kerosene-air mechanisms. The new mechanism consists of 30 species and 77 irreversible reactions and is developed to accurate reproduce key flame parameters yet being small enough to be used in finite rate Large Eddy Simulations (LES), Direct Numerical Simulations (DNS) and in Reynolds Average Navier-Stokes (RANS) simulations. The well-proven development technique uses a refined fuel breakdown oxidation sub-mechanism, a simplified C2 intermediate species sub-mechanism and a more detailed set of reactions for the H/C1/O chemistry. The mechanism has been modified to be able to predict ignition delay times for a wide range of temperatures, including in the negative temperature regime.The mechanism has been evaluated for combustion parameters related to flame propagation and ignition over a wide range of equivalence ratios, initial gas temperatures and pressures. Agreements to experimental data and a set of detailed and skeletal mechanisms are good for all target parameters. The proposed mechanism shows good agreement at a computational cost far below all tested reference mechanisms, making it highly suitable for use in combustion computational fluid dynamic (CFD) simulations.

On the Prediction of Standard Enthalpy of Formation of C2-C4 Oxygenated Species.

In this study, to determine an efficient and accurate method for predicting standard enthalpy of formation (ΔfHo) of oxygenated species, we calculated ΔfHo for several typical C2-C4 oxygenated species using atomization and isodesmic reactions in combination with various quantum chemical methods, including six density functional theory methods, three compound methods, and CCSD(T)/CBS. Compared with experimental values, at the same quantum chemical level, ΔfHo values predicted by using isodesmic reactions are more accurate than those using atomization reactions. Comparing various quantum chemical methods when isodesmic reactions are used, the performance of G4 is the best with a mean unsigned deviation (MUE) of 0.3 kcal/mol and a standard deviation (SD) of 0.3 kcal/mol, while M06-2X can predict ΔfHo efficiently and accurately with an MUE of 0.6 kcal/mol and SD of 0.5 kcal/mol. Using the best methods we have found, we calculated the enthalpies of formation and other thermodynamic properties for dimethyl carbonate (DMC) and its associated species and then applied them in a DMC combustion model for predicting ignition delay times. Better agreement with the experiments is achieved when the newly computed thermodynamic properties are adopted.

Effects of organic silicon compounds on syngas auto-ignition behavior

Siloxanes are a significant impurity in syngas feedstocks and an important source of silicon in combustion synthesis applications. However, little is known regarding the fundamental combustion chemistry of siloxane compounds. The impact of two organic silicon species with different but related chemical structures, trimethylsilanol (TMSO) and hexamethyldisiloxane (HMDSO), on syngas auto-ignition behavior was investigated in this study using physical and computational experiments. A rapid compression facility (RCF) was used to create temperatures of 1010–1070 K and pressures of 8–10.3 atm for auto-ignition experiments. Experiments with trace concentrations of TMSO (100 ppm, mole basis) or HMDSO (100 ppm) were added to a surrogate syngas blend (CO and H2 with a molar ratio of 2.34:1, air levels of dilution, with molar equivalence ratios of ϕ = 0.1). The measured ignition delay times showed both siloxane species promote ignition behavior with TMSO yielding faster ignition delay times by approximately 37% and HMDSO yielding faster times by approximately 50% compared with the reference syngas mixture which contained no siloxanes. A computational study was conducted to interpret the results of the ignition experiments. Because detailed chemistry does not exist for these organo-silicon compounds, the effects of the addition of CH3, H, and OH radicals to H2 and CO mixtures were explored to simulate the potential rapid decomposition of the siloxanes in the H2 and CO system. Addition of the radicals decreased the predicted ignition delay times when compared with the H2 and CO mixture simulations without the radical species, but the simulations did not fully capture the behavior observed in the experiments, indicating the siloxane chemistry is more complex than providing a rapid source of radicals.