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Ethylene plays a crucial role as an intermediate component in the cracking and combustion processes of large molecular alkane and olefins. In this article, the laminar flame speed of ethylene–air mixtures was measured using the heat flux method. The mechanism of ethylene was simplified by utilizing the error propagation directed relationship graph (DRGEP) and sensitivity analysis (SA), and the Arrhenius pre‐exponential factors for 20 selected reactions in the skeletal mechanism were optimized using the particle swarm optimization (PSO) algorithm. Finally, an ethylene optimization mechanism including 39 species and 85 reactions was obtained. The prediction results for flame speed, ignition delay time, and species concentration were compared with experimental data and other mechanisms, covering a wide range of temperatures (298–1725 K), pressures (1–22.8 atm), and equivalence ratios (0.5–2.0). The findings demonstrate that the optimization mechanism not only improves the prediction results of laminar flame speed in the rich combustion zone and low oxygen environment but also enhances the prediction accuracy of the ignition delay time at high pressure and in the lean combustion zone, as well as the prediction accuracy of C2H4 and H2O radicals. In conclusion, the optimized mechanism exhibits higher accuracy and broader applicability. [ABSTRACT FROM AUTHOR]

In this contribution, dedicated to the memory of Prof. Gian Gualberto Volpi, we provide a short review of recent work carried out in our laboratory on reactive scattering studies of the reaction dynamics of atomic oxygen with nitrogen-bearing organic molecules. Specifically, we focus on the polyatomic bimolecular reactions of atomic oxygen, both in the ground and first excited state, O(3P) and O(1D), with the simplest unsaturated nitriles, namely HCCCN (cyanoacetylene) and CH2CHCN (cyanoethylene, or acrylonitrile), and with the simplest six-member ring N-heterocyclic compound, pyridine (C5H5N). Using the crossed molecular beam (CMB) scattering technique with universal electron-impact ionization mass-spectrometric detection and time-of-flight analysis to measure product angular and velocity distributions, the primary product channels and their branching fractions were determined, thus assessing the central role played by intersystem-crossing (ISC) in this class of reactions. The experimental work was synergistically accompanied by theoretical calculations of the relevant triplet and singlet potential energy surfaces (PESs) to assist the interpretation of experimental results and elucidate the reaction mechanism, including extent of ISC. Cyanoacetylene and cyanoethylene are of considerable interest in astrochemistry being ubiquitous (and relatively abundant) in space including comets and the upper atmosphere of Titan. Being oxygen the third most abundant element in space, the title reactions are of considerable relevance in the chemistry of extraterrestrial environments. In addition, they are also important in combustion chemistry, because thermal decomposition of pyrrolic and pyridinic structures present in bound N-containing fuels generates N-bearing compounds including, in particular, the above two nitriles. [ABSTRACT FROM AUTHOR]

Stereochemical effects significantly influence chemical processes, yet it is not well understood if they are a leading source of uncertainty in combustion modeling. Stereochemistry influences a combustion model (i) at the earliest stage of its construction when mapping the reaction network, (ii) in the computation of individual thermochemical and rate parameters, and (iii) in the prediction of combustion observables. The present work reviews the importance of enumerating stereochemical species and reactions at each of these steps. Further, it analyzes the separate influence of several types of stereochemistry, including geometric, optical, and fleeting transition state diastereomers. Three reaction networks serve to examine which stages of low-temperature oxidation are most affected by stereochemistry, including the first and second oxidation of n-butane, the third oxidation of n-pentane, and the early stages of pyrolysis of 1- and 2-pentene. The 149 reactions in the n-butane mechanism are expanded to 183 reactions when accounting for diastereomerism. Each of these 183 reactions is parameterized with ab initio kinetics computations to determine that, for the n-butane mechanism, the median factor of diastereomeric deviation is 3.5 at 360 K for rate constants and as high as 1.6 for mechanism reactivity, in terms of ignition delay times, as opposed to a mechanism without stereochemical expansion. This article is categorized under: Theoretical and Physical Chemistry > Reaction Dynamics and Kinetics Theoretical and Physical Chemistry > Thermochemistry Software > Simulation Methods [ABSTRACT FROM AUTHOR]

Unsaturated hydrocarbons are major components of transportation fuels, combustion intermediates, and unburnt exhaust emissions. Conversely, NOx species are minor species present in the residual and exhaust gases of gasoline-fueled engines and gas turbines. Their co-existence in transportation engines is quite common, particularly with exhaust gas recirculation, which can greatly influence engine combustion characteristics. Therefore, this paper presents a review on the combustion chemistry of unsaturated hydrocarbons and NOx mixtures, with a focus on their chemical kinetic interactions. First, a comprehensive overview of fundamental combustion experiments is provided, covering mixtures of C2–C5 unsaturated/oxygenated species (namely alkenes, alkynes, dienes, alcohols, ethers, ketones, and furans) and three major NOx species (namely NO, NO2, and N2O), as well as reactors including jet-stirred reactors, flow reactors, burners, shock tubes, and rapid compression machines. Then, two widely adopted nitrogen chemistry models are evaluated in conjunction with a core chemistry model (i.e., NUIGMech1.1) via detailed chemical kinetic modeling, and the model similarities and differences across broad temperature ranges are highlighted. Thereafter, the unique interconversions between the three major NOx species are presented. In particular, the controversy regarding the pathways governing NO and NO2 conversion is discussed. Following this, the key direct interaction reactions between unsaturated species and NOx species are overviewed. Finally, the distinguishing features of the combustion chemistry for unsaturated hydrocarbon and NOx mixtures are summarized, and recommendations for future research on this topic are highlighted. [ABSTRACT FROM AUTHOR]

The ethylene inverse diffusion sooting flame chemistry was studied for the first time experimentally and numerically in detail. Effects of oxygen index on flame major species and important small intermediate species formation were analyzed. Inverse diffusion flames were detected by online gas chromatograph to quantitatively measure the mole fraction distributions of major and important intermediate species. With the increase of oxygen index, More C 2 H 4 is consumed and most of intermediate species are promoted. However, the formation of CO 2 on the centerline, C 2 H 6 in the radial direction, and C 3 H 6 in both directions are inhibited. The modified inverse diffusion flame model can successfully reproduce the main variation trend of species mole fraction using the KM2 mechanism. The predicted results by the inverse diffusion flame simulation with the KM2 mechanism were used for further discussion of the combustion chemistry in detail. Two-dimensional predictions indicate that the O 2 oxidant is completely depleted in the reaction zone, and peak mole fractions of most species appear on the fuel side of the flame front or the centerline of the flame. According to the analysis of the reaction path for species formation under different oxygen indexes, A1 formation process is mostly dominated by the even carbon atom path, and the odd carbon atom path only contributes to the formation of A1 at a lower height. [ABSTRACT FROM AUTHOR]

Carrying out in-depth research on the formation characteristics and mechanisms of organic products in coal combustion will help to provide theoretical guidance for the emission reduction of coal-fired pollutants. Since the combustion of coal volatiles plays a vital role in the whole process of coal combustion, this work took coal pyrolysis gases as fuels and focused on the organic pollutants such as alkanes, olefins and aromatics in the combustion of coal pyrolysis gases. To explore the effects of three pyrolysis temperatures of 600 °C, 750 °C and 900 °C on the formation characteristics and mechanisms of hydrocarbon products during coal pyrolysis gases combustion, the online detection was carried out by gas chromatography (GC) and gas chromatography-mass spectrometer (GC-MS) on the counterflow diffusion flame platform, combined with chemical reaction kinetics simulations. The results showed that rising pyrolysis temperature had an inhibitory effect on the formation of light hydrocarbons and aromatic hydrocarbons, such as acetylene (C 2 H 2), propyne (PC 3 H 4), 1,3-butadiene (C 4 H 6 -13), vinylacetylene (C 4 H 4), diacetylene (C 4 H 2), benzene (C 6 H 6), toluene (C 6 H 5 CH 3) and naphthalene (C 10 H 8), during the combustion of pyrolysis gases. Among three mechanisms used in this study, Creck mechanism was more accurate and applicable than KM2 mechanism and AramcoMech 3.0 mechanism. As the pyrolysis temperature rose, the rate of the reactions which dominated the formation of C 2 H 2 , propargyl (C 3 H 3), C 6 H 6 and C 10 H 8 decreased continuously, because the decrease of CH 3 concentration and the increase of H concentration inhibited the forward progress of these reactions. In addition, with rising pyrolysis temperature, less C 3 H 3 was converted to butadiene (C 4 H 6), C 6 H 6 and Fulvene, and more inclined to its upstream reactants, allene (AC 3 H 4) and PC 3 H 4. The tendency of phenyl (C 6 H 5) to form C 6 H 6 was strengthened. Meanwhile, less C 10 H 8 was generated from C 4 H 4 and C 6 H 5 , but the dominant role of this reaction was still enhanced. •Effects of pyrolysis temperature on coal volatiles flame chemistry was revealed. • Light and aromatic hydrocarbons in coal pyrolysis gas flames were analyzed. • Kinetic analysis was performed to explore pollutants formation mechanisms. • Results can serve for future coal volatiles combustion mechanisms optimization. [ABSTRACT FROM AUTHOR]

Development and validation of a new reduced dimethyl ether-air (DME) reaction mechanism is presented. The mechanism was developed using a modular approach that has previously been applied to several alkane and alkene fuels, and the present work pioneers the use of the modular methodology, with its underlying H/C1/O base mechanism, on an oxygenated fuel. The development methodology uses a well-characterized H/C1/O base mechanism coupled to a reduced set of fuel and intermediate product submechanisms. The mechanism for DME presented in this work includes 30 species and 69 irreversible reactions. When used in combustion simulation the mechanism accurately reproduced key combustion characteristics and the small size enables use in computationally demanding Large Eddy Simulations (LES) and Direct Numerical Simulations (DNS). It has been developed to accurately predict, among other parameters, laminar burning velocity and ignition delay times, including the negative temperature regime. The evaluation of the mechanism and comparison to experimental data and several detailed and reduced mechanisms covers a wide range of conditions with respect to temperature, pressure and fuel-to-air ratio. There is good agreement with experimental data and the detailed reference mechanisms at all investigated conditions. The mechanism uses fewer reactions than any previously presented DME-air mechanism, without losing in predictability.

[Display omitted] • An axial local dilution (ALD) inverse diffusion flame (IDF) burner is manufactured. • The model structure in the CoFlame code is modified based on the ALD-IDF burner. • The concentrations of radial species are quantitatively measured. • The impact of various injection rates and locations on flame structure is studied. • The inhibitory effects of CO 2 on key species vary with injection rate and height. To investigate the combustion chemistry of methane inverse diffusion sooting flames with local CO 2 injection, this study designs an axial local dilution (ALD) inverse diffusion flame (IDF) burner capable of precisely controlling the diluent injection rate and location along the flame centerline using a diluent injection pipe. Then the flame generated by the ALD-IDF burner is simulated by a modified model based on the CoFlame code. The distributions of flame temperature and major species mole fractions are measured using a thermocouple and online gas chromatography, and the measurements are compared with numerical results. The impact of different injection flow rates and locations on the flame structure is studied. Results show that injecting CO 2 close to the high-temperature region significantly affects the oxidizer flow field. The predictions of the main trends of species mole fractions show good agreement with experimental results. The oxidizer stream structure variety caused by CO 2 injection is the main reason for affecting the spatial distribution of species in the flame. The local CO 2 injection inhibits the formation of some small molecules and radicals (H 2 , H, OH) as well as intermediate species (C 2 H 2 , C 2 H 4 , C 4 H 2 , A1) close to the flame centerline region. In addition, the inhibitory effect of CO 2 on the aforementioned species increases with higher injection flow rates and heights. However, injecting CO 2 at different heights results in moderate differences in the inhibition of A1 formation. Kinetic analysis of the A1 formation pathways indicates that this phenomenon is due to the varying degrees of suppression on the main A1 formation reaction (2C 3 H 3 = A1) at different injection heights. This study presents a novel method for adding diluents within the flame and analyzes the underlying regulatory mechanisms. The obtained insights are expected to expand the existing pollutant control technologies. [ABSTRACT FROM AUTHOR]

The diversity of reactivities, intermediates, and pathways associated with the low-temperature (low-T) oxidation of various component classes that constitute real fuels is perhaps the most challenging aspect of modeling their combustion chemistry. Unlike high-temperature oxidation (T > 1100 K), where the combustion properties of multicomponent fuels are relatively insensitive to compositional variations, reactions governing low-T oxidation exhibit pronounced sensitivity to fuel composition. Despite the fuel specificity, intermediate formation during low-T oxidation exhibits characteristic behaviors. Combining such observations and the already mature Hy brid Chem istry (HyChem) methodology for high-temperature oxidation of real fuels [ 1 ], we propose a framework to develop simplified, physics-based chemical kinetic models for low-T oxidation of real fuels. The proposed model captures the complexity of low-T oxidation through concise, fuel-specific reactions whose stoichiometric parameters and rate constants are experimentally constrained. Shock tube experiments needed for constraining model parameters are identified and plausible validation targets are discussed. The present paper outlines the model's description, its underlying physical principles, and an initial application to a simple, multi-component mixture, TPRF-60. Detailed uncertainty analyses and application to three real fuels will be presented in a companion paper. [ABSTRACT FROM AUTHOR]

Unsaturated hydrocarbons are major components of transportation fuels, combustion intermediates, and unburnt exhaust emissions. Conversely, NOx species are minor species present in the residual and exhaust gases of gasoline-fueled engines and gas turbines. Their co-existence in transportation engines is quite common, particularly with exhaust gas recirculation, which can greatly influence engine combustion characteristics. Therefore, this paper presents a review on the combustion chemistry of unsaturated hydrocarbons and NOx mixtures, with a focus on their chemical kinetic interactions. First, a comprehensive overview of fundamental combustion experiments is provided, covering mixtures of C2–C5 unsaturated/oxygenated species (namely alkenes, alkynes, dienes, alcohols, ethers, ketones, and furans) and three major NOx species (namely NO, NO2, and N2O), as well as reactors including jet-stirred reactors, flow reactors, burners, shock tubes, and rapid compression machines. Then, two widely adopted nitrogen chemistry models are evaluated in conjunction with a core chemistry model (i.e., NUIGMech1.1) via detailed chemical kinetic modeling, and the model similarities and differences across broad temperature ranges are highlighted. Thereafter, the unique interconversions between the three major NOx species are presented. In particular, the controversy regarding the pathways governing NO and NO2 conversion is discussed. Following this, the key direct interaction reactions between unsaturated species and NOx species are overviewed. Finally, the distinguishing features of the combustion chemistry for unsaturated hydrocarbon and NOx mixtures are summarized, and recommendations for future research on this topic are highlighted.

The study of fuel chemistry and soot inception in non-premixed combustion can be advanced by characterizing flame configurations in which the advection and diffusion transport can be finely controlled, with the ability to decouple pyrolysis from oxidation. Also, the ideal flames to be investigated should be perturbed minimally by probes and thick enough for sampling techniques to yield spatially resolved measurements of their structure. The Planar Mixing Layer Flame (PMLF) configuration introduced herein is established between a fuel and an oxidizer slot jet adjacent to each other and shielded from the ambient air by annularly co-flowing inert nitrogen. The PMLF flow is kept laminar and steady by an impinging flat plate equipped with a rectangular exhaust slit opening which anchors the position of the hot combustion products via buoyancy. The PMLF is accessible to sampling and its flow stability is preserved when using any tested probe. The experiments are complemented with 2D- Computational Fluid Dynamics (CFD) modeling with detailed chemical kinetics. The results demonstrate that the PMLF has a self-similar boundary layer structure whose horizontal cross-sections are equivalent to properly selected and equally thick 1D- Counterflow Flames (CFs). The equivalence allows for excellent predictions of the PMLF thermochemical structure characterized experimentally but at a small fraction of the 2D-CFD computational cost. The 1D-CF equivalence affects even aromatics less than twofold despite their kinetics being known to be very sensitive to the temperature field. Importantly, the PMLF thickness is several millimeters and grows at increasing HABs so that the equivalent 1D-CFs have strain rates as small as 7.0 /s which cannot be studied in CF experiments. As a result, the PMLF emerges as a promising canonical non-premixed flame configuration for studying flame chemistry and soot inception on time scales of tens of milliseconds typical of many combustion applications. [ABSTRACT FROM AUTHOR]

The extinction limits of non-premixed counterflow flames of methane (CH 4)/ammonia (NH 3)/nitrogen (N 2) versus high-temperature air (T Air = 700 K and 1000 K) were investigated both experimentally and numerically. Extinction stretch rates of non-premixed counterflow flames of CH 4 /NH 3 mixtures decreased greatly as the ammonia mixing ratio increased. Recent chemical kinetic models could well predict measured extinction limits of non-premixed counterflow flames of CH 4 /NH 3 mixtures, especially for T Air = 1000 K. Chemical kinetic analyses indicated that the nature of NH 3 consuming active radicals but not regenerating them through its oxidation is the primary reason for the drastic decreases in extinction stretch rates of non-premixed counterflow flames of CH 4 /NH 3 mixtures. Furthermore, the combined metric of the transport weighted enthalpy (TWE) and the radical index (RI) is introduced for non-premixed counterflow flames of CH 4 /NH 3 mixtures. The OH-radical index (RI OH), previously used in the combined metric for extinction limits of non-premixed counterflow flames of large hydrocarbons, expresses linear relationships with extinction limits for both T Air conditions. According to further investigations on heat releases from individual reactions, the contribution to heat releases from a reaction involving O radicals, i.e., CH 3 + O ⇄ CH 2 O + H, becomes large in addition to reactions involving OH radicals, i.e., CO + OH ⇄ CO 2 + H and H 2 + OH ⇄ H + H 2 O. Based on the analyses, a new radical index based on the combination of OH and O radicals (RI OH&O) is proposed. The RI OH&O better expresses the linear relationship between extinction limits and the combined metric of RI OH&O and TWE. [ABSTRACT FROM AUTHOR]

We provide an overview of 70 years of evaluated chemical kinetic data published in the Journal of Physical and Chemical Reference Data (dating to 1972), the National Standard Reference Data System series (dating to 1965), as part of the National Bureau of Standards Chemical Kinetics Data Center (dating to 1951), and the National Institute of Standards and Technology Chemical Kinetics Database (SRD 17) (dating to 1990). [ABSTRACT FROM AUTHOR]

As an alternative fuel, dimethyl ether (DME) has attracted much attention due to its high efficiency and low emission. In this study, first, a skeletal mechanism of DME is obtained using a directed relation graph with error propagation (DRGEP) and sensitivity analysis (SA). Then the Arrhenius preexponential factors of the selected reactions in the skeletal mechanism are adjusted by the particle swarm optimization (PSO) algorithm to improve the prediction accuracy. Finally, an optimized mechanism with 30 species and 65 elementary reactions is proposed to describe the combustion of DME–air mixtures. This reaction mechanism is validated against the experimental data of ignition delay time, species concentration, and laminar flame speed, covering a wide range of initial temperatures (298–1600 K), pressures (1.0–60 bar), and equivalence ratios (0.5–2.0). Comparing the previous skeletal mechanisms, the optimized mechanism can excellently reproduce the experiment results, especially in the prediction of the ignition delay time of low‐temperature zone and laminar flame speed of high‐pressure conditions. It means that this mechanism corresponds to higher precision and more extensive applicability. [ABSTRACT FROM AUTHOR]

The present work focused on the combustion chemistry in sooting transition process in iso -octane counterflow diffusion flames. A series of iso -octane flames ranging from sooting-free to heavy-sooting were investigated by stepwise increasing the oxygen concentration (X O) in the oxidizer stream. In particular an optical approach with two different algorithms based on the three wavelengths distribution of flame images captured by digital single lens reflex (DSLR) camera was proposed to identify the critical condition for the sooting transition process. The results showed that the sharp increase in signal ratio of red luminosity with simultaneously obvious decrease in signal rations of blue luminosity could be used as an effective indicator of sooting onset in flames. On-line gas chromatography (GC) measurements were used to investigate the flame chemistry and determine quantitative mole fraction profiles of stable and reactive species formed in the combustion process. Particular attention was given to the combustion chemistry changes caused by the occurrence of sooting transition. Numerical kinetic simulations were performed along with the comparisons with experimental results to obtain deep insights into the sooting transition process, with focus on benzene chemistry considering the critical role of benzene formation in the subsequent large aromatic species/soot formation process. In most cases, the trend observed in the experiments could be satisfactorily reproduced and explained by the models. For major species and C1–C4 species, it can be seen that the concentration increased with the increase of X O in sooting transition process. For the decomposition of fuel and the formation of benzene, the influence in sooting transition process was mainly reflected in the contributions of different pathways due to temperature effects. The impact of sooting transition on flame chemical structure was mainly reflected in the growth of PAHs. The increasing trend for the PAHs growth caused by the increase of oxygen concentration in the transition process was more and more obvious. • Critical sooting transition condition was identified with a novel optical method. • Combustion chemistry in the sooting transition process was studied. • Kinetic analysis was performed to get insights into sooting transition process. • The growth of PAHs was promoted during the sooting transition process. [ABSTRACT FROM AUTHOR]

Abstract Products formed in the reaction of C2H radicals with 1,3-butadiene at 4 Torr and 298 K are probed using photoionization time-of-flight mass spectrometry. The reaction takes place in a slow-flow reactor, and products are ionized by tunable vacuum-ultraviolet light from the Advanced Light Source. The principal reaction channel involves addition of the radical to one of the unsaturated sites of 1,3-butadiene, followed by H-loss to give isomers of C6H6. The photoionization spectrum of the C6H6 product indicates that fulvene is formed with a branching fraction of (57 ± 30)%. At least one more isomer is formed, which is likely to be one or more of 3,4-dimethylenecyclobut-1-ene, 3-methylene-1-penten-4-yne or 3-methyl-1,2-pentadien-4-yne. An experimental photoionization spectrum of 3,4-dimethylenecyclobut-1-ene and simulated photoionization spectra of 3-methylene-1-penten-4-yne and 3-methyl-1,2-pentadien-4-yne are used to fit the measured data and obtain maximum branching fractions of 74%, 24% and 31%, respectively, for these isomers. An upper limit of 45% is placed on the branching fraction for the sum of benzene and 1,3-hexadien-5-yne. The reactive potential energy surface is also investigated computationally. Minima and first-order saddle-points on several possible reaction pathways to fulvene + H and 3,4-dimethylenecyclobut-1-ene + H products are calculated.

Abstract Products formed in the reaction of C2H radicals with 1,3-butadiene at 4 Torr and 298 K are probed using photoionization time-of-flight mass spectrometry. The reaction takes place in a slow-flow reactor, and products are ionized by tunable vacuum-ultraviolet light from the Advanced Light Source. The principal reaction channel involves addition of the radical to one of the unsaturated sites of 1,3-butadiene, followed by H-loss to give isomers of C6H6. The photoionization spectrum of the C6H6 product indicates that fulvene is formed with a branching fraction of (57 ± 30)%. At least one more isomer is formed, which is likely to be one or more of 3,4-dimethylenecyclobut-1-ene, 3-methylene-1-penten-4-yne or 3-methyl-1,2-pentadien-4-yne. An experimental photoionization spectrum of 3,4-dimethylenecyclobut-1-ene and simulated photoionization spectra of 3-methylene-1-penten-4-yne and 3-methyl-1,2-pentadien-4-yne are used to fit the measured data and obtain maximum branching fractions of 74%, 24% and 31%, respectively, for these isomers. An upper limit of 45% is placed on the branching fraction for the sum of benzene and 1,3-hexadien-5-yne. The reactive potential energy surface is also investigated computationally. Minima and first-order saddle-points on several possible reaction pathways to fulvene + H and 3,4-dimethylenecyclobut-1-ene + H products are calculated.

A strategy based on machine learning is discussed to close the gap between the detailed description of combustion chemistry and the numerical simulation of combustion systems. Indeed, the partial differential equations describing chemical kinetics are stiff and involve many degrees of freedom, making their solving in three-dimensional unsteady simulations very challenging. It is discussed in this work how a reduction of the computing cost by an order of magnitude can be achieved using a set of neural networks trained for solving chemistry. The thermochemical database used for training is composed of time evolutions of stochastic particles carrying chemical species mass fractions and temperature according to a turbulent micro-mixing problem coupled with complex chemistry. The novelty of the work lies in the decomposition of the thermochemical hyperspace into clusters to facilitate the training of neural networks. This decomposition is performed with the Kmeans algorithm, a local principal component analysis is then applied to every cluster. This new methodology for combustion chemistry reduction is tested under conditions representative of a non-premixed syngas oxy-flame.

We report a first attempt to investigate the reaction mechanisms of hydroborate iron compound (HIC), a promising candidate for combustion catalyst, in AP/HTPB/Al composite solid propellants using ammonium perchlorate (AP) as the oxidizer and aluminum as the metal fuel. The effects of HIC on the combustion properties of propellants were experimentally examined to determine burning rates, combustion flame structures and flame temperature distribution. The combustion residues were collected and examined; the mechanical sensitivity of propellant samples were tested. It was found that the addition of HIC particles can increase the burning rates with only minor changes of the pressure exponent. When 5.0 wt% of Al is replaced by HIC, the pressure exponent increases from 0.31 to 0.35 over the pressure range 1–12 MPa while the burning rate increases from 33 mm s−1 to 40 mm s−1 at 7 MPa. The combustion flame images of burning propellants surface loaded with HIC at various pressures present multi-flame structures, and the flame brightness increases with increasing the pressure. Many sparks are observed near the burning surface during the propellant combustion, due to burning aluminum particles in gas phase. With 5.0 wt% HIC included, the propellant produces very different condensed combustion products (CCPs) with a variety of morphologies, and the most typical ones are "porous" shape, large alveolate and loose structures. The relative combustion residue mass decreases with increasing pressure (1–5 MPa) and HIC mass fraction in the formulation (0–5%); While HIC powder is insensitive to friction and impact stimuli (0% at height is higher than 125 cm), the tested propellant formulations containing different mass fraction of HIC particles result sensitive to impact and friction a bit more than the reference formulation without HIC particles. [ABSTRACT FROM AUTHOR]

Direct Numerical Simulation (DNS) data obtained by Dave and Chaudhuri (2020) from a lean, complex-chemistry, hydrogen-air flame associated with the thin-reaction-zone regime of premixed turbulent burning are analyzed (by adapting five alternative definitions of combustion progress variable c) in order to examine three different models that (i) are based on the flamelet paradigm and (ii) aim at evaluating mean concentrations of various species in applied CFD research into turbulent combustion. Mean mole fractions of all considered species and mean density are predicted if the laminar-flame profiles of species mole fractions and density, respectively, are directly averaged using a Probability Density Function (PDF) P (c). The best predictions are obtained by extracting P (c) from the DNS data and defining c based on hydrogen mass fraction. These predictions suggest that mean mole fractions of various species in a premixed turbulent flame can be evaluated at a post-processing stage of a CFD study by adopting P (c), obtained at the major stage of the simulations, to average a flamelet library. When applied in such a way, the flamelet paradigm is useful even for lean hydrogen-air flames and even at Karlovitz number as large as 13. If the same PDF is applied to average reaction rates from the same flamelet library, the mean rates of production/consumption of species n are poorly predicted, e.g. for radicals H, O, OH, HO 2 , and H 2 O 2 if c is defined using hydrogen mass fraction. A hypothesis that conditioned rates 〈 W n | c 〉 can be predicted using conditioned mole fractions 〈 X n | c 〉, temperature 〈 T | c 〉, and density 〈 ρ | c 〉 is not supported either, e.g. for radicals O and OH. These differences between predictive capabilities of the first approach (directly averaging concentration profiles) and two other approaches (averaging reaction rates) are attributed to weakly (highly) non-linear dependencies of the concentrations (rates, respectively) on c. [ABSTRACT FROM AUTHOR]

The present work quantifies the impact of fuel chemistry on burning modes using premixed dimethyl ether (DME), ethanol (EtOH) and methane flames in a back-to-burnt opposed jet configuration. The study considers equivalence ratios 0 ≤ Φ ≤ 1, resulting in a Damköhler (Da) number range 0.06 ≤ Da ≤ 5.1. Multi-scale turbulence (Re ≃ 19,550 and Re t ≃ 360) is generated by means of a cross fractal grid and kept constant along with the enthalpy of the hot combustion products (T HCP = 1700 K) of the counterflow stream. The mean turbulent rate of strain exceeds the laminar extinction rate for all flames. Simultaneous Mie scattering, OH-PLIF and PIV are used to identify reactants, mixing, weakly reacting, strongly reacting and product fluids. The relative balance between conventional flame propagation and auto-ignition based combustion is highlighted using suitably defined Da numbers and a more rapid transition towards self-sustained (e.g. flamelet type) combustion is observed for DME. The strain rate distribution on the reactant fluid surface for methane remains similar to the (non-reactive) mixing layer (Φ = 0), while DME and EtOH flames gradually detach from the stagnation plane with increasing Φ leading to stabilisation in regions with lower compressive rates of strain. The study further provides information on the conditions leading to burning mode transitions via (i) multi-fluid probabilities, (ii) structural flow field information and turbulence-flame interactions delineated by means of conditional (iii) velocity statistics and (iv) the rate of strain along fluid iso-contours. [ABSTRACT FROM AUTHOR]

Combustion involves chemical reactions that are often highly exothermic. Combustion systems utilize the energy of chemical compounds released during this reactive process for transportation, to generate electric power, or to provide heat for various applications. Chemistry and combustion are interlinked in several ways. The outcome of a combustion process in terms of its energy and material balance, regarding the delivery of useful work as well as the generation of harmful emissions, depends sensitively on the molecular nature of the respective fuel. The design of efficient, low-emission combustion processes in compliance with air quality and climate goals suggests a closer inspection of the molecular properties and reactions of conventional, bio-derived, and synthetic fuels. Information about flammability, reaction intensity, and potentially hazardous combustion by-products is important also for safety considerations. Moreover, some of the compounds that serve as fuels can assume important roles in chemical energy storage and conversion. Combustion processes can furthermore be used to synthesize materials with attractive properties. A systematic understanding of the combustion behavior thus demands chemical knowledge. Desirable information includes properties of the thermodynamic states before and after the combustion reactions and relevant details about the dynamic processes that occur during the reactive transformations from the fuel and oxidizer to the products under the given boundary conditions. Combustion systems can be described, tailored, and improved by taking chemical knowledge into account. Combining theory, experiment, model development, simulation, and a systematic analysis of uncertainties enables qualitative or even quantitative predictions for many combustion situations of practical relevance. This article can highlight only a few of the numerous investigations on chemical processes for combustion and combustion-related science and applications, with a main focus on gas-phase reaction systems. It attempts to provide a snapshot of recent progress and a guide to exciting opportunities that drive such research beyond fossil combustion. [ABSTRACT FROM AUTHOR]

We have used laser-photolysis/photoionization mass spectrometry to measure the kinetics of the reaction of 1-methylpropargyl (but-3-yn-2-yl, ▪) radicals with oxygen molecules as a function of temperature (T = 200 − 685 K) and bath gas density (1.2 − 15 × 10 16 cm − 3 ). The low temperature (T ≤ 304 K) kinetics is dominated by oxygen addition to the ▪ carbon of the radical to form a peroxyl radical, and the measured ▪ bimolecular rate coefficient exhibits negative temperature dependence and depends on bath gas density. At slightly higher temperatures (335 − 396 K), where the redissociation rate of the peroxyl is already observable, we measured the ▪ equilibrium constant as a function of temperature. At even higher temperatures (T = 479 − 685 K), the loss rate of 1-methylpropargyl is determined by the addition of oxygen to the terminal ▪ carbon and the reaction is observed to produce methylketene. The high-temperature ▪ bimolecular rate coefficient is independent of bath gas density and the temperature dependence is weakly positive. To explain our experimental findings, we performed quantum chemical calculations together with master equation simulations. By using our experimental data to constrain key parameters, the master equation model was able to reproduce experimental results reasonably well. We then extended the conditions of our simulations up to 2000 K and 100 bar. The results of these simulations are provided in ChemKin compatible PLOG format. [ABSTRACT FROM AUTHOR]

Bioalcohols are a promising family of biofuels. Among them, 1-butanol has a strong potential as a substitute for petrol. In this manuscript, we report on a theoretical and experimental characterization of 1-butanol thermal decomposition, a very important process in the 1-butanol combustion at high temperatures. Advantage has been taken of a flash pyrolysis experimental set-up with mass spectrometric detection, in which the brief residence time of the pyrolyzing mixture inside a short, resistively heated SiC tube allows the identification of the primary products of the decomposing species, limiting secondary processes. Dedicated electronic structure calculations of the relevant potential energy surface have also been performed and RRKM estimates of the rate coefficients and product branching ratios up to 2,000 K are provided. Both electronic structure and RRKM calculations are in line with previous determinations. According to the present study, the H2O elimination channel leading to 1-butene is more important than previously believed. In addition to that, we provide experimental evidence that butanal formation by H2 elimination is not a primary decomposition route. Finally, we have experimental evidence of a small yield of the CH3 elimination channel.

The identification of the primary products and the determination of their branching ratios as a function of translational energy (temperature) for multi-channel elementary (bimolecular) reactions of importance in combustion flames and plasma-assisted combustion still represent a challenge for traditional kinetics experiments. On the other hand, this kind of information is central for the detailed modeling of combustion/plasma systems. In this short review, the significant contribution provided in this area by the crossed molecular beam (CMB) scattering method with "universal" mass-spectrometric detection and time-of-flight analysis is illustrated. In particular, we describe the basics of the CMB technique empowered with "soft" electron-impact ionization as recently implemented in our laboratory, and report on its application to the study of the multi-channel elementary reactions of ground state atomic oxygen, O(3P), with unsaturated hydrocarbons containing two carbon atoms (acetylene and ethylene), three carbon atoms (propyne, propene, and allene), and also four carbon atoms (1-butene, 1,2-butadiene, and 1,3-butadiene), which are of paramount interest in combustion flames and plasma-assisted combustion of hydrocarbons. These studies are usually complemented in a synergistic manner by high-level electronic structure calculations of the underlying potential energy surfaces and related statistical (and dynamical when feasible) calculations of product branching ratios. The complementarity to kinetics studies and the implications of the dynamics results for the modeling of combustion/plasma chemistry will be commented on. [ABSTRACT FROM AUTHOR]

Abstract We have measured the kinetics and thermochemistry of the reaction of 3-methylpropargyl radical (but-2-yn-1-yl) with molecular oxygen over temperature (223–681 K) and bath gas density (1.2 − 15.0 × 10 16 cm − 3 ) ranges employing photoionization mass-spectrometry. At low temperatures (223–304 K), the reaction proceeds overwhelmingly by a simple addition reaction to the − CH 2 end of the radical, and the measured CH 3 CCCH 2 • + O 2 reaction rate coefficient shows negative temperature dependence and depends on bath gas density. At intermediate temperatures (340–395 K), the addition reaction equilibrates and the equilibrium constant was determined at different temperatures. At high temperatures (465–681 K), the kinetics is governed by O 2 addition to the third carbon atom of the radical, and rate coefficient measurements were again possible. The high temperature CH 3 CCCH 2 • + O 2 rate coefficient is much smaller than at low T , shows positive temperature dependence, and is independent of bath gas density. In the intermediate and high temperature ranges, we observe a formation signal for ketene (ethenone). The reaction was further investigated by combining the experimental results with quantum chemical calculations and master equation modeling. By making small adjustments (2 − 3 kJ mol − 1 ) to the energies of two key transition states, the model reproduces the experimental results within uncertainties. The experimentally constrained master equation model was used to simulate the CH 3 CCCH 2 • + O 2 reaction system at temperatures and pressures relevant to combustion. [ABSTRACT FROM AUTHOR]

The formulation of reactive flow problems can be both quite challenging and important to the efficiency and robustness of solution algorithms. In this article, we focus on the choice of the thermochemical state vector as it relates to recently-developed computational techniques for complex combustion chemistry problems. We identify over-specification of the state vector as a source of both ambiguity and error in the partial derivatives used in forming analytical forms of the chemical source Jacobian matrix. We review and compare several approaches taken to increase sparsity of the Jacobian matrix, as it relates to the use of Newton–Krylov methods for implicit time integration, and identify proper techniques for achieving sparsity that do not rely on ad-hoc choice of state variables with inconsistent Jacobians. Chemical explosive mode analysis and linearly-implicit methods, such as Rosenbrock methods, are identified as areas where Jacobian accuracy may be critical. The distinction between how Jacobian exactness impacts Rosenbrock and Newton–Krylov methods is demonstrated with a simple example. We demonstrate the errors in conservation obtained from over-specification of the state vector with auto-ignition calculations for hydrogen, ethylene, and n-heptane chemistry with a high-order implicit Runge–Kutta method. [ABSTRACT FROM AUTHOR]

Double-imaging photoelectron/photoion coincidence (i²PEPICO) spectroscopy using a multiplexing, time-efficient, fixed-photon-energy approach offers important opportunities of gas-phase analysis. Building on successful applications in combustion systems that have demonstrated the discriminative power of this technique, we attempt here to push the limits of its application further to more chemically complex combustion examples. The present investigation is devoted to identifying and potentially quantifying compounds featuring five heavy atoms in laminar, premixed low-pressure flames of hydrocarbon and oxygenated fuels and their mixtures. In these combustion examples from flames of cyclopentene, iso-pentane, iso-pentane blended with dimethyl ether (DME), and diethyl ether (DEE), we focus on the unambiguous assignment and quantitative detection of species with the sum formulae C5H6, C5H7, C5H8, C5H10, and C4H8O in the respective isomer mixtures, attempting to provide answers to specific chemical questions for each of these examples. To analyze the obtained i2PEPICO results from these combustion situations, photoelectron spectra (PES) from pure reference compounds, including several examples previously unavailable in the literature, were recorded with the same experimental setup as used in the flame measurements. In addition, PES of two species where reference spectra have not been obtained, namely 2-methyl-1-butene (C5H10) and the 2-cyclopentenyl radical (C5H7), were calculated on the basis of high-level ab initio calculations and Franck-Condon (FC) simulations. These reference measurements and quantum chemical calculations support the early fuel decomposition scheme in the cyclopentene flame towards 2-cyclopentenyl as the dominant fuel radical as well as the prevalence of branched intermediates in the early fuel destruction reactions in the iso-pentane flame, with only minor influences from DME addition. Furthermore, the presence of ethyl vinyl ether (EVE) in DEE flames that was predicted by a recent DEE combustion mechanism could be confirmed unambiguously. While combustion measurements using i2PEPICO can be readily obtained in isomer-rich situations, we wish to highlight the crucial need for high-quality reference information to assign and evaluate the obtained spectra. [ABSTRACT FROM AUTHOR]

In the past decades, combustion chemistry research grew rapidly due to the development of combustion diagnostic methods, quantum chemistry methods, kinetic theory, and computational techniques. A lot of kinetic models have been developed for fuels from hydrogen to transportation fuel surrogates. Besides, multi-scale research method has been widely adopted to develop comprehensive models, which are expected to cover combustion conditions in real combustion devices. However, critical gaps still remain between the laboratory research and real engine application due to the insufficient research work on high pressure and low temperature combustion chemistry. Besides, there is also a great need of predictive pollutant formation model. Further development of combustion chemistry research depends on a closer interaction of combustion diagnostics, theoretical calculation and kinetic model development. This paper summarizes the recent progress in combustion chemistry research briefly and outlines the challenges and perspectives. [ABSTRACT FROM AUTHOR]

Gasoline fuels are complex mixtures that vary in composition depending on crude oil feedstocks and refining processes. Gasoline combustion in high-speed spark ignition engines is governed by flame propagation, so understanding fuel composition effects on premixed flame chemistry is important. In this study, the combustion chemistry of low-pressure, burner-stabilized, premixed flames of two gasoline fuels was investigated under stoichiometric conditions. Flame speciation was conducted using vacuum-ultraviolet synchrotron photoionization time-of-flight molecular beam mass spectroscopy. Stable end-products, intermediate hydrocarbons, and free radicals were detected and quantified. In addition, several isomeric species in the reaction pool were distinguished and quantified with the help of the highly tunable synchrotron radiation. A comparison between the products of both flames is presented and the major differences are highlighted. Premixed flame numerical simulations were conducted using surrogate fuel kinetic models for each flame. Furthermore, a new approach was developed to elucidate the main discrepancies between experimental measurements and the numerical predictions by comparing quantities of interest. [ABSTRACT FROM AUTHOR]

The chemical modeling of combustion treats the chemical conversion of hundreds of species through thousands of reactions. Recent advances in theoretical methodologies and computational capabilities have transformed theoretical chemical kinetics from a largely empirical to a highly predictive science. As a result, theoretical chemistry is playing an increasingly significant role in the combustion modeling enterprise. The accurate prediction of the temperature and pressure dependence of gas phase reactions requires state-of-the-art implementations of a variety of theoretical methods: ab initio electronic structure theory, transition state theory, classical trajectory simulations, and the master equation. In this work, we illustrate the current state-of-the-art in predicting the kinetics of gas-phase reactions through sample calculations for some prototypical reactions central to combustion chemistry. These studies are used to highlight the success of theory, as well as its remaining challenges, through comparisons with experiments ranging from elementary reaction kinetics studies through to global observations such as flame speed measurements. The illustrations progress from the treatment of relatively simple abstraction and addition reactions, which proceed over a single transition state, through to the complexity of multiwell multichannel reactions that commonly occur in studies of the growth of polycyclic aromatic hydrocarbons. In addition to providing high quality rate prescriptions for combustion modelers, theory will be seen to indicate various shortcomings in the foundations of chemical modeling. Future progress in the fidelity of the chemical modeling of combustion will benefit from more widespread applications of theoretical chemical kinetics and from increasingly intimate couplings of theory, experiment, and modeling. [ABSTRACT FROM AUTHOR]

The present study was aiming to obtain the effects of magnetic gradients with varying direction on counterflow diffusion ethylene flames, including the flame shape, chemical structure and soot concentration distribution. Experiments were performed using flame imaging with a digital camera, two-color pyrometry, and flame speciation with gas chromatography (GC)/mass spectrometer (MS) system. When upward and horizontal outward magnetic gradients were introduced to flames, it was discovered that the mean flame temperature rose and the mean soot concentration dropped in the sooting region, while an opposite trend was observed with the addition of downward magnetic gradient. Meanwhile, results revealed that the presence of the upward or horizontal outward magnetic gradients could accelerate the consumption of ethylene and encourage the conversion of intermediate hydrocarbon species to CO and CO 2 in the flame area, while the oxidation of fuel and intermediate hydrocarbon species were inhibited and the formation of the aromatics was promoted with the application of the downward magnetic gradient. Furthermore, it was noted that the changing trend of aromatics was consistent with that of soot. The reason might be understood from the variation of magnetic force imposed on oxygen, which in turn affected the fuel consumption and soot distribution. • The influence of different magnetic gradients on flame shape was investigated. • Effects of magnetic gradients on temperature and soot concentration were examined. • Flame chemical structures with the addition of magnetic gradients were analyzed. • The influence mechanism of magnetic field on combustion chemistry was explored. [ABSTRACT FROM AUTHOR]

Aromatic hydrocarbons are important components of petroleum-based transportation fuels, biomass, coal, and solid waste, etc. The reaction kinetics of aromatic hydrocarbons largely determine the combustion characteristics and pollutant emission of vehicle/jet engines, power plants, and industrial reactors. While a few reviews have recently focused on aromatic hydrocarbons in gasoline surrogate fuels, thermochemical conversion of biomass/coal/solid waste, and combustion soot formation, a dedicated overview of research on the combustion chemistry of aromatic hydrocarbons is still lacking. In the last decades, valuable investigations addressing the reaction kinetics were reported based on the measurements from pyrolysis, oxidation, flames, shock tubes, and rapid compression machines, complemented by quantum chemistry and detailed kinetic modeling. Significant advances have allowed a better understanding of such physicochemical reacting system, from aromatic decomposition, oxidation, to pollutants formation. In the present review, aromatic hydrocarbons are systematically categorized to five common classes: basic, mono-substituted, multi-substituted, hydrogenated, and polycyclic aromatics. Fundamental aromatic combustion chemistry consists of the reactions of basic aromatic molecular structures. Then the aryl group strongly influences the reaction kinetics of aromatic derivates, which leads to very different combustion performance from those ordinary paraffins, olefins, and naphthenes. This paper seeks to provide an introduction to the knowledge gathered in the recent research, highlight pertinent aspects of this rapidly enriching information, and outlook the challenges towards fundamentally comprehensive aromatic combustion chemistry and practically efficient aromatic combustion model. [ABSTRACT FROM AUTHOR]