Detailed Chemical Kinetic Mechanism Research Articles

Effects of combustion heat release on velocity and scalar statistics in turbulent premixed jet flames at low and high Karlovitz numbers

Theoretical scaling arguments for turbulent premixed combustion indicate that the pressure-dilatation source of turbulent kinetic energy becomes significant at low Karlovitz numbers, leading to potential invalidation of commonly-used turbulence models developed for non-reacting flow. Based on these arguments, a critical Karlovitz number is defined, below which dilatation effects are expected to become significant. Velocity and scalar statistics are obtained from Direct Numerical Simulation (DNS) calculations of low Mach number spatially-evolving turbulent premixed planar jet flames. At fixed bulk Reynolds number and stoichiometric equivalence ratio, two simulations are performed at Karlovitz numbers above and below the critical Karlovitz number. Hydrogen combustion with detailed transport is modeled using a detailed nine-species chemical kinetic mechanism, and coflows of combustion products are used to ensure flame stability at uniform equivalence ratio. The analysis of these statistics focuses on three key areas. First, the influence of the velocity-pressure gradient source of turbulent kinetic energy is confirmed at a low Karlovitz number, and the unimportance of these effects is confirmed at a high Karlovitz number. Similar effects are observed for the chemical source term in the scalar variance budgets. Second, the degree of alignment between the Reynolds stress tensor (scalar flux) and the strain-rate tensor (scalar gradient), the foundation of a majority of the turbulence models used in reacting flows, is assessed with the DNS databases. Additionally, consistency of anisotropic Reynolds stress and strain-rate tensor invariants is assessed using invariant maps. While good alignment and consistency are obtained for statistics and invariants at a high Karlovitz number, both alignment and consistency degrade at a low Karlovitz number. Third, turbulence models formulated for non-reacting flow are modified algebraically in the Bray–Moss–Libby (BML) formalism for turbulent premixed combustion. A variable efficiency function is defined to capture the regime dependence of heat release effects in these models. Model performance is evaluated at Karlovitz numbers above and below the critical Karlovitz number using the DNS databases, and satisfactory prediction of counter-gradient transport in the flame-normal direction is obtained. However, heat release effects are also observed in the flame-parallel directions in the low-Karlovitz number simulation, and the models developed in the formalism for statistically planar flames fail to capture these effects. Furthermore, in the low-Karlovitz number case, redistributive effects are active on the shear components of the Reynolds stress, which are not considered in the BML formalism. More advanced turbulence models are therefore necessary for turbulent premixed jet flames below the critical Karlovitz number.

A Shock Tube Experimental and Modeling Study of Multicomponent Gasoline Surrogates Diluted with Exhaust Gas Recirculation

The ignition delay for pure cyclohexane and two quaternary gasoline surrogate fuels CTRF1 (isooctane/n-heptane/toluene/cyclohexane, 37.070/11/40.076/11.854 by mole fraction), CTRF2 (isooctane/n-heptane/toluene/cyclohexane, 30.538/11/43.077/15.385 by mole fraction) with the same research octane number of 95 (RON = 95) in air is measured under lean, stoichiometric, and rich conditions behind reflected shock waves, at temperatures of 1027 K–1400 K, and pressures of 10, 15, and 19 bar/20 bar. To analyze the effects of exhaust gas recirculation upon ignition, CTRF1/air mixtures are diluted with CO2 to simulate different exhaust gas recirculation (EGR) loadings (0, 20%, 40%, and 60%). The experimental data are compared to the predictions calculated by a detailed chemical kinetic mechanism with 526 species and 2763 reactions generated in this work, which is also validated by the autoignition characteristics of pure cyclohexane, iso-octane, n-heptane, toluene, and their binary and ternary mixtures. The simulation...

The Effect of Methyl Pentanoate Addition on the Structure of a Non-Premixed Counterflow n-Heptane/O2 Flame

The influence of methyl pentanoate (MP) addition to n-heptane on the species pool in a nonpremixed counterflow flame fueled with n-heptane at atmospheric pressure has been investigated experimentally and numerically. Two non-premixed flames in counterflow configuration have been examined: (1) n-heptane/Ar (5.3%/94.7%) vs O2/Ar (24.1%/75.9%) and (2) n-heptane/MP/Ar (2.5%/2.5%/95%) vs O2/Ar (19.6%/80.4%). Both flames had similar strain rates and stoichiometric mixture fractions to allow an adequate comparison of their structures. The mole fraction profiles of the reactants, major products, and intermediates in both flames were measured using flame sampling molecular beam mass spectrometry. These experimental data were used for validation of a detailed chemical kinetic mechanism, which was proposed earlier for prediction of combustion characteristics of n-heptane/iso-octane/toluene/MP mixtures. The addition of MP to n-heptane reduced the flame temperature and the peak mole fractions of many flame intermediat...

Detailed Chemical Kinetic Mechanisms for Hydrocarbon and Oxygenated Hydrocarbon Fuel Combustion

Detailed Chemical Kinetic Mechanisms for Hydrocarbon and Oxygenated Hydrocarbon Fuel Combustion

CFD simulations of Rapid Compression Machines using detailed chemistry: Evaluation of the ‘crevice containment’ concept

The use of creviced pistons in Rapid Compression Machines (RCMs) has proven to be very efficient in making the temperature homogeneous inside the reaction chamber but has the disadvantage of inducing an unintended mass transfer from the reaction chamber to the crevice, especially during the preliminary heat release of two-stage ignition processes. Aiming to mitigate this mass transfer, the technique of ‘crevice containment’ (CC) has been proposed. It consists of a physical separation between the reaction chamber and the crevice region that is engaged at the end of the compression, physically preventing any mass transfer between both parts of the geometry. In order to numerically assess this novel design concept across a broader range of conditions than previously investigated, reactive simulations using detailed chemical kinetic mechanisms are performed for n-heptane and iso-octane mixtures. It is found that for compressed temperatures outside of the NTC (negative temperature coefficient) region, the CC approach is very effective in suppressing the influence of the crevice mass transfer and thus increases the validity of the widely-used 0-D model based on the adiabatic core assumption. Still, for most of the temperature cases inside the NTC region, ignition appears to be prematurely initiated in the residual vortex region formed after the seal engagement, possibly inducing very significant differences with the 0-D model. The benefits of eliminating the post-compression crevice mass transfer appear to be counter-balanced by effects that have not been previously identified.

Effects of Hydrogen Addition on the Standoff Distance of Premixed Burner-Stabilized Flames of Various Hydrocarbon Fuels

The quenching distance of premixed hydrocarbon flames is of significant importance for studying flame/wall interactions and for understanding the unburned hydrocarbon emissions of internal combustion engines. Motivated by the fact that the standoff distance of premixed burner-stabilized flames could be used to investigate the behavior of head-on quenching distance of freely propagating flames despite the different physics involved, a parametric investigation on the standoff distances of methane, ethane, and propane burner-stabilized flames was conducted numerically using a detailed chemical kinetic mechanism, with a focus on the effects of hydrogen addition. Specifically, the minimum standoff distance was found to quantitatively correlate with the head-on quenching distance of premixed flames. The variations of the minimum standoff distance as a function of hydrogen fractions were then investigated in detail. The results showed that as hydrogen fraction increased, the minimum standoff distances decreased ...

Numerical Simulation of Stoker-type Incinerator including Waste Bed Combustion and NO<SUB>X</SUB> Prediction using a Detailed Chemical Kinetics Mechanism

Numerical Simulation of Stoker-type Incinerator including Waste Bed Combustion and NO<SUB>X</SUB> Prediction using a Detailed Chemical Kinetics Mechanism

Multi-fuel surrogate chemical kinetic mechanisms for real world applications.

The most important driving force for development of detailed chemical kinetic reaction mechanisms in combustion is the desire by researchers to simulate practical systems. This paper reviews the parallel evolution of kinetic reaction mechanisms and applications of those models to practical, real engines. Early, quite simple, kinetic models for small fuel molecules were extremely valuable in analyzing long-standing, poorly understood applied ignition and flame quenching problems, and later kinetic models have been applied to much more complex flame propagation, problems including autoignition in spark-ignition engines and issues related to octane numbers and knock in modern, high compression ratio and other engines. The recent emergence of very large, multi-fuel surrogate kinetic mechanisms that can address many different fuel types and real engine applications is discussed as a modern analytical tool that can be used for a wide variety of practical applications.

Experimental and modeling studies of a biofuel surrogate compound: laminar burning velocities and jet-stirred reactor measurements of anisole

Lignocellulosic biomass is a promising alternative fuel source which can promote energy security, reduce greenhouse gas emissions, and minimize fuel consumption when paired with advanced combustion strategies. Pyrolysis is used to convert lignocellulosic biomass into a complex mixture of phenolic-rich species that can be used in a transportation fuel. Anisole (or methoxybenzene) can be used as a surrogate to represent these phenolic-rich species. Anisole also has attractive properties as a fuel component for use in advanced spark-ignition engines because of its high blending research octane number of 120. Presented in the current work are new measurements of laminar burning velocities, jet-stirred reactor (JSR) speciation of anisole/O2/N2 mixtures, and the development and validation of a detailed chemical kinetic mechanism for anisole. Homogeneous, steady state, fixed gas temperature, perfectly stirred reactor CHEMKIN simulations were used to validate the mechanism against the current JSR measurements and published JSR experiments from CNRS-Nancy. Pyrolysis and oxidation simulations were based on the experimental reactant compositions and thermodynamic state conditions including P = 1 bar and T = 675–1275 K. The oxidation compositions studied in this work span fuel-lean (ϕ = 0.5), stoichiometric, and fuel rich (ϕ = 2.0) equivalence ratios. Laminar burning velocities were measured on a heat flux stabilized burner at an unburnt T = 358 K, P = 1 bar and simulated using the CHEMKIN premixed laminar flame speed module. Ignition delay times of anisole were then simulated at conditions relevant to advanced combustion strategies. Current laminar burning velocity measurements and predicted ignition delay times were compared to gasoline components (e.g., n-heptane, iso-octane, and toluene) and gasoline surrogates to highlight differences and similarities in behavior. Reaction path analysis and sensitivity analysis were used to explain the pathways relevant to the current studies. Under pyrolysis and oxidative conditions, unimolecular decomposition of anisole to phenoxy radicals and methyl radicals was found to be important due to the relatively low bond strength between the oxygen and methyl group, ∼65 kcal/mol. Reactions of these abundant phenoxy radicals with O2 were found to be critical to accurately reproduce anisole's reactivity.

In Situ Flame Temperature Measurements Using a Mid-Infrared Two-Line H2O Laser-Absorption Thermometry

ABSTRACTA mid-infrared two-line thermometry based on H2O absorption was developed for in situ and sensitive temperature measurements of laminar flames. The H2O absorption lines in v3 fundamental band were systematically investigated and a new promising line pair (4029.52 cm–1 and 4030.73 cm–1) was selected to achieve strong absorption line-strength and high-temperature sensitivity over the temperature range of 1000–3000 K. This line pair was readily assessed by a single current scan of a distributed feedback laser near 2.5 μm. The scanned-wavelength laser absorption strategy was adopted for temperature measurements for both laminar sooting and non-sooting flames. This optical sensor was applied in measuring various flame conditions including CH4/air and C2H4/air premixed flames (Ф = 0.8–1.2), C2H4/air sooting flames (Ф = 1.78–2.38), and partially premixed flames. Our measurements were in good agreement with thermocouple measurements for all the flame conditions. In addition, numerical simulations using the detailed chemical kinetic mechanisms (CH4 flame: GRI 3.0, C2H4 flame: USC 2.0) were conducted to validate the experimental results.

A Numerical Study on Hypergolic Combustion of Hydrazine Sprays in Nitrogen Tetroxide Streams

ABSTRACTUnsteady simulations of hydrazine (N2H4) sprays in nitrogen tetroxide (NTO, NO2-N2O4) streams were conducted to explore the hypergolic combustion in bipropellant thrusters. The Navier–Stokes equations were solved using a detailed chemical kinetics mechanism and dispersed droplets were modeled through direct numerical simulations. Auto-ignition occurred when the sum of the heat transfer from the ambient gas and the heat release from hydrogen abstraction reactions exceeded the latent heat of the droplets. Although the evaporation of the droplets was enhanced as the droplet size decreased, the ignition delay time increased due to the lower temperatures of the mixtures of the N2H4 vapor and nitrogen tetroxide. After the flames reached a steady state, a double flame structure appeared, comprised of outer diffusion and inner decomposition flames. The inner decomposition flame and N2H4 vapor flow exhibited a sinusoidal behavior at a certain droplet size. This behavior was initiated by the locally expanded decomposition gases and developed by the supply of N2H4 droplets to the decomposition gases at relatively high temperatures. In cases of larger and smaller droplet sizes, the sinusoidal behavior was not significant due to less evaporation of the N2H4 droplets and a lower temperature of the N2H4 vapor, respectively. The sinusoidal behavior of the decomposition flames enhanced the mixing and reactions of the fuel components (i.e., N2H4, NH3, and H2). The present study demonstrated a large impact of droplet size on flame dynamics, suggesting that a fine spray is not always better for hypergolic propellant combustion to consume the fuel components quickly.

Identification of initial decomposition reactions in liquid-phase HMX using quantum mechanics calculations

Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine, (HMX), also known as octogen, is a commonly used ingredient in solid propellants and explosives. As a result, many experimental and theoretical studies have investigated HMX to elucidate its liquid-phase and gas-phase decomposition behavior. In various experimental efforts, including both fast and slow thermolysis, the results indicate that ring opening occurs very early due to the presence of carbon-containing species, such as HCN, H2CO, HNCO, CO and CO2. After a lengthy search using quantum mechanics, ring-opening and subsequent reactions have been identified which may occur in both liquid and gas phases. Two existing pathways – 1) HONO elimination and 2) NNO2 homolysis – are re-examined for liquid-phase decomposition, and early ring-opening reactions have been identified. Three additional pathways – 1) reaction with NO and formation of ONTNTA, 2) prompt oxidation via HONO and ONNO2 addition, and 3) hydrogen abstraction via NO2– are also identified along with early ring-opening reactions in each pathway. The proposed pathways for decomposition of HMX are similar to that of RDX presented earlier [37]. The quantum mechanics calculations are performed using density functional theory (DFT) at the B3LYP/6-311 + + G(d,p) level. Conductor-like Polarizable Continuum Model (CPCM) is used to account for solvation effects with water as solvent within the Gaussian program package. Intrinsic reaction coordinate calculations have also been performed to verify that the reactants are indeed connected to the expected products. Similar to RDX, HONO elimination appears to be the initiation reaction followed by early ring-opening reactions. Results also indicate simultaneous formation of formaldehyde and N2O from early ring-opening reactions. Competing bimolecular reactions with NO2, HONO and ONNO2 are also investigated in the liquid phase, which explain the experimentally observed autocatalytic behavior. These reactions can assist in the development of a detailed liquid-phase chemical kinetics mechanism of nitramine propellants containing RDX and HMX.

Effect of methane-hydrogen mixtures on flow and combustion of coherent jets

Coherent jets are widely used in electric arc furnace (EAF) steelmaking to increase the oxygen utilization and chemical reaction rates. However, the influence of fuel gas combustion on jet behavior is not fully understood yet. The flow and combustion characteristics of a coherent jet were thus investigated at steelmaking temperature using Fluent software, and a detailed chemical kinetic reaction mechanism was used in the combustion reaction model. The axial velocity and total temperature of the supersonic jet were measured via hot state experiments. The simulation results were compared with the experimental data and the empirical jet model proposed by Ito and Muchi and good consistency was obtained. The research results indicated that the potential core length of the coherent jet can be prolonged by optimizing the combustion effect of the fuel gas. Besides, the behavior of the supersonic jet in the subsonic section was also investigated, as it is an important factor for controlling the position of the oxygen lance. The investigation indicated that the attenuation of the coherent jet is more notable than that of the conventional jet in the subsonic section.

Study of combustion behaviors for dimethyl ether as an alternative fuel using CFD with detailed chemical kinetics

In the present analysis, the dissemination of dimethyl ether (DME) asan alternative fuel with the charged air inside the HCCI engine chamber until accomplishing the burning procedure have been numerically expected by utilizing CFD with detailed chemical kinetics mechanism. The physical technique of the ignition and pollution arrangement in the engine barrel with DME fuel is examined with a developed hydrocarbon reaction mechanism at various HCCI engine loads. The mechanism is including 81 chemical species and 362 basic chemical reactions. Along these lines, the used CFD/discipline code predicts the admittance of the fuel spray, ignition, and pollutant development of DME asa biodiesel fuel. The split injection technique has been used to inject the dimethyl ether remotely at the engine admission tube. The in-cylinder flow field enhances farther the engine chamber fuel/air distribution. The outcomes showed that the blend of DME fuel with charged air is exact and efficient to accomplish the in-barrel blend homogeneity. It is anticipated that, under every engine working condition the production of the methyl radical is considered as imperative part in dimethyl ether pyrolysis and oxidation. In the meantime, the formaldehyde sub-component is a huge division of the general dimethyl ether (DME) ignition system.

Numerical validation of the liquid-phase decomposition mechanism of guanidinium azotetrazolate

The objective of this work is to numerically validate a previously formulated detailed chemical kinetics mechanism for the liquid phase decomposition of guanidinium azotetrazolate (GzT). GzT is of keen interest to researchers because of its energetic properties therefore it is critical to thoroughly understand the chemistry involved in its decomposition. The present work is a step in this direction. The reaction mechanism used in the present model, which consists of 76 species and 107 elementary reactions along with their kinetic parameters, was compiled from two previously published works. Simulations of the mass loss in thermogravimetric analysis and heat flow in differential scanning calorimetry were carried out using a numerical model. Results of the simulations were found to match satisfactorily with those from the experiments. Critical pathways in the GzT reaction mechanism were also identified by carrying out a sensitivity analysis.

Assessment of uncertainties of laminar flame speed of premixed flames as determined using a Bunsen burner at varying pressures

Laminar methane/air premixed flames at different pressures in a newly developed high-pressure Bunsen flame rig are studied using detailed numerical simulations and laser diagnostics. In the numerical simulations, one-dimensional and two-dimensional axisymmetric configurations were considered employing detailed transport properties and chemical kinetic mechanisms. In the measurements, OH PLIF was employed. The aims are to improved the understanding of the structures of the flames at varying pressures, to measure the laminar flame speed at different pressures, and to quantify the accuracy of the Bunsen flame method for measurement of laminar flame speed at different pressures. The stoichiometric and fuel-rich flames were found to exhibit a two-reaction-zone structure: an inner premixed flame in which the fuel was converted to CO and H2, and an outer diffusion flame in which CO and H2 were oxidized further to form combustion products. With increasing pressure, the inner premixed flame becomes thinner and the flame as a whole has the tendency to become unstable. Using the numerical and the experimental data, the methods of flame-cone-angle and flame-area were used to extract the laminar flame speed for different equivalence ratios and pressures. The flame-cone-angle method showed slightly better accuracy than the flame-area method did. The accuracy of both methods became lower under high pressure conditions. The inlet velocity of the burner was shown to affect the accuracy of the extracted laminar flame speed. For a stoichiometric atmospheric flame it was found that the most suitable inlet velocity for the fuel/air mixture was about 6 times the laminar flame speed, yielding a flame length about 7 times the radius of the burner. With appropriate flame length, the mid-height of the flame showed a rather low flame stretch rate, the laminar flame speed being in close agreement with the unstretched laminar flame speed, the error being less than 6% for the flames that were studied.

Study of the Chemical Structure of Laminar Premixed H2/CH4/C3H8/O2/Ar Flames at 1–5 atm

The paper presents an experimental and modeling study of the chemical structure of laminar premixed stoichiometric H2/CH4/C3H8/O2/Ar flames stabilized on a flat burner at 1, 3, and 5 atm. The flame structure was simulated using four different detailed chemical kinetic mechanisms proposed in the literature for oxidation of small hydrocarbons. The width of the zone of consumption of the fuel components was shown to differ appreciably at the three pressures. Hydrogen was shown to have the largest consumption zone, while propane has the smallest one. The kinetic analysis provided an explanation for the observed phenomenon, which assumes the formation of additional pathways for hydrogen and methane production in the flames of ternary fuel mixtures. Comparison of the measured and simulated flame structures shows that all the mechanisms satisfactorily predict the mole fraction profiles of the reactants, products, and some intermediates at atmospheric and elevated pressures. It is noteworthy that the mechanisms a...

Combined Ab Initio, Kinetic Modeling, and Shock Tube Study of the Thermal Decomposition of Ethyl Formate.

The potential energy surfaces (PESs) and reaction rate constants of the unimolecular decomposition of ethyl formate (EF) were investigated using high-precision theoretical methods at the CCSD(T)/CBS(T-Q)//M06-2X/6-311++G(d,p) level of theory. The calculated PESs of EF dissociation and molecular decomposition reactions indicate that the intramolecular H-shift to produce formic acid and ethylene is the dominant decomposition pathway. A detailed chemical kinetic mechanism for EF pyrolysis was constructed by incorporating the important reactions of EF and its radicals into an existing mechanism previously developed for small methyl esters. The updated mechanism was first used to reproduce CO, CO2, and H2O concentration time histories during EF pyrolysis in the shock tube reported by Ren et al. [ Ren , W. , Mitchell Spearrin , R. , Davidson , D. F. , and Hanson , R. K. J. Phys. Chem. A 2014 , 118 , 1785 - 1798 ]. The rate of production and sensitivity analyses show that the competing dehydration and decarboxylation channels of the intermediate formic acid control the final product yields of EF pyrolysis. The EF mechanism was further validated against the shock tube data of OH, CO, CO2, and H2O time histories measured during EF oxidation (equivalence ratio Φ = 1.0) at 1331-1615 K and 1.52-1.74 atm. This revised EF mechanism captured all of the species' time histories over the entire temperature range. Such modeling capability was due to the more accurate rate constants of EF reactions determined by high-precision theoretical calculations and a high-fidelity C0-C2 basis mechanism.

Study on premixed combustion characteristics of co-firing ammonia/methane fuels

Ammonia is believed to eventually play an important role in substituting conventional fossil fuels for future energy systems. In this study, to gain a deep insight into the combustion properties of co-firing ammonia/methane fuel blends for the power and steel industry, a detailed chemical-kinetics mechanism model was developed for comprehensively modelling ammonia/methane fuels combustion. Characteristics of ignition delay time, unstretched laminar flame speed and NO, CO2 and CO emissions in the exhaust gas were obtained over a wide range of equivalence ratios and ammonia fractions. High NO emissions will be a main problem as CO and CO2 emissions tend to drop when adding ammonia into methane. To gain a further understanding of the effect of ammonia substituting methane for combustion use, analyses of laminar premixed flame structures were performed. The impact of ammonia substitution was illustrated by analysing relevant specific radicals. Furthermore, to study the combustion characteristics of ammonia/methane under more practical conditions, effects of engine relevant conditions (elevated pressure and initial temperature) were also studied. Results indicate that pressure has a more prominent effect than initial temperature and there is a good potential that unwanted emissions can be reduced significantly under industrial conditions.

CSP-based chemical kinetics mechanisms simplification strategy for non-premixed combustion: An application to hybrid rocket propulsion

A set of simplified chemical kinetics mechanisms for hybrid rocket applications using gaseous oxygen (GOX) and hydroxyl-terminated polybutadiene (HTPB) is proposed. The starting point is a 561-species, 2538-reactions, detailed chemical kinetics mechanism for hydrocarbon combustion. This mechanism is used for predictions of the oxidation of butadiene, the primary HTPB pyrolysis product. A Computational Singular Perturbation (CSP) based simplification strategy for non-premixed combustion is proposed. The simplification algorithm is fed with the steady-solutions of classical flamelet equations, these being representative of the non-premixed nature of the combustion processes characterizing a hybrid rocket combustion chamber. The adopted flamelet steady-state solutions are obtained employing pure butadiene and gaseous oxygen as fuel and oxidizer boundary conditions, respectively, for a range of imposed values of strain rate and background pressure. Three simplified chemical mechanisms, each comprising less than 20 species, are obtained for three different pressure values, 3, 17, and 36 bar, selected in accordance with an experimental test campaign of lab-scale hybrid rocket static firings. Finally, a comprehensive strategy is shown to provide simplified mechanisms capable of reproducing the main flame features in the whole pressure range considered.