Skeletal Chemical Kinetic Mechanism Research Articles

Combustion of spherical PMMA samples in still air simulated using a skeletal chemical kinetic mechanism

The combustion of spherical samples of polymethyl methacrylate (PMMA) under natural convection was studied experimentally and by CFD simulation. Species mole fraction and temperature profiles in the flame were measured by microprobe mass spectrometry and microthermocouples. Gas flow around a solid sphere was modeled using the full Navier–Stokes equations for a multicomponent mixture, with a skeletal chemical kinetic mechanism taking into account heat transfer, convection, and radiation. A kinetic model has been developed that includes the surface pyrolysis of PMMA and a skeletal kinetic mechanism for the gas-phase combustion of its monomer, methyl methacrylate (MMA). Satisfactory agreement between the experimental data and the results of 3D modeling for the chemical and thermal structure of the flame has proven the efficiency of the proposed chemical kinetic model. An analysis of the obtained data has shown that in the diffusion combustion of the spherical samples, the region of the stoichiometric composition of the gas mixture does not coincide with the region of maximum temperature. It has been found that under the conditions studied, gas-phase MMA decomposition proceeds mainly through radical reactions and unimolecular decomposition plays only a minor role.

A skeletal mechanism for gasoline surrogates: Development, validation, and CFD application

In this study, a skeletal chemical kinetic mechanism for gasoline surrogates is developed from a detailed mechanism by applying several reduction techniques. Directed relation graph (DRG) routines and DRG-aided sensitivity analysis methods are applied with worst-case error tolerances equal to 30% and 40%, respectively. Issues with reaction dead-ends in the oxidation paths are resolved by adding adequate intermediate species and reactions. Sensitivity analyses are conducted to identify the most impactful fuel-dependent reactions to ignition delay, and the specific reaction rates of such reactions are optimized to replicate the ignition behavior of detailed mechanism. The final version of the skeletal mechanism consists of 152 species and 563 reactions including NOx chemistry. Experiments are carried out in Sandia's low-temperature gasoline combustion research engine with an E10 regular-grade gasoline, termed RD5-87, and the results are replicated in 0D simulations with good accuracies using both detailed mechanism and the skeletal mechanism although a slight over-estimation of the low-temperature heat release is observed. Finally, the performance of the skeletal mechanism is also tested in 3D CFD simulation using LES. Simulations are able to predict the pressure evolution and heat release rate, proving that the proposed skeletal mechanism is adequate for simulations.

A skeletal chemical kinetic mechanism for ammonia/n-heptane combustion

Progressively stricter pollutant emission targets in international agreements have shifted the focus of combustion research to low carbon fuels. Ammonia is recognized as one of the promising energy vectors for next-generation power production. Due to the low flame speed and high auto-ignition temperature, ammonia is often burned with a high reactivity pilot fuel (e.g. diesel). However, chemical kinetic mechanisms describing the combustion of ammonia and large hydrocarbon fuels (such as n-heptane, a surrogate of diesel) are less developed. In this work, a skeletal chemical kinetic mechanism for n-heptane/ammonia blend fuels is proposed using a joint decoupling methodology and optimization algorithm. A sensitivity analysis of the ignition delay times of the ammonia/n-heptane mixture is performed to identify the dominant reactions. A genetic algorithm is used to optimize the mechanism further. The final skeletal mechanism is made up of 69 species and 389 reactions. The skeletal ammonia/n-heptane mechanism is validated against the experimental data for combustion of pure ammonia, ammonia/hydrogen and ammonia/n-heptane mixtures, including the global combustion characteristic parameters such as ignition delay times measured in shock tubes or rapid compression machines, laminar burning velocities measured in heat flux burners or spherical flame vessels, and species data measured in jet-stirred reactors. Comparing the results from the skeletal mechanism with those from other mechanisms from the literature is conducted to evaluate the mechanism further. The present skeletal mechanism can well predict the combustion processes for a wide range of conditions, and the mechanism is computationally efficient, showing good potential to model ammonia/n-heptane combustion with good accuracy and efficiency.

Usage of Converter Gas as a Substitute Fuel for a Tunnel Furnace in Steelworks.

Converter gas (BOFG) is a by-product of the steel manufacturing process in steelworks. Its usage as a substitute fuel instead of natural gas for fueling a metallurgical furnace seems to be reasonable due to potential benefits as follows: CO2 emission reduction into the ambient air and savings in purchasing costs of natural gas. Results of theoretical analysis focused on implementing converter gas as a fuel for feeding a tunnel furnace for either steel plate rolling, steel sheet hardening in its real working condition or both, are discussed. The analysis was focused on the combustion chemistry of the converter gas and its potential ecological and economic benefits obtained from converter gas usage to heat up steel in a tunnel furnace. Simulations of combustion were conducted using a skeletal chemical kinetic mechanism by Konnov. The directed relation graph with error propagation aided sensitivity analysis (DRGEPSA) method was used to obtain this skeletal kinetic mechanism. Finally, the model was validated on a real tunnel furnace fueled by natural gas. Regarding exhaust emissions, it was found that nitric oxide (NO) dropped down from 275 to 80 ppm when natural gas was replaced by converter gas. However, carbon dioxide emissions increased more than three times in this case, but there is no possibility of eliminating carbon dioxide from steel manufacturing processes at all. Economic analysis showed savings of 44% in fuel purchase costs when natural gas was replaced by converter gas. Summing up, the potential benefits resulting from substituting natural gas with converter gas led to the conclusion that converter gas is strongly recommended as fuel for a tunnel furnace in the steel manufacturing process. Practical application requires testing gas burners in terms of their efficiency, which should provide the same amount of energy supplied to the furnace when fed with converter gas.

Analysis of laminar premixed flame structure of isooctane/2-methylfuran/air mixtures with a skeletal mechanism

The prospects of 2-methylfuran (2MF) as a biofuel and gasoline blend stock are quite high. Therefore, a skeletal chemical kinetic mechanism – containing 238 species and 1156 reactions – for the simulation of premixed flames involving isooctane (representing gasoline)/2MF blends is proposed in the present work. The proposed model has been validated against a wide range of experimental data at various pressures, temperatures and compositions. It is demonstrated in the present work that the proposed mechanism can predict the concentrations of gaseous soot precursors as well. Moreover, the compatibility of the NOx sub-mechanism in the present model has been assured through validations against a wide range of recent experimental data. The results obtained in this work suggest that there are little chances of co-oxidation reactions among the species generated from the initial decomposition of both isooctane and 2MF. Furthermore, there is around 26% variation in the peak laminar burning velocity when the mole fraction of 2MF is increased from 10% to 90% in isooctane/2MF blend.

The Role of Multiple Injections on Combustion in a Light-Duty PPC Engine

This paper presents a numerical investigation of the ignition and combustion process of a primary reference fuel in a partially premixed light-duty internal combustion (PPC) engine. Partially pre-mixed combustion is achieved by employing a multiple injection strategy with three short injection events of fuel pulses. The timing of the first two fuel pulses, 48 and 22 crank angle degrees before top dead center, are chosen with the purpose to stratify the fuel and air charge, whereas the third injection, at five crank angle degrees before top dead center, serves as an actuator of the main heat release. In addition to this baseline injection, three alternative injection strategies are studied, including a split-fuel two-injection strategy and modified triple-injection strategies. Large eddy simulations are employed utilizing a skeletal chemical kinetic mechanism for primary reference fuel capable of capturing the low-temperature ignition and the high temperature combustion. The large eddy simulation (LES) results are compared with experiments in an optical accessible engine. The results indicate that the first ignition sites are in the bowl region where the temperature is relatively higher, and the reaction fronts thereafter propagate in the swirl direction and towards the centerline of the cylinder. The charge from the first two injections initially undergoes low-temperature reactions and thereafter high-temperature reservoirs are formed in the bowl region. The main heat-release is initiated in the engine when the fuel from the third injection reaches the high-temperature reservoirs. Finally, the remaining fuel in the lean mixtures from the first two injections is oxidized. By variation of the injection strategy, two trends are identified: (1) by removing the second injection a higher intake temperature is required to enable the ignition of the charge, and (2) by retarding second injection, a longer ignition delay is identified. Both can be explained by the stratification of fuel and air mixture, and the resulting reactivity in various equivalence ratio and temperature ranges. The LES results reveal the details of the charge stratification and the subsequent heat release process. The present results indicate a rather high sensitivity of partially premixed combustion process to the injection strategies.

Skeletal chemical kinetic mechanism generation for methanol combustion and systematic analysis on the ignition characteristics

AbstractA skeletal chemical kinetic mechanism for methanol combustion is developed based on a comprehensive detailed mechanism (AramcoMech 3.0), which consists of 581 species and 3,037 reactions. The systematic skeletal mechanism reduction methods are applied to the detailed mechanism to obtain a minimal skeletal mechanism. Systematic mechanism analysis including global reaction path and sensitivity analysis are performed to identify key reactions relevant to methanol oxidation. Species relevant to the combustion chemistry of methanol are analyzed through global reaction path and compared with other reduced mechanisms, which highlights the importance of maintaining realistic chemistry in skeletal mechanism. Brute‐force sensitivity analysis and chemical explosive mode analysis are employed to analyze the ignition properties of methanol. The performance of the skeletal mechanism is validated against a wide range of pressures, temperatures, and equivalence ratios for ignition, species concentrations, and laminar flame speeds. This work not only presents a minimal skeletal mechanism for methanol combustion based on systematic mechanism reduction methods from very large detailed mechanisms but also provides insight into the chemical kinetics of methanol combustion.

Effects of blending 2,5-dimethylfuran on the laminar burning velocity and ignition delay time of isooctane/air mixture

The prospects of 2,5-dimethylfuran (DMF) as a bio-derived fuel that can be blended with gasoline are believed to be impressive. However, the effects of blending DMF on the key combustion parameters like the laminar burning velocity and ignition delay time of gasoline/air mixture need to be studied extensively for the successful implementation of the fuel mixture in spark ignition engines. Therefore, a skeletal chemical kinetic mechanism, comprising of 999 reactions among 218 species, has been developed in the present work for this purpose. The proposed chemical kinetic model has been validated against a wide range of experimental data for the laminar burning velocity and ignition delay time of isooctane (representing gasoline), DMF and their blends. It has been found from the present study that the thermal diffusivity of the unburnt gas mixture changes by a very small amount from the corresponding value for the pure isooctane/air mixture when DMF is added. Unlike isooctane, the DMF molecule does not consume H radicals during its primary breakup. Therefore, the maximum laminar burning velocity increases marginally when 50% DMF is blended with isooctane due to the increased presence of H radicals in the flame. The negative temperature coefficient behaviour in the ignition delay time of the isooctane fuel vanishes when 30% DMF (v/v) is blended to it.

Damkohler Number Analysis in Lean Blow-Out of Toroidal Jet Stirred Reactor

Lean blowout (LBO) prediction based on the local parameters in the laboratory toroidal jet-stirred reactor (TJSR) is investigated. The reactor operated on methane is studied using three-dimensional computational fluid dynamics (CFD); the results are compared with the experimental data. Skeletal chemical kinetic mechanism with the eddy dissipation concept (EDC) model is used. Flow bifurcation in the radial (poloidal) plane due to the interaction between counter-rotating vortices creates one dominating poloidal recirculation zone (PRZ) and one weaker toroidal recirculation zone (TRZ). The Damkohler (Da) number in the reactor is the highest in the stabilization vortex; it varies from about Da ∼ 2 at ϕ = 0.55 to Da ∼ 0.2–0.3 at LBO conditions. Due to the reduced turbulent dissipation rate in PRZ, the Da number is an order of magnitude higher than in TRZ. The global blowout event is predicted at the local Da = 0.2 in PRZ. Local blowout events in the regions of low Da can lead to flame instability and to a global flame blowout at a higher fuel–air ratio than predicted by the CFD. Local Da nonuniformity can be used for optimization and analysis of combustion system stability. Further research in the process parameterization and application to the practical combustion system is needed.

Development of an Optimized Skeletal Chemical Kinetic Mechanism for Methane Combustion for Marine Engine Applications

The goal of the present study is to develop an optimized skeletal chemical kinetic mechanism for methane combustion, for conditions relevant to dual-fuel marine engines. To this end, a systematic approach is developed, consisting of the following steps: (a) assessment of three widely used detailed mechanisms, by comparing simulation results against three sets of indirect experimental data pertinent to methane combustion, (b) sensitivity analysis, with identification of important reactions (species), (c) selection of one detailed mechanism and production of a skeletal mechanism by means of the simulation error minimization connectivity method, (d) uncertainty analysis of the rate constants of important reactions, and (e) optimization of the skeletal mechanism for the rate constant parameters of the important reactions. The resulting optimized skeletal mechanism, consisting of 28 species and 119 elementary reactions, accurately reproduces experimental data in a wide range of conditions and is an important development for computational fluid dynamics studies in dual-fuel marine engines.

A computational study of the effects of DC electric fields on non-premixed counterflow methane-air flames

Two-dimensional axisymmetric simulations for counterflow non-premixed methane-air flames were undertaken as an attempt to reproduce the experimentally observed electro-hydrodynamic effect, also known as the ionic wind effect, on flames. Incompressible fluid dynamic solver was implemented with a skeletal chemical kinetic mechanism and transport property evaluations. The simulation successfully reproduced the key characteristics of the flames subjected to DC bias voltages at different intensity and polarity. Most notably, the simulation predicted the flame positions and showed good qualitative agreement with experimental data for the current–voltage curve. The flame response to the electric field with positive and negative polarity exhibited qualitatively different characteristics. In the negative polarity of the configuration considered, a non-monotonic variation of the current with the voltage was observed, along with the existence of an unstable regime at an intermediate voltage level. With positive polarity, a typical monotonic current–voltage curve was obtained. This behavior was attributed to the asymmetry in the distribution of the positive and negative ions resulting from ionization processes. The present study demonstrated that the mathematical and computational models for the ion chemistry, transport, and fluid dynamics were able to describe the key processes responsible for the flame-electric field interaction.

Computational Study of the Laminar Reaction Front Properties of Diluted Methanol–Air Flames Enriched by the Fuel Reforming Product

The current work investigates the flame structure and the propagation of premixed laminar flame fronts for mixtures of diluted methanol–air enriched by fuel reforming products under spark ignition engine conditions. Two engine concepts were investigated: one with external fuel reforming (EFR) and one with reformed exhaust gas recirculation (R-EGR). Here, the fuel reformate (or syngas) is a mixture of H2, CO, and CO2 with a CO selectivity of 6.5%, which is used to represent the products of methanol steam reforming over a Cu–Mn/Al foam catalyst. The simulations were exercised over a wide range of dilution level and unburned gas temperature at 40 bar with a skeletal chemical kinetic mechanism using zero/one-dimensional codes. Two types of dilution—air and EGR—were compared at the same fuel-to-charge equivalence ratio (ϕ′). The results showed that the knock limit for spark-ignited operation is extended with rising dilution levels, especially when diluted by an R-EGR mixture. Syngas addition also leads to a re...

Asymptotic analysis of quasi-steady n-heptane droplet combustion supported by cool-flame chemistry

A skeletal chemical-kinetic mechanism for n-heptane cool flames is simplified to the maximum extent possible by introduction of steady-state approximations for intermediaries, following procedures employed previously in addressing two-stage ignition. A pair of ordinary differential equations in mixture-fraction space is thereby obtained, describing the quasi-steady structures of the temperature and heptylketohydroperoxide fields. Application of activation-energy asymptotics for the partial-burning regime to this pair of equations is shown to provide convenient expressions for flame structures and the extinction condition associated with maximally reduced chemistry. With the mixture-fraction co-ordinate related to radius, these results are used to address droplet-combustion experiments that have been performed in the International Space Station. Droplet diameters at extinction are predicted as functions of the oxygen concentration in the atmosphere and are compared with experiment. While the results are encouraging concerning qualitative predictions of dependences of extinction diameters on atmospheric conditions, there are noticeable quantitative differences that point to deficiencies in the analysis, likely resulting from a number of oversimplifications. Further investigation is therefore recommended.

Development of a New Skeletal Chemical Kinetic Mechanism for Ethanol Reference Fuel

A new skeletal chemical kinetic mechanism of ethanol reference fuel (including ethanol, iso-octane, n-heptane, and toluene combustion mechanisms) consisting of 62 species and 194 reactions is developed for oxidation and combustion of gasoline blend surrogate fuels. The skeletal ethanol chemical kinetic mechanism is added to the toluene reference fuel (TRF) mechanism (including iso-octane, n-heptane, and toluene combustion mechanisms) using reaction paths and semidecoupling model. The ignition delay and laminar flame speed of the new combustion mechanism were modeled by using several fuel surrogates at different pressures, temperatures, and equivalence ratios. The skeletal chemical kinetic mechanism ignition delay and laminar flame speed are validated by comparison to the available experimental data of the shock tube and plate burner. The results indicate that satisfactory agreement between predictions and experimental measurements are achieved.

The role of cool-flame chemistry in quasi-steady combustion and extinction of n-heptane droplets

Experiments on the combustion of large n-heptane droplets, performed by the National Aeronautics and Space Administration in the International Space Station, revealed a second stage of continued quasi-steady burning, supported by low-temperature chemistry, that follows radiative extinction of the first stage of burning, which is supported by normal hot-flame chemistry. The second stage of combustion experienced diffusive extinction, after which a large vapour cloud was observed to form around the droplet. In the present work, a 770-step reduced chemical-kinetic mechanism and a new 62-step skeletal chemical-kinetic mechanism, developed as an extension of an earlier 56-step mechanism, are employed to calculate the droplet burning rates, flame structures, and extinction diameters for this cool-flame regime. The calculations are performed for quasi-steady burning with the mixture fraction as the independent variable, which is then related to the physical variables of droplet combustion. The predictions with the new mechanism, which agree well with measured autoignition times, reveal that, in decreasing order of abundance, H2O, CO, H2O2, CH2O, and C2H4 are the principal reaction products during the low-temperature stage and that, during this stage, there is substantial leakage of n-heptane and O2 through the flame, and very little production of CO2 with no soot in the mechanism. The fuel leakage has been suggested to be the source of the observed vapour cloud that forms after flame extinction. While the new skeletal chemical-kinetic mechanism facilitates understanding of the chemical kinetics and predicts ignition times well, its predicted droplet diameters at extinction are appreciably larger than observed experimentally, but predictions with the 770-step reduced chemical-kinetic mechanism are in reasonably good agreement with experiment. The computations show how the key ketohydroperoxide compounds control the diffusion-flame structure and its extinction.

A Skeletal Chemical Kinetic Model of PRF Oxidation for HCCI Engines

A skeletal chemical kinetic mechanism, including 42 species and 71 reactions for the oxidation of primary reference fuel (PRF), was developed and applied to model homogeneous charge compression ignition (HCCI) combustion after various experiments and available mechanisms for PRF oxidation being reviewed and the performance of mechanisms applied to experiments relevant to HCCI engines being analyzed. The ignition delay predicted by the skeletal mechanism showed good agreement with experiments for different fuels over the temperature range 667-1350K. Validation was also performed with experiments from HCCI engines, and good agreement was obtained for the primary reference fuels. The results show that the present PRF mechanism gives reliable performance for HCCI combustion predictions. Sensitivity analysis indicates that for PRF70 and PRF91.8 under HCCI conditions, H-atom abstraction from iso-octane molecule reaction and OH addition to n-heptane molecule have large influence, and CH2O, CH3 are very important intermediate species.

Ignition delay of fatty acid methyl ester fuel droplets: Microgravity experiments and detailed numerical modeling

Recent optical engine studies have linked increases in NO x emissions from fatty acid methyl ester combustion to differences in the premixed autoignition zone of the diesel fuel jet. In this study, ignition of single, isolated liquid droplets in quiescent, high temperature air was considered as a means of gaining insight into the transient, partially premixed ignition conditions that exist in the autoignition zone of a fatty acid methyl ester fuel jet. Normal gravity and microgravity (10 −4 m/s 2) droplet ignition delay experiments were conducted by use of a variety of neat methyl esters and commercial soy methyl ester. Droplet ignition experiments were chosen because spherically symmetric droplet combustion represents the simplest two-phase, time-dependent chemically reacting flow system permitting a numerical solution with complex physical submodels. To create spherically symmetric conditions for direct comparison with a detailed numerical model, experiments were conducted in microgravity by use of a 1.1 s drop tower. In the experiments, droplets were grown and deployed onto 14 μm silicon carbide fibers and injected into a tube furnace containing atmospheric pressure air at temperatures up to 1300 K. The ignition event was characterized by measurement of UV emission from hydroxyl radical (OH*) chemiluminescence. The experimental results were compared against predictions from a time-dependent, spherically symmetric droplet combustion simulation with detailed gas phase chemical kinetics, spectrally resolved radiative heat transfer and multi-component transport. By use of a skeletal chemical kinetic mechanism (125 species, 713 reactions), the computed ignition delay period for methyl decanoate (C 11H 22O 2) showed excellent agreement with experimental results at furnace temperatures greater than 1200 K.

Low temperature n-butane oxidation skeletal mechanism, based on multilevel approach

In order to reconcile an increasingly large deviation (order of magnitude) of the ignition delay time at decreasing initial temperature, computed using the prior art kinetic schemes, with the available experimental values, a new skeletal mechanism (54 species, 94 reactions) for low-temperature (500–800 K) ignition of n- butane in air based on ab initio calculations is developed. The skeletal mechanism obtained accurately reproduces n-butane combustion kinetics for the practically important ranges of pressure, temperature and fuel–air equivalence ratio, especially in the low-temperature range. The elaborated first principal skeletal chemical kinetic mechanism of n-butane oxidation was validated against available experimental results for normal and elevated initial pressure (1–15 atm) using the Chemical Work Bench code. A good agreement with experiments was shown.

Characteristics of microjet methane diffusion flames

Characteristics of microjet methane diffusion flames stabilized on top of the vertically oriented, stainless-steel tubes with an inner diameter ranging from 186 to 778 μ m are investigated experimentally, theoretically and numerically. Of particular interest are the flame shape, flame length and quenching limit, as they may be related to the minimum size and power of the devices in which such flames would be used for future micro-power generation. Experimental measurements of the flame shape, flame length and quenching velocity are compared with theoretical predictions as well as detailed numerical simulations. Comparisons of the theoretical predictions with measured results show that only Roper's model can satisfactorily predict the flame height and quenching velocity of microjet methane flames. Detailed numerical simulations, using skeletal chemical kinetic mechanism, of the flames stabilized at the tip of d = 186, 324 and 529 μ m tubes are performed to investigate the flame structures and the effects of burner materials on the standoff distance near extinction limit. The computed flame shape and flame length for the d = 186 μm flame are in excellent agreement with experimental results. Numerical predictions of the flame structures strongly suggest that the flame burns in a diffusion mode near the extinction limit. The calculated OH mass fraction isopleths indicate that different tube materials have a minor effect on the standoff distance, but influence the quenching gap between the flame and the tube.

Modeling concepts for larger carbon number alkanes: A partially reduced skeletal mechanism for n-decane oxidation and pyrolysis

A new, partially reduced skeletal chemical kinetic mechanism for the oxidation and pyrolysis of n -decane was developed. This mechanism was validated against newly obtained n -decane flow reactor data. The proposed model was also compared against n -decane oxidation jet-stirred reactor data and n -decane/air shock tube ignition delay data obtained from the literature. The mechanism is an extension of a similar mechanism developed for n -heptane recently published by this laboratory. The current conceptual approach differs from that in our previous modeling of n -heptane oxidation in that it includes detailed chemistry of n -decane and the five n -decyl radicals, and it incorporates both internal hydrogen isomerization reactions and β -scission pathways for the various system radicals. To include this additional detailed reaction information and simultaneously minimize the number of species present in the model, an important assumption was made regarding the distribution of radical isomers. It was assumed that the different isomers of a given alkyl radical are in equilibrium at each carbon number above the C 4 level, thereby allowing the inclusion of the reaction channels associated with each isomer, without imposing the computational penalty associated with including each isomer as a separate species in the mechanism. As a result, only a single radical is needed to represent all the isomers associated with it. Thus, the new mechanism contains detailed reaction chemistry information, while maintaining the compactness necessary for use in combined fluid mechanical/chemical kinetic computational simulations. In addition to verifying this approach for n -decane, the approach was shown to be compatible with the modeling of n -heptane. The hierarchical nature of this modeling technique should prove amenable for use in developing compact mechanisms for other large n -alkanes, and this partially reduced skeletal model can serve as a validated starting model for developing more compact representations.