Development Of Chemical Kinetic Mechanism Research Articles

Development of a reduced chemical kinetic mechanism for ammonia combustion using species-based global sensitivity analysis

The increasing demands on efficiency and accuracy for numerical studies are urgently driving the development of chemical kinetic mechanisms with reliable prediction performance and compact size. In this work, a reduced kinetic mechanism was developed to comprehensively predict the combustion characteristics of ammonia. First, the performances of a series of detailed ammonia mechanisms were evaluated based on the average error function between mechanism predictions and experimental data from various typical reactors. The Han Mech., Mei Mech., and Zhang Mech. were identified to best predict the ignition delay time, laminar flame speed, and concentration profiles under a wide range of operating conditions, respectively. Then, the species-based path sensitivity analysis and global sensitivity analysis were utilized to identify the importance of species in these three selected detailed kinetic mechanisms on specific targets. The redundant species are eliminated until the relative errors between the predicted results using the interim reduced mechanisms and the detailed mechanisms exceed 10%. Finally, by merging and optimizing the three interim reduced mechanisms, a new reduced model was developed, which consists of 25 species and 174 reactions. Extensive validations against experimental data in shock tubes, premixed laminar flames, and jet-stirred reactors over broad operating conditions show satisfactory agreements, thereby substantiating the good practicability of the developed mechanism.

Development of Natural Gas Chemical Kinetic Mechanisms and Application in Engines: A Review.

In this paper, the brief development of chemical kinetic modeling of natural gas is discussed, with emphasis on the development of chemical kinetic mechanisms describing fuel oxidation. The addition of ethane and/or propane to natural gas not only decreases the ignition delay times but also increases flame speeds. Thus, the mixture of methane, ethane, and propane rather than bare methane obtains more accurate predictions for the combustion and emission characteristics of natural gas. This paper also evaluates different comprehensive mechanisms employed for natural gas engines and pointed out their advantages and disadvantages, giving guidance for the selection of mechanisms during the development of natural gas engines.

A multi-wavelength speciation framework for high-temperature hydrocarbon pyrolysis

A framework for the study of high-temperature hydrocarbon pyrolysis is presented. The proposed framework details a multi-wavelength speciation method using convex optimization, a review of complementary fixed-wavelength laser diagnostics, and a database of high-temperature absorption cross-sections to enable use of the framework for 11 different species. The proposed speciation method involves a vectorized Beer-Lambert system solved under solution constraints using convex optimization. In support of the proposed method, a review of pertinent laser diagnostics and a database of high-temperature absorption cross-sections are also provided. The database includes measured absorption cross-sections of methane (CH4), ethylene (C2H4), propene (C3H6), isobutene (i-C4H8), 1-butene (1-C4H8), benzene (C6H6), toluene (C7H8), and four jet fuels (samples of Jet A, JP5, JP8, and Gevo ATJ) at the wavelengths of 3.1758 µm, 3.17595 µm, 3.283 µm, 3.392 µm, 3.410 µm, 10.532 µm, 10.675 µm, 10.89 µm, 10.958 µm, 10.962 µm, and 11.345 µm within the range of 300–1600 K and 0.25–4 atm. The database is comprised of over 1000 high–temperature absorption cross-section measurements from new shock tube experiments, new Fourier transform infrared spectrometer (FTIR) experiments, and an aggregated dataset from the literature. The convex speciation method and cross-section database provided will facilitate laser absorption studies of hydrocarbon pyrolysis used in the development of chemical kinetic mechanisms.

Developing detailed chemical kinetic mechanisms for fuel combustion

This paper discusses a brief history of chemical kinetic modeling, with some emphasis on the development of chemical kinetic mechanisms describing fuel oxidation. At high temperatures, the important reactions tend to be those associated with the H2/O2 and C1–C2 sub-mechanisms, particularly for non-aromatic fuels. At low temperatures, and for aromatic fuels, the reactions that dominate and control the reaction kinetics are those associated with the parent fuel and its daughter radicals. Strategies used to develop and optimize chemical kinetic mechanisms are discussed and some reference is made to lumped and reduced mechanisms. The importance of accurate thermodynamic parameters for the species involved is also highlighted, as is the little-studied importance of collider efficiencies of different third bodies involved in pressure-dependent reactions.

Low-temperature speciation and chemical kinetic studies of n-heptane

Although there have been many ignition studies of n-heptane—a primary reference fuel—few studies have provided detailed insights into the low-temperature chemistry of n-heptane through direct measurements of intermediate species formed during ignition. Such measurements provide understanding of reaction pathways that form toxic air pollutants and greenhouse gas emissions while also providing key metrics essential to the development of chemical kinetic mechanisms. This paper presents new ignition and speciation data taken at high pressure (9atm), low temperatures (660–710K), and a dilution of inert gases-to-molecular oxygen of 5.64 (mole basis). The detailed time-histories of 17 species, including large alkenes, aldehydes, carbon monoxide, and n-heptane were quantified using gas chromatography. A detailed chemical kinetic mechanism developed previously for oxidation of n-heptane reproduced experimentally observed ignition delay times reasonably well, but predicted levels of some important intermediate chemical species that were significantly different from measured values. Results from recent theoretical studies of low temperature hydrocarbon oxidation reaction rates were used to upgrade the chemical kinetic mechanism for n-heptane, leading to much better agreement between experimental and computed intermediate species concentrations. The implications of these results to many other hydrocarbon fuel oxidation mechanisms in the literature are discussed.

An experimental and reduced modeling study of the laminar flame speed of jet fuel surrogate components

The laminar flame speed is an essential combustion parameter used in the validation of chemical kinetic mechanisms. In recent years, mechanisms tailored for jet fuel surrogate components have been partially validated using the laminar flame speeds of pure components, which were derived using both linear and non-linear extrapolation techniques. However, there remain significant deviations between the results from different studies that motivate further investigation. In this study, laminar, atmospheric pressure, premixed stagnation flames are investigated for the surrogate fuels n-decane, methylcyclohexane and toluene, which are representative of the alkane, cycloalkane and aromatic components of conventional aviation fuel, respectively. Numerical simulations are directly compared to velocity profile measurements to assess the predictive capabilities of the recently proposed JetSurF 2.0 chemical kinetic mechanism. Simulations of each experiment are carried out using the CHEMKIN-PRO software package together with the detailed mechanism, with accurate specification of the necessary boundary conditions from experimental measurements. Furthermore, a skeletal version of the detailed mechanism is deduced for improved computational speed using a species sensitivity reduction method, here referred to as Alternate Species Elimination (ASE). Toluene experimental data are further compared to a detailed toluene mechanism, termed the Stanford mechanism. The experimental and numerical reference flame speeds are used to infer the true laminar flame speed of the compounds following a recently proposed direct comparison technique that is similar to a non-linear extrapolation to zero flame stretch. JetSurF 2.0 and the skeletal ASE mechanisms demonstrate excellent overall agreement with experiment for n-decane and methylcyclohexane flames, for which the original model was optimized, but poor agreement for toluene, which was not an optimization target. Improved agreement for toluene is observed between the Stanford mechanism and experiment. Results confirm that the direct comparison method yields consistent laminar flame speed data irrespective of the reactivity accuracy of the chemical kinetic model employed. The laminar flame speed results from this study are essential for the further development of chemical kinetic mechanisms and contribute to the surrogate modeling of jet fuel combustion.

Formation of secondary aerosols over Europe: comparison of two gas-phase chemical mechanisms

Abstract. The impact of two recent gas-phase chemical kinetic mechanisms (CB05 and RACM2) on the formation of secondary inorganic and organic aerosols is compared for simulations of PM2.5 over Europe between 15 July and 15 August 2001. The host chemistry transport model is Polair3D of the Polyphemus air-quality platform. Particulate matter is modeled with a sectional aerosol model (SIREAM), which is coupled to the thermodynamic model ISORROPIA for inorganic species and to a module (MAEC) that treats both hydrophobic and hydrophilic species for secondary organic aerosol (SOA). Modifications are made to the gas-phase chemical mechanisms to handle the formation of SOA. In order to isolate the effect of the original chemical mechanisms on PM formation, the addition of reactions and chemical species needed for SOA formation was harmonized to the extent possible between the two gas-phase chemical mechanisms. Model performance is satisfactory with both mechanisms for speciated PM2.5. The monthly-mean difference of the concentration of PM2.5 is less than 1 μg m−3 (6%) over the entire domain. Secondary chemical components of PM2.5 include sulfate, nitrate, ammonium and organic aerosols, and the chemical composition of PM2.5 is not significantly different between the two mechanisms. Monthly-mean concentrations of inorganic aerosol are higher with RACM2 than with CB05 (+16% for sulfate, +11% for nitrate, and +10% for ammonium), whereas the concentrations of organic aerosols are slightly higher with CB05 than with RACM2 (+22% for anthropogenic SOA and +1% for biogenic SOA). Differences in the inorganic and organic aerosols result primarily from differences in oxidant concentrations (OH, O3 and NO3). Nitrate formation tends to be HNO3-limited over land and differences in the concentrations of nitrate are due to differences in concentration of HNO3. Differences in aerosols formed from aromatic SVOC are due to different aromatic oxidation between CB05 and RACM2. The aromatic oxidation in CB05 leads to more cresol formation, which then leads to more SOA. Differences in the aromatic aerosols would be significantly reduced with the recent CB05-TU mechanism for toluene oxidation. Differences in the biogenic aerosols are due to different oxidant concentrations (monoterpenes) and different particulate organic mass concentrations affecting the gas-particle partitioning of SOA (isoprene). These results show that the formulation of a gas-phase chemical kinetic mechanism for ozone can have significant direct (e.g., cresol formation) and indirect (e.g., oxidant levels) effects on PM formation. Furthermore, the incorporation of SOA into an existing gas-phase chemical kinetic mechanism requires the addition of reactions and product species, which should be conducted carefully to preserve the original mechanism design and reflect current knowledge of SOA formation processes (e.g., NOx dependence of some SOA yields). The development of chemical kinetic mechanisms, which offer sufficient detail for both oxidant and SOA formation is recommended.