| 1. |
- Pignatelli, Francesco, et al.
(author)
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Predictions of Spray Combustion using Conventional Category A Fuels and Exploratory Category C Fuels
- 2023
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In: AIAA SCITECH 2023 Forum. - Reston, Virginia : American Institute of Aeronautics and Astronautics. - 9781624106996
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Conference paper (peer-reviewed)abstract
- Aviation currently contributes about 3% of the world’s CO2 emissions, 5% of the global warming, and 35% of the trade. Reducing the emissions and the global warming from aviation is thus essential. Many approaches to achieve this goal are underway, including H2, fuel cells, and batteries, but also by replacing the fossil jet fuel with sustainable jet fuel from non-fossil feedstocks. This involves many challenges, and among them we have the issue of current jet engines being developed for existing fossil jet fuels. To facilitate the change towards sustainable jet fuel, typically having different thermophysical and combustion properties compared to fossil jet fuels, we need to analyze the sensitivity of combustion to other fuels, having a wider range of thermophysical specifications. Here, we examine combustion of n-heptane, n-dodecane, Jet A, and two test fuels, C1 and C5 in three different combustors. The first and second cases are axisymmetric and rectilinear pre-vaporized premixed bluff-body stabilized flames, whereas the third is a single sector helicopter combustor for liquid fuel. A Finite Rate Chemistry (FRC) Large Eddy Simulation (LES) model is used together with small comprehensive reaction mechanisms of ~300 reactions. Comparison with experimental data is performed for the pre-vaporized combustor configurations. Good agreement is generally observed, and small to marginal differences in combustion behavior is observed between the different fuels.
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| 2. |
- Ahmed, Ahfaz, et al.
(author)
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Kinetic modelling and experimental study of small esters : Methyl acetate and ethyl acetate
- 2017
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In: 11th Asia-Pacific Conference on Combustion, ASPACC 2017. ; 2017-December
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Conference paper (peer-reviewed)abstract
- A detailed chemical kinetic mechanism comprising methyl acetate and ethyl acetate has been developed based on the previous work by Westbrook et al. [1]. The newly developed kinetic mechanism has been updated with new reaction rates from recent theoretical studies. To validate this model, shock tube experiments measuring ignition delay time have been conducted at 15 & 30 bar and equivalence ratio 0.5, 1.0 and 2.0. Another set of experiments measuring laminar burning velocity was also performed on a heat flux burner at atmospheric pressure over wide range of equivalence ratios [ ~ 0.7-1.4]. The new mechanism shows significant improvement in prediction of experimental data over earlier model across the range of experiments.In this study, a detailed chemical kinetic model for methyl and ethyl acetate (Fig. 1) has been developed. This model is advanced from the mechanism proposed for laminar premixed flames by Westbrook and coworkers in 2009 [1]. Acetates studied in this work are both high RON fuels with suitable physical and chemical properties [Table 1] to be considered as potential fuels in advanced gasoline engines [4]. Shock tube experiments measuring ignition delay time have been conducted at 15 & 30 bar and equivalence ratio 0.5, 1.0 and 2.0. Another set of experiments measuring laminar burning velocity have also been performed on a heat flux burner at atmospheric pressure over wide range of equivalence ratios. The model developed in this work shows good agreement with ignition data and laminar burning velocity data across the temperature and equivalence ratio range respectively.
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| 3. |
- Ahmed, Ahfaz, et al.
(author)
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Small ester combustion chemistry : Computational kinetics and experimental study of methyl acetate and ethyl acetate
- 2019
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In: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 37:1, s. 419-428
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Journal article (peer-reviewed)abstract
- Small esters represent an important class of high octane biofuels for advanced spark ignition engines. They qualify for stringent fuel screening standards and could be synthesized through various pathways. In this work, we performed a detailed investigation of the combustion of two small esters, MA (methyl acetate) and EA (ethyl acetate), including quantum chemistry calculations, experimental studies of combustion characteristics and kinetic model development. The quantum chemistry calculations were performed to obtain rates for H-atom abstraction reactions involved in the oxidation chemistry of these fuels. The series of experiments include: a shock tube study to measure ignition delays at 15 and 30 bar, 1000-1450 K and equivalence ratios of 0.5, 1.0 and 2.0; laminar burning velocity measurements in a heat flux burner over a range of equivalence ratios [0.7-1.4] at atmospheric pressure and temperatures of 298 and 338 K; and speciation measurements during oxidation in a jet-stirred reactor at 800-1100 K for MA and 650-1000 K for EA at equivalence ratios of 0.5, 1.0 and at atmospheric pressure. The developed chemical kinetic mechanism for MA and EA incorporates reaction rates and pathways from recent studies along with rates calculated in this work. The new mechanism shows generally good agreement in predicting experimental data across the broad range of experimental conditions. The experimental data, along with the developed kinetic model, provides a solid groundwork towards improving the understanding the combustion chemistry of smaller esters.
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| 4. |
- Aidas, Kestutis, et al.
(author)
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On the performance of quantum chemical methods to predict solvatochromic effects: The case of acrolein in aqueous solution.
- 2008
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 128:19, s. 1-194503
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Journal article (peer-reviewed)abstract
- The performance of the Hartree-Fock method and the three density functionals B3LYP, PBE0, and CAM-B3LYP is compared to results based on the coupled cluster singles and doubles model in predictions of the solvatochromic effects on the vertical n-->pi(*) and pi-->pi(*) electronic excitation energies of acrolein. All electronic structure methods employed the same solvent model, which is based on the combined quantum mechanics/molecular mechanics approach together with a dynamical averaging scheme. In addition to the predicted solvatochromic effects, we have also performed spectroscopic UV measurements of acrolein in vapor phase and aqueous solution. The gas-to-aqueous solution shift of the n-->pi(*) excitation energy is well reproduced by using all density functional methods considered. However, the B3LYP and PBE0 functionals completely fail to describe the pi-->pi(*) electronic transition in solution, whereas the recent CAM-B3LYP functional performs well also in this case. The pi-->pi(*) excitation energy of acrolein in water solution is found to be very dependent on intermolecular induction and nonelectrostatic interactions. The computed excitation energies of acrolein in vacuum and solution compare well to experimental data.
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| 5. |
- Alekseev, Vladimir A., et al.
(author)
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Nitric oxide formation in flames of NH3/DME binary mixtures : Laser-induced fluorescence measurements and detailed kinetic analysis
- 2024
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In: Fuel. - 0016-2361. ; 371
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Journal article (peer-reviewed)abstract
- Binary mixture of ammonia (NH3) and dimethyl ether (DME) has been considered in literature as a potential fuel for practical use. Nitric oxide (NO) is a major product of combustion of NH3-containing fuels, and its formation routes have to be comprehensively studied. In this work, concentration profiles of NO were experimentally measured in laminar axisymmetric flames using planar laser-induced fluorescence. The molar percentage of NH3 in the NH3/DME fuel mixture varied from 10% to 60%. Emission levels of NO have reached as much as around 1% for mixtures with around 50% NH3. NO formation was analyzed with numerical simulations of 1D laminar flames and several detailed kinetic mechanisms. Modeling was performed in Chemkin with the steady-state burner-stabilized and free-propagating planar laminar flame reactor models. It was observed that the most recent versions of the contemporary NH3/DME models are able to reproduce the experiments, and their predictions agree with each other due to similarities in the NH3 submechanisms. Kinetic analysis has revealed some disagreement was observed in terms of how much direct chemical coupling between carbon- and nitrogen-containing species affects NO formation.
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| 6. |
- Alekseev, Vladimir A., et al.
(author)
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Reduced kinetics of NH3/n-heptane : Model analysis and a new small mechanism for engine applications
- 2024
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In: Fuel. - 0016-2361. ; 367
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Journal article (peer-reviewed)abstract
- A compact reduced mechanism covering a wide range of conditions is developed for use in simulations of NH3/n-heptane combustion in engines. Reduction targets were selected after reviewing available experimental studies of NH3 combustion in engines. Ignition, flames and oxidation of NH3/n-heptane mixtures were targeted. Particularly, mixtures with very low molar percentage of n-heptane which are important for the applications were considered. They have been observed to have a distinct ignition behavior. Target quantities also included pollutants with a goal to account for two possible mechanisms of N2O formation in engines, discussed in literature. The reduced mechanism of this study was developed with ant colony reduction method. It consists of 57 species and 159 reactions, and its range of applicability is 10–100 atm pressure and 0–100 % NH3 in the fuel mixture. The performance of the mechanism was found comparable to larger models from literature. Importance of carbon–nitrogen interactions, influence of key reactions in the NH3 subset and effect of CO on N2O formation were analyzed and discussed in terms of the predictive ability of the reduced mechanism of the present study and those available from literature.
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| 7. |
- Andersen, Christina, et al.
(author)
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Atmospheric Chemistry of Tetrahydrofuran, 2-Methyltetrahydrofuran, and 2,5-Dimethyltetrahydrofuran : Kinetics of Reactions with Chlorine Atoms, OD Radicals, and Ozone
- 2016
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In: The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory. - : American Chemical Society (ACS). - 1089-5639. ; 120:37, s. 7320-7326
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Journal article (peer-reviewed)abstract
- FTIR smog chamber techniques were used to study the kinetics of the gas-phase reactions of Cl atoms, OD radicals, and O3 with the five-membered ring-structured compounds tetrahydrofuran (C4H8O, THF), 2-methyltetrahydrofuran (CH3C4H7O, 2-MTHF), 2,5-dimethyltetrahydrofuran ((CH3)2C4H5O, 2,5-DMTHF), and furan (C4H4O). The rate coefficients determined using relative rate methods were kTHF+Cl = (1.96 ± 0.24) × 10-10, kTHF+OD = (1.81 ± 0.27) × 10-11, kTHF+O3 = (6.41 ± 2.90) × 10-21, k2-MTHF+Cl = (2.65 ± 0.43) × 10-10, k2-MTHF+OD = (2.41 ± 0.51) × 10-11, k2-MTHF+O3 = (1.87 ± 0.82) × 10-20, k2,5-DMTHF+OD = (4.56 ± 0.68) × 10-11, k2,5-DMTHF+Cl = (2.84 ± 0.34) × 10-10, k2,5-DMTHF+O3 = (4.58 ± 2.18), kfuran+Cl = (2.39 ± 0.27) × 10-10, and kfuran+O3 = (2.60 ± 0.31) × 10-18 molecules cm-3 s-1. Rate coefficients of the reactions with ozone were also determined using the absolute rate method under pseudo-first-order conditions. OD radicals, in place of OH radicals, were produced from CD3ONO to avoid spectral overlap of isopropyl and methyl nitrite with the reactants. The kinetics of OD radical reactions are expected to resemble the kinetics of OH radical reactions, and the rate coefficients of the reactions with OD radicals were used to calculate the atmospheric lifetimes with respect to reactions with OH radicals. The lifetimes of THF, 2-MTHF, and 2,5-DMTHF are approximately 15, 12, and 6 h, respectively.
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| 8. |
- Brackmann, Christian, et al.
(author)
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Experimental studies of nitromethane flames and evaluation of kinetic mechanisms
- 2018
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In: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 190, s. 327-336
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Journal article (peer-reviewed)abstract
- The present work reports new experimental data for premixed flames of nitromethane, CH3NO2, at atmospheric pressure, and an evaluation of two contemporary kinetic mechanisms based on these new flame studies as well as previously published experimental data on laminar burning velocity and ignition. Flames of nitromethane + air at lean (ϕ = 0.8) and rich (ϕ = 1.2) conditions were stabilized on a flat-flame burner, where profiles of CH2O, CO and NO were obtained using laser-induced fluorescence and temperature profiles using coherent anti-Stokes Raman spectroscopy. Laminar burning velocities for nitromethane + O2 + CO2 were measured using the heat flux method for ϕ = 0.8–1.3 at 348 K and ϕ = 0.8–1.6 at 358 K, and an oxidizer composition of 35% O2 and 65% CO2. In addition, the effect of the oxidizer composition was examined for a stoichiometric flame at 358 K by varying oxygen fraction from 30% to 40%. The mechanism by Mathieu et al. (Fuel 2016, 182, 597), previously not validated for flames, was able to reproduce experimental laminar burning velocities for nitromethane + air, but under predicted new results for CH3NO2 + O2 + CO2 mixtures. The mechanism by Brequigny et al. (Proc. Combust. Inst. 2014, 35, 703) under predicted experimental laminar burning velocities significantly at all investigated conditions. Previous studies have shown that none of the mechanisms can accurately predict ignition delay time over a wide range of conditions with respect to pressure, temperature, diluent and dilution ratio. The evaluation of the mechanisms reveals that the understanding of nitromethane combustion is at the present time not sufficient to produce a widely applicable mechanism.
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| 9. |
- Brackmann, Christian, et al.
(author)
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Formation of NO and NH in NH3-doped CH4 + N2 + O2 flame : Experiments and modelling
- 2018
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In: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 194, s. 278-284
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Journal article (peer-reviewed)abstract
- Co-combustion of 5200 ppm NH3 with a stoichiometric, atmospheric pressure, CH4 + N2 + O2 flame has been investigated with experiments and kinetic modelling. Profiles of the amidogen (NH) radical and nitric oxide (NO) have been measured using laser-induced fluorescence, the latter being quantitatively determined. Temperature profiles were measured using Rayleigh scattering and thermocouple, the nonintrusive measurements were considered more reliable and were used for evaluation of LIF data as well as input for flame modelling. Experimental results are compared with predictions of a chemical mechanism developed by Mendiara and Glarborg (2009), with simulations based on solution of energy equation as well as on experimental temperature profiles as input. Compared with a neat flame, the NH3-doped flame shows a shift in position ∼0.7 mm downstream, as established from the measurements of the NH profile. Modelling prediction of post-flame NO concentrations in the NH3-doped flame, around 1160 ppm, was within the evaluated uncertainty with experimental data (1460 ppm). Reaction path analysis indicated NH2 as a key species in the formation of NO and N2 from the nitrogen added to the flame by NH3. Altogether, the mechanism predicts concentration levels rather well but fails to predict the shift in flame position obtained with addition of NH3 to the rather slowly burning hydrocarbon flame.
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| 10. |
- Dakshnamurthy, Shanmugasundaram, et al.
(author)
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Experimental Study and a Short Kinetic Model for High-Temperature Oxidation of Methyl Methacrylate
- 2019
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In: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 191:10, s. 1789-1814
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Journal article (peer-reviewed)abstract
- Synthetic and natural polymeric esters find applications in transport and construction sectors, where fire safety is an important concern. One polymer that is widely used is poly (methyl methacrylate) (PMMA), which almost completely undergoes thermal decomposition into methyl methacrylate (its monomer) CH2=C(CH3) - C(= O) - O - CH3 (MMA) at ~250–300°C. In order to analyze the high-temperature gas-phase oxidation of PMMA, and thereby predict its fire behavior (such as burning rate, temperature of the material, and heat fluxes) with less computational effort, a compact kinetic model for the oxidation of its primary decomposition product, MMA, is most essential. This is accomplished in the present work by obtaining a reduced mechanism for MMA oxidation from a detailed mechanism from the Lawrence Livermore National Laboratories group. To extend the available data base for model validation and present validation data at atmospheric pressure conditions, for the first time, (i) detailed measurements of species profiles have been performed in stoichiometric laminar flat flames using flame sampling molecular beam mass spectrometry (MBMS) technique and (ii) laminar burning velocities have been obtained using the heat flux method for various unburnt mixture temperatures. Evaluating the model against these data sets point to the need to revise the kinetic model, which is achieved by adopting rate constants of key reactions among analogous molecules from recent literature. The updated compact kinetic model is able to predict the major species in the flat flame as well as the burning velocity of MMA satisfactorily. The final “short MMA mechanism” consists of 88 species and 1084 reactions.
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