439 research outputs found
Contribution of the radical-complex mechanism to the rate of the reaction CH3 + O-2 (+ M) -> CH3O2 (+ M) at high pressures.
No full textEarlier experimental studies of the falloff curves of the reaction CH3 + O-2 M) -> CH3O2 ( M) in the bath gases M = Ar and N-2 (Fernandes et al., J. Phys. Chem. A 2006, 110, 4442), in addition to the usual behavior of the energy-transfer (ET) mechanism, showed first evidence for a participation of the radicalcomplex (RC) mechanism in the reaction at pressures above about 300 bar and at temperatures below 400 K. By extending these measurements to the bath gas M = CO2, more pronounced deviations from the ET mechanism were now observed. This unambiguously confirms the presence of the RC mechanism at high pressures in a medium-sized molecular system, analogous to earlier observations for larger systems such as the dimerization of benzyl radicals (Luther et al., Phys. Chem. Chem. Phys. 2004, 6, 4133)
Falloff curves for the reaction CH3 + O2 (+M) -> CH3O2 (+M) in the pressure range 2-1000 bar and the temperature range 300-700 K.
No full textThe reaction CH3 + O-2 (+M) -> CH3O2 (+M) was Studied in the bath gases Ar and N-2 in a high-temperature/high-pressure flow cell at pressures ranging from 2 to 1000 bar and at temperatures between 300 and 700 K. Methyl radicals were generated by laser flash photolysis of azomethane or acetone. Methylperoxy radicals were monitored by UV absorption at 240 nm. The falloff curves of the rate constants are represented by the simplified expression k/k(infinity) approximate to [x/(1 + x)]F-cent(1/{1+[(logx)/N]2}) with x = k(0)/k(infinity) F-cent approximate to 0.33, and N approximate to 1.47, where k(0) and k(infinity) denote the limiting low and high-pressure rate constants, respectively. At low temperatures, 300400 K, and pressures > 300 bar, a fairly abrupt increase of the rate constants beyond the values given by the falloff expressions was observed. This effect is attributed to a contribution from the radical complex mechanism as was also observed in other recombination reactions of larger radicals. Equal limiting low-pressure rate constants k(0) = [M]7 x 10(-31)(T/300 K)(-3.0) cm(6) molecule(-2) s(-1) were fitted for M = Ar and N-2 whereas limiting high-pressure rate constants k(infinity) = 2.2 x 10(-12)(T/300 K)(0.9) cm(3) molecule(-1) s(-1) were approached. These values are discussed in terms of unimolecular rate theory. It is concluded that a theoretical interpretation of the derived rate constants has to be postponed until better information of the potential energy surface is available. Preliminary theoretical evaluation suggests that there is an "anisotropy bottleneck" in the otherwise barrierless interaction potential between CH3 and O-2
Experimental and modelling study of the recombination reaction H+O(2)(+ M) -> HO(2)(+ M) between 300 and 900 K, 1.5 and 950 bar, and in the bath gases M = He, Ar, and N(2)
No full textThe recombination reaction H + O(2) (+ M)-HO(2) (+ M) was studied by laser. ash photolysis in a high pressure flow cell, over the temperature range 300-900 K, the pressure range 1.5-950 bar and in the bath gases M = He and N(2). Earlier experiments by Hahn et al. ( Phys. Chem. Chem. Phys. 2004, 6, 1997) in the bath gas M = Ar were also extended. The data were analyzed in terms of unimolecular rate theory employing new calculations of relevant molecular parameters. Improved energy transfer parameters for the bath gases M = He, Ar, N(2), and H(2)O could thus be obtained and complete fallo. curves were constructed. In the case of water, the high pressure rates well connect with pulse radiolysis results obtained in supercritical water by Janik et al
The reaction of hydroxyethyl radicals with O2: A theoretical analysis and experimental product study
No full textReactions of α-hydroxyethyl (CH3CHOH) and β-hydroxyethyl (CH2CH2OH) radicals with oxygen are of key importance in ethanol combustion. High-level ab initio calculations of the potential energy surfaces of these two reactions were coupled with master equation methods to compute rate coefficients and product branching ratios for temperatures of 250–1000 K. The α-hydroxyethyl + O2 reaction is controlled by the barrierless entrance channel and shows negligible pressure dependence; in contrast, the reaction of the β isomer displays pronounced pressure dependence. The high pressure limit rate coefficients of both reactions are about the same at the temperatures investigated. Products of the reactions were monitored experimentally at 4 Torr and 300–600 K using tunable synchrotron photoionization mass spectrometry. Hydroxyethyl radicals were produced from the reaction of ethanol with chlorine atoms and the β isomer was also selectively produced by the addition reaction C2H4 + OH → CH2CH2OH. Formaldehyde, acetaldehyde, vinyl alcohol and H2O2 products were detected, in qualitative agreement with the theoretical predictions
Experimental and modeling study of the temperature and pressure dependence of the reaction C2H5 + O2 (+ M) → C2H5O2 (+ M).
No full textThe reaction C2H5 + O2 (+ M) → C2H5O2 (+ M) was studied at 298 K at pressures of the bath gas M = Ar between 100 and 1000 bar. The transition from the falloff curve of an energy transfer mechanism to a high pressure range with contributions from the radical complex mechanism was observed. Further experiments were done between 188 and 298 K in the bath gas M = He at pressures in the range 0.7-2.0 Torr. The available data are analyzed in terms of unimolecular rate theory. An improved analytical representation of the temperature and pressure dependence of the rate constant is given for conditions where the chemical activation process C2H5 + O2 (+ M) → C2H4 + HO2 (+ M) is only of minor importance
Evolução química e história de formação estelar no universo local
No full textTese (doutorado) - Universidade Federal de Santa Catarina, Centro de Ciências Físicas e Matemáticas, Programa de Pós-Graduação em Física, Florianópolis, 2010Analisamos as galáxias do Sloan Digital Sky Survey com o nosso código de síntese espectral Starlight. O Starlight acha a combinação de populações estelares simples de diferentes idades e metalicidades que melhor modela o espectro de uma galáxia. Ele permite derivar vários parâmetros associados a uma galáxia, como a massa em estrelas, a história de formação estelar e a evolução química. A partir do espectro residual puramente nebular (subtraindo do espectro observado o modelado), medimos as linhas de emissão, das quais derivamos propriedades do gás dentro das galáxias. Estudamos a evolução das galáxias com formação estelar. Constatamos que as galáxias de maior massa formaram suas estrelas e seus metais mais rapidamente. A evolução da metalicidade das estrelas é estudada diretamente. Calibramos também a taxa de formação estelar atual medida pela síntese com a medida pela luminosidade de Halfa. Derivamos a relação massa estelar-metalicidade estelar (M*-Z*) em diferentes redshifts. Esta é a primeira vez que a relação M*-Z* é calculada para o mesmo conjunto de galáxias. Observamos que a metalicidade estelar observada tem uma evolução compatível com um modelo simples de evolução química de caixa fechada. Para as galáxias classificadas como LINERs, encontramos que a luminosidade observada em Halfa é compatível com o número de fótons ionizantes emitidos pelas populações estelares velhas dessas galáxias. Este resultado implica em uma profunda revisão da taxa de atividade nuclear nas galáxias do Universo local
Eine experimentelle und modellgestützte Studie über die Verbrennungseigenschaften von C3-C4-Alkoholen und C4-C5-Furanen für die nachhaltige Luftfahrt
No full textOver the years, the surge in air traffic has increased CO2 emissions, necessitating a focus on combustion efficiency and the abatement of pollutant emissions in combustion applications. Electrofuels (e-fuels), harnessed through renewable sources like wind and solar energy using PtX technology, emerge as a promising avenue for emission reduction and the advancement of sustainable aviation. Integrating the lean premixed pre-vaporized (LPP) concept with e-fuels demonstrates potential in curbing soot formation and NOx emissions. Identifying suitable e- fuels involves an assessment of crucial combustion properties such as ignition delay time (IDT). Employing measurement techniques like shock tubes (ST) ensures a uniform gas phase environment, particularly when coupled with tunable diode laser absorption spectroscopy (TDLAS) for precise time-resolved detection of dynamic species. TDLAS, characterized by its high responsiveness and non-invasive nature, is a valuable diagnostic technique that effectively complements the brief measurement durations in shock tubes (ST). On the other hand, the rapid compression machine (RCM), designed to mimic engine working conditions closely, proves instrumental in probing the IDT of fuels within the low-to-intermediate temperature range. This investigation, conducted within the framework of the cluster of excellence "Sustainable and energy-efficient aviation (SE2A)" at TU Braunschweig University in Germany, focuses on the ignition properties of propanol, butanol isomers, and furans like tetrahydrofuran (THF) and 2-methyl tetrahydrofuran (2-MTHF). These properties are scrutinized at intermediate-high temperatures (600 - 1550 K), atmospheric-high pressures (1-10 bar, 20, and 40 bar), and under LPP conditions, considering their potential application in jet engines. The research utilizes a combination of ST and RCM, with TDLAS providing real-time species history during measurements. Among alcohols, all measured IDTs under different experimental conditions exhibit no Negative Temperature Coefficient (NTC) behavior. Significantly, 1-butanol displays the highest reactivity due to a lower activation energy, followed by iso-butanol, which has a similar reactivity to 2-butanol. 1-propanol has higher reactivity than 2-propanol, whereas tert-butanol was the least reactive, considering its higher activation energy than other butanol. The study observes a decrease in IDTs with an increase in equivalence ratio within the investigated temperatures, and this effect is less pronounced with increasing pressure. The analysis of CO profiles reveals a temperature correlation, with a distinct plateau near stoichiometric conditions for both 1- and 2-butanol. However, this plateau is absent for 1- propanol and 2-propanol. Validation of Experimental IDTs was conducted using various literature models1–3, with each model demonstrating good predictions for specific fuel types. The rate of production analysis highlights differences in radical species production, contributing to variations in IDTs. The model by Sarathy et al.1 well-predicted the IDTs of 1-and 2-butanol, Nadiri et al.2 predicted the IDTs better for iso-butanol, and Van geem et al. 3 model captured the IDTs well for tert-butanol. Through the rate of production analysis, it was understood that 2-butanol produced much fewer reacting radicals, H radicals, and relatively more stable radical species such as C2H5 than 1-butanol, leading to the longer IDT of 2-butanol. The production rate of CO species indicated that HCO and HCCO were the major decomposition species that led to the formation of CO. For propanol isomers, the rate of production analysis using the Johnson et al.4 model indicates significant impacts on the IDTs due to CO formation and consumption reactions such as “C2H2+OCH2+CO” and “HCO+OHCO+H2O” had an impact on the IDTs of the fuel and consumption reaction such as CO + OH CO2 + H to have a high impact on the magnitude of CO peak. In the case of furans, namely THF and 2-MTHF, measured IDTs exhibit NTC behavior at low- intermediate temperatures, where THF was highly reactive. Three-stage ignition is observed in both THF and 2-MTHF. The kinetic modeling study showed that the Fenard et al. 5 model showed better predictions for THF and Wu et al. 6 for 2-MTHF. The inhibiting reaction HO2 + OH = H2O + O2 and the explosion reaction CO + OH = CO2 + H were significant contributors that led to multiple stages of ignition.Der zunehmende Luftverkehr hat im Laufe der Jahre zu einem Anstieg der CO2-Emissionen geführt. Aus diesem Grund sind die Verbrennungseffizienz und die Verringerung der Schadstoffemissionen zu einem wichtigen Anliegen für Verbrennungsanwendungen geworden. Elektro-Treibstoffe (E-Fuels) sind vielversprechende Konzepte, die die Emissionen reduzieren und die nachhaltige Luftfahrt fördern. Hochwertige Kraftstoffe wie E-Fuels werden durch die Nutzung erneuerbarer Energien wie Wind- und Sonnenenergie und geringwertige Kraftstoffe in Form von Flüssigkeiten oder Gasen mit Hilfe der PtX-Technologie erzeugt. Darüber hinaus verspricht das Konzept der mageren vorgemischten Verdampfung (LPP), die Rußbildung und NOx-Emissionen in Kombination mit E-Kraftstoffen zu reduzieren. Um die potenziellen E-Kraftstoffe zu identifizieren, muss daher die Schlüsseleigenschaft der Verbrennung wie die Zündverzögerungszeit (IDT) bestimmt werden. Messtechniken wie Stoßrohre (ST) werden häufig eingesetzt, um eine einheitliche Gasphasenumgebung für einen breiten Temperatur- und Druckbereich zu erzeugen, die sich für das Verständnis der Chemie bei Reaktionen bei höheren Temperaturen mit anschließender zeitaufgelöster Detektion eignet. Die Kombination aus der Stoßrohrtechnik und der abstimmbaren Diodenlaser- Absorptionsspektroskopie (TDLAS) bietet eine Plattform für die genaue Messung des zeitlichen Verlaufs dynamischer Spezies. TDLAS ist eine hochempfindliche, nicht-invasive Diagnosetechnik, die die kurze Messzeit der ST ergänzt. Die Schnellkompressionsmaschine (RCM) arbeitet unter triebwerksnahen Bedingungen. RCM wird normalerweise zur Untersuchung der IDT von Kraftstoffen bei niedrigen bis mittleren Temperaturen eingesetzt. Der Schwerpunkt dieser Arbeit liegt auf der Messung der Zündeigenschaften der hier betrachteten potenziellen E-Kraftstoffkandidaten, die im Rahmen des Exzellenzclusters "Nachhaltige und energieeffiziente Luftfahrt (SE2A)" an der TU Braunschweig von Interesse sind. Als potenzielle Kraftstoffe für den Einsatz in Strahltriebwerken sind die Zündeigenschaften von Propanol, Butanol-Isomeren und Furanen wie Tetrahydrofuran und 2- Methyl-Tetrahydrofuran noch nicht sehr umfassend untersucht worden, insbesondere bei hohen Zwischentemperaturen (600 - 1550 K), hohen Drücken (1-10 bar, 20 und 40 bar) und unter Berücksichtigung der LPP-Bedingungen. Die Technologie der mageren Verbrennung hat die Vorteile eines verbesserten thermischen Wirkungsgrads und niedriger Emissionen. Daher wurden die Selbstzündungseigenschaften von Propanol, Butanolisomeren und Furanen wie Tetrahydrofuran (THF) und 2-Methyl-Tetrahydrofuran (2-MTHF) in einem ST und einem RCM untersucht. Optische Diagnostik wie die TDLAS-Technik wird bei den Messungen angewandt, um die Speziesgeschichte in Echtzeit zu erhalten. Die Temperatur ist auf 600 bis 1000 K in der RCM und 1100 bis 1550 K in der ST eingestellt, die untersuchten Drücke sind 1-10 bar in der ST und 20 und 40 bar in der RCM mit einem Äquivalenzverhältnis von 0,25, 0,5 und 0,9. Unter den Alkoholen zeigten alle gemessenen IDTs bei verschiedenen Versuchsbedingungen kein Verhalten des negativen Temperaturkoeffizienten (NTC). 1-Butanol hatte die höchste Reaktivität aufgrund der niedrigeren Aktivierungsenergie, gefolgt von Iso-Butanol, das eine ähnliche Reaktivität wie 2-Butanol aufweist. 1-Propanol hat eine höhere Reaktivität als 2- Propanol, während tert-Butanol aufgrund seiner höheren Aktivierungsenergie als andere Butanol- und Propanolisomere bei allen Äquivalenzverhältnissen am wenigsten reaktiv war. Innerhalb der untersuchten Temperaturen sinken die IDTs mit einem Anstieg des Äquivalenzverhältnisses. Die Auswirkung des Äquivalenzverhältnisses innerhalb der untersuchten Bedingungen war mit steigendem Druck geringer. Die gemessenen CO-Profile wiesen eine gute Temperaturkorrelation auf, wobei für φ nahe der stöchiometrischen Bedingung ein scharfes Plateau im CO-Bildungsprofil sowohl für 1- als auch für 2-Butanol beobachtet wurde, das bei einem Druck von 3 bar vorherrschte. Bei 1- Propanol und 2-Propanol war dieses Plateau jedoch nicht vorhanden. Zur Validierung der gemessenen experimentellen IDTs wurden verschiedene Literaturmodelle verwendet. Das Modell von Sarathy et al.1 lieferte eine gute Vorhersage der IDTs von 1- und 2-Butanol. Das Modell von Van Geem et al. 3 erfasste die IDTs für tert-Butanol gut, und Nadiri et al. 2 sagte die IDTs für Iso-Butanol besser voraus. Durch die Analyse der Produktionsrate wurde deutlich, dass 2-Butanol viel weniger reaktive Radikale, H-Radikale und relativ stabilere Radikalspezies wie C2H5 als 1-Butanol produziert, was zu der längeren IDT von 2-Butanol führt. Die Produktionsrate der CO-Spezies zeigte, dass HCO und HCCO die wichtigsten Zersetzungsspezies waren, die zur Bildung von CO führten. Im Falle der Propanol-Isomere zeigte die ROP von CO, die mit dem Modell von Johnson et al. 4 durchgeführt wurde, dass die CO-Bildungsreaktion "C2H2+OCH2+CO" und "HCO+OHCO+H2O" einen Einfluss auf die IDT des Kraftstoffs und die Verbrauchsreaktion wie CO + OH CO2 + H einen großen Einfluss auf die Größe des CO- Peaks haben. Die gemessenen IDTs von THF und 2-MTHF zeigten ein NTC-Verhalten im Niedrig-Zwischen-Temperaturbereich bei 20 und 40 bar. THF erwies sich unter den untersuchten Bedingungen als reaktionsfreudiger als 2-MTHF. Außerdem wurde bei THF und 2-MTHF eine dreistufige Zündung beobachtet. Die Studie zur kinetischen Modellierung zeigte, dass das Modell von Fenard et al. 5 bessere Vorhersagen für THF und Wu et al. 7 für 2-MTHF lieferte. Die hemmende Reaktion wie HO2 + OH = H2O + O2 und die Explosionsreaktion CO + OH = CO2 + H waren die Hauptfaktoren, die zu mehreren Zündstufen führten
Uma ferramenta de apoio ao processo de aprendizagem de algoritmos
No full textDissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico. Programa de Pós-graduação em Ciência da Computaçã
Pore structure characterization of low permeability rocks
No full textTese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Ciência e Engenharia de Materiais, Florianópolis, 2014.Hoje as pesquisas em rochas de baixa permeabilidade (grande tendência no mundo e em breve na indústria petrolífera brasileira) se voltam à escala de poros seja para investigação petrofísica, morfológica, de distribuição de tamanhos de grãos ou poros ou escoamento de fluidos, prática descrita pelos valores de permeabilidade. A avaliação destas propriedades por sua vez, é essencial ao desenvolvimento e exploração de reservas de hidrocarbonetos. No entanto, a determinação de parâmetros do sistema poroso nessas rochas, arenitos de baixa permeabilidade (TGS) e rochas selantes (SR), continua a ser um grande desafio devido à extrema variabilidade de ambientes deposicionais e complexa microestrutura composta por argilas e tamanhos de poros de submícrons a ångströms. Nesta tese empregou-se um conjunto de técnicas experimentais para a caracterização da estrutura porosa de TGS e SR. De tal modo, o trabalho foi dividido em dois tópicos principais: (i) Caracterização do sistema poroso e propriedades petrofísicas em TGS utilizando-se as técnicas de permeabilidade por decaimento de pulso (PDP), NMR de baixo campo, adsorção gasosa N2 (N2GA), porosimetria por intrusão Hg (MICP), nano- e microtomografia de raios X (res. Abstract : Nowadays, significant research effort in low-permeability rocks (a wide tendency elsewhere and soon in the Brazilian petroleum industry) has been focused on pore-scale petrophysics, morphologies and distributions, as well as fluid flow circulation described by the values of permeability. The evaluation of these properties in turn is essential for the assessment and exploitation of hydrocarbon reserves; however, determining pore system parameters in such rocks as tight gas sandstones (TGS) and seal rocks (SR) remains challenging because of the extreme variability in depositional environments resulting in complex pore structures comprised by clays and length scales from sub-microns to Angstroms. In this work we applied a set of techniques to characterize submicron-pore structures in TGS and SR. Therefore it was divided into two main topics of interest: (i) Characterization of petrophysical properties and pore systems in very low permeability TGS using Pulse-Decay Permeability (PDP), Low Field Nuclear Magnetic Resonance (LFNMR), Nitrogen Gas Adsorption (N2GA), Mercury Intrusion Capillary Pressure (MICP) and Multi-scale 3D X-ray Nano- and MicroCT (down to 0.7 µm resolution) techniques; (ii) Study of Photoacoustic Spectrometry (PAS) for determining thermal diffusivity (TD) and porosity in three seal rocks originating from dissimilar fields as a key issue for safe exploration, storage purposes (CO2 sequestration) and developments in shale characterization. The values obtained for TD were between 0.01667 and 0.09298 (cm2/s) while porosity ranged from 1.42 to 9%. For the analyzed TGS the 3D pore-structure characterization lead to pore tortuosity and shape factors ranges of 2.19-5.47 and 3.2-8.5, respectively, and pore size distributions tended to be bimodal for MICP, trimodal for 3D multi-scale and tetramodal for LFNMR measurements. The porosity values ranged from 1.94 to 11.96% obtained by the combination of N2GA and MICP techniques and permeability from 0.036 to 0.00066 mD by PDP technique. The measured pore-structure parameters were also used to predict empirical permeability in TGS (using e.g. Carman-Kozeny (Dullien, 1992) and Coates (1999) models). The set of applied methods has shown to be a useful tool for the unconventional reservoir characterization since it allows obtaining pore morphological and quantitative parameters which account for the permeability values
An experimental and kinetic modeling study of 2-methyltetrahydrofuran flames
No full textMoshammer K, Vranckx S, Chakravarty HK, Parab P, Fernandes RX, Kohse-Höinghaus K. An experimental and kinetic modeling study of 2-methyltetrahydrofuran flames. COMBUSTION AND FLAME. 2013;160(12):2729-2743
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