• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 84
  • 22
  • 12
  • 8
  • 2
  • 1
  • 1
  • 1
  • 1
  • 1
  • Tagged with
  • 170
  • 60
  • 47
  • 42
  • 31
  • 27
  • 23
  • 23
  • 21
  • 19
  • 18
  • 18
  • 17
  • 17
  • 17
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
61

Measurements and modeling of turbulent consumption speeds of syngas fuel blends

Venkateswaran, Prabhakar 19 February 2013 (has links)
Increasingly stringent emission requirements and dwindling petroleum reserves have generated interest in expanding the role of synthesis gas (syngas) fuels in power generation applications. Syngas fuels are the product of gasifying organic-based feedstock such as coal and biomass and are composed of mainly H₂ and CO. However, the use of syngas fuels in lean premixed gas turbine systems has been limited in part because the behavior of turbulent flames in these mixtures at practical gas turbine operating conditions are not well understood. This thesis presents an investigation of the influence of fuel composition and pressure on the turbulent consumption speed, ST,GC, and the turbulent flame brush thickness, FBT, for these mixtures. ST,GC and FBT are global parameters which represent the average rate of conversion of reactants to products and the average heat release distribution of the turbulent flame respectively. A comprehensive database of turbulent consumption speed measurements obtained at pressures up to 20 atm and H₂/CO ratios of 30/70 to 90/10 by volume is presented. There are two key findings from this database. First, mixtures of different H₂/CO ratios but with the same un-stretched laminar flame speeds, SL,0, exposed to the same turbulence intensities, u'rms , have different turbulent consumption speeds. Second, higher pressures augment the turbulent consumption speed when SL,0 is held constant across pressures and H₂/CO ratios. These observations are attributed to the mixture stretch sensitivities, which are incorporated into a physics-based model for the turbulent consumption speed using quasi-steady leading points concepts. The derived scaling law closely resembles Damkhler's classical turbulent flame speed scaling, except that the maximum stretched laminar flame speed, SL,max, arises as the normalizing parameter. Scaling the ST,GC data by SL,max shows good collapse of the data at fixed pressures, but systematic differences between data taken at different pressures are observed. These differences are attributed to non-quasi-steady chemistry effects, which are quantified with a Damkhler number defined as the ratio of the chemical time scale associated with SL,max and a fluid mechanic time scale. The observed scatter in the normalized turbulent consumption speed data correlates very well with this Damkhler number, suggesting that ST,GC can be parameterized by u'rms/SL,max and the leading point Damkhler number. Finally, a systematic investigation of the influence of pressure and fuel composition on the flame brush thickness is presented. The flame brush thickness is shown to be independent of the H₂/CO ratio if SL,0 is held constant across the mixtures. However, increasing the equivalence ratio for lean mixtures at a constant H₂/CO ratio, results in a thicker flame brush. Increasing the pressure is shown to augment the flame brush thickness, a result which has not been previously reported in the literature. Classical correlations based on turbulent diffusion concepts collapse the flame brush thickness data obtained at fixed u'rms/U₀ and pressure reasonably well, but systematic differences exist between the data at different u'rms/U₀ and pressures.
62

Numerical Simulation of Flame-Vortex Interactions in Natural and Synthetic Gas Mixtures

Weiler, Justin D. 17 August 2004 (has links)
The interactions between laminar premixed flames and counter-rotating vortex pairs in natural and synthetic gas mixtures have been computationally investigated through the use of Direct Numerical Simulations and parallel processing. Using a computational model for premixed combustion, laminar flames are simulated for single- and two-component fuel mixtures of methane, carbon monoxide, and hydrogen. These laminar flames are forced to interact with superimposed laminar vortex pairs, which mimic the effects of a pulsed, two-dimensional slot-injection. The premixed flames are parameterized by their unstretched laminar flame speed, heat release, and flame thickness. The simulated vortices are of a fixed size (relative to the flame thickness) and are parameterized, solely, by their rotational velocity (relative to the flame speed). Strain rate and surface curvature measurements are made along the stretched flame surfaces to study the effects of additive syngas species (CO and H2) on lean methane-air flames. For flames that share the same unstretched laminar flame speed, heat release, and flame thickness, it is observed that the effects of carbon monoxide on methane-air mixtures are essentially negigible while the effects of hydrogen are quite substantial. The dynamics of stretched CH4/Air and CH4/CO/Air flames are nearly identical to one another for interactions with both strong and weak vortices. However, the CH4/H2/Air flames demonstrate a remarkable tendency toward surface area growth. Over comparable interaction periods, the flame surface area produced during interactions with CH4/H2/Air flames was found to be more than double that of the pure CH4/Air flames. Despite several obvious differences, all of the interactions revealed the same basic phenomena, including vortex breakdown and flame pinch-off (i.e. pocket formation). In general, the strain rate and surface curvature magnitudes were found to be lower for the CH4/H2/Air flames, and comparable between CH4/Air and CH4/CO/Air flames. Rates of flame stretching are not explicitely determined, but are, instead, addressed through observation of their individual components. Two different models are used to determine local displacement speed values. A discrepancy between practical and theoretical definitions of the displacement speed is evident based on the instantaneous results for CH4/Air and CH4/H2/Air flames interacting with weak and strong vortices.
63

Density functional theory study of alcohol synthesis reactions on alkali-promoted Mo2C catalysts

Li, Liwei 08 June 2015 (has links)
As an important chemical raw material, alcohols can be used as fuels, solvents and chemical feedstocks to produce a variety of downstream products. With limited fossil fuel resources, alcohol synthesis from syngas reactions can be a potential alternative to the traditional petroleum based alcohol synthesis. Among many catalysts active for syngas to alcohol processes, alkali promoted Mo2C has shown promising performance. More interestingly, the alkali promoter was found to play an important role in shifting the reaction selectivity from hydrocarbons to alcohols. However, limited understanding of the mechanism of this alkali promoter effect is available due to the complexity of syngas reaction mechanism and low content of alkali added to the catalysts. In this thesis, we performed a comprehensive investigation of the alkali promoter effect with density functional theory (DFT) calculations as our primary tool. We first examine various Mo2C surfaces to determine a representative surface structure active to alkali adsorption. On this particular surface, we develop a syngas reaction network including relevant reaction mechanisms proposed in previous literature. With energetics derived from DFT calculations and a BEP relation, we predict the syngas reaction selectivity and find it to be in excellent agreement with experimental results. The dominant reaction mechanism and selectivity determining steps are determined from sensitivity analysis. We also propose a formation mechanism of alkali promoters on Mo2C catalysts that shows consistency between experimental IR and DFT computed vibrational frequencies. Finally, the effect of alkali promoters on the selectivity determining steps for syngas reactions are investigated from DFT calculations and charge analysis. We are able to rationalize the role of alkali promoters in shifting the reaction selectivity from hydrocarbons to alcohols on Mo2C catalysts.
64

Superadiabatic combustion in counter-flow heat exchangers

Schoegl, Ingmar Michael 22 March 2011 (has links)
Syngas, a combustible gaseous mixture of hydrogen, carbon monoxide, and other species, is a promising fuel for efficient energy conversion technologies. Syngas is produced by breaking down a primary fuel into a hydrogen-rich mixture in a process called fuel reforming. The motivation for the utilization of syngas rather than the primary fuel is that syngas can be used in energy conversion technologies that offer higher conversion efficiencies, e.g. gas turbines and fuel cells. One approach for syngas production is partial oxidation, which is an oxygen starved combustion process that does not require a catalyst. Efficient conversion to syngas occurs at high levels of oxygen depletion, resulting in mixtures that are not flammable in conventional combustion applications. In non-catalytic partial oxidation, internal heat recirculation is used to increase the local reaction temperatures by transferring heat from the product stream to pre-heat the fuel/air mixture before reactions occur, thus increasing reaction rates and allowing for combustion outside the conventional flammability limits. As peak temperatures lie above the adiabatic equilibrium temperature predicted by thermodynamic calculations, the combustion regime used for non-catalytic fuel reforming is referred to as 'superadiabatic'. Counter-flow heat exchange is an effective way to transfer heat between adjacent channels and is used for a novel, heat-recirculating fuel reformer design. An analytical study predicts that combustion zone locations inside adjacent flow channels adjust to operating conditions, thus stabilizing the process for independent variations of flow velocities and mixture compositions. In experiments, a reactor prototype with four channels with alternating flow directions is developed and investigated. Tests with methane/air and propane/air mixtures validate the operating principle, and measurements of the resulting syngas compositions verify the feasibility of the concept for practical fuel-reformer applications. Results from a two-dimensional numerical study with detailed reaction chemistry are consistent with experimental observations. Details of the reaction zone reveal that reactions are initiated in the vicinity of the channel walls, resulting in "tulip"-shaped reaction layers. Overall, results confirm the viability of the non-catalytic reactor design for fuel reforming applications. / text
65

Anode materials for H2S containing feeds in a solid oxide fuel cell

Roushanafshar, Milad Unknown Date
No description available.
66

Extinction Limits of Laminar Diffusion Counterflow Flames of Various Gaseous Fuels including Syngas and Biogas

Kwan, Timothy 29 November 2013 (has links)
This work investigates the extinction limits of laminar diffusion counterflow flames for various gaseous (methane, syngas, biogas) fuels using a high flow rate counterflow burner designed and built for this work. Equal momenta of the fuel and oxidizer streams were not maintained to provide data to check the fidelity of the numerical schemes and their chemical mechanisms at "non-standard" conditions. Strain rate values at extinction were obtained as a function of fuel mole fraction. Preliminary work with the new burner found that the methane extinction limit results were consistent with results from literature. The results provide insight into the extinction limit conditions of the aforementioned fuels. The strain rate was found to increase with increasing fuel mole fraction. Extinction limit results indicated that fuels with the highest concentration of hydrogen have the greatest extinction limit, which is believed to be attributed to the high diffusivity and reactivity of hydrogen.
67

Extinction Limits of Laminar Diffusion Counterflow Flames of Various Gaseous Fuels including Syngas and Biogas

Kwan, Timothy 29 November 2013 (has links)
This work investigates the extinction limits of laminar diffusion counterflow flames for various gaseous (methane, syngas, biogas) fuels using a high flow rate counterflow burner designed and built for this work. Equal momenta of the fuel and oxidizer streams were not maintained to provide data to check the fidelity of the numerical schemes and their chemical mechanisms at "non-standard" conditions. Strain rate values at extinction were obtained as a function of fuel mole fraction. Preliminary work with the new burner found that the methane extinction limit results were consistent with results from literature. The results provide insight into the extinction limit conditions of the aforementioned fuels. The strain rate was found to increase with increasing fuel mole fraction. Extinction limit results indicated that fuels with the highest concentration of hydrogen have the greatest extinction limit, which is believed to be attributed to the high diffusivity and reactivity of hydrogen.
68

Application de la diffusion Rayleigh induite par laser à la caractérisation des fronts de flamme laminaire de prémélange H2/CH4/Air et H2/CO/Air

Ponty, Ludovic 14 June 2011 (has links) (PDF)
Ce travail de Thèse est consacré à la caractérisation de la structure thermique des fronts de flammelaminaire de prémélange H2/CH4/Air et H2/CO/Air pauvres. L'étude a été réalisée sur un brûleur à jets opposés, permettant de stabiliser des flammes planes stationnaires, dans des conditions quasi-adiabatiques, pour différentes conditions d'étirement. Un diagnostic de Vélocimétrie par Imagerie de Particule (PIV) et un diagnostic bidimensionnel de diffusion Rayleigh induite par laser ont été utilisés successivement pour étudier l'influence de la richesse, de la concentration en hydrogène dans le combustible et de l'étirement sur le profil de température normal au front de flamme. Trois grandeurs fondamentales ont été étudiées : la température des gaz brûlés, le gradient maximum de température et l'épaisseur de flamme au sens de Spalding. Une attention particulière a été portée à l'interprétation du signal Rayleigh. Ce dernier dépendant notamment de la composition du gaz qui évolue à travers le front de flamme. Dans ce travaille de thèse, cette évolution a été évaluée numériquement (simulations 1D : CANTERA et OPPDIF) puis prise en compte pour améliorer le traitement des données expérimentales. Les résultats expérimentaux couvrent une gamme de richesses s'étalant pour H2/CH4/Air et H2/CO/Air, respectivement de 0.6 à 0.8 et de 0.4 à 0.6. Les concentrations en hydrogène dans le combustible s'étalent respectivement de 0 à 50% et de 10 à 50%. Une comparaison systématique a été faite avec les résultats de simulation numérique 1D (OPPDIF).
69

Lean blowoff characteristics of swirling H2/CO/CH4 Flames

Zhang, Qingguo 05 March 2008 (has links)
This thesis describes an experimental investigation of lean blowoff for H2/CO/CH4 mixtures in a swirling combustor. This investigation consisted of three thrusts. The first thrust focused on correlations of the lean blowoff limits of H2/CO/CH4 mixtures under different test conditions. It was found that a classical Damköhler number approach with a diffusion correction could correlate blowoff sensitivities to fuel composition over a range of conditions. The second part of this thesis describes the qualitative flame dynamics near blowoff by systematically characterizing the blowoff phenomenology as a function of hydrogen level in the fuel. These near blowoff dynamics are very complex, and are influenced by both fluid mechanics and chemical kinetics; in particular, the role of thermal expansion across the flame and extinction strain rate were suggested to be critical in describing these influences. The third part of this thesis quantitatively analyzed strain characteristics in the vicinity of the attachment point of stable and near blowoff flames. Surprisingly, it was found that in this shear layer stabilized flame, flow deceleration is the key contributor to flame strain, with flow shear playing a relatively negligible role. Near the premixer exit, due to strong flow deceleration, the flame is negatively strained i.e., compressed. Moving downstream, the strain rate increases towards zero and then becomes positive, where flames are stretched. As the flame moves toward blowoff, holes begin to form in the flame sheet, with a progressively higher probability of occurrence as one moves downstream. It is suggested that new holes form with a more uniform probability, but that this behavior reflects the convection of flame holes downstream by the flow. It has been shown in prior studies, and affirmed in this work, that flames approach blowoff by first passing through a transient phase manifested by local extinction events and the appearance of holes on the flame. A key conclusion of this work is that the onset of this boundary occurs at a nearly constant extinction strain rate. As such, it is suggested that Damköhler number scalings do not describe blowoff itself, but rather the occurrence of this first stage of blowoff. Given the correspondence between this first stage and the actual blowoff event, this explains the success of classical Damköhler number scalings in describing blowoff, such as shown in the first thrust of this thesis. The physics process associated with the actual blowoff event is still unclear and remains a key task for future work.
70

S?ntese e caracteriza??o de cer?micas nanom?tricas para produ??o de g?s de s?ntese

Oliveira, Rosane Maria Pessoa Bet?nio 04 October 2010 (has links)
Made available in DSpace on 2014-12-17T14:07:04Z (GMT). No. of bitstreams: 1 RosaneMPBO_TESE_partes_autorizadas.pdf: 623811 bytes, checksum: 3724885447cd319f325ecaa9845943f0 (MD5) Previous issue date: 2010-10-04 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / In this work, ceramic powders belonging to the system Nd2-xSrxNiO4 (x = 0, 0.4, 0.8, 1.2 and 1.6) were synthesized for their use as catalysts to syngas production partial. It was used a synthesis route, relatively new, which makes use of gelatin as organic precursor. The powders were analyzed at several temperatures in order to obtain the perovskite phase and characterized by several techniques such as thermal analysis, X-rays diffraction, Rietveld refinement method, specific surface area, scanning electron microscopy, energy dispersive spectroscopy of X-rays and temperature programmed reduction. The results obtained using these techniques confirmed the feasibility of the synthesis method employed to obtain nanosized particles. The powders were tested in differential catalytic conditions for dry reforming of methane (DRM) and partial oxidation of methane (POM), then, some systems were chosen for catalytic integrals test for (POM) indicating that the system Nd2-xSrxNiO4 for x = 0, 0.4 and 1.2 calcined at 900 ?C exhibit catalytic activity on the investigated experimental conditions in this work without showing signs of deactivation / Neste trabalho, p?s cer?micos pertencentes ao sistema Nd2-xSrxNiO4 (x = 0; 0,4; 0,8; 1,2 e 1,6) foram sintetizados visando sua utiliza??o como catalisadores para produ??o de g?s de s?ntese. Foi utilizada uma rota de s?ntese, relativamente nova, na qual faz uso de gelatina como precursor org?nico. Os p?s cer?micos sintetizados foram calcinados a v?rias temperaturas visando ? obten??o da fase perovsquita e ent?o caracterizados por v?rias t?cnicas, tais como: An?lise t?rmogravim?trica, difra??o de raios X, refinamento Rietveld, ?rea espec?fica (m?todo BET), microscopia eletr?nica de varredura, espectroscopia por dispers?o de energia de raios X e redu??o ? temperatura programada. Os resultados obtidos atrav?s destas t?cnicas confirmaram a viabilidade do m?todo de s?ntese empregado para obten??o de part?culas nanom?tricas. Os p?s foram submetidos a testes catal?ticos em condi??es diferenciais para rea??o de reforma a seco (RSM) e oxida??o parcial do metano (OPM), em seguida alguns sistemas foram escolhidos para testes catal?ticos integrais de OPM indicando que o sistema Nd2-xSrxNiO4 para x = 0, 0,4 e 1,2 calcinados a 900 ?C apresentam atividade catal?tica nas condi??es experimentais investigadas nesse trabalho sem mostrar sinais de desativa??o

Page generated in 0.0351 seconds