126 research outputs found
Computations on Endohedral Metallofullerenes: Characterization, Properties and Growth
No full textEls ful•lerens són caixes closes de carboni formades per un nombre parell d’àtoms. Una de les propietats mes interessants és la capacitat d’encapsular àtoms i petites molècules en la cavitat interior. El primer ful•lerè endoèdric fou proposat just després del descobriment del C60. Els metal•loful•lerens endoèdrics han atret atenció de gran part de la comunitat científica per les seves propietats i potencials aplicacions en camps com la medicina o la ciència de materials. En aquesta tesi s’hi mostra un extensiu treball combinant experiments i computació per a la caracterització i modelatge de noves especies de metal•loful•lerens endoèdrics. En la majoria dels casos, la síntesi de noves espècies necessita d’aquesta combinació d’experiments i computació per poder la identificació i caracterització de les estructures. Així doncs, aquí es presenta el treball per l’estudi de les propietats i caracterització des de sistemes petits com Ti@C2n (2n =26-50) i altres endoedres monometàl•lics (M@C2n), així com l’estudi detallat dels sistemes Sc2S@C70, Sc2S@C72, Ti2S@C78 i la modelització d’algunes de les seves propietats mes rellevants.Los fulerenos son poliedros esféricos formados por un nombre par de átomos de carbono distribuidos en pentágonos y hexágonos. Una de sus propiedades más atractiva es la capacidad de atrapar átomos y pequeñas moléculas en su interior. Estos fueron descubiertos rápidamente después del descubrimiento del C60. El Sc3N@C80 es el fulereno endoédrico más abundante, y el tercer sistema más abundante en toda la familia de fulerenos, solo por detrás de C60 y C70. Los fulerenos endoédricos han captado la atención de gran parte de la comunidad científica debido a sus propiedades y potenciales aplicaciones en campos como la medicina y la ciencia de materiales. La caracterización de nuevas especies es difícil debido al bajo rendimiento, por eso, la combinación de experimentos con trabajos computacionales es esencial para la exitosa identificación de nuevos sistemas. En esta tesis se incluye un extenso trabajo combinando estudios computacionales con varios tipos de experimentos. Entre las especies estudiadas y caracterizadas se encuentran desde los pequeños fulerenos monometálicos Ti@C2n (2n=26-50), hasta especies más grandes cómo Sc2S@C70, Sc2S@C72 y [email protected] are closed carbon cages constituted by an even number of atoms. One of the attractive properties of the hollow carbon clusters is the possibility to use them as robust containers for other species. The first proposal of an endohedral fullerene was given only a few days after the discovery of C60. Endohedral metallofullerenes have attracted the attention of the scientific community not only because their unique host-guest behaviors, but also because the properties and, thus, the applications are significantly different from those of the empty cages. The formal electron transfer that has been found to happen between the trapped unit and the carbon cages is determinant for the understanding of these new properties. Herein we report an extensive study combining computations and experiments. This combination is a powerful tool for the structural characterization of new species. The computation of the structures and the modeling of the properties, allow us to compare the experimental and computational data to identify the new systems. In this thesis the reader will find a complete study from small endohedrals, Ti@C2n (2n=26-50) and other M@C2n, to larger fullerenes as Sc2S@C70, Sc2S@C72 and Ti2S@C78
Risk analysis for reuse of a Dutch natural gas transmission pipeline for 100% hydrogen transport
No full textAs an alternative energy carrier alongside electricity, hydrogen is considered one of the key factors in the impending required changes in the energy system transition. Due to the decrease in natural gas extraction in the Netherlands, the reuse of the natural gas network which eventually will become available for hydrogen transmission has emerged as a topic, recently. Although it seems to be possible to reuse the existing natural gas transmission network for the transmission of hydrogen, a quantitative risk analysis is still lacking and there is uncertainty about the safety of hydrogen transmission. This report aims to quantify the risk of the reuse of an existing natural gas pipeline for 100% hydrogen transmission and compare it with the natural gas case. A risk analysis entails the quantification of both the probability of failure and the adverse consequences. This research investigated the current methodology to determine the probability of failure for hydrogen transmission and proposed a new calculation method using a Bayesian network and historical natural gas data to derive new failure frequencies. The consequence of failure for both hydrogen and natural gas will be modelled using the SAFETI-NL; the influence of the gas different characteristics on the lethality will be quantified and risk values are produced, assuming the probability of failure to be similar for both gases when transported through natural gas pipelines. This research illustrated the promising application of a Bayesian network is a promising method to derive failure frequencies while taking into the significant influence of the pipeline characteristics. However, from the results, it emerges that further research is required with more redundant data. Also, future research should investigate the application of natural gas data since some failure scenarios are not entirely independent of the transmission gas, which could result in less favourable failure frequencies. Furthermore, calculations in SAFETI-NL have shown that hydrogen release is associated with a shorter duration and lower release power than natural gas, resulting in less significant consequences in the vicinity of the pipelines. Also, the low weight and high diffusive coefficient of hydrogen cause the consequences to decrease much faster with distance. As a result, there is concluded that hydrogen transmission is accompanied by lower risk values than natural gas, assuming the failure frequencies to be similar when both gases transported through natural gas pipelines. Although, numerous input parameters affect the risk values determined, of which at least the probability of ignition for hydrogen release has to be investigated in future research.Electrical Engineering | Sustainable Energy Technolog
Experimental Study of Pressure and Main Gas Ingestion Distributions in a Model Rotor-Stator Disk Cavity
No full textabstract: Ingestion of high temperature mainstream gas into the rotor-stator cavities of a gas turbine is one of the major problems faced by the turbine designers. The ingested gas heats up rotor disks and induces higher thermal stresses on them, giving rise to durability concern. Ingestion is usually reduced by installing seals on the rotor and stator rims and by purging the disk cavity by secondary air bled from the compressor discharge. The geometry of the rim seals and the secondary air flow rate, together, influence the amount of gas that gets ingested into the cavities. Since the amount of secondary air bled off has a negative effect on the gas turbine thermal efficiency, one goal is to use the least possible amount of secondary air. This requires a good understanding of the flow and ingestion fields within a disk cavity. In the present study, the mainstream gas ingestion phenomenon has been experimentally studied in a model single-stage axial flow gas turbine. The turbine stage featured vanes and blades, and rim seals on both the rotor and stator. Additionally, the disk cavity contained a labyrinth seal radially inboard which effectively divided the cavity into a rim cavity and an inner cavity. Time-average static pressure measurements were obtained at various radial positions within the disk cavity, and in the mainstream gas path at three axial locations at the outer shroud spread circumferentially over two vane pitches. The time-average static pressure in the main gas path exhibited a periodic asymmetry following the vane pitch whose amplitude diminished with increasing distance from the vane trailing edge. The static pressure distribution increased with the secondary air flow rate within the inner cavity but was found to be almost independent of it in the rim cavity. Tracer gas (CO2) concentration measurements were conducted to determine the sealing effectiveness of the rim seals against main gas ingestion. For the rim cavity, the sealing effectiveness increased with the secondary air flow rate. Within the inner cavity however, this trend reversed -this may have been due to the presence of rotating low-pressure flow structures inboard of the labyrinth seal.Dissertation/ThesisM.S. Mechanical Engineering 201
Hypercrosslinked Additives for Ageless Gas‐Separation Membranes
No full textThe loss of internal pores, a process known as physical aging, inhibits the long-term use of the most promising gas-separation polymers. Previously we reported that a porous aromatic framework (PAF-1) could form a remarkable nanocomposite with gas-separation polymers to stop aging. However, PAF-1 synthesis is very onerous both from a reagent and reaction-condition perspective, making it difficult to scale-up. We now reveal a highly dispersible and scalable additive based on α,α'-dichloro-p-xylene (p-DCX), that inhibits aging more effectively, and crucially almost doubles gas-transport selectivity. These synergistic effects are related to the intimately mixed nanocomposite that is formed though the high dispersibility of p-DCX in the gas-separation polymer. This reduces particle-size effects and the internal free volume is almost unchanged over time. This study shows this inexpensive and scalable polymer additive delivers exceptional gas-transport performance and selectivity
Experimental Study of Main Gas Ingestion in a Subscale 1.5-stage Axial Flow Air Turbine
No full textabstract: Gas turbine efficiency has improved over the years due to increases in compressor
pressure ratio and turbine entry temperature (TET) of main combustion gas, made viable
through advancements in material science and cooling techniques. Ingestion of main
combustion gas into the turbine rotor-stator disk cavities can cause major damage to the
gas turbine. To counter this ingestion, rim seals are installed at the periphery of turbine
disks, and purge air extracted from the compressor discharge is supplied to the disk
cavities. Optimum usage of purge air is essential as purge air extraction imparts a penalty on turbine efficiency and specific fuel consumption.
In the present work, experiments were conducted in a newly constructed 1.5-stage
axial flow air turbine featuring vanes and blades to study main gas ingestion. The disk
cavity upstream of the rotor, the 'front cavity', features a double seal with radial clearance
and axial overlap at its rim. The disk cavity downstream of the rotor, the 'aft cavity', features a double seal at its rim but with axial gap. Both cavities contain a labyrinth seal radially inboard; this divides each disk cavity into an 'inner cavity' and a 'rim cavity'.
Time-averaged static pressure at various locations in the main gas path and disk
cavities, and tracer gas (CO2) concentration at different locations in the cavities were
measured. Three sets of experiments were carried out; each set is defined by the main air flow rate and rotor speed. Each of the three sets comprises of four different purge air flow rates, low to high.
The mass flow rate of ingested main gas into the front and aft rim cavities is
reported at the different purge air flow rates, for the three experiment sets. For the present stage configuration, it appears that some ingestion persisted into both the front and aft rim cavities even at high purge air flow rates. On the other hand, the front and aft inner cavity were completely sealed at all purge flows.Dissertation/ThesisMasters Thesis Engineering 201
Industry structure and regulation
No full textAs private firms become increasingly involved in the development of key infrastructure, redefining the role of government from that of serviceprovider to regulator presents both challenges and opportunities. The factors that give rise to sector reforms color how much policymakers invest in regulatory design during the reform process. Nevertheless, two factors are essential to sustainable sector and regulatory reform. First, the right structure must be established for the industry concerned, a structure that allows competition appropriate for that industry. Second, the objectives of regulation must be well defined, with a clear distinction between policymaking, policy implementation, and operations. The extent to which competition can be harnessed to help make regulation efficient, effective, and sustainable depends on the intrinsic technical characteristics of the sector. Each decision affects the sustainability of the regulatory regime in the face of the threat of regulatory capture (both political and commercial). Careful regulatory design is crucial not only for successful sectoral reform but also to balance the interests of various actors (government, consumers, developers, investors, and financiers). One model that has been relatively successful combines new entry, unbundled services, and the unambiguous spelling out of the legal rights and duties for both public and private service providers, administered by an autonomous regulatory authority. Problems with regulation often result as much from inadequate attention to sector structure and fostering competition as from weaknesses in the regulatory authority's institutional capacity. As for the tools of regulation, despite differences in some details between licenses and concessions (and their many contractual variations), these are basically instruments that establish the rights and obligations of contracting parties. Choices about where these rights and obligations are located in the legal hierarchy are shaped by a country's institutional capacity and legal traditions. But the existence of instruments to establish those rights and obligations does not eliminate the need for institutionsto administer them, and thus carry out the regulatory function. Establishing effective sectorwide regulation can be difficult in a developing country, but it is necessary. Policymakers will be able to create effective regulatory regimes where adequate attention is given to sector structure, competition, and institution-building.Environmental Economics&Policies,Trade Finance and Investment,Knowledge Economy,ICT Policy and Strategies,Decentralization,Environmental Economics&Policies,Administrative&Regulatory Law,ICT Policy and Strategies,Water and Industry,Knowledge Economy
Hypercrosslinked additives for ageless gas-separation membranes
No full textThe loss of internal pores, a process known as physical aging, inhibits the long-term use of the most promising gas-separation polymers. Previously we reported that a porous aromatic framework (PAF-1) could form a remarkable nanocomposite with gas-separation polymers to stop aging. However, PAF-1 synthesis is very onerous both from a reagent and reaction-condition perspective, making it difficult to scale-up. We now reveal a highly dispersible and scalable additive based on α,α′-dichloro-p-xylene (p-DCX), that inhibits aging more effectively, and crucially almost doubles gas-transport selectivity. These synergistic effects are related to the intimately mixed nanocomposite that is formed though the high dispersibility of p-DCX in the gas-separation polymer. This reduces particle-size effects and the internal free volume is almost unchanged over time. This study shows this inexpensive and scalable polymer additive delivers exceptional gas-transport performance and selectivity
INVESTIGATION INTO FACTORS CONTRIBUTING TO THE PERFORMANCE OF HOUSEHOLD LIQUEFIED PETROLEUM GAS IN DURBAN
No full textDECLARATION: This dissertation represents the original work of the author and has not been submitted to this or any other University. Wherever use was made of work of others, it was duly acknowledged in the text. Background The Liquefied Petroleum Gas (LPG) cylinder market consist of a group of consumers with similar yet differentiated needs, namely commercial customers (small industries, restaurants, wholesalers and nature reserves), low-income customers (such as the rural and township households with a monthly income ofR748- R2288) and high-income customers (such as the urban households with a monthly income above R9743). LPG is mainly used for cooking, lighting, heating, leisure and as a preferred form of energy in the commercial segment. The key issues currently facing the industry are: minimum growth, high capital requirement, rising cost of product and squeezmg profitability. Objective
Effect of atmospheric boundary layer inflow and wind yaw angle on the aerodynamics of heavy duty vehicles and attached boat-tails
No full textGreenhouse gas emissions have globally increased temperatures and have led to global warming. A major part of these greenhouse gas emissions is emitted by motor vehicles in Europe. With an aim to limit global temperature increase by targeting emissions from heavy duty vehicles (HDV) which travel long-haul international journeys, this thesis aims to study the aerodynamics of heavy duty vehicles subjected to the inflow of an atmospheric boundary layer approaching the vehicle at a non-zero wind yaw angleAerospace Engineerin
Molecular dynamics as a tool to study heterogeneity in zeolites - Effect of Na cations on diffusion of CO and N in Na-ZSM-5
No full textZeolites typically contain extra-framework cations to charge-compensate for trivalent Al atom substitutions in the SiO framework. These cations, such as Na, directly interact with quadrupolar guest molecules, such as CO and N, which move through their micropores, causing energetic heterogeneity. To assess the effects of heterogeneity in Na-ZSM-5 on diffusion of CO and N, molecular dynamics (MD) simulations are carried out. In silicalite-1, the pure-silicon form of ZSM-5, the self-diffusivity exhibits a monotonic decrease with molecular loading, while the corrected diffusivity shows a relatively constant value. In contrast, the Na cations cause a maximum or a flat profile over molecular loading for the self- and corrected diffusivities of CO at T=200 and 300K, while the cations only have minimal impact on the diffusivity of N. The MD simulations allow us to identify energy basins or sites at which guest molecules spend a relatively long time, and construct a coarse-grained lattice representation for the pore network. Average residence times at these sites are calculated for both species. The trends observed in the residence times correlate to the trends observed in the diffusivity. The residence times for CO at T=200K are long at low loading, but decrease with loading as additional CO molecules compete to stay close to a cation. In contrast, the residence times for N are relatively insensitive to the cations, only mildly increasing near a cation. This difference in behavior can be associated to the quadrupole moments of these molecules
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