Dissertations / Theses on the topic 'Chaos experiment'

There are still many open questions regarding spatiotemporal chaos although many well developed theories exist for chaos in time. Rayleigh-B'enard convection is a paradigmatic example of spatiotemporal chaos that is also experimentally accessible. Discoveries uncovered using numerics can often be compared with experiments which can provide new physical insights. Lyapunov diagnostics can provide important information about the dynamics of small perturbations for chaotic systems. Covariant Lyapunov vectors reveal the true direction of perturbation growth and decay. The degree of hyperbolicity can also be quantified by the covariant Lyapunov vectors. To know whether a dynamical system is hyperbolic is important for the development of a theoretical understanding. In this thesis, the degree of hyperbolicity is calculated for chaotic Rayleigh-B'enard convection. For the values of the Rayleigh number explored, it is shown that the dynamics are non-hyperbolic. The spatial distribution of the covariant Lyapunov vectors is different for the different Lyapunov vectors. Localization is used to quantify this variation. The spatial localization of the covariant Lyapunov vectors has a decreasing trend as the order of the Lyapunov vector increases. The spatial localization of the covariant Lyapunov vectors are found to be related to the instantaneous Lyapunov exponents. The correlation is stronger as the order of the Lyapunov vector decreases. The covariant Lyapunov vectors are also computed using a spectral element approach. This allows an exploration of the covariant Lyapunov vectors in larger domains and for experimental conditions. The finite conductivity and finite thickness of the lateral boundaries of an experimental convection domain is also studied. Results are presented for the variation of the Nusselt number and fractal dimension for different boundary conditions. The fractal dimension changes dramatically with the variation of the finite conductivity.
Ph. D.

Laser-cooled atoms offer an excellent platform for testing new ideas of quantum control and measurement. I will discuss experiments where we use light and magnetic fields to drive and monitor non-trivial quantum dynamics of a large spin-angular momentum associated with an atomic hyperfine ground state. We can design Hamiltonians to generate arbitrary spin states and perform a full quantum state reconstruction of the results. We have implemented and verified time optimal controls to generate a broad variety of spin states, including spin-squeezed states useful for metrology. Yields achieved are of the range 0.8-0.9.We present a first experimental demonstration of the quantum kicked top, a popular paradigm for quantum and classical chaos. We make `movies' of the evolving quantum state which provides a direct observation of phase space dynamics of this system. The spin dynamics seen in the experiment includes dynamical tunneling between regular islands, rapid spreading of states throughout the chaotic sea, and surprisingly robust signatures of classical phase space structures. Our data show differences between regular and chaotic dynamics in the sensitivity to perturbations of the quantum kicked top Hamiltonian and in the average electron-nuclear spin entanglement during the first 40 kicks. The difference, while clear, is modest due to the small size of the spin.

Les processus hors d’équilibre conduisent à la formation de structures spatiales périodiques et étendues, dites patterns. La création d’un pattern a lieu à partir de la brisure spontanée, d’une ou plus des symétries qui caractérisent l’état homogène. La variation d’un paramètre de contrôle peut conduire à la déstabilisation du pattern vers un régime de chaos spatio-temporel ou bien à sa localisation dans une région restreinte de l’espace disponible, ainsi que nous avons des structures localisées plutôt que des structures étendues. L’objectif des travaux présentés dans ce mémoire de thèse est de réaliser le contrôle aussi bine des structures localisées que du chaos spatio-temporel dans une expérience d’optique non linéaire. L’expérience consiste en une valve à cristal liquide (LCLV, de l’anglais Liquid Crystal Light Valve) insérée dans une boucle de rétroaction optique. En utilisant un modulateur spatial de lumière, nous avons démontré une méthode pour réaliser le contrôle du chaos spatio-temporel et des états localisés. Le système est modélisé sur la base d’une équation physique pour le cristal liquide, qui est couplée à la propagation de la lumière dans la boucle de rétroaction. Les études numériques de ce modèle montrent un accord quantitatif avec les observations expérimentales
Non equilibrium processes lead in nature to the formation of spatially periodic and extended structures, so-called patterns. The birth of pattern takes place through the spontaneous breaking of one or more symmetries characterizing the homogeneous state. Changing one of more parameters leads eventually to the destabilization of the pattern in favour of a regime of space-time chaos. In other cases, the pattern is localized in a particular region of the available space, so what we deal with localized instead of extended structures. The goal of the work presented in this dissertation is to realize the control of both localized and spatiotemporal chaotic states in a nonlinear optical experiment. The experiment consists in a Liquid Crystal Light Valve (LCLV) inserted in a optical feedback loop. By means of a spatial light modulator, inserted in the optical path of the input beam, we have realized the control space-time chaos and localized structures. The system is modelled on the basis of a liquid crystals physical equation, which is coupled to the light propagation in the feedback loop. Numerical studies of the model are performed, showing quantitative agreements with the experimental findings

Construímos um arranjo experimental para o estudo de ressonadores acústicos, que tem sido considerados como análogos clássicos de bilhares quânticos. O equipamento mantém estabilidade durante vários dias, o que é uma condição necessária para a obtenção de espectros de autofreqüências com a resolução requerida para a caracterização precisa destes sistemas. Caracterizamos 7 amostras, que são placas de alumínio com espessura < 2 mm e que possuem as seguintes geometrias: dois estádios de Sinai, com e sem dessimetrização planar; três triângulos sendo um equilátero, um retângulo e outro escaleno, este com todos os ângulos agudos e irracionais em unidades de ; além de duas amostras circulares, com e sem dessimetrização planar. Observamos que três amostras apresentam estatísticas GOE, uma 2GOE, uma semi-Poisson, uma Poisson com perda de níveis, e outra aparentemente intermediária entre a GOE e a 2GOE, que nao foi possível classificar. A qualidade dos dados também permitiu a obtenção das energias dos espectros, onde obtivemos resultados coerentes com a classificação a
We have built an experimental apparatus to study acoustic resonators which have been considered as classical analogs of quantum billiards. The equipment was able to keep the stability during several days, which is a requirement to the precise eigenfrequency measurements allowing a characterization of the systems. We have characterized 7 samples made of aluminum plates with thickness smaller than 2 mm having the following geometries: two Sinai\'s stadiums (with and without planar symmetry), an equilateral triangle, a rectangle triangle, and a scalene triangle with three acute and irrational angles, and two circular shaped samples, with and without planar symmetry. We observed that three of the samples followed the GOE statistics (the asymmetrical Sinai stadium, the rectangle triangle and the scalene one). The asymmetrical Sinai stadium was described by 2GOE statistics, the equilateral triangle by the semi-Poisson, the symmetrical circle by a Poisson with missing levels and the asymmetrical circle has statistics apparently between 1GOE and 2GOE which was not possible to classify. The high quality of data allowed us to calculate the spectra energies and we found these results compatible with the previous one.

Computer simulation of real world phenomena is now ubiquitous in science, because experimentation in the field can be expensive, time-consuming, or impossible in practice. Examples include climate science, where future climate is examined under global warming scenarios, and cosmology, where the evolution of galaxies is studied from the beginning of the universe to present day. Combining complex mathematical models and numerical procedures to solve them in a computer program, these simulators are computationally expensive, in that they can take months to complete a single run. The practice of using a simulator to understand reality raises some interesting scientific questions, and there are many sources of uncertainty to consider. For example, the discrepancy between the simulator and the real world process. The field of uncertainty quantification is concerned with the characterisation and reduction of all uncertainties present in computational and real world problems. A key bottleneck in any uncertainty quantification analysis is the cost of evaluating the simulator. The solution is to replace the expensive simulator with a surrogate model, which is computationally faster to run, and can be used in subsequent analyses. Polynomial chaos and Gaussian process emulation are surrogate models developed independently in the engineering and statistics communities respectively over the last 25 years. Despite tackling similar problems in the field, there has been little interaction and collaboration between the two communities. This thesis provides a critical comparison of the two methods for a range of criteria and examples, from simple test functions to simulators used in industry. Particular focus is on the approximation accuracy of the surrogates under changes in the size and type of the experimental design. It is concluded that one method does not unanimously outperform the other, but advantages can be gained in some cases, such that the preferred method depends on the modelling goals of the practitioner. This is the first direct comparison of polynomial chaos and Gaussian process emulation in the literature. This thesis also proposes a novel methodology called probabilistic polynomial chaos, which is a hybrid of polynomial chaos and Gaussian process emulation. The approach draws inspiration from an emerging field in scientific computation known as probabilistic numerics, which treats classical numerical methods as statistical inference problems. In particular, a probabilistic integration technique called Bayesian quadrature, which employs Gaussian process emulators, is applied to a traditional form of polynomial chaos. The result is a probabilistic version of polynomial chaos, providing uncertainty information where the simulator has not yet been run.

L'objectif de cette thèse est de traiter la problématique de la caractérisation et du traitement de la variabilité de l'exposition humaine aux ondes radio à travers l'utilisation de la dosimétrie numérique. En effet, si les progrès dans le domaine du calcul hautes performances ont contribué à significativement réduire les temps de simulation pour l'évaluation de l'exposition humaine, ce calcul du débit d'absorption spécifique reste un processus coûteux en temps. Avec la grande variabilité des usages, cette contrainte fait que la prise en compte de l'influence de paramètres d'entrée aléatoires sur l'exposition ne peut se faire par des méthodes classiques telles que les simulations de Monte Carlo. Nous proposons dans ces travaux deux approches pour répondre à cette problématique. La première s'appuie sur l'utilisation et l'hybridation de méthodes de construction de modèles de substitution afin d'étudier l'influence globale des paramètres d'entrée. La deuxième vise à l'évaluation efficace et parcimonieuse des quantiles à 95% des distributions de sortie et s'appuie sur le développement d'une méthode de planification d'expériences adaptative et orientée couplée à la construction de modèles de substitution. Les méthodes proposées dans ce manuscrit sont comparées et testées sur des exemples analytiques et ensuite appliquées à des problèmes concrets issus de la dosimétrie numérique
The purpose of this thesis is to deal with the problem of the management and the characterization of the variability of the human exposure to radio frequency waves through the use of the numerical dosimetry. As a matter of fact, if the recent advances in the high performance computing domain led to reduce significantly the simulation duration for the evaluation of the human exposure, this computation of the specific absorption rate remains a time-consuming process. With the variability of the usage, this constraint does not allow the analysis of the influence of random input parameters on the exposure to be achieved with classical approaches such as Monte Carlo simulations. In this work, two approaches are proposed to address this problem. The first one is based on the use and the hybridization of construction methods of surrogate models in order to study the global influence of the input parameters. The second one aims at assessing efficiently the 95th-percentiles of the output distributions in a parcimonous way. It is based on the development of an adaptive and oriented methodology of design of experiments combined with the construction of surrogate models. In this manuscript, the proposed methods are compared and tested on analytical examples and then applicated to full-scale problems from the numerical dosimetry

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The present work proposes a study of the dynamics of a gyroscope using simulated data of an analytical model by comparing with experimental data. Classical mechanical modeling approaches are used to identify the equilibrium points, stability and verification of the regions where the motion equations of the gyroscope can present regular or chaotic behavior. The Lyapunov exponents are identified through the standard method, Eckmann-Ruelle Method, Wolf method with time series and the 0-1 test. The results achieved illustrate the main advantages and drawbacks of each method and allow to observe qualitatively and quantitatively information about the motion of the gyroscope used.
Este trabalho propõe um estudo da dinâmica de um giroscópio usando dados de simulação de um modelo analítico comparando com dados experimentais. Verifica-se a modelagem usando mecânica clássica, estudo de pontos de equilíbrio, estabilidade e verificação de regiões onde o movimento do giroscópio pode ficar regular ou caótico. Os expoentes de Lyapunov são identificados usando o método padrão, método de Eckmann-Ruelle, método deWolf com séries temporais e o teste 0-1. Os resultados alcançados nesta dissertação permitiram comparar as principais vantagens e desvantagens de cada um dos métodos e extrair informações qualitativas e quantitativas sobre o movimento do giroscópio em estudo.

Grace aux progres de la technologie laser permettant l'obtention de faisceaux ultraviolets accordables, et de la microelectronique fournissant des detecteurs tres performants, on est aujourd'hui capable d'etudier des etats vibrationnellement tres excites dans les molecules. Nous avons mis au point et utilise une chaine lasers organisee autour d'un laser a vapeur de cuivre a fort taux de repetition permettant l'obtention de six watts de faisceau laser accordable dans le rouge et cinq cents milliwatts de faisceau accordable dans l'ultraviolet a trois cent vingt nanometres avec une largeur de raie de deux giga-hertz. Ceci a permis l'obtention de neuf cent soixante cinq niveaux vibrationnels de symetrie particuliere de la molecule de disulfure de carbone. Ces niveaux s'etalent depuis le fond du puits de potentiel de l'etat fondamental de la molecule jusqu'a une energie de dix neuf mille nombres d'onde par centimetre. L'etude detaillee de l'etat rovibronique de la molecule de disulfure de carbone preparee par excitation du faisceau ultraviolet nous a permis de choisir un etat de symetrie particuliere. Le choix judicieux de l'etat intermediaire nous a permis l'observation directe, sur le spectre de fluorescence dispersee, de la brisure de symetrie responsable de l'apparition du chaos quantique dans l'etat fondamental lineaire de la molecule etudiee. La serie de tests statistiques mettent en evidence le caractere chaotique de la molecule a haute energie vibrationnelle. Cette experience nous a permis d'observer pour la premiere fois la transition continue d'un regime regulier vers un regime chaotique purement vibrationnel dans une molecule triatomique

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
The Chua s circuit is an electronic oscillator that has a non-linearity coupled to an LC oscillator, enabling the study of experimental chaos. Using the method of feedback with sine function of the voltage from C1 capacitor, indirectly alters the nonlinear curve of the circuit, with new operation points allowing the emergence of four scroll attractors. A consequence of this feedback is the emergence of a new chaotic region in the parameter space. We present here a qualitative numerical study simulated via MULTISIM/SPICE. It also presents the study by fourth order Runge-Kutta numerical integration for the construction of the parameter space of the largest Lyapunov exponent and bifurcation diagram. It explores the crisis region in the numerical study and show the experimental attractors in this phenomenon. We seek to compare the crisis phenomenon relating the number of visits that system is in the regions +V1 and −V1.
O circuito de Chua é um oscilador eletrônico que possui uma não-linearidade acoplada a um oscilador LC, viabilizando o estudo de caos experimental. Utilizando o método de realimentação por função senoidal da tensão do capacitor C1, altera-se indiretamente a curva não linear do circuito, com novos pontos de operação do oscilador, possibilitando o surgimento de atratores de quatro rolos. Uma consequência desta realimentação ´e o surgimento de uma nova região de caos no espaço de parâmetros. Apresenta-se neste trabalho, o estudo numérico qualitativo simulado via MULTISIM/SPICE. Também apresenta o estudo por integração numérica pelo método Runge-Kutta de quarta ordem, para a construção do espaço de parâmetros do maior expoente de Lyapunov e diagrama de bifurcação. Explora-se a região de crise no estudo numérico e mostra-se atratores experimentais em tal fenômeno. Buscamos comparar o fenômeno de crise relacionando o numero de visitas que o atrator faz nas regiões +V1 e −V1.

O Texas Helimak é uma máquina toroidal de confinamento de plasma cujas linhas de campo magnético têm forma helicoidal e no qual parâmetros do plasma (como a densidade e a temperatura) são similares aos da borda e da região externa (scrape-off-layer) de um tokamak. Nesta tese foram analisados o equilíbrio e as flutuações do plasma no Texas Helimak. São apresentadas a análise e a interpretação do controle da turbulência eletrostática e do transporte turbulento de partículas pelo potencial elétrico, bias, aplicado externamente. As alterações na turbulência e no transporte causadas pela mudança do potencial elétrico externo foram investigadas em uma região do plasma com gradientes radiais uniformes. As flutuações em descargas com bias positivo ou nulo apresentam espectros de potência de banda larga e uma PDF com uma cauda acentuada que revela a ocorrência intermitente de eventos extremos. Por outro lado, as flutuações em descargas com bias negativo possuem um espectro de potência mais estreito, uma PDF mais Gaussiana e um diagrama de recorrência com mais estruturas. Um modelo de quatro ondas acopladas foi utilizado para relacionar a largura de banda da turbulência com o tempo de interação entre as ondas do modelo. Perfis radiais do transporte turbulento de partículas na direção radial foram calculados, como função do bias, e comparados com os perfis da velocidade do fluxo de plasma e seu cisalhamento. Foi mostrado que o transporte depende do perfil radial da velocidade de fluxo do plasma. Nas descargas em que essa velocidade apresenta um ponto de máximo em seu perfil radial, o perfil do transporte possui um mínimo que foi interpretado, utilizando um modelo Hamiltoniano de ondas de deriva, como sendo resultado de uma barreira de transporte onde o cisalhamento do fluxo é nulo. Em outras descargas, máximos no perfil radial do transporte foram relacionados a ressonâncias que ocorrem onde a velocidade de fase da onda é igual à velocidade do plasma.
The Texas Helimak is a toroidal confinement of plasma device with helically magnetic field lines and which plasma parameters (like the density and temperature) are similar to the edge and the scrape-off-layer of a tokamak. This thesis analyzed the equilibrium and fluctuations in the Helimak Texas plasma. We present the analysis and interpretation of electrostatic control of turbulence and turbulent particle transport by electrical potential, bias, applied externally. Changes in transport and turbulence caused by the change of external electric potential were investigated in a region of uniform radial gradients plasma. Fluctuations in discharges with positive or zero bias have broadband power spectra and a PDF with an accentuated tail that reveals the intermittent occurrence of extreme events. Moreover, fluctuations in discharges with negative bias have narrow power spectra, a more Gaussian PDF and more structures in the recurrence diagram. A four coupled wave model was used to relate the bandwidth of turbulence with the interaction time between the waves in the model. Radial profiles of turbulent transport of particles in the radial direction were calculated as a function of bias and compared to the profiles of the plasma flow velocity and its shear. It was shown that the transport depends on the radial profile of the plasma flow velocity. In that discharges where the velocity radial profile presents a point of maximum, the transport profile has a minimum which was interpreted, using a drift wave model Hamiltonian, as a result of a shearless flow transport barrier. In other discharges, the maxima in the transport radial profiles were related with resonances that occur where the wave phase velocity is equal to the plasma velocity.

The objective of this thesis is to experimentally investigate the performance of steam gasification of chars of pure coal (lignite, sub-bituminous), pure biomass (radiata pine, eucalyptus nitens) and their blends. The influences of gasification temperature, types of coal and biomass, coal-biomass blending ratio, alkali and alkaline earth metal (AAEM) in lignite, on specific gasification characteristics (producer gas composition and yield, char reactivity) were studied. In addition, synergistic effects in co-gasification of coal-biomass blend char were also investigated. This project is in accordance with objectives of the BISGAS Consortium. In this study, experiments were performed in a bench-scale gasifier at gasification temperatures of 850°C, 900°C and 950°C, respectively. Two types of coals (lignite and sub-bituminous) and two kinds of biomass (radiata pine and eucalyptus nitens) from New Zealand were selected as sample fuels. From these raw materials, the chars with coal-to-biomass blending ratios of 0:100 (pure coal), 20:80, 50:50, 80:20 and 100:0 (pure biomass), which were derived through the devolatilization at temperature of 900°C for 7 minutes, were gasified with steam as gasification agent. During the gasification tests, the producer gas composition and gas production were continuously analysed using a Micro gas chromatograph. When the gas production was undetectable, the gasification process was assumed to be completed and the gasification time was recorded. The gasification producer gas consisted of three main gas components: hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2). The results from gasification of chars of individual solid fuels (coal or biomass) confirmed that biomass char gasification was faster than coal char gasification. The influences of gasification temperatures were shown as: when gasification temperature increased, the H2 yield increased in coal char gasification but decreased in biomass char gasification. In the meantime, CO yields increased while CO2 yields decreased in both coal char and biomass char gasification. In addition, the char reactivity of all the pure fuel samples increased with elevated gasification temperatures. The results from co-gasification of coal-biomass blend char exhibited that the syngas production rate, which is defined as the total gas production divided by the gasification completion time, was enhanced by an increase in gasification temperatures as well as an increase in the biomass proportion in the blend. The AAEM species played a significant catalytic role in both gasification of pure coal chars and co-gasification of coal-biomass blend chars. The presence of AAEM increased the producer gas yield and enhanced the char reactivity. The positive synergistic effects of the coal-biomass blending char on syngas production rate only existed in the co-gasification of lignite-eucalyptus nitens blend chars. The other blend chars showed either insignificant synergistic effects or negative effects on the syngas production rate.

Lorsqu'un fil chaud est placé juste sous la surface libre d'un fluide, des ondes de chaleur se propageant le long du fil peuvent être observées. Ce travail porte sur la recherche et la caractérisation de comportements chaotiques ainsi que sur l'étude des mécanismes physiques mis en jeu dans cette expérience. Par exemple, l'écoulement du fluide et le champ de température de la surface sont étudiés. Plusieurs routes vers le chaos sont mises en évidence. En particulier, la première observation de l'intermittence de type-II dans un système hydrodynamique est rapportée. Les comportements résultant du couplage non-linéaire de deux oscillateurs sont étudiés dans l'expérience du fil chaud et comparés au modèle théorique de Curry et Yorke. Enfin, des comportements chaotiques spatio-temporels sont observés. Le nombre de degrés de liberté décrivant l'état du système est alors de l'ordre de 6

Depuis la mise en evidence de la transition vers le chaos vibrationnel dans l'etat electronique fondamental de la molecule de bisulfure de carbone, il semblait impossible d'attribuer des niveaux vibrationnels situes au dessus de cette limite. Grace a une excitation plus selective, des progressions regulieres et identifiables ont ete observees jusqu'a tres haute energie, avant l'apparition d'un regime plus perturbe, par la methode de spectroscopie de fluorescence dispersee induite par laser. Les progres experimentaux reposent entre autres sur l'utilisation d'un nouveau laser cu/hbr comme laser de pompe et d'une camera ccd refroidie a l'azote liquide. La grande sensibilite et la possibilite de travailler a haute resolution ont permis egalement d'observer des anomalies rotationnelles, dues a l'interaction entre rotation et vibration. Ainsi, la resonance du type l fait apparaitre un systeme supplementaire de niveaux vibrationnels en emission, ce qui a permis de determiner pour la premiere fois le parametre de ce couplage sur un tres grand domaine spectral. Dans le but d'etudier les etats electroniques excites de la molecule de bisulfure de carbone et leur interaction avec l'etat fondamental, une experience de double resonance infrarouge - ultraviolet a ete construite et mise en oeuvre. Elle utilise un laser a monoxide de carbone et la chaine laser composee du laser cu/hbr et d'un laser a colorant double en frequence. Grace a sa grande selectivite, cette technique permet de s'affranchir de la congestion rotationnelle dans les etats excites et de simplifier les spectres.

The key controlling factor in the effective energy conversion of coal to combustible gases during the UCG process is the behaviour of the pyrolysed char in the reduction zone of the UCG cavity, which has not been published in available academic literature. This study investigates the impact of the operating parameters during the reduction zone of UCG using a bespoke high pressure high temperature rig which was developed as part of this research work. This rig, operating at temperatures of up to 900 oC and at pressures up to 5.0 MPa, simulates the UCG process including each UCG zone individually for a broad range of underground conditions to a depth of 500 m. Carbon dioxide and steam were used as the primary reductants with char derived from dry steam coal and anthracite sample. Carbon dioxide and steam were injected at a variety of pressures and temperatures, plus at a range of relative H2O/CO2 proportions. The composition of the resulting product gas of both coals was measured and subsequently used to calculate carbon conversion (X), carbon conversion of combustible gases ( ), cold gas efficiency (CGE) and low heating value (LHV) of the product gas. Optimal operating conditions were determined for the dry steam coal and anthracite that produced the best gas composition both at atmospheric and elevated pressure and are unique for each UCG system. A shrinking core model was employed to describe the behaviour of the pyrolised char to determine the activation energy and pre-exponential factor at atmospheric pressure for both coals. The evolution of the volatile matter of both coals and its contribution to the overall UCG performance was also determined. An optimum H2O/CO2 ratio was determined for both coals which enhanced the gasification rate of both coal chars up to the ratio of 2:1, above this ratio the effect saturated for both coals. It was shown that pressure increases the reduction-gasification process of the chars which suggests that there is an optimum operating pressure which produces a peak in carbon conversion, CGE and LHV for the product gas over the conditions tested that differs for each coal. Therefore UCG projects aiming at reaching higher pressures will not achieve an increase in the output, unless there are some new effects occurring above 4.0 MPa. Pressure enhances the gas solid reactions and almost doubles the max carbon conversion ( of combustible gases achieved at elevated pressure compared to that at atmospheric pressure. A shrinking core model was modified to take into account the effect of total pressure to the gasification rate of dry steam coal at 900 oC and pressures ranging from 0.7 to 1.65 MPa. Reaction constants for various pressures at 900 oC were determined for both coal chars. Analysis of data shown that typical UCG operations on low rank coals provides a combustible product gas that relies heavily on releasing the volatile matter from the coal and does not depend on the carbon conversion of char to gas which justifies the high CGE and LHV of the product gas found in the field trials. It was found that carbon conversion X is not significantly affected by the type of coal and that the carbon converted during UCG is between approximately 45% for high rank coals up to 55% for low rank coals. Experimental results were used to calculate the output, size and UCG model of a potential power plant which produced realistic solutions and proves that high rank coals can be suitable for UCG projects. Anthracite can produce almost the same amount of combustible gases as the dry steam coal operating under specific conditions but with a lower CGE and LHV which suggests that anthracite may be found to be more suitable for producing hydrocarbons with UCG than energy.

L'effet d'un champ magnetique alternant a basse frequence sur un metal liquide presentant une surface libre, est etudie a l'aide d'une cuve cylindrique contenant du mercure et placee dans un solenoide alimente par un courant monophase de frequence 1 a 22 hz. Les mouvements internes turbulents, preponderant pour les valeurs de frequence comprises entre 10 et 22 hz, sont, sur de nombreux points, similaires a ceux observes, dans les etudes anterieures, aux plus grandes valeurs de frequence. La principale particularite est la rapide decroissane de la vitesse moyenne typique et du taux de dissipation turbulente. Ce phenomene semble etre correle a la presence de vitesses oscillatoires forcees par la partie pulsatoire des forces de lorentz devient predominante et engendre des mouvements de surface dont la configuration et l'amplitude dependent de l'itensite i et de la frequence f du courant electrique inducteur applique. L'espace des parametres (i, f) peut etre divise en quatre regions correspondant a quatre regimes. Une analyse theorique de la stabilite d'une surface libre soumise a un champ electromagnetique basse frequence montre qu'il apparait un systeme d'ondes concentriques stationnaires forces de frequence 2f et que la croissance des modes azimutaux est gouvernee par un systeme couple d'equations de mathieu. Sous certaines conditions, les ondes azimutales deviennent instables et l'instabilite apparait avec une transition sous-harmonique. La comparaison entre la theorie et l'experience montre que le modele theorique permet une bonne prediction qualitative des phenomenes observes.

La structure vibronique de la molecule de dioxyde d'azote a ete etudiee dans deux regions spectrales grace a deux techniques spectroscopiques differentes combinees toutes deux a l'utilisation d'un jet supersonique (qui a pour effet de simplifier enormement le spectre rotationnel): la spectroscopie ultrasensible d'absorption intra cavite laser (iclas), combinee a l'utilisation d'un jet supersonique en fente, a permis d'observer, avec une resolution de 0. 05 cm#-#1, 175 bandes vibroniques froides entre 11200 et 16150 cm#-#1. La spectroscopie d'excitation par fluorescence induite par laser (lif) a permis d'etendre le spectre au dela de 16000 cm#-#1 a plus haute resolution (0. 003 cm#-#1): nous disposons dorenavant de 350 bandes vibroniques froides entre 16000 et 19363 cm#-#1. Un modele de densite d'etats integree a ete elabore d'une part pour estimer le nombre de niveaux vibroniques observes par rapport au nombre de niveaux vibroniques attendus et d'autre part pour effectuer le redressement de la densite d'etats integree experimentale, etape indispensable a l'etude des proprietes statistiques des niveaux d'energie observes. D'apres ce modele pres de 96% des bandes vibroniques ont ete detectees entre 16578 et 19363 cm#-#1. La purete de cet ensemble de niveau permet une interpretation fiable des analyses statistiques (distribution du premier voisin, transformee de fourier, etc) qui mettent en evidence l'existence d'un chaos vibronique fortement developpe des 17000 cm#-#1. Les transformees de fourier des differents spectres vibroniques de la molecule ont par ailleurs permis de proposer des valeurs experimentales pour les frequences d'elongation symetrique, de pliage et d'elongation antisymetrique de l'etat electronique excite a#2b#2

Pour les etats de vibration tres excites de certaines molecules, l'attribution en termes des nombres quantiques usuels n'est plus valable. Dans cs#2, les modes d'elongation symetrique et de pliage sont fortement couples par une resonance non lineaire de fermi. Au moins a basse energie le mode d'elongation antisymetrique reste faiblement couple par des anharmonicites. La dynamique quantique, classique et semiclassique de ce modele tres important en physique moleculaire est etudie en detail. Il est montre comment les bifurcations successives des orbites periodiques permettent de rendre compte de l'evolution de la dynamique quantique en fonction de l'energie. L'etude du vibrogramme permet egalement d'evaluer la validite du modele de fermi entre 12000 et 19000 cm##1 dans un domaine ou elle n'etait pas connue. Nous avons ensuite effectue des experiences de fluorescence resolue a plus de 19000 cm##1 au-dessus du fondamental afin de pratiquer les statistiques spectrales permettant de caracteriser le chaos quantique. Pres de 400 raies experimentales ont ete obtenues. L'analyse demontre plusieurs points importants. En premier lieu, la transition vers le chaos quantique de la molecule vers 13000 cm##1 est remise en question a cause d'une excitation impure. D'autre part, nous avons montre le role de la resolution sur les statistiques spectrales. Des spectres mal resolus peuvent presenter des correlations qui sont des artefacts. Ceci nous a conduit a remettre en question toutes les preuves experimentales existantes de l'existence de chaos quantique vibrationnel dans les molecules. Afin d'obtenir des spectres vibrationnels purs, nous avons developpe deux nouvelles experiences pour la spectroscopie de cs#2 : la double resonance i-r u-v et un jet supersonique. Si les transferts rotationnels observes en double resonance rendent difficiles l'utilisation de cette technique, le jet supersonique permet d'exciter veritablement une raie unique.

Cette these a ete motivee par le desir d'etudier l'effet de proximite entre un metal normal (n) et un supraconducteur (s). Pour cela, nous avons construit un microscope a effet tunnel (stm) pour mesurer la densite d'etats electroniques locale d'un metal. Celui-ci fonctionne a 100 mk dans un cryostat a dilution renverse. Pour respecter la philosophie de ce cryostat original, le stm est thermalise par contacts mecaniques sur la boite de melange sans gaz d'echange. Avant de coupler le stm a la dilution, nous avons procede a une mise au point prealable dans un cryostat a 4he a des temperatures comprises entre 4,2 k et 1,5 k. Nous avons donc etudie l'effet de proximite entre de l'or (n) en bon contact electrique avec du niobium (s) a 1,5 k. La reflexion d'andreev est le mecanisme microscopique qui regit cet effet de proximite. Elle permet a electron incident d'energie au dessus du niveau de fermi qui arrive sur une interface avec un supraconducteur d'etre retro-reflechit en un trou d'energie -. La paire electron-trou correles ainsi creee dans le metal normal se propage de maniere coherente sur une distance l hd/. Ces correlations conduisent a un affaiblissement de la densite d'etats a une particule dans le metal normal. Lorsque l'or est de taille infinie, 1 m dans notre cas, nous observons un affaiblissement de la densite d'etats electroniques locale dont l'extension spectrale diminue lorsque l'on s'eloigne de l'interface. Physiquement, c'est une manifestation de la longueur l. Les spectres mesures sont quantitativement expliques par le formalisme quasi-classique de l'equation d'usadel. Lorsque l'or est en contact avec le niobium est de faibles dimensions, quelques dizaines de nanometres, la densite d'etats montre un minigap

碩士
國立高雄師範大學
物理學系
97
Abstract In this study, we will present the evidence of suppressing chaotic motion by adding periodic driving pulse signal. Both of the simulation and experimental results will be discussed with the concept of entropy. Due to the convenience, we employed the electronic circuit as the experiment platform. In the thesis, we will make a conclusion of possible reasons of suppressing chaos. Keyword: suppression chaos, entropy, Chen system

碩士
國立中山大學
物理研究所
82
In this thesis study, the generation of synchronized chaos and its application to secure communications have been experimentally investigated and successfully verified based on an electronic circuit implementation of Lorenz model. The theory of Pecora and Carroll ( Phys. Rev. Lett. 64(1990)821 ) has been applied to the circuit analysis and the secure communications proposed by Cuomo and Oppenheim ( Phys. Rev. Lett. 71(1993)65 ) has been verified. We have also explored the possibility of synchronized chaos by replacing electric signals with light .

In this research we consider synchronization of oscillators. We use mechanical metronomes that are coupled through a mechanical medium. We investigate the problem for three different cases: 1) In passive coupling of two oscillators, the coupling medium is a one degree of freedom passive mechanical basis. The analysis of the system is supported by simulations of the proposed model and experimental results. 2) In another case, the oscillator is forced by an external input while the input is also affected by the oscillator. This feedback loop introduces dynamics to the whole system. For realization, we place the mechanical metronome on a one degree of freedom moving base. The movements of the base are a function of a feedback from the phase of the metronome. We study a family of functions for the reactions of the base and their impact on the behavior of the metronome. 3) We consider two metronomes located on a moving base. In this case the two metronomes oscillate and as the base is not freely moving, they are not directly coupled to each other. Now based on the feedbacks from the vision system, the base moves and hence the phases of the metronomes are affected by these movements. We study the space of possibilities for the movements of the base and consider impacts of the base movement on the synchronization of metronomes. We also show how such a system evolves in time.

The focus of this dissertation is on phenomena exhibited by multistable systems. Two phenomena of particular importance are chaos control and stochastic resonance. In this work, both models that can predict ordered responses and experiments in which ordered responses occur are explored. In addition, parameter identification methods are presented and improved.

Chaos control, when implemented with delays, can be an effective way to stabilize unstable periodic orbits within a multistable system experiencing a chaotic response. Delayed control is easy to implement physically but greatly increases the complexity of analyzing such systems. In this work, the spectral element method was adapted to evaluate unstable periodic orbits stabilized by feedback control implemented with delays. Examples are presented for Duffing systems in which the delay is equal to the forcing period. The spectral approach is also extended to analyze the control of chaos with arbitrary delays. Control with arbitrary delays can also be used to stabilize equilibria within the chaotic response. These methods for arbitrary delays are explored in self-excited, chaotic systems.

Stochastic resonance occurs in multistable systems when an increase in noise results in an ordered response. It is well known that noise excitation of multistable systems results in the system escaping from potential wells or switching between wells. In stochastic resonance, a small external signal is amplified due to these switching events. Methods for modeling stochastic resonance in both underdamped and overdamped systems are presented. In addition, stochastic resonance in a bistable, composite beam excited by colored noise is investigated experimentally. The experimental results are compared with analytical models, and the effect of modal masses on the analytical expressions is explored. Finally, an alternative approach for calculating the effect of colored noise excitation is proposed.

In order to implement analysis methods related to delay differential equations or stochastic resonance, the parameters of the system must be known in advance or determined experimentally. Parameter identification methods provide a natural connection between experiment and theory. In this work, the harmonic balance parameter identification method was applied to beam energy harvesters and is improved using weighting matrices. The method has been applied to a nonlinear, bistable, piezoelectric beam with a tip mass. Then, an experimental method of determining the number of restoring force coefficients necessary to accurately model the systems was demonstrated. The harmonic balance method was also applied to a bistable, beam system undergoing stochastic resonance. Finally, a new weighting strategy is presented based on the signal to noise ratio of each harmonic.

Dissertation

The objective of this investigation is to measure and analyze surface temperature fluctuations in pool boiling. The surface temperature fluctuations were recorded on silicon surfaces with and without multi-walled carbon nanotubes (MWCNT). Novel Thin Film Thermocouples (TFT) are micro-fabricated on test substrates to measure surface temperatures. A dielectric liquid refrigerant (PF-5060) is used as test fluid. Both nucleate and lm boiling regimes are investigated for the silicon test substrates. Dynamics of nucleate boiling is investigated on the CNT coated substrates. High frequency temperature fluctuation data is analyzed for the presence of determinism using non-linear time series analysis techniques in TISEAN(copyright) software. The impact of subcooling and micro/nano-scale surface texturing using MWCNT coatings on the dynamics of pool boiling is assessed. Dynamic invariants such as correlation dimensions and Lyapunov spectrum are evaluated for the reconstructed attractor. A non-linear noise reduction scheme is employed to reduce the level of noise in the data. Previous investigations in pool boiling chaos, reported in literature were based on temperature measurements underneath the test surface consisting of single or few active nucleation sites. Previous studies have indicated the presence of low-dimensional behavior in nucleate boiling and high-dimensional behavior in CHF and film boiling. Currently, there is no study detailing the effects of multiple nucleation sites, subcooling and surface texturing on pool boiling dynamics. The investigation comprises of four parts: i) in situ micro-machining of Chromelalumel (K-type) TFT, ii) calibration of these sensors, iii) utilizing these sensors in pool boiling experiments iv) analysis of these fluctuations using techniques of nonlinear time series analysis. Ten TFT are fabricated on a rectangular silicon surface within an area of ~ 3.00 cm x 3.00 cm. The sensing junctions of the TFT measure 50 mm in width and 250 nm in depth. Surface temperature fluctuations of the order of i) 0.65-0.93 degrees C are observed near ONB ii) 2.3-6.5 degrees C in FDNB iii) 2.60-5.00 degrees C at CHF and iv) 2.3-3.5 degrees C in film boiling. Investigations show the possible presence of chaotic dynamics near CHF and in film-boiling in saturated and subcooled pool boiling. Fully-developed nucleate boiling (FDNB) is chaotic. No clear assessment of the dynamics could be made in the onset of nucleate boiling (ONB) and partial nucleate boiling (PNB) regimes due to the effects of noise. However, the frequency spectra in these regimes appear to have two independent frequencies and their integral combinations indicating a possible quasiperiodic bifurcation route to chaos. The dimensionality in FDNB, at CHF and in film-boiling is lower in saturated pool boiling as compared to values in corresponding regimes in subcooled pool boiling. Surface temperature fluctuations can damage electronic components and need to be carefully controlled. Understanding the nature of these fluctuations will aid in deciding the modeling approach for surface temperature transients on an electronic chip. Subsequently, the TFT signals can be employed in a suitable feedback control loop to prevent the occurrence of hotspots.

More than ever, organisations are recognising that they need to build and sustain healthy relationships with stakeholders in order to survive, grow and be successful. When an organisation is threatened by environmental changes – such as a crisis or competition as a result of information technology developments, increased customer demands, new legislations, even the threat of AIDS – the need for better communication increases. Successful organisations use the potential of communication management, not only to ease the transformation process, but also to improve their relationships with key stakeholders and the environment, and uphold their reputation. In this sense, communication practitioners are playing managerial, ethical and strategic roles during times of instability because change complexities involve having to deal with stakeholders’ trust and commitment. This thesis attempts to clarify the growing importance of communication management, particularly the role of relationship management. Proposed here is a different way of thinking about change communication strategies and building healthy relationships when organisations and their stakeholders have to make or adjust to change. Existing literature shows that most organisations tend to take a planned approach to change which is structured, consists of specific goals and objectives, and tightly controlled by management. Management sees its role within this paradigm as reducing conflict, creating order, controlling chaos and simplifying all the complexities created by the environment. Possible outcomes are predicted and alternatives for action are planned. Structures determine the information needed, and perceptions are managed by feeding the ‘right’ information or withholding information that might give rise to disorder and chaos. An alternative paradigm is the postmodern perspective, drawing specifically from chaos and complexity theories. These ‘emergent’ approaches to management consider organisations as living and holistic systems, more organic and ecological, seeking less control and more freedom. Organisations that operate like living systems are open, flexible, creative, caring and willing to adjust their strategies to adapt to the environment. While strategic planning is still considered important within the positivistic paradigms of management, it is nevertheless moving from the basic premise of control and prediction to scenario planning and processes of open communication, facilitation and networking. The emphasis is on relationship building via the full participation of the stakeholders involved. This two-way, symmetrical approach is also considered the more ethical. When an organisation is confronted by a problem, and if the constraints on communication are low, the organisation’s publics (employees/stakeholders) tend to feel connected to the problem and want to do something about it. Their need to actively seek information about the problem opens up many communication potentialities, including a willingness to change their attitudes, beliefs and behaviours. The result is a culture of shared responsibility, participation in decision-making, open and honest communication, which leads to a positive working climate and higher productivity. So an important assumption that can be made here is that an empowered public will strive for a positive relationship with the organisation. It can further be derived that a positive relationship between an organisation and its publics, particularly its internal stakeholders (employees), will lead to greater communication and a greater willingness to change. These assumptions were tested in this study. The research questions were: (1) What is the connection between the communication management strategy followed during change in organisations and the relationship and behavioural effects on internal stakeholders (employees)? (2) What are the effects of the communication management strategy followed during high change on relationships and behaviours with internal stakeholders (employees)? The methodology was an experimental approach which allows for the manipulation of independent variables and measurement of influences thereof on dependent variables. The independent variables were the communication and change strategies followed in organisations; the dependent variables were the relationship characteristics (trust, control mutuality, commitment, and satisfaction). The use of scenarios was most effective in this experimental context because different scenarios can project different outcomes. The researcher, therefore, had the opportunity to analyse the effects of the change strategies, as well as the changes that would take place. Data collection from 9 different organisations resulted in, more or less, 10–40 employees from different levels of management and non-management of each organisation. Each respondent evaluated 2 different scenarios, which resulted in 372 evaluations in total (186 for each scenario). The scenarios addressed changes in general and were about various issues. Basic descriptive statistics, as well as hypotheses testing using MANOVA (to test for meaningful differences between groups), were conducted. Other data analyses included testing for validity and reliability, analysis of variance and the Scheffe’s Test for significance of correlations. The findings showed significant proof of the internal validity of the experimental design used, indicating that: (a) the experimental manipulation (the two different change management strategies) had a definite effect on the relationship that internal stakeholders would have with organisations, and that except for the size of the organisation and the educational level of the employees, no other variables had an influence; (b) strong correlations exist between the strategy followed during change and the resulting projected relationships with internal stakeholders of the organisation; (c) high participation during high change led to significantly more positive overall relationships between an organisation and its internal stakeholders, as compared to low participation with a planned approach. In brief, the findings support a strong participative, two-way public relations strategy to be followed when organisations go through major change processes. The significance of these findings calls for a new paradigm in strategic communication and relationship management. Change cannot be solely based on plans and projections, but rather on understanding the complexities of situations and weighing different options available. Well-developed organisational change, therefore, is a strategically managed process that takes into consideration all the possibilities of change in the environment. Traditional studies and models of change management have either ignored the importance of strategic communication as a contribution to successful change, or saw communication as only a tool in the first stages of ransformation. However, to facilitate successful strategic change management is to recognise communication management as contributing significantly to guiding the complete transformation process in building important relationships internally and externally. Alternative emergent approaches to change recognise that change and, more specifically, transformation should be viewed as a continuous process linking to the complexities of the changing market, nature of work environments, new management approaches, organisational boundaries and relationships. Chaos and complexity theories, in particular, stress the importance of interconnectivity between subsystems of societies and organisations. Central to these theories is the observation that relationships built on open communication have the potential of producing something greater for an organisation. The complex and dynamic nature of the environment, structural alteration, and the need for employee flexibility are all recognised. Another important insight is the view that organisations can create visions and perform strategic planning around scenarios that guide actions. Another way of adapting to change is to influence back on change, that is, steer change through relationship building and participative decision-making. To become a true learning organisation requires the building of knowledge architecture with a strong supporting technical infrastructure. The main function of the communication manager, therefore, is to establish networks and structures for the collection and dissemination of information, and ultimately, the translation to knowledge. Communication leaders can connect teams and workgroups by driving communication and building trust. Involving staff in change management decision-making stimulates debate and criticisms, thus creates opportunities for innovation and revolutionary change. These are some of the main preconditions for sustainable change, and all can be achieved through sound communication management and the building of relationships with stakeholders.
Thesis (PhD)--University of Pretoria, 2006.
Communication Management
PhD
Unrestricted

博士
國立高雄師範大學
物理學系
104
This study aims to explore the cognitive change processes of junior high school students in chaos concepts when the designed Chaotic System Dynamics and Computer Simulation Experimental Courses are practiced with Cooperative Learning(CL), fused with Inquiry Teaching(IT) mode. This study first chooses proper Chaotic System Dynamics concepts in designing a "double pendulum" and a "chaotic pendulum" experiments and compiles the Chaotic System Dynamics experimental courses, including also reading materials-"What is Chaos?" and an instructional video- "cracking butterfly effect". The participant teachers then use team cooperative learning method combined with inquiry teaching method to teach the reading materials and practice the experimental courses. Finally, with hands-on experiments and auxiliary computer simulations, this study aims to enhance chaos concepts and scientific epistemological views of students and, in the meantime, assess the feasibility of such chaotic system dynamics and computer simulation experimental courses in the junior high schools. A total of 29 students participate in the learning activities in this study, during which learning tables of the students, questionnaire answers, classroom observation data, pre-and post-assessments and student interview records are collected for classifying and observing the applications of teaching strategies that cause the cognitive changes in chaos concepts of students. The results reveal that the designed teaching strategy, combining team cooperative learning and the inquiry teaching model, can effectively help teachers teach Chaos Theory concepts and become teachers’ teaching scaffold, under which operative teaching strategy adjustments can be done to provide innovative teaching strategies of the chaos concepts to break through the esoteric dilemma of the Chaos Theory. The designed teaching strategy for the experimental courses can help students’ cognitive changes of chaos concepts and enhance their understanding of chaos phenomena and scientific epistemological views; this can be observed from students’ learning performance improvement sowing to the process of implementing the teaching strategy. Based on the results and the interpretation in teaching process and students' cognitive changes, this study argues in three aspects that (1)Through appropriate teaching activities and course materials, e.g. video tutorials, even junior high school students can understand chaos concepts and chaotic phenomena and can easily identify chaotic phenomena in their daily life. (2)Through practical hands-on experiments and computer simulations, students can effectively enhance their understanding of chaos concepts and chaotic phenomena. (3) Through team cooperative learning and inquiry teaching modes, it can be observed that there are some positive cognitive changes to students’ concepts of chaos and to their scientific epistemological views.

Advances in the knowledge of chemical structure of coal and development of high performance computational techniques led to more than hundred and thirty four proposed molecular level representations (models) of coal between 1942 and 2010. These models were virtually on the carboniferous coals from the northern hemisphere. There are only two molecular models based on the inertinite- and vitrinite-rich coals from the southern hemisphere. The current investigation is based on the chars derived from the Permian-aged coals in two major South African coalfields, Witbank #4 seam and Waterberg Upper Ecca. The two coals were upgraded to 85 and 93% inertinite- and vitrinite-rich concentrates, on visible mineral matter free basis. The coals were slow heated in inert atmosphere at 20 ℃ min-1 to 450, 700 and 1000 ℃ and held at that temperature for an hour. After the HCl-HF treatment technique at ambient temperatures, the characteristics of the coals and chars were examined with proximate, ultimate, helium density, porosity, surface area, petrographic, solid-state 13C NMR, XRD and HRTEM analytical techniques. The results largely showed that substantial transitions occurred at 700-1000 ℃, where the chars became physically different but chemically similar. Consequently, the chars at the highest temperature (1000 ℃) drew attention to the detailed study of the atomistic properties that may give rise to different reactivity behaviours with CO2 gas. The H/C atomic ratios for the inertinite- and vitrinite-rich chars were respectively 0.31 and 0.49 at 450 ℃ and 0.10 and 0.12 at 1000 ℃. The true density was respectively 1.48 and 1.38 g.cm-3 at 450 ℃ and 1.87 and 1.81 g.cm-3 at 1000 ℃. The char form results from the petrographic analysis technique indicated that the 700-1000 ℃ inertinite-rich chars have lower proportions of thick-walled isotropic coke derived from pure vitrinites (5-8%) compared with the vitrinite-rich chars (91-95%). This property leads to the creation of pores and increases of volume and surface area as the softening walls expand. It was found that the average crystallite diameter, La, and the mean length of the aromatic carbon fringes from the XRD and HRTEM techniques, respectively, were in good agreement and made a definite distinction between the 1000 ℃ inertinite- and vitrinite-rich chars. The crystallite diameter on peak (10) approximations, La(10), of 37.6Å for the 1000 ℃ inertinite-rich chars fell within the HRTEM’s range of minimummaximum length boundary of 11x11 aromatic fringes (27-45Å). The La (10) of 30.7Å for the vitrinite-rich chars fell nearly on the minimum-maximum length range of 7x7 aromatic fringes (17-28Å.) The HRTEM results showed that the 1000 ℃ inertinite-rich chars comprised a higher distribution of larger aromatic fringes (11x11 parallelogram catenations) compared with a higher distribution of smaller aromatic fringes (7x7 parallelogram catenations). The mechanism for the similarity between the 700-1000 ℃ inertinite- and vitrinite-rich chars was the greater transition occurring in the vitrinite-rich coal to match the more resistant inertinite-rich coal. This emphasised that the transitions in the properties of vitrinite-rich coals were more thermally accelerated than those of the inertinite-rich coals. The similarity between the inertinite- and vitrinite-rich chars was shown by the total maceral reflectance, proximate, ultimate, skeletal density and aromaticity results. Evidence for this was the carbon content by mass for the inertinite- and vitrinite-rich chars of respectively 90.5 and 85.3% at 450 ℃ and 95.9 and 94.1% at 1000 ℃. The aromaticity from the XRD technique was respectively 87 and 77% at 450 ℃ and 98 and 96% at 1000 ℃. A similar pattern was found in the hydrogen and oxygen contents, the atomic O/C ratios and the aromaticity from the NMR technique. The subsequent construction of large-scale molecular structures for the 1000 ℃ inertinite-rich chars comprised 106 molecules constructed from a total of 42929 atoms, while the vitrinite-rich char model was made up of 185 molecules consisting of a total of 44315 atoms. The difference between the number of molecules was due to the inertinite-rich char model comprising a higher distribution of larger molecules compared with the vitrinite-rich char model, in agreement with the XRD and HRTEM results. These char structures were used to examine the behaviour on the basis of gasification reactivity with CO2. The density functional theory (DFT) was used to evaluate the interactions between CO2 and the atomistic representations of coal char derived from the inertinite- and vitrinite rich South African coals. The construction of char models used the modal aromatic fringes (fringes of highest frequencies in size distributions) from the HRTEM, for the inertinite- and vitrinite-rich chars, respectively (11x11 and 7x7 parallelogram-shaped aromatic carbon rings). The structures were DFT geometrically optimized and used to measure reactivity with the Fukui function, f+(r) and to depict a representative reactive carbon edge for the simulations of coal gasification reaction mechanism with CO2 gas. The f+(r) reactivity indices of the reactive edge follows the sequence: zigzag C remote from the tip C (Czi = 0.266) > first armchair C (Cr1 = 0.087) > tip C (Ct = 0.075) > second armchair C (Cr2 = 0.029) > zigzag C proximate to the tip C (Cz = 0.027). The DFT simulated mean activation energy, ΔEb, for the gasification reaction mechanism (formation of second CO gas molecule) was 233 kJ mol-1. The reaction for the formation of second CO molecule is defines gasification in essence. The experimental activation energy determined with the TGA and random pore model to account essentially for the pore variation in addition to the gasification chemical reaction were found to be very similar: 191 ± 25 kJ mol-1 and 210 ± 8 kJ mol-1; and in good agreement with the atomistic results. The investigation gave promise towards the utility of molecular representations of coal char within the context of fundamental coal gasification reaction mechanism with CO2.
PhD (Chemical Engineering), North-West University, Potchefstroom Campus, 2015