Estudamos o comportamento termodinâmico de um modelo de Ising numa rede cübica simples com interações competitivas entre primeiros e segundos vizinhos ao longo de uma das direções (modelo ANNNI). Para isto desenvolvemos uma nova aproximação de campo médio, incluindo flutuações de spins nos planos normais ao eixo de anisotropia do sistema. Construímos um diagrama de fases exibindo uma serie de fases moduladas e um ponto de Lifshitz. Concluímos que, em comparação com os resultados de campo media simples, o diagrama de fases obtido na nossa aproximação está mais perto do previsto por amostragens de Monte Carla e análises de séries de altas temperaturas. / We study the thermodynamic behavior of a layered Ising model in a simple cubic lattice with competing interaction between first and second neighbours along a single direction (ANNNI model). We develop a new mean field approximation which includes spin fluctuations in the planes perpendicular to the axis of anisotropy. We construct a phase-diagram which displays a succession of modulated phases and a Lifshitz point. We conclude that, in comparison with the simple mean-field results, the phase diagram obtained in the new approximation is closer to the predictions of the Monte Carlo simulations and the analysis of high temperature series expansions.

A programme of work was instigated to investigate and improve the efficiency, and stability of intra-cavity étalons for the Argon ion laser, especially in the far blue to near ultra-violet wavelength region, by using sol-gel prepared silica. The original program has been modified by experiences outlined below. We have attempted to differentiate between the sol-gel derived and the high temperature prepared silica. We confirm that relatively small changes in thickness of silica monoliths can give rise to proportionally much larger changes of refractive index. Our main target has been to assess whether it is possible to make a new optical material that shows insignificant changes in optical path length or refractive index with temperature.

The work described in this dissertation is concerned with some aspects of heat transfer in shallow gas fluidised beds. Results from the experimental investigations with the single heater indicated that fluidising velocity, particle size, bed height, heater diameter and heater position influence heat transfer. As expected, heater material has no influence. Unlike previous work, this work has concentrated upon the range of dependency of heat transfer on each of the tested parameters (e.g., heater diameter has no influence on heat transfer with heater diameter greater than 25.3 mm and bed depth has no influence on heat transfer for depths greater than 150 mm). The variation of this dependency with respect to the other parameters (e.g., influence of fluidising velocity on heat transfer coefficient increased with an increase in bed depth. The influence of heater location decreased with an increase in particle size, fluidising velocity and heater diameter), The effect of horizontal rod spacing on heat transfer was also examined, when unheated rods were used to complete the rod bundle. The effect of rod spacing was dependent on the other parameters (e.g., fluidising velocity, bed depth and heater diameter). Since the heat transfer depends primarily on frequent renewal of the particles at the heat transfer surface and particle mixing, these were improved by the use of gas jets. Heat transfer was found to increase by up to 50%. The following parameters were investigated: volumetric flow rate, particle size, heater size, bed height, jets position and direction and jets velocity to find their effect on the influence of gas jets on heat transfer. The data were compared with some earlier correlations and two dimensionless correlations developed. One was for a single heater and the other was for one row of horizontal rods. Unlike previous correlations, these correlations took account of both the effect of heater size and bed depth. The data for the single heater were compared with some earlier models and the fit with one of these models is good.

No description available.

No description available.

Estudamos o comportamento termodinâmico de um modelo de Ising numa rede cübica simples com interações competitivas entre primeiros e segundos vizinhos ao longo de uma das direções (modelo ANNNI). Para isto desenvolvemos uma nova aproximação de campo médio, incluindo flutuações de spins nos planos normais ao eixo de anisotropia do sistema. Construímos um diagrama de fases exibindo uma serie de fases moduladas e um ponto de Lifshitz. Concluímos que, em comparação com os resultados de campo media simples, o diagrama de fases obtido na nossa aproximação está mais perto do previsto por amostragens de Monte Carla e análises de séries de altas temperaturas. / We study the thermodynamic behavior of a layered Ising model in a simple cubic lattice with competing interaction between first and second neighbours along a single direction (ANNNI model). We develop a new mean field approximation which includes spin fluctuations in the planes perpendicular to the axis of anisotropy. We construct a phase-diagram which displays a succession of modulated phases and a Lifshitz point. We conclude that, in comparison with the simple mean-field results, the phase diagram obtained in the new approximation is closer to the predictions of the Monte Carlo simulations and the analysis of high temperature series expansions.

Thesis (Ph.D.)--University of Missouri-Columbia, 2006. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file viewed on (February 27, 2007). Vita. Includes bibliographical references.

Conventional combustion processes are known to be highly irreversible processes. The potential to obtain useful work from the fuel is degraded during the combustion process. For example, for a reciprocating internal combustion engine, about 20% or more of the potential work from the fuel is destroyed during the combustion process. This potential work is known as availability (a thermodynamic property). The motivation for the current work was to develop a conceptual model of a set of processes related to reciprocating engines that would eliminate this destruction of availability. One conceptual model, proposed by Keenan, suggested that a preselected set of “reactants” could be compressed (at constant composition) to a high temperature and pressure. At this high temperature and pressure, the “reactants” would be in chemical equilibrium. At this point, the “reactants” would be expanded back to the original volume. The expansion process would consist of a “shifting” chemical equilibrium such that the composition during expansion would continue to change. At the end of the compression and expansion, net work would be available without destroying any of the work potential of the fuel. The purpose of the current work was to develop a quantitative model of this concept, and to use the model in a series of computations to examine the effects of temperature, pressure, and other parameters on the work production capability of the concept. The concept was studied for eight different fuels for various conditions. In general, the net work output increased as the temperature, pressure and compression ratio increased. For low compression temperatures and pressures, the concept resulted in a small amount of net work produced without destroying any fuel availability. For sufficiently high compression pressure and temperature (e.g., 10 MPa and 6000 K, respectively), however, the thermal efficiency was ~28% for isooctane and was ~40% for hydrogen and methane, for air as the oxidant, an equivalence ratio of 1.0, and a compression ratio of 18. Although the temperatures and pressures considered are well beyond practical values for the materials and designs of today, the general result of the study is that conditions can be identified to eliminate the combustion irreversibility.

Thèse : Mécanique : Faculté des Sciences de Paris : 1839. / Titre provenant de l'écran-titre.

Abhandlung--Technische Hochschule, Stuttgart. / Includes bibliographical references (p. 117-120).