Thesis (M.S.)--University of Wyoming, 2008. / Title from PDF title page (viewed on August 3, 2009). Includes bibliographical references (p. 65-68).

In this study, the applicability of the Lattice Boltzmann Method to neutron transport is investigated. The transport model used, is derived from the Boltzmann equation for neutral particles by inverting the streaming operator and casting the integral transport equation into an operator form. From the operator equation, an iterative solution to the transport problem is presented, with the first collision source as the starting point for the iteration scheme. One of the main features of the method is the simultaneous discretization of the phase space of the problem, whereby particles are restricted to move on a lattice. A full description of the discretization scheme is given along with the iterative procedure and quadrature set used for the angular discretization. To mitigate lattice ray effects, an angular refinement scheme is introduced to increase the angular coverage of the problem phase space. The method is then applied to a model problem to investigate its applicability to neutron transport. Three cases are considered where constant, linear and exponential interpolants are used to account for the accumulation of flux due to the streaming of particles between nodes. The results obtained are compared to a reference solution, that was calculated by using the MCNP code and to the values calculated using a nodal SN method. Finally, areas of improvement are identified and possible extensions to the algorithm are provided. / Thesis (MIng (Engineering Sciences in Nuclear Engineering))--North-West University, Potchefstroom Campus, 2013

In this study, the applicability of the Lattice Boltzmann Method to neutron transport is investigated. The transport model used, is derived from the Boltzmann equation for neutral particles by inverting the streaming operator and casting the integral transport equation into an operator form. From the operator equation, an iterative solution to the transport problem is presented, with the first collision source as the starting point for the iteration scheme. One of the main features of the method is the simultaneous discretization of the phase space of the problem, whereby particles are restricted to move on a lattice. A full description of the discretization scheme is given along with the iterative procedure and quadrature set used for the angular discretization. To mitigate lattice ray effects, an angular refinement scheme is introduced to increase the angular coverage of the problem phase space. The method is then applied to a model problem to investigate its applicability to neutron transport. Three cases are considered where constant, linear and exponential interpolants are used to account for the accumulation of flux due to the streaming of particles between nodes. The results obtained are compared to a reference solution, that was calculated by using the MCNP code and to the values calculated using a nodal SN method. Finally, areas of improvement are identified and possible extensions to the algorithm are provided. / Thesis (MIng (Engineering Sciences in Nuclear Engineering))--North-West University, Potchefstroom Campus, 2013

Thesis (M. S.)--Paper Science Engineering, Georgia Institute of Technology, 2010. / Committee Chair: Aidun, Cyrus; Committee Member: Graham, Samuel; Committee Member: Joshi, Yogendra. Part of the SMARTech Electronic Thesis and Dissertation Collection.

En raison de la tendance positives pour le développement durable, les systèmes solaires(capteurs solaires, concentrateur solaire, etc.) Intègrent (et demandent d'intégrer encore plus)intensivement les résidences et les industries. Dans ce contexte, les systèmes diphasiques comme le caloduc semblent être très efficaces en raison de leurs capacités élevées de transport de chaleur et de leur fonctionnement passif appliqués aux capteurs. Compte-tenu de la complexité des caloducs à structure poreuse dans ce type d'application, la plupart des systèmes existants sur le marché utilisent des thermosiphons. Ainsi, le besoin croissant de solutions de contrôle thermique fiables et plus efficaces croit rapidement pour de tels systèmes.Ce travail de thèse porte sur la caractérisation des performances des caloducs à structure poreuse utilisés dans les applications solaires. Une étude numérique a été réalisée pour modéliser et simuler le comportement d'un caloduc typique à l'aide de la méthode Lattice Boltzmann. Une étude expérimentale a également été réalisée pour caractériser les performances de trois prototypes testes dans différentes conditions (température du condenseur, puissance introduite et angle d'inclinaison). Les effets induits par plusieurs paramètres incluant le taux de remplissage, le fluide de travail et la symétrie de la puissance appliquée sur les performances de ces dispositifs ont également été étudiés. En particulier, l'asymétrie du chauffage induit un assèchement plus précoce, toutes choses étant égales par ailleurs. L'inclinaison optimale est également déterminée là où est équilibrée la chaleur solaire maximale disponible et reçue par le caloduc et l'écoulement de liquide assisté par gravité à l'intérieur de ce dispositif. / Owing to the trend to development sustainability, solar systems (solar collector, solar concentrator, etc.) Are integrating (and asked to integrate even more) intensively residences and industries. In this context, two-phase systems like heat pipe seem highly effective because of their high heat transport capabilities and their passive operation in collectors’ technology. In view of the complexity of the heat pipes with a porous structure in this kind of application,most of the existing systems on the market use thermosyphons. Thus, the growing need of reliable and more efficient thermal control solutions is increasing for such systems. This thesis work focuses on the performance characterization of heat pipes with porous structure used in solar applications. A numerical study has been performed to model and simulate the behavior of a typical heat pipe using the Lattice Boltzmann method. An experimental study has also been done to characterize the performance of three prototypes tested under different conditions (condenser temperature, heat input and inclination angle). The effects induced by several parameters including the filling rate, working fluid and symmetry of the applied heat on the performance of these devices has also been investigated. In particular, heating asymmetry is found to induce dry-out earlier, all other things being equal. Optimal inclination is also determined where is balanced the maximum solar heat available and received by the heat pipe and the gravity-assisted liquid flow inside that device.

No description available.

Thesis (Ph.D.). / Written for the Computational Acoustic Modeling Laboratory, School of Music. Title from title page of PDF (viewed 2008/01/12). Includes bibliographical references.

The motion of microorganisms presents interesting and diffcult problems ranging from mechanisms of propulsion to collective effects. Experimentally, some of the complicating factors, such as death, reproduction, chemotaxis, etc., can be suppressed through genetic manipulation or environmental control. Nonequilibrium statistical mechanics has been used to study simple models, however proceeding analytically is extremely challenging. Thus simulations, where one has total control over and knowledge of the system, are a compelling method for examining models of their behaviour. In this work I present simulations of minimal, self-propelled particles, while ensuring realistic hydrodynamic behaviour using the lattice Boltzmann method (LBM), a well-studied method for simulating fluid flows that scales linearly in computational effort with the system volume. The derivation of the LBM is reviewed, including the addition of forces in a consistent, accurate manner as well as thermal fluctuations that satisfy the fluctuation-dissipation theorem. It is extended to include singular forces via a regularization of the Dirac δ-function. This is implemented and extensively tested for agreement with low Reynolds number hydrodynamics. The regularized singularities are used to develop an effcient algorithm for pointlike particles which move under the influence of an external force, such as gravity, or thermal fluctuations of the fluid. The method is compared to theoretical results and simulations using a well-studied algorithm that resolves the particle, finding good agreement in the dilute limit and significantly reduced computational requirements. Using the singular forces, we then construct a minimal model for self-propelled particles, that may also experience forces or undergo random changes of orientation (modelling the “run-and-tumble” dynamics observed in swimming bacteria such as E. coli). The collective behaviour of these model swimmers is studied in three situations: sedimentation under gravity; in a central, harmonic trap; and in a Poiseuille flow between parallel plates. For sedimentation, the behaviour is not very different from that expected of non-interacting run-and-tumble particles, except that total collapse to the container bottomwhen the weight of the particles equals the propelling force is prevented by the velocity fluctuations caused by the particles’ activity. The trapped particles, for runlengths comparable to the trap size, self-assemble into a pump-like structure, while for short run-lengths an approximately Gaussian distribution seenwithout hydrodynamic interactions, is maintained. In Poiseuille flows we find the particles orient upstream; forweak flows this results in a net upstreamcurrent. We find significant hydrodynamic effects, in the dilute limit, only when there is some mechanism that causes alignment of the particles.

Liquid water management in the cathode of polymer electrolyte membrane fuel cells (PEMFC) is crucial to efficient transport of gases and to maintaining electrochemical activity in the catalyst layer. Cracks and interfacial voids are typical of catalyst layers in operating cells, and are thought to affect water management and other transport properties such as gas diffusion and conductivity. This thesis investigates the effect of such morphological imperfections on liquid water transport using a combination of numerical techniques. Both the catalyst layer and microporous layer parts of the cathode are considered. The layers are first numerically reconstructed using data from advanced microscopy, and cracks, perforations and interfacial voids are created. Lattice Boltzmann simulations of the dynamics liquid water imbibition process are performed to study the effect of characterizing features of the cracks and interfacial voids such as aperture area, degree of protrusion, and tortuosity. The resulting liquid water distributions were then input into a pore scale model to characterize the effect of the morphological features on other transport properties, such as effective diffusivities and conductivities. Larger crack apertures were found to increase liquid water uptake, and elongated cracks allowed for faster breakthrough at lower saturation levels. A notable observation is that short and large interfacial cracks have a higher liquid water uptake potential due to the lower effective capillary pressures. It was also found that elongated cracks aligned with the pressure gradient provide preferential pathway, and a capillary pressure increase that favours liquid water transport towards the membrane and mitigates flooding. The effective diffusivity increased for all crack protrusion depths, even for the wet catalyst layer, likely due to low liquid water saturation. The geometry with the most elongated crack showed a significant increase in gas diffusion under wet conditions, indicating that enhanced gas transport is achievable when liquid water removal is effective. Protonic and electrical conductivities decreased for all crack shapes due to higher contact resistance. / Graduate / 0548 / vlosier@uvic.ca

In this thesis, two numerical modeling methods are used to investigate the thermal conductivity of the polymer electrolyte membrane (PEM) fuel cell gas diffusion layer (GDL). First, an analytical model is used to study the through-plane thermal conductivity from representative physical GDL models informed by microscale computed tomography imaging of four commercially available GDL materials. The effect of the heterogeneity of the through-plane porosity of the GDL and polytetrafluoroethylene (PTFE) treatment is studied and it is noted that the high porosity surface transition regions have a dominating effect over the addition of PTFE in impacting the overall thermal conductivity. Next, the lattice Boltzmann method (LBM) is employed to study both the in-plane and through-plane thermal conductivity of stochastic numerically generated GDL modeling domains. The effect of GDL compression, binder content, PTFE treatment, addition of a microporous layer (MPL), heterogeneous porosity distributions, and water saturation on the thermal conductivity are investigated.