<p> Interest in thermoelectric devices (TEDs) for waste-heat recovery applications has recently increased due to a growing global environmental consciousness and the potential economic benefits of increasing cycle efficiency. Unlike conventional waste-heat recovery systems like the organic Rankine cycle, TEDs are steady-state, scalable apparatus that directly convert a temperature difference into electricity using the Seebeck effect. The benefits of TEDS, namely steady-state operation and scalability, are often outweighed by their low performance in terms of thermal conversion efficiency and power output. To address the issue of poor device performance, this dissertation takes a multi-faceted approach focusing on device modeling, analysis and design and material processing.</p><p> First, a complete one-dimensional thermal resistance network is developed to analytically model a TED, including heat exchangers, support structures and thermal and electrical contact resistances. The purpose of analytical modeling is twofold: to introduce an optimization algorithm of the thermoelectric material geometry based upon the realized temperature difference to maximize thermal conversion efficiency and power output; and to identify areas within the conventional TED that can be restructured to allow for a greater temperature difference across the junction and hence increased performance. Additionally, this model incorporates a component on the numerical resolution of radiation view factors within a TED cavity to properly model radiation heat transfer. Results indicate that geometric optimization increases performance upwards of 30% and the hot-side ceramic diminishes realized temperature difference. The resulting analytical model is validated with published numerical and comparable analytical models, and serves as a basis for experimental studies.</p><p> Second, an integrated thermoelectric device is presented. The integrated TED is a restructured TED that eliminates the hot-side ceramic and directly incorporates the hot-side heat exchanger into the hot-side interconnector, reducing the thermal resistance between source and hot-side junction. A single-state and multi-stage pin-fin integrated TED are developed and tested experimentally, and the performance characteristics are shown for a wide range of operating fluid temperatures and flow rates. Due to the eliminated to thermal restriction, the integrated TED shows unique performance characteristics in comparison to conventional TED, indicating increased performance.</p><p> Finally, a grain-boundary engineering approach to material processing of bulk bismuth telluride (Bi₂Te₃) is presented. Using uniaxial compaction and sintering techniques, the preferred crystallographic orientation (PCO) and coherency of grains, respectively, are controlled. The effect of sintering temperature on thermoelectric properties, specifically Seebeck coefficient, thermal conductivity and electrical resistivity, are determined for samples which exhibited the highest PCO. It is shown the performance of bulk Bi₂Te₃ produced by the presented method is comparable to that of nano-structured materials, with a maximum figure of merit of 0.40 attained at 383 K.</p>

Thesis (Ph. D.)--University of Toronto, 2008. / Includes bibliographical references.

<p> It is proposed that both the adhesive interface geometry and the mismatch of elastic moduli influence the tensile strengths of dental bonds attaching restorative ceramics to dentin. Prior calculations indicate this to be due to peripheral interface stress singularities. A physical testing approach to examine and validate the theoretical conclusions utilizing a microtensile testing system is presented. </p><p> Considering the choice of shear versus tensile and then macro versus micro tensile testing, reasons were identified for choosing micro tensile testing. Specimen dimensions and shapes were developed to optimize the adjustment of the interface geometries and information that could be obtained therein. Here a dumbbell structure is best suited to the testing needs. </p><p> Dumbbell specimens were first fabricated using the ceramic press technique, and then mini-CNC machining. Specimens fabricated by each technique were examined, showing that the mini-CNC machining methodology was superior. </p><p> Significant problems in instrumentation were overcome by the design and fabrication of two testing fixtures: 1. A collet based design with independent upper and lower mechanical grips for each end of the dumbbell, to be used in conjunction with a loading device; 2. A screw based clamping design similar to previous jigs, using two screw clamps on V-channels connected by sliding rods. Testing revealed that the collet-based design shows the most promise because of its distributed gripping load. Further tests that can evaluate the effectiveness of this device for microtensile testing are outlined.</p><p>

<p> Image-based modeling and simulation has become an important analytic and predictive tool for patient-specific medical applications, including large-scale in silico patient studies, optimized medical device design, and custom surgical guides and implants via additive manufacturing. The pipeline for patient-specific modeling and simulation is: image acquisition, image segmentation, surface generation, mesh generation, physics-based modeling and simulation, and clinical application. This research establishes a semi-automatic workflow for these steps, which includes a novel image-based meshing tool <i> Shabaka.</i> The toolchain is demonstrated by modeling the mechanics of a beating human heart based on magnetic resonance imaging (MRI) data. </p><p> The Shabaka workflow ensures robust execution of each step of the pipeline. Medical images are processed and segmented using thresholding, region-growing, and manual techniques. Watertight surface meshes are extracted from image masks using a novel Voronoi-based algorithm. For scientific computing purposes, surface meshes are supplied either to tetrahedral meshing routines for conventional finite element approaches, or to a robust polyhedral mesh generation tool for a novel polyhedral finite element approach. A polyhedral finite element code is explored, that retains most of the favorable properties of conventional finite element methods, while reducing the system size by up to an order of magnitude compared to conventional techniques for the same input surface. </p><p> In conjunction with a cardiac simulation code, the workflow is utilized to model finite-deformation cardiac mechanics. A quadratic tetrahedral mesh is generated from MRI data of the human heart ventricles. The constitutive law is comprised of an incompressible orthotropic hyperelastic stress response for the myocardium, plus an electrical activation-dependent active stress for the muscle fibers. Muscle fiber orientations are generated using a rule-based approach. Electrical activation times are read from a coupled electrophysiology code. A lumped circulatory model is used to impose time-dependent ventricular volume constraints. Simulation results are presented. The same mechanics are also implemented for the polyhedral finite element mesh, and preliminary verification results are presented. </p><p> The toolchain used in performing image-based cardiac mechanics simulations makes important improvements to the speed and robustness of image-based modeling techniques. As efforts continue to mature, so too does the promise for simulation to impact and improve healthcare.</p><p>

<p> This dissertation is primarily concerned with the development of an effective methodology for reducing structure-borne sound radiation from an arbitrarily shaped vibrating structure. There are three major aspects that separate the present methodology from all the previous ones. Firstly, it is a non-contact and non-invasive approach, which is applicable to a class of vibrating structures encountered in engineering applications. Secondly, the input data consists of a combined normal surface velocity distribution on a portion of a vibrating surface and the radiated acoustic pressure at a few field points. The normal surface velocities are measured by using a laser vibrometer over a portion of the structural surface accessible to a laser beam, while the field acoustic pressures are measured by a small array of microphones. The normal surface velocities over the rest surface of the vibrating structure are reconstructed by using the Helmholtz Equation Least Squares (HELS) method. Finally, the acoustic pressures are correlated to structural vibration by decomposing the normal surface velocity into the forced-vibro-acoustic components (F-VAC). These F-VACs are mutually orthogonal basis functions that can uniquely describe the normal surface velocity. The weightings of these F-VACs represent the relative contributions of structural vibrations into the sound radiation. This makes it possible to suppress structure-borne acoustic radiation in the most cost-effective manner simply by controlling the key F-VACs of a vibrating structure. The effectiveness of the proposed methodology for reducing structure-borne acoustic radiation is examined numerically and experimentally, and compared with those via traditional experimental modal analyses. Results have demonstrated that the proposed methodology enables one to reduce much more acoustic radiation at any selected target frequencies than the traditional approach.</p><p>

<p> A common technique to improve the performance of shell and tube heat exchangers (STHE) is by redirecting the flow in the shell side with a series of baffles. A key aspect in this technique is to understand the interaction of the fluid dynamics and heat transfer. Computational fluid dynamics simulations and experiments were performed to analysis the 3-dimensional flow and heat transfer on the shell side of an STHE with and without baffles. Although, it was found that there was a small difference in the average exit temperature between the two cases, the heat transfer coefficient was locally enhanced in the baffled case due to flow structures. The flow in the unbaffled case was highly streamed, while for the baffled case the flow was a highly complex flow with vortex structures formed by the tip of the baffles, the tubes, and the interaction of flow with the shell wall.</p>

<p> Different positions of a material used for structures experience different stresses, sometimes at both extremes, when undergoing processing, manufacturing, and serving. Taking the three-point bending as an example, the plate experiences higher stress in the middle span area and lower stress in both sides of the plate. In order to ensure the performance and reduce the cost of the composite, placement of different composite material with different mechanical properties, i.e. selective reinforcement, is proposed. </p><p> Very few study has been conducted on selective reinforcement. Therefore, basic understanding on the relationship between the selective reinforcing variables and the overall properties of composite material is still unclear and there is still no clear methodology to design composite materials under different types of loads. </p><p> This study started from the analysis of composite laminate under three point bending test. From the mechanical analysis and simulation result of homogeneously reinforced composite materials, it is found that the stress is not evenly distributed on the plate based on through-thickness direction and longitudinal direction. Based on these results, a map for the stress distribution under three point bending was developed. Next, the composite plate was selectively designed using two types of configurations. Mathematical and finite element analysis (FEA) models were built based on these designs. Experimental data from tests of hybrid composite materials was used to verify the mathematical and FEA models. Analysis of the mathematical model indicates that the increase in stiffness of the material at the top and bottom surfaces and middle-span area is the most effective way to improve the flexural modulus in three point bending test. At the end of this study, a complete methodology to perform the selective design was developed.</p>

<p> Developing light metals that have desirable mechanical properties is always the object of the endeavor of materials scientists. Magnesium (Mg), one of the lightest metals, had been used widely in military and other applications. Yet, its relatively poor formability, as well as its relatively low absolute strength, in comparison with other metals such as aluminum and steels, caused the use of Mg to be discontinued after World War II. Owing to the subsequent energy crisis of the seventies, recently, interest in Mg development has been rekindled in the materials community. The main focus of research has been quite straight-forward: increasing the strength and formability such that Mg and its alloys may replace aluminum alloys and steels to become yet another choice for structural materials. This dissertation work is mainly focused on fundamental issues related to Mg and its alloys. More specifically, it investigates the mechanical behavior of different Mg-based materials and the corresponding underlying deformation mechanisms. In this context, we examine the factors that affect the microstructure and mechanical properties of pure Mg, binary Mg-alloy (with addition of yttrium), more complex Mg-based alloys with and without the addition of lanthanum, and finally Mg-based metal matrix composites (MMCs) reinforced with ex-situ ceramic particles. More specifically, the effects of the following factors on the mechanical properties of Mg-based materials will be investigated: addition of rare earths (yttrium and lanthanum), in-situ/ex-situ formed particles, particle size or volume fraction and materials processing, effect of thermal-mechanical treatment (severe plastic deformation and warm extrusion), and so on and so forth. </p><p> A few interesting results have been found from this dissertation work: (i) although rare earths may improve the room temperature ductility of well-annealed Mg, the addition of yttrium results in ultrafine and un-recrystallized grains in the Mg-Y alloy subjected to equal channel angular pressing (ECAP); (ii) the reverse volume fraction effect arises as the volume fraction of nano-sized ex-situ formed reinforcements is beyond 10%; (iii) nano-particles are more effective in strengthening Mg than micro-particles when the volume fraction is below 10%; (iv) complete dynamic recovery and/or recrystallization is required to accomplish the moderate ductility in Mg, together with a strong matrix-particle bonding if it is a Mg-based composite; and (v) localized shear failure is observed in all Mg samples, recrystallized completely, which is attributed to the reduced strain hardening rate as a result of the exhaustion of twinning and/or dislocation multiplication.</p>

Thesis (M.S. in Mechanical Engineering)--Naval Postgraduate School, September 2002. / Thesis advisor(s): Young W. Kwon. Includes bibliographical references (p. 55-56). Also available online.

<p> It has become apparent in recent years the advancement of manufacturing processes in the aerospace industry. Sealant nozzles are a critical device in the use of fuel tank applications for optimal bonds and for ground service support and repair. Sealants has always been a challenging area for optimizing and understanding the flow patterns. A parametric study was conducted to better understand geometric effects of sealant flow and to determine whether the sealant rheology can be numerically modeled. The Star-CCM+ software was used to successfully develop the parametric model, material model, physics continua, and simulate the fluid flow for the sealant nozzle. The simulation results of Semco sealant nozzles showed the geometric effects of fluid flow patterns and the influences from conical area reduction, tip length, inlet diameter, and tip angle parameters. A smaller outlet diameter induced maximum outlet velocity at the exit, and contributed to a high pressure drop. The conical area reduction, tip angle and inlet diameter contributed most to viscosity variation phenomenon. Developing and simulating 2 different flow models (Segregated Flow and Viscous Flow) proved that both can be used to obtain comparable velocity and pressure drop results, however; differences are seen visually in the non-uniformity of the velocity and viscosity fields for the Viscous Flow Model (VFM). A comprehensive simulation setup for sealant nozzles was developed so other analysts can utilize the data.</p><p>