Open cell metal foams are a type of engineered material can be characterized by high porosity, high strength to weight ratio, tortuous flow paths and high surface area to volume ratio. It is the structure that gives the metal foams the characteristics that make them well suited for many application including heat exchangers. In this work, the structure of open celled metal foams is quantitatively characterized using an image analysis based method in order to predict the evaporative heat transfer of the metal foam using the fluid permeability. Several image processing algorithms were developed to quantitatively characterize the porosity, surface area per unit volume and the tortousity of metal foams from digital images of the cross sections of the material, and an expression was used to calculate the fluid permeability. An algorithm was developed to partion the pore space in the digital images so that individual cells within the structure could also be quantitatively characterized. Tools were also developed to predict the structure of open celled foam processed using the sacrificial template method by digitally constructing microstructures based the particle packing of the sacrificial templating material.

This thesis deals with metal foam, a relatively new material whose tremendous potential has been identified early on. The material is an excellent shock absorber and also has a very high strength-to-weight ratio, properties that are highly desirable particularly within the aerospace and automotive industries. Despite the material’s immense potential, hardly any metal foam products have made it past the prototype stage. The reason is that the material is difficult to manufacture in the shapes required in industrial applications. Oftentimes, applications require sheets to be bent into specific shapes, yet bending is not possible with conventional methods. Laser forming is currently the only method that shows promise to bend metal foam panels to a range of shapes. In this thesis, the analysis of laser forming of metal foam was taken far beyond the experimental work that has been delivered thus far. A thorough analysis was performed of the thermo-mechanical bending mechanism that governs the deformation of metal foam during laser forming. This knowledge was then used to explain the effect of the process condition on the bending efficiency and the bending limit. Additionally, the impact of laser forming on the metal foam properties was explored. Experimental results were complemented by numerical results that were validated both thermally (using infrared imaging) as well as mechanically (using digital image correlation). Numerical models with different levels of geometrical complexities were used, and the effect of the model geometry on the predictive accuracy was explored. In the second half of the thesis, the aforementioned effort was extended to metal foam sandwich panels, in which metal foam is sandwiched between two sheets of solid metal. The material again has a high strength-to-weight ratio and excellent shock absorption capacity, while also being stiff and core-protective. Just like metal foam alone, metal foam sandwich panels are typically manufactured in flat sheets, and failure-free bending can only be achieved using lasers. The analysis was again initiated with the bending mechanism. It was revisited whether the foam core still follows the same bending mechanism, and how its deformation is affected by the interaction with the solid facesheets. This insight was then used to elucidate the bending efficiency and limit at different process conditions, as well as the impact of the process on the material performance. Additionally, the effect of the sandwich panel manufacturing method on the process outcome was investigated. This was achieved by contrasting two sandwich panel types with a different foam core structure, foam core composition, facesheet composition and facesheet attachment method. Lastly, three-dimensional deformation of metal foam sandwich panels into typical non-Euclidean shapes such as bowl and saddle shapes was explored. It was shown that a significant amount of 3D deformation can be induced. At the same time, it was discussed that the achievable deformation is limited to moderate curvatures, since only a limited amount of in-plane strains may be induced using laser forming. The aforementioned experimental efforts were again accompanied by numerical efforts. Sandwich panel models with different levels of geometrical complexity were used to study all aspects pertaining to the process, and the properties to the facesheet/foam core interface were discussed. Overall, the work in this thesis demonstrated that laser forming is capable of bending metal foam panels and metal foam sandwich panels up to large bending angles without causing failures, while maintaining the favorable properties of the material. Conceptual, experimental and numerical groundwork was laid towards a successful implementation of the material in industrial applications.

Thesis (Ph. D.)--University of Washington, 2007. / Vita. Includes bibliographical references (p. 120-127).

Dissertation (PhD)--University of Stellenbosch, 2000. / ENGLISH ABSTRACT: A mathematical model is presented which conceptualises fluid flow and heat transfer in cellular metallic foams completely saturated with a fluid in motion. The model consists of a set of elliptic partial differential governing equations describing, firstly, a momentum balance in the fluid by the spatial distribution of its locally mean velocity, and secondly, an energy balance in the fluid and in the solid matrix of the metallic foam, by the spatial and temporal distribution of their locally mean temperatures. The separate energy balance descriptions for the fluid and the solid matrix extend the application of the model to conditions of thermal equilibrium and thermal non-equilibrium between the fluid and the solid matrix. A computational solution algorithm is presented which allows the universal application of the model to porous domains of arbitrary shape, with spatially and temporally variable heat loads in a variety of forms. / AFRIKAANSE OPSOMMING: 'n Wiskundige model word voorgestel wat vloei en warmteoordrag voorspel in sellulêre metaalsponse wat in geheel gevul is deur 'n bewegende vloeier. Die vloeier kan in gasof vloeistoffase verkeer. Die model bestaan uit 'n stel elliptiese parsiële differensiaalvergelykings wat in die eerste plek 'n momentum-ewewig in die vloeier beskryf in terme van 'n ruimtelike, lokaal-gemiddelde snelheidsveld, en wat tweedens 'n energie-ewewig in die vloeier en in die soliede matriks van die metaalspons beskryf in terme van ruimtelike en tydelike lokaal-gemiddelde temperatuur verspreidings. Die aparte energie-ewewig beskrywings vir die vloeier en vir die soliede matriks van die metaalspons brei die aanwending van die model uit na gevalle waar die vloeier en die soliede matriks in termiese ewewig of in termiese onewewig verkeer. 'n Numeriese oplossingsalgoritme word ook voorgestel vir die universele toepassing van die model op ruimtelik-arbitrêre metaalspons geometrië wat onderwerp word aan 'n aantal verskillende ruimtelik-en tydveranderlike termiese laste.

L'hydrogène n'étant pas une énergie primaire, il faut donc le produire, le transporter et le stocker avant de l'utiliser. Il peut être produit par des procédés chimiques, électrolytiques ou biologiques à partir de ressources renouvelables, ou non. Les énergies fossiles représentent la première ressource d'hydrogène, avec 96% de la production totale mondiale, dont 48% se fait à base de gaz naturel qui contient essentiellement du méthane. Dans cette thèse, nous nous intéressons à la génération de l'hydrogène par le procédé de vaporeformage du méthane qui reste le procédé le plus utilisé pour sa conversion. Les objectifs consistent premièrement à explorer, par des études numériques, les performances thermiques et massiques d'un vapo-reformeur à parois catalytiques, dans lequel une répartition discrète du catalyseur est adoptée, combinée ou non, avec une insertion d'un matériau cellulaire à haute porosité, de type mousse métallique, et deuxièmement à analyser, par une approche expérimentale complétée par une procédure numérique inverse, afin d'estimer le flux de chaleur inconnu reçu par le mélange gazeux. Les configurations géométriques adoptées dans les études numériques sont modélisées par les équations deconservation et complétées par les conditions aux limites. La cinétique de la réaction est régie par un modèle basé sur les lois de puissance, et le système d'équations est résolu par la méthode des volumes finis. Pour l'estimation du flux de chaleur, un dispositif expérimental approchant le système de chauffage du réacteur est conçu afin de mesurer la distribution de la température et un code de calcul inverse basé sur la méthode spécification de fonctions. Les résultats montrent que les performances du procédé de vaporeformage peuvent être améliorées en adoptant une bonne distribution du catalyseur sur les parois du réacteur muni d'une mousse métallique dans sa région catalytique. Les améliorations obtenues en termes de conversion de méthane, par rapport à une configuration classique, sont de l'ordre de 44.6%. De plus, la combinaison des approches expérimentale et numérique a permis de déterminer la quantité de chaleur nette transférée par le système de chauffage du vaporeformeur. / Hydrogen is not a primary energy; we must produce it, transport it and store it before use. It cans be produced by chemical, biological or electrolytic processes from renewable resources or not. Fossil fuels represent the first hydrogen resource, with 96% of total world production, which 48% is made from natural gas containing methane. In this thesis, we focus on the generation of hydrogen by the steam-methane reforming process, which is the most used conversion method. The aims consist first to explore, through numerical studies, the thermal and mass performances of a wall coated steam-methane reformer, wherein a discrete distribution of the catalyst is adopted, combined or not, with an insertion of a highly porous metal foam, and secondly to analyze, by an experimental approach completed by a numerical inverse procedure to estimate the unknown heat flux received by the gas mixture. The geometric configurations adopted in the numerical studies are modeled by the conservation equations and the boundary conditions. The kinetic reaction is governed by a model based on power laws, and the system of equations is solved by the finite volume method. For the estimation of heat flux, an experimental device approachingthe reactor heating system is designed to measure the temperature distribution, and an inverse code based on the function specification method. The results show that the steam methane reforming process performances can be improved by adopting a good distribution of the catalyst on the walls of the reactor fitted on its catalytic region with metal foam. The improvements obtained in terms of methane conversion, compared to a conventional configuration, are of the order of 44.6%. In addition, the combination of experimental and numerical approaches was used to determine the net quantity of heat transferred from the heating system to the steam reformer.

Nanoporous (NP) metal foams combine desirable characteristics of metals with unique nanoarchitectural features to yield weight normalized properties far superior than either dense metals or bulk metal foams. Due to their high surface to volume ratios these structures show great promise as components of fuel cells, as sensors and have been suggested for use in biological applications, for example as antimicrobial scaffolds or as platforms on which to explore biological material behavior. While most NP metal foams are isotropic, structures with anisotropic features spanning different length scales can further extend applications. This work examines the parameters controlling the synthesis of transversely isotropic NP Platinum foam by dealloying an amorphous Pt-Si alloy. The structure that is examined in this work is hierarchical with Voronoi polyhedra that form on the free surface and under each polyhedral hyper-structure, nanocrystalline NP Pt foam forms with radial struts of length ~60 nm and grain size of 5 nm. The size of the polyhedra can be tailored by changing the dealloying potential. In turn, the mechanical properties of these structures as assessed by nanoindentation can range from 1 to 3GPa depending on the geometric arrangement of the struts. Finally, the initiation location of these structures and the relationship between electrochemical parameters and dealloying front evolution is examined.

The major challenge associated with renewable-energy systems especially solar, is the supply intermittency. One effective solution is to incorporate thermal energy storage components utilizing phase change materials (PCMs). These materials have the potential to store large amounts of energy in relatively small volumes and within nearly an isothermal storage process. The primary drawback of today’s PCMs is that their low thermal conductivity values critically limit their energy storage applications. Also, this grossly reduces the melting/ solidification rates, thus making the system response time to be too long. So, the application of heat transfer enhancement is very important. To improve the PCM storage performance, an efficient performing containment vessel (triplex-tube) along with applications of various heat transfer enhancement techniques was investigated. The techniques were; (i) dispersion of solid nanoparticles, (ii) incorporation of metal foam with nanoparticle dispersion, and (iii) insertion of longitudinal fins with nanoparticle dispersion. Validated simulation models were developed to examine the effects of implementing these techniques on the PCM phase-change rate during the energy storage and recovery modes. The results are presented with detailed model description, analysis, and conclusions. Results show that the use of nanoparticles with metal foam or fins is more efficient than using nanoparticles alone within the same volume usage. Also, employing metal foam or fins alone results in much better improvement for the same system volume.

Exponential growth of heat flux densities in commercial and industrial electronics, and compact heat exchangers demand surfaces and heat sinks with high dissipation rate capabilities. Among different technologies proposed to meet these demands, high-porosity metal foams have attracted the attention of many investigators due to their higher surface area densities as well as higher thermal performance due to the turbulence and tortuosity generated in the flow due to their structure. One of the disadvantages of such metal foams, however, is the attendant higher pressure drop or pumping power penalty. This thesis was undertaken to investigate whether channels partially filled with metal foams can reduce the required pumping power with a minimal loss in thermal performance. The thermo-hydraulic (T-H) performance factor J/F<sup>1/3, where J is the Colburn-J factor and F is the friction factor, was used to compare the relative performance of foams for various values of blocking fractions (B), where B is defined as the ratio of the height of the foam to the height of the channel. The metal foam samples considered were 10 PPI (pores per inch) 6101-T6 Aluminum, with porosity of ∼ 94 − 96%, and B of 1/6, 1/3, 2/3, 5/6, and 1. Each of these samples was attached to an aluminum slab embedded in one of the walls, which had a patch heater that acted as a heat source. A modification was made to all B < 1 configurations by attaching an aluminum plate on top, which then separated the foam-free and the foam-filled flows completely. These configurations are denoted by a 'P' in their names (e.g. B = 1/3P is the plated modification of B = 1/3). Experiments were conducted in an in-house designed wind tunnel, with a test section of 45" in length and a cross-section of 3"X3". Reynolds number (based on channel hydraulic diameter and inlet velocity) was varied from 1,000 to 15,000 to capture the flow domains from laminar to turbulent. The data obtained for the three scenarios namely - 1. Controlled-Flow Scenario 2. Pumping Power Variation with Temperature Difference, and 3. Fan-Based System were analyzed for their thermo-hydraulic performance. The extreme low blocking fractions are evaluated and compared against the dimpled/protruded surfaces, and were found to give superior performance, hence displaying potential as good turbulators. The plated configurations were found to perform better in almost all scenarios when compared to their non-plated counterparts. Furthermore, a new simplified analytical model is introduced that considers the flow in the partially-blocked region as two separate 'parallel' flows, one in the foam-free region and the other in the foam-filled region. The comparison between this novel approach and the analytical solution from the literature shows good agreement, suggesting that this simplified model may be appropriate. This model is then used for determining the foam-filled region flow ratios for the performed experiments, and a correlation is presented. / Master of Science / Portable devices, such as laptops, and mobile phones are trending towards miniaturization and simultaneously becoming more power-hungry, leading to ever-increasing heat flux densities. Growing energy and technology demands require high thermal dissipation rates to be achieved in equipment such as industrial and commercial electronics, data centers, heat exchangers in automobiles, and power plants - both renewable and non-renewable. One of the best ways to enhance convective heat transfer is by increasing the heat transfer surface area. This is traditionally done using fins. A much higher surface area can be achieved using a metal foam instead, along with improving the turbulent mixing of the fluid. The flow through the metal foam, however, faces a higher pressure drop penalty which is one of the major reasons for a continued preference for fins. In this experimental study, we aim at minimizing this pressure drop penalty of a metal-foam heat-sink along with maintaining a respectable heat transfer performance through 'partial-blocking' (filling) of the channel, where the height of the foam is lower than the total channel height. The ratio of metal foam height to the channel height is named as blocking fraction B. A general comparison of the hydraulic, thermal, and thermo-hydraulic (T-H) performance reveals that the ∼ 83.3% plated configuration is capable of superseding the baseline of full blockage. The 'plating' here denotes a slight modification - a solid plate rests on top of the metal foam, separating the foam-free and foam-filled flow. For applications with Re > 10000, ∼ 33.3% plated configuration is highly recommended. For fan-based systems, ∼ 83.3% plated, ∼ 33.3% plated, and 33.3% non-plated configurations emerge as possible alternatives to the fully-blocked case. Furthermore, while considering partial configurations, it is shown that one should go for lower PPI metal foams to improve the flow ratio inside the metal foam. For pressure-drop critical equipment, ∼ 16.7% configuration is found to perform better than the conventional double-protruded walls and other turbulence-enhancing surface treatments. Finally, this thesis presents a novel and simplified approach for estimating the flow ratios for partially-blocked channels using scaling analysis.

Thermal energy storage (TES) systems based on renewable energy sources (concentrated solar, wind, and photovoltaic etc.) are crucial to reducing dependence on conventional energy generation systems and reducing renewable energy’s intermittent nature. TES can be utilized in conjunction with concentrated solar power (CSP) in particle-based power cycles where the particles can be charged (heat addition) using solar energy and then discharged (heat extraction) using particle-based heat exchangers (HX). Efficient particle based HXs are vital in coupling heat transfer fluid (HTF) from thermal receivers to power cycle working fluid (WF). Heat transfer enhancement is essential for adopting particle-based moving packed-bed heat exchangers (MPBHXs) in next-generation TES systems, as MPBHXs usually exhibit low particle bed-to-wall heat transfer coefficients and total heat transfer rate. This dissertation focuses on addressing the limitations of MPBHXs by computationally studying the heat transfer performance enhancement due to granular flows in metal foam-based MPBHXs and reactive flow-based MPBHXs. Comprehensive multidimensional, multiscale, and multiphysics models are developed to predict the TES/TCES (Thermochemical energy storage) performance accurately. First, the flow properties through metal foams are determined, followed by granular flow through metal foam-based particle-to-sCO2 HXs to predict the heat transfer enhancement. Then, granular flows with reactive and sensible heat-only particles are studied in particle-to-sCO2 HXs to predict the heat transfer enhancement, followed by the development of discrete element models (DEM) in inclined moving bed granular flows to study particle-scale heat and mass transfer. Overall, this study provides valuable insights into effective modeling of granular flows from continuum to discrete scales and improved design and operation of particle-based heat exchangers and thermochemical reactors.

Superconductors present great potential for weight reduction and increased power delivery when compared to traditional copper power delivery systems, but current systems require cryogenic cooling systems. Traditional superconductor cooling systems consist of helium cooled by helical heat exchangers made of Oxygen Free High thermal Conductivity (OFHC) copper tube. The helium is cooled by bulky heat exchangers consisting of OFHC copper coils wrapped around a cryogenic cooler heat sink for heat transfer into the working fluid. Metal foams have recently been studied in a variety of heat transfer applications, and could greatly reduce the weight of heat exchanger modules in superconductor cooling systems while simultaneously providing increased heat transfer effectiveness. Aluminum and Copper foams have been available for several years, but more recently, graphite foams, such as PocoFoam™, have been developed which have particularly good heat transfer characteristics. Using Computational Fluid Dynamics (CFD) to model a cryogenic heat exchanger application, this study examines the effectiveness and pressure drop of several metal foam heat exchangers, and compares their performance with the traditional helical coil design for superconductor cooling applications. The CFD simulation results show that a heat exchanger with the same heat sink contact area as existing helical heat exchangers weighs up to 95 percent less and can be up to 25 percent more effective, depending on system conditions such as pressure, cryogenic cooler temperature and helium inlet temperature. Aluminum and copper foam heat exchangers had comparable weight to the PocoFoam heat exchanger, but were significantly less effective than the helical or PocoFoam heat exchanger models.