Open Cell Conducting Foams for High Synchrotron Radiation Beam Liners

The possible use of open-cell conductive foams in high synchrotron radiation particle accelerator beam liners is considered. Available materials and modeling tools are reviewed, potential pros and cons are discussed, and preliminary conclusions are drawn.Comment: 21 pages, 5 figures, 3 tables, 51 references, submitted to PR-STA

Advances in Experimental Methods for Characterisation of Porous Solids

This thesis describes work on the development of new integrated methods for the characterisation of porous media. Porous media are of wide importance in a variety of applications including oil and gas production, tissue engineering, filtration and separation and ground water hydrology. Such porous media are characterised by the fraction of their volume occupied by pores (the porosity) and by parameters characterising the ease of flow and diffusion through the medium (the permeability and the diffusivity). However, the flow processes are very complex, reflecting the complex nature of the pore structure. The objective of the work described in this thesis was to develop and apply two new integrated pieces of apparatus which were aimed at elucidating several aspects of the complex flow processes. The first integrated apparatus was aimed at the study of gaseous transport and the second at the study of mercury penetration, flow and electrical conduction, in the pores of selected media. Thin-section imaging was also applied to obtain supplementary information. The integrated gaseous transport apparatus was designed not only to measure permeability (by the pressure rise technique), but also to study both steady state and (importantly) transient diffusion of oxygen in nitrogen in the pores of the selected media. The system was capable of operating with media with a wide range of permeabilities and yielded accurate values of the viscous permeability and the slip flow coefficient. In addition, experiments were carried out in the partially turbulent flow region. The diffusion measurements yielded information on the ratio (rD) of the diffusion coefficient in the media to that in free space; this ratio is also a specific property of the medium. Combining the steady state and transient diffusion measurements, it was possible to deduce the effective porosity and the pore length. The second integrated apparatus was for the study of porous media subjected to mercury penetration under pressure. This apparatus allowed conventional mercury porosimetry measurements to be performed (i.e. measurements of the pore volume occupied as a function of pore size penetrated) but, crucially, it also allowed measurements of the permeability to mercury and of the conductivity with mercury to be made simultaneously. The permeability to mercury approaches the gaseous value when complete saturation of the medium is achieved; however the manner in which mercury permeability varies with pore size gives a striking indication of the role of the various pores in the flow process. In the conductivity experiments, the ratio (rCp) of the conductivity of the medium penetrated with mercury at pressure p to the conductivity of pure mercury is determined as a function of p. At high penetration pressures, the value of rCp approaches that of rD reflecting the analogy between conduction and diffusion when the medium is saturated with mercury. However, the variation of rCp with pore size penetrated gives a remarkable indication of the significance of the pores in given size ranges in the diffusion process (analogous to the information yielded by the mercury permeability measurements relating to the overall permeability)

Evaluation of nonmetallic thermal protection materials for the manned space shuttle. Volume 1, task 1: Assessment of technical risks associated with utilization of nonmetallic thermal protection system

Technical problems of design and flight qualification of the proposed classes of surface insulation materials and leading edge materials were reviewed. A screening test plan, a preliminary design data test plan and a design data test plan were outlined. This program defined the apparent critical differences between the surface insulators and the leading edge materials, structuring specialized screening test plans for each of these two classes of materials. Unique testing techniques were shown to be important in evaluating the structural interaction aspects of the surface insulators and a separate task was defined to validate the test plan. In addition, a compilation was made of available information on proposed material (including metallic TPS), previous shuttle programs, pertinent test procedures, and other national programs of merit. This material was collected and summarized in an informally structured workbook

Heat transfer enhancement in phase change materials (PCMs) by metal foams and cascaded thermal energy storage

Low heat transfer performance has been the main problem restricting the use of Phase Change Materials (PCMs) in situations requiring rapid energy release or storage. Three innovative solutions are studied in this Thesis to improve heat transfer in PCMs. These include combining PCMs with metal foams, Cascaded Thermal Energy Storage (CTES) and Metal Foam-enhanced Cascaded Thermal Energy Storage (MF-CTES). Heat conduction is investigated in Chapter 3, in which it was found that metal foams can improve heat conduction of PCMs by 5–20 times. Natural convection is investigated in Chapter 4, in which metal foams were found to suppress natural convection due to their large flow resistances. Nevertheless, metal foams can still achieve a higher overall heat transfer rate (3–10 times) than PCMs without metal foams. CTES is examined in Chapter 5, with results showing that CTES has a higher heat transfer rate (30%) and a higher exergy transfer rate (22%) than Single-stage Thermal Energy Storage (STES). MF-CTES is proposed in Chapter 6; this is, to the best knowledge of the author, the first time that it has been investigated. MF-CTES was found to further improve the heat and exergy transfer of CTES by 2–7 times, meanwhile reducing melting time by 67%–87%

Wet steam drying: Microwave-assisted droplet evaporation in open-cell ceramic foams

In many energy and process engineering systems where fluids are processed, droplet-laden gas flows may occur. As droplets are often detrimental to the system’s operation, they are required to be removed. According to the state-of-the-art, industrial droplet removal is achieved through a sequential arrangement of several separators followed by droplet collection and discharge. This results in a high-quality gas stream, yet at the expense of bulky and expensive systems that are difficult to retrofit to existing facilities. In addition, the multiple sequential separators produce high pressure drops, further increasing operating costs. Alternatively, a single droplet separation stage and in situ evaporation would provide compact solutions for facilities. However, compact engineering solutions for the removal of entrained droplets are difficult to achieve with conventional flow control and conduction heat transfer approaches such as Joule heating. Joule heating requires a well-defined and homogeneous electrical resistance to ensure uniform heating, which is technically challenging to apply in fine separators and thus compact removal devices are hence often costly and ineffective. Therefore, it becomes necessary to investigate alternative heating approaches to overcome these challenges, such as volumetric heating using microwaves. The research conducted in this thesis aims to analyze the potential of a compact microwave solution approach for droplet removal. The compactness of the approach relies on a novel fine separator structure enhanced by microwave-heat transfer for efficient in-flow droplet evaporation. The investigation targets at fundamental studies of the combined effect of droplet flow filtering and heat transfer from numerical calculations and experimentation. As novel fine separators, solid open-cell foams are a promising alternative for the separation of liquid droplets suspended in gas flows at comparably low pressure drops. Using susceptors, such as dielectric materials, for the skeleton and exposing them to microwaves is an efficient way to use them as heating elements. Silicon carbide (SiC) based open-cell foam samples were considered for the study as they are good susceptor materials. First, pore-scale fluid numerical simulations on representative foam models were used to obtain a deeper insight into the effects of pore size and pore density on the droplet retention time within foams. Numerical findings were reported considering the pressure gradient and the residence time distribution of droplets under different superficial flow velocities, droplet sizes, porosities and pore densities. Next, the temperature-dependent permittivity of SiC-based foam materials was determined by the cavity perturbation technique using a waveguide resonator at a microwave frequency of 2.45 GHz up to 200 °C. The permittivity was of particular interest as it is a crucial parameter for predicting and designing systems utilizing microwave heating. Along the permittivity measurements, electromagnetic wave propagation simulations were used to derive novel mixing relations describing the effective permittivity of foams while considering their skeletal morphology. The derived relations facilitate an efficient and reliable estimation of the effective permittivity of open-cell foams, producing good agreement to experimental data. Using the foams dielectric properties and the fluid characteristics of droplet-laden streams, a microwave applicator was designed to concentrate the electric field on the open-cell foams. The applicator was constructed for carrying out experimental studies on droplet evaporation removal under different flow velocities, microwave power and different SiC-based foams. Measurements of droplet size, velocity, number density and flux at the inlet and outlet streams of the applicator were performed using a 2D-phase Doppler interferometer. Eventually, it was found from the experimental data analysis that the application of open-cell ceramic foams as a filter medium reduced 99.9 % of the volumetric flow of droplets, while additional microwave exposure increased the reduction to 99.99 %. In addition, microwave-heated foams prevent droplet re-entrainment and structure-borne liquid accumulation within foams, thus avoiding water clogging and flooding. Hence, open-cell foams can be used as fine droplet separators as long as microwave heating may effectively evaporate accumulations of liquid. An important factor in designing future devices based on this microwave heating approach is the temperature, as it changes the arcing breakdown voltage of the gas, thus limiting the microwave input power and droplet flow velocity. Although more investigations are needed to develop an applicable and optimal product, the results presented in this thesis provide a first insight into the viability of using microwave heating and fine filtering as a compact solution for droplet removal

Fluid Flow and Heat Transfer in Cellular Solids

To determine the characteristics and properties of cellular solids for an application, and to allow a systematic practical use by means of correlations and modelling approaches, we perform experimental investigations and develop numerical methods. In view of coupled multi-physics simulations, we employ the phase-field method. Finally, the applicability is demonstrated exemplarily for open-cell metal foams, providing qualitative and quantitative comparison with experimental data

Characterization of Ceramic Foam Filters Used for Liquid Metal Filtration

In the current study, the morphology including tortuosity, and the permeability of 50-mm thick commercially available 30, 40, 50,and 80pores per inch (PPI) alumina ceramic foam filters (CFFs) have been investigated. Measurements have been taken of cell (pore), window, and strut sizes, porosity, tortuosity, and liquid permeability. Water velocities from ~0.015 to 0.77m/s have been used to derive both first-order (Darcy) and second-order (Non-Darcy) terms for being used with the Forchheimer equation. Measurements were made using 49-mm "straight through” and 101-mm diameter "expanding flow field” designs. Results from the two designs are compared with calculations made using COMSOL 4.2a® 2D axial symmetric finite element modeling (FEM), as a function of velocity and filter PPI. Permeability results are correlated using directly measurable parameters and compared with the previously published results. Development of improved wall sealing (49mm) and elimination of wall effects (101mm) have led to a high level of agreement between experimental, analytic, and FEM methods (±0 to 7pct on predicted pressure drop) for both types of experiments. Tortuosity has been determined by two inductive methods, one using cold-solidified samples at 60kHz and the other using liquid metal at 50Hz, giving comparable result

Thermal Lattice Boltzmann Methods for the Simulation of Turbulent Flows with Conjugate Heat Transfer – Application to Refrigerated Vehicles

In dieser Arbeit wird eine thermische Lattice-Boltzmann-Methode (TLBM) für die instationäre Simulation turbulenter Strömungen mit natürlicher Konvektion und konjugierter Wärmeübertragung vorgestellt. Turbulente Strömungen mit ihren chaotischen Druck- und Geschwindigkeitsschwankungen stellen eine besondere Herausforderung für numerische Simulationen dar, wobei turbulente Strömungen, angetrieben durch thermische Auftriebskräfte, eine besonders schwierige Aufgabe darstellen. Wie in dieser Arbeit gezeigt wird, ermöglicht TLBM Large Eddy Simulationen (LES) solcher Probleme im industriellen und technischen Maßstab unter Verwendung eines Smagorinsky-Feinstruktur-Modells und unter Ausnutzung seiner intrinsischen Parallelisierbarkeit sowie der Möglichkeit, mehrere tausend Prozessorkerne zu verwenden. Die Eignung der vorliegenden Methode wird in dieser Arbeit anhand von Anwendungen zur Simulation der Innenluftströmung und der Isolationseffizienz eines Kühlwagens, des Wärmetransports im Luftspalt zwischen Rotor und Stator bei Elektromotoren, der Weiterentwicklung hocheffizienter Isolation auf der Basis von Vakuumisolationspaneelen (VIP) und Latentwärmespeichern sowie deren Anwendung in Kühlwagen gezeigt. Eine umfassende Validierung der Methode und ihrer Implementierung im Open-Source-Framework OpenLB wird durchgeführt. Gitterkonvergenz zweiter Ordnung wird gegen das analytische Porous Plate Problem demonstriert, während stabile Simulationen auch bei grober Diskretisierung mit hohen Reynolds- und Rayleigh-Zahlen erreicht werden. Eine sehr gute Übereinstimmung wird für natürliche Konvektion in einem quadratischen Hohlraum, ein bekannter Benchmark-Fall, vom laminaren zum turbulenten Regime mit 10^3 <= Ra <= 10^10 und bei Auflösungen von y+ ~ 2 gezeigt. Im ersten Teil der Ergebnisse werden Simulationen eines leeren Kühlaufbaus für einen Kühllastwagen vorgestellt. Das Strömungsfeld und der Wärmeübergang innerhalb eines gegebenen Kühllastwagens zeigt eine sehr gute Übereinstimmung mit den Messergebnissen, insbesondere den experimentellen Daten für ein Kühlfahrzeug bei Re ~ 53000 an vier charakteristischen Geschwindigkeits- und 13 Temperaturpositionen im Lastwagen. Die Wärmeübertragung durch die Wände wird in den Simulationen durch konjugierte Wärmeübertragung aufgelöst. Dies ermöglicht nun die präzise Vorhersage von Wärmeströmen nahe von Nusselt-Korrelationen für den gegebenen Aufbau, aber - im Gegensatz zu gewöhnlichen Nusselt-Korrelationen - wird der Wärmestrom in der Simulation räumlich aufgelöst. Im zweiten Teil der Ergebnisse wird die Strömung und der Wärmeübergang in einem Ringspalt mit innen rotierendem Zylinder untersucht. Die besondere Herausforderung bei der Simulation dieser Taylor-Couette-Strömung ist die Bildung von Taylor-Wirbeln, die durch ihre Rotation senkrecht zur Hauptströmungsrichtung den entsprechenden Wärmeübergang deutlich erhöhen. Detaillierte instationäre Simulationen werden über einen weiten Drehzahlbereich von fast schleichender Strömungen bis hin zum Auftreten von Taylor-Wirbeln durchgeführt. Es wird eine gute Übereinstimmung mit bisherigen Ergebnissen für die Strömungsstrukturen und die Verbesserung des Wärmeübergangs durch Taylor-Wirbel festgestellt. Insbesondere wird die vorliegende Methode mit Messungen, einer Korrelation und Simulationen unter Verwendung des Scherspannungstransport-Turbulenzmodells (SST) verglichen. Besonderes Augenmerk wird auf die Vorhersage der kritischen Taylor-Zahl gelegt. Während direkte numerische Simulationen (DNS) mit LBM die kritische Taylor-Zahl aus den Experimenten nahezu identisch vorhersagen, wird sie von LBM-LES leicht und vom SST-Modell weiter überschätzt, was auf die übermäßig dissipative Natur der Turbulenzmodelle für die Transition zurückzuführen ist. Im dritten Teil der Ergebnisse werden innovative Konzepte für verbesserte, nachhaltigere Kühlfahrzeuge numerisch untersucht. Um den Kraftstoffverbrauch und die damit verbundenen Emissionen zu reduzieren, werden zwei Ansätze als vielversprechend angesehen: (a) der Einbau von Vakuum-Isolationspaneelen (VIP) in die Wände des Kühlkoffers und (b) die Einführung eines Latentwärmespeichers (LHS) zum Austausch der kraftstoffbetriebenen Klimaanlage (AC). Die Verwendung des vorliegenden TLBM erlaubt in den Simulationen die Auflösung der durch die AC und die natürliche Konvektion induzierten turbulenten Luftströmung, des Wärmeflusses innerhalb der Isolierwände und der tiefgefrorenen Ladung. Dies liefert neue Erkenntnisse über den Einfluss der Konzepte auf die Wärmeübertragung in verschiedenen Kühlaufbauten. Die Simulationen zeigen einen stark reduzierten und homogenisierten einströmenden Wärmestrom für das kombinierte PUR- und VIP-Isoliermaterial im Vergleich zu einer reinen PUR-Isolierung. Die Dämmung des Kühlaufbaus mit VIPs halbiert daher die erforderliche Kühlenergie. Dies ermöglicht den Ersatz der AC durch einen LHS in Dachnähe und ein zusätzliches Lüftungssystem mit deutlich geringerer Gesamtleistung. Unter Berücksichtigung der Temperaturhomogenität von Tiefkühlprodukten wird eine leichte Umströmung des Kühlgutes als notwendig erachtet. Die maximal zulässige Ausfallzeit der AC wird in den Simulationen mit jeweils ca. 3,3 min (PUR), 8 min (PUR+VIP) und 11 min (PUR+VIP+LHS) ermittelt. Im vierten Teil der Ergebnisse wird eine LBM zur Simulation des Schmelzens und des konjugierten Wärmeübergangs auf der Basis des Transports der Gesamtenthalpie vorgestellt, welche bei Validierung gegen die analytische Lösung des zeitabhängigen Stefan-Problems präzise Ergebnisse liefert. Die in dieser Arbeit entwickelte Methode zeigt geringe Grenzflächendiffusion für einen weiten Bereich von Relaxationszeiten und Stefan-Zahlen. Weiterhin wird eine enge Übereinstimmung für das Schmelzen von Gallium einschließlich der natürlichen Konvektion in 2D und 3D mit Messungen und Simulationen mit unterschiedlichen Ansätzen gezeigt. Das Modell wird ferner auf das Schmelzen von Paraffin in zwei komplexen Metallschaumgeometrien angewendet. Es wird eine Voxel-basierte parallele Vernetzung vorgestellt, die eine schnelle und automatisierte Verarbeitung der komplexen Geometrie in wenigen Minuten ermöglicht. Die Simulationen erfassen erfolgreich den materialübergreifenden Wärmetransfer in 3D, wobei die Wärmeleitfähigkeit des Schaums mehr als 1000-mal größer als die des Paraffins ist. Die Form der Schmelzfront und der Einfluss der spezifischen Oberfläche der verschiedenen Metallschäume stehen in enger Übereinstimmung mit früheren Simulationen