Universität Stuttgart
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Item Open Access Modeling and simulation of closed low-pressure adsorbers for thermal energy storage(2019) Schäfer, Micha; Thess, André (Prof. Dr. rer. nat.)Closed low-pressure adsorption systems can be applied for thermal energy storage. Their performance is determined by the mass and heat transport processes in the adsorber. Therefore, thorough knowledge of these transport processes is required for further storage development. The present thesis contributes to this by providing detailed models of closed low-pressure adsorbers and by conducting simulations over a broad range of parameters and configurations. The focus is on adsorbers of larger scale (length L = 0.1 . . . 1 m) and on the discharging process. As the adsorption pair, binderless zeolite 13X with water is examined. The models are developed in a stepwise manner from pore to storage scale. The Finite-Difference-Method is implemented to numerically solve the models. Simulations are conducted for defined reference cases as well as over a broad range of geometric and process parameters. The reference cases are analyzed in detail to gain a better understanding of the transport processes. Furthermore, the results are analyzed with respect to two particular modeling aspects: equilibrium assumptions and rarefaction effects (e. g. slip effect). With respect to the application, the discharging performance is analyzed in terms of thermal power and a defined discharging degree. Both the adsorber and the adsorbent configurations are varied. In addition, the effect of the discharging conditions is evaluated. Finally, one exemplary charging process is examined. The detailed analysis of the reference cases reveals that the mass and heat transport and the adsorption processes are strongly coupled and can only be understood in their interaction. For onedimensional adsorber configurations, that is the mass and heat transport are in the same direction, the discharging process is generally limited by the heat transport. This leads to insufficient thermal power and unsuitable discharging durations of up to one year. In contrast, for two-dimensional adsorber configurations, that is the mass and heat transport are in perpendicular directions, the discharging process can be limited either by the mass or heat transport or by the adsorption. The limitation depends on the configuration of the adsorber and adsorbent. Moreover, the twodimensional adsorber configurations can provide sufficient thermal power. With respect to the modeling, it is found that the assumption of a uniform pressure distribution is applicable for one-dimensional adsorber configurations. In contrast, for two-dimensional configurations, no equilibrium assumptions can be applied in general. However, for powder adsorbent it is always valid to assume local adsorption equilibrium. Regarding the rarefaction effects in twodimensional adsorber configurations with honeycombs and granules, the slip effect is relevant for small channel and particle diameters (d = 1 mm). For adsorbers with powder adsorbent, the reduction of the effective heat conductivity due to the rarefaction effect becomes relevant. With respect to the application, the variation of the adsorber configuration shows that the volumetric thermal power generally decreases with increasing adsorber length. Furthermore, the power decreases with increasing width between the parallel heat exchanger plates in the adsorber. Regarding the adsorbent configuration in two-dimensional adsorber configurations, it is found that the volumetric thermal power can be optimized by variation of the channel or particle diameter. Interestingly, the optima for peak and mean power do not coincide. In addition, the discharging degree is found to strongly depend on the discharging conditions in terms of discharging temperature and volume flow of the heat transfer fluid extracting the heat from the adsorber. In general, the discharging degree decreases with increasing discharging temperature. Similarly, the discharging degree decreases with increasing volume flow of the heat transfer fluid. Finally, the analysis of an exemplary charging process revealed that the pressure in the adsorber can increase significantly (> 50%) due to the desorption.Item Open Access Development and application of PICLas for combined optic-/plume-simulation of ion-propulsion systems(2019) Binder, Tilman; Fasoulas, Stefanos (Prof. Dr.-Ing.)Electric propulsion systems are an efficient option for altitude/attitude control and orbit transfers of spacecraft. One example is the gridded ion thruster which ionizes the propellant and accelerates the ions of the generated plasma by a high-voltage grid system. This work deals with the numerical simulation of the plasma flow starting near the grid system in the ionization chamber and leaving the thruster with high velocity. These simulations give direct insight into the modeled, physical interrelationships and can be used to investigate questions arising in the industrial development process of ion propulsion systems. The required simulation method is challenging due to the high degree of flow rarefaction and the plasma state itself, including freely moving ions and electrons. Applicable simulation methods belong to a particle-based, gas-kinetic approach, such as Particle-In-Cell (PIC) for the simulation of electromagnetic interaction and the Direct Simulation Monte Carlo (DSMC) for inter-particle collisions. The effects resulting from the finite size of a real system can only be investigated by simulating the complete, three-dimensional thruster geometry which requires a large and complex simulation domain. Acceptable simulation times are realized by expanding and using the framework of the coupled PIC-DSMC code PICLas in combination with high performance computing systems.Item Open Access Präzise Fahrzeugpositionierung durch Entzerrung der gepulsten magnetischen Flussdichteverteilung einer Ladespule(2017) Martinovic, Dean; Reuss, Hans-Christian (Prof. Dr.-Ing.)Elektrofahrzeuge werden in Zukunft nicht mehr per Kabel, sondern mittels induktiver Ladesysteme mit Strom versorgt. Um eine hohe Ladeleistung sicher übertragen zu können, müssen die Spulen hinreichend genau übereinander positioniert werden, was für den Fahrer eine kaum lösbare Aufgabe darstellt. Das allgemeine Ziel der vorliegenden Arbeit ist es daher, eine neue Methode zu untersuchen, die ein gepulstes Magnetfeld der Ladespule zu dessen Ortung nutzt. Hierbei wird das magnetische Pulssignal durch den ferromagnetischen Unterboden des Elektrofahrzeugs verzerrt. Dieser verändert die Pulsamplitude entsprechend einer unbekannten Abbildung, ohne deren Kenntnis eine präzise und eindeutige Positionierung nicht möglich ist. Die Herausforderung der vorliegenden Arbeit ist daher die Bestimmung dieser Abbildung samt ihrer Eigenschaften und Abhängigkeiten. Theoretische Untersuchungen zeigen, dass die Abbildung allgemein vom nicht-deterministischen magnetischen Zustand des Unterbodenmaterials abhängt und dessen messtechnische Erfassung kaum möglich ist. Im weiteren Verlauf der Untersuchungen wird jedoch hergeleitet, dass die Ladespule, das Elektrofahrzeug und die umgebende Atmosphäre zusammen einen magnetischen Kreis bilden, der aufgrund der sehr hohen Reluktanz der Atmosphäre linear ist. Änderungen des magnetischen Zustands haben folglich keinen Einfluss auf die Abbildung. Diese ist somit reproduzierbar und kann messtechnisch einfach erfasst werden. Die These wird für unterschiedliche magnetische Zustände experimentell nachgewiesen. Basierend auf den Forschungsergebnissen wird ein vollständiger Prototyp entwickelt und in ein Versuchsfahrzeug integriert. Das Gesamtsystem wird anschließend erfolgreich getestet. Die gefundenen Ergebnisse zeigen, dass mittels gepulster magnetischer Felder eine universelle, kostengünstige, sichere und präzise Positionierung von Elektrofahrzeugen möglich ist. Dies unterstreicht das Potential des neuen, komfortablen Positionierungsverfahrens eine Schlüsseltechnologie für die Elektromobilität zu werden.Item Open Access Dynamics of finite-dimensional mechanical systems(2019) Winandy, Tom; Leine, Remco I. (Prof. Dr. ir. habil.)This monograph deals with the description of mechanical systems having finitely many degrees of freedom using the language of global differential geometry. The mechanical systems may be explicitly time-dependent and involve nonpotential forces. The focus is on the mathematically rigorous formulation of the physical theory dealing with the aforementioned mechanical systems with the objective to introduce the involved physical quantities as well-defined mathematical objects. The geometric presentation of the physical theory is erected upon a generalized space-time known as Galilean manifold. The state space of a mechanical system is defined as an affine subbundle of the tangent bundle of its associated Galilean manifold. The system's motion is considered to be an integral curve of a second-order vector field on the state space. With the coordinate-free characterization of the motion in terms of second-order vector fields, differential forms appear on stage. A one-to-one correspondence between second-order vector fields and action forms is established. Action forms are differential two-forms with additional properties. The definition of action forms and the derivation of this bijective relation relies on the geometry of double tangent bundles, in which vector bundle homomorphisms and their differential concomitants play an important role. A coordinate-free definition of forces is given and different geometric interpretations are discussed. With the definition of kinetic energy and of potential forces, the equations of motion are postulated in a coordinate-free way using the action form of the mechanical system. Lagrange's, Hamel's, and Hamilton's equations become local representations of this postulate in terms of a respective chart of the state space. Moreover, the connection between action forms and the concept of virtual work is established. This allows to obtain Lagrange's and Hamel's central equation. This variational perspective is pursued by showing that motions characterized by an exact action form satisfy Hamilton's principle. For this purpose, a coordinate-free definition of the action integral is given. Finally, constraints are defined as distributions compatible with the time structure of the Galilean manifold on which they are defined. Consequently, the distinction between holonomic and nonholonomic constraints is made using the Frobenius theorem.Item Open Access Wetting, de-icing and anti-icing behavior of microstructured and plasma-coated polyurethane films(2019) Grimmer, Philipp E. S.; Hirth, Thomas (Prof. Dr. rer. nat.)Ice build-up on surfaces, for example on wings of airplanes or on rotor blades of wind turbines, impairs the functionality of transportation vehicles or technical systems and reduces their safety. Therefore, functional anti-ice surfaces are being researched and developed, which shall enable an easy removal or reduce the amount of ice on the surfaces at risk. The starting hypothesis for this work is that superhydrophobic polyurethane (PU) films with microstructure base diameters of 35 µm or more reduce the wetting by water, show a low ice adhesion for easy removal of ice and reduce or delay icing. Superhydrophobic PU films for passive anti- and de-icing were created by hot embossing and plasma enhanced chemical vapor deposition (PECVD). The hot embossing process as well as the plasma coating and etching processes were analyzed for the dependence of the surface characteristics on different process parameters. The functionalized PU films were characterized for their surface topography, surface chemistry, stability against erosion, wettability, ice adhesion and icing behavior. For comparison, the ice adhesion and icing behavior were examined on relevant technical materials (aluminum, titanium, copper, glass, epoxy resin of carbon fiber reinforced polymer and other fluoropolymers) and on some commercial anti-ice coatings. The PU films were chemically analyzed by IR spectroscopy. As the first process step for functionalization, microstructures of cylindrical, elliptical or linear shape were imprinted in PU films by a hot embossing technique with different ns-pulsed laser-drilled stamps and characterized by several microscopy methods. The microstructures had heights of 15 µm to 140 µm, diameters or widths of 35 µm to 300 µm and distances (pitch values) of 50 µm to 500 µm. The embossing process was analyzed and optimized in terms of the process parameters temperature, pressure, time, PU film release temperature and reproducibility of the microstructures. In a second functionalization step (PECVD) the microstructured surfaces were coated with thin, hydrophobic plasma polymers using different fluorocarbon precursors (CHF3, C3F6 and C4F8) or hexamethyldisiloxane (HMDSO). Different process parameters for plasma coating and etching (Ar or O2 plasmas) were used in order to create various nanoscale roughness values. Electron spectroscopy for chemical analysis (ESCA), spectroscopic ellipsometry and atomic force microscopy (AFM) were used for analysis of the chemical composition, the thickness and the nanoroughness of the plasma polymers. The functionalizations, especially the plasma coatings, were completely worn off by a UV/water weathering test (1000 h, X1a CAM 180 Test, SAE J-2527), but showed sufficient stability against sand erosion (DIN 52348), in a long-term outdoor test for 13.5 months and against fivefold repeated pull-off of ice. The silicone-like plasma coatings were more stable than the fluorocarbon plasma coatings. The wetting behavior of water was determined by static, advancing and receding contact angle measurements. Static contact angle measurements with diiodomethane (DIM) were made for determination of the surface free energies of the relevant surfaces. Advancing contact angles of over 150° and very low contact angle hysteresis values below 10° were reached on some of the cylindrically and elliptically structured PU samples with microstructure base diameters in the range of 35 µm to 50 µm. The measured water advancing contact angles did not reach the theoretical values of the Cassie-Baxter state. Starting from a mixed wetting state near Cassie-Baxter in case of the superhydrophobic PU surfaces, they approached the Wenzel state with an increasing pitch/diameter (P/d) factor. Fluorescence laser scanning microscopy images were taken of some microstructured, uncoated or plasma coated samples during the wetting by a water drop containing a fluorescent dye. These images show the Wenzel state or a mixed wetting state by visualization of the interface between the water droplet and the surface. A new icing test chamber and a test setup were developed for characterization of the ice adhesion and the icing behavior. The tensile ice adhesion was measured at -20 °C by pull-off of ice cylinders (highly purified water, (<0.056 µS/cm, diameter of 4 mm, similar to the diameter of large raindrops) and compared to the theoretical values and the wetting behavior. The technical material surfaces measured for comparison showed a high ice adhesion, which led to cohesive fractures especially on the metal surfaces, whereas some of the commercial anti-ice coatings showed lower ice adhesion values. The flat, plasma coated PU surfaces showed adhesive fractures with a reduced ice adhesion compared to the technical material surfaces and uncoated PU and revealed a good correlation of the ice adhesion with the wetting behavior of water (work of adhesion). On the other hand, the microstructured PU surfaces showed a greatly increased ice adhesion in comparison to the flat PU and technical material surfaces which was enhanced even further by the plasma coatings and did not correlate with the wetting behavior. The reason for this is the wetting transition from the Cassie-Baxter to the Wenzel state during the cooling or freezing process, leading to an increased ice-surface contact area and mechanical interlocking of the ice with the micro- and nanostructures. The freezing of water drops was examined in thermodynamic equilibrium (static experiment) and under quasi-steady conditions (dynamic experiment). In the static experiment, 15 µl water drops (corresponding to medium to large raindrops) at room temperature were dispensed onto a cold surface at a constant temperature of -20 °C. The freezing delay times, the crystallization times and the total freezing times were measured and compared to calculated expected values. On the flat samples, the freezing delay times could be extended by the plasma treatments. On the microstructured samples, the freezing (nucleation) could sometimes be delayed even further, but not always reproducible because of an unstable Cassie-Baxter state. In the dynamic experiment, 25 µl water drops (corresponding to large raindrops) were cooled down in quasi-steady conditions with the surface and the surrounding atmosphere by a constant, low cooling rate of 1 K/min while the water drop temperature was measured by an IR camera for determination of the surface-specific nucleation temperature and crystallization time. A lower nucleation temperature could be measured on the flat, plasma coated PU surfaces compared to uncoated PU and the hydrophilic glass and metal surfaces. The superhydrophobic PU surfaces did not show a further reduction of the nucleation temperature because of an unstable Cassie-Baxter state. The resulting measured nucleation temperatures were compared to the expected values calculated with an enhanced nucleation theory including a quasi-liquid interfacial layer of the ice nucleus and a Poisson process. Overall, it is shown that hot embossing and PECVD are useful processes for creating superhydrophobic PU surfaces with regard to a roll-to-roll process. The flat, plasma coated PU films show a reduced ice adhesion and lowered nucleation temperature compared to the relevant technical material surfaces. The microstructured, plasma coated PU films are far more water repellent than the flat, plasma coated PU surfaces or the other technical materials. However, the microstructures with base diameters of 35 µm or more and the nanoroughness of the plasma coatings cannot stabilize the Cassie-Baxter state of a freezing water drop enough for a low ice adhesion or a significant decrease of the nucleation temperature. These superhydrophobic PU films are therefore not more icephobic than the flat, plasma coated PU films. In the outlook, the reduction of the geometrical parameters of the microstructures (diameter D, distance P) and nanostructures (curvature radius R) of the surface functionalizations for lower ice adhesion values and nucleation temperatures is proposed.Item Open Access Entwicklung eines Echtzeit-Triggersystems für Strahl- und Trajektoriensensoren zur Messung von geladenen Mikropartikeln(2018) Strack, Heiko; Srama, Ralf (PD Dr.-Ing.)Item Open Access Charakterisierung von Kohlendioxid-Plasmaströmungen zur Simulation von Marseintrittsmanövern(2017) Marynowski, Thomas; Fasoulas, Stefanos (Prof. Dr.-Ing.)Das Thema dieser Arbeit ist die Charakterisierung von CO2-Plasmaströmungen, die die Simulation von Eintrittsmanövern an Planeten ermöglichen. Die Planeten Venus und Mars besitzen eine von CO2 dominierte Atmosphäre und besonders unser direkter Nachbarplanet Mars steht momentan im Fokus aktueller explorativer Missionen. Für eine sicherere und umfangreichere Erkundung der Planeten sind effiziente Technologien erforderlich. Dabei spielen Hitzeschutzmaterialien (engl. Thermal Protection Systems, TPS) eine wichtige Rolle, denn sie ermöglichen erst die Eintrittsmanöver und machen einen erheblichen Masseanteil der Raumfahrzeuge aus. Durch Verbesserung und effizienteren Einsatz der Hitzeschutzmaterialien kann der Nutzlastanteil gesteigert und durch Erhöhung der Sicherheit die Erfolgschancen der Missionen verbessert werden. Das Testen und die Weiterentwicklung solcher Hitzeschutzmaterialien sind mit Hilfe des induktiven Plasmagenerators IPG4 am Plasmawindkanal PWK3 möglich. Die Voraussetzung für solche Tests ist die Kenntnis der wichtigsten Parameter des Freistrahls. Die Messung der Parameter wird mit zwei unterschiedlichen Gruppen von Messmethoden durchgeführt. Als Teil der nicht intrusiven Messmethoden und Schwerpunkt dieser Arbeit wird die laserspektroskopische Methode der Zwei-Photonen laserinduzierten Fluoreszenz (engl. Two-Photon Absorption Laser-Induced Fluorescence, TALIF) eingesetzt. Damit wird zum ersten Mal bei Eintrittsbedingungen in einem induktiv geheizten CO2-Plasma die Grundzustandsdichte von Sauerstoff, als eines der wichtigsten Dissoziationsprodukte einer CO2-Strömung, gemessen. Absolute Aussagen (Grundzustandsdichte, translatorische Temperatur und Geschwindigkeit) über den atomaren Sauerstoff werden durch Messungen an Xenon ermöglicht, das einen passenden Zweiphotonenübergang besitzt und so zur Kalibrierung benutzt werden kann. Zur Erweiterung der Charakterisierung werden auch weitere Messmethoden genutzt. Die optische Emissionsspektroskopie (OES) und ein Hochgeschwindigkeitskamerasystem (HSC) werden als weitere nicht intrusive Diagnostiken eingesetzt. OES ermöglicht die Identifizierung der vorkommenden Spezies sowie die Bestimmung von Vibrations-, Rotations- und elektronischen Anregungstemperaturen. Die Daten der Hochgeschwindigkeitsaufnahmen geben orts- und zeitaufgelöste Informationen über Emissionsverteilungen einzelner identifizierter Spezies in der Strömung, was durch den Einsatz von schmalbandigen Spektralfiltern erreicht wird. Darüber hinaus werden intrusive, also in die Strömung gebrachte, Sonden verwendet, um Totaldruck, Wärmestromdichte und massenspezifische Enthalpie zu messen. Die massenspezifische Enthalpie wird dabei auf zwei unterschiedliche Weisen ermittelt. Dazu wird einerseits eine spezielle Enthalpiesonde und andererseits ein indirekter semiempirischer Ansatz, der sich auf die Messung von Totaldruck und Wärmestromdichte sowie eine benötigte Konstante stützt, verwendet. Durch die Sondenmessung der massenspezifischen Enthalpie ist es möglich, die Konstante aus den Daten dieser Arbeit, durch eine Rückrechnung neu zu ermitteln und mit der Literatur zu vergleichen. Insgesamt geben die Ergebnisse Aufschluss über wichtige Parameter der Strömung wie Geschwindigkeit, Temperaturen, Teilchendichte, Totaldruck, Wärmestromdichte und massenspezifische Enthalpie. Weiter sind über die identifizierten Atome und Moleküle Aussagen über die chemische Zusammensetzung der Strömung möglich, wodurch Betrachtungen des thermo-chemischen Zustandes der Plasmaströmung ermöglicht werden. Für die supersonische Strömung zeichnet sich das Bild eines Nichtgleichgewichtszustandes, das im Einzelnen (chemisch und thermisch) betrachtet wird. Es wird ein Vergleich der vorliegenden Strömungsdaten zu Daten der vergangenen erfolgreichen Marsmissionen sowie weltweit anderer Bodentestanlagen dargestellt. Dabei wird gezeigt, dass der Plasmawindkanal PWK3 mit dem induktiven Plasmagenerator IPG4 in der Lage ist, die Wärmestromdichte und die massenspezifische Enthalpie der bisherigen Eintrittsmissionen im vollen Umfang zu reproduzieren, jedoch der Totaldruck nur auf die frühen Phasen der Eintrittstrajektorien beschränkt simulierbar bleibt. Das Ergebnis dieser Arbeit ist eine sehr gut charakterisierte CO2-Plasmaströmung, die zur Erprobung von Hitzeschutzmaterialien für zukünftige Flüge zum Mars und der Venus verwendet werden kann.Item Open Access Emissionsspektroskopische Analyse einer hyperbolischen Wiedereintrittsströmung im Plasmawindkanal(2017) Hermann, Tobias; Fasoulas, Stefanos (Prof. Dr.-Ing.)Die Arbeit beschäftigt sich mit der experimentellen Simulation einer hyperbolischen Wiedereintrittsströmung in einem Plasmawindkanal. Die Strömung wurde anhand verschiedener emissionsspektroskopischer Methoden charakterisiert und analysiert. Die Analyse hat zur quantitativen Bestimmung von absoluten Teilchendichten und Anregungstemperaturen verschiedener Plasmaspezies geführt. Schließlich wurde der Einfluss ablativer Materialproben auf die körpernahe Strömungsgrenzschicht untersucht.Item Open Access Ablenk-Systeme für die Multi-Elektronenstrahllithografie auf Basis CMOS-kompatibler Fertigungsprozesse(2017) Jurisch, Michael; Burghartz, Joachim N. (Prof. Dr.-Ing.)Item Open Access Laser-induced thermal acoustics : simultaneous velocimetry and thermometry for the study of compressible flows(2016) Förster, Felix Johannes; Weigand, Bernhard (Prof. Dr.-Ing. habil.)Air-breathing propulsion concepts, such as scramjets, provide a promising alternative to conventional systems for a faster and economically as well as ecologically more efficient transportation of passengers and cargo to any destination on the globe. Furthermore, scramjets are an important supplement to existing rocket-based systems to increase the payload and reduce operational costs of space transportation systems. The development of a scramjet engine is, however, challenging and involves the knowledge of many disciplines. One of the most critical problems is a stable and reliable combustion. The flows relevant to this thesis are therefore characterized by high speeds, high temperatures and chemical reactions. Obtaining quantitative data of such a flow field, sufficiently resolved in time and space, is a difficult task for any measurement technique. However, the continuous study of the occurring flow phenomena as well as the requirement to validate advanced numerical simulations demand the development of new diagnostic methods to provide more sophisticated experimental data sets. The focus of this thesis is the development, evaluation and application of Laser-Induced Thermal Acoustics (LITA) as a promising diagnostic tool for the study of compressible flows. LITA allows non-intrusive and remote measurements of speed of sound, flow velocity, Mach number and temperature – resolved both spatially and temporally. A thorough validation of the setup was conducted for reference cases at flow conditions comparable to the intended application verifying that very accurate and detailed data sets can be obtained with LITA. Three different applications are investigated in this thesis. In the first case, time-resolved speed of sound, flow velocity and Mach number measurements were conducted in a hydrogen/air free jet flame. Flow profiles were obtained at different axial positions showing the evolution of the combustion zone. The second application is dedicated to the flow field inside scramjet combustor models. Detailed experimental data sets were provided for the validation of complementary CFD simulations. In addition, a precise reconstruction of the flow field and the shock system resulting from a jet injected into the supersonic cross flow was possible. These results motivated to use LITA in a shock tube facility. Measurements were successfully conducted behind the incident and reflected shock wave proving the technique’s potential for shock-heated flows.