Universität Stuttgart
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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 Chiral metamaterials(2016) Eslami, Sahand; Fischer, Peer (Prof. Dr.)Item Open Access Thermo-hydraulic analysis of wall bounded flows with supercritical carbon dioxide using direct numerical simulation(Stuttgart : Institute of Nuclear Technology and Energy Systems, 2018) Pandey, Sandeep; Laurien, Eckart (Prof. Dr.-Ing. habil.)The power cycle based on supercritical carbon dioxide technologies promises a higher thermal efficiency and a compact plant layout. However, heat transfer and hydraulic characteristics are peculiar in the near-critical region due to the sharp variation of thermophysical properties in a narrow temperature and pressure range. Therefore, this works presents the results of several direct numerical simulations (DNS) of turbulent wall-bounded flow at supercritical pressure. The spatially developing pipe flows are simulated with the low Mach number approximation to characterize the cooling process of supercritical carbon dioxide. The upward and downward flow of carbon dioxide in vertical orientation has been considered. Heat transfer deterioration followed by recovery is observed in the downward flow while enhancement occurs in the upward flow as compared to forced convection. During the heat transfer deterioration, sweep and ejection events are decreased greatly, triggering the reduction in turbulence. The recovery in turbulence is brought by the Q1 and Q3 (also known as outward and inward interaction) events, contrary to the conventional belief about turbulence generation. The turbulence anisotropy of the Reynolds stress tensor showed that the turbulence structure becomes rod-like during the deteriorated heat transfer regime in the downward flow and disc-like for the upward flow. In addition to low Mach number DNS, a framework for using fully-compressible discontinuous Galerkin spectral element method for DNS of supercritical carbon dioxide is presented. A turbulent channel flow is considered to demonstrate the ability of this framework and to observe the effects of Mach number in the supercritical fluid regime. The increase in the Mach number increases the turbulence in the flow for a given Reynolds number. Finally, a computationally light data-driven approach for heat transfer and hydraulic characteristics modeling of supercritical fluids is presented based on the deep neural network. This innovative approach has shown remarkable prediction capabilities.Item Open Access Novel X-ray lenses for direct and coherent imaging(2019) Sanli, Umut Tunca; Schütz, Gisela (Prof. Dr.)Item Open Access Interfacial mechanics and liquid crystal structure of liquid gallium(2021) Yunusa, Muhammad; Sitti, Metin (Prof. Dr.)This dissertation aims to shed light on the fundamental aspects of supercooled liquid gallium. First, the mechanical properties of the oxide skin encapsulating the supercooled liquid gallium droplet is investigated. By leveraging the supercooling behavior of gallium and the formation of its oxide skin, we characterized the emergence of wrinkling at the interface and adhesion energy between the liquid gallium and rigid substrate. An interfacial energy of 0.238 ± 0.008 J m-2 was measured between gallium droplet and flat glass. Second, seed induced crystallization of supercooled liquid gallium on the different substrate materials revealed the dependence of the kinetics of crystallization on thermal behavior of the substrate. Such approach further elucidates on the importance of interface temperature during crystallization with a preferred orientation. The third part of the dissertation reports an unprecedented liquid crystal structure in supercooled liquid gallium. By the use of techniques applied in liquid crystal (LC) research, observation of LC texture in supercooled liquid gallium was realized. Reflective polarized optical microscopy (R-POM) on liquid gallium sandwiched between glasses treated with rubbed polymers reveals the onset of an anisotropic reflection of possible dimer molecules or clusters at 120°C that increases on cooling and persists down to room temperature or below. On the other hand, when gallium is sandwiched between substrates that align conventional liquid crystal molecules normal to the surface, the reflection is isotropic. This observation of LC structure of a highly electrically conductive supercooled liquid gallium provides an unexpected new field of materials science and liquid crystal research.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 Computational studies of massively separated wake flows of transport aircraft(2021) Waldmann, Andreas; Krämer, Ewald (Prof. Dr.)This work focuses on the investigation of flow phenomena associated with low speed stall using a representative commercial transport aircraft configuration. Subsonic stall at high Reynolds number involves a highly complex turbulent flow field, which is difficult to analyze in ist entirety via experimental methods. Various computational approaches based on URANS and hybrid RANS/LES were evaluated, utilizing validation data from the European Transonic Windtunnel. Scale-resolving computational approaches were leveraged to gain deeper insight into the processes occurring in such a wake. DDES-based methods were found to be able to resolve the flow features occurring at the separation location and in the wake. An extensive study on the impact of solver settings, computational grids, model geometry and inflow Reynolds number was carried out in order to permit a validation of the chosen approach. Using these findings, the massively separated wake flow was studied at three angles of attack in post stall conditions. Three different regimes of formation of the separated wake were identified via the main locations where turbulence kinetic energy is produced. Analysis of anisotropy, turbulence length scales and signal characteristics provided insight into the propagation of the wake and the mixing processes. Modal analysis of the wake dynamics enabled the detection of a near-wing recirculation area and a von Kármán vortex street in the wake. Flow structures associated with both phenomena result in tailplane load fluctuations at their respective characteristic frequencies.Item Open Access Ressourceneffiziente Erzeugung ultra-transparenter Elektroden durch perkolierende Nanostrukturen(2016) Ackermann, Thomas; Westkämper, Engelbert (Prof. a. D. Dr.-Ing. Prof. E. h. Dr.-Ing. E. h. Dr. h. c. mult.)Transparente leitfähige Schichten (transparente Elektroden) sind elementare Bauteile in Touch-Modulen, Displays und Solarzellen. Die vorliegende Arbeit beschäftigt sich mit der Erzeugung transparenter Elektroden auf Basis alternativer Materialien, um die Defizite - insbesondere die Brüchigkeit und die relativ hohen Herstellungskosten - des konventionellen Materials Indiumzinnoxid zu umgehen. Zweidimensionale Netzwerke aus stäbchenförmigen elektrischen Leitern werden ausgehend von einer Dispersion durch Nassfilmbeschichtung hergestellt und hinsichtlich ihrer Eignung als transparente Elektroden untersucht. Dabei handelt es sich Netzwerke aus Silbernanodrähten und um Hybrid-Schichten aus Silbernanodrähten und Kohlenstoffnanoröhren (Co-Perkolation). Neben der Ableitung und Umsetzung Produkt- und Prozess-orientierter Ziele liefert die Arbeit einen Beitrag zum Verständnis der zweidimensionalen elektrischen Perkolation in Netzwerken aus stäbchenförmigen elektrischen Leitern, insbesondere nahe an der Perkolationsschwelle, bei der die Netzwerke eine sehr hohe Transparenz aufweisen, weshalb derartige Schichten als ultra-transparent bezeichnet werden. Diese Arbeit entstand an der Graduate School of Excellence advanced Manufacturing Engineering (GSaME) der Universität Stuttgart in Kooperation mit dem Fraunhofer-Institut für Produktionstechnik und Automatisierung (IPA) in Stuttgart.Item Open Access Investigations and technical development of adsorption thermal energy storage systems with simulation and different control strategies(2021) Abou Elfadil, Mazen; Hirth, Thomas (Prof. Dr.)Thermal energy storage (TES) has been receiving an increasing worldwide attention, especially with the growing concerns about environmental problems caused by an inefficient utilization of energy. A major part of the energy consumption is considered as low temperature thermal energy. Thus, a better management of this energy by using thermal energy storage could provide a significant contribution to improve the overall efficiency of energy utilization in industrial processes and economies. The thermal energy can be stored in different forms e.g. sensible heat, latent heat or thermo-chemical, allowing variety of choices depending on the application. While the sensible and latent heat storage technologies are standard products, the thermo-chemical energy storage is still under development. Based on the method used, thermo-chemical energy storage can be divided into absorptive and adsorptive thermal energy storage systems. The adsorptive thermal energy storage systems have a great potential in both daily (short term) and seasonal (long term) applications. However, their implementation is still limited due to their low degree of applicability caused by lack of scientific knowledge on the thermal analysis level, as well as the absence of knowledge on the level of system integration, which has prevented the heat storage systems from reaching their maximum potential and from being fully commercialized. Consequently, there is still a big necessity for research and development in this field [Salvatore Vasta, 2018]. The principle of the adsorption storage system is based on a gaseous working fluid (e.g. water resp. vapor) which gets adsorbed by a highly porous material (e.g. zeolite). This adsorption process is an exothermal one, thus heat is being released and can be transferred and used. In order to recharge the heat storage system, desorption of the working fluid is done by heating the porous material. The heat storage system consists mainly of a reactor (where the porous material is located), a condenser/evaporator and other auxiliary components (e.g. water tank, pumps, sensors…). Efforts of development of the adsorptive TES were concentrated mainly on developing the adsorptive material, as the performance of the storage material has been the priority so far [Salvatore Vasta, 2018]. Little focus was put on heat power analysis and temperature behavior in the different system components, which have an impact on the overall system efficiency. Thus, system approach is still needed in order to combine and integrate this technology into industrial applications and products [Hauer, Andreas 2020] [Michelangelo Di Palo 2020]. With the aim of improving the heat storage efficiency (recovered heat to stored heat ratio), both numerical (simulations) and experimental (technical modifications) approaches were applied, which have enabled the system to achieve an optimal operational status in terms of energy utilization and efficiency. These approaches were later on used to define a fully automated control system assisting the adsorption TES to instantly react with the continuously varying parameters in such a way to assure an optimal performance. Hence, in the first stage of this investigation, process-modeling and simulation of the whole heat storage system were carried out, so that the total performance of the heat storage system can be predicted and evaluated for any future applications, including the possibility of combining different reactors or heat storage units. In the second stage, different experiments and technical modifications of the system were conducted. This includes testing various possibilities of TES setups (e.g. storage cascades), where the different pressure and temperature behavior in the reactor were evaluated. With the help of experiments, a detailed numerical 3D-model of the packed bed was created, giving an insight into the heat and mass transfer in the reactor during both adsorption and desorption. As a result, a new heat exchanger design was developed, which has improved the temperature distribution and the heating/cooling power. Additionally, the simulation’s results suggested the separation between the evaporator and the condenser to achieve an enhanced water vapor transfer between the reactor and condenser. On a parallel stage of this investigation, comprehensive heat power analysis during both adsorption and desorption processes was carried out, which has showed that the sensible heat left in the reactor, contributes to ca. 50% of the total stored heat. Consequently, multiple reactor concept was introduced, in order to enable the sensible heat recovery. As a conclusion, process simulation enabled tests with different parameters to be performed within much shorter time than the real experimental time. Thus, it was possible to cover numerous application-scenarios and help improving the system overall efficiency. The experimental results have shown that the developed heat exchanger design has increased the maximum power of the heat exchanger about 74%. Moreover, by improving the fluid dynamics between the reactor and condenser, the efficiency of desorption ηd and overall efficiency ηo were increased by 32% and 9% respectively. Furthermore, about 36% of the sensible heat left in the reactor after desorption was recovered by using multiple reactors with sequential configuration, which has led to a reduction in the total invested heat by ca. 9%. For future work it’s recommended to investigate the possibility of controlling the amount of discharged heat from the system by regulating the water uptake during adsorption. In addition, trying a different approach to the reactor’s design (e.g. moving bed reactor) could bring significant improvements to the system.Item Open Access Ablenk-Systeme für die Multi-Elektronenstrahllithografie auf Basis CMOS-kompatibler Fertigungsprozesse(2017) Jurisch, Michael; Burghartz, Joachim N. (Prof. Dr.-Ing.)