Recent Submissions
Untersuchung chemischer Absorptionsverfahren auf Aminbasis zur CO2-Abtrennung aus Rauchgasen
(2024) Schmid, Marc Oliver; Scheffknecht, Günter (Prof. Dr. techn.)
Der anthropogene Klimawandel durch Emissionen an klimawirksamen Gasen, insbesondere CO2 aus der Nutzung fossiler Brennstoffe, stellt eine der großen globalen Herausforderungen des 21. Jahrhunderts dar. Um die negativen Auswirkungen des Klimawandels auf Menschen und Umwelt zu begrenzen, soll die Erderwärmung bezogen auf das vorindustrielle Zeitalter, nicht mehr als 1,5 °C betragen.
Fossil befeuerte Kraftwerke sind für ca. 40 % des globalen Ausstoßes an klimawirksamem CO2 verantwortlich. Zur Senkung der CO2-Emissionen von Kraftwerken und als Maßnahme gegen den Klimawandel werden gegenwärtig verschiedene Verfahren diskutiert und erforscht. Neben der Substitution fossiler Brennstoffe, der Effizienzsteigerung und neuen Verbrennungsverfahren, stellt die nachgeschaltete CO2-Abtrennung aus Rauchgasen eine vielversprechende Maßnahme dar. Die Abtrennung erfolgt hierbei durch einen der konventionellen Rauchgasreinigung nachgeschalteten, chemischen Waschprozess mit wässrigen Aminlösungen. Mit Hilfe von Amingruppen wird das CO2 aus dem Rauchgas absorbiert und nachfolgend in einem Regenerationsprozess durch Temperaturerhöhung und Strippen mit Dampf wieder ausgetrieben, wodurch die Lösung erneut als Absorptionslösung verwendet werden kann. Vorteile dieses Verfahrens sind die Möglichkeit einer Nachrüstung von bestehenden Kraftwerksanlagen sowie die Bereitstellung eines reinen CO2-Stroms für die zunehmend diskutierte stoffliche Nutzung.
Im Rahmen dieser Arbeit wird als verfahrenstechnische Alternative zur Aminwäsche mit konventionellen Packungs- oder Füllkörperkolonnen der Einsatz eines Sprühwäschers im Technikumsmaßstab untersucht. Sprühwäscher weisen unter anderem einen geringeren gasseitigen Druckverlust und einen flexibleren Betrieb auf, allerdings ist aufgrund der fehlenden Einbauten die wirksame Stoffaustauschfläche im Wesentlichen auf das Spray begrenzt.
Der Fokus dieser Arbeit liegt auf der Entwicklung und experimentellen Erprobung verschiedener Maßnahmen, um den Stoffübergang in Sprühwäscheanlagen zur Abtrennung von CO2 mit wässrigen Aminlösungen zu steigern. Als Absorptionslösung wird eine wässrige 30 Gew.-%ige Monoethanolaminlösung (MEA) verwendet. Hierfür werden nach einer umfangreichen Untersuchung des Einflusses des L/G-Verhältnisses und der spezifischen Heizleistung in einer Sprühwäscheanlage eine interne Absorptionsmittel-Rezirkulation innerhalb der Absorberkolonne sowie ein Tray-Einsatz untersucht. Abschließend wird der zusätzliche Einfluss einer Bündeldüse aus einzelnen Hohlkegeldüsen erprobt und die Ergebnisse bewertet.
Measuring network effects of digital industrial platforms : towards a balanced platform performance management
(2023) Schüler, Fabian; Petrik, Dimitri
Firms increasingly establish digital industrial platforms to cope with the adaption of the industrial internet of things (IIoT) paradigm. The tremendous success of digital platforms in many platform-mediated industries can be traced back to the ignition of network externalities. However, the impact of network externalities is still under discussion in the IIoT domain, and their measurement remains a challenge for platform companies. This paper outlines how network effects were measured in the existing research, deriving three dimensions of network effects for IIoT: (1) ecosystem utility, (2) complementarity, and (3) compatibility. This conceptualization is further used in an empirical study with practitioners from digital industrial platform organizations to enable performance measurement of network effects in IIoT by developing 20 key performance indicators (KPIs). Based on the empirical study results, this paper proposes a framework for balanced platform management. Utilizing the goals of a balanced scorecard, the framework emphasizes the trade-off between the contradicting perspectives on costly network effect simulation and platform earnings that platform managers need to balance. The KPI portfolio can support platform managers in implementing the framework.
Lokale Hauptwahl oder nationale Nebenwahl? : der Einfluss von Gemeindegröße, Wahlsystem und
Parteiensystem auf die Beteiligung an Kommunalwahlen
(2023) Graeb, Frederic; Bernhagen, Patrick
Kommunalwahlen werden nicht zuletzt aufgrund ihrer geringen Wahlbeteiligung häufig als nationale Nebenwahlen betrachtet. Die Fokussierung der Nebenwahlperspektive auf nationale Einflussfaktoren führt jedoch zu einer Unterbewertung lokaler Heterogenität. Der Artikel untersucht, inwiefern lokale Kontextfaktoren, insbesondere Gemeindegröße, Wahl- und Parteiensysteme sowie die wahrgenommene Bedeutung der kommunalen Ebene einen Beitrag zur Erklärung von Beteiligungsunterschieden zwischen Kommunalwahlen leisten. Dazu analysieren wir die Wahlbeteiligung bei den Kommunalwahlen 2014 in neun Bundesländern. Die Ergebnisse zeigen, dass Wahlsysteme freier Listen einen negativen Einfluss auf die Wahlbeteiligung haben. Die Erwartung, dass die kommunale Wahlbeteiligung mit steigender Gemeindegröße abnimmt, kann hingegen nur mit Einschränkung bestätigt werden. Auch kann ein Einfluss des lokalen Parteiensystems nur unter Vorbehalt nachgewiesen werden. Gemeinden mit nationalisiertem Parteiensystem genießen zwar eine höhere Wahlbeteiligung, aber der Effekt ist nur von geringem Umfang und kann lediglich auf der Aggregatdatenebene geschätzt werden. In Einklang mit der Nebenwahlthese zeigt sich schließlich, dass die Wahrscheinlichkeit der individuellen Wahlteilnahme mit der wahrgenommenen Bedeutung der Kommunalwahlen steigt.
Experimental investigations on structural changes of CaO/Ca(OH)2 bulks in thermochemical energy storage
(2025) Gollsch, Marie; Thess, André (Prof. Dr.)
Storage of thermal energy has the potential to become a major part of future energy systems due to versatile integration possibilities as well as relatively low cost. Thermochemical energy storage (TCS), in which thermal energy is stored as chemical potential, is particularly promising for certain applications such as long-term storage. However, this technology still requires considerable efforts in research and development in order to deliver market-ready solutions.
This thesis investigates a widely recognised factor that forms a major obstacle to the technical implementation of TCS based on chemical reactions: As the TCS materials react during charging and discharging of the storage, they change their chemical composition and thus their chemical and physical properties. In case of gas-solid reaction systems, which represent a majority of the materials studied, the changes in solid morphology can be significant, leading to correlating changes in any related properties of the storage material, often in the order of magnitudes. Design and predictive modelling of reactors and storage systems are therefore challenging. To improve the understanding of the processes leading to changes in reaction and transport behaviour within TCS material bulks, the reaction system CaO/Ca(OH)2 is studied in depth, as challenges caused by restructuring of this system's solids are widely reported in literature.
For that purpose, bulk storage materials consisting of powder, nanoparticle-modified powder, shaped and encapsulated granules are investigated over the course of several reaction cycles. Changes in gas permeability and thermal conductivity are discussed qualitatively and - where feasible - quantitatively. Additional analyses are carried out with regard to reactivity, morphology and phase composition. In summary, it is concluded that the inhomogeneities that develop within reactors are likely to be so pronounced that they lead to reaction temperature differences in the range of tens of Kelvin within a distance of centimetres. More detailed studies on the microstructural evolution of reacting storage materials are identified as promising to further TCS technologies. Based on the results regarding material tailoring, a combination of the studied approaches is proposed. Shaped materials such as granules with diameters in the range of a few millimetres are suitable to prevent agglomeration and improve handling while minimising limitations due to insufficient heat and mass transfer. Mechanical stabilisation can be achieved by a “breathing” nanocoating or/and an inert component within the particle. However, ideal material properties depend on the specific reactors and operating conditions. The proposed approach of nanocoated granules has already been demonstrated on a technical scale following the work described in this thesis.
Investigating 3-D effects on flashing cryogenic jets with highly resolved LES
(2023) Gärtner, Jan Wilhelm; Kronenburg, Andreas; Rees, Andreas; Oschwald, Michael
For the development of upper stage rocket engines with laser ignition, the transition of oxidizer and fuel from the pure cryogenic liquid streams to an ignitable mixture needs to be better understood. Due to the near vacuum conditions that are present at high altitudes and in space, the injected fuel rapidly atomizes in a so-called flash boiling process. To investigate the behavior of flashing cryogenic jets under the relevant conditions, experiments of liquid nitrogen have been performed at the DLR Lampoldshausen. The experiments are accompanied by a series of computer simulations and here we use a highly resolved LES to identify 3D effects and to better interpret results from the experiments and existing 2D RANS. It is observed that the vapor generation inside the injector and the evolution of the spray in the combustion chamber differ significantly between the two simulation types due to missing 3D effects and the difference in resolution of turbulent structures. Still, the observed 3D spray dynamics suggest a suitable location for laser ignition that could be found in regions of relative low velocity and therefore expected low strain rates. Further, measured droplet velocities are compared to the velocities of notional Lagrangian particles with similar inertia as the measured droplets. Good agreement between experiments and simulations exists and strong correlation between droplet size and velocity can be demonstrated.
On the asymmetric troposphere modeling in PPP
(2025) He, Shengping; Hobiger, Thomas (Prof. Dr. techn.)
Tropospheric asymmetry is a crucial error term which needs to be considered for the refinement of tropospheric modeling in Precise Point Positioning (PPP). Wet asymmetry can account for more than one-fifth of the wet delay, causing residuals ranging from centimeters to decimeters at low elevation angles. Tropospheric asymmetry significantly impacts high-precision positioning applications and meteorological research. First models of tropospheric asymmetry are based on the concept of tropospheric horizontal gradient, which has prompted the development of many new models, including the widely used two-axis gradient model. However, the traditional two-axis gradient model is insufficient to represent the complex azimuthal variation of tropospheric delays. To address this issue, a directional mapping function based on cyclic B-spline functions, the so called the B-spline Mapping Function (BMF), is proposed. BMF enables a continuous characterization of tropospheric delay across any azimuth direction. The effectiveness of BMF has been validated using both numerical weather model data and Global Navigation Satellite System (GNSS) data from International GNSS Service (IGS) stations in Europe and Africa. Results reveal that compared to the conventional gradient model, BMF improves coordinate repeatability by approximately 10% horizontally and 5% vertically. The improvement in 3D-RMSE can reach up to 15% under heavy rainfall conditions.
While the functional model of the asymmetric troposphere has been extensively studied, the stochastic model of the asymmetric troposphere remains unexplored. The absence of a suitable stochastic model for asymmetric troposphere reduces the accuracy of positioning and Zenith Total/Wet Delay (ZTD/ ZWD) estimates. In this work, an Azimuth-Dependent Weighting (ADW) scheme is introduced with the purpose to adaptively weight GNSS observations affected by azimuth-dependent errors using parameters from asymmetric mapping functions. Validated using NWP and IGS data, ADW improves PPP solution coordinate repeatability by approximately 10% horizontally and 20% vertically. ADW also improves ZWD estimates during the PPP convergence period and yields smoother results. Thus, this new weighting scheme is recommended for PPP applications when the asymmetric mapping functions are used.
In the currently used conventional PPP processing strategies, ZWD is usually dynamically estimated as a stochastic parameter. During the convergence period, ZWD could become negative due to the lack of physical constraints. This problem increases the convergence time and reduces short-term accuracy of PPP. To address this issue, a method which incorporates physical constraints on ZWD using Non-negative Least Squares (NNLS) methods and Karush–Kuhn–Tucker (KKT) conditions is proposed. This method reduces ZWD outliers during the PPP convergence period, and effectively improves the short-term accuracy for the Up component of the position up to 20%.
Similar to tropospheric delays, there are some other systematic errors that also have azimuth-dependent characteristics, such as multipath error. Consequently, when using an asymmetric troposphere model, the gradient parameters may absorb some of the multipath error, leading to biases in ZWD/ZTD estimates. Due to the site-specific nature of multipath errors, establishing a universal mathematical model is challenging. Therefore, in this thesis, a commercial simulator is employed to simulate multipath signals under different scenarios. The impact of multipath errors on estimated ZTD time series and methods to mitigate this effect by adjusting the process noise of ZWD are studied.
This thesis provides an in-depth exploration of asymmetric troposphere modeling in PPP from four perspectives: functional models, stochastic models, constraint conditions, and systematic errors. It investigates methods for refining troposphere modeling and analyzes their effectiveness in PPP and GNSS meteorology.
Delay of laminar-turbulent transition by counter-rotating cylindrical roughness elements in a laminar flat plate boundary layer
(2023) Römer, Tristan M.; Schulz, Kai A.; Wu, Yongxiang; Wenzel, Christoph; Rist, Ulrich
Delaying laminar-turbulent transition in boundary layers is of great interest since the skin-friction coefficient can be reduced by up to one order of magnitude. In this experimental research, it is shown that counter-rotating cylindrical roughness elements are able to delay transition under realistic flow conditions. Evidence is given by the intermittency, evaluated from hot-film measurements in a laminar water channel. An increase in rotation speed results in a delay of transition of up to 6.5%in the center of the plate. This trend can be explained by the streaks amplified by the rotating cylinders, resulting in a damping of the fluctuation amplitude in the boundary layer. The advantage of this method is that the transition delay can be actively controlled with conventional cylindrical roughness elements.
ReViewer : an AI-assisted interactive document editor
(2024) Trinh, Hoai Nam
As neural language models become increasingly effective, their integration into real-world applications is expanding. Despite this, many of these applications are often limited in the types of interactions they facilitate, with text-based chats being one of the most common modes of engagement. One area that remains largely unexplored is the use of AI in the scientific peer review process, particularly in the crucial editing phase. There have been many different tools that support the writing process, but recent advances in AI have created the potential for novel features. This thesis introduces ReViewer, an AI-assisted interactive editing tool designed to enhance the editing stage of the peer review process. ReViewer leverages the capabilities of large language models (LLMs) to assist editors by automating content analysis and extracting actionable suggestions from reviewer feedback. The application offers several innovative features, including statement-based highlighting, severity classification, contextual improvement suggestions, and visualizations for review overlap analysis. These features empower editors to make informed, objective decisions while maintaining control through a human-in-the-loop system that supports but does not replace their expertise. To assess the effectiveness and usability of ReViewer, we conducted a preliminary study involving both novice and expert participants in the context of scientific peer review. This study offered valuable insights into users' attitudes towards integrating AI-assisted features into their scientific editing workflows, as well as their experiences with the specific features we implemented. The findings not only highlight the benefits and challenges of using AI tools in the review process but also provide a deeper understanding of how these tools can complement human expertise. Finally, we outline potential directions for future research and development, focusing on refining AI-driven features further to improve their usability and trust for AI-assisted editing.
Greedy-kernel algorithms for data mapping in multiphysics simulations
(2025) Tucciarone, Fabio
Data mapping in multiphysics simulation coupling describes the transfer of data between possibly nonconforming meshes. Choosing a numerical approximation method for data mapping is always a trade-off between accuracy and performance. Lower-accuracy methods include, for example, the first-order nearest-neighbour mapping, whereas higher accuracies can often be achieved with a computationally expensive radial basis function interpolation.
We extend the multiphysics coupling library preCICE with a greedy approach to radial basis function interpolation. We implement and evaluate the P- and f-greedy methods, which aim to reduce the size of a radial basis function interpolant using a greedy vertex selection approach. The greedy selection is terminated when a user-defined tolerance for a greedy criterion is reached. We compare this method to the nearest neighbour, as well as a global-direct solution and a partition-of-unity approach to radial basis function mapping.
We find, that the greedy selection process is computationally expensive for small error tolerances. At the same time, we often see an improvement in mapping time compared to a global-direct solution after the interpolant has been constructed in an offline stage. For high error tolerances, a nearest-neighbour mapping is typically the cheaper option. The partition-of-unity method can achieve comparatively small errors for better runtimes.
VR maze with omnidirectional treadmill
(2025) Alkadre, Mohamad
Diese Ausarbeitung beschäftigt sich mit der Entwicklung eines Labyrinthspiels für die virtuelle Realität (VR), das speziell für das omnidirektionale Laufband Omnideck konzipiert wurde und auf der Unity Engine basiert. Durch die Integration von physischer Bewegung in das Gameplay wird ein immersives Erlebnis geschaffen, das den Spieler aktiv in die digitale Welt eintauchen lässt. Ein zentrales Element dieses Projekts ist die Entwicklung eines Tools zur Generierung von 3D-Modellen aus Vektoren, das die Erstellung und Anpassung von Labyrinthstrukturen vereinfacht. Das Tool wird zudem um eine Funktion erweitert, die es ermöglicht, 3D-Modelle aus Bildern zu generieren. Dabei kann ein 2D-Labyrinth-Modell als Eingabe verwendet werden, welches das Tool pixelweise analysiert und vektorisiert. Diese Vektordaten dienen dann als Grundlage für die Erstellung eines 3D-Modells. Durch die pixelgenaue Analyse des Bildes können Strukturen und Konturen des Labyrinths präzise erfasst und in die dritte Dimension übertragen werden. Des Weiteren wird der gesamte Entwicklungsprozess von der Konzeptualisierung über die technische Implementierung bis zur abschließenden Integration des Omnidecks erläutert. Herausforderungen und Lösungen, die sich aus der Zusammenführung von Hardware- und Softwareelementen ergeben, werden ebenfalls behandelt.