04 Fakultät Energie-, Verfahrens- und Biotechnik

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/5

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Institute: search.filters.UBSInstitut.Institut für Chemische VerfahrenstechnikAuthor: search.filters.author.Nieken, Ulrich

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Now showing 1 - 10 of 23
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    Constitutive correlations for mass transport in fibrous media based on asymptotic homogenization
    (2023) Maier, Lukas; Kufferath-Sieberin, Lars; Pauly, Leon; Hopp-Hirschler, Manuel; Gresser, Götz T.; Nieken, Ulrich
    Mass transport in textiles is crucial. Knowledge of effective mass transport properties of textiles can be used to improve processes and applications where textiles are used. Mass transfer in knitted and woven fabrics strongly depends on the yarn used. In particular, the permeability and effective diffusion coefficient of yarns are of interest. Correlations are often used to estimate the mass transfer properties of yarns. These correlations commonly assume an ordered distribution, but here we demonstrate that an ordered distribution leads to an overestimation of mass transfer properties. We therefore address the impact of random ordering on the effective diffusivity and permeability of yarns and show that it is important to account for the random arrangement of fibers in order to predict mass transfer. To do this, Representative Volume Elements are randomly generated to represent the structure of yarns made from continuous filaments of synthetic materials. Furthermore, parallel, randomly arranged fibers with a circular cross-section are assumed. By solving the so-called cell problems on the Representative Volume Elements, transport coefficients can be calculated for given porosities. These transport coefficients, which are based on a digital reconstruction of the yarn and asymptotic homogenization, are then used to derive an improved correlation for the effective diffusivity and permeability as a function of porosity and fiber diameter. At porosities below 0.7, the predicted transport is significantly lower under the assumption of random ordering. The approach is not limited to circular fibers and may be extended to arbitrary fiber geometries.
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    Thin organic‐inorganic anti‐fouling hybrid‐films for microreactor components
    (2022) Neßlinger, Vanessa; Welzel, Stefan; Rieker, Florian; Meinderink, Dennis; Nieken, Ulrich; Grundmeier, Guido
    Deposit formation and fouling in reactors for polymer production and processing especially in microreactors is a well‐known phenomenon. Despite the flow and pressure loss optimized static mixers, fouling occurs on the surfaces of the mixer elements. To improve the performance of such parts even further, stainless steel substrates are coated with ultra‐thin films which have low surface energy, good adhesion, and high durability. Perfluorinated organosilane (FOTS) films deposited via chemical vapor deposition (CVD) are compared with FOTS containing zirconium oxide sol‐gel films regarding the prevention of deposit formation and fouling during polymerization processes in microreactors. Both film structures led to anti‐adhesive properties of microreactor component surfaces during aqueous poly(vinylpyrrolidone) (PVP) synthesis. To determine the morphology and surface chemistry of the coatings, different characterization methods such as X‐ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy as well as microscopic methods such as field‐emission scanning electron microscopy (FE‐SEM) and atomic force microscopy (AFM) are applied. The surface free energy and wetting properties are analyzed by means of contact angle measurements. The application of thin film‐coated mixing elements in a microreactor demonstrates a significant lowering in pressure increase caused by a reduced deposit formation.
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    Acid-base flow battery, based on reverse electrodialysis with bi-polar membranes : stack experiments
    (2020) Xia, Jiabing; Eigenberger, Gerhart; Strathmann, Heinrich; Nieken, Ulrich
    Neutralization of acid and base to produce electricity in the process of reverse electrodialysis with bipolar membranes (REDBP) presents an interesting but until now fairly overlooked flow battery concept. Previously, we presented single-cell experiments, which explain the principle and discuss the potential of this process. In this contribution, we discuss experiments with REDBP stacks at lab scale, consisting of 5 to 20 repeating cell units. They demonstrate that the single-cell results can be extrapolated to respective stacks, although additional losses have to be considered. As in other flow battery stacks, losses by shunt currents through the parallel electrolyte feed/exit lines increases with the number of connected cell units, whereas the relative importance of electrode losses decreases with increasing cell number. Experimental results are presented with 1 mole L^-1 acid (HCl) and base (NaOH) for open circuit as well as for charge and discharge with up to 18 mA/cm2 current density. Measures to further increase the effciency of this novel flow battery concept are discussed.
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    Coupled electrohydrodynamic and thermocapillary instability of multi-phase flows using an incompressible smoothed particle hydrodynamics method
    (2022) Almasi, Fatemeh; Hopp-Hirschler, Manuel; Hadjadj, Abdellah; Nieken, Ulrich; Safdari Shadloo, Mostafa
    This paper concerns the study of coupled effects of electrohydrodynamic (EHD) and thermocapillary (TC) on the dynamic behaviour of a single liquid droplet. An incompressible Smoothed Particle Hydrodynamic (ISPH) multiphase model is used to simulate EHD-TC driven flows. The complex hydrodynamic interactions are modeled using the continuum surface force (CSF) method, in which the gradient of the interfacial tension and the Marangoni forces are calculated with an approximated error or 0.014% in the calculation of Marangoni force compared to the analytical solutions which is a significant improvement in comparison with previous SPH simulation studies, under the assumption that the thermocapillarity generates sufficiently large stress to allow droplet migration, while the electrohydrodynamic phenomena influences the droplet morphology depending on the electrical and thermal ratios of the droplet and the ambient fluid. This study shows that, when applying a vertical electric field and thermal gradient, the droplet starts to stretch horizontally towards a break-up condition at a high rate of electrical permitivity. The combined effect of thermal gradient and electric field tends to push further the droplet towards the break-up regime. When the thermal gradient and the electric field vector are orthogonal, results show that the droplet deformation would take place more slowly and the Marangoni forces cause the droplet to migrate, while the stretching in the direction of the electric field is not seen to be as strong as in the first case.
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    Regelung eines instationär betriebenen Festbettreaktors mit Fuzzy-Kontrollregeln
    (1993) Nieken, Ulrich; Kolios, Gregorios; Eigenberger, Gerhart
    Katalytiscbe Abluftreinigung in Reaktoren mit periodischer Strömungsumkehr erlaubt einen autothermen Betrieb bis zu niedrigen Schadstoffkonzentrationen. Bei höheren Schadstoffkonzentrationen ermöglicht ein Mittenabzug die vollständige Auskopplung der Verbrennungswärme. Aufgabe der Regelung ist es, den gezündeten Zustand bei geringen Rohgaskonzentrationen durch Zugabe von Stützgas aufrechtzuerhalten und bei höheren Konzentrationen Reaktionswärme in Form eines heißen Teilgassstromes zurückzugewinnen. Ein konventioneller Reglerentwurf gestaltet sich in diesem Fall als schwierig, da der Reaktor keinen definierten Arbeitspunkt besitzt, sondern nur im gezündeten Zustand gehalten werden muß, wobei er sich aufgrund der direkten Kopplung von chemischer Reaktion und Wärmetausch stark nichtlinear verhält. Aus diesem Grund werden die Methoden der Fuzzy-Regelung ausgewählt und an einem Reaktor im Labormaßstab erprobt. Im folgenden wird zunächst kurz auf das Prinzip der katalytischen Oxidation mit periodisch wechselnder Strömungsrichtung eingegangen, um die Wahl der Reglerein- und Ausgangsgrößen zu begründen. Anschließend werden der Aufbau des Fuzzy-Reglers skizziert und die erzielten Versuchsergebnisse diskutiert.
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    Modeling of diffusive transport of polymers moments using limiting cases of the Maxwell-Stefan model
    (2022) Welzel, Stefan; Säckel, Winfried; Nieken, Ulrich
    A polymer distribution is usually represented by its moments. Thus, to calculate transport in a polymer system, a formulation for the transport of moments of the polymer is needed. This is only possible if the moments close or if there is a suitable closing condition. To archive this, two simplifications of the Stefan-Maxwell diffusion are derived, which convert the transport equation of polymeric species to a closed set of transport equations for the polymer moments. The first approach corresponds to an infinitely diluted polymer system, whereas the second one describes a highly concentrated polymer system. Both formulations are compared with the full Stefan‐Maxwell model of a ternary mixture of a solvent and two polymer species of different chain length.
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    Abluftoxidation in Monolith-Katalysatoren mit periodischem Wechsel der Strömungsrichtung
    (1988) Eigenberger, Gerhart; Nieken, Ulrich
    Mit steigenden gesetzlichen Anforderungen zur Reduzierung des Schadstoffgehalts gewinnen Verfahren zur oxidativen Umwandlung von Schadstoffen in Abluft zunehmend an Bedeutung. Typische Aufgaben der Abluftreinigung sind gekennzeichnet durch hohe Luftdurchsätze und niedrige Schadstoffkonzentrationen; die adiabate Temperaturerhöhung der Totaloxidation beträgt häufig weniger als 100°C. Das bedeutet, daß der Druckverlust in der Reinigungsanlage niedrig und der Warmerücktausch zwischen Zu- und Ablauf hoch sein muß, um die Betriebskosten niedrig zu halten. Im folgenden wird eine neuartige Betriebsweise der katalytischen Oxidation untersucht, die sich durch effektiven Warmerücktausch, niedrigen Druckverlust und geringes Bauvolumen auszeichnet. Sie beruht auf der Benutzung von Monolith-Katalysatoren in Verbindung mit der von Macros und Mitarb. entwickelten Prozeßführung mit periodischem Wechsel der Strömungsrichtung. Dabei wirkt jeweils der vordere und der hintere Teil des Katalysatorbettes als Regenerator-Wärmeaustauscher.
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    An application of the Cahn-Hilliard approach to smoothed particle hydrodynamics
    (2014) Hirschler, Manuel; Huber, Manuel; Säckel, Winfried; Kunz, Philip; Nieken, Ulrich
    The development of a methodology for the simulation of structure forming processes is highly desirable. The smoothed particle hydrodynamics (SPH) approach provides a respective framework for modeling the self-structuring of complex geometries. In this paper, we describe a diffusion-controlled phase separation process based on the Cahn-Hilliard approach using the SPH method. As a novelty for SPH method, we derive an approximation for a fourth-order derivative and validate it. Since boundary conditions strongly affect the solution of the phase separation model, we apply boundary conditions at free surfaces and solid walls. The results are in good agreement with the universal power law of coarsening and physical theory. It is possible to combine the presented model with existing SPH models.
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    Electrically heated oxide ceramic tubes for high temperature reactions
    (2023) Matthies, Jörn; Schall, Thomas; Pritzkow, Walter; Tuttlies, Ute; Nieken, Ulrich
    Endothermic high temperature reactions are usually carried out in metal tubes heated by gas burners. Electrical heating allows substantial reduction of CO2 emissions. We propose the usage of a composite tube, where a thin metallic layer is embedded between an inner and outer ceramic layer. While monolithic ceramics suffer from brittleness and low tolerance to thermal stress, only the inner layer is made from monolithic ceramics, while the outer layer is made of fiber reinforced oxide ceramics. In first tests the hybrid ceramic tube was electrically heated to 1250 °C with a maximum heat release of 85 kW m-2.
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    Carbon‐methanol based adsorption heat pumps : identifying accessible parameter space with carbide‐derived carbon model materials
    (2020) Träger, Lisa; Gläsel, Jan; Scherle, Marc; Hartmann, Julian; Nieken, Ulrich; Etzold, Bastian J. M.
    In adsorption heat pumps, the properties of the porous adsorbent and the refrigerant determine the performance. Major parameters for this working pair are the total uptake of the adsorptive, its kinetics, and the heat transfer characteristics. In the technical application despite powdered adsorbents, thin consolidated layers of the adsorbent can be attractive and obtained by a binder‐based approach but likely result in competing material properties. Thus, for a process optimization, the accessible parameter space and interdependencies have to be known and were deduced in this work for model porous carbons (carbide‐derived carbons derived from TiC and ZrC) and methanol as well as the addition of different amounts of boron nitride, silver, and graphite as heat‐conductive agents and the use of two binders.