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Item type:Publication, Access status: without files , Experimental Investigation of Reactive Bubbly Flows—Influence of Boundary Layer Dynamics on Mass Transfer and Chemical Reactions(Springer, 2021-07-30); ; ; ; Bubbly flows are extensively used processes in the chemical industry. Since the complex interaction of fluid dynamics, mass transfer and chemical reaction is not yet fully understood, a reliable prediction of yield and selectivity is not possible. Within this work different benchmark experiments are developed, allowing the investigation of the interplay of mixing and chemical reactions. For precise predictions of the chemical process, a detailed knowledge about the intrinsic kinetics is essential. Therefore, the guiding measure “SuperFocus Mixer” (SFM) has been developed and successfully tested by determining the kinetics of a model system and of the oxidation of a temperature sensitive copper complex. In a second step, the identified reaction is transferred into the Taylor bubble setup, marking the second benchmark system. Here the effect of mixing on the production of the products in consecutive and competitive-consecutive reaction is investigated. The conducted experiments show significant influence of the mixing intensity on the production of the first reaction product MNIC and the side product DNIC, favoring the first product at intensified mixing. Finally, the local mass transfer at freely ascending bubbles superimposed by a chemical reaction is determined by applying planar-LIF, and the influence of bubble–bubble bouncing is quantified. In addition, a novel method, the Time Resolved Scanning-LaserInduced Fluorescence (TRS-LIF) for the visualization of 3D concentration fields, is introduced and tested at single rising oxygen bubbles.Publicationtype: Book Part109 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Experimental and numerical investigation of reactive species transport around a small rising bubbleIn this article, we present experimental and numerical techniques to investigate the transfer, transport, and reaction of a chemical species in the vicinity of rising bubbles. In the experiment, single oxygen bubbles of diameter d b =0.55…0.85mm are released into a measurement cell filled with tap water. The oxygen dissolves and reacts with sulfite to sulfate. Laser-induced fluorescence is used to visualize the oxygen concentration in the bubble wake from which the global mass transfer coefficient can be calculated. The ruthenium-based fluorescent dye seems to be surface active, such that the rise velocity is reduced by up to 50% compared to the experiment without fluorescent dye and a recirculation zone forms in the bubble wake. To access the local mass transfer at the interface, we perform complementary numerical simulations. Since the fluorescence tracer is essential for the experimental method, the effect of surface contamination is also considered in the simulation. We employ several improvements in the experimental and numerical procedures which allow for a quantitative comparison (locally and globally). Rise velocity and mass transfer coefficient agree within a few percents between experiment, simulation and literature results. Because the fluorescence tracer is frequently used in mass transfer experiments, we discuss its potential surface activity. © 2019 The AuthorsPublicationtype: Journal ArticleScopus© Citations 29 230 463 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Optimization of a split and recombine micromixer by improved exploitation of secondary flows(2018-02-15); ; ; ; Flow pattern in two split and recombine (SAR) micromixers were experimentally studied by μ-LIF measurements to validate corresponding CFD simulations. The original design of the investigated micromixer type is based on the repetition of identical SAR segments of a specific geometry. Aside a micromixer following this original design, the second micromixer investigated alternates two SAR segments versions, one being the original one, the other being an inverted and rotated version of it. The results of the simulations indicate that the secondary flows developing in the original design reduce the mixing performance for Re < 700. This knowledge and the improved understanding of the mixing principle in this specific kind of SAR micromixer was used to develop the design of an optimized version of this split and recombine micromixer type for Re < 700.Publicationtype: Journal Article156 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , 53 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Reactive Liquid-Flow Simulation of Micromixers Based on Grid Deformation Techniques(2017-08-01); ; ; ; Process intensification of engineering applications in the framework of reacting flows in micromixer devices attracts the attention of engineers and scientists from various fields. With the steadily increasing available computational resources, the traditional experimentally supported investigations may be extended by computational ones. For this purpose, a simulation framework based on state-of-the-art numerical techniques extended with special grid deformation techniques has been developed. Its validation in terms of comparison with computational and experimental results in reacting as well as in non-reacting frameworks has been performed on the basis of the T-mixer and SuperFocus mixer, respectively. The computational efficiency of the developed tool is shown to be applicable for optimization tasks, such as reverse engineering purposes.Publicationtype: Journal Article131 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Test system for the investigation of reactive taylor bubbles(Wiley-VCH, 2017-07-06); ; ; ; Detailed analysis of reactive multiphase flows is still challenging due to the complex linked transport processes. Until now, there is no reliable and reproducible experimental method for gas-liquid flows, which enables the local measurement of dissolved species concentrations of educts, products, and side products at well-defined and reproducible hydrodynamic conditions. A vertically arranged glass channel is used to capture gas bubbles with well-defined wake structures. The hydrodynamic conditions in the wake can be predicted by dimensionless numbers and adapted for most industrially relevant systems. An experimental apparatus is presented for the investigation of the interdependency between hydrodynamics, mass transfer, and chemical reaction. The applicability of the setup is demonstrated with a chemical test system that shows a strong color shift from colorless to blue when the gas-liquid reaction takes place.Publicationtype: Journal Article98 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Simultaneous local determination of mass transfer and residence time distributions in organic multiphase systems(Elsevier, 2017-04-01); ; ; ; A new method for simultaneous local determination of mass transfer coefficients and residence time distributions in continuously operated bubble columns has been developed. Since for organic liquids not many experimental data sets are available in respect to mass transfer, measurements were carried out with the organic gas/liquid multiphase system consisting of cumene (1-methylethylbenzene) and carbon dioxide. The concentration of carbon dioxide in the gas phase was adjusted by mixing with nitrogen. To measure concentrations of compounds in the liquid phase, attenuated total reflectance Fourier transform infrared spectroscopy (ATR FTIR) was applied: five ATR probes integrating silver-halide optical fibers with a length of 1.5 m were integrated in a bubble column of 128.5 cm height and a diameter of 10 cm. The probes were distributed over the height of the reactor and could be shifted within the reactor to measure radial profiles of the mass transfer coefficients in steady state operation mode at different positions. The superficial liquid velocity of the system was adjusted in the range of 0.7–1.4 cm s−1. The used superficial gas velocities were between 1.5 cm s−1 and 2.1 cm s−1 to ensure a homogeneous flow regime. Furthermore, tracer experiments with benzaldehyde for the investigation of the residence time distributions were performed. From the FTIR spectral data, CO2 and benzaldehyde tracer concentration data were determined simultaneously by conducting multivariate chemometric models. The corresponding mass transfer coefficients were calculated using a compartmental method for stationary flow. The residence time distributions were calculated from the benzoic aldehyde concentration data. Both parameters could be determined successfully. For the mass transfer coefficient of carbon dioxide into cumene, an interesting profile could be observed, which arose from several influences like areas of different bubble densities, caused by bubble breakage and coalescence, superficial velocities of gas and liquid as well as the applied type of sparger.Publicationtype: Journal Article96 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Growth model of a carbon based 3D structure (Aerographite) and electrical/mechanical properties of compositesAerographite is a 3D interconnected carbon based tetrapod structure. The production of Aerographite can be divided in two steps. First, the production of zinc oxide (ZnO) templates in a flame transport synthesis followed by the replication into the carbon structure in a chemical vapor deposition process (rCVD). During the rCVD process carbon deposits on the surfaces of the zinc oxide template, with the simultaneous reduction of zinc oxide into gaseous zinc. This replication process starts from the tetrapod base and continues alongside the tetrapod arms towards its top. Based on interrupted synthesis processes investigated by intense scanning electron microscopy (SEM) we present a simple model of the growth mechanisms of Aerographite. In a CFD (Computational Fluid Dynamics) simulation we could demonstrate that the initial flow of the cold Ar and H2 gas, when entering the reactor, sinks to the bottom, heats up to at the center of the reactor (where we have the maximum temperature) and advances to the top of the reactor. Vortexes are induced, leading to a partly backstream of the gas. The gas flow hits the samples from the back, so the conversion process of the zinc oxide templates starts from here towards the front of the samples. Furthermore, the electrical and mechanical properties of neat Aerographite and polymer composites will be shown.Publicationtype: Conference Paper74 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Influence of Bubble Bouncing on Mass Transfer and Chemical Reaction(2016-10-01); ; The hydrodynamics of bubbly flows is dominated by bubble-induced turbulence and bubble-bubble interactions. Both phenomena influence the gas-liquid mass transfer as well as the mixing of reactants. If the time scales of mass transfer and mixing are in the same order as the time scales of a parallel-consecutive reaction, the yield and selectivity will be affected by the local hydrodynamics. An experimental setup is presented that enables the investigation of mass transfer during well-defined and adjustable bubble collisions. The influence of CO2 bubble collisions on mass transfer is measured and modeled with a modified Sherwood number correlation. Further visualization of the concentration field in the vicinity of O2 bubbles by means of laser-induced fluorescence demonstrates the dependency of mass transfer from a chemical reaction and permits the development of a first model approach.Publicationtype: Journal Article53