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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: without files , 63 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Mass transfer from freely rising microbubbles in aqueous solutions of surfactant or salt(2020-05-01); ; ; ; Microbubbles are used in many applications. One of the major advantages of using microbubbles is to improve the mass transfer of gases to the bulk phase. In this study, mass transfer from single microbubbles that rise in ultrapure water and aqueous solutions of surfactants (SDS, Pluronic F68) or salt (NaCl) was investigated. The rising and dissolution behaviors of single air microbubbles were observed using a high-speed imaging technique by which the rising velocity, U B , and the mass transfer coefficient, k L , were optically evaluated. We found that the rising velocity of microbubbles obeys Stokes’ law, indicating that the surface of the microbubbles used in this study is immobile, despite their rising in ultrapure water. The obtained mass transfer coefficients were in good agreement with those predicted by Ranz and Marshall's correlation for the Sherwood number, Sh. Using the two correlations for U B and Sh, a theoretical model that describes the mass transfer from a microbubble was constructed. The proposed model can predict the dissolution of single microbubbles rising in ultrapure water and aqueous solutions with an accuracy of ±10% in terms of dissolution time.Publicationtype: Journal Article136 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Potential of lagrangian analysis methods in the study of chemical reactors(Wiley-VCH, 2020-03-02); ; ; ; In most chemical reactors the homogeneous distribution of reactants is a desired prerequisite for guaranteed product quality and purity. However, oftentimes, especially in new reactor concepts this condition may not be met due to evolving or existing structures in the transport patterns. In this article the usage of Lagrangian analysis techniques in order to analyze chemical and biochemical reactors regarding the behavior of Lagrangian tracers is explored. A Lagrangian analysis on two different datasets is performed trying to show the large versatility of the concepts. The Lagrangian analysis techniques allow new insights into the reactor dynamics and could be a valuable tool to identify and characterize areas of different mixing performance.Publicationtype: Journal ArticleScopus© Citations 10 182 228 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , How coherent structures dominate the residence time in a bubble wake: An experimental exampleTimescales and residence times in reactive multiphase flows are essential for product selectivity. For instance when a gas species is consumed, e.g., by a competitive consecutive reaction with moderate reaction kinetics where reaction timescales are comparable to relevant mixing timescales. To point out the importance of the details of the fluid flow, we analyze experimental velocity data from a Taylor bubble wake by means of Lagrangian methods. By adjusting the channel diameter in which the Taylor bubble rises, and thus the rise velocity, we obtain three different wake regimes. Remarkably, the residence times of passive particles advected in the bubble wake's velocity field show a peak for intermediate rise velocities. This observation seems unintuitive at first glance because one expects a faster removal of passive tracers for a faster overall flow rate. However, the details of the flow topology analyzed using Finite Time Lyapunov Exponent (FTLE) fields and Lagrangian Coherent Structures (LCS) reveal the existence of a coherent vortical pattern in the bubble wake which explains the long residence times. The increased residence times within the vortical structure in combination with the close bubble interface acting as a constant gas species source could enhance side product generation of a hypothetical competitive consecutive reaction, where the first reaction with the gas species forms the desired product and the second the side product.Publicationtype: Journal Article196 - 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 , Mass Transfer from Single Taylor Bubbles in MinichannelsMass transfer from single CO2 Taylor bubbles in vertical minichannels was measured for various channel hydraulic diameters Dh. The effects of channel geometries on the mass transfer were also investigated by using square ducts and circular pipes. Bubble rising velocities, vB, in the ducts were much faster than those in the pipes due to large liquid flow areas in the corners of the ducts. The values of mass transfer coefficients in the pipes were almost the same as those in the ducts, in spite of a large difference in vB. Sherwood numbers, ShD, using Dh as a characteristic length, are well-correlated in terms of the Eötvös number. The proposed ShD correlation can well predict a long-term dissolution process of a Taylor bubble.Publicationtype: Journal Article61