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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 , How Do Vortex Structures Influence Boundary Layer Dynamics in Gas-Liquid Systems?(2019-07); ; In many process engineering applications, the dynamics of gas-liquid two-phase flows plays a crucial role. The transfer of mass from the gaseous phase into the liquid phase is often a limiting step. This communication deals with the question how vortex structures, which are ubiquitous in engineering fluid flows, influence the transport processes of mass and momentum close to the interface. The question is approached by means of a methodical series of experiments with increasing optical access. By utilizing particle image velocimetry and laser-induced fluorescence, detailed investigations of hydrodynamics and mass transfer are carried out.Publicationtype: Journal Article91 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Laser-Induced Fluorescence in Multiphase Systems(2018-08); ; ; Nowadays, multiphase flows occur in many different applications in everyday life or industrial context. Consequently, the understanding of transport phenomenolgy between the participating phases is a crucial task of recent and prospective research. When it comes to the optimization of absorption and chemical reaction tasks in process industry, in-depth knowledge concerning mass transfer is required. The laser-induced fluorescence (LIF) imaging is therefore a promising optical measurement technique to characterize concentration fields with high spatial and temporal resolution. This review gives an overview of the different fields of research within which LIF measurements are carried out, with focus on gas-liquid systems. Chances and obstacles of recently publicated results as well as experiences of LIF methods regarding multiphase flows are presented and discussed.Publicationtype: Journal Article44 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Experimental analysis of a bubble wake influenced by a vortex street(Multidisciplinary Digital Publishing Institute (MDPI), 2018-01-20); ; Bubble column reactors are ubiquitous in engineering processes. They are used in waste water treatment, as well as in the chemical, pharmaceutical, biological and food industry. Mass transfer and mixing, as well as biochemical or chemical reactions in such reactors are determined by the hydrodynamics of the bubbly flow. The hydrodynamics of bubbly flows is dominated by bubble wake interactions. Despite the fact that bubble wakes have been investigated intensively in the past, there is still a lack of knowledge about how mass transfer from bubbles is influenced by bubble wake interactions in detail. The scientific scope of this work is to answer the question how bubble wakes are influenced by external flow structures like a vortex street behind a cylinder. For this purpose, the flow field in the vicinity of a single bubble is investigated systematically with high spatial and temporal resolution. High-speed Particle Image Velocimetry (PIV) measurements are conducted monitoring the flow structure in the equatorial plane of the single bubble. It is shown that the root mean square (RMS) velocity profiles downstream the bubble are influenced significantly by the interaction of vortices. In the presence of a vortex street, the deceleration of the fluid behind the bubble is compensated earlier than in the absence of a vortex street. This happens due to momentum transfer by cross-mixing. Both effects indicate that the interaction of vortices enhances the cross-mixing close to the bubble. Time series of instantaneous velocity fields show the formation of an inner shear layer and coupled vortices. In conclusion, this study shows in detail how the bubble wake is influenced by a vortex street and gives deep insights into possible effects on mixing and mass transfer in bubbly flows.Publicationtype: Journal ArticleScopus© Citations 5 396 484 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Investigation of the influence of vortex structures on transport processes at fluidic interfacesPublicationtype: Doctoral Thesis47