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    Mixing structures in bubble wakes of single bubbles and bubble swarms and their influence on gas-liquid mass transfer and chemical reactions
    The main objective of this proposal is to unveil how mixing structures in the wakes of freely rising single bubbles and in bubble swarms can be incorporated into compact mass transfer correlations for process engineering. In this way the impact of the wakes on chemical reactions can be estimated more easily. The knowledge about the detailed mass transfer is crucial to produce a vast proportion of bulk chemicals used in everyday life that are synthesized in gas-liquid reactions like hydrogenation, oxidation, or chlorination. Furthermore, contained high gas concentrations in bubble wakes can also be potentially harmful to microorganisms and enzymes in gassed reactors leading to a substrate inhibition. To predict how much gas is transferred from a bubble to a surrounding liquid the mass transfer coefficient (or the nondimensionalized Sherwood number )has to be determined. Todays mass transfer models (Sherwood correlations) either build on the two-film theory, on the penetration theory, or on the surface-renewal theory with varying parameters in dependence on bubble relative velocity to the surrounding liquid and laminar or turbulent surrounding flow conditions. Adapted models exist to account for the effect of surfactants onto the mass transfer. All theories assume, however, that once the gas is dissolved in the liquid phase it gets immediately well mixed and that all fluid parcels passing the bubble interface experience the same conditions in the bubble wake once they leave the bubble surface. This picture neglects effects of a heterogeneous concentration wake that will likely influence a reaction taking place close to the bubble interface and in the bubble wake for small to intermediate Hatta numbers as according to Levenspiel. So far, the details of the structure of the mixing in the bubble wake are not considered for mass transfer correlations. In this research proposal, the Lagrangian coherent structures (LCS) in the bubble wake of 3D freely rising bubbles and bubble swarms in stagnant liquid will be analyzed by evaluating 4D-PTV measurements using recently developed mathematical tools stemming from dynamical systems theory. Additionally, a quantitative evaluation of the concentration of the dissolved gas will be obtained using Time Resolved Laser Induced Fluorescence (TRS-LIF). The LCS will be compared to the concentration wakes and both measurements supply input for the development of new Sherwood correlations that incorporate the effect of coherent flow structures in bubble wakes.
      19
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    Fine bubbles for biocatalytic processes: Microscale phenomena and novel applications
    Biotransformations, where one of the reactants is supplied via the gas phase, especially in the case of oxidations, are still a challenge. The aeration of enzymatic reactions using fine bubbles, whose diameters are less than 100 µm, has been investigated by focusing on hydrodynamics and mass transfer effects as well as reaction kinetics of enzymatic systems. However, the fundamental scientific questions regarding the local processes taking place at the gas-liquid interface of a dissolving fine bubble are still to be targeted. This is even more important, if the interaction between a fine bubble and an immobilized enzyme on a surface is addressed, especially, on the background of increasing attention to structured packings as carrier materials for enzymes. The local mass transfer rate and oxygen concentration gradient at the interface is expected to be strongly affected by the Laplace pressure with decreasing bubble diameter. For mass transfer limited enzymatic reactions this mass transfer enhancement and the interaction between the enzymes and the gas-liquid interface are central aspects influencing the effectivity of the reaction. Using Laser Induced Fluorescence (LIF) measurements local mass transfer phenomena can be studied. With the help of these measurements the effect that enzymes are more stable during fine bubble aeration compared to conventional aeration, as demonstrated in the predecessor project, will be analyzed. In this context, investigation of the concentration boundary layer conditions on enzyme denaturation will be carried out for a better understanding of the acting mechanisms.
      68
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    Aerosol Particles and their Dispersion - Scientific Network to Support Interdisciplinary Exchange in the DFG Focus Funding COVID-19
    During the Corona pandemic, it became clear that aerosols contribute significantly to the infection process, but their propagation kinetics and ageing in connection with the infectivity of the virus are still largely unknown scientifically. In particular questions on the formation of aerosol particles during breathing, speaking, coughing and sneezing, as well as theirkinetics and dispersal dynamics (turbulent mixing) in rooms have so far remained largely unanswered. In addition, there are still many uncertainties regarding the efficientremoval of aerosol particles from indoor air and the infectivity of aerosol-borne viruses as well as their possible inactivation by means of multiphysical approaches (radiation, charge, temperature, etc.). The deposition and transport of aerosol particles in the respiratory tract is also largely unexplored. To support the fight against the COVID-19 pandemic, the DFG initiated a focus funding COVID-19 "Aerosol particles and their dispersion", in the framework of which 15 projects will be funded. Within the scope of the focus funding, the projects are expected to use model systems (e.g. virus-like particles) that can be transferred to SARS-CoV-2, and an interdisciplinary cooperation between engineering and/or life sciences. This scientific network is intended to support interdisciplinary exchange in the "DFG-Fokusförderung" program. Furthermore, the networking should enable the initiation of a DFG priority program for the interdisciplinary investigation of virus-carrying aerosols.
      68
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    Fine Bubbles for Biocatalytic Processes
    The rising demand in process engineering for aeration with high mass transfer performance with low pressure drop, low shear stress and avoidance of foaming opens an interesting field for new technologies. One possibility to achieve these goals is the aeration with fine bubbles whose diameter is less than 100 micrometers. In Japan, many applications of fine bubbles can be found and several companies have been established that are offering fine bubble generators (FBG) and analyzing devices for fine bubbles. In 2013, the technical committee for Fine Bubble Technologies (FBT) was established and approved by the Organization for Standardization (ISO). In Germany fine bubbles are known at the most in connection with flotation processes but their potential for mass transfer in two phase flows has not been addressed so far. Especially the question of physical properties and effectiveness has not been answered satisfactorily. Several authors are reporting a lifetime of fine bubbles in the range of several weeks and months, which is much longer than the predicted one. This effect is discussed very contradictorily and attributed to contaminations or surface charging. Nevertheless, the introduction of small-sized bubbles leads to large volume-specific interfacial areas and therefore to high mass transfer rates. The strong collaboration between Germany and Japan in the field of Multiscale Multiphase Process Engineering (MMPE joint conferences 2011 and 2014, supported by the DFG) provides a unique possibility to intensively exchange knowledge about fine bubbles and its applications between Germany and Japan and enables efficient research in this emerging field of technology. This project is intended to investigate the potential of fine bubbles for biocatalytic processes. High mass transfer performance, negligible two-phase pressure drop and shear stress as well as the avoidance of foaming and reactant evaporation are the most promising advantages of fine bubble aeration in comparison to conventional systems. While the measurement methods of the Institute of Multiphase Flows will provide a deep insight into hydrodynamics and mass transfer processes in fine bubble two-phase flows with high temporal and spatial resolution, the reaction engineering expertise and knowledge in biocatalysis of the Institute of Technical Biocatalysis will enable the detailed investigation of the effect of fine bubbles on biocatalytic reactions in stirred tanks as well as packed bed reactors. For the transfer of knowledge, Prof. Koichi Terasaka from Keio University, Japan is intended to join the project as Mercator Fellow. Furthermore, an intensive scientific exchange of students, PhD students and professors between Keio University and TUHH will foster research between the Universities and the presentation of results at the next International Symposium of MMPE 2017 in Toyama, Japan, is intended to motivate further initiatives related to research on fine bubbles.
      49
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    SPP 2170: Multiscale experimental analysis and simulation of lifelines in bioreactors to study their impact on the cultivation per-formance of Chinese Hamster Ovary (CHO) cells
    The CHOLife+ project aims to achieve full spatiotemporal resolution for bioreactor characterisation across scales—from laboratory-scale 3 L systems to industrially relevant 15,000 L reactors. Central to this work is the investigation of mixing heterogeneities, flow behaviour, and multiscale mixing phenomena using Lagrangian Sensor Particles (LSPs) and Lattice Boltzmann Large Eddy Simulations (LB-LES). This includes the detailed analysis of particle lifelines to determine their impact on CHO cell cultivation performance. A key element of the research is improving the reproducibility and robustness of LES simulations, ensuring that computational predictions remain reliable across scales and operating conditions. This also extends to exploring multiphase flow operation, reflecting the complexities of true industrial fermentation environments where gas–liquid interactions significantly influence mixing, transport, and residence time distributions. The work further involves mapping and fully characterising stirred-tank reactors (STRs) and their spatiotemporal gradients, including detailed distributions of flow structures and residence times of cells and molecules based on Lagrangian lifelines. Experimental and numerical insights are leveraged to advance the design and operation of single multi-compartment bioreactors (SMCBs) used as scale-down models at the University of Stuttgart. These systems replicate essential hydrodynamic and environmental features of large-scale industrial reactors under controlled laboratory conditions. By integrating sensor-based measurements (LSPs), LB-LES simulations, and complementary diagnostic methods, this research provides a multi-faceted and mechanistic understanding of bioreactor performance. This includes transport and mixing efficiency, compartmentalisation, and the dynamic trajectories experienced by cells in realistic single- and multiphase environments.
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