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Item type:Publication, Access status: with files , An experimental study on the time dependence of diffusive mass transfer of single oxygen bubbles(Elsevier, 2026-12-15); ; ; ; In the vast majority of gas–liquid engineering applications, the liquid phase contains a range of dissolved gaseous species. These dissolved process gases transfer from the liquid phase to the gaseous phase countercurrent to the typically desired mass transfer of gas to liquid. In process design, the resulting change in the composition of the gaseous phase is usually neglected, although a temporal change in the composition of the gaseous phase can directly influence the mass transfer performance over time. The current fundamental study quantifies the mass transfer performance of oxygen bubbles to liquid phases saturated with another gas. For this purpose, the oxygen mass transfer from a bubble to degassed, helium-, nitrogen-, argon- and carbon dioxide-saturated water is studied. Light Sheet Fluorescence Microscopy is used as imaging system for Planar Laser-induced Fluorescence measurements of dissolved oxygen concentration fields, delivering local instantaneous Sherwood numbers, diffusion coefficients and mass transfer coefficients. For the first time, the study showcases that the mass transfer performance from a gaseous dispersed to a liquid continuous phase is independent of time if mass transfer occurs in one direction only. If mass transfer occurs in both directions, the mass transfer performance of the dispersed phase is significantly lower and its time dependence higher to liquids containing gaseous species with high solubilities, such as carbon dioxide in water. Furthermore, the results suggest that applying intrinsic values, such as diffusion or mass transfer coefficients, obtained for binary systems in multicomponent systems can lead to high uncertainty.Publicationtype: Journal Article75 18 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Lagrangian μ-particle tracking velocimetry analysis of microscale flow over structured surfaces(American Chemical Society (ACS), 2026-10-08); ; ; ; This work aims to provide an experimental basis for the improvement of (bio)reactors by studying flow over micro- and nanostructured surfaces, with a focus on microscale mass transport toward and away from the surface. μ-Particle tracking velocimetry is employed in a microreactor to experimentally obtain Lagrangian trajectories of flow-following tracer particles. The method also resolves three-dimensional, time-dependent velocity fields. The derived mean velocity profiles agree well with the theory of flow over porous walls. Beyond averaged Eulerian velocities, the Lagrangian framework captures near-surface interactions. The Lagrangian analysis of different surfaces indicates that rectangular structured microgrooves increase the wall contact of the tracer particles, especially when the flow is oriented perpendicular to the microgrooves. Particularly, this approach enables resolution of trapping phenomena and localized recirculation dynamics. Nanostructuring by growing a carbon nanotube forest increases surface area and modifies local fluid dynamics, thereby enhancing contact with (bio)catalysts that are potentially functionalized on the nanotubes in future applications.Publicationtype: Journal Article40 13 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , A method for optical quantification of local oxygen limitations in multiphase bioreactors(Wiley, 2026-06-23); ; ; ; Spatial oxygen limitation is a major source of physiological stress and metabolic, scale-dependent heterogeneity in aerobic bioprocesses. In gas-liquid stirred tank reactors, oxygen availability is governed by hydrodynamics and mass transfer and is therefore inherently non-uniform. Despite its importance, experimentally accessing spatial information on oxygen local supply remains challenging, as most established techniques rely on point-wise sensing or intrusive tracer-based methods. This work presents an optical method for visualizing local oxygen limitation based on the reversible redox chemistry of methylene blue in alkaline glucose solutions. After driving the reactor into a uniformly reduced, colorless state, the spatial reoxidation of the indicator during oxygen aeration is recorded and evaluated pixel-wise. The resulting two-dimensional fields of colorization time provide a spatially continuous indicator of local oxygen supply governed by hydrodynamic transport and gas-liquid mass transfer. Systematic experiments were conducted to assess the influence of glucose and sodium hydroxide concentration on the stability and reproducibility of the redox system, enabling the selection of a robust reaction medium. The method was then applied to a laboratory-scale stirred tank operated at different impeller speeds. The resulting colorization fields reveal characteristic spatial structures associated with oxygen-limiting regions, including insufficiently supplied zones under weak agitation, progressive homogenization with increasing turbulence, and residual transport limitations near reactor walls at high agitation rates. The presented approach requires no local tracer injection and operates under normal aeration conditions, providing qualitative yet physically meaningful insight into oxygen-supply and mixing related heterogeneities. It therefore offers a simple and cost-effective tool for identifying limiting operating points, supporting future scale-transfer studies, and complementing CFD-based analyses of oxygen transport in stirred tank bioreactors.Publicationtype: Journal Article34 9 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Experimental studies on microscopic, multiphase transport phenomena: novel applications of light sheet fluorescence microscopy(Springer Science and Business Media Deutschland GmbH, 2026-05-26); ; ; ; Over the past two decades, light sheet fluorescence microscopy has developed to a powerful tool for studies of dynamics in biological systems. In a new development, we apply light sheet fluorescence microscopy as a novel experimental measurement technology within the field of multiphase process engineering and fluid dynamics. The technology enables a number of novel studies of single- and multiphase transport phenomena on a microscopic scale. In the current publication, we introduce the first implementation of light sheet fluorescence microscopy in the field and demonstrate its applicability on one exemplary measurement of diffusive oxygen mass transfer from an oxygen bubble to degassed water. The results prove that such measurements can be conducted with a high spatial resolution with a submicron pixel pitch and enable precise studies on microscopic transport phenomena. Besides measurements of mass transfer, on which we lay the focus here, light sheet fluorescence microscopy further enables studies of fluid dynamics on a microscopic scale.Publicationtype: Journal Article73 16 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Comprehensive study of 3D liquid flow fields in additively manufactured structures for SMART reactors using large-scale vertical magnetic resonance imaging and computational fluid dynamics(Elsevier, 2026-04-24); ; ; ; Triply Periodic Minimal Surface (TPMS) structures have emerged as a new class of porous materials with variable geometries and favourable transport properties, making them promising for reactor internals in chemical engineering. However, experimental data on internal TPMS flow behaviour are still limited. To address this gap, the flow behaviour in additively manufactured TPMS structures is analysed using three-dimensional Magnetic Resonance Imaging (MRI) velocimetry in a large-bore vertical 3 T MRI system, in cylindrical columns of 38 mm diameter and Reynolds numbers between 50 and 300. Three different TPMS geometries are investigated, and consistency between Computational Fluid Dynamics (CFD) simulations and experimentally measured MRI velocity fields is established through cross-validation. The MRI system provides fully three-dimensional velocity fields with a divergence deviation below 6 %. MRI revealed distinct flow features: the Gyroid TPnS exhibited pronounced channelling, while the Schwarz-Diamond TPSf showed merge-split behaviour, achieving a 46 % increase in lateral mixing compared to the Gyroid TPnS structures. Numerical simulations reproduce the flow features and show agreement with the MRI data. The combined methodology demonstrates the suitability of MRI velocimetry for the experimental validation of CFD simulations and establishes a robust foundation for future studies of heat and mass transfer, as well as reactive flow, in structured reactor systems.Publicationtype: Journal Article68 85 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Experimental analysis of time resolved three-dimensional velocity and vorticity fields behind single rising bubbles using Lagrangian particle tracking velocimetry(Elsevier, 2026-01-29); ; ; ; The present study utilizes highly resolved Lagrange particle tracking measurements to analyze the time-dependent three-dimensional fluid dynamics in the wake of air bubbles rising in quiescent bidistilled water. Two bubble sizes are studied with regard to the velocity fields and the temporal evolution of the generated vorticity in their wakes. The range of bubble sizes is of particular interest, as it corresponds to other recent studies. The bubbles are reconstructed in three dimensions to determine the bubble rise trajectory and the bubble shape. The analysis of the aforementioned measurements, particularly the calculation of the Q-criterion, supports prior numerical findings of a wake mode exhibiting secondary vortex loops. Additionally, the energy spectra of the velocity fields and their temporal evolution are analyzed, providing a unique experimental dataset for the validation of numerical simulations and the further study of bubbly flows.Publicationtype: Journal Article148 58 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Lagrangian sensor particles for detecting hydrodynamic heterogeneities in industrial bioreactors: experimental analysis and Lattice-Boltzmann simulations(Elsevier, 2025-04-19); ; ; ; This study analyzes trajectories of three particle types in an industrial-scale bioreactor, equipped with a Rushton turbine and a pitched blade turbine, to characterize hydrodynamic compartments. The trajectories obtained from measurements with Lagrangian Sensor Particles (LSP,exp) are compared to those generated by Lattice-Boltzmann large eddy simulations (LB LES). The latter method is used to reproduce analogous simulated LSPs (LSP,sim) as resolved particles. Additionally, for benchmarking purposes, massless tracer particles (tracer,sim) are incorporated to accurately represent fluid flow dynamics. Discrepancies in the axial probability of presence and velocity between LSP,exp and LSP,sim likely stem from differences in mass distribution, density, number of particles, and ratio of particle size to grid. A necessarily high LSP,sim volume fraction in LB LES leads to increased collisions and clustering, negatively impacting flow dynamics, and reducing turbulent kinetic energy by at least 3%. Circulation and residence time distributions for the three types of particles identify three hydrodynamic compartments within the bioreactor, validated by local mixing time distributions. The ratio of overall average circulation time to global mixing time is Θglob,95≈3.0⋅t¯circ for LSP,exp, which largely corresponds to literature results. A theoretical LSP size of dp,th≈1mm is estimated to be flow following on micro-scale in the bulk phase, if a Stokes number of St=0.1 is assumed. However, Stokes number estimations confirm that LSP,exp are capable to follow flow patterns on the meso-scale and macro-scale with St≈0.2 and St≈0.002, respectively. Hence, hydrodynamic structures at length scales greater than or equal to the size of the impeller can be investigated by current state-of-the-art LSPs, which proves their technological readiness for industrial bioreactors.Publicationtype: Journal Article53 26 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , High-pressure reactor technology for aerated biotransformationsUtilizing pressure as a process parameter can make biotechnological processes more efficient and attractive compared to established ones. This paper presents a high-pressure reactor setup for enzymatically catalyzed gas–liquid reactions, which can be operated up to 15.0 MPa. The reactor is equipped with optical measurement technology for inline and in situ monitoring of the oxygen concentration under high-pressure conditions. The setup is characterized by assessing the influence of the process parameter pressure on the conversion of the glucose oxidation to d-glucono-δ-lactone by immobilized glucose oxidase. The study demonstrates that the increased oxygen availability due to higher solubility reduces the reaction time in a batch reactor from 270 to 90 min.Publicationtype: Journal Article65 14 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Four-dimensional Flow MRI-based Computational Fluid Dynamics Simulation for Noninvasive Portosystemic Pressure Gradient Assessment in Patients with Cirrhosis and Transjugular Intrahepatic Portosystemic Shunt(Radiological Society of North America Inc., 2024-10-01); ; ; ; Background Transjugular intrahepatic portosystemic shunt (TIPS) dysfunction in patients with liver cirrhosis and recurrent symptoms of portal hypertension is primarily assessed with US and confirmed with invasive catheter venography, which can be used to measure the portosystemic pressure gradient (PSPG) to identify TIPS-refractory portal hypertension. To avoid the risks and costs of invasive catheter venography, noninvasive PSPG evaluation strategies are needed. Purpose To demonstrate the feasibility of the combination of four-dimensional (4D) flow MRI with computational fluid dynamics (CFD) for noninvasive PSPG assessment in participants with cirrhosis and TIPS. Materials and Methods Abdominal 4D flow MRI was performed prospectively in participants with cirrhosis and TIPS between January 2019 and September 2020. Flow rates were measured within the TIPS and inferior vena cava (IVC). The portal vein (PV), TIPS, right hepatic vein, and IVC were segmented on MRI scans to create a CFD mesh. The PV and infrahepatic IVC were defined as inflows for 4D flow MRI-derived flow rates. The suprahepatic IVC was defined as the outflow. CFD simulations were used to noninvasively estimate PSPG as the difference between the simulated pressures in the PV and suprahepatic IVC. Invasive venographic measurements of the PSPG served as the reference standard, and Pearson correlation analysis was conducted to evaluate the relationship between noninvasive estimates and invasive measurements. Results In all 20 participants with cirrhosis (mean age, 58 years ± 9 [SD]; 11 men), 4D flow MRI-based CFD simulations enabled visualization of flow velocities and pressure distributions within the segmented vasculature and TIPS. Noninvasive estimates and invasive measures of PSPG were strongly correlated (r = 0.77; P < .001). The 4D flow MRI-based CFD simulations correctly classified the presence or absence of a post-TIPS PSPG greater than 12 mm Hg in 16 of 20 participants (80%). Conclusion The combination of 4D flow MRI and CFD was feasible for noninvasive PSPG assessment in participants with cirrhosis, portal hypertension, and TIPS.Publicationtype: Journal Article44 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Dynamics of Lagrangian sensor particles : the effect of non-homogeneous mass distribution(Multidisciplinary Digital Publishing Institute (MDPI), 2024-08-01); ; ; ; The growing demand for bio-pharmaceuticals necessitates improved methods for the characterization of stirred tank reactors (STRs) and their mixing heterogeneities. Traditional Eulerian measurement approaches fall short, culminating in the use of Lagrangian Sensor Particles (LSPs) to map large-scale STRs and track the lifelines of microorganisms such as Chinese Hamster Ovary cells. This study investigates the hydrodynamic characteristics of LSPs, specifically examining the effects that the size and position of the Center of Mass (CoM) have on their flow-following capabilities. Two Lagrangian Particle (LP) designs are evaluated, one with the CoM and a Geometric Center aligned, and another with a shifted CoM. The experimental study is conducted in a rectangular vessel filled with deionized water featuring a stationary circular flow. Off-center LPs exhibit higher velocities, an increased number of floor contacts, and moreover, a less homogeneous particle probability of presence within the vessel compared to LPs with CoM and Geometric Center aligned. Lattice Boltzmann Large Eddy Simulations provide complementary undisturbed fluid velocity data for the calculation of the Stokes number St. Building upon these findings, differences in the Stokes number St between the two LP variants of ΔSt = 0.01 (25 mm LP) and ΔSt = 0.13 (40 mm LP) are calculated, highlighting the difference in flow behavior. Furthermore, this study offers a more representative calculation of particle response time approach, as the traditional Stokes number definition does not account for non-homogeneous particles, resulting in an alternative Stokes number (ΔStalt = 0.84 (25 mm LP) and ΔStalt = 2.72 (40 mm LP)). This study contributes to the improved characterization of STRs through the use of Lagrangian Sensor Particles. Results highlight the implications the internal mass distribution has on LSP design, offering crucial considerations for researchers in the field.Publicationtype: Journal Article32 16