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It is more and more possible to design bubbly flow reactors with methods of the computational fluid dynamics (CFD). Measurements that can be used for model validation, however, are often missing, especially for complex setups like airlift reactors. Such measurements include locally resolved information about the dispersed and continuous phase, particularly the information about the flow field and interface structures are important. In the present work Reynolds stresses, liquid velocity and gas void fraction profiles as well as bubble size distributions are provided at several positions in the riser and the downcomer in a rectangular airlift reactor for this purpose. In addition, the hydrodynamics inside this airlift reactor are described in detail by the measured values. [ABSTRACT FROM AUTHOR]

CFD simulations using the Euler–Euler approach are performed to model the gas–liquid bubbly flow in a helical static mixer. The model validation work was based on experiments, which are carried out in a column of diameter 0.08 m packed with helical static mixer elements (length 80 mm/diameter 80 mm). Measurements of gas volume fractions, gas velocities and bubble size distributions at several planes within the mixer elements by in-house developed ultrafast X-ray electron beam tomography were taken (Rabha et al., 2015. Chem. Eng. J., 15, 527–540). The predicted axial and radial gas phase distribution considering different mono-disperse bubble sizes (3, 5.8 and 8 mm) is studied and validated against the experimental results. The dependency of non-drag forces on the bubble size was considered. Consequently, the bubble size dependent effects of the non-drag forces on the flow and on the cross–sectional gas volume fraction distribution are shown. Despite obvious shortcomings of the models for this application, some conclusions on the suitability of certain mixer designs for gas–liquid dispersion may be drawn already. The swirling flows created by the twist and turn of the helical mixer elements, which in turn pushes the lighter phase towards the center of the pipe is well predicted and validated. Further investigations have to consider the bubble size distribution e.g. by a population balance model to accurately predict the dispersion of the gas phase within and downstream the helical static mixer. [ABSTRACT FROM AUTHOR]

• Experimental determined lift coefficient C L of single ellipsoidal bubbles in water • Improved C L determination through simultaneous bubble and liquid velocity measurements • Investigation of the influence of impurities on C L with the use of different water qualities • New correlation to calculate the lift coefficient in air/water flows for ellipsoidal bubbles For the simulation of bubbly flows, knowledge of the lift force as an interaction between gas bubbles and a surrounding shear field is of great importance. The sign of the lift coefficient C L changes with increasing bubble size, i.e. with more pronounced bubble deformation. Beside this, impurities in terms of surface-active components are well-known to change the complete hydrodynamic behavior of a bubble even if the amount is very small. In the present work, the lift coefficient of single ellipsoidal bubbles is determined with a recently developed method, which is suitable to overcome difficulties connected to low viscous systems. In order to investigate the influence of impurities on the lift force, we conducted experiments with single bubbles of different sizes in purified, deionized and tap water. Overall, the determined lift coefficients show no difference between deionized and tap water but reveal differences to results obtained with purified water. As no significant differences in shape and velocity are found between the different water qualities, it remains unclear how the impurities cause the observed differences. For the deionized and tap water results that are more relevant in practice, a new correlation is proposed to account for the observed differences in comparison to data from the literature. It can be used to calculate C L of ellipsoidal bubbles in the investigated size range. [ABSTRACT FROM AUTHOR]

• The two phase flow was studied simultaneously with a combination of PIV and PSV. • The formation of a tailing vortex behind the impeller blade was resolved. • The computed single-phase flow velocities agreed will with the experiments. • A "CFD-grade" database for a two phase stirred tank was generated. Computational Fluid Dynamics (CFD) simulations of solid–liquid flows in stirred tanks are feasible with appropriate closure models. However, no systematic assessment of different models has appeared yet because of lacking validation data. The present study accumulates a "CFD-grade" database on solid–liquid two-phase flow experiments in a stirred tank (diameter = 90 mm), including single-phase flows for comparison. The velocity fields of the liquid and solid phases are measured with Particle Image Velocimetry and Particle Shadow Velocimetry, respectively. The experiments cover a range of density ratios, particle diameters, solid volume fractions and impeller rotation speeds. The mean and fluctuating liquid and solid velocities are obtained by time-averaging and angle-resolved averaging, as well as the time-averaged local solid fraction. The experimental data for the single-phase flows is compared with CFD simulations which show reasonably good predictions. A systematic assessment of CFD models for solid–liquid flows will appear as a sequel. [ABSTRACT FROM AUTHOR]

The two- or multi-fluid approach is frequently used for Nuclear Reactor Safety (NRS)-related simulations of gas-liquid flows. To enable reliable predictions the closure models have to reflect the involved local physical phenomena at the non-resolved scale properly. To consolidate the CFD-modelling in the frame of the multi-fluid approach the so-called baseline model strategy was recently proposed (Lucas et al., 2016). The present technical note discusses a long-term strategy for the baseline model development and ways to obtain or improve closure models. Guidelines for the model development are given by listing requirements for appropriate closure models as well as frequently made mistakes. This is illustrated by examples for recent developments done for HZDR baseline models for poly-disperse bubbly flows. [ABSTRACT FROM AUTHOR]

Using Brownian dynamics simulations, we investigate how hydrodynamic interaction (HI) affects the behavior of tethered polymers in uniform flow. While it is expected that the HI within the polymer will lead to a dependency of the polymer's drag coefficient on the flow velocity, the interchain HI causes additional screening effects. For the case of two polymers in uniform flow with their tether points a finite distance apart, it is shown that the interchain HI not only causes a further reduction of the drag per polymer with decreasing distance between the tether points but simultaneously induces a polymer-polymer attraction as well. This attraction exhibits a characteristic maximum at intermediate flow velocities when the drag forces are of the order of the entropic forces. The effects uniquely attributed to the presence of HI can be verified experimentally.

The Brownian motion of a particle in a harmonic potential, which is simultaneously exposed either to a linear shear flow or to a plane Poiseuille flow is investigated. In the shear plane of both flows the probability distribution of the particle becomes anisotropic and the dynamics is changed in a characteristic manner compared to a trapped particle in a quiescent fluid. The particle distribution takes either an elliptical or a parachute shape or a superposition of both depending on the mean particle position in the shear plane. Simultaneously, shear-induced cross-correlations between particle fluctuations along orthogonal directions in the shear plane are found. They are asymmetric in time. In Poiseuille flow thermal particle fluctuations perpendicular to the flow direction in the shear plane induce a shift of the particle's mean position away from the potential minimum. Two complementary methods are suggested to measure shear-induced cross-correlations between particle fluctuations along orthogonal directions.

We discuss relaxation of conformational fluctuations around deformed polymer states. To this end, Brownian dynamics simulations of bead-spring models including a finite extensibility of the springs as well as excluded volume and hydrodynamic interactions between the beads have been performed. Complete spectra of relaxation times as well as corresponding relaxation modes are obtained from the simulation data by applying the static dynamics formalism, which rigorously describes the initial decay of correlations between the bead positions. As shown here, this procedure amounts to using a generalized Rouse-Zimm-like model which is governed by linear effective equations of motion having the same initial decay of correlations as the full nonlinear bead-spring model used in the simulations. In thermal equilibrium, the well-known scaling laws in the presence of excluded volume and hydrodynamic interactions between the beads are recovered, but, in addition, the static dynamics method also yields numeric values for the nonuniversal prefactors of the respective laws. The method is equally applicable to a broad range of problems, where the polymer is deformed by the action of flows or forces. Two examples of recent interest are considered: a tethered polymer pulled at its free end and one which is stretched by a uniform flow. It is shown that in both cases, the relaxation process is dominated by a finite extensibility of the springs.