This review discusses and compares different measurement techniques for liquid foam flow experiments. Particular emphasis is put on measurements of foam velocity and liquid fraction. Because foam is opaque, complex and fragile, many of the established tools of fluid mechanics are not directly applicable. Consequently, these techniques had to be adapted or new approaches had to be developed. This review elucidates the most common techniques and approaches based on optical imaging, electrical conductivity, X-ray and neutron imaging, ultrasound, magnetic resonance imaging, positron emission tomography, and small-angle neutron scattering. Each technique has its specific advantages and limitations, and needs to be chosen wisely depending on the measurement requirements. To that end, this review provides some guidelines to choose the most appropriate technique for a specific measurement.

This study focuses on a new optical measurement technique for understanding the flow structure in an opaque suspension. Such opaque systems occur frequently within flotation cells. Adding fluorescent dye to the liquid phase of the suspension enhances the visibility of the particles significantly, at a solid concentration of 10% by mass. The image contrast is sufficient to derive the velocity field from Particle Image Velocimetry (PIV), based on cross-correlation of subsequent images. We studied the correlation quality as a function of the dyes molar concentration and added solid concentration of Magnetite or Quartz. At ideal dye concentration, crosscorrelation coefficients in the range of 0.4 to 0.8 can be reached, allowing for reliable determination of flow fields. The technique is simple, cheap and can enable experiments to understand the complex flow conditions in flotation cells with up to at least 10% solid mass fraction.

Improvement in resolving hydrodynamic variables in multiphase flows is key to optimizing flotation performance. However, due to equipment complexity and opacity of three-phase systems, in situ measurements become challenging. Therefore, by using a novel multi-sensor approach, the aim of this study is to spatially resolve key hydrodynamic and gas dispersion parameters in a mechanical flotation cell such as superficial gas velocity (Jg), g ), gas holdup (epsilon g), g ), bubble size distribution (BSD), and bubble surface area flux (Sb). b ). A high-resolution inline endoscope (SOPAT), Jg g and epsilon g g sensors were fixed at multiple axial positions in a 6L nextSTEPTM TM flotation cell. This multi-sensor concept has been applied to a simplified benchmark flotation scenario, as part of a binary (pyrite-quartz) flotation test campaign (30 % solid load). Varying operating conditions include tip speed (4.7 - 5.5 m/s), air flow (0.4 - 0.5 cm/s), frother (MIBC: 30 - 60 g/ton), and collector concentrations (PAX: 30 - 60 g/ ton). Sb b is a good indicator of gas dispersion efficiency in flotation, and local measurements indicated that there are significant differences in the local superficial gas velocities which can be measured with our adapted sensor. Real-time bubble size measurements reflected the high shear rates near the rotor-stator region. Overall, the gas flow rate and frother concentration were shown to have the most significant effect on the gas dispersion in the benchmark flotation tests.

The analysis of particle-bubble collisions in turbulent flow is a fundamental problem of high technological relevance, e.g., for the separation of valuable mineral particles by froth flotation. This relevance contrasts with an apparent lack of experimental data and understanding of this collision process. To this end, a periodic bubble chain was used to study the collision of millimeter-sized bubbles with polystyrene particles. The collision process between these entities was measured using 4D particle tracking velocimetry (PTV). By analyzing the collision data as a function of the polar angle along the bubble surface, we show that the collision took place not only at the leading edge but also at the trailing edge of the bubble. To understand the underlying mechanisms of the trailing edge collision, the flow field around a rising bubble chain was measured with Tomographic Particle Image Velocimetry (TPIV). The vortex formed in the bubble wake led to a velocity in the direction of the bubble surface that enabled trailing edge collisions. This effect is amplified by an increase in the turbulent kinetic energy and dissipation rate in the bubble wake. Overall, the investigation reveals different collision mechanisms and advances our understanding of the role of the wake in the bubble-particle collision.

The dynamics of a sub-millimeter air bubble rising at a bubble Reynolds number of about 100 in water in an inclined, laminar channel flow is investigated experimentally. In this configuration which is relevant in modern separation technologies for valuable particles, the bubble is undergoing a cross-stream motion, as the buoyancy force is not aligned with the undisturbed liquid flow. From measurements of bubble velocities and trajectories we estimate the drag and lift forces on the bubble at two different channel Reynolds numbers. The results are compared with their streamwise counterparts, i.e. in the configuration where the bubble rises largely along a streamline of the undisturbed liquid flow. For the lower channel Reynolds number, the cross-stream effects are only small. For the larger channel Reynolds number however, the drag coefficient is found to be notably larger than its streamwise counterpart. The lift coefficient may be either larger or smaller than its streamwise counterpart depending on the detailed local flow conditions. In particular, its value is non-zero when the bubble crosses the channel centerline where the shear rate is zero. These deviations are found to be closely connected with the bending of the bubble wake as well as the finite value of the angle formed between the bubble slip velocity and the velocity of the liquid flow.

Multiphase computational fluid dynamics (CFD) simulation is a useful tool to study the hydrodynamics in a bubble column if appropriate closure models are known. Systematic assessment of different models is an ongoing venture that benefits from improved validation data. The present study accumulates a database on two-phase flow experiments in a bubble column. This is achieved by using a combination of particle image velocimetry and shadowgraphy to measure the liquid velocity field and gas dispersion properties simultaneously. This methodology is applied for different needle diameters and gas flow rates. The experimental data are compared with CFD simulations which show good predictions. A systematic investigation of the three-phase flow in the bubble column will appear as a sequel.

Simulations of solid-liquid flow on industrial scales are feasible within the Euler-Euler / RANS approach. The reliability of this approach depends largely on the closure models applied to describe the unresolved phenomena at the particle scale, in particular the interfacial forces. In this work, a set of closure models reviewed previously for this kind of application (Shi and Rzehak, 2020) is further validated by comparing the predictions to a recent experiment on stirred-tank flows (Sommer et al., 2021), which focuses on dilute suspensions. The dataset used for validation comprises 14 different experimental cases, covering a wide range of particle Reynolds number, impeller Reynolds number, and particle Stokes number. For each case, simulation results on the solid velocity and volume fraction as well as liquid velocity and turbulence are compared with the experimental data. It turns out that by and large the experimental data are reasonably well reproduced. However, the measurements show a small but clear effect of modulation of the liquid phase turbulence by the particles. Therefore, several particle induced turbulence (PIT) models based on the available literature are assessed as well. Our results indicate a reduction in the predicted fluctuations by all PIT models, which improves the results in cases with turbulence suppression but deteriorates those with turbulence augmentation.

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 com-parison. 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. (c) 2021 Elsevier Ltd. All rights reserved.

The original manuscript contains scribal errors in the main equation. This corrigendum contains the correct equations. Results and conclusions in the original manuscript are not affected.

In a flotation cell, turbulence influences the motion of solid particles relative to the bubble surface, and, thus, affects the recovery rate. But, the impact of turbulence on the probability of a bubble-particle aggregation is still difficult to measure, especially in a dense flow. Therefore, the focus of this work was to apply Positron Emission Particle Tracking (PEPT) as a method to investigate the effect of turbulence on the particle movement and bubble-particle interaction in an opaque flow. Single air bubbles (d(b) = 2. 5 mm) were generated on a needle in a water flow channel. Upstream, a grid produced an isotropic turbulent flow with 5% to 15% turbulence intensity and a Kolmogorov microscale of 20 mu m. Depending on the distance to the grid, the flow near the captive bubble (R-eb = 450) was characterized by eddies of different length scales and magnitude with tomographic Particle Image Velocimetry (PIV). The solid suspension contained up to 0.3% polymethylmethacrylate (PMMA) particles (d(p) = 200-400 mu m) and up to six radiolabelled particles (d(p) = 300-400 mu m) coated with PMMA. The trajectories of the labelled particles were used to determine the average particle distribution in the turbulent field and describe the bubble-particle interactions. These results provide valuable information on the applicability of PEPT in turbulent and dense flow fields as well as on particle trajectories close to bubbles, enhancing our understanding of key flotation phenomena.

The original manuscript contains scribal errors in the main equation. This corrigendum contains the correct equations. Results and conclusions in the original manuscript are not affected.

This work focuses on analysing the collection process in flotation by means of a simultaneous time-resolved measurement of particle and bubble trajectories. We introduced a new method that determined the probability of collision and attachment using a 3D particle-tracking method with high temporal resolution (1000 fps) and spatial (0.03 mm/pixel) resolution in a dense particle flow (5000 particles/ml). A 4D particle image tracking device with three high-speed cameras recorded the three-phase flow in a rectangular bubble column (2 mm, bubble chain). Particles made of fluorescent polystyrene were employed so that particles appeared bright and bubbles dark on the captured images. An attachment occurred if the trajectory of a particle coincided with that of a bubble. The recovery was calculated based on the number of particles attached to a bubble compared to the total number of particles within a reference volume. With this method, the true flotation depending on the particle diameter (30-100 mu m) was investigated and the results compared with an existing model of the bubble-particle collection microprocess.

In this work, a model for the interaction force between a small bubble and a wall or another bubble is presented. The formulation is especially designed for Lagrangian calculations of bubble or soft sphere trajectories, with or without resolution of the continuous fluid. The force only relies on position and velocity of the bubble. The model does not include any empirical parameter that would have to be calibrated. Therefore, this force model is easy to implement. The formulation of the force is explicit, which means low computational effort. The collision of a small bubble with an inclined top wall is investigated numerically and experimentally. The computational results achieved with the new collision model show good agreement with the experiment.

Up to 80% of the total energy budget of wastewater treatment plants is consumed by the activated sludge process. Current optimizations are mostly based on limited instrumentation for single points of measurement which cannot express the complex hydrodynamic and biochemical processes. Therefore, the ultrafast electron beam X-ray tomography system ROFEX of HZDR is used as a new measurement technique to capture the temporal evolution of the multiphase flow in the opaque active sludge. A detailed study has been carried out in a vertical column of 3.5m height at HZDR to obtain an improved understanding of the hydrodynamics of aerated sludge and an evaluation of different aerators. The target parameters are bubble size distribution, equivalent Sauter mean diameter of the bubbles, bubble rise velocity and local gas hold-up under the variation of sparger type (rubber, monolithic material), gas flow rate, rheology of the fluid (deionized water, salty water, sludge) and height in the liquid column. Therefore, in-house developed advanced image analysis algorithms were applied to the reconstructed tomographic images, which are also presented in the paper. The experiments showed that with ROFEX reliable measurement data of opaque multiphase flows is produced and is expected to be used in further investigations for the validation of computational fluid dynamics (CFD) models. The different sparger types showed comparable hydrodynamic performance.