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Abstract: Present work describes the development of a non-intrusive image analysis technique to measure the bubble size distribution in bubbly flows. Generally image analysis methods for bubble characterization in bubbly flows are suitable at low void fractions. These methods depend mainly on the appropriate thresholding/conversion of the image into a binary image, dividing liquid and bubble objects. Clusters of overlapping bubbles are discarded using the criteria of a shape factor, thus leaving only solitary bubbles within an image. When the void fraction is low, the amount of discarded bubble data is very small compared with the obtained solitary bubble data. For bubbly flows with larger void fractions, the discarded data of overlapping/clustering bubbles is considerable. In this work the overlapping clusters are passed through a watershedding algorithm in order to segment the groups of bubble object areas into individual bubbles. The developed image algorithm is verified with synthetic bubble images and applied for an actual system involving bubbly flow in a pseudo-2D bubble column. [Copyright &y& Elsevier]

In this work the concept of multi-scale modeling is demonstrated. The idea of this approach is to use different levels of modeling, each developed to study phenomena at a certain length scale. Information obtained at the level of small length scales can be used to provide closure information at the level of larger length scales. In this work the front tracking model is used to simulate the rise of a single bubble in a quiescent liquid and in a shear field. The results of the front tracking model are used to obtain coefficients for the drag, virtual mass and lift forces that are required in the Euler–Lagrange model used to study the behavior of dispersed gas–liquid flow on a larger scale. Finally, the Euler–Lagrange model, equipped with the correct closure information is used to simulate the flow in a square cross-sectioned bubble column. It is found that the results of the Euler–Lagrange model are in good agreement with experimental data. [Copyright &y& Elsevier]

Gas-liquid-solid flows are frequently encountered in chemical, petrochemical and biochemical industries. To overcome the heat and mass transfer limitations in trickle bed reactors and bubble slurry columns, respectively, a micro-structured bubble column (MSBC) can serve as an attractive alternative. In a MSBC, wire meshes are introduced to cut the bubbles in smaller bubbles and enhance the surface renewal (and hence gas-liquid mass transfer) rates, by deformation of the bubbles. Earlier (Jain et al., 2013) modeling efforts using the Euler-Lagrange approach to simulate a micro-structured bubble column employed a bubble cutting closure based on purely geometrical considerations. To improve on this ad hoc procedure in this paper we explore the possibilities of Direct Numerical Simulations to gain more insight in this complex phenomenon with the ultimate aim to develop improved closures. A combined Volume of Fluid-Immersed Boundary method was applied to simulate the interactions between bubbles and wire meshes. When the bubbles are aligned with the opening of the wire mesh, cutting of the bubbles is not observed in our simulations, while cutting was expected based solely on geometrical considerations. When the Eötvös number, Eo , is larger than 4, the bubbles are highly deformable and squeeze themselves through the opening of the wire mesh. In addition, the bubble gets stuck underneath the mesh when the bubbles are small ( Eo ⩽ 4 ) and/or the opening is in the wire mesh is small. Almost all bubbles that hit the intersection of two crossing wires get stuck underneath the mesh, except for large bubbles ( Eo = 15 ), which get cut by the mesh. Based on these results, it is concluded that the cutting of bubbles depends on the Eötvös number, the opening of the wire mesh and geometrical considerations. However, the results also seem to indicate that the diameter of the wire mesh will also influence the cutting behavior. [ABSTRACT FROM AUTHOR]

Abstract: In this work we present a numerical model to predict the bubble size distribution in turbulent bubbly flows. The continuous phase is described by the volume-averaged Navier–Stokes equations, which are solved on an Eulerian grid, whereas the dispersed or bubble phase is treated in a Lagrangian manner, where each individual bubble is tracked throughout the computational domain. Collisions between bubbles are described by means of a hard-sphere model. Coalescence of bubbles is modeled via a stochastic inter-particle encounter model. A break-up model is implemented with a break-up constraint on the basis of a critical Weber value augmented with a model for the daughter size distribution. A numerical parameter study is performed of the bubble break-up model implemented in the deterministic Euler–Lagrange framework and its effect on the bubble size distribution (BSD) is reported. A square bubble column operated at a superficial gas velocity of 2cm/s is chosen as a simulation base case to evaluate the parameters. The parameters that are varied are the values of the critical Weber number ( ), the daughter size distribution (β) and the superficial gas velocity ( ). Changes in the values of and have a significant impact on the overall BSD, while a different shaped β did not show a significant difference. [Copyright &y& Elsevier]

Abstract: In many different fields of research, the interactions between two immiscible fluids are of importance. To study these flows in industrial equipment, a multi-scale modeling approach is often used. In this approach, the smallest scale models apply detailed information in the form of closure equations for the larger scale models, which can model complete industrial equipment. This paper will focus on the improvement of the smallest scale model; direct numerical simulations employing the Volume of Fluid model. In this model, mass is inherently conserved because of the surface treatment, but this treatment also poses a challenge in calculating the surface properties like the surface tension. In this paper, three different surface tension models for the Volume of Fluid were tested: the generally used Continuum Surface Force (CSF) model, the height function model and the novel tensile force method. From the verification tests, it was concluded that both the height function model and the tensile force method are an improvement of the CSF model. The single bubble simulations showed that the height function method works best for small bubble . This is due to problems with connectivity for the tensile force method. While for the larger bubbles , the tensile force method is the best functioning surface tension model, because the calculation of the curvature in the height function method uses a stencil in which the distance between two interfaces in the direction of the normal should at least be four grid cells. In all the other tested cases, the height function model and the tensile force method perform equally well. The Morton number changes the ranges for the region of use of the surface tension models slightly when (the height function model can only be used when , while the tensile force method can only be used at ) and (the height function model can in this region also be used when ). [Copyright &y& Elsevier]

Abstract: The present paper describes an Euler–Lagrange model utilizing a drag closure derived from direct numerical simulations (front-tracking model) for (i) single isolated bubbles and (ii) bubbles rising in bubble swarms, expressed as a function of the local gas fraction. The model is applied to the prediction of an air/water system in a bubble column and for which experimental data is available. The effect of variation in size of the mapping window for the interphase coupling between the Eulerian and Lagrangian framework is investigated for both closure relations. It is found that the drag closure as a function of the local gas fraction is an improvement over the use of the drag closure for isolated single bubbles for the prediction of bubbly flow. [Copyright &y& Elsevier]

Many gas–liquid–solid contactors, such as trickle bed and bubble slurry columns, suffer from heat and mass transfer limitations. To overcome these limitations, new micro-structured bubble column reactor is proposed. In this reactor, a catalyst coated wire mesh is introduced in a bubble column to cut the bubbles and enhance the interface dynamics. In this work, the effect of the simplest form of a wire mesh, i.e. a single wire, on a single bubble is studied experimentally and using Direct Numerical Simulations. The combined Volume of Fluid and Immersed Boundary method shows a good agreement with the experimentally obtained bubble sizes after cutting, bubble shapes, trajectory and bubble rise velocities. Only small differences are found in the bubble rise velocity in close proximity of the wire. Various effects were determined both experimentally and numerically: the effects of Eötvös number, Morton number, bubble rise velocity, wire diameter and the obliqueness of the bubble wire interactions. From the results, it can be concluded that the main influences on the bubble cutting are the Eötvös number, the wire diameter and the obliqueness of the interaction. However, a quantitative closure for the bubble cutting using single wires could not be obtained. [ABSTRACT FROM AUTHOR]

Abstract: Gas–liquid–solid three phase flows are encountered in for example the Fischer–Tropsch process for the production of synthetic fuels in bubble slurry columns. To predict the hydrodynamics in large slurry bubble columns a multi-scale modeling approach can be used, which accounts for the large variation in time and length scales. In this paper, the smallest scale model has been developed using the Front Tracking model of Dijkhuizen et al. (2010b) and the Immersed Boundary model of Kriebitzsch (2011). In the Front Tracking model, each bubble is tracked separately. Furthermore, the Immersed Boundary method introduces the particle–fluid and the particle–particle (via a hard sphere model) interactions in the model. The resulting hybrid Front Tracking Immersed Boundary model is able to simulate dense three-phase flows and accounts for swarm effects in a fundamental manner. From our simulations we found that the relative drag coefficient of bubbles in three-phase flows seems to increase with increasing solids volume fraction. However, longer averaging periods are needed to derive a fully predictive correlation for the relative drag coefficient with respect to the solid volume fraction. [Copyright &y& Elsevier]

Abstract: This work presents an experimental study of the bubble size distribution of a bubbly flow using digital image analysis (DIA). In order to facilitate the image measurement technique a pseudo-2D bubble column is chosen for the experiments. To obtain well-defined inlet conditions a gas sparger, consisting of 20 needles, is used. By employing DIA, the bubble size distribution (BSD) has been measured for a range of superficial gas velocities. The resulting BSD's are expressed in terms of a probability density function (PDF). For low superficial gas velocities of 5 and 10mm/s the PDF has a unimodal shape, while for higher superficial gas velocities of 15 and 20mm/s the PDF has a bimodal shape. The effects of coalescence and break-up of bubbles are visible by evaluating the changes of the resulting BSDs for increasing superficial gas velocity. A comparison of gas hold-ups is made between the calculated BSD and the liquid expansion height. This comparison shows how well the BSD obtained with DIA describes the actual gas hold-up in the column. [Copyright &y& Elsevier]