In this experimental study the segregation behavior for fluidized mixtures of spherical and cylindrical particles is investigated. In industry, fluidization of particles featuring a wide range of shapes is common in various applications such as biomass gasification, drying applications, food processing and production of pharmaceuticals. Earlier publications have mainly focused on segregation of spherical particles of different volume or density. The particles used in this study have equal volume and density but a different shape. The main purpose of this work is to study de-mixing driven by particle shape. To analyze the particle distributions inside the fluidized bed, a Digital Image Analysis (DIA) technique has been developed, capable of capturing the particle positions and orientations within the bed over time. The experiments show that in the non-bubbling flow regime (at low fluidization velocities) rod-shaped particles may segregate, sinking to the bottom of the bed. In the bubbling flow regime (at higher fluidization velocities) segregation does not occur, because of bubble-induced mixing. Here strong alignment of the cylindrical particle's long axis with the flow is observed. The experimental results obtained give qualitative and quantitative insight in the behavior of non-spherical particles in fluidized beds and can be used for validation of numerical models concerning non-spherical particle mixing.

It is known that viscoelastic fluids exhibit elastic instabilities in simple shear flow and flow with curved streamlines. During flow through a straight microchannel with pillars, we found strikingly strong hydrodynamic instabilities characterized by very large transversal excursions, even leading to a complete change in lanes, and the presence of fast and slowmoving lanes. Particle image velocimetry measurements through a pillared microchannel provide experimental evidence of these instabilities at a very low Reynolds number (< 0.01). The instability is characterized by a rapid increase in spatial and temporal fluctuations of velocity components and pressure at a critical Deborah number. We characterize under which conditions these strong instabilities arise.

A complete knowledge of the effect of droplet viscosity on droplet-droplet collision outcomes is essential for industrial processes such as spray drying. When droplets with dispersed solids are dried, the apparent viscosity of the dispersed phase increases by many orders of magnitude, which drastically changes the outcome of a droplet-droplet collision. However, the effect of viscosity on the droplet collision regime boundaries demarcating coalescence and reflexive and stretching separation is still not entirely understood and a general model for collision outcome boundaries is not available. In this work, the effect of viscosity on the droplet-droplet collision outcome is studied using direct numerical simulations employing the volume of fluid method. The role of viscous energy dissipation is analysed in collisions of droplets with different sizes and different physical properties. From the simulations results, a general phenomenological model depending on the capillary number (Ca, accounting for viscosity), the impact parameter (B), the Weber number (We), and the size ratio (Δ) is proposed.

The large-scale hydrodynamic behavior of relatively dense dispersed multiphase flows, such as encountered in fluidized beds, bubbly flows, and liquid sprays, can be predicted efficiently by use of Euler-Lagrange models. In these models, grid-averaged equations for the continuous-phase flow field are solved, where the grid size is larger than the discrete phase size, while the discrete phase is explicitly tracked and experiencing forces in a Lagrangian fashion. In this chapter, we provide a summary of our own efforts in this field, including details which we deem necessary for a novice to be aware of. We start with a theoretical introduction to Euler-Lagrange models, emphasizing the importance of the availability of high-quality correlations for the interphase momentum transfer and the outcome of binary interactions between members of the discrete phase. Then, in three topical sections, we discuss implementations of the methods which are used intensely in our group: the computational fluid dynamics/discrete element method (CFD-DEM), discrete bubble method (DBM), and direct simulation Monte Carlo (DSMC). CFD-DEM is most suitable for solid particles moving in a gas. The interplay between hydrodynamic flow and dissipative collisions between these particles leads to inhomogeneities at meso- and larger scale. DBM applies to bubbly flows, where the additional complication of coalescence and splitting of bubbles needs to be taken into account accurately. DSMC is suitable for not-too-dense systems of particles or droplets in a gas (dispersed volume fraction less than 10%). Collisions between the discrete phase elements are detected stochastically from the local number density, relative velocities, and sizes of neighboring dispersed elements, leading to a considerable saving of computer time. We end with an outlook into directions of research which would lead to an even more comprehensive use of Euler-Lagrange models in the future.

Pneumatic conveying of particles is generally applied in large ducts. However, new applications are emerging which benefit from millimeter-sized ducts; for example, triboelectric separators where intensive wall-particle contact is desirable. An optical method is proposed to measure the distribution of the position and velocity of 100-1000 μm particles in such narrow ducts. Images of the system are captured using a digital camera on which a Hough transform is applied to detect the particles and their positions. The velocities are acquired by applying a hybrid particle tracking and particle image velocimetry approach. This made it is possible to overcome challenges caused by suboptimal lighting, nonsmooth background, and a large ratio between particle and duct diameter (>O(0.1)). It is shown that the algorithm is subpixel accurate when sufficient particles can be sampled. Finally, typical results are shown to illustrate the method's capabilities.

An Euler-Lagrange model is presented that describes the dynamics of liquid droplets emerging from a high-pressure spray nozzle in a relatively large volume (of the order of almost a cubic meter). In the model, the gas phase is treated as continuum, solved on an Eulerian grid, and the liquid phase is treated as a dispersed phase, solved in a Lagrangian fashion, with interphase coupling through state-of-the-art drag relations obtained from direct numerical simulations. The droplets are introduced into the system at high velocities, leading to a turbulent self-induced gas flow which is solved using large eddy simulation. Despite the relatively low liquid volume fraction in the spray, the number density of droplets at the nozzle is still more than 10¹⁰m^-3, which is why we employ a highly efficient stochastic Direct Simulation Monte Carlo approach to track collisions between droplets. The droplet collision frequency is calculated on the basis of local droplet number density, droplet size and relative velocities of neighbouring droplets within a dynamically adapting searching scope, as described in Pawar et al. (2014. Chem. Eng. Sci. 105, 132-142). We use known correlations from literature to determine the outcome of a binary droplet collision, which depending on characteristic dimensionless numbers can be coalescence, bouncing or, for high velocity impacts, stretching or reflexive separation leading to formation of satellite droplets. Our simulation model is compared with droplet velocities and size distributions obtained from phase Doppler interferometry experiments on an industrial scale hollow-cone pressure swirl nozzle spray. We find semi-quantitative agreement for spray characteristics such as the axial and radial spray velocity, spray jet width, and the dependence of the droplet size distribution on position within the spray. The simulation model enables us to study the relative importance of different droplet collision events occurring in the spray volume.

Research on the dynamic flow behaviour in spray dryers has a long history. Interest in describing these flows originates from problems like roof and wall fouling. The aim of the present study is to experimentally investigate the dynamic jet behaviour and turbulent flow in a scaled-down cold flow model of a spray dryer in order to better understand and optimize spray drying units. Dynamic jet behaviour and turbulent flow features (i.e., RMS velocities) were studied by particle image velocimetry (PIV) using water as the continuous phase. To obtain more insight in the jet dynamics, we analyzed the turning point, the width and shape, and the velocity profiles of the turbulent jet at different heights and the turbulence characteristics. We found that at higher Reynolds numbers, the jet penetrates further along the downward direction with a time-averaged profile which is symmetric at the centre. In addition, we investigated the effect of the expansion ratio via proper orthogonal decomposition (POD). Outcomes of different characteristics of the dynamic jet, like steady, transient, regular, and complex precession, can be collapsed by proper scaling. These results can be used for validation of computational fluid dynamics simulations and facilitate the design (identification of jet operation boundaries) of new spray dryer configurations.

The bubble characteristics in a 3-D cylindrical fluidized bed have been investigated both experimentally and numerically. Experiments were performed on a 0.1 m diameter fluidized bed, with alumina oxide particles (diameter ~1 mm) as a fluidizing material. Measurements were done at a spatial resolution of 1 mm and a temporal resolution of 1000 cross-sectional images per second, using an ultrafast electron beam X-ray computed tomography (XRT) setup (Fischer and Hampel 2010). A two-fluid model using kinetic theory of granular flow (Verma et al., 2013) was used to predict the bed dynamics numerically. The equivalent bubble diameter as a function of height is in close agreement with Darton et al. (1977) and Werther (1975) correlations. The bubble size distribution predicted from simulations is broader compared to experiments. Both the bubble rise velocity and the bubble size increase with increase in excess gas velocity. The experimental measurements and simulation predictions are in fair agreement with the Hilligardt and Werther (1986) correlation.

We apply a recently developed two-fluid continuum model (TFM) based on kinetic theory of granular flow (KTGF) in three dimensional cylindrical coordinates, to investigate bubble formation through a single central orifice in a gas-solid fluidized bed. A comprehensive study for Geldart D type particles, revealing the influence of particle diameter, jet injection flow rate, and bed size on bubble characteristics have been investigated. At a given gas injection flow rate, the bubble diameter continuously increases while gas leakage from the bubble to the emulsion phase decreases with time. With increasing particle diameter, leakage fraction increases and hence a smaller bubble diameter is predicted. These results are consistent with DPM simulations, experimental results and approximate bubble formation models reported previously in the literature.

This paper reviews the use of direct numerical simulation (DNS) models for the study of mass, momentum and heat transfer phenomena prevailing in dense gas-solid flows. In particular, we consider the DNS models as the first important step in a multiscale modeling strategy. Both the merits and the limitations of different DNS methods are discussed, in particular for the field of fluidized bed modeling. The importance of the closures for interfacial transfer of mass, momentum and heat, obtained from DNS and applied in coarser scale models, is demonstrated with illustrative examples. Finally, we present our view on required future developments of DNS models for the investigation of various chemical engineering problems.