Experimental and numerical studies found that fractures of particle assemblies occur often in dense gas-solid flows systems, leading to considerable heterogeneity in the configuration. We have studied the influence of such heterogeneities on the hydrodynamic drag by detailed investigation of an idealized case consisting of random configurations of spheres with a channel-like void region. Highly resolved lattice Boltzmann simulations were carried out to solve fluid flow through static homogeneous and heterogeneous particle arrangements respectively. We consider the overall pressure drop dependence on the characteristic width of the channel, the superficial Reynolds number (30 ≤ Re ≤ 300) and the solid volume fraction in the dense region (0.4 ≤ φp ≤ 0.55). The numerical results are in good agreement with previously reported works [1-3]. However, the overall momentum exchange obtained for configurations containing a heterogeneity is significantly lower. A significant reduction is shown in overall pressure drop even for a channel width of only one particle diameter. In addition to the numerical simulations, we have further supplemented our numerical findings with a semi-analytical approach combining the correlations for pressure drop in homogeneous particle beds and in channels. We estimate the channel pressure drop with the appropriate correlations selected according to the superficial Reynolds number. This work underlines the significance of channel/crack formation on fluid-solid momentum exchange and overall pressure drop in dense particle arrangements. In a large-scale granular system, the channel formation can develop on a scale smaller than the grid resolution of unresolved simulations. Consequently, such sub-grid heterogeneities are often overlooked in unresolved CFD-DEM simulations. Our proposed analytical description of overall fluid-solid momentum exchange might open the path towards the development of specific sub-grid models for unresolved CFD-DEM simulations.

Fluidized beds are widely used in the chemical and process industries for a large variety of processes. Good understanding of the transport phenomena in these systems is of great importance to improve design and scale-up procedures [1]. The focus of the work is on the investigation of interaction between cohesive particles and interstitial gas by means of non-resolved CFD-DEM simulation and the experimental investigation of the spout bed, where such interaction can be observed. Gas-solid flow heterogeneity, such as particle clustering, can have significant impact on interphase transport properties. It is of interest to establish which parameters have influence on the channel properties and distribution, and to what extent, so a correlation can be derived as a function of the properties (such as fluid velocity and viscosity, solid volume fraction etc.). Experimental measurements are conducted in a lab-scale 2D fluidized bed test facility. The main purpose of the experiment is to obtain the information on the overall dynamics of the spout bed with dry and moisturized particles. With wet particles, it is expected to observe the effects related to the particle cohesion, such as channeling and formation of particle clusters. Beside images captured by high speed camera, data will be obtained from pressure sensors to get pressure drop through the bed. Inlet velocity is controlled by controlling the fan power and measured with the pitot tube. The recorded images were processed in MATLAB using a script for digital image processing. For each experimental run, an initial snapshot of the bed is captured and initial bed height is calculated. Mean images and pixel variance images are calculated and such images provide a reproducible result that can be compared with previous measurements. Image processing also allows the identification of different zones inside the bed. In the mean images, it is possible to observe central spout, blurred moving annulus region and sharp dead zones. The problem will also be addressed by non-resolved CFD-DEM simulation. Particles are modeled with the Discrete Element Method (DEM), where one integrates Newton's law of motion for each particle under the forces due to the surrounding particles. This method is based on the use of an explicit numerical scheme in which the interaction of the particles is monitored contact by contact [2, 3]. CFDDEM simulations allow the incorporation of single-particle properties and modifications of their interaction, and are as such suitable for this study. Numerical simulation can be validated by the experimental measurement.

Manufacturing process of medical grade silicon rubber trileaflet valves for VADs could propitiate important leaflet thickness variations which could result in partial opening of the valve and affect its hydrodynamic performance. The leaflets of a total of 10 valves were measured to assess its thickness variability. Two experiments were performed to asses the impact of the leaflets thickness variation under hypothetical situations. The first experiment was divided into three hypothetical cases. In each case either none, one or two leaflets of different valves were mechanically blocked, resembling possible critical working circumstances. The second experiment was intended to know how the variation on the leaflets thickness affects the hydrodynamic performance of the valves. The results demonstrated a significant variation on the leaflets thickness was found. As for the first experiment, a small variation on the hydrodynamic performance was found above 4 L/min flow rates and a slightly higher energy loss was found in one of the cases. As for the second experiment, the results showed that the variation of the leaflet thickness does not affect the general hydrodynamic performance of the valves. No relationship between the thickness variability and the hydrostatic performance of the valves was found