Homogeneous to Bubbling Regime Transition in Gas- and Liquid-Fluidized Beds Through DEM-CFD Simulations

DEM-CFD simulations are carried out for the water fluidization of 200 micron glass ballotini and air fluidization of cohesionless alumina 70 micron powders. In the first case, homogeneous expansion is found throughout the whole investigated range of water velocity. Alumina powders exhibits a transition to bubbling regime at a voidage value in very good agreement with results of the theory of particle bed stability. The simulated kinematic and dynamic wave propagation velocities are in good agreement with theoretical predictions

Coarse-Grain DEM Modelling in Fluidized Bed Simulation: A Review

In the last decade, a few of the early attempts to bring CFD-DEM of fluidized beds beyond the limits of small, lab-scale units to larger scale systems have become popular. The simulation capabilities of the Discrete Element Method in multiphase flow and fluidized beds have largely benefitted by the improvements offered by coarse graining approaches. In fact, the number of real particles that can be simulated increases to the point that pilot-scale and some industrially relevant systems become approachable. Methodologically, coarse graining procedures have been introduced by various groups, resting on different physical backgrounds. The present review collects the most relevant contributions, critically proposing them within a unique, consistent framework for the derivations and nomenclature. Scaling for the contact forces, with the linear and Hertz-based approaches, for the hydrodynamic and cohesive forces is illustrated and discussed. The orders of magnitude computational savings are quantified as a function of the coarse graining degree. An overview of the recent applications in bubbling, spouted beds and circulating fluidized bed reactors is presented. Finally, new scaling, recent extensions and promising future directions are discussed in perspective. In addition to providing a compact compendium of the essential aspects, the review aims at stimulating further efforts in this promising field

Force on a large sphere immersed in an expanded water-fluidized bed over a wide range of voidage values

The presence of large objects immersed in a fluidized bed has been long studied in order to verify to what extent can the analogy with a buoyant body in a liquid represent the object-to-suspension interaction (1). One of the most useful information resulting from such study is the effective drag force exerted on the object and how it is related to the suspension properties, particularly with respect to the expansion degree. Implications are found also in the formulation of drag force expressions for homogeneous polydisperse systems. In the present work an experimental series of tests has been conducted on a 10 cm diameter, 2 m height fluidization column using glass beads in two sizes (300 and 600 m) as solid and water as fluidizing medium. A 2.1 cm diameter sphere held by a balance was immersed and kept fixed at the center of the cross-section but free to move axially. Measurements of the hydrodynamic force were carried out at voidage values as high as 0.94, allowing the full range of bed expansion conditions to be covered. Different vertical positions of the large particle was also considered. Results for the drag force indicate that while at low and intermediate expansions the analogy with buoyancy works quite well, at higher voidage values, starting from about = 0.8, the trend departs and additional force contributions are required to explain, though partially, the observed deviations. REFERENCES R. Di Felice, P.U. Foscolo, L.G. Gibilaro. The experimental determination of the interaction force on spheres submerged in liquid fluidized beds. Chem. Eng. Process. 25: 27-34, 1989

Microscopic modelling of capillary liquid bridge force and volume repartition for DEM simulations of polydisperse systems

Many fluidization processes like granulation, agglomeration, encapsulation or coating exhibit conditions where liquid bridges form between particles (1), resulting in strong particle-particle interactions at the microscopic scale. DEM and DEM-CFD simulations are able to show how the particle-scale phenomena determine the overall behavior of a wet particulate material in industrial systems. Description of the capillary interaction between spherical particles in the pendular regime (i.e. with liquid bridges forming on a one-to-one basis) has been long studied and characterized, particularly in the case of spheres of the same size. When polydisperse systems are considered, the formation, interaction force and rupture of a liquid bridge follow paths that can differ significantly from the monodisperse case. For example, the validity of the toroidal or parabolic approximation of the bridge shape (2) is much more limited. Additionally, upon rupture the relative amount of liquid remaining with the bigger or smaller particles has not been addressed in general. In the present work, the rigorous solutions of the Young-Laplace equations for the determination of the liquid bridge shape, its interaction force and rupture characteristics are found by means of an effective continuation technique. Use of the code allows critical (e.g. rupture) conditions to be found relatively easily as a function of the prescribed size ratio, liquid volume, contact angle, inter-particle distance. Also, it is shown that the volume repartition upon rupture can be estimated by assuming the bridge to break at the neck. Exemplary results for various ranges of the parameters will be shown and discussed. REFERENCES L. Fries, S. Antonyuk, S. Heinrich, G. Niederreiter, S. Palzer. Product design based on discrete particle modeling of a fluidized bed granulator. Particuology, 12: 13-24, 2014. D. Megias-Alguacil, L.J. Gauckler. Accuracy of the toroidal approximation for the calculus of concave and convex liquid bridges between particles. Granular Matter, 13: 487-492, 2011

COMPUTATIONAL STUDY OF LAYER INVERSION IN TWO-COMPONENT LIQUID-FLUIDIZED BEDS BY DEM-CFD

In the present work the layer inversion phenomenon observed in experiments from the literature is reproduced via discrete element simulations, in which a novel drag force model valid for bi- and poly-disperse particle systems is used. The simulations serve both as validation of the drag model and as a tool to analyze the dynamics of the phenomenon. The comparison with published data is carried out in terms of bed height and component distributions as functions of the liquid velocity, showing very good agreement

Design of Reconfigurable Intelligent Surfaces by Using S-Parameter Multiport Network Theory -- Optimization and Full-Wave Validation

Multiport network theory has been proved to be a suitable abstraction model for analyzing and optimizing reconfigurable intelligent surfaces (RISs), especially for studying the impact of the electromagnetic mutual coupling among radiating elements that are spaced less than half of the wavelength. Both representations in terms of $Z$-parameter (impedance) and $S$-parameter (scattering) matrices are widely utilized. In this paper, we embrace multiport network theory for analyzing and optimizing the reradiation properties of RIS-aided channels, and provide four new contributions. (i) First, we offer a thorough comparison between the $Z$-parameter and $S$-parameter representations. This comparison allows us to unveil that the typical scattering models utilized for RIS-aided channels ignore the structural scattering from the RIS, which results in an unwanted specular reflection. (ii) Then, we develop an iterative algorithm for optimizing, in the presence of electromagnetic mutual coupling, the tunable loads of the RIS based on the $S$-parameters representation. We prove that small perturbations of the step size of the algorithm result in larger variations of the $S$-parameter matrix compared with the $Z$-parameter matrix, resulting in a faster convergence rate. (iii) Subsequently, we generalize the proposed algorithm to suppress the specular reflection due to the structural scattering, while maximizing the received power towards the direction of interest, and analyze the effectiveness and tradeoffs of the proposed approach. (iv) Finally, we validate the theoretical findings and algorithms with numerical simulations and a commercial full-wave electromagnetic simulator based on the method of moments.Comment: Submitted for journal publicatio

Analysis and Optimization of Reconfigurable Intelligent Surfaces Based on SS-Parameters Multiport Network Theory

In this paper, we consider a reconfigurable intelligent surface (RIS) and model it by using multiport network theory. We first compare the representation of RIS by using $Z$-parameters and $S$-parameters, by proving their equivalence and discussing their distinct features. Then, we develop an algorithm for optimizing the RIS configuration in the presence of electromagnetic mutual coupling. We show that the proposed algorithm based on optimizing the $S$-parameters results in better performance than existing algorithms based on optimizing the $Z$-parameters. This is attributed to the fact that small perturbations of the step size of the proposed algorithm result in larger variations of the $S$-parameters, hence increasing the convergence speed of the algorithm.Comment: Submitted to a conference (invited

DENDRITIC CELL DIFFERENTIATION BLOCKED BY PRIMARY EFFUSION LYMPHOMA-RELEASED FACTORS IS PARTIALLY RESTORED BY INHIBITION OF P38 MAPK

To better understand the molecular mechanisms underlying the dendritic cell (DC) defects in cancer, we analyzed which signaling pathway is implicated in the abnormal monocyte differentiation into DC determined by the presence of Primary effusion lymphoma (PEL) released factors. Our results indicate that the DC, obtained in this condition, together with phenotypic abnormalities and reduced allostimulatory function, showed hyperphosphorylation of signal transducer and activator of transcription 3 (STAT3) and p38 mitogen-activated protein kinase (MAPK) molecules, in comparison to the DC differentiated in the absence of PEL-released factors. The inhibition of p38 MAPK but not of STAT3 phosphorylation, with specific inhibitors, was able to revert the effect of the PEL-released factors on the DC phenotype. This study suggests that p38 MAPK signaling pathway is an important contributor to the abnormal differentiation of DC in PEL