Jürgen Tomas’s research while affiliated with Otto-von-Guericke University Magdeburg and other places

In most industrial solid processing operations, the classification of particles is important and designed based on the terminal settling velocity as the main control parameter. This settling velocity is dependent on characteristic particle properties like size, density, and shape. Turbulent particle diffusion is the other key property controlling the efficiency of the separation. In this project, multi-stage separation experiments of a variety of materials have been performed using different flow velocities, mass loadings of the air, number of stages. Separation has been investigated separately concerning particle size, particle density, and particle shape. Continuous operation in terms of solid material and airflow has been mostly considered. However, variations in mass loading and pulsating operation of the fan have been investigated as well. The performance has been analyzed and discussed with respect to the separation functions, for instance regarding separation sharpness. Several modelling approaches have been checked and/or developed to describe theoretically the corresponding observations. After fitting the free model parameters, a very good agreement has been obtained compared to experimental measurements. Finally, the reduced model has been implemented into the central software DYSSOL.

This paper focuses on the effect of ultra-fine (d < 10 µm) powders in mixtures with fine (d < 100 µm) bulk material on compression processes and also evaluates the re-fluidization behavior of the compressed bed (history effect). Achieving this goal, different mixtures of fine and ultra-fine Ground-Carbonate-Calcium were compressed at three pressure levels. The results show that by increasing the applied pressure, the compressibility decreases due to change in compaction regime. Subsequently, for the higher pressure, the slope of packing density versus applied stress curves is noticeably different. However, this slope does not depend on the size distribution of mixtures, but on the type of material. Comparing fluidization and re-fluidization curves (bed pressure drop vs. gas velocity) shows an increase in the maximum bed pressure drop (ΔPpeak) for re-fluidization. By increasing the portion of ultra-fine particles in the binary mixture, ΔPpeak increases in a non-linear manner. Furthermore, the incipient fluidization point moves to a higher gas velocity. After compression, the peak of the bed pressure drop in the re-fluidization test happens at a lower gas velocity than in the initial fluidization test. Thus, the slope of the loading curve is much larger for re-fluidization. The opposite is observed for the unloading curves.

The effect of the presence of ultra-fines (d < 10 μm) on the fluidization of a bed containing fine particles (d < 100 μm), is the subject of this paper. Practically, it can happen due to breakage or surface abrasion of the fine particles in some processes which totally changes the size distribution and also fluidization behaviour. The materials used in this study are both ground calcium carbonate (GCC); fine is CALCIT MVT 100 (Geldart’s group A) and ultra-fine is CALCIT MX 10 (group C). The experimental results for different binary mixtures of these materials (ultra-fines have 30%, 50%, or 68% of the total mixture weight) show that the physical properties of the mixtures are close to those of pure ultra-fine powders. Using mean values of the bed pressure drop calculated from several independent repetitions, the fluidization behaviour of different mixtures are compared and discussed. The fluidization behaviour of the mixtures is non-reproducible and includes cracking, channelling and agglomeration (like for pure ultra-fine powders). Increasing the portion of ultra-fine materials in the mixture causes a delay in starting partial fluidization, an increase in the bed pressure drop as well as a delay in reaching the peak point.

In most industrial processes dealing with particles, separation of fine (or light) and coarse (or heavy) particles in a fluid flow is an essential part to obtain the required product quality. These separation processes are typically designed based on the particles' terminal settling velocity as the dominant property. In this work, sand and gravel have been selected as suitable model grains representing mineral and agricultural raw materials and products. The terminal settling velocity depends on stochastically distributed physical and granulometric particle properties like size, density, and shape. Separation experiments in a pilot-scale zigzag separator have been performed using different channel velocities and solid mass loadings in order to improve the understanding of the separation process in this complex turbulent flow. Then, classification has been modeled as a multistage cross-flow separation process in a turbulent air flow. Performance has been analyzed and discussed with respect to separation functions and characteristic parameters like cut size, separation sharpness, separation stage utilization, and product quality, quantified by cumulative product purities of over- and underflow as well as specific energy consumption. Finally, a SMART analysis has been used to identify optimal process parameters. The obtained cut sizes differ significantly from those found in literature. Thus, a more detailed analysis accounting for aerodynamics and turbulence is developed to show the significant influence of channel geometry on separation efficiency.

To investigate the permanent compressive stress of single particles in a cohesive bulk solid during handling or transport, uniaxial compression tests were carried out with cyclic loading/unloading. This involves a continuation of our previous studies (contact properties determination of macroscopic fine disperse glass particles via compression tests in normal direction). The experiments were implemented by means of a home-built compression tester. By repeated loading/unloading cycles with spherical, dry and smooth soda lime glass particles with a mean diameter of d50,3 = 513.20 μm, a set of micromechanical contact properties was determined. It is a model-based back-calculation of experimentally generated normal force-displacement curves of the elastic-plastic sample material. For this purpose, the contact model ‘stiff particles with soft contacts’, which was developed by Tomas, is used. In order to influence the flowability of soda lime glass particles and to verify the effect of the characteristic adhesion force on the elastic-plastic contact, the particles were altered to be hydrophilic and hydrophobic.

The paper describes the deformation behavior of spherical, dry and non-porous particles during a single particle compression test in normal direction. Therefore a compression tester was built. Industrial used soda lime glass particles with two macroscopic fine disperse sizes (d1,50,3 = 284,30 µm and d2,50,3 = 513,20 µm) were applied as model material to investigate the micromechanical contact behavior. In order to influence the elastic-plastic contact properties of particles, the surfaces were altered with chemical modification by means of silanization. The determination of various micromechanical contact properties (e.g. adhesion force, modulus of elasticity and contact stiffness) was carried out model-based with the contact model ‘stiff particles with soft contacts’ by means of a back-calculation. It could be shown that the model-based determination of material properties was a good alternative compared to the comprehensive tensile tests and pull-off force measurements. In addition to the gained normal force-displacement data in normal direction, the friction limits for tangential loading and rolling with the load-dependent adhesion force were model-based determined.

At in vivo contacts with operational and surgical instruments, intra-abdominal organs are mechanically stressed to varying degrees. Owing to the distributed mechanical constitutive properties (such as stiffness and strength) of abdominal organs and the usage of different types of surgical instruments which may create unique contacts, the transmitted load on the organ may often be substantially sufficient to cause tissue damage or initiate rupture. Thus, characterization of the rupture probability with respect to different loading conditions is an inevitable necessity to prevent damage of organs during surgical procedures. In this article, measurements of the material behavior (upon loading until failure) of whole porcine livers as well as cut samples in vitro using quasi-static uniaxial compression and indentation tests are presented. The measured force-displacement responses have been approximated using the first-order Mooney-Rivlin constitutive model and thereby, the characteristic hyperelastic mechanical properties have been determined. Using lognormal and Weibull distributions, a comparison of the rupture probability functions of the loading force, the contact displacement, the contact strength and the volumetric toughness at uniaxial compression and indentation are presented. It is shown that larger loads and correspondingly higher deformation work are necessary to initiate rupture in the liver by uniaxial compression (planar loading) in comparison to indentation (almost point loading). Besides the experimental results, finite element method based simulations which describe the stress distribution, sample deformation and sample slipping due to frictional effects are presented. Further aspects such as histological image analysis of ruptured samples, prediction of the rupture origin as well as rupture path propagation are presented.

Essential physical product properties that depend on particle size distributions are one of the reasons that some products do not meet the application requirements. Fine particle fractions often have unsuitable effects in the processing or make the final product undesirable for specified applications. Such products require additional processing efforts to meet the desired product size distributions. Usually, fine and coarse particles with sizes beyond 100 μm can be classified without difficulties using various sieving machines. Air classifiers are suitable to classify fine particles (d < 100 μm). However, classification of ultra-fine adhesive particles shows totally different features, impacting processing performance, like insufficient separation characteristics and enormous product impurities. The inter-particle forces lead to additional difficulties; most conventional methods then cannot meet the required processing results. In this paper, ...

Bei der verfahrenstechnischen Behandlung partikulärer Feststoffprodukte treten permanent Stoßvorgänge auf. Bei einem großen Inelastizitätsmodulus kann dabei ein signifikanter Anteil der kinetischen Aufprallenergie in elastische Biegewellen dissipiert werden. Die Stoßzahl charakterisiert die auftretende Energiedissipation bzw. Dämpfung des Stoßes und ist ein essentieller Parameter bei der numerischen Simulation von Partikelsystemen. Es wird eine sehr präzise analytische Näherungslösung für die Stoßzahl des Biegemodells von Zener hergeleitet. Experimente mit elastisch-plastischen Granulaten und Acrylglasplatten bestätigen, dass elastische Wellen in Form von Biegewellen nicht nur bei elastischen Materialien wie Stahl oder Glas signifikanten Einfluss haben können. Collisions permanently occur during the processing of particulate solids. With a large inelasticity parameter, a significant content of the kinetic energy of the impact can be dissipated into elastic flexural waves. The coefficient of restitution characterizes the energy dissipation or more precisely the damping of the impact and is an essential parameter for the numerical simulation of particulate systems. A novel analytical approach to solve the otherwise mathematically strenuous Zener model for elastic sphere impacts on large, thin plates has been proposed. Experimental investigations using elastic-plastic granules and acryl glass plates confirm that elastic waves in terms of flexural waves can have a significant influence not only for elastic materials such as steel or glass.

The understanding of effects occurring during low velocity normal particle impact on a hard elastic plate is important for the description of different processes occurring during pneumatic conveying, in jet mills, mixers, or fluidized bed apparatuses. In this study, a suitable semi-empiric method is proposed to calculate the coefficient of restitution. This coefficient of restitution is modelled by means of combination of two different approaches. The first approach is based on dissipation of impact energy due to bending wave excitation in the plate described only by one inelasticity parameter. The second approach is based on viscoelastic mechanism of energy dissipation, represented by an additional viscoelastic damping coefficient. Thus, these two coefficients are responsible for bending and viscoelastic energy dissipation and are calculated independently from each other. The summarized energy dissipation is used to determine both material parameters and coefficients. A good agreement between calculated and measured coefficient of restitution was observed. In this way, the interaction between steel spheres impacting on glass plates with a thickness comparable to the diameter of the sphere was calculated as an example.