1,720,972 research outputs found
Dynamic ball indentation: advancing powder flowability characterization through complete impact dynamics analysis
Full text linkPredicting how particulate materials behave under applied forces is challenging due to their complex rheology. Flowability—defined as a material's ability to initiate and sustain motion under stress—is critical in many traditional and emerging industrial applications, such as additive manufacturing. Traditional shear testers are limited in their ability to assess dynamic flow behaviour. The dynamic ball indentation method, which measures the penetration of an indenter into a powder bed, presents a promising alternative. This study enhances the method by tracking the full indentation process to more accurately calculate dynamic hardness. By monitoring the indenter's trajectory, we obtain detailed data on material flowability, which can provide deeper insights for industrial applications. These insights become even more significant when integrated with information obtained from traditional methods such as shear cell testing
Particle Size Segregation During the Discharge of Binary Mixtures and the Role of Void Saturation
No full textParticles differing in size, under external mechanical activation, typically segregate rather than mix. This phenomenon, known as particle size segregation, is spontaneous and unavoidable in practice. In this work, we present an experimental investigation aiming at better understanding the segregation mechanisms occurring in industrial devices related to the storage of particulate materials. Even if storage in closed vessels is often considered a static operation, loading and unloading steps are always present and they must be considered as inherent part of the whole storage operation. These steps are clearly dynamic in nature. This work concentrates on the discharge step in which the particulate material is strongly sheared, especially close to the outlet of the storage bin giving high chances of segregation to the material. Segregation in dense sheared flows under gravity occurs mainly through the percolation of smaller particles in the voids existing between larger ones. The level of saturation of voids by small particles can strongly impact the segregation rate. It appears therefore that the relative amount of fines with respect to large particles, the level of bed dilatancy induced by shear, the interparticle friction are all variables that can affect fine particles mobility and therefore the extent of segregation. For these reasons, we carried out experiments on small-scale vessels discharging in a funnel flow regime. Binary mixtures at different fine compositions and particle size ratios have been considered. Furthermore, we carried out experiments to a better knowledge of the mixtures voidage. The correlations found in the literature fail to predict the critical fine concentration at which fine particle segregation stops. An Alternative correlation has been therefore proposed based on a micromechanical analysis of particles filling
Investigation on particle size and packing tortuosity by coupling image analysis and permeability tests
Full text linkThis experimental study aims to enhance the understanding of the correlation among equivalent particle diameters measured using two analytical techniques: optical analysis (assisted by computer aided image analysis) and permeability tests. The presence or absence of a specific analytical method or instrument can lead to the use of an incorrect equivalent diameter. Therefore, it can be beneficial to establish conversion rules between different equivalent particle diameters obtained through various methods and instruments. The optical analysis returns an equivalent diameter value inherently independent of particle arrangement since it deals with isolated particles. In contrast, the permeability test offers an equivalent mean diameter dependent not only on the size of the particles but also on their packed arrangement. A suitable correlation between the two diameters has been proposed, shown to be a decreasing function of porosity following a power law. An unexpected outcome of the comparison between the optical method and permeametry is the possibility to isolate and characterize the effect that the packing arrangement has on pressure losses and to characterize it in terms of the tortuosity of the path that the fluid must travel through the packed bed. Our findings confirm a strong alignment between our tortuosity model, which contains the ratio between the two equivalent diameters considered here, and an empirical correlation from literature often utilized for predicting packed bed tortuosity
Characterization of Powder Flowability for Additive Manufacturing
No full textPowders have always been used in industry, spanning from food to metallurgic ones, and in recent decades they have become the protagonists of other innovative process: additive manufacturing (AM). Knowing the flowability of a powder in a specific piece of equipment is fundamental to achieve better manufacturing efficiency and product quality in powder processing industries. A powder used in AM machines is subjected to different flow and stress regimes. There are commercial devices able to measure the flowability of the powder under different levels of consolidation stresses, but they are not able to work at very low stresses. Among the available, alternative, non-commercial tests dynamic BIM is the most promising. In dynamic BIM a spherical indenter impinges on the powder with the velocity of a falling body; it is possible to determine the flowability (through the hardness) of the powder, but the evaluation is complex and must be improved. In this work we investigate the dynamic impact of an indenter in a packed powder, measuring the geometrical characteristic of the crater forming on the powder surface. Furthermore, we try to improve the definition of dynamic hardness and shear rate by rewriting them in terms of granular velocity rather than indenter impact velocity as the most common reports in the literature
Particle Size Characterization through Bed Permeability Tests
No full textThis is an experimental study aiming at understanding the relationship between particle diameters measured with different analytical techniques: optical analysis (microscopy aided by computer image analysis), laser light scattering and permeability tests. Permeability test can provide an equivalent mean diameter that can be used in several cases where particle specific surface is the relevant property to focus on (for example problems related to particle reactivity or involving interactions with fluids as in pneumatic transport or in fluidization). This diameter can be determined by measuring the pressure loss in a granular bed of known porosity at various gas flow rate and by using a proper mathematical model (e.g., Darcy or Karman-Kozeny models) to correlate them. Four different materials (glass beads, granulated microcrystalline cellulose, MCC, sodium chloride crystals, tetraacetylethylenediamine powders, TAED) in the size range 700-1000 μm (by sieve analysis) were analysed. A strong discrepancy was observed by calculating the Sauter diameters from image analysis (and laser light scattering) and those from permeability measurements. The introduction of a function considering the pores morphology and connectivity was able to reconcile the size measured with the three different methods
Characterising powder flowability at high shear rates by the ball indentation method
No full textUnreliable powder flow is a major problem during processing of powders. The shear cell is the most widely used method for powders subjected to moderate or high stresses, and under quasi-static conditions, with established methods for designing large bins and hoppers based on the measurement. However, this method is not suitable for measuring the flowability of dynamic systems, such as powder mixing. Here, the ball indentation method is investigated as a technique for evaluating powders in the intermediate and dynamic regime of flow. The method, which simply consists of dropping a ball onto a cylindrical bed of powder previously consolidated, directly measures hardness, which is related to the unconfined yield stress of the powder by the constrain factor (Hassanpour and Ghadiri, 2007). The impact of the ball on the bed is recorded with a high-speed camera to determine velocity and penetration depth. The shear rate is varied by using a range of indenter materials and sizes, and a range of drop heights. The hardness against the strain rate is considered for several materials. It was found that the indenter size does not influence the hardness results, which are consistent with the flowability evaluation achieved with the rheometer. Furthermore the hardness, which is independent of the strain rate in quasi-static conditions, becomes shear rate dependent in intermediate regime of flow. Further work is needed to evaluate hardness in the rapid granular flow regime
Shear-induced particle segregation in binary mixtures: Verification of a percolation theory
No full textGranular materials composed of different-sized grains may experience undesired segregation. Segregation is detrimental for a lot of industries because it leads to an increase in production costs and wastes. For these reasons, the segregation phenomena have been intensively studied in the last decades, and a lot of models have been provided by many researchers. However, these models are mainly based on empirical relations rather than physical considerations. This paper aims to confirm the main assumptions made by Volpato, Tirapelle, and Santomaso (2020) in their percolation theory by means of DEM simulations. The simulated geometry is a tilting shear box filled with few tracer particles in a bed of coarser sized grains, and simulations are performed for a range of tilting frequencies and size ratios. The results provide meaningful insight on the mathematical model parameters and allow us to say that the percolation theory relies on physically consistent assumptions
Modeling and experimental investigation of shear-induced particle percolation in diluted binary mixtures
No full textExperimental investigation and numerical modelling of density-driven segregation in an annular shear cell
No full textGranular materials segregate spontaneously due to differences in particle size, shape, density and flow behaviour. In this paper we experimentally investigate density-difference-driven segregation for a range of density ratios and a range of heavy particle concentrations. The experiments are conducted in an annular shear cell with rotating bumpy bottom that yields an exponential shear profile. The cell is initially filled with a layer of light particles and an upper layer of heavier grains and, on top, a load provides confinement. The segregation process is filmed through the transparent side-wall with a camera, and the evolution of particle concentration in space and time is evaluated by means of post-processing image analysis. We also propose a continuum-approach to model density-driven segregation. We use a segregation-diffusion transport equation, constitutive relations for effective viscosity and friction coefficient, and a segregation velocity analogous to the Stokes’ law. The model, which is validated by comparison with experimental findings, can successfully predict density-driven segregation at different density ratios and volumetric fraction
- …
