1,720,985 research outputs found
CFD-DEM Modelling: Clogging of Triplet Particles in a Microchannel with a Constriction
No full textClogging represents an important issue in the field of chemical engineering, particularly concerning microfluidic technologies such as microreactors. This phenomenon consists in the the interruption of the flow in confined geometries due to the complete blockage of the channel cross-section. Since experiments can be expensive and difficult to implement, mathematical modelling has become more and more used. In this work, the numerical method called CFD-DEM (combining Computational Fluid Dynamics and Discrete Element Method) is employed to simulate clogging in a microchannel with sudden contraction, considering a suspension of triplet particles and constant pressure drop. The influence of particle volume fraction, cohesion energy density, and pressure drop on the flow rate is investigated. A study concerning the orientation of the triplets in the microchannel is also carried out
Rheology of a Dilute Suspension of Aggregates in Shear-Thinning Fluids
No full textThe prediction of the viscosity of suspensions is of fundamental importance in several fields. Most of the available studies have been focused on particles with simple shapes, for example, spheres or spheroids. In this work, we study the viscosity of a dilute suspension of fractal-shape aggregates suspended in a shear-thinning fluid by direct numerical simulations. The suspending fluid is modeled by the power-law constitutive equation. For each morphology, a map of particle angular velocities is obtained by solving the governing equations for several particle orientations. The map is used to integrate the kinematic equation for the orientation vectors and reconstruct the aggregate orientational dynamics. The intrinsic viscosity is computed by a homogenization procedure along the particle orbits. In agreement with previous results on Newtonian suspensions, the intrinsic viscosity, averaged over different initial orientations and aggregate morphologies characterized by the same fractal parameters, decreases by increasing the fractal dimension, that is, from rod-like to spherical-like aggregates. Shear-thinning further reduces the intrinsic viscosity showing a linear dependence with the flow index in the investigated range. The intrinsic viscosity can be properly scaled with respect to the number of primary particles and the flow index to obtain a single curve as a function of the fractal dimension
Numerical simulation of clogging in a microchannel with planar contraction
No full textClogging is the mechanism that interrupts the flow in confined geometries due to the complete blockage of the channel cross section. It represents a critical issue in the processing of particle suspensions for both industrial and biological applications, and it is particularly relevant in microfluidics and membrane technology due to the high particle confinement and the difficult device cleaning. Although numerous experimental and numerical studies have been carried out to understand the mechanism governing this complex multiscale phenomenon, the picture is not yet clear and many questions still remain, especially at the particle level. In this regard, the numerical simulations may represent a useful investigation tool since they provide a direct insight to quantities not easily accessible from experiments. In this work, a detailed computational fluid dynamics-discrete element method simulation study on the clogging mechanism in a microchannel with planar contraction is carried out. Both constant flow rate and constant pressure drop conditions are investigated, highlighting the effect of flow conditions, particle volume fraction, cohesion forces, and contraction angle. The onset of clogging conditions is discussed
Sedimentation of Fractal Aggregates in Shear-Thinning Fluids
No full textSolid–liquid separation is a key unit operation in the wastewater treatment, generally consisting of coagulation and flocculation steps to promote aggregation and increase the particle size, followed by sedimentation, where the particles settle due to the effect of gravity. The sedimentation efficiency is related to the hydrodynamic behavior of the suspended particles that, in turn, depends on the aggregate morphology. In addition, the non-Newtonian rheology of sludges strongly affects the drag coefficient of the suspended particles, leading to deviations from the known settling behavior in Newtonian fluids. In this work, we use direct numerical simulations to study the hydrodynamic drag of fractal-shaped particles suspended in a shear-thinning fluid modeled by the power-law constitutive equation. The fluid dynamics governing equations are solved for an applied force with different orientations uniformly distributed over the unit sphere. The resulting particle velocities are interpolated to compute the aggregate dynamics and the drag correction coefficient. A remarkable effect of the detailed microstructure of the aggregate on the sedimentation process is observed. The orientational dynamics shows a rich behavior characterized by steady-state, bistable, and periodic regimes. In qualitative agreement with spherical particles, shear-thinning increases the drag correction coefficient. Elongated aggregates sediment more slowly than sphere-like particles, with a lower terminal velocity as the aspect ratio increases
Spreading of powders in powder bed additive manufacturing: An experimental approach
No full textPowder bed additive manufacturing allows for the production of fully customizable parts and is of great interest for industrial applications. However, the repeatability of the parts and the uniformity of the mechanical properties are still an issue. More specifically, the physical mechanism of the spreading process of the powders, which significantly affects the characteristics of the final part, is not completely understood. In powder bed fusion technologies, the spreading is performed by a device, typically a roller or a blade, that collects the powders from the feedstock and successively deposits them in a layer of several dozens of microns that is then processed with a laser beam. In this work, an experimental approach is developed and employed to study the powder spreading process and analyze in detail the motion of the powders from the accumulation zone to the deposition stage. The presented experiments are carried out on a home-made device that reproduces the spreading process and enables the measurement of the characteristics of the powder bed. Furthermore, the correlation with the process parameters, e.g., the speed of the spreading device, is also investigated. These results can be used to obtain useful insights on the optimal window for the process parameters
CFD simulations of the effect of dust diameter on the dispersion in the 1 m3 Explosion Vessel
Full text linkThere are at least two main requirements for repeatable and reliable measurements of flammability and explosibility parameters of dusts: A uniform dispersion of solid particles inside the test vessel and a homogeneous degree of turbulence. Measurements of these parameters are performed in spherical vessels (20 L sphere or 1 m3 sphere). In several literature works, it has been shown that, in the standard 20 L sphere, the dust injection system generates a non-uniform dust cloud, while high gradients characterize the turbulent flow field. In our recent work, CFD simulations of flow field and dust concentration distribution in the 1 m3 spherical vessel were carried out and the results compared to the data previously obtained for the 20 L. It has been found that in the 1 m3 vessel, the spatial distribution of the turbulent kinetic energy is lower and much more uniform. Concerning the dust distribution, as in the case of the 20 L, dust is mainly concentrated at the outer zones of the vortices generated inside the vessel. In this work we use the previously validated CFD model to simulate the dust dispersion inside the 1 m3 vessel at different dust diameters. Results show that on increasing the dust diameter, the dust paths are different from those of the fluid flow until the sedimentation effect prevails and the turbulence field becomes similar to the dust-free air case. Since the spatial distribution of the turbulent kinetic energy is lower and much more uniform than in the 20 L sphere, the 1 m3 vessel is less susceptible to variations in the dust intrinsic properties, making parameter measurements more reliable and repeatable
Nanoparticles synthesis in wet-operating stirred media: Investigation on the grinding efficiency
No full textThe use of nanomaterials, thanks to their peculiar properties and versatility, is becoming central in an increasing number of scientific and engineering applications. At the same time, the growing concern towards environmental issues drives the seeking of alternative strategies for a safer and more sustainable production of nanoparticles. Here we focus on a low-energy, magnetically-driven wet milling technique for the synthesis of metal nanoparticles starting from a bulky solid. The proposed approach is simple, economical, sustainable, and provides numerous advantages, including the minimization of the nanoparticles air dispersion and a greater control over the final product. This process is investigated by experiments and discrete element method simulations to reproduce the movement of the grinding beads and study the collision dynamics. The effect of several parameters is analyzed, including the stirring bar velocity, its inclination, and the grinding bead size, to quantify the actual frequency, energy, and angle of collisions. Experiments reveal a non-monotonous effect of the stirring velocity on the abrasion efficiency, whereas numerical simulations highlight the prevalent tangential nature of collisions, which is only weakly affected by the stirring velocity. On the other hand, the stirring velocity affects the collision frequency and relative kinetic energy, suggesting the existence of an optimal parameters combination. Although a small variation of the stirring bar length does not significantly affect the collision dynamics, the use of grinding beads of different dimensions offers several tuning opportunities
CFD simulations of dust dispersion in the 1 m3 explosion vessel
Full text linkAccording to standard procedures, flammability and explosion parameters for dusts and dust mixtures are evaluated in 20 L and/or 1 m3 vessels, with equivalent results provided a correct ignition delay time (60 ms in the 20 L vessel; 600 ms in the 1 m3 vessel). In this work, CFD simulations of flow field and dust concentration distribution in the 1 m3 spherical vessel are performed, and the results compared to the data previously obtained for the 20 L. It has been found that in the 1 m3 vessel, the spatial distribution of the turbulent kinetic energy is lower and much more uniform. Concerning the dust distribution, as in the case of the 20 L, dust is mainly concentrated at the outer zones of the vortices generated inside the vessel. Furthermore, an incomplete feeding is attained, with most of the dust trapped in the perforated annular nozzle. Starting from the maps of dust concentration and turbulent kinetic energy, the deflagration index KSt is calculated in both vessels. In the conditions of the present work, the KSt is found to be 2.4 times higher in the 20 L than in the 1 m3 vessel
Nanoparticles synthesis in wet-operating stirred media: Preliminary investigation with DEM simulations
No full textThe growing demand of nanomaterials is pushing towards the development of alternative strategies for the safe and sustainable production of nanoparticles. At the same time, to ensure high performances, a fine control over the product specifications is required. We focused on a bottom-up method combined with a mechanical disaggregation technique using a wet bead-stirring process, since it provides numerous advantages over other approaches, including the minimization of the nanoparticles air dispersion and a greater control over the final product. However, given the broad variability of the parameters involved in both the setup and operation of the process, it is essential to combine the experiments with a theoretical-simulative study to optimize the design. The present activity consists in the preliminary simulation of the interactions among the grinding beads, modelled through the discrete element method (DEM), and the magnetic stirrer. This approach, providing information regarding the frequency and energy of collisions, which can be related to the properties of the produced nanoparticles, allows a fine tuning of the process parameters
- …
