Interplay between hydrodynamic and Brownian fluctuations in sedimenting colloidal suspensions

We apply a hybrid molecular dynamics and mesoscopic simulation technique to study the steady-state sedimentation of hard sphere particles for Peclet number (Pe) ranging from 0.08 to 12. Hydrodynamic backflow causes a reduction of the average sedimentation velocity relative to the Stokes velocity. We find that this effect is independent of Pe number. Velocity fluctuations show the expected effects of thermal fluctuations at short correlation times. At longer times, nonequilibrium hydrodynamic fluctuations are visible, and their character appears to be independent of the thermal fluctuations. The hydrodynamic fluctuations dominate the diffusive behavior even for modest Pe number, while conversely the short-time fluctuations are dominated by thermal effects for surprisingly large Pe numbers. Inspired by recent experiments, we also study finite sedimentation in a horizontal planar slit. In our simulations distinct lateral patterns emerge, in agreement with observations in the experiments

The influence of particle surface friction on the behavior of gas-fluidized beds: Development of a two fluid model

The influence of physically realistic collisional properties on the hydrodynamics in a bubbling dense gas-solid fluidized bed is investigated using both a Discrete Particle Model (DPM) and a Two Fluid Model (TFM) incorporating a kinetic theory of granular flow (KTGF) for rough spheres by Lei et al. (1). The validated KTGF accounts for particle rotation and particle surface friction expilicitly. Comparisons between the two models are carried out to investigate the influence of particle friction on axial particle velocity, solids circulation pattern, and bubble behavior. The simulated results from both models reveal that the friction coefficient plays an important role in the formation of heterogeneous structures in a bubbling bed. When the friction coefficient is increased, larger bubbles appear and the fluidization in the bed is more vigorous. In addition, the time-averaged gas-solid flow field and time-averaged solids volume fraction vary significantly with different friction coefficient. Less dense zones are found in the bed for larger values of the friction coefficient. Please click Additional Files below to see the full abstract

Partial slip boundary conditions for collisional granular flows at flat frictional walls

Generally, slip occurs at the boundary wall for the granular flows, and the boundary may provide fluctuation energy to the flow. Wall boundary conditions (BCs) for the solids phase have significant effects on numerical predictions of various gas-solids fluidized beds. In this work, we derive new boundary conditions for collisional granular flows of spheres at flat frictional walls. New theory for the solids stress tensors, energy dissipation rates per unit area and the fluxes of fluctuation energy is proposed distinguishing sliding and sticking collisions and including particle rotation. Comparisons between the theory and the literature simulation data from Louge (1) show that an excellent prediction for stress ratio can be obtained. We propose an approximated expression for the mean rotational velocity in the bubbling fluidized beds using discrete particle model. The theory also predicts better agreement for the fluxes of fluctuation energy and energy dissipation rates for relative rough spheres with the expression. Finally, we determine new BCs with an extra BC for the rotational granular temperature based on the theory within the framework of kinetic theory of granular flow. Please click Additional Files below to see the full abstract

Numerical investigation of the vertical plunging force of a spherical intruder into a granular bed

The plunging of a large sphere into a prefluidized granular bed with various constant velocities is investigated using a state-of-theart hybrid Discrete Particle and Immersed Boundary Method (DPIBM), with which both the gas-induced drag force and the contact force exerted on the intruder can be investigated separately. Our simulation method has been validated by comparison with the existing experimental results. Current simulation results show a concave-to-convex plunging force as a function of depth and in the concave region the force fits to a power-law with exponent around 1.3, which is in good agreement with existing experimental observations

Modeling of tribo-electrification of a pneumatically conveyed powder in a squared duct using DEM-CFD

Dry separation technology is a sustainable alternative to conventional wet separation technology for production of food ingredients. This paper is concerned with the exploration of a new driving force for dry separation, i.e. triboelectrification. To investigate the possibilities of this driving force, we modified our in-house DEM-CFD code to model a learning system where powder is tribo-electrically charged by conveying it pneumatically through a squared tube. The charged particles will electrostatically interact with both other charged particles, as well as their induced charges on the conducting walls. We show that the amount of acquired charge depends on the electrostatic interaction between particles and walls and show the corresponding spatial distribution of the particles. They depend both highly on the (mean) charge of the particles. We observed a critical charge per particle after which particles charged rapidly to their saturation charge. This critical charge is delicate and lower than expected from first order derivations

Experimental and computational investigation of droplet-particle interactions

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An experimental study of droplet-particle collisions

When spray drying a liquid slurry such as milk, collisions between droplets, partially dried particles and completely dry particles are important because coalescence, agglomeration and breakup events influence the size and morphology of the produced powder. When modelling such a spray drying process, it is therefore important to be able to predict the outcomes of individual binary collisions. Both binary dry particle collisions and binary droplet collisions have individually been thoroughly researched over the years due to their widespread occurrence. The importance of understanding binary particle-droplet collisions has been emphasized more recently. However, the number of available studies is limited and simulation studies usually focus on relatively high capillary number. A theory explaining the transition between different regimes is still lacking. The goal of this study is to provide an experimental data set at low capillary number. These results can be used to validate future theories and simulations. To produce and record particle-droplet collisions, an experimental setup that enables synchronized release of both a particle and a droplet was used. One single hanging droplet was released from above onto a particle that initially was held in place by vacuum suction. A high speed camera was synchronized with the setup, and recorded the collisions. Image files were then analysed in Matlab to find velocities and sizes of the particle and droplet before and after impact. The contrast of particle and droplet against the illuminated background was a key factor in succeeding with this. Different collision outcomes were identified as either agglomeration (merging), where the whole droplet would stick to the surface of the particle, or a stretching separation (breaking), where the droplet collides with the particle in an oblique position and stretches out until a part of the droplet detaches from the liquid sticking to the particle. The formation of satellite droplets, i.e. droplets with a radius significantly smaller than the leaving droplet, was also detected. The relation of these collision outcomes to impact conditions such as Weber number and impact parameter was reviewed and put into regime maps

A direct numerical simulation method for complex modulus of particle dispersions

We report an extension of the smoothed profile method (SPM)[Y. Nakayama, K. Kim, and R. Yamamoto, Eur. Phys. J. E {\bf 26}, 361(2008)], a direct numerical simulation method for calculating the complex modulus of the dispersion of particles, in which we introduce a temporally oscillatory external force into the system. The validity of the method was examined by evaluating the storage $G'(\omega)$ and loss $G"(\omega)$ moduli of a system composed of identical spherical particles dispersed in an incompressible Newtonian host fluid at volume fractions of $\Phi=0$, 0.41, and 0.51. The moduli were evaluated at several frequencies of shear flow; the shear flow used here has a zigzag profile, as is consistent with the usual periodic boundary conditions