Phase-coexisting patterns, horizontal segregation and controlled convection in vertically vibrated binary granular mixtures

We report new patterns, consisting of coexistence of sub-harmonic/harmonic and asynchronous states [for example, a granular gas co-existing with (i) bouncing bed, (ii) undulatory subharmonic waves and (iii) Leidenfrost-like state], in experiments on vertically vibrated binary granular mixtures in a Heleshaw-type cell. Most experiments have been carried out with equimolar binary mixtures of glass and steel balls of same diameter by varying the total layer-height ($F$) for a range of shaking acceleration ($\Gamma$). All patterns as well as the related phase-diagram in the ($\Gamma, F$)-plane have been reproduced via molecular dynamics simulations of the same system. The segregation of heavier and lighter particles along the horizontal direction is shown to be the progenitor of such phase-coexisting patterns as confirmed in both experiment and simulation. At strong shaking we uncover a {\it partial} convection state in which a pair of convection rolls is found to coexist with a Leidenfrost-like state. The crucial role of the relative number density of two species on controlling the buoyancy-driven granular convection is demonstrated. A possible model for spontaneous horizontal segregation is suggested based on anisotropic diffusion

Patterns and Collective Behavior in Granular Media: Theoretical Concepts

Granular materials are ubiquitous in our daily lives. While they have been a subject of intensive engineering research for centuries, in the last decade granular matter attracted significant attention of physicists. Yet despite a major efforts by many groups, the theoretical description of granular systems remains largely a plethora of different, often contradicting concepts and approaches. Authors give an overview of various theoretical models emerged in the physics of granular matter, with the focus on the onset of collective behavior and pattern formation. Their aim is two-fold: to identify general principles common for granular systems and other complex non-equilibrium systems, and to elucidate important distinctions between collective behavior in granular and continuum pattern-forming systems.Comment: Submitted to Reviews of Modern Physics. Full text with figures (2Mb pdf) avaliable at http://mti.msd.anl.gov/AransonTsimringReview/aranson_tsimring.pdf Community responce is appreciated. Comments/suggestions send to [email protected]

Particle simulation of vibrated gas-fluidized beds of cohesive fine powders

We use three-dimensional particle dynamics simulations, coupled with volume-averaged gas phase hydrodynamics, to study vertically vibrated gas-fluidized beds of fine, cohesive powders. The volume-averaged interstitial gas flow is restricted to be one-dimensional (1D). This simplified model captures the spontaneous development of 1D traveling waves, which corresponds to bubble formation in real fluidized beds. We use this model to probe the manner in which vibration and gas flow combine to influence the dynamics of cohesive particles. We find that as the gas flow rate increases, cyclic pressure pulsation produced by vibration becomes more and more significant than direct impact, and in a fully fluidized bed this pulsation is virtually the only relevant mechanism. We demonstrate that vibration assists fluidization by creating large tensile stresses during transient periods, which helps break up the cohesive assembly into agglomerates.Comment: to appear in I&EC Research, a special issue (Oct. 2006) in honor of Prof. William B. Russe

Dynamically structured fluidization: Oscillating the gas flow and other opportunities to intensify gas-solid fluidized bed operation

Various approaches to structure gas-solid fluidized beds are reviewed, followed by detailed discussion on the use of gas pulsation to induce dynamic structuring. Granular media are dissipative systems, which develop complex spatiotemporal patterns when excited by an oscillating energy source. Here, we discuss how such perturbations initiate surface patterns and how these could propagate into a macroscopically organized flow. We call this dynamically structured fluidization. Vibrated shallow granular layers form ordered surface waves. The hydrodynamics of pulsed gas-fluidized layers are related, but more complex: Under appropriate conditions, surface waves transition into a three-dimensionally ordered bubbling flow. This occurs in much deeper granular beds than under vibration, indicating distinct physics. In this dynamically structured state, bubbles organize into a scalable sub-harmonic, triangular lattice that is highly predictable and responsive to changes in oscillation parameters, allowing for an unprecedented level of control. Structured bubbling is observed only under sufficiently dense conditions; thus, a dynamically structured fluidized bed sits between fixed and fluidized beds, offering opportunities for process intensification, due to less macromixing than traditional fluidization, but a higher level of control through micromixing. This informs new intensified designs for processes that are highly exothermic, involve particle formation, thermally sensitive or high-value materials

Volume fraction variations and dilation in colloids and granulars

Discusses the importance of spatial and temporal variations in particle volume fraction to understanding the force response of concentrated colloidal suspensions and granular materials

Designing and assessing a novel vertical vibrated particle separator

Uncontrolled segregation in particulate mixtures has long been considered as an annoying, and costly, feature encountered in many materials handling operations and although the onset is not clear, many believe it to be driven by the differences in particulate physical properties. An increasing number of usefully scaled laboratory and computer simulation investigations are being carried, particularly by the physics community, to help our understanding of this phenomenon. Physicists at the University of Nottingham have identified that through careful control of frequency and acceleration during vertical vibration, different types of particles can be positioned and/or segregated in a small rectangular cell. An extension of this work resulted in the design of a new small scale batch separator capable of recovering at least one separated particle layer in a different chamber. This work has explored the scale up of the small particle separator to operate in a semi-continuous mode. Since complete experimental know how of particle segregation phenomena is still deficient an empirical design strategy was used. This scaled up particle separator was driven by a pneumatically powered vertical vibration bench in which dry, non-cohesive particulate mixtures of varying densities and sizes (<1000µm) were vertically vibrated under different conditions to assess their separation behaviours. Experiments with regular (e.g. glass and bronze) and irregular shaped particle mixtures (e.g. comminuted glass and bronze) showed that lower magnitudes of vertical vibration frequency (30±10%), dimensionless acceleration (3±10%), particle bed heights (20 and 40mm in majority of the investigated cases) and partition gap sizes (5 and 10mm) were important for separation. Finally, the technique was employed to separate various industrially relevant particle mixtures (shredded printed circuit boards, iridium and aluminium oxide and shredded personal computer wires). Two-dimensional Discrete Element Modelling (DEM) with interstitial fluid interactions simulated with a maximum of 1000 virtual glass and bronze particles showed some important aspects of particle segregation such as; layered particle separation, high density particles ending on top and bottom of the particle bed, convection currents, particle bed tilting and partitioned particle separation. The application of Positron Emission Particle Tracking (PEPT) to glass, bronze, ilmenite and sand particles showed distinct trajectory maps in three dimension (X,Y and Z) with varying particle speeds in the vertically vibrated particle mixtures. The low density particles were mostly observed to move in the middle while the high density particles patrolled in the outer periphery of the separation cell. These distinct particle motions suggested that convection currents played an important role in controlling segregation. Furthermore, the application of a smoke blanket visualization technique showed the existence of air convection currents on top of the vertically vibrating particle mixtures. The experiments on the scaled up semi-continuous particle separator confirmed what was identified previously in that good particle separation could be achieved through careful control of the frequency and acceleration during vertical vibration. This information lays the foundations for a new breed of low cost, dry separator for fine particulate mixtures. Key Words: Vertical vibration, particle separator, fine particle mixtures, dry separation, PEPT, DEM, smoke visualizations

Cooperative effects in vibrated granular systems

In this thesis we describe experimental studies carried out in three different granular systems. Firstly, we describe the results of experiments and computer simulations carried out to test the validity of a randomly-forced model to describe the behaviour of a vertically vibrated, granular monolayer. We study how a single particle moves across a vibrated roughened surface and show that both a random force and viscous dissipation are needed to match the experimental data. We then simulate a collection of particles driven in this way and compare the structure factor S(k) obtained from simulation with that measured experimentally. The small k behaviour of S(k) reveals a quasi long-range structure which has not been observed previously. Secondly, we study the behaviour of water-immersed granular beds. The first system consisted of spherical barium titanate particles contained in a rectangular cell which is divided into two columns, linked by two connecting holes, one at the top and one at the bottom of the cell. Under vibration the grains spontaneously move into just one of the columns via a gradual transfer of grains through the connecting hole at the base of the cell. We have developed numerical simulations that are able to reproduce this behaviour and provide detailed information on the instability mechanism. We use this knowledge to propose a simple analytical model for this fluid-driven partition instability based on two coupled granular beds vibrated within an incompressible fluid. In the second system to be studied a water-enhanced Brazil nut effect, which occurs when the vibrated granular bed is fully immersed in a liquid, will be describe. We use a bed of glass beads immersed in water and monitor the behaviour of a large steel intruder as the system is vibrated vertically. To aid our understanding, we have developed numerical simulations to model this system and provide detailed information about the fluid and grain motion. The mechanism responsible for the rapid rise of the intruder is shown to be fluid-enhanced ratcheting rather than simple differential drag. Lastly, we describe experiments carried out in a levitation magnet to investigate the behaviour of spheres suspended magnetically in a viscous fluid. Under vibration the spheres attract and for sufficiently large vibration amplitudes the spheres are observed to spontaneously orbit each other. Data collapse shows that the instability occurs at a critical value of the streaming Reynolds number. Simulations are used to provide a detailed understanding of the cause of this instability