Fragmentation of grains under impact
Luisa Orozco, Jean-Yves Delenne, Farhang Radjai, Philippe Sornay
Many industrial granular processes involve desired or undesired fragmentation of grains. However, despite experimental measurements and numerical modelling approaches, the mechanisms of single grain fragmentation and its effects on the behaviour of granular materials are still poorly understood. In this work, we investigate the fracture and fragmentation of a single grain due to impact at low energies, using three dimensional DEM simulations by means of the contact dynamics method. The grains are assumed to be perfectly rigid but modelled as an assembly of glued polyhedral Voronoï cells. The strength of the glue represents the internal cohesion of the grain along normal and tangential directions. The numerical method allows us to calculate the forces and torques at the interface zones between cells. The inter-cell joints can open either in tension (mode 1) or by slippage (mode 2) when the fracture strength is reached. A series of simulations for a range of different values of parameters (number of cells, fracture strength, impact velocity) were performed. The efficiency of the process has been defined as the ratio between the energy liberated by the fracture and the kinetic energy at the impact. It was found that the efficiency increases with the number of cells. Also, the process efficiency is inversely proportional to the internal cohesion. Finally, an impact velocity that maximizes the efficiency have been found at 0.08 m/s.
Particle dynamics simulation of wet granulation in a rotating drum
Thanh Trung Vo, Saeid Nezamabadi, Patrick Mutabaruka, Jean-Yves Delenne, Edouard Izard, Roland Pellenq, Farhang Radjai
We simulate the granulation process of solid spherical particles in the presence of a viscous liquid in a horizontal rotating drum by using molecular dynamics simulations in three dimensions. The numerical approach accounts for the cohesive and viscous effects of the binding liquid, which is assumed to be transported by wet particles and re-distributed homogeneously between wet particles in contact. We investigate the growth of a single granule introduced into the granular bed and the cumulative numbers of accreted and eroded particles as a function of time for a range of values of material parameters such as mean particle size, size polydispersity, friction coefficient, and liquid viscosity. We find that the granule growth is an exponential function of time, reflecting the decrease of the number of free wet particles. The influence of material parameters on the accretion and erosion rates reveals the nontrivial dynamics of the granulation process. It opens the way to a granulation model based on the realistic determination of particle-scale mechanisms of granulation.
Modeling particle breakage inside rotating drums
Luisa Orozco, DH Nguyen, Jean-Yves Delenne, Philippe Sornay, Farhang Radjai`
Rotating drums are systems often used in industry for processes that require mixing and grinding of materials. Laboratory tests show that the extrapolation of the observed behavior to the industrial scale does not produce the expected results in terms of grinding performance (particle size distribution, specific surface, etc). It is difficult to measure experimentally the crushing evolution, but, using numerical simulations, we are able to follow the breakage processes that take place at different scales. We use the Contact Dynamics method (CD) and the Bonded Cell Method (BCM) in order to simulate breakable grains. The study of a single grain impact enhances the understanding of the breakage process under dynamic conditions. Then, the evolution of material properties is compared for different rotating drum configurations.
A particle is considered as an assembly of independent cells generated using Voronoï tessellation. The intercell cohesive behavior is governed by two independent strength thresholds: Cn (preventing tensile failure) and Ct (preventing shear failure). The effective contact strength depends on the contact surface (s). Once one of the thresholds is reached, the contact can break irreversibly when the work performed by the cells relative movement reaches the fracture energy (Gf= Gn= Gt). Then, the non-cohesive interfaces follow a purely frictional contact law.
Two-dimensional numerical simulation of chimney fluidization in a granular medium using a combination of discrete element and lattice Boltzmann methods
Jeff Ngoma, Pierre Philippe, Stéphane Bonelli, Farhang Radjaï, Jean-Yves Delenne
We present here a numerical study dedicated to the fluidization of a submerged granular medium induced by a localized fluid injection. To this end, a two-dimensional (2D) model is used, coupling the lattice Boltzmann method (LBM) with the discrete element method (DEM) for a relevant description of fluid-grains interaction. An extensive investigation has been carried out to analyze the respective influences of the different parameters of our configuration, both geometrical (bed height, grain diameter, injection width) and physical (fluid viscosity, buoyancy). Compared to previous experimental works, the same qualitative features are recovered as regards the general phenomenology including transitory phase, stationary states, and hysteretic behavior. We also present quantitative findings about transient fluidization, for which several dimensionless quantities and scaling laws are proposed, and about the influence of the injection width, from localized to homogeneous fluidization. Finally, the impact of the present 2D geometry is discussed, by comparison to the real three-dimensional (3D) experiments, as well as the crucial role of the prevailing hydrodynamic regime within the expanding cavity, quantified through a cavity Reynolds number, that can presumably explain some substantial differences observed regarding upward expansion process of the fluidized zone when the fluid viscosity is changed.