On the modelling of granular flows in industrial applications via the Particle Finite Element Method

Abstract

The aim of this work is to present a new procedure for modelling industrial processes that involve granular material flows, using a numerical model based on the Particle Finite Element Method (PFEM). The numerical results herein presented show the potential of this methodology when applied to different branches of industry. Due to the phenomenological richness exhibited by granular materials, the present work will exclusively focus on the modelling of cohesionless dense granular flows.

The numerical model is based on a continuum approach in the framework of large-deformation plasticity theory. For the constitutive model, the yield function is defined in the stress space by a Drucker-Prager yield surface characterized by two constitutive parameters, the cohesion and the internal friction coefficient, and equipped with a non-associative deviatoric flow rule. This plastic flow condition is considered nearly incompressible, so the proposal is integrated in a u- p mixed formulation with a stabilization of the pressure term via the Polynomial Pressure Projection (PPP). In order to characterize the non-linear dependency on the shear rate when flowing a visco-plastic regularization is proposed.

The numerical integration is developed within the Impl-Ex technique, which increases the robustness and reduces the iteration number, compared with a typical implicit integration scheme. The spatial discretization is addressed within the framework of the PFEM which allows treating the large deformations and motions associated to granular flows with minimal distortion of the involved finite element meshes. Since the Delaunay triangulation and the reconnection process minimize such distortion but do not ensure its elimination, a dynamic particle discretization of the domain is proposed, regularizing, in this manner, the smoothness and particle density of the mesh. Likewise, it is proposed a method that ensures conservation of material or Lagrangian surfaces by means of a boundary constraint, avoiding in this way, the geometric definition of the boundary through the classic -shape method.

For modelling the interaction between the confinement boundaries and granular material, it is advocated for a method, based on the Contact Domain Method (CDM) that allows coupling of both domains in terms of an intermediate region connecting the potential contact surfaces by a domain of the same dimension than the contacting bodies. The constitutive model for the contact domain is posed similarly to that for the granular material, defining a correct representation of the wall friction angle. In order to validate the numerical model, a comparison between experimental results of the spreading of a granular mass on a horizontal plane tests, and finite element predictions, is carried out. These sets of examples allow us validating the model according to the prediction of the different kinematics conditions of granular materials while spreading – from a stagnant condition, while the material is at rest, to a transition to a granular flow, and back to a deposit profile.

The potential of the numerical method for the solution and optimization of industrial granular flows problems is achieved by focusing on two specific industrial applications in mining industry and pellet manufacturing: the silo discharge and the calculation of the power draw in tumbling mills. Both examples are representative when dealing with granular flows due to the presence of variations on the granular material mechanical response.

References

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