Building a numerical model for bulk materials from standard shear test data

While numerical methods have become an integral part of everyday work in process engineering for fluid processes, there is a curious lack of such methods in the field of solid handling. One of the reasons may be the inability of the most often used CFD codes to handle bulk solids. In the paper an attempt is made to show how the behaviour of bulk solids can be modelled using a CFD code without a specific constitutive model for bulk solids. Another reason for not using numerical tools to handle bulk materials is the difficulty of generating the necessary material parameters. Those material models suitable for bulk solids that are available in commercial packages are mostly derived from soil mechanics. Their parameters are determined using a triaxial cell. This device is generally not available in the chemical industries and most often not suitable for bulk solids, due to the high stress levels applied in those tests. In the paper a method is presented which allows the use of standard shear test data, supplemented by data from a compression tests in a "lambda-meter", to determine the parameters of an extended Drucker-Prager model with a compressive yield cap. Model equations are given and parameters are determined for white polymer powder. With these parameters a simulation of silo discharge has been performed successfully using a CFD code. To make CFD codes, which already have the much-needed multi-phase capability, capable of handling bulk solid flow, significant work remains to be done (e.g. shear stresses at rest and anisotropic stress tensors).