Jin Y. Ooi

Influence of discrete element model parameters on bulk behavior of a granular solid under confined compression

Recent decades have witnessed considerable research effort in the development and application of the discrete element method (DEM) for modeling granular materials. One key aspect that appears to have not been addressed adequately is the choice or determination of the DEM model parameters to produce satisfactory quantitative predictions. In many DEM studies, the input parameters were often not measured and the values were sometimes assumed without any proper justification. This article describes a careful validation study in which DEM was used to model confined compression of spherical (glass beads) and nonspherical (corn grains) particles. The key properties of the particles were independently measured. The experimental data on the bulk compressibility and the corresponding load transfer to the cylindrical wall and the bottom platen provide quantitative information for direct comparison with the DEM results. Initial particle packing structure is expected to influence the bulk behavior of the assembly, so several methods of particle generation were used to investigate the sensitivity of particle generation in DEM. An investigation of the influence of interparticle friction and particle stiffness in DEM simulations provided an insight into the bulk response of the granular assemblies under confined compression. Comparison between the simulations and the experiments revealed several useful observations, including the roles of these model parameters in the predicted bulk responses.

Finite element analysis of wall pressure in imperfect silos

It is generally accepted that the pressures exerted by stored bulk solid on silo walls after filling are closely predicted by the Janssen theory. However, recent experimental observations of silo wall pressure have shown significant deviations from Janssen pressures even after careful symmetrical filling. Both meridional and circumferential variations from the Janssen values have been found, which induce bending of the wall in reinforced concrete silos and high membrane stresses in steel silos. It has been suggested that the presence of geometric imperfections in the silo wall is a likely cause of these pressure variations. However, no rigorous investigation of the effects of uneven surface profile on wall pressures appears to have been performed, though some simple calculations have been produced. This paper presents a finite element study of the effects of a local axisymmetric wall imperfection on pressures acting on the wall of a circular silo. The effects of imperfection height and imperfection geometry on wall pressures are also investigated.

Experimental and Simulation Studies of Dilute Horizontal Pneumatic Conveying

Dilute horizontal pneumatic conveying has been the subject of this experimental and numerical study. Experiments were performed utilising a 6.5 m long, 0.075 m diameter horizontal pipe in conjunction with a laser-Doppler anemometry (LDA) system. Spherical glass beads with three different sizes 0.8–1 mm, 1.5 mm, and 2 mm were used. Simulations were carried out using the commercial discrete element method (DEM) software, EDEM, coupled with the computational fluid dynamics (CFD) package, FLUENT. Experimental results illustrated that, for mass solid loading ratios (SLRs) ranging from 2.3 to 3.5, the higher the particle diameter and solid loading ratio, the lower the particle velocity. From the simulation investigations it was concluded that the inclusion of the Magnus lift force had a crucial influence, with observed particle distributions in the upper part of the conveying line reproducible in the simulation only by implementing the Magnus lift force terms in the model equations.

Spatial and Temporal Coarse-Graining for DEM Analysis

The discrete element method is used extensively for the simulation of granular materials. The great majority of engineering applications require bulk design parameters to be determined which requires some averaging or coarse graining techniques to be applied to the particle data from DEM computations. Although numerous papers have been published on these techniques, we suggest that there are still significant challenges in the calculation of the continuum quantities from the particle data. This is chiefly because DEM computes at grain contact level and at the order of 10−5 or smaller computational time step, whereas important engineering events often occur at much larger timescale, so the question of what temporal and spatial averaging scales should be adopted is not so clear. This paper outlines an implementation of coarse graining methods for the projection of discrete quantities into continuum fields. Two DEM simulation examples with a quasi-static and a rapid dynamic flow conditions are used to study ...

Establishing predictive capabilities of DEM – Verification and validation for complex granular processes

The discrete element method has been increasingly used to model granular processes of increasing complexity, from dry to wet to multiphase systems. Whilst DEM has been shown to produce qualitatively many of the complex phenomena observed in these complex granular systems, establishing the extent of the predictive capability of DEM model is still in its infancy. This summary paper discusses the need to develop verification and validation methodologies with regard to DEM computations. It proposes the need to first verify that a DEM code with its implemented mathematical model and computation algorithm is accurately reproducing the conceptual model and its solution, before validating to determine the degree to which the computational model accurately represent the physics being modelled. It also discusses the associated issue of data analysis to extract the parameters of interest from DEM particle scale data.

Correlation analysis of solid particles' permittivity and composition using electrical capacitance tomography and Maxwell Garnett formula

The solid volume fraction and volume composition ratio of different solid particles in a silo are important parameters indicating the character of granular mixtures. In this paper, measurement of the two parameters is studied by utilizing Electrical Capacitance Tomography (ECT). This preliminary work focuses on comparing normalized permittivity changes from ECT with those predicted by the Maxwell Garnett formula (MGF) relating to solid volume fraction and volume composition ratio. Experimental results show that normalized permittivity changes from the two methods have a strong linear correlation in gas-solid two- and three-phase systems. This work demonstrates that ECT is a promising technique for monitoring solid volume fraction of single solid particles and the volume composition ratio of mixtures of two different solid particles in a silo.

Numerical analysis of impact events in a centrifugal impact pin mill

Milling is an important operation in many industries, such as mining and pharmaceutical. Although the comminution process during milling has been extensively studied, the material fragmentation mechanisms in a mill are still not well understood partly because of the lack of an understanding on the local stressing and dynamic information under operational conditions in mills. This paper presents a DEM simulation of particle dynamics and impact events in a centrifugal impact pin mill. The main focus is the statistical characteristics of the dominant stressing modes during the milling process. The frequency, velocity and force of the different impact events between particles and mill components, or between particles, are analysed.