Jenike & Johanson project engineer Virat Gurung discusses a study of moisture–particle interactions in bulk materials using coupled Discrete Element Method and Smoothed Particle Hydrodynamics.
Bulk materials inherently contain varying levels of moisture throughout the supply chain, spanning stages such as storage, processing, and transportation. The moisture content in bulk materials can range from dry to saturated.
It is very well established through bulk material flow testing that the flowability of a bulk material is significantly influenced by the moisture content of the material. Increase in moisture content typically results in increased cohesive strength, particularly in materials with a high proportion of fine particles. This increased cohesion complicates handling and transportation, potentially leading to operational inefficiencies (loss of production) and maybe resulting in safety issues.
A common problem during storage and transportation of bulk materials is associated with moisture migration. Moisture migration within a material is often encountered, where the initial state of the bulk material marked by a relatively uniform moisture distribution undergoes transformation into a non-homogenous moisture distribution.
This non-homogenous moisture distribution creates zones of varying saturation levels.
This not only complicates handling and transport operations, but may in some applications result in degraded product quality. Some common flow problems that are often encountered in a storage bin as a result of moisture migration during extended storage include arching and ratholing (Figure 1).
Studying moisture–particle interactions
In the mining industry, understanding the interaction between fluid and granular materials is critical for applications such as transfer chutes, storage bins, pneumatic conveying, wet screening, and slurry transport in SAG mills. Traditionally, the design of equipment for such applications has relied heavily on empirical models.
However, with the advancement in computational power, numerical modelling techniques such as Discrete Element Method (DEM) and Smoothed Particle Hydrodynamics (SPH) have emerged as powerful tools for studying particle and fluid flow, respectively. Furthermore, these methods are now being coupled to study solid-fluid interactions to simulate complex solid-fluid mixture flow in various material handling and processing systems.
Overview of DEM modelling
DEM is a numerical method used to simulate the behaviour of particulate systems, including powder and granular materials. DEM method operates by discretising a system into individual elements or particles that interact through contact forces.
The motion of each particle is governed by Newton’s second law, where the forces and torques acting on each particle determine its acceleration and trajectory over time. The interactions between particles are handled using contact models which define how forces are transmitted during collision or separation.
Overview of SPH modelling
SPH is a Lagrangian, particle-based method originally developed to simulate astrophysical phenomena.
In SPH formulation, a continuous fluid is represented by a set of discrete particles, each carrying physical properties such as mass, density, velocity, and pressure.
The field variables representing in-situ conditions are approximated using a smoothing kernel function, where each particle’s properties are computed as a weighted sum of contributions from neighbouring particles located within the smoothing length distance.
The method then solves the Navier-Stokes equations which govern particle motion. Particle trajectories are updated through time integration techniques, by updating their positions and velocities.
Applications and limitations of the coupled DEM–SPH method
The coupling of DEM and SPH has facilitated the investigation of a wide range of applications involving the flow of solid-fluid mixtures. This integrated DEM–SPH approach has been employed to study slurry flow in SAG mills and tower mills, mixing dynamics in feed boxes, wet banana screening processes, and moisture migration within bulk materials. However, accurate simulation outcomes rely on proper model calibration.
In DEM, this typically involves a bulk calibration approach which is done through bulk material flow properties testing, where particle friction coefficients are systematically adjusted to replicate macroscopic behaviours observed experimentally.
In SPH, calibration focuses on refining two key independent length scales: the particle size, which determines the resolution of the fluid domain, and the smoothing length, which defines the interaction radius used in kernel interpolation.
Conclusion
While numerical modelling tools offer valuable insights into the behaviour of multiphase flows, several limitations remain. A primary constraint of both DEM and SPH simulations is the high computational demand. In both techniques, simulation time scales approximately linearly with the number of particles involved.
This makes large-scale or highly detailed simulations computationally expensive. Furthermore, accurately representing a complete particle size distribution (PSD) of bulk materials is often impractical due to limitations in available computational resources.
Despite existing limitations such as computational expense and challenges in accurately capturing full PSD, coupled DEM–SPH method is a valuable tool for studying complex solid-fluid mixture flows.
When supported by well-calibrated parameters, these simulations provide insights into solid-fluid dynamics that are often difficult to capture experimentally.
References
Cundall, P.A., and O.D.L. Strack. 1979. “A discrete numerical model for granular assemblies.” Geotechnique 29 (1): 47-65.
Gingold, R.A., and J.J. Monaghan. 1977. “Smoothed particle hydrodynamics: theory and application to non-spherical stars.” Royal Astronomical Society 181 (3): 375-389.
Hartford, Carrie. 2016. Bulk solids handling system design. 28 November. https://www.processingmagazine.com/material-handling-dry-wet/powder-bulk-solids/article/15586872/bulk-solids-handling-system-design.