Granular and Bulk Flow Simulation

Granular materials and the associated processing processes are highly significant to the economy and, at the same time, they are an exciting area of Approximately 60 percent of the products in the chemical industry are granular materials – another 20 percent of the products contain powdered forms.

GRAIN – Granular and Bulk Flow Simulation

The Complex Rheology GRAIN Solver is developed to deal with the flows of granular materials (for example sand). To deal with industrial problems two competing goals have to be met:

  • The whole range of the complex three dimensional dynamic behaviour of granular flow has to be reproduced as realistic as possible.
  • The computation time must be short enough for the use in an industrial environment.

Both requirements are best fulfilled in GRAIN by using a special nonlinear hydrodynamic model, which has been developed in our Institute. Our hybrid model combines the characteristics of rapid granular flow with approaches of soil mechanics of dense, slow flows. It reproduces known results from granular dynamics as e.g. dilatancy, existence of shear bands and solid like behaviour.

This model and appropriate numerical methods (i.e. several nonlinear finite volume methods) are implemented in GRAIN  – a module of the CoRheoS software platform. CoRheoS is able to cover the entire simulation of granular flows, from the acquisition of arbitrary geometries to visualization.

Advantages Over Other Methods

An important advantage of GRAIN in comparison to particle-based methods like DEM is the treatment of the granular material as a continuum. This enables the simulation of fast, dilute as well as slow, dense granular processes with industrially-relevant material volumes and realistic particle sizes of different granulates and powders in the customary CFD context with similar computation times.

The microscopic interaction of particles becomes part of the continuum modelling here. In addition, the parallelisability of CFD processes can be achieved very effectively and we are researching intensively. These results can be applied directly to the method in GRAIN for the further reduction of simulation times.

 

Interaction of Moving Components

In recent years the application field for GRAIN was greatly expanded once again by the interaction of moving components with granular media. Building on the initial success of promising testing, an industrial project for mixing machines with rapidly moving components was successfully completed.

Services:

  • Spatially resolved dynamic 3D simulation of
    • granular bulk flow processes
    • two and three-phase air-driven granular flow processes
    • mechanically driven mixing and milling processes
  • Virtual testing of design, upscaling, flow patterns and residence time distributions.
  • Project-based collaborations and scientific consulting for
    • Material characterization targeted towards simulation in collaboration with Fraunhofer IKTS
    • Modeling extensions to customer-specific process conditions
    • On-Site licensing, installation and running of the software

Simulation of Mixing Processes and Devices

The simulation of mixers presents the unique challenge of combining granular flows and rapidly moving mixing devices that generate enormous shear forces in a solid. Our GRAIN software module is able to simulate such processes using realistic rotational speeds and material properties.

 

Results:

  • Spatially resolved information on local density, velocity, shear forces, pressure and component load
  • Virtual tests for the design of the mixing machine, upscaling of the mixer for different sizes and different process conditions

The example shows the simulation of an EIRICH mixer from Maschinenfabrik Gustav Eirich GmbH & Co KG.

Simulation of Segregation Processes

Granular systems of different particle types – mainly differences in size and density – are subject to the phenomenon of segregation, which leads to the segregation of the components. A process that can be used to advantage in parts of the coal and steel industry leads to ongoing challenges in other industrial sectors. Even well-mixed systems can later lose quality during loading and unloading or transport processes.

The segregation module of the GRAIN software is able to simulate the separation behaviour of particle mixtures caused by particle interaction during shear in flowing granular systems.

The video shows the proportional distribution of large (blue) and small particles (red) during the mixing process in differently designed mixers.

Results:

  • Spatially resolved information on the local distribution of different particle types
  • Virtual tests to validate the quality of mixing or separation processes

Simulation and Design of Silos

The example shows the simulation of the residence time of a recycling process in a silo with conical installation at the Chair of Mechanical Process Engineering (MVT), TU Kaiserslautern.

Based on our unique model for flows of granular materials, our GRAIN software allows the simulation of material influence, installations, fixtures and silo design itself on the flow field in a silo.

In particular, GRAIN is able to predict silo-specific behaviour such as core and mass flow and to calculate the residence time of a silo. In contrast to DEM simulations, we are able to consider silos with industrial size and realistic particle size and obtain computation times comparable to complex CFD simulations.

The injection of air to loosen the bulk material or to dry the bulk material can be simulated by a coupled simulation with FLUID.    

 

Results:

  • 3D simulation with spatial resolution of the flows in the silo with local information about density, velocity, pressure, component load
  • Virtual tests and evaluation of residence time distribution, flow pattern and flow path for arbitrarily set tracers
Residence Time
© Fraunhofer ITWM
Distribution of the Residence Time.

Simulation of Agitator Bead Mills

Mill Simulation
© Fraunhofer ITWM
The example shows the simulation of the flow of a TiO2 suspension through granular agitator balls in a vertically rotating disk mill of KRONOS INTERNATIONAL, INC.

The simulation of the flow in agitator bead mills is a particular challenge. Here, several research areas of the calculation of complex flows are combined within one simulation:

  • Two-phase suspension flow of powder and water, modelled as a non-Newtonian fluid
  • a third phase of spheres modelled as granular, fast moving components
  • a fourth phase of enclosing air

The modelling and numerical difficulties involved are challenging. The interaction of four spatially and temporally fully resolved phases must be modelled both among each other and with the fast moving components. We meet the resulting phase-dependent demands on the numerical methods by means of an extended time step control.

For the first time we have combined the modules FLUID, i.e. multi-phase, non-Newtonian modelling together with a third Newtonian phase, and GRAIN – for the simulation of the agitator balls in such a complex problem. In cooperation with the project partner, the resulting simulation results were successfully compared with existing measurements.

 

Results:

  • Dynamic calculation of local variables
  • Density distribution of the suspension and agitator balls
  • Velocity fields and pressure distribution
  • Shear forces and energy dissipation in all phases
  • Virtual tests of the performance of the mill in terms of:
    • Local stresses and local power loss to characterize the grinding performance
    • Local shear forces on the plates and cylinder walls to illustrate the abrasive wear

Simulation of the Pneumatic Transport

In pneumatic transport, granular material is transported through a pipe system by means of a gas flow – this is a frequently used and particularly product-friendly method of transport. However, pneumatic transport requires a careful and material-specific design, as the pipe may be blocked by blockages or unfavourable flow conditions.

The GRAIN software module is the first holistic approach to design support, which is intended to assist engineers in the material- and system-specific adjustment of the flow boundary conditions (such as granular inflow or gas velocity).

The example shows the flow fields in a pipe system at the Chair for Process Engineering of Disperse Systems at the TU Munich.

Results:

Spatially resolved information for

  • local gas and granular velocity
  • pressure drop of the gas phase
  • local porosity of the granular material
  • granular mass flow