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Diskrete Element Method

Numerous industrial processes are based on solid flows or very dense gas-solid flows. Examples of such moved bulk materials from the energy process engineering are the bed material in fluidised bed combustions or solids treated thermally or burned in rotary kilns or on grate systems (wood chips, pellets from waste processing). The theoretical description of such granular media is only at the beginning of the development.

At the Department of Energy Plant Process Engineering (LEAT) at the Ruhr-University Bochum a discrete element code (DEM) code has been developed, which simulates the real behavior of moving bulk materials through the individual consideration of each particle of the bed and the calculation for two and three dimensional applications possible. The developed DEM code can,in addition to the mechanical simulation of the particle movement, also trace any movement of enclosing walls. This possibility is essential for many industrial applications, since the movement of granular media is often induced by moving walls. The existing model is fully parallelized, hence realistic calculations with more than 10 millions of individual particles can be made on parallel computers and workstation clusters.

The discrete element method is based on pursuing each individual particle of a particle system and its interaction with the environment over time continuously. This makes it possible to determine information on the position, the translational and rotational velocity, and the time-resolved forces acting on each individual particle. The mentioned quantities are obtained by integration of the Newtonian equations of motion. Here, it is necessary to model the forces that act between a particle and its surrounding. Generally, it is possible to determine these forces directly from the deformations experienced by a particle during the contact. The difficulty of such an approach lies in the fact that it is very complicated to find appropriate generic models, which have to be solved numerically, as well. Such an approach must be rejected solely, due to computing time. As an alternative, one chooses the approach to calculate the contact forces from a virtual overlap instead of the deformation of the particles.

These forces can be decoupled in the normal and the tangential direction, so that the two directions are treated separately and can independently be solved numerically.

Through this detailed analysis of the overall system with its mechanical interactions results in the characteristic overall behavior are given. Only in this way phenomena such as segregation, the emergence of force chains or even transient heating processes can be simulated. At the same time additional information such as local loads in silos and transport systems are available, which are very difficult to determine experimentally.

Example of a two-dimensional calculation of a moving grate with DEM