CFD-DEM: Exploring how a coupled model can help you better solve solid-fluid flows

Learn how CFD-DEM coupling provides an intermediate approach between using the sub-particle resolution for the fluid and the mesh resolution for both fluids and particles

Many processes in various industries involve the simultaneous flow of fluids and particles. Just to cite a few examples:

  • Slurry mills (mining industry)
  • Cyclones, desanders, and drill cutting removal (oil & gas industry)
  • Pneumatic conveyors (multiple industries)
  • Wastewater management (waste disposal industry)
  • Grains drying and sorting (agriculture and food industries)
  • Biomass reactors (energy industry)
  • Fluidized beds and catalytic reactors (chemical and nuclear industries)

In all these cases it is important to take the fluid flow into account in order to get the correct behavior of the particles. Design and scale-up, as well as optimization of such processes, require a deep understanding of the thermo-hydrodynamics, which is determined by the particle-level interactions between the fluids, particles, and boundaries.

Why is it so complicated to model these systems?

The complexity of the fluid-solid flow present in these systems makes modeling them a challenging task. The primary source of difficulties is due to differences in order of magnitudes amongst the characteristic scales existing in the problem.

First of all, there is the device scale, which is naturally respected. Secondly, the typical fluid-flow scales are captured in a CFD solution by solving the flow at the mesh scale, which is generally much bigger than the particle but yet quite small when compared to the device scale. Finally, there is the fluid-particle interactions scale, which has the magnitude of the smaller particles, therefore making it computationally prohibitive to solve the flow in a sub-particle resolution for most industrial applications.
These difficulties are what makes the coupled CFD-DEM approach so promising: it provides an intermediate level between using the sub-particle resolution for the fluid and the mesh resolution for both fluids and particles.

Why not just use CFD alone?

There are two main approaches to deal with solids in CFD: The Eulerian approach and the Lagrangian approach.
In the Eulerian approach, both the fluids and particles are treated as continuums and therefore continuum equations are solved for both phases with an appropriate interaction term to model the, so constitutive equations for inter- and intra-phase interaction are needed.
The problem relies on the fact that finding general equations for granular systems is hard due to the changing nature of how solids flow. Moreover, due to the continuum interpenetrating approach, no individual particle information is available, and this might be the data sought. Not to mention that prescribing a particle size distribution can increase your computational cost, since in general several phases are created to model several particle sizes.
In the Lagrangian approach, the fluid is still treated as continuum, but the particulate phase is treated as individual particles (or parcels modeling a group of particles) and these are tracked along the fluid phase by the result of forces acting on them. However, due to the fact that there is no particle-to-particle interaction, this approach is limited to very dilute flows, which is not the reality in the majority of the industrial applications.

Why couple CFD and DEM together?
The coupled CFD-DEM approach is a promising alternative for modeling granular-fluid systems since it can capture the discrete nature of the particle phase while maintaining the computational tractability. This is accomplished by not solving the detailed fluid field at the particle level, which enlarges the range of equipment and processes that can be studied with numerical simulations.

Some specific benefits to using the CFD-DEM coupled method vs. using CFD alone are listed below.

  • Unlike continuum methods, the motion of every particle is simulated – so particle-to-particle interactions are taken into account by using this “brute force approach” and there is no need to provide state equations of granular systems, which, again, are quite difficult to derive.
  • By the same token, there is no limitation of low particle size concentration and PSD distribution is easily prescribed without increasing CFD solver computational cost.
  • You can deal with non-spherical particles and more than that, you can model adhesive/cohesive materials by modeling the attractive force between a pair of particles and between particles and walls.
  • Additionally, you can compute particle-to-particle and particle-to-walls heat transfer as well as the convective heat transfer with the fluid.

What are some examples of coupled CFD-DEM applications?

In this example, a wind shifter (typically used in industrial waste processes to separate light from heavy particles) is modeled using the Fluent One Way Steady State coupling mode found in Rocky DEM. Different material properties and shapes for the particles are used, while air flows from the bottom. The use of a drag law that takes into account particle shape and alignment of the particle with the flow allows the correct prediction of the separation efficiency.

Fluidized bed
This video shows a Fluent Two Way coupling simulation in Rocky of a fluidized bed with increasing gas flow rate. The intimate mixing behavior shown is one of the main advantages of using fluidized beds in industrial processes.

Ring within fluidized bed
In this case, a ring is kept suspended in a fluidized bed due to the collisions of the small particles against the ring surface. As it was mentioned before, not only is shape taken into account, but also several particle groups can be modeled without creating several additional phases on the CFD side.

Slurry mill
In this example,  two fluid phases plus particles flow in a rotating domain. Rocky-Fluent coupling is used to predict slurry pooling and power draw in a slurry mill. In this device, ore particles are inserted into a mill that contains steel balls and also water (that later becomes mud due to the finer particles) in order to break ore into the desired size range.

Potatoes in an oven
This last example shows a one-way coupling case in which potatoes are heated due to their passage through hot air in an oven. As expected, smaller particles reach a higher temperature faster than the bigger potato particles.

Want to know more about the CFD-DEM coupling formulation?
Curious about how the CFD-DEM coupling works and the mathematical modeling behind it? Check out this webinar, where the coupling approach between Rocky and ANSYS Fluent is addressed. Mathematical modeling for the coupling itself is described, and a  few application examples are presented. Finally, a hands-on demonstration of a two-way coupling setup is provided.


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