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

Updated from original publication on: July 19, 2017

In this blog post, we’ll discuss how CFD-DEM coupling expands the range of granular-fluid systems that can be simulated quickly and effectively.

The simultaneous flow of fluids and particles is common in many processes across multiple industries. For example:

  • Slurry mills (mining industry)
  • Cyclones, desanders, and drill cutting removal (oil & gas industry)
  • Pneumatic conveyors (multiple industries)
  • Wastewater management (waste disposal industry)
  • Grain drying and sorting (agriculture and food industries)
  • Tablet and candy coating (pharmaceutical 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 obtain the correct behavior of the particles. Design, scale-up, and optimization of such processes require a deep understanding of the thermo-hydrodynamics of the system, 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 in these systems makes their modeling a challenging task. The primary source of difficulties involves the differences in the orders of magnitude among the characteristic scales existing in the problem. 

First of all, there is the device scale, which is naturally respected. Second, the typical fluid-flow scales, which are generally much bigger than the particle but quite small when compared to the device scale, are captured in a CFD solution by solving the flow at the mesh scale. Finally, there is the scale of the fluid-particle interaction that has the magnitude of the smaller particles, making it computationally prohibitive to solve the flow in a sub-particle resolution for most industrial applications. 

These difficulties are what make the coupled CFD-DEM approach so promising: The approach provides an intermediate level between using the sub-particle resolution for the fluid and the mesh resolution for both fluids and particles. We’ll cover this in more detail a bit later in the post.

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, so constitutive equations for inter- and intra-phase interactions are needed. The problem stems from the fact that finding general equations for granular systems is hard due to the changing nature of how solids flow. 

In addition, prescribing a particle size distribution can increase your computational cost, since several phases are required to model several particle sizes. Moreover, due to the continuum interpenetrating approach, no individual particle information is available, and this might be the data you’re seeking.

In the Lagrangian approach, the fluid is still treated as a continuum, but the particulate phase is treated as individual particles (or parcels modeling a group of particles) and the particles are tracked along the domain by the result of forces acting on them. 

However, because particle-to-particle interactions are not solved, this approach is limited to very dilute flows, which is not the reality in most industrial applications. 

Why couple CFD and DEM together?

The coupled DEM-CFD 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. 

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

  • Unlike continuum methods, the motion of every particle is simulated – particle-particle and particle-boundary interactions are solved – and there is no need to provide equations of state for granular systems, which, again, are quite difficult to derive.
  • Since all forces acting on particles are computed by the DEM solver, cases with unique, non-spherical-shaped particles can be accurately solved using Rocky’s precise shape representation.
  • There is no limitation on the particle size concentration, and particle size distribution is easily prescribed without increasing CFD solver computational cost. 
  • Different particle shapes can be simultaneously used in the DEM solver without increasing the CFD cost. 
  • Adhesive/cohesive materials can be modeled using one of the adhesion models available in Rocky DEM. 
  • Because convective heat transfer between particles and fluids can be solved, along with the conductive heat exchanged during collisions, processes that involve temperature changes (such as drying) can be modeled with increased accuracy.   
  • As the complete history is available for all particles inside the domain (for example, velocities, temperatures, and contact data), the extensive set of post-processing tools available in Rocky enhances the level of information that can be extracted from the coupled simulation, providing better insight into your problem.
  • The association of Rocky’s multi-GPU capabilities for the DEM solver with the Ansys Fluent distributed parallel option for solving the CFD equations, brings into reality the simulation of cases with a huge number of particles.
  • Finally, Rocky features such as detailed particle collision statistics, energy spectra analysis, and breakage modeling can be simultaneously used with the Fluent-Rocky coupling solution, broadening the range of cases that can be numerically modeled and expanding your analyses to the next level.

 

Examples of simulations using the coupled CFD-DEM approach

Windshifter

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 approach. Different material properties and shapes for the particles are used, while air flows from the bottom. The ability to choose separate drag laws for each of the particle shapes allows you to 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.

 

 

 

Rings suspended in a fluidized bed

In this coating equipment, small spherical particles are fluidized due to the air flowing from the bottom, while some ring-shaped particles are kept suspended due to the collisions of the small particles against the ring surface. Particles are colored in Rocky by their initial position. This simulation helped engineers predict scenarios in which the rings were not suspended and fell near the gas inlet, blocking the gas flow and collapsing the bed.

Flexible hair strands in a cyclonic vacuum cleaner

In this simulation performed by BISSELL, a large number of hair strands are modeled using Rocky’s flexible fiber particles and the one-way coupling with Fluent. A specific drag model for long and slender particles was adopted to incorporate the effect of the flow on the hair. Using Fluent-Rocky coupling as a tool for hair modeling enables conceptual device testing, reducing the number of lab-tested prototypes and minimizing development time and cost.

Pharmaceutical tablet coating device

In this study, Rocky DEM was fully coupled with Ansys Fluent to study the drying process in a tablet coater. Custom particle shapes accurately predict the tablet heating as hot air flows within the equipment, and both convective and conductive heat transfers are taken into account. The coating liquid was injected and allowed to dry due to the heat provided by the hot air stream. During this process, the solute on the coating liquid was incorporated to the tablet, and coating homogeneity was then investigated by measuring the final particle mass variability.

Fine particles in human airways

The coupling framework between Rocky DEM and Ansys Fluent accurately predicts the transport and deposition in human airways of cohesive pharmaceutical powders with different shapes, such as spherical particles or multi-element flexible fibers. More details on how this multiphysics modeling provides insight into fine-particle transport in human airways can be found in this post.

Riding lawn mower

Rocky DEM flexible fibers are used to model grass in this simulation of a riding lawnmower. One-way coupling between Fluent and Rocky is adopted to incorporate the fluid forces on grass particles near the rotating blades. A breakage model allows engineers to model the cutting of the grass.

 

Want to know more about the CFD-DEM coupling formulation?

DEM and CFD Coupling webinar

In this webinar, the mathematical modeling for the coupling itself is briefly described, and few application examples and a hands-on demonstration of a 2-way coupling setup are presented.

Ansys Fluent (CFD) and Rocky DEM coupling for modeling fluid particulate systems

In this post, you will learn how the Ansys Fluent-Rocky DEM coupling solution works and under what conditions it is best to employ it. At the end of the article, you will also see some industry examples of the Ansys Fluent-Rocky DEM coupling solution in action.


Lucilla Almeida

CAE Specialist at ESSS, D.Sc.

Lucilla holds a BE (Chemical) undergraduate degree, an M.Sc. in Chemical Engineering and a Ph.D. in Nuclear engineering from the Federal University of Rio de Janeiro. She joined ESSS in 2008 and has spent 5 years focused on applying CFD to solve common engineering problems in the Oil and Gas industry, dealing with turbulent and multiphase flow simulations. Since 2013, she is an Application Engineer for Rocky DEM Business Unit, supporting users, working on consultancy projects and validating models implemented for the CFD-DEM coupling.


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