Simulating evaporation with the EDEM-Fluent coupling

Since the first introduction of the EDEM – Fluent coupling back in 2006, EDEM has been working closely with ANSYS to continually expand the capability of the coupling to meet customers’ needs – including requirements for advanced applications and for fast coupled simulations. Over the years, the EDEM-Fluent coupling has been widely adopted by engineers and researchers to investigate a range of phenomena including fluid and particles such as fluidized beds, pneumatic transport, spray coating and other processes.

One experienced user of the EDEM-Fluent coupling has been Andrew Hobbs, Chief DEM/CFD Engineer at Astec, Inc., for modeling the aggregate drying process. Andrew has been involved in developments to the coupling over the years and, in this post, he describes the latest enhancements to the coupling that include a chemical species transfer capability.

The release of version 2.2 of the EDEM CFD Coupling for ANSYS Fluent introduced some fundamental improvements in the coupling methodology.  In this approach, particle information from EDEM is converted into particles in Fluent’s discrete phase model (DPM). This then allows access to all of the DPM models available in Fluent including UDF options for drag and heat transfer. A feature that was missing was a way to link custom particle properties from EDEM to additional particle quantities in the DPM.  This functionality would allow the simulation of mass transfer from the particle to gas phase. So in 2018 a collaboration between EDEM, ANSYS, and Astec began to implement this connection between EDEM custom properties and species in Fluent’s particle model.

To make the mass transfer model as generic as possible we chose to use the multi-component DPM particle model. This model affords the most flexibility in terms of the number of evaporating species and is not bound to any of the dedicated particle models. For simple drying only one evaporating component is necessary, but the multi-component model allows many different liquid species and combinations of species, for example urea and water.

Once the EDEM coupling has been loaded in Fluent and the multi-component particle type selected in the DPM model with a  mixture species defined, custom properties for the evaporating species and the non-evaporating species will be created automatically as custom particle properties in EDEM representing the mass fraction of each species. The initial mass fractions of the particles are set in the EDEM factory settings. The mass transfer is calculated according to the law chosen in Fluent (such as the Diffusion controlled model) and the species mass fraction of each particle is updated in the custom properties in EDEM each time Fluent passes back the particle information.  The video below shows the model being used for a simple fluidized bed heating and drying wet particles. The coupling is calculating the particle drag, convective heat transfer, and evaporative mass loss.

Since aggregate drying is a key part of the process of producing hot mix asphalt, a CFD-DEM model that could capture evaporative drying is a key capability for Astec. Using an aggregate dryer model previously described (see recent article by Dr. Marina Sousani, et al[1]) we used the multi-component DPM to add 5% liquid fraction to the particles fed into the drum. The results capture the evaporative phase change as the aggregate particles dry and the gas phase gains water vapor as shown in the video below.

As with any engineering endeavor there is always more to do and ways to improve. The next phase of development will be to link other DPM particle models, specifically particle combustion, to simulate solid fuel burners for power generation. This phase will also see improvements in the EDEM coupling API that will allow particle sizes to be modified directly by Fluent when particle sizes are changed substantially.

[1] M. Sousani, A.M. Hobbs, A. Anderson, R. Wood (2019) ‘Accelerated heat transfer simulations using coupled DEM and CFD’, Powder Technology

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