Perspectives on Particle–Fluid Coupling at Varying Resolution in CFD-DEM Simulations of Thermochemical Biomass Conversion

In computational fluid dynamics (CFD) simulations of thermochemical biomass conversion using the discrete element method (DEM), the need to establish adequate coupling schemes for the fluid-to-particle and particle-to-fluid information exchange arises. Simultaneously, the requirements on the resolution of the fluid fields typically vary throughout the reactor, and the particulate flow regimes can vary from dilute to dense. For such conditions, there is currently a lack of consensus in the literature about the appropriate length scale to establish the aforementioned coupling and on the issue of whether the coupling in the two directions can be chosen independently or not. In the current work, we review the recent literature on coupling in CFD-DEM simulations, with a particular focus on semiresolved approaches. We discuss the different coupling methods available and identify requirements arising from dealing with thermochemical conversion processes. Finally, we establish some general guidelines for the simulation of these processes, while noting that it is impossible to simultaneously enforce symmetric couplings, sampling of the undisturbed fluid state (as sometimes required by correlations), and localized influence of the particle on the fluid fields. It is suggested that future model development shall focus on reintroducing the effects of varying local microstructure of the particulate flow that is filtered out in the particle–fluid coupling whenever excessive coupling lengths are used to stabilize the numerical solution procedure. Data-driven approaches, in combination with nonintrusive experimentation at high spatial and temporal resolution, are identified as a promising route to realize these ambitions.