The design of novel apparatuses can benefit from numerical simulations. These allow for greater insight into the process, rapid evaluation of design variants and enable scale-up after lab-scale vavalidation, reducing the need for costly experiments.
The state of the art in granular flow simulation is the Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) which allows for the accurate description of all granular flow phenomena. The fluid phase is described using the Navier-Stokes equations while the granular phase is described using discrete particles obeying the Newtonian equations of motion, with momentum exchange modelled by a drag closure.
The degree of accuracy and applicability requires the resolution of small-scale phenomena like particle collisions and clustering which result in small timesteps and high computational demand. To allow for the treatment of large-scale systems, coarse-graining techniques are applied that attempt to reproduce the bulk behavior by tracking representative computational parcels instead of all individual particles. The Lagrangian nature of CFD-DEM preserves particle identities and enables tracking of physical or chemical processes occurring within the particle, thereby predicting product quality.
Based on these highly-resolved simulations, time extrapolation can be performed using Recurrence CFD (rCFD). This novel approach captures pseudo-periodic flow patterns like bubbling or spouting to allow for the long-term description of transport processes within the system and its constituents.
- Implementation of Recurrence CFD in OpenFOAM.
- Development and implementation of a new communication scheme for scaling the CFD-DEM software CFDEMcoupling to hundreds of cores.
- Modelling of heat and mass transfer within pilot scale fluidized and spouted bed granulators.
- Investigation of intra-particle transport processes over the course of granulation.
An implementation of the rCFD method was developed and applied to two promising target cases: spray coating and residence time behavior prediction. The particle dynamics is not influenced by the deposition of a thin coating layer in the first case and by the motion of particle through the system in the second one.
For spray coating, a batch spouted bed equipped both with and without stabilizing draft plates was first simulated using state-of the art CFD-DEM and subsequently extrapolated in rCFD. Here, spray parcels were injected and droplet deposition was modelled using a filter correlation, stripping mass from the parcels. The particle surface percentage coated is estimated using a statistical approach that obeys the asymptotic behavior of increasing overlapping droplet impacts. The surface coverage distribution after 1 h of coating is shown in Fig. 1. The unstabilized system shows much more homogenous coating due to inhibited mixing in the system with draft plates. The present rCFD implementation achieved a speedup of 2100x over pure CFD-DEM, without which the study at hand would not have been feasible.
- Kieckhefen, P., Lichtenegger, T., Pietsch, S., Pirker, S., Heinrich, S.: Simulation of Spray Coating in a Spouted Bed using Recurrence CFD, Particuology, (2018), DOI: j.partic.2018.01.008.
- Kieckhefen, P., Pietsch, S., Höfert, M., Schönherr, M., Heinrich, S., Kleine Jäger, F.: Influence of gas inflow modelling on CFD-DEM simulations of three-dimensional prismatic spouted beds, Powder Technology, 328 (2018), DOI: j.powtec.2018.01.048
- Pietsch, S., Kieckhefen, P., Heinrich, S., Müller, M., Schönherr, M., Kleine Jäger, F.: CFD-DEM modelling of circulation frequencies and residence times in a prismatic spouted bed, Chemical Engineering Research and Design, (2018), in press.