Separation By Design - Towards Simulation Guided Engineering of Coiled Channels for Precise Particle Separation

Coiling a straight channel around an axis, i.e. forming a torus, gives rise to additional fluid motion, changing the trajectories of suspended particles. Most significant differences between straight and coiled channel flow are (i) the deflection of the velocity maximum to the outer bent, and (ii) the occurrence of secondary motion within the cross-sectional plane caused by centrifugal forces. Consequently, suspended particles are subject to a non-trivial interplay of various hydrodynamic forces, most notably drag and lift, resulting in an interesting orbiting behavior of suspended particles. Most important, such behavior can be used for size-based particle separation or fractionation (Yoon 2009, Di Carlo 2009). Previous studies found the process to be sensitive to the Reynolds number, and the channel’s curvature, as well as the channel cross sectional shape. However, most studies followed a purely experimental pathway, and captured the particle motion by imaging methods (Guan 2013, Martel 2013). Unfortunately, experimental methods do not allow the precise measurement of certain flow details (e.g., hydrodynamic forces acting on the particles) and hence often fail to identify the true mechanism that leads to, e.g., particle separation. To highlight on such mechanisms, we follow a bottom-up approach focusing on the hydrodynamic forces acting on particles of different size. We use Euler-Lagrange simulations to quantify the relevance of these forces, and how they affect particle orbiting behavior.

The flow field was directly simulated using OpenFOAM® (Greenshields, 2016). Particles were then placed into the developed fluid flow field. Fluid-particle forces were calculated using CFDEM®coupling (www.cfdem.com), and the particle motion was calculated using LIGGGHTS® (Kloss 2012). In a sub-set of numerical experiments, we performed computationally extremely demanding particle-resolved suspension flow simulations to guide our choice of proper hydrodynamic interaction models with a special focus on lift forces. An array of particle-unresolved simulations was performed to study cases of different channel aspect ratio, curvature, and Reynolds number. Forces acting on the particles were recorded and carefully analyzed, i.e., they were correlated with certain features of the fluid flow field, as well as the particles’ position. Finally, we condense our findings in general guidelines for the design of coiled channels to separate particles of given size.

Acknowledgements

The authors gratefully acknowledge support from NAWI Graz by providing access to dcluster.tugraz.at.

References
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