Mechanistic understanding of size-based fiber separation in coiled tubes

Understanding separation of poly-disperse particle suspensions according to the particles size is of great importance to product quality. Previous experimental studies of suspension flow through coiled tubes report different results for spherical and elongated particles, e.g., larger and thus heavier elongated particles are faster than smaller ones.

We use Euler–Lagrange simulations, as well as experiments, to measure the residence time distribution of fibers with different size in coiled tubes with different curvatures. Fluid flow through the coiled tubes was simulated as toroidal flow, i.e., the pitch of the tube was neglected. Fibers are one-way coupled to the fluid, and their movement in the cross section, as well as their orientation is predicted based on the assumption of an infinitely dilute suspension.

We find that in coiled, dilute suspension flow of fibers the ratio of particle settling velocity to the secondary flow speed determines the fiber motion in the tube cross section. For low Reynolds number and thus larger effect of gravitation, fibers are found to concentrate in distinct orbits. Long fibers form flocs propagating through the torus whilst small fibers are well mixed and thus retained in the tube. We found that fiber–fiber interaction and the formation of flocs and not fiber–fluid interaction is key to the size based separation.