Numerical study on development of particle concentration profiles in a curved microchannel

The functional section of a microseparator/classifier is a semicircular microchannel whose downstream end bifurcates to separate/classify the particles in a slurry [Ookawara, S., Higashi, R., Street, D., Ogawa, K., 2004a. Feasibility study on concentration of slurry and classification of contained articles by microchannel. Chemical Engineering Journal 101, 171-178 and Ookawara, S., Higashi, R., Street, D., Ogawa, K., 2004b. The Influence of channel depth on the performance of a microseparator/classifier. Kagaku Kougaku Ronbunshu 30, 135-141.]. Previous numerical studies, based on an Eulerian-Eulerian approach, showed how the particle lift force was an indispensable factor for the separation/classification [Ookawara, S., Street, D., Ogawa, K., 2004c. A practical application of the Euler-granular model to a microseparator/classifier. In: Proceedings of the Fifth International Conference on Multiphase Flow, CD-ROM, #206.]. The present numerical study, by consistently employing the Eulerian-Eulerian approach, extensively examines the development of particle concentration profiles and the effects of feed concentration at various cross-sections in a curved microchannel for De=30(Re=450). The necessary arc length for particle concentration profiles to be fully established increases with the particle decreasing size. Particles become most concentrated at the centers of secondary Dean vortices. The dimensions of concentration region depend on the particle size and the feed concentration. In spite of the small particle relaxation time in water and the laminar flow nature, steep shear rates in a microchannel cause a collision interval comparable to the relaxation time of the particles that can be separated. To characterize the effect a newly defined Stokes number is based on the shear-induced particle-particle collisions in liquid laminar flow. A concentration efficiency is also defined as the normalized ratio of the maximum concentration to the feed concentration and it is approximately 1.0 below a Stokes number of 0.1. However, beyond the Stokes number of 0.1 the concentration efficiency decreases linearly as the log of the Stokes number increases independently of the particle size. This is because the particle to particles collision in a concentrated slurry adversely influences the efficiency of the separator.