Enhancement of laminar mixing by an anchor impeller with rotationally reciprocating motion

Abstract

Rotationally reciprocating mixing, in which a plate impeller slowly rotates back and forth for only half a rotation, shows excellent fluid mixing performance at Re > 40. On the other hand, since vertical flow disappears at Re < 10, two-dimensional fluid mixing proceeds in the horizontal cross section of a cylindrical vessel, producing segmented mixing regions with a pair of central poor mixing zones. In this study, we investigated the usefulness of an anchor-type impeller in the expectation of enhancing two-dimensional fluid mixing under a laminar regime by utilizing the fluid flow through the central clearances. First, we examined the effect of the clearance size on the spreading behavior of tracer particles by numerical simulation. Time series of Poincaré sections are used to classify the tracer particles depending on their spreading behavior, which is then quantified by newly defined mixing ability. It is then elucidated that even a narrow central clearance could avoid the segmentation of the mixing region, while a vast central clearance produces additional isolated mixing regions in the clearance due to stable vortex flows. The tracer particles could uniformly spread in the entire circular cross section by increasing the impeller width and adjusting the flow rate in the central and wall clearances without segmentation. Furthermore, the experimental observation of streaklines could also demonstrate that the uniformity of the fluid mixing performance is improved considerably.