(40d) The Simplest Stirred TANK: Structure and Mixing Dynamics in Eccentricaly Agitated Vessels

Eccentric agitation in stirred tank systems has been proposed as a cost effective way to improve mixing performance in laminar and turbulent systems. For example, at some eccentricity values, segregated regions in stirred tanks are drastically minimized, and flow separatrixes are eliminated. The flow structure, and therefore the mixing topology, of eccentrically agitated systems are far more complicated than those observed in concentrically stirred tanks. Manifolds in eccentric flows are highly asymmetrical, and dynamically more complex that concentric flows. Indeed, it is not intuitive to predict, a priori, the flow structure in an eccentric system. In this contribution, we use planar Laser Induced Fluorescent experiments (p-LIF) to visualize the flow structure originated by tracer injections in eccentrically agitated stirred tanks in the laminar regime (Re≈40). Eccentricity is defined in terms of the parameter E; where E=distance between the center line and the axes of rotation divided by the tank radius. Results from LIF experiments conducted at E=0.0 (concentric case), E=0.13, E=0.21, E=0.42, and E=0.50 are presented, and a mixing time is calculated based on image analysis techniques. 

In addition, we use CFD to describe the short term mixing dynamics of tracers injected at different points in eccentrically agitated tanks. In the mixing literature, there is abundant discussion on the relevance of the location of injection points for the achievement of rapid dispersion. However, most of the previous work has been circumscribed to conventional stirred tank geometries, namely stirred tanks agitated concentrically, with typically used impellers. In this contribution, we extend the discussion on the relevance of a proper selection of injection points to eccentrically agitated systems. The manifold of the flow determines the fate of any tracer injected into the system. Therefore, a detailed calculation of this manifold is required to precisely diagnose the effectiveness of an injection point. Through CFD studies of the short term evolution/dispersion of tracer injections, composed of multiple particles, we compare the rate and extent of mixing at four different injection points and three eccentricities values in stirred tanks operated at Re≈4000. The rate of mixing is faster in the vicinity of E=0.42. Counter-intuitively, at E=0.50 the mixing performance is inferior than the one observed at E=0.42.