Fluid trajectories in a stirred vessel of non-Newtonian liquid using positron emission particle tracking

The technique of positron emission particle tracking (PEPT) enables the position of a small radioactive particle tracer to be detected many times per second and its trajectory followed. The technique was employed to determine the flow field of viscous non-Newtonian CMC solutions inside a 290 mm diameter vessel using a 600 mum almost neutrally buoyant tracer. Two axial flow impellers, Lightnin A410 and A320, were used. Their mixing effectiveness was characterised qualitatively by a study of the flow patterns generated within the vessel, and quantitatively by a statistical analysis of the trajectories of the Lagrangian tracer tracked over a test period of about an hour. Zones of effective fluid agitation as well as zones of stagnation and poor mixing were clearly identified. Analysis of particle trajectory-length distributions showed that an increase in agitation speed gave rise to a wider trajectory distribution in the tank. Both agitators behaved predominantly as radial flow impellers as the fluid viscosity was increased. This feature seems to be specific to highly viscous fluids. The ratio of the mean fluid-circulation velocity and the impeller tip velocity is proposed as an index of agitation. (C) 2000 Elsevier Science Ltd. All rights reserved.