Characterization of mixing in upflow solids-contacting clarifier

It has been recognized that flocculation performance can be related to the mixing process. This process has traditionally been described by vessel ave rage parameters such as the root-mean square velocity gradient (G) over bar , which may not represent local mixing conditions within a vessel, particul arly in the impeller vicinity. The analysis of turbulence in the flocculato r allows a more refined specification of flow, energy, and dissipation, whi ch have long been known to be important to the flocculation process. Presen ted is a study to characterize hydrodynamic conditions in the impeller zone of an upflow solids-contacting clarifier using a laser Doppler anemometer. Results were analyzed based on an analogy of the impeller-generated flew t o a swirling radial jet. It was found that mean and fluctuating velocities scaled on the tip speed of the impeller. Use of the swirling radial jet ana logy also allowed the scaling of local dissipation rates. Results compared favorably to other impeller studies and true radial jets, indicating the ap plicability of concepts for the assessment of other flocculation impellers. It was found that local dissipation rates near the impeller were significa ntly higher than the vessel average values. These high values highlight the need for further study of the impact of local mixing conditions on floccul ation performance.