Measurement of Turbulent Flow in Standard Jar Test Apparatus

Abstract

Although the jar test has long been used in coagulation and flocculation, there is little available information on vessel hydrodynamics. Vessel-average parameters such as the root-mean-square velocity gradient, \iG¯, have traditionally been used for hydraulic simultude between the jar test and the full scale. With the recognition that vessel-average parameters may not truly represent hydraulic conditions within a vessel, there is a need to characterize jar test hydrodynamics. Analysis of turbulent flow in a standard jar test apparatus was completed using a two-dimensional laser doppler anemometer. Three dimensional velocity measurements were used to determine the general flow field, dissipation rates, and turbulent length scales within the jar. Substantial variations in turbulent characteristics were found to occur within the vessel. Scaling relations were developed for various turbulent parameters. Values for scaling constants were found to be different than those reported for the Rushton turbine, which is by far the most studied mixing impeller. For example, “scaling constants” for local turbulent dissipation rates in the impeller zone were found to be in the order of five times less than the Rushton turbine. The result is significant because many researchers have accounted for inhomogeneties in mixing characteristics within the jar test apparatus by using findings from available data on the Rushton turbine.