Concentration fluctuations in a stirred baffled vessel



A stirred vessel is maintained in a quasisteady state by continuously introducing water and a conductive tracer solution and continuously removing the resulting mixture by overflow. Detailed properties of the system are elucidated by measuring tracer concentration changes with a conductivity probe in a volume element of the order of 0.3 cu. mm. By the use of suitable electronic equipment the following statistical concentration parameters are measured in experiments with three diameters of flat-bladed turbines and with a tenfold range of rotational speed: temporal mean, total root-mean-square fluctuation, spectral distribution of fluctuations, and Eulerian micro time scale. Results are interpreted in terms of behavior of time-average and fluctuation measures in three regions, the generation region within the impeller volume, the decay region which starts in the horizontal fluid sheet issuing from the impeller, and the recirculation flow region in the vessel.

Generation of concentration fluctuations within the confines of the impeller is described in terms of a model involving gross mixing, without significant decay, in the wake of each flat blade. Distributions of mean and fluctuating concentrations are suggested to provide a measure of uniformity in the vessel as a whole. The former, in particular, was remarkably constant throughout the vessel.