Turbulent mixing of unpremixed reactants in stirred tanks.

Abstract

The turbulent mixing process between two liquid streams in a standard tank stirred by a Rushton turbine has been studied. Experimental measurements of concentration and segregation (fluctuating concentration) have been made for both reacting and non-reacting flows. For the non-reacting case, one stream was marked with a fluorescent dye; the local concentration was measured using a fluorescence technique and a bifurcated fiber optic probe of custom design. Measurements were taken at two axial-radial planes within the tank. In the reacting case, the second-order reaction between sodium hydroxide and hydrochloric acid was studied, and urinine acted as a fluorescent indicator which became non-fluorescent as the reaction proceeded. Numerical studies of the mixing in the laboratory-scale vessel were made. FLUENT, a general-purpose fluid flow modelling program, was used to simulate the flow within the tank. This program uses a k-epsilon closure of the turbulent momentum equations. The program was modified to allow the inclusion of a segregation balance equation. Using this segregation balance technique, the turbulent species balance equations were solved. The results of these simulations agreed with the experimental measurements in all regions except the region near the entrance jets, where the model could not adequately predict the fluid behavior. This study has successfully predicted the behavior of reacting fluids in a bench-scale turbulently mixed stirred tank by the implementation of a segregation balance throughout the entire domain.