Analysis of scalar mixing and turbulent flow in a continuous stirred tank by simultaneous PIV and PLIF

This work deals with the velocity field of the liquid and the scalar transport of a passive tracer in a continuously fed standard configuration stirred vessel under turbulent impeller Reynolds number. The aim of the investigation is twofold. The influence of a liquid jet entering the vessel from the top lid on the fluid flow patterns is assessed first. Afterwards, a finer analysis of the turbulent characteristics of the flow and of the scalar concentration fluctuations is performed. As fed-batch and continuous stirred tanks are widely adopted in chemical and bio-chemical processes, the results of this work provide useful indication for assessing to what extend the current knowledge on batch stirred tanks can be applied to continuous systems. Besides, the results on the turbulent mixing of the tracer provide a very useful benchmark for the Reynolds-averaged scalar transport equations, which are widely adopted for the industrial Computational Fluid Dynamics (CFD) simulation of stirred vessels, but their reliability would be significantly improved by quantitative validations of the turbulent fluctuations modelling. The data are collected by 2D Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF). The results of the first part of the investigation show that the inlet jet affects the fluid flow field with respect to batch conditions. As expected, the variations of the flow field in the upper zone of the vessel are significant, while the impeller pumping action is slightly affected by the liquid jet, as the impeller speed is higher than that of the liquid feed stream. The simultaneous PIV and PLIF measurements in the vertical plane passing through the inlet jet axis allow the evaluation of the Reynolds fluxes and the Reynolds stress components in the axial and radial directions. The results provide insight into the interaction between the jet and the flow features due to the mechanical agitation.