Effects of baffle on flow structure and cyclic variation in stirred tanks with Rushton turbine

Abstract

In order to reveal the effect of baffles on flow structure and cyclic variation stirred by Rushton turbines, particle image velocimetry experiments were performed in baffled and unbaffled stirred tanks. In a baffled tank, the peak value of time-averaged velocity obviously increases, the high velocity region enlarges significantly, and the fluctuation of fluid elements near the turbine increases significantly. In the 20° phase plane of the baffled tank, the high swirl strength region slightly enlarges, but the peak of phase-resolved swirl strength is lower. The peak value and high value regions of turbulent kinetic energy are larger in the baffled stirred tank at various phase planes. Using quadruple proper orthogonal decomposition, the flow field is decomposed into a small-scale part, a transition part, a large-scale part, and a mean part with different spatial scales. Adding the baffle, the mean part with largest spatial scale obviously increases (it is favorable for mass transfer at different liquid levels) and the transition part, which mainly includes the random fluctuation of a turbulent cascade, slightly enhances (it is conducive to diffusion). The main influence of cyclic variation for various blade passages is the flow with large-scale, and the standard deviation of time coefficients decreases by 3%–4% for the mean part and the large-scale part by adding the baffle.