Effect of Temperature Change on Geometric Structure of Isolated Mixing Regions in Stirred Vessel

The present work experimentally investigated the effect of temperature change on the geometric structure of isolated mixing regions (IMRs) in a stirred vessel by the decolorization of fluorescent green dye by acid-base neutralization. A four-bladed Rushton turbine was installed in an unbaffled stirred vessel filled with glycerin as a working fluid. The temperature of working fluid was changed in a stepwise manner from 30°C to a certain fixed value by changing the temperature of the water jacket that the vessel was equipped with. The step temperature change can dramatically reduce the elimination time of IMRs, as compared with a steady temperature operation. During the transient process from an initial state to disappearance of IMR, the IMR showed interesting three-dimensional geometrical changes, that are, simple torus with single filament, simple torus without filaments, a combination of crescent shape and circular tori, and doubly entangled torus. 1. Introduction Stirred vessels are frequently used to homogenize different substances, conduct chemical reactions, and enhance mass transfer between different phases. These vessels are versatile and they are available in a wide variety of sizes and impeller configurations for use in industrial processes. Although turbulent flow is efficient for mixing, laminar mixing is required in some cases such as for high-viscosity fluids and shear-sensitive materials. Koiranen et al. [1] proposed specific principles for effective mixing of highly viscous liquids or shear-sensitive materials in laminar flow mixing regimes. In these regimes, global mixing is inefficient due to the existence of isolated mixing regions (IMRs). Makino et al. [2] characterized IMRs in a stirred vessel using radial flow impellers and found that IMRs consisted of various Kolmogorov-Arnold-Moser (KAM) tori. Ohmura et al. [3] reported the existence of KAM tori as island structures in a phase-locked orbit that has a rational relation of the time period between the primary and secondary circulation flows. Noui-Mehidi et al. [4] found that the mechanism of IMR disappearance could be described by the formation of a period-doubling locus in the physical space when using a six-blade Rushton turbine impeller. Hashimoto et al. [5] successfully visualized a three-dimensional structure of thin filaments spirally wrapping around the core of toroidal region. They formulated and estimated the relation between mixing conditions and filament numbers and/or wire turns. The elimination of IMR at low Reynolds numbers has also been studied extensively. Lamberto et