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[Display omitted] • Combined with RGB brightness and quantitative area analysis, the mixing performance of the stirred reactor was evaluated. • There is an inherent axial mixing limit in traditional stirred reactors under laminar flow conditions. • The DSO mixing system exhibits enhanced mass transfer efficiency and overall mixing performance. • The flow pattern from radial flow to axial flow is adjusted by changing the interface evolution. Efficiently realizing laminar flow mixing of non-Newtonian fluids is a key challenge faced by conventional stirred reactors. In this study, an innovative strategy is proposed to regulate the flow pattern from radial to axial flow by changing the interface evolution of the flow field, so as to solve the problem of uneven mixing materials in laminar flow. An innovative combination of the RGB (Red, Green, Blue) brightness analysis and quantitative analysis area method was used to quantitatively describe the mixing performance of the three stirred reactor in the laminar flow. We found that the coloring area ratio of the dual shaft off-centred mixer (DSO) mixer was approximately 165.7 % and 93.8 % higher than that of the single shaft central (SSC) and single shaft off-centred mixer (SSO), respectively. Results showed that the DSO mixer can directionally adjust the stable interface of the flow field, and then obtain the ideal velocity distribution and flow pattern. Importantly, it is found that the mixer has an inherent axial mixing limit in the laminar flow. Increasing the Reynolds number can only shorten the time for the mixing system to reach the steady state, and cannot further improve the axial transport capacity of the system. Compared to the SSC and SSO systems, the DSO mixer demonstrated a reduction of nearly 20 % in overall mixing time and power consumption. Through comparative analysis of pressure distribution and Poincaré cross section, the DSO mixing system can switch chaos oscillation and realize the "globally chaotic mixing" from "locally chaotic mixing". Remarkably, this work highlights the potential of DSO mixer as a simple and efficient system for laminar flow mixing applications, such as polymerization processes, biological fermentation. [ABSTRACT FROM AUTHOR]

• Squeezing-induced chaotic advection mixing was proposed. • Mixing performance in rectangular stirred tanks was investigated. • Segregation region was eliminated in rectangular stirred tanks. • Mixing time decreased as Length-to-Width ratio increased. • Fluid mechanism in rectangular stirred tanks is similar to that in eccentric ones. The phenomenon of chaotic advection mixing in laminar stirred tanks, particularly when handling high-viscosity fluids, represents a common and challenging issue in various chemical and biochemical industrial processes. The effectiveness of these mixing processes is often constrained by the stretching, folding, and squeezing of fluid particles. Previous research efforts have predominantly concentrated on the design and/or optimization of impellers and operation modes to increase the stretching and folding of fluid particles. However, the mechanism of triggering chaos through the squeezing of fluid particles remains comparatively less explored. This study endeavors to address the intricate problem of laminar mixing by proposing the utilization of unbaffled rectangular stirred tanks. Via a combined approach of experimental and computational investigations, we illustrate that the implementation of rectangular stirred tanks yields significant improvements in chaotic mixing within laminar flow regimes. This type of tank not only enhances the convective transport of fluids but also distorts or completely eliminates the segregation regions by squeezing. Remarkably, as the tank L / W ratio increases from 1 to 2.5, the observed asymptotic area coverage from a frontal perspective escalates from approximately 55 % to about 85 %. Concurrently, the dimensionless mixing time decreased significantly from 1950 to 350. Of particular note, macroscopic homogeneous mixing is observed in stirred tanks with L / W ratios of 2 and 2.5 at R e = 29, with dimensionless mixing times of 550 and 350, respectively. This work highlights the potential of rectangular stirred tanks as a simple and cost-effective system for laminar mixing applications. Furthermore, it also hints at the broader applicability of the approach in various industrial mixing contexts, such as the suspension of floating granules, entrainment of air, and others. [ABSTRACT FROM AUTHOR]