Revealing Chaotic Advection Dynamics in the Chaotic Mixing of Non-Newtonian Fluids within a Stirred Tank Employing a Twisted-Punched Impeller

Chaotic advection provides a natural way to enhance mixing efficiency by stretching and folding the fluid particles in laminar flows. This investigation addresses the development of a rigid–flexible impeller using a torsional-punched structure as the flexible connection pieces (TPRFI system), aiming at accelerating the chaotic mixing in non-Newtonian fluids by chaotic advection. Experiments and computational fluid dynamics (CFD) were employed to reveal the chaotic behavior. The results show that TPRFI is clearly superior to RFI, PRFI, and TRFI in terms of largest Lyapunov exponent (LLE), NTm, and WV values and exhibits a better capability in improving mixing efficiency, especially for the near-wall region. Specifically, at Re values nearing 500, the LLE value in TPRFI exceeds that of RFI, PRFI, and TRFI by approximately 22.23%, 18.69%, and 10.01%, respectively. This is due to the large velocity gradient generated by TPRFI that makes the dynamic flow more complex and disordered, enabling a better energy distribution. Both Eulerian and Lagrangian analyses demonstrated that the distance of two particles whose initial positions are very close to each other changes continuously in TPRFI, which confirmed the existence of chaotic advection flow. The chaotic motions provide a natural way to enhance mixing efficiency by stretching and folding the fluid particles in laminar flows. The study provides a new insight into chaotic advection-enhanced mixing and enlightens the application of the torsional-punched impeller in laminar systems.