2D-unsteady chaotic flows for mixing two different non-Newtonian fluids: optimization and flow control

Abstract

Mixing of non-Newtonian fluids has been widely studied, yet numerical investigations of the simultaneous mixing of two different shear-thinning fluids inside chaotic mixers remain limited. In this work, we numerically investigate, for the first time, the mixing of two shear-thinning fluids with distinct rheological properties inside an unsteady 2D two-rod chaotic mixer. The mixer domain is divided vertically, with each half containing a different fluid. Two stirring protocols that generate chaotic advection are compared: continuous modulation (CM) and alternating modulation (ALT). Mixing performance is evaluated using the mixing index, while energy consumption is analyzed separately. The results show that the choice of protocol depends on the trade-off between mixing time and energy consumption. The CM protocol is significantly faster, reducing mixing time by 29% to 39% compared to ALT. However, the ALT protocol is more energy efficient, saving between 9% and 25% of energy. Furthermore, the final produced fluid remains shear-thinning, and its rheological behavior is governed by the more viscous fluid of the two (the one with the higher power-law index). These findings provide useful guidance for selecting an appropriate stirring protocol in industrial applications, depending on whether mixing speed or energy efficiency is the primary objective.