Combining two orthogonal secondary flows to enhance the mixing in an annular duct

• Mixing enhancement technique for laminar flows in an annular tube.
• Combination of two orthogonal secondary flows in radial and tangential directions.
• Scalar homogenisation to show the mixing efficiency.
• Chaotic advection spotted through Lagrangian analysis and Poincaré sections.

Laminar flows in annular tubes can be encountered in industrial situations. In order to improve static and continuous mixers or multifunctional heat exchangers, several ways have been investigated. In the present article, two already well-known kinds of passive enhancement devices are set on a laminar annular flow and numerically studied. Alternate elliptic wall deformations are applied to the external wall as the internal wall is swirled. These geometrical perturbations both create secondary flows that are in mutually orthogonal directions. Their combination is numerically evaluated in this paper and is found to lead to a significant increase of the mixing. A nearly uniform mixing is even achieved for a reduced tube length as shown by considering scalar mixing for different inlet configurations. The analysis, based on Lagrangian tracking and Poincaré sections, shows that the mixing enhancement can be explained by the formation of zones of chaotic advection in the flow.

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