Measurement of heat transfer and mixing enhancement by chaotic advection: Experimental methods

The enhancement of heat and mass transfer is key to designing energy-efficient industrial devices with the lowest environment impact, a challenge for sustainable development and energy transition. Several approaches can be used, either by extending the transfer surface and/or by acting on the fluid flow to increase the heat transfer rate, in passive or active manners. In this Review, the focus is placed on one type of passive technique, known as chaotic advection, which generates unsteady and irregular fluid motion at relatively low flow rates, with negligible additional energy input, thus enhancing mixing and mass transfer. Chaotic advection is used in a wide range of applications and fields, including fluid mixing, heat exchangers, and microfluidics. In this context, experimental methods offer a direct means of observing, quantifying, and unraveling the complexity of fluid dynamics coupled with heat transfer and ultimately finding effective strategies for intensifying exchanges. The main purpose of this review paper is to summarize experimental techniques used in the literature, both local and global, enabling an in-depth examination of the interplay between chaotic advection, heat transfer, and mixing efficiency. To this end, a compilation of methods for visualizing and quantifying transfer mechanisms is detailed in this Review. They give access to local velocity and scalar fields and help depict chaotic transfer mechanisms. Several global parameters and performance coefficients are also defined, providing valuable information on overall thermohydraulic efficiency. The advantages and drawbacks of the different experimental techniques presented are discussed. Finally, this Review also discusses some promising experimental techniques that have not yet been used in the literature, in the context of chaotic exchangers, but hold the potential to deepen our understanding of both external and internal mixing and thermal behavior of such systems.