Mass transfer and emulsification by chaotic advection

This study characterizes a new mixing process based on chaotic advection for the production of water/oil (w/o) emulsified engine fuel. At low and intermediate Reynolds numbers, in curved pipes, Dean roll-cells induce radial convective mass transfer, a mechanism exploited in the technology of helical static mixers. The succession of bends of alternating curvature planes produces in addition spatially chaotic flow trajectories that enhance the mixing over and above that in the reference helically coiled geometry made of coplanar bends. This feature is assessed in the present work by comparison of the droplet sizes obtained in helical (regular) and alternating (chaotic) static mixers of the same cross section and the same tube length. The coils are assembled from 90 degrees bends, and the chaotic configuration is obtained by turning each bend by +/- 90 degrees with respect to the previous one. To cover a large range of Reynolds numbers [30-350] and capillary numbers [0.1-1], a lipophilic solvent (Butanol) is added to the continuous phase (oil) to decrease the viscosity and a surfactant (Tween 20) is used beyond the critical concentration (covering the whole w/o interface) to decrease the interfacial tension. Drop-size measurements at the exit of the mixer in the chaotic advection configuration show substantial drop-size reduction and tightening in drop-size distribution. Moreover, it is shown that mixing intensification by chaotic advection is almost independent of Reynolds number in the studied range, with a mixing efficiency enhancement around 30%. (C) 2013 Elsevier Ltd. All rights reserved.