Flow and mixing characteristics of σ-type plate static mixer with splitting and inverse recombination

A new type of demountable plate static mixer composed of σ-shaped elements was developed, which was designed to perform multi-lamination of two fluids through systematic splitting and inverse recombination. Its flow and mixing characteristics have been investigated by conducting the decolourisation reaction of iodine with sodium thiosulfate and CFD analysis. Visualized mixing patterns demonstrate that splitting and inverse recombination progress as designed in the flow of Re less than 10, where the number of mixing elements required for complete mixing increases with Re. At Re > 10, secondary flows are generated due to the two- and three-dimensionally curved portion in the mixer, distorting the systematic splitting and recombination. They contribute to accelerating mixing progress through the deformation and elongation of the interfaces between the fluids to be mixed. The number of elements for complete mixing becomes maximum at Re of 10–20 and then decreases with further increasing Re. Mixing progress in the flow direction and cross-sectional mixing patterns obtained by CFD analysis show that splitting and inverse recombination do not perform satisfactorily in a shallow channel, which suggests that the aspect ratio of the mixer channel should be set around one if heat transfer is simultaneously taken into account. Based on the CFD velocity data, fluid particles disposed on the inlet cross-section at a time were traced in the mixer with the aspect ratio of one. Distributions of travelling or residence time of fluid particles reveal that the overall flow behaviour in the mixer approaches the plug flow with increasing Re as well as the number of mixing elements. The friction factor, which was obtained by applying the calculated pressure loss to the Fanning's equation, varies in inverse proportion to Re at Re < 10 and its variation becomes smaller with further increasing Re.