Flow regimes, mixing and reaction yield of a mixture in an X-microreactor

In the present work, experimental flow visualizations and numerical simulations are carried out jointly to investigate how flow regimes affect the reaction yield in an X-microreactor. The X-microreactor is fed with an aqueous solution of ascorbic acid into one of the inlet channels and an aqueous solution of methylene blue and hydrochloric acid into the other which, upon reacting, produce a colorless reaction product that can be directly measured in experiments. Different kinetic constants are considered. At low Reynolds number, Re, the two reactant streams remain stratified in the mixing channels due to their different densities and mixing is promoted only by diffusion. By increasing the Reynolds number, the engulfment regime occurs, which is characterized by the presence of a single vortical structure in the confluence region and in the outlet channels, enhancing mixing thanks to convection. The reaction yield is studied both numerically and experimentally showing a remarkable agreement between the results. In the stratified regime, the reaction yield decreases with the Reynolds number due to a decrease in residence time. Instead, with the onset of the engulfment regime, the reaction yield starts to increase with Re because enhancement in the mixing degree between the streams overcomes the reduction of the residence time. However, due to the opposite behaviors of mixing efficiency and residence time with Re, the reaction yield does not monotonically increase with Reynolds number within the engulfment regime, but it reaches a maximum and then slightly decreases. The reaction yield obtained in the X-microreactor is significantly larger than that of a T-microreactor having the same cross-section geometry and operating in the same conditions.