Passive mixing of co-flowing slugs in grooved microchannels

Abstract

The anisotropic grooves in a microchannel induce spiral circulation around the flow axis. The stretch and fold of the streams result in a passive mixing. This article describes the effectiveness of induced crossflow, arising from the grooves, placed at the floor of the microchannel. The results from numerical simulation show the extent of local mixing achieved at various positions around the groove. The emphasis here is to understand the mixing when the two streams enter the channel, and displace air while flowing alongside each other. Microchannels with slanted grooves and staggered herringbone type grooves are considered here. The FLUENT 6.2 software and the Open FOAM software were used for the simulation. The latter was primarily utilized for tracking of the interface when a liquid slug displaced air from the micromixer. At a Reynold’s number of about 1.0, low velocity pockets could be identified near the two ends of the ridges. These pockets did not support crossflow and mixing. The herringbone type micromixer was found to have the best mixing effectiveness, when the advection dominates mixing. This advantage is mostly lost at the higher values of diffusivity, as a crossflow became less important for mixing. These observations were made when one of the streams is 1.5 times more viscous than the other, and the mixture-rule governed the viscosity of the mixed stream. When the liquid front enters the microchannel for the first time, the front had to displace air from the channel. At first, the upper part of the channel was swept. This was immediately followed by the sweeping of the groove-part in the floor. Higher hydraulic resistance in the grooves resulted in this non-uniform movement of the front. The co-flow of streams could either be continuous over the entire length of the channel, or in the form of slugs that maintained an interface with air. At a particular position in the channel, the extents of mixing were compared for the two modes of co-flow. The extent of mixing was found smaller in the slug, next to the interface with air. Lack of axial dispersion with preceding liquid stream seems to be the reason for this trend.