Diffusion and flow development in co-flowing microchannel streams

Abstract

A study of the momentum diffusion and flow development of two co-flowing streams in a microchannel is presented. The microchannel has a high-aspect-ratio (20) rectangular cross section with a height of 50 μm and a width of 1,000 μm. A splitter plate is used to obtain fully developed flow in each of two channels prior to the flows merging. Experimental results are obtained illustrating the streamwise variation of the interface between the streams for a range of flow rate ratios ranging from 1 to 9. Numerical simulations were used to determine details of the velocity and pressure fields in the region of momentum mixing. When there is a velocity difference between each stream there is a cross-stream pressure gradient just downstream of the splitter plate that causes the faster-moving stream to expand very rapidly into the slower-moving stream. The faster-moving stream is shown to recirculate into the slower-moving stream, resulting in a stagnation region. The two streams do not mix during this process, although mass diffusion is observed. The merged streams, although unmixed, have a fully developed velocity profile within approximately 4 to 14 hydraulic diameters downstream of the end of the splitter plate, depending on the flow rate ratio. This is shown to be significantly further than predicted using existing entrance length relationships in the literature.