Flow characteristics in the curved rectangular channels Visualization of secondary flow

In order to clarify the secondary flow pattern which appears in curved rectangular channels, flow visualization is made in a fully developed laminar flow. The channels have aspect ratios ranging from 0.5 to 2.5, curvature ratios ranging from 5 to 8 and a width of 20 mm. From the experimental results the developing process of secondary flow vortices becomes clear. At a retarded layer near the outer concave wall, additional counter-rotating pairs of vortices are observed. The critical Dean number takes the minimum and maximum values at the aspect ratios of about 1 and 2, respectively. The critical Dean number increases rapidly with decreasing aspect ratio when it is smaller than 1.