Flow structures of a lobed mixer and effects of streamwise vortices on mixing enhancement

Abstract

Large-scale vortices downstream of a lobed mixer are investigated experimentally using a nanoparticle-based planar laser scattering experimental system. The results are contrasted with additional measurements of a flat plate and a convoluted plate. Three streamwise vortices form downstream of the lobed mixer due to the pressure difference between the peak and trough regions of the trailing edge. This is not the case for the convoluted plate, where the streamwise structures are suppressed and do not appear downstream. Comparison of vortex sizes indicates that streamwise vortices contribute almost 80% of mixing enhancement, which is substantially higher than what could be expected from the increase of the interfacial surface area, especially in the far field of the present compressible mixing layer. It is also quite different from the effects of streamwise vortices on the mixing enhancement in incompressible mixing layers. That no streamwise vortices appear downstream of the convoluted plate indicates that the pressure difference around the corners of the peak and trough regions in the lobed mixer plays an important role for the appearance of the streamwise vortices. Three-dimensional views of the streamwise vortices show that a significant amount of fluid is entrained from the lower stream to the upper stream, resulting in the development of the streamwise vortices for the lobed mixer. The interaction of the streamwise and spanwise vortices then leads to the formation of a large number of small-scale vortices. These two mechanisms enlarge the interfacial surface area of the two streams greatly and substantially improve the turbulent mixing prominently. In addition, the presence of vortex clusters and T-shape vortices in the mixing layer indicate that they are common topological structures in such flows.