Confined coaxial nozzle flow with central lobed mixer at different velocity ratios

In a continuing effort to study the mixing enhancement by streamwise vorticity, time-average velocity measurements of turbulent, confined, coaxial nozzle flows with a central four-lobe forced mixer were obtained by a two-component fiber-optic laser-Doppler anemometer at a Reynolds number of 5.1 × 104 (based on the baseline circular nozzle diameter Di = 30 mm and bulk mean velocity Ur of the two streams at 1.7 m/s). Ratios of the annular mean to the core mean velocity (?) were kept at 0.4 1.1, and 2.0, respectively. Results showed that the mixing processes were greatly enhanced by the formation and interaction of the azimuthal and streamwise vorticities. In particular, the deformation and the subsequent stretching of the azimuthal vortices shed at the nozzle walls by the streamwise vortices played a crucial role in the mixing processes. The higher-speed stream appeared to determine the form of mixing downstream. At ? = 2.0 and 1.1, the generation of the streamwise vorticity by the lobes and that from the gap between the lobe nozzle peaks and the inner annular wall enhanced the mixing between the two coflowing streams as well as with the surrounding still fluid. At ? = 0.4, the generation of vorticity was by only the geometry of the lobe, which shed streamwise vorticity that was confined radially within each lobe as the flow traveled downstream. Mixing between the central stream and the surrounding fluid was better than that between the two coflowing streams. For all of the cases considered, streamwise vortices were dissipated within 6Di from the nozzle exit plane. Finally, attempts were made to identify the role of streamwise and azimuthal vorticities using the inviscid vortex dynamics.