Flow characteristics of a confined co-axial nozzle with a central lobed mixer at different velocity ratios

Measurements of turbulent, confined, coaxial nozzle flows with a central four-lobe forced mixer have been obtained by a two-component fibre-optic laser-Doppler anemometer at a Reynolds number of 5.1 x 10⁴ (based on the baseline circular nozzle diameter, D_j = 30 mm, and bulk mean velocity, U_r, of the two streams at 1.7 m/s). Ratios of the annular mean to the core mean velocity (λ) 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 normal and streamwise vorticity. In particular, the deformation and the subsequent stretching of the normal vortices shed at the nozzle walls by the streamwise vortices played an crucial role in the mixing processes. The higher speed stream appeared to determine the form of mixing downstream. At λ = 2.0 and 1.1, the generation of the streamwise vorticity was found to originate from two sources: the geometry of the lobe and the gap between the lobe nozzle peaks and the inner annular wall. The former was important to enhance the mixing between the two co-flowing streams at both velocity ratios while the latter was more important for λ, = 2.0 to the mixing between the annular flow and the surrounding still fluid. At λ = 0.4, only the geometry of the lobe shed streamwise vorticity which was confined radially within each lobe as the flow traveled downstream. Mixing between the core and the surrounding fluid was better than that between the two co-flowing streams. For all the cases considered, streamwise vortices were dissipated within dD_jfrom the nozzle exit plane.