Dual stream axisymmetric mixing in the presence of axial vorticity

An experimental investigation was undertaken to study mixing in a coaxial jet where the inner nozzle is a lobed mixer. Four inner jet nozzle geometries were explored: a baseline circular jet and three different six-lobed nozzles, Also, three velocity ratios of 3:1, 1:1, and 1:3 (inner:outer) were examined. Flow visualizations using a passive scalar were performed using a laser sheet from a Nd:YAG laser, The laser has a 9-ns pulse duration that effectively freezes the flow. Instantaneous cross-sectional images were taken as well as streamwise views. The effects of various parameters such as the interfacial area increase due to the lobed nozzle geometry, the strength of streamwise vortices, and the large-scale structures on the mixing process are evaluated and discussed. Results showed highly enhanced mixing as the strength of the streamwise vortices increased. In addition, the presence of large-scale structures due to the Kelvin-Helmholtz instabilities, and their interaction with streamwise vortices generated bg the lobes were found to be crucial for enhanced mixing, The fraction of mixing enhancement due to streamwise vorticity (relative to mixing enhancement due to increased interfacial contact area) was found to increase as velocity ratio increased. This fraction also increased with downstream distance.