An innovative vortex generator for promoting subsonic and sonic jet mixing

Mixing promoting capability of a pair of tabs with truncated projections located asymmetrically on a rectangular stem fixed diametrically opposite at the exit of a circular nozzle has been investigated experimentally for the Mach numbers of 0.6, 0.8, and 1.0. The centerline pressure decay of the sonic jet at under-expanded conditions corresponding to nozzle pressure ratios (NPR) 2, and 3 was also considered in this study. It is found that the mixing promoting small-scale vortices shed by the tabs appreciably promotes the jet mixing at all Mach numbers. The highest mixing promotion is for the correctly expanded sonic jet, leading to a core length reduction of about 87%. In contrast, for Mach 0.8 and 0.6 jets, the core length reduction achieved is about 85% and 77%, respectively. The favourable pressure gradient at the nozzle exit is found to be of no effect on the mixing promotion caused by the tabs. At NPRs 2 and 3, the core length reduction induced by the tabs is only 85% and 88%, comparable to that at correctly expanded sonic state. The shadowgraph images of the under-expanded sonic jet show that the shock waves in the jet core become weaker due to the enhanced mixing caused by the tabs. Also, the tabs weaken the Mach disc formed in the core of a highly under-expanded sonic jet operating at NPR 4.