Passive mixing control via lobed injectors in high-speed flow

The near-field flow, and mixing characteristics of nonreactive lobed fuel injectors in subsonic and transonic airstreams were studied experimentally. Three alternative injector geometries were explored, two of which had lobed shapes. These lobed injectors mixed gas-phase injectant (nitrogen) and coflowing air to different extents, straining fluid interfaces due to streamwise vorticity generation. The experiments were conducted in a "trisonic" wind tunnel, with coflowing airstream Mach numbers ranging from 0.4 to 1.2. Visualization of the downstream evolution of,the injectant was achieved via planar laser-induced fluorescence imaging of acetone seeded in the nitrogen. Comparisons of near-field flow evolution, mixing properties, and scalar dissipation and strain rates were made among different injectors for the different experimental flow conditions. It was observed that lobed injector geometries produced greater near-field mixing as well as higher effective interfacial strain rates than nonlobed injectors in the subsonic (Mach 0.4) regime, although at higher subsonic Mach numbers mixing increases were accompanied by a reduction in the effective strain rate. Flow evolution observed in the experiments was also compared with numerical simulations of vorticity evolution and rollup, with good correspondence.