Mixing Effects of Pylon-aided Fuel Injection Located Upstream of a Flameholding Cavity in Supersonic Flow

The Air Force Research Lab (AFRL), Propulsion Directorate, Wright-Patterson Air Force Base, Ohio is conducting ongoing research into propulsive efficiency in supersonic ramjet (scramjet) technology. One current focus of this research is the usefulness of flameholding cavities implemented in the supersonic hydrocarbon-fueled combustion chamber. Because good mixing and proper cavity-core interaction lead to more efficient combustion, methods of optimizing fuel/air mixing both within and upstream the cavity are investigated. In a cooperative effort with the Air Force Institute of Technology (AFIT), AFRL provided a supersonic wind tunnel outfitted with an existing cavity design. A circular injection port was placed upstream of the cavity, and a series of three pylons (medium, tall, wide geometries) were in turn fitted just upstream of the port to improve mixing and penetration of the fuel into the core airflow. The main goals of this experiment were to characterize the mixing ability of injected fuel with the core flow as it propagated downstream of the pylon and to analyze the effects, if any, of this mixing strategy on cavity flow and overall efficiency compared to a no pylon case. The experiment was a non-reacting mixing study. Measurements were obtained from pressure transducers, Planar Laser-Induced Fluorescence (PLIF), and Mie scattering visualization. Of the three pylon geometries tested, the wide pylon provided the best combination of fuel penetration into the freestream (135% increase) and mixing potential. The taller pylon provided more penetration but incurred a large loss penalty. All pylons lifted the fuel from the injector to prevent flashback, and all pylons demonstrated equivalent or better mixing potential than the flat reference at various tunnel conditions