Mixing of Supersonic Jets in a RBCC Strutjet Propulsion System

The Strutjet approach to Rocket Based Combined Cycle (I&CC) propulsion depends upon fuel-rich flows from the rocket nozzles and turbine exhaust products mixing with the ingested air for successful operation in the ramjet and scramjet modes. It is desirable to delay this mixing process in the air-augmented mode of operation present during take-off and low speed flight. A scale model of the Strutjet device was built and tested to investigate the mixing of the streams as a function of distance from the Strut exit plane in simulated sea level take-off conditions. The Planar Laser Induced Fluorescence (PLIF) diagnostic method was employed to observe the mixing of the turbine exhaust gas with the gases from both the primary rockets and the ingested air, The ratio of the pressure in the turbine exhaust to that in the rocket nozzle wall at the point of their intersection was the independent variable in these experiments. Tests were accomplished at values of 1 .O (the original design point), 1.5 and 2.0 for this parameter and images of the degree of mixing taken at 8 locations downstream of the nozzle exit plane. The results illustrate the development of the mixing zone from the exit plane of the strut to a distance of about 18 equivalent rocket nozzle exit diameters downstream (18"). These images show the mixing to be confined until a short distance downstream of the nozzle for a single nozzle geometry set. The lateral expansion is more pronounced at pressure ratios of 1.0 and 1.5 indicating that mixing with the ingested air flow would be likely to begin at an L/D of approximately 1 downstream of the nozzle exit plane. Of the pressure ratios tested in this research, a value of 2.0 delays the mixing until 2" downstream and is the best value at the operating conditions considered.