Strut Fuel Injector Penetration and Mixing Enhancement for a Dual-mode Scramjet

To meet the needs of future high-speed, air-breathing aerospace applications, large-scale dual-mode scramjet engines are being developed. These engines tend to have large combustor cross sections where flush wall injectors are not adequate for delivering the fuel to the core flow. In these applications, struts are typically used to span the gap, however, these intrusive devices add drag to the internal flow, and have high thermal loads to manage. In this study a tip injector was designed to penetrate fuel above the strut into the core flow of the combustor in order to limit the exposure of mechanical surfaces to the flow. Since the flowfield surrounding the tip injector is vastly different from that of a flush wall injector, this study provides a detailed investigation into those physics with both computational and experimental results. Background Oriented Schlieren (BOS) and in-stream gas sampling was used to visualize and measure the flowfield and mixing. Both time-averaged and instantaneous images were reported, and the gas sampling data was correlated to the BOS image data showing that a calibration of the BOS was possible in order to extract quantifiable information about the mixing. Comparing the results of the strut tip injector to a flush wall injector showed that the strut has a reduction in mixing, and a benefit in the jet penetration.

The flowfield of a dual-mode scramjet fuel injector is affected by the combustion process, which increases the static pressure. The experiments conducted for this study, simulated the combustor pressure by backpressuring the flowpath. The mixing results show differences in the behavior between the strut and flush wall injector due to boundary layer differences. The jet penetration from the strut is entirely dependent on the jet-to-air dynamic pressure ratio, Q because it is far from the wall relative to the wall boundary layer thickness.

The specific case of a backpressure induced shock interacting with the injector bow shock is also considered. Instantaneous BOS images of the flush wall injector under these conditions show an increase in turbulence in the mixing jet, implying that the shock enhances the mixing rate. The results also show that the shock has no impact on the jet penetration for the strut injector.

This study provides the first detailed investigation into a strut tip injector, and the first examination of a strut injector in a backpressured flowpath. The results indicate that for a strut injector the jet penetration can be accurately predicted over the range of dual-mode scramjet flight conditions.