Comparison of Scramjet Engine Performance in Mach 6 Vitiated and Storage-Heated Air

To investigate the sensitivity of combustion to the test gas, an H2-fueled scramjet engine was tested at a Mach 6 flight condition with the air supplied by a combustion heater (V mode) and a storage heater (S mode). The fuel self-ignited without the assistance of igniters in the V mode. However, self-ignition was difficult in the S mode. The easier ignition with vitiated air was caused by radicals supplied from the combustion heater. The combustion behavior was also affected by the test air, which suggests that the combustion was not fully mixing-controlled. As the fuel flow rate increased, the combustion changed from a weak mode, delivering a lower thrust, to an intensive mode, with a higher thrust. Gas sampling showed that the weak combustion was caused by autoignition in the boundary layer on the engine walls. In the intensive combustion mode, the flame was anchored near the backward-facing step on the sidewalls. However, the flame partially detached from the step on the top wall in the combustor. The detached flame may make the combustion kinetically controlled to produce the sensitivity to the test air. (Author)