The supersonic combustion ramjet, or scramjet, is an air-breathing engine designed for cruise at Mach number of 5 and above. The high-speed flow within the combustor poses a major issue with regard to combustion, because the air and fuel must mix and ignite within a time frame of typically less than 1 millisecond. This makes flame anchoring and efficient combustion very difficult. Moreover, due to the elevated air stagnation temperatures at flight of high Mach number, thermal management becomes a big challenge. This study addresses these three topics using a new experimental setup.
It was found that it is possible to alter the flame stabilization mode by distributing the fuel between two fuel injectors, one in the cavity and one upstream of the cavity. When a sufficiently high fuel flow rate in the cavity led to flame stabilization in the fuel jet’s wake, and a more confined combustion zone. Shadowgraph imaging showed that injection of the fuel from the cavity floor at a sufficient momentum penetrated the shear layer over the cavity and enabled flame propagation upstream of the cavity. It was concluded that the heat release distribution and pressure profile in the combustor could be significantly influenced by the fuel injection distribution through its impact on the flame stabilization mode.
In addition to hydrocarbon gaseous fuel (ethylene), simulated synthetic gas (mixture of hydrogen and methane) was used. Syngas is a product of an endothermic chemical reaction between water and hydrocarbon fuel, called steam reforming, enabling up to 3 times higher heat sink than fuel cracking. It was found that the simulated syngas did not burn on its own in the supersonic chamber due to the relatively long ignition delay of the methane-hydrogen mixture compared to ethylene.
Forced acoustic excitation was investigated as a method for mixing enhancement in scramjet engines for the first time. It was found that acoustic forcing at frequencies of below ~100 Hz during transition between 2 combustion modes led to a higher probability of the more efficient combustion mode. This led to an average pressure-rise of 14% compared the non-excited baseline. Analysis of the chemiluminescence and pressure data suggested that the acoustic forcing excited heat release oscillations at ~10 Hz at the cavity shear layer, which were enhanced further downstream leading to a more efficient combustion.