Transition dynamics of ethylene combustion in a Mach 8 scramjet combustor: From autoignition to flame stabilization

This work delved into the combustion and heat release process in a scramjet combustor under Mach 8 flight conditions. The flow and combustion characteristics were diagnosed using CH* chemiluminescence imaging, schlieren visualization, and pressure measurements. The Scram, Weak dual, and Strong dual modes were successively identified with an increase in combustion heat release. While the Scram mode exhibited smooth combustion transition, the dual modes displayed distinct staged characteristics. The entire combustion evolution process was segmented into five stages: Injection, Ignition, Transition, Boundary layer separation, and Steady combustion. Notably, it took more than 50 % additional time for the intense combustion modes to reach the final steady stage. Comparative analysis demonstrated that the establishment of the strong combustion experienced some stages of the weak combustion, verifying the universal transitional mechanisms in supersonic combustion. During the Boundary layer separation stage, the variation of wall pressure lagged behind the flame morphology, which suggested that the flow and combustion were not synchronized. The supersonic mainstream brought some instability to the steady flame under the Scram mode, while the boundary layer separation upstream of the cavity induced periodic flame flashback. Among the three combustion modes, the Weak dual mode displayed the most moderate flame oscillation, which was beneficial for the stable operation of scramjet.