Effects of Additional Cavity Floor Injection on the Ignition and Combustion Processes in a Mach 2 Supersonic Flow

Effects of additional cavity floor injection on the ethylene ignition and combustion processes in a cavity-based scramjet combustor are investigated experimentally in a Mach 2.0 supersonic flow using flame luminosity and CH* (CH radical) spontaneous emission methods and static pressure measurements. Numerical calculation is performed to study the non-reacting flow-field structures prior to ignition. Two injection schemes, including the cavity upstream injection scheme and the combined injection scheme with an additional cavity floor injection, are compared to study the effects of the additional cavity floor injection on the ignition and combustion processes. It is found that there exists an equivalence ratio upper limit for maintaining stable combustion for the cavity upstream injection scheme. As the equivalence ratio further increases, the fuel jet penetration is improved accordingly, and thus, the interaction between the fuel jet and the cavity is weakened, which can lead to the ignition failure and flame blowout during combustion. On the contrary, although the combined injection scheme has a minor effect on combustion enhancement at the same global equivalence ratio, it can also provide a more favorable flow-field environment that enables more successful ignitions and better flame stabilizations. For the combined injection scheme, as the equivalence ratio increases, the initial flame propagations are observed to perform different routines during the ignition process, and the major combustion reaction zone tends to move further downstream the cavity shear layer. It is concluded that the advantages of the combined injection scheme with an additional cavity floor injection are more significant when the equivalence ratio is higher, as well as that the interaction between the fuel jet and the cavity becomes weaker.