Combustion modes and oscillation characteristics in a rocket-dominated scramjet

To investigate the combustion modes and the corresponding oscillation characteristics in various operational states of a rocket-dominated scramjet, ground direct-connect experiments are carried out. CH* chemiluminescence images, time-frequency analysis, and pressure distributions are employed. The impact of factors, including the MFR (mass flow rate ratio of secondary fuel to the rocket) and the relative position of the secondary fuel injection to the rocket plume, on the combustion and flow process in the rocket-dominated scramjet is obtained. The results indicate that: (i) Three combustion modes are identified in the rocket-dominated scramjet. The cavity stabilized combustion mode is observed in the “rocket-off” case. The combustion exhibits the cavity assisted plume stabilized combustion mode while the MFR is 0.7 in the “rocket-on” case. Reducing the MFR to 0.3 transforms the combustion to the plume stabilized combustion mode. (ii) The cavity assisted plume stabilized combustion mode intensifies the low-frequency oscillations (below 150 Hz), particularly in the fuel combustion region and the region influenced by backpressure, while suppressing oscillations of higher frequency (beyond 300 Hz) presented in the cavity stabilized combustion mode. (iii) The plume stabilized combustion mode effectively inhibits low-frequency oscillations, including reducing both the oscillation frequency range and the oscillation energy. However, the plume stabilized combustion mode based on a high-mass-flow rate rocket exhibits oscillations of higher frequency (ranging from 300 to 400 Hz) in the fuel-burning region at the tail of the plume. (iv) Positioning the fuel injector upstream of the plume enhances the overall flow stability, which significantly suppresses low-frequency oscillations in the cavity.