Combustion Characteristics in Scramjet Combustor Operating at Different Inflow Stagnation Pressures

In this study, combustion processes in a cavity-based scramjet combustor are experimentally studied based on the inflow conditions of different stagnation pressures (, 1.3, and 1.6 MPa), which correspond to a Mach 6 flight at trajectories of different dynamic pressures of 0.25, 0.34, 0.43 atm, respectively. Three combustion modes are identified according to flame distribution, flowfield structure, and wall pressure. Cases at the same equivalence ratio and combustion mode with different inflow stagnation pressures are selected to compare flame characteristics. For each inflow condition, flame structure, combustion intensity, and coherence between upstream and downstream flames are analyzed based on CH* chemiluminescence and schlieren imaging measurements. Combustion instability is examined according to statistical results, frequency domain and spatial distribution. It is found that the linearly increased inflow stagnation pressure causes irregularity variance in the combustion oscillation spectra, indicating the quasi-linearly enhanced combustion induces complicated aerodynamic and thermodynamic variances. Chaos of the combustion oscillation is quantitatively evaluated by a classical nonlinear analysis. The dynamic mode decomposition analysis presents that the high-frequency oscillation induced by the self-sustaining shear-layer instability exist mainly in the combustor upper region while the low-frequency oscillation that could be caused by the injector–flame feedback is predominant in the cavity region. In addition, thermo-acoustic couplings, which are in connection with these two kinds of oscillations, have been analyzed. It is further revealed that the variance of the shock waves caused by the backpressure destroys the existing thermo-acoustic feedback related to the high-frequency oscillation, which is the main reason for the chaotic phenomena shown in the transitional state between the scramjet mode and the ramjet mode.