Coupling of isolator flow with heat release rate and equivalence ratio oscillations in the dual-mode scramjet

The dual-mode scramjet engine has become an emerging technology due to its capabilities in running at a wider speed range and a broader altitude range. The present work studies the occurrence of combustion oscillation in dual-mode scramjet. Synchronized time-resolved schlieren imaging (for the isolator), time-resolved CH* and OH* chemiluminescence, and acoustic pressure measurements were used to capture the dynamic interaction between the shock front oscillation in the isolator and combustion processes in the combustor. The oscillations of the shock front in the isolator were obtained using Schlieren imaging, while CH* characterized heat release rate oscillations, and the normalized ratio (Z) of OH* to CH* was employed to represent qualitatively equivalence ratio fluctuations. A characteristic shock front oscillation frequency around 335 Hz was identified. Both Fast Fourier Transform (FFT) and a novel Principal Correlation Decomposition (PCD) method were employed and reveal that the region upstream of the strut exhibits the strongest CH* oscillations, linked to shock front oscillations in the isolator. The first PCD mode suggests that heat release rate oscillations in this region are out of phase with those downstream of the strut, likely due to recirculated fuel transport between the regions. Additionally, the Z signal oscillates primarily at 335 Hz downstream of the combustion chamber, indicating that shock front oscillations induce equivalence ratio fluctuations. Conversely, the pressure oscillations at the cavity's bottom, with a frequency of 1715 Hz, are attributed to flow-induced acoustic pressure oscillation. In summary, this work demonstrates simultaneous time-resolved measurements of multiple time-resolved physical quantities together with the employment of signal processing techniques, the PCD analysis in particular, providing insights in revealing correlations between fundamental processes in a scramjet engine environment.