Dynamic Mode Decomposition of the Combustion Flow Field in a Scramjet Combustor with a Cavity Flameholder

Abstract

To clarify the forced ignition instability and combustion instability by the micro-rocket torch using modal analysis, dynamic mode decomposition (DMD) was applied to the OH* chemiluminescence images measured by previous supersonic combustion experiments. Open-type cavities with length-to-depth (L/D) ratios of 4 and 8 were installed in the constant area section of the scramjet combustor. In addition, at 5.0 mm downstream from the cavity step, the micro-rocket torch was installed at the bottom of the cavity. DMD analysis was conducted using a snapshot-based method for OH* chemiluminescence images. In two types of cavities, DMD analysis clarified the forced ignition instability and combustion instability in the cavity installed with the micro-rocket torch. For example, oscillations based on low frequencies, which are under 1000 Hz, were formed by injector flame feedback from the fuel and micro-rocket torch and flow fluctuations around the cavity. Moreover, oscillations based on high frequencies, which are over 1000 Hz, were formed by the incoming turbulent boundary layer. It is therefore speculated that in low frequency regions, a major part of the forced ignition instability and combustion instability occurred via combustion gas injection from the micro-rocket torch.