Nonlinear mode decomposition of combustion instabilities under various jet-to-crossflow momentum ratios in a hydrogen-rich ram combustor using a deep Koopman network

The excitation mechanism of the thermoacoustic combustion instability (TCI) in a hydrogen-rich ram combustor was investigated experientially, focusing on the effects of jet-to-crossflow momentum ratios on combustion instabilities. The momentum ratios were varied using two orifice diameters: 1.0 mm and 1.4 mm, while maintaining the constant equivalence ratio. Combustion experiments were performed using a high-enthalpy wind tunnel. To capture the transition from combustion noise to the TCI, the equivalence ratio was increased gradually from 1.5 to 2.7 for each orifice diameter. To understand the unsteady initiation process of TCI, the time-resolved OH* chemiluminescence images and combustion pressure were analyzed using a neural network inspired by Koopman theory (Koopman network). The experimental results showed that TCI was excited at higher equivalence ratio for the larger injection orifice. Prior to the initiation of TCI, a low frequency oscillation at less than 400 Hz was observed. The frequency increased with the momentum ratios and finally reached the frequency of the longitudinal oscillation mode of the combustor (440 Hz). Koopman network analysis implied that the low frequency oscillation mode was induced by the oscillation of the shear layer of the hydrogen jets impinging on the wall, which was characterized by the Strouhal number based on the shear layer length. As the momentum ratios increased, shear layer length decreased due to the increase in the penetration height of hydrogen jets. This led to the increase in the frequency of the impinging shear layer oscillation. When the frequency reached that of the longitudinal oscillation mode, the local flame blowout was observed, involving the oscillation of the impinging shear layer. Koopman network analysis also indicated that the oscillation of the impinging shear layer and the local flame blowout induced the TCI.