Effects of Ambient Ventilation and Oxygen Enrichment/depletion on Self-Excited Oscillations and Flame Modes in Laminar Flame Driven Rijke Tube

Thermoacoustic instabilities have been one of the major concerns in combustors. Given the relationship between the oscillation behaviour and combustion characteristics, strategies for combustion control have considerable potential for modulating these oscillations. In this study, we experimentally investigated the effects of co-flow conditions on self-excited thermoacoustic oscillations driven by a laminar premixed fuel-rich flame in a Rijke tube, aiming to identify the mechanisms governing changes in oscillation behaviour and to clarify the driving mechanism of the oscillation under laminar flame conditions. The co-flow conditions were varied by changing air co-flow strength and oxygen concentration in the N2-O2 co-flow. Oscillation behaviour was characterised by the oscillation eigenfrequency (feigen) and amplitude. The thermal and combustion characteristics were examined using the tube-end temperature and global OH*, CH* and C2* chemiluminescence signals. Flame modes were identified from high-speed flame images using proper orthogonal decomposition (POD), and the local flame-pressure coupling was quantified using a windowed POD based local coupling coefficient to reveal the oscillation driving mechanism. The nonlinear dynamical properties of the system were analysed using the Wayland method and recurrence analysis. The results demonstrated that the co-flow conditions can modify feigen and suppress self-excited oscillations. The oscillation amplitude was found to be more sensitive to changes in combustion characteristics and strongly correlated with the overall oxygen-fuel relationship. The oscillations induced by laminar flames were primarily driven by the local flame-root fluctuations rather than by global heat release fluctuations. Changes in equivalent average sound speed, originating from variations in temperature conditions, were found to play a dominant role in affecting feigen, with the equivalent-average-sound-speed model predicting feigen within 1 %. These findings support the control of eigenfrequency and amplitude through ventilation and oxygen conditioning in practical combustors.