Experimental study on combustion instability and flame dynamics in a multi-nozzle staged combustor

Lean premixed prevaporized multi-nozzle staged combustors are a good choice for maritime power, but their operation is prone to thermoacoustic instabilities. In this study, we investigated flame dynamics and instability mechanisms of a staged combustor operated under preheated atmospheric conditions with varying pilot and main stage equivalence ratios. High-speed OH* chemiluminescence (CL) imaging and dynamic pressure measurements are used, with the data analyzed through phase-space reconstruction, spectral proper orthogonal decomposition (SPOD), local Rayleigh index, and phase-difference probability density functions. The results reveal a nonlinear transition behavior characterized by a Hopf bifurcation, where increasing the main equivalence ratio at high pilot equivalence ratios induces an abrupt transition from stable operation to intermittent oscillations and high-amplitude limit cycles. Time-averaged OH* CL images show that the main flame stabilization mode evolves from interaction-zone anchoring to a dual-root mode consisting of a main flame inner root (MIR) and outer root (MOR), while the pilot flame weakens under certain high-load conditions. SPOD and phase-averaged analyses demonstrate that higher oscillation amplitudes amplify the harmonic content, lateral expansion, and phase differences between flame regions. Furthermore, coupling analysis further indicates that thermoacoustic driving is dominated by the pilot flame, assisted by the MIR, whereas the MOR consistently acts as a damping region. These findings provide new insights into staged combustor instability and guidance for lean-premixed combustion system design.