Velocity-forced nonlinear heat release response of transverse reacting jet in an axial fuel-staged combustion system

Heavy-duty gas turbines integrated with axial fuel staging functionality outperform conventional non-staged systems in terms of controlling nitrogen oxides emissions under high turbine inlet temperature conditions, while maintaining reliable part load operations. Substantial developments have been achieved in the understanding of nitrogen oxide reaction pathways and autoignition-related stabilization mechanisms, whereas limited attention has been paid to thermoacoustic interactions driven by coupled primary-secondary flame dynamics. In particular, flame describing functions (FDF) and underlying mechanisms of a second-stage transverse reacting jet in high-temperature vitiated crossflow remain unknown. To address these problems, here we perform velocity-forced nonlinear heat release response measurements in the absence and presence of crossflow velocity modulations, in conjunction with phase-resolved flame surface density (FSD) characterization and dynamic mode decomposition (DMD)-based representation of complex modal dynamics. Experimentally, we show that high-amplitude velocity perturbations cause the FDF gain to be reduced to some degree with no sign of strong saturation; the transverse reacting jet’s nonlinear response is dictated by non-axisymmetric vortex-flame interactions. The coexistence of two different sources of upstream velocity disturbances – including the primary flame dynamics-induced mono-frequency crossflow modulations and the preexisting velocity fluctuations propagating normally from the secondary injector’s inlet plenum – is observed to promote the secondary flame’s FDF gain reduction, as well as to reduce the characteristic response time. Taken together, these results provide previously unidentified nonlinearity-relevant information, pivotal to improving our understanding of acoustically-constrained interactions between two axially staged flames.