Identification of flame dynamics in a technically premixed combustor with compliant fuel injection

This work investigates the response of a turbulent, technically premixed methane air flame to flow perturbations, with a focus on how fuel injector impedance affects system dynamics. The flame transfer function (FTF) is determined via system identification (SI) applied to time-series data from forced, high-fidelity large-eddy simulations (LES) and is validated against experimental mean flame shape and position. A Multiple Input, Single Output SI approach is used to separately identify the flame’s response to fluctuations in premixture velocity and equivalence ratio. The simulated premixture velocity FTF agrees well with the premixed experimental FTF, confirming that the simulations have a reasonable capture of flame dynamics. However, the airline fluctuation results show poorer agreement with the technically premixed measurements, indicating that injector impedance plays a significant role in those experiments. A low-order acoustic network model (LOM) is employed to evaluate the system transfer matrix, incorporating injector impedance along with the extracted transfer functions. This allows the injector impedance and the effects of fuel-line aperture and fuel-flow perturbations to be inferred. The results show that fuel injector impedance is significant even in the cold flow measurements, and that its combined effect with the flame’s response to fuel flow perturbations substantially influences the dynamics of the reacting case. These findings highlight the importance of ensuring injector acoustic stiffness across the full operating range in technically premixed systems, or alternatively, of characterizing the response to fuel flow perturbations and accounting for their influence on the measured flame dynamics.