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This paper proposes and compares two nonlinear time-domain models of self-excited thermoacoustic oscillations of laminar premixed flames. Both models are hybrid formulations, where the flame and its immediate vicinity are resolved with a reactive flow simulation, while the acoustic field is modeled with a low-order model that is coupled to the reactive flow simulation. Firstly, a flame model based on the fully compressible Navier–Stokes equations is investigated. In this case, the flame simulation is coupled to the low-order model via the characteristic wave amplitudes at the inlet boundary. Secondly, the flame is resolved with a low Mach number reactive flow simulation. In order to include two-way thermoacoustic feedback, this flame model is coupled with an acoustic network model via the global heat release rate and the fluctuation of the axial velocity at a reference position upstream of the flame. A bifurcation analysis using the plenum length as bifurcation parameter is conducted. Both models exhibit complex nonlinear oscillations and are in good agreement with each other. Therefore, we conclude that the coupling of a linear acoustic model and a nonlinear flame model via reference velocity and global heat release rate is sufficient to accurately capture thermoacoustic oscillations of the configuration investigated. This implies that the most important nonlinearities can be attributed to hydrodynamic effects and flame kinematics. Furthermore, the study corroborates that premixed flames respond predominantly to fluctuations of the upstream flow velocity. [ABSTRACT FROM AUTHOR]

Premixed flames are velocity sensitive, i.e., they react to a velocity perturbation at the burner mouth, say, with fluctuations in heat release rate. Unsteady heat release generates acoustic waves that travel back from the flame to the burner mouth, where they modulate the velocity and thereby close an intrinsic thermoacoustic (ITA) feedback loop. The present paper demonstrates that corresponding ITA eigenmodes are in general important for the dynamics and stability of premixed combustion systems. It is shown that the complete set of eigenmodes of a combustor test rig should be interpreted as the sum of acoustic and ITA eigenmodes. A procedure is presented which allows to distinguish between eigenmodes that may be considered as acoustic modes driven by the flame, versus those resulting from ITA feedback (but influenced by the acoustic properties of the combustor). This procedure is based on a factorization of the dispersion relation of the thermoacoustic model. Differences between the acoustic and intrinsic eigenmodes of a combustor test rig, in particular the corresponding mode shapes, are discussed. The paradoxical observation that increased acoustic losses at the boundaries may destabilize a combustion system is explained as an instability of the dominant ITA mode. [ABSTRACT FROM AUTHOR]