Hybrid CFD/ low order modeling of thermoacoustic limit cycles

This paper proposes and compares two nonlinear time-domain models of self-excited thermoacoustic instabilities of laminar premixed flames. We resolve the flame and its immediate vicinity with a CFD simulation. Simultaneously, the acoustic field is modeled with a low-order model that is coupled to the CFD over the inlet boundary condition. The first model is based on a fully compressible CFD solver. Here, the low-order model describes the plenum of the combustor and is coupled via the characteristic wave amplitudes using the newly developed Characteristic Based State-Space Boundary Conditions. This reduces the computational costs and allows to change the plenum length of the combustor without changing the computational grid. The second model resolves the flame with an incompressible CFD solver. In order to include the thermoacoustic feedback this model is coupled on-line with an acoustic network model over the global heat release rate and an acoustic reference velocity according to the Rankine-Hugoniot equations. A bifurcation analyses using the plenum length as bifurcation parameter is conducted. Both models exhibit complex nonlinear oscillations. A comparison in terms of a root mean square (RMS), dominant frequency, power spectrum and phase portraits show that both models are in good agreement.