Combustion instabilities in propulsion systems are often manifested through high amplitude pressure oscillations that can severely compromise performance and even lead to mechanical failure. Such instability arises from the development of large-scale coherent structures and their breakdown into fine scale turbulence that can alter the flame structure and affect turbulent mixing. When in phase with the pressure, the modulated heat release rate fluctuations can drive the system to the point where it reaches a limit cycle.
Using high fidelity CFD, the present investigation describes the occurrence of combustion-driven instability in bluff-body stabilized turbulent premixed flames, in which there is dynamic coupling between the preferred hydrodynamic modes and the acoustics of the duct. A URANS approach is adopted, using a second moment closure to solve for the anisotropic turbulent Reynolds stresses. This is combined with the Bray-Moss-Libby (BML) combustion model with a modified reaction rate closure that aims to capture the changes in the flame surface density due to external flow perturbations. Two different geometries are used for the investigation: the first is a laboratory-scale planar bluff-body flameholder ; and the second is the well-known Volvo afterburner experiment . Four different conditions are presented to illustrate the various self-excited instabilities that can appear depending on the coupling mechanisms between the different fluid-mechanical and acoustic phenomena.
For the planar geometry, a self-sustained hydrodynamic instability induced by large-scale coherent structures occurs under fuel-lean conditions. When the equivalence ratio is increased, the flame becomes strongly wrinkled due to velocity perturbations arising from the Kelvin-Helmholtz (K-H) instability of the shear layer. The combustion heat release becomes modulated such that its phase relationship with the pressure fluctuations is sufficient to trigger thermoacoustic instability. For the Volvo experiment, symmetric shedding takes place and an acoustic mode of the duct is excited when the mixture strength is lean. At higher equivalence ratio, the flame is perturbed by the hydrodynamic instabilities of the most amplified mode. Small scale structures can be seen in the vicinity of the flameholder, and larger fluctuations in the flame occur further downstream. No appreciable feedback from the acoustic modes is present to sustain combustion instabilities.