Damping combustion instabilities with perforatesat the premixer inlet of a swirled burner

摘要

Despite extensive efforts, controlling combustion instabilities in modern gas turbines remains a challenge at the design stage. The strong coupling between unsteady combustion and acoustics that leads to the growth of such instabilities is not yet fully mastered even though acoustics in complex geometries and combustion dynamics of turbulent swirled flames are now reasonably well understood. Comparatively, the effects of the acoustic boundary conditions on the system stability are much less studied. They are nonetheless of prime importance when a global acoustic energy balance is to be written in a combustor, as they determine the acoustic fluxes at the inlets and outlets of the combustor. The present study describes a reliable solution to efficiently control the acoustic properties of a boundary condition upstream of the combustion zone, like the premixer manifold or the inlet of a combustor. Effects of the acoustic reflection coefficient on self-sustained combustion oscillations are investigated and a passive control solution is proposed, using perforated plates with bias flow. Performances of this system are characterized on an existing turbulent swirl-stabilized combustion facility which exhibits strong unstable regimes. Tuning the upstream reflection coefficient leads to strong damping of the main resonant modes of the combustion instabilities, while modifications of the initial geometry and flow operating conditions are minimal. The combustion facility and the design of the control system are first described. Efficient control of the reflection coefficient is then assessed in an impedance tube, with large amplitudes of pressure fluctuations, typical of those encountered in practical systems. The influence of this control method on unstable regimes in the turbulent combustor facility is then presented. Finally the acoustic energy budget is examined and discussed at limit cycles for different values of the burner inlet reflection coefficient:|R|= 0.2 to 0.8.