Effect of fuel–air mixture velocity on combustion instability of a model gas turbine combustor

Abstract

Nowadays, it is easy for unstable combustion phenomenon to develop in a gas turbine that is working in a lean premixed condition. To eliminate the onset of these instabilities and develop effective approaches for control, the mechanisms responsible for their occurrence must be understood. The flame recirculation zone is very important, as it can modulate the fuel flow rate and may be the source of instability, plus its flame structure has a major impact on heat release rate oscillation and flame stabilization. In this study, we conducted experiments under various operating conditions with a model gas turbine combustor to examine the relation of combustion instability and flame structure by OH chemiluminescence. Swirling CH4 - air flame was investigated with an overall equivalence ratio of 1.2 to lean blowout limit and dump plane velocity of 30–70 m/s. Phase-locking analysis was performed to identify structural changes at each phase of the reference dynamic pressure sensor under conditions of instability. At an unstable condition, flame root size varies a lot compared to stable condition which is because of air and fuel mixture flow rate changes due to combustor pressure modulation. After this structural change, local extinction and re-ignition occur and it can generate a feedback loop for combustion instability. This analysis suggests that pressure fluctuation of combustion causes deformation of flame structure and variation of flame has a strong effect on combustion instability. In this study, we observed two types of combustion instability characteristics related to the instability of both the thermo-acoustic and flame vortex type.