Active control of combustion instabilities in gas turbine burners

This paper gives an overview of open and closed loop active control methodologies used to suppress symmetric and helical thermoacoustic instabilities in an experimental low-emission swirl-stabilized gas turbine combustor The controllers were based on fuel (or equivalence ratio) modulations in the main premixed combustion or alternatively in secondary pilot fuel. For the main premix fuel supply two methods of fuel injection modulations were tested: symmetric and asymmetric injection. The tests showed that the closed loop asymmetric modulations were more effective in the suppression of the symmetric mode instability than symmetric fuel excitation. Symmetric excitation was quite efficient in abating the symmetric mode as well, however at a certain range of phase shift the combustion was destabilized to an extent that caused blow out of the flame. Using premixed open loop fuel modulations the symmetric instability mode responded to symmetric excitation only when the two frequencies matched. The helical fuel injection affected the symmetric mode only at frequencies that were much higher than that of the instability mode. The asymmetric excitation required more power to obtain the same amount of reduction as that required by symmetric excitation. Unlike the symmetric excitation which destabilized the combustion when the modulation amplitude was excessive, the asymmetric excitation yielded additional suppression as the modulation level increased. The NOx emissions were reduced to a greater extent by the asymmetric modulation. Secondary fuel injection in a pilot flame was used to control low frequency symmetric instability and high frequency helical instability. Adding a continuous flow of fuel into the pilot flame controlled both instabilities. However modulating the fuel injection significantly decreased the amount of necessary fuel. The reduced secondary fuel resulted in a reduced heat generation by the pilot diffusion flame and therefore yielded lower NOx emissions. The secondary fuel pulsation frequency was chosen to match the time scales typical to the central flow recirculation zone which stabilizes the flame in the burner Suppression of the symmetric mode pressure oscillations by up to 20 dB was recorded.