Changes to Flame Instability Characteristics with Hydrogen Blending and Diluents in a Swirl-Stabilized Flame

Abstract Thermoacoustic combustion instability is one of the most challenging issues for gas turbine engines. This study focuses on the impact of both hydrogen blending in the fuel and diluent blending into air on the flame dynamics during self-excited thermoacoustic combustion instability in a piloted swirl-stabilized flame. Hydrogen is blended into natural gas up to 40% by volume and CO2 and N2 diluents are blended into air at different levels such that the lowest mole fraction of oxygen is 15%. Experiments are run in a variable-length combustor such that several six different oscillation modes are observed. High-speed chemiluminescence imaging is used to measure flame oscillations at each condition over a range of fuel and diluent compositions. The resulting flame oscillations are interpreted in the context of several fundamental flame parameters, particularly the mixture's extinction strain rate. All but one mode aligns closely with a small range of extinction strain rates, allowing us to connect the mode oscillation with the kinematics of the flame. The remaining mode exists over a wide range of extinction strain rates; analysis of high-speed imaging allows us to understand the reason for this unique behavior. In particular, the dynamics of the pilot flame relative to the main flame seems to be a key contributor to the ability of this mode to strongly oscillate. The paper concludes with a discussion of the implication of these findings and the role of the pilot flame in the stability of swirl-stabilized flames.