Flame stabilization and thermoacoustic instability during operating condition modulations: Roles of pilot and main flames

Operating condition modulation plays a key role in the startup process of practical gas turbine applications. The present article investigates the operating condition modulations of a centrally staged swirl burner regarding the flame stabilization and thermoacoustic instability, in which the roles of pilot and main flames have been distinguished. The global characteristic of thermoacoustic instabilities during operating condition modulations has been explored in a wide operating range by incrementally increasing equivalence ratios in the pilot stage ( ϕ p) and main stage ( ϕ m). Transitions of stability regimes under self-excited conditions have been observed with increasing ϕ m and show difference with ϕ p = 1.1–1.3. Two modes of stable flame stabilizations under un-excited conditions have been identified with ϕ m variation, including pilot and staged flames. Their main difference is the varied attachment of flame roots along pilot shear layers and main shear layer. At self-excited conditions, three different instability modes have been revealed with increasing ϕ m, i.e., pilot mode, hybrid mode, and staged mode. The periodic interaction between the pilot flame and the main stage stream is shown to be necessary for sustaining a large-scale convective motion of pilot flame to trigger strong thermoacoustic instabilities in pilot and staged modes. For the hybrid mode, the periodic attachment of the main flame root has suppressed the convective motion and, thus, caused weakened thermoacoustic couplings. The present investigation reveals the unique and various interactions between pilot and main flames during operating condition modulations, which can assist startup design in practical centrally staged combustion systems.