Ignition of a sequential combustor: Evidence of flame propagation in the autoignitable mixture

Ignition of the second stage in a lab-scale sequential combustor is investigated experimentally. A fuel mixing section between jet-in-cross-flow injection and the second stage chamber allows the fuel and vitiated, hot cross-flow to partially mix upstream of the main heat release zone. The focus of the present work is on the transient ignition process leading to a stable flame in the second stage. High-speed OH-PLIF as well as OH chemiluminescence imaging is applied to obtain complementary planar and line-of-sight integrated information on the ignition. We find experimental evidence for the co-existence of two regimes dominating the chamber ignition, i.e. autoignition and flame propagation. As the mass flow of the dilution air injected downstream of the first stage is increased (i.e. mixing temperatures in the fuel mixing section are decreased), we transition from an autoignition to a flame propagation dominated regime. Hysteresis in the ignition behavior is observed indicating that the first stage in a sequential combustor may be operated at leaner conditions than required for ignition of the second stage. The time traces of integral heat release obtained simultaneously with a photomultiplier tube show distinct features depending on the dominating regime, which is important for high-pressure testing with limited optical access.