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This numerical study investigates the combustion modes in the second stage ofa sequential combustor at atmospheric and high pressure. The sequential burner(SB) features a mixing section with fuel injection into a hot vitiatedcrossflow. Depending on the dominant combustion mode, a recirculation zoneassists flame anchoring in the combustion chamber. The flame is locatedsufficiently downstream of the injector resulting in partially premixedconditions. First, combustion regime maps are obtained from 0-D and 1-Dsimulations showing the co-existence of three combustion modes: autoignition,flame propagation and flame propagation assisted by autoignition. These regimemaps can be used to understand the combustion modes at play in turbulentsequential combustors, as shown with 3-D large eddy simulations (LES) withsemi-detailed chemistry. In addition to the simulation of steady-statecombustion at three different operating conditions, transient simulations areperformed: (i) ignition of the combustor with autoignition as the dominantmode, (ii) ignition that is initiated by autoignition and that is followed by atransition to a propagation stabilized flame, and (iii) a transient change ofthe inlet temperature (decrease by 150 K) resulting into a change of thecombustion regime. These results show the importance of the recirculation zonefor the ignition and the anchoring of a propagating type flame. On thecontrary, the autoignition flame stabilizes due to continuous self-ignition ofthe mixture and the recirculation zone does not play an important role for theflame anchoring.

Combustion regimes in sequential combustors: Flame propagation and autoignition at elevated temperature and pressure