Experimental assessment of the lean blow-off in a fully premixed annular combustor

The behaviour of the flame in an annular combustor with multiple bluff-body injectors with swirl was investigated to provide insights into lean blow-off (LBO) mechanisms when flames interact. Two different configurations, with 12 and 18 burners, and various bulk velocities and equivalence ratios were tested. Flame shape and main features were studied by means of 5 kHz OH* chemiluminescence imaging and the stability limits were identified and compiled into stability regime diagrams. As the equivalence ratio of the mixture was reduced the individual flames would first exhibit a transition from a stable "W-shape" state to a stable "V-shape" state before becoming unstable close to extinction. In the 18-burner configuration LBO was characterised by random detachment and re-stabilisation of the flames over multiple burners across the chamber, until complete lift-off. In the 12-burner configuration the flame anchors on a few burners in azimuthally symmetric locations, making the overall flame less prone to global extinction. Finally, the stability curves were computed using a correlation based on the Damkholer (Da) number and compared to single burner configurations. The beginning of the blowoff transient was found to be similar to the LBO condition for a single burner in the 12-burner setup, while the 18-burner configuration was less stable for all the conditions investigated. However, it was found that correlations based on single burner extinction data do not fully work for the extinction of interacting flames. The results provide insights into the blow-off of realistic gas turbine engines and can be used for validating models of such processes.