Ignition Probability and Lean Ignition Behavior of a Swirled Premixed Bluff Body Stabilized Annular Combustor

An experimental investigation was performed in a premixed annular combustor equipped with multiple swirl, bluff body burners to assess the ignition probability and to provide insights into the mechanisms of failure and of successful flame propagation. The experiments are done at conditions that are close to the lean blow-off (LBO) limit, and hence, the ignition is difficult and close to the limiting condition when ignition is not possible. Two configurations were employed, with 12 and 18 burners, the mixture velocity was varied between 10 and 30 m/s, and the equivalence ratio (⁠$ϕ$⁠) between 0.58 and 0.68. Ignition was initiated by a sequence of sparks (2 mm gap, 10 sparks of 10 ms each) and “ignition” is defined as successful ignition of the whole annular combustor. The mechanism of success and failure of the ignition process and the flame propagation patterns were investigated via high-speed imaging (10 kHz) of OH^* chemiluminescence. The lean ignition limits were evaluated and compared to the LBO limits, finding the 12-burner configuration is more stable than the 18-burner. It was found that failure is linked to the trapping of the initial flame kernel inside the inner recirculation zone (IRZ) of a single burner adjacent to the spark, followed by localized quenching on the bluff body probably due to heat losses. In contrast, for a successful ignition, it was necessary for the flame kernel to propagate to the adjacent burner or for a flame pocket to be convected downstream in the chamber to grow and start propagating upward. Finally, the ignition probability (P_ign) was obtained for different spark locations. It was found that sparking inside the recirculation zone resulted in $Pign∼0$ for most conditions, while P_ign increased moving the spark away from the bluff body or placing it between two burners and peaked to $Pign∼1$ when the spark was located downstream in the combustion chamber, where the velocities are lower and the turbulence less intense. The results provide information on the most favorable conditions for achieving ignition in a complex multiburner geometry and could help the design and optimization of realistic gas turbine combustors.