In this paper, we study the impact of injecting diluent gas into the outer recirculation zone of a swirl stabilized premixed methane/air flame on the flame shape and combustion dynamics. Two modes of instability were observed, a low frequency mode at 8???12 Hz which corresponds to a flame flickering mode in the outer recirculation zone and a high frequency thermo-acoustic instability at 250???275 Hz. Furthermore, air, N 2 and CO 2 were injected into the outer recirculation zone, at mass flow rates of at 1% and 3% compared with the main flow of premixed methane/air. The diluent addition shifts the outer recirculation zone flickering mode of instability to higher equivalence ratios and completely damps the thermoacoustic instability for the current experimental conditions. Changing the diluent composition from air to N 2 and to CO 2 improved the extent of instability mitigation displaying the influence of diluent gas on the chemical kinetic in the ORZ region. Furthermore, the impact of the different diluent compositions on mitigating the combustion instability scaled according to the extinction strain rate of the diluted mixture in the outer recirculation zone. ?? 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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