EFFECT OF PREHEATING ON FLAME STRUCTURE OF A SWIRL STABILIZED COMBUSTOR WITH POROUS INSERT TO CONTROL THERMOACOUSTICS

Lean premixed (LPM) combustion is a common strategy in the turbine industry for power generation to reduce emissions of nitric oxides and other pollutants. LPM combustion tends to produce thermoacoustic instabilities under specific conditions. Previously we have shown that an appropriately designed ring-shaped porous insert located on the dump plane can mitigate thermoacoustic instabilities in LPM swirl-stabilized combustion for a range of operating conditions, and explained results based on time-resolved flow-field measurements. In this study, experiments are conducted at higher inlet air temperatures than used before, and the flame structure in the combustor without and with porous insert is investigated for the first time using time-resolved OH planar laser-induced fluorescence technique operated at 10 kHz. Large pressure oscillations in the fuel-air mixing tube demonstrate the existence of thermoacoustic instabilities without the porous insert. The pressure oscillations diminish with the porous insert, which is attributed to the changes in the flow field and flame structure.