Dynamic Response of Stratified Flames to Acoustic Excitation in a Multi-Swirler Model Combustor

Abstract

Flame response is a key element in predicting thermoacoustic instabilities in gas turbine combustors. Flame dynamic response of single swirling flames to acoustic excitation was well studied in the past decades, while the unsteady dynamic of multi-swirling flames, such as stratified flames, is not fully reported. This paper presents dynamic response of stratified flames in a multi-swirler combustor which includes a main stage and a pilot stage. The stratified flame contains an outer main flame and an inner pilot flame. The overall Flame Transfer Function (FTF) of the stratified flame is extracted during the experiment. High-speed camera and high-frequency Particle Image Velocimetry (PIV) are used to capture the evolution of the flame and flow structure. Experimental results show the overall flame transfer function of the stratified flame features several discrete peaks and valleys in a narrow frequency range which is slightly different with a typical simple swirling flame. The main flame is stabilized at the inner shear layer region of the main flow while the pilot flame settles at a position where turbulent flame speed equals to the local pilot flow speed. The effect of the acoustic driving on the topology structure of the stratified flame is not apparent. Proper orthogonal decomposition of the stratified flame shows a wave of alternating positive and negative values across the flame indicating flame fluctuations are in axial modes. Proper orthogonal decomposition of the multi-swirling flow reveals coherent structures are formed in the shear layer of the main flow which dominates the stratified flame response.