Experimental study of the effect of air split ratio on thermoacoustic instability in a centrally-staged swirl burner

This study focuses on one of the critical design parameters of lean premixed (LPM) combustors-the air split ratio (ASR) between the pilot and main stages. The effect of the air split ratio on thermoacoustic instability is experimentally studied based on a centrally staged model combustor. It is found that the thermoacoustic instability is suppressed with the ASR higher than 20%. The acoustic modes of ASR lower than 20% are dominated by the main stage mode, while the others are controlled by the pilot stage mode. The time-averaged flame shapes and the flame dynamics are processed and discussed. With the increase in ASR, the pilot flame becomes longer, whereas the main flame becomes shorter. Meanwhile, the pilot and main flames become less separated. Moreover, the flame in the outer shear layer (OSL) is extinguished, which is the key to suppressing the thermoacoustic instability. The flame with lower ASR exhibits large-scale periodic axial motion of flame dynamics. The flow fields of ASR = 15% (unstable) and ASR = 30% (stable) are analyzed and discussed to help reveal the differences in thermoacoustic instability between the two cases. It is found that the primary recirculation zone is smaller with ASR = 30%. Higher vorticity and strain rate distributions found in the OSL are believed to quench the flame there. The present study illustrates that a higher air split ratio improves the thermoacoustic stability, which is helpful in designing LPM combustors