Interaction among flame structures and thermoacoustic instabilities in a centrally staged combustor

Abstract

The centrally staged combustor is an effective way to reduce pollutant emissions. However, combustion instability is one of the prominent and unavoidable problems for lean premixed combustion. The present paper presents an experimental investigation of combustion instability, flame dynamics and emissions in a three-stage central-graded lean combustion system with strong coupling of inner and outer premixed flames. The flame structure was captured by OH* chemiluminescence images using a high-speed camera. The effects of stratified ratios (SR) under different thermal powers on the thermoacoustic oscillations, flame dynamics and emissions are analyzed. The results indicate that the effect of stratification ratio on thermoacoustic instability, flame dynamics, and emission characteristics is significantly different at various thermal powers. Limit circle oscillations are more likely to occur under high thermal power conditions, and an increase in the stratification ratio leads to a shift in the flame structure from an M-shaped, two-branch stratified flame structure to a V-shaped structure. At low thermal power, a change in the stratification ratio does not lead to a shift in the flame structure, and heat release is enhanced with an increase in the stratification ratio and accompanied by a thermoacoustic state that becomes unstable. In this study of central stratified combustion featuring tightly coupled inner and outer flames, the thermoacoustic instability is closely associated with the dynamic characteristics of the flames. Three unstable combustion mechanisms are identified and confirmed. The interactions between the inner and outer flames, the main shear layer flame and the wall, as well as the inner and outer shear layer flames, are the primary causes inducing instability in the unstable V-shaped flames, the M-shaped flames, and the two-branched stratified flame, respectively. Emission measurements show that under low thermal power conditions, the out-enriched flame primarily leads to elevated CO emission levels, with a limited impact on NO emissions. Conversely, the enrichment of the inner flame contributes significantly to an increase in NO emissions.