Effect of confinement size on flame macrostructures and thermoacoustic instability in a centrally-staged burner

Flame macrostructures and thermoacoustic instabilities of swirl flames in a centrally-staged burner fueled with methane at atmospheric pressure are experimentally investigated by varying the stratification ratio (SR) and global equivalent ratio (ϕ_glo), covering three different confinement diameters:100 mm (D=100 mm), 140 mm (D=140 mm), and 180 mm (D=180 mm). At first, the flame macrostructures are investigated and discussed under a short flame tube (200 mm). Three flame types, i.e., stratified flame (S-flame), V-shaped flame (V-flame), and lifted flame (L-flame), are observed in test conditions. A smaller confinement diameter could strengthen the degree of flame impinges upon the wall and cause a more vital outer shear layer (OSL) flame. The ratio of CH* Chemiluminescence intensity near the wall to the global of the detached flame is larger than the attached flame in D=100 and 140 mm, and in S-flame, it decreases with the increase in SR. Thermoacoustic instabilities are then investigated. The result shows that the D=180 mm has the best thermoacoustic stability among the three cases. There is a smaller amplitude of D=140 mm than 100 mm, and almost all conditions reach the limit cycle. Furthermore, more vital oscillation with multi-mode is found in D=100 mm. Finally, Simplified thermoacoustic network analysis is proposed to provide an insight into the origin of thermoacoustic instabilities in three confinement sizes.