  The present article investigates the interactions between the pilot and main
flames in a novel stratified swirl burner using both experimental and numerical
methods. Experiments are conducted in a test rig operating at atmospheric
conditions. The system is equipped with the BASIS (Beihang Axial Swirler
Independently-Stratified) burner fuelled with premixed methane-air mixtures. To
illustrate the interactions between the pilot and main flames, three operating
modes are studied, where the burner works with: (i) only the pilot flame, (ii)
only the main flame, and (iii) the stratified flame (with both the pilot and
main flames). We found that: (1) In the pilot flame mode, the flame changes
from V-shape to M-shape when the main stage is switched from closed to
supplying a pure air stream. Strong oscillations in the M-shape flame are found
due to the dilution of the main air to the pilot methane flame. (2) In the main
flame mode, the main flame is lifted off from the burner if the pilot stage is
supplied with air. The temperature of the primary recirculation zone drops
substantially and the unsteady heat release is intensified. (3) In the
stratified flame mode, unique beating oscillations exhibiting dual
closely-spaced frequencies in the pressure spectrum. is found. This is observed
within over narrow window of equivalence ratio combinations between the pilot
and main stages. Detailed analysis of the experimental data shows that flame
dynamics and thermoacoustic couplings at these two frequencies are similar to
those of the unstable pilot flame and the attached main flame cases,
respectively. Large Eddy Simulations (LESs) are carried out with OpenFOAM to
understand the mechanisms of the time averaged flame shapes in different
operating modes. Finally, a simple acoustic analysis is proposed to understand
the acoustic mode nature of the beating oscillations.
