Effects of swirl numbers on the unsteady characteristics of pilot flame in a triple-swirler staged combustor

Abstract

This paper studies the effects of the inner and outer pilot swirlers on the flame structures and thermo-acoustic instabilities in the triple-swirler staged combustors under various inner and outer pilot swirl numbers. We obtain energetic modes in the combustor's velocity and the OH radical mass fraction fields, utilizing the proper orthogonal decomposition (POD) technique. With the increment of the pilot inner swirl number, the vortex shedding mode and flame radial propagation mode for the flame fluctuation is enhanced, and the axial propagation mode is weakened. Nevertheless, the increased pilot outer swirl number primarily impacts the vortex shedding mode's enhancement while positively influencing the other flame fluctuation modes. Specifically, the vortex shedding in the inner shear layer is the leading factor affecting the thermo-acoustic instability in the combustion chamber. Nevertheless, coupling this mode and the pressure oscillation weakens the area downstream of the central recirculation zone. Thus, due to the comprehensive impacts of the vortex shedding mode, spatial distribution, and the premixed flame area proportion, the impact of this mode's energy ratio on the thermo-acoustic instability increases non-linearly with the increased pilot swirl number.