Dynamics of a Transversely Excited Swirling, Lifted Flame: Part I — Experiments and Data Analysis

This paper is the first of two parts, and describes measurements of the response of a transversely forced, single nozzle, swirling flame. This study is motivated by combustion instabilities coupling with azimuthal combustor modes. Two different forcing configurations are applied, where the flame/nozzle are located at a pressure node and anti-node, respectively. High speed velocimetry and chemiluminescence measurements were made of the forced and unforced flow in multiple orthogonal planes, revealing both the axial and azimuthal development of the unsteady flow. Spectra and azimuthal mode decompositions of these data show the dominance of the m = 1 helical mode in the unforced flow. Depending upon the nature of the forcing, the flow response at the forcing frequency can be dominated by the axisymmetric, m = 0 mode, or the m = 1 mode. These results clearly show that the dominant fluid mechanic structures exciting flames during transverse instabilities varies from nozzle to nozzle, depending upon the phase characteristics of the acoustic excitation field. Part II of this paper uses these time averaged and fluctuating measurements as inputs to a flame response model of the unsteady global heat release fluctuations.