Impact of turbulence on the coherent flame dynamics in a bluff-body stabilized flame

Coherent flame oscillations generated by large-scale flow instabilities have been shown to significantly influence combustor performance and thermoacoustic instability. This study examines the influence of turbulence intensity on the large-scale dynamics of rod-stabilized flames. Instability in the flow field of a bluff-body stabilized flame, which is a function of mean shear in the flow field, turbulence intensity of the incoming flow, and the location of the flame with respect to the shear layer, manifests as coherent vortex shedding. However, this vortex shedding is not self-excited if the flow is globally stable, as is often the case in reacting flows. In this experiment, time-resolved, three-component velocity measurements from high-speed stereoscopic particle image velocimetry are taken at three turbulent inlet flow conditions and at three bulk flow velocities. To identify whether these instabilities occur, the velocities fields are filtered using a wavelet transform around select spectral bands and then decomposed using proper orthogonal decomposition to extract the most energetic motion in the flow; this filtering method retains any time-dependent, intermittent behavior. The results show that vortex shedding is intermittent and the degree of intermittency is dependent on the in-flow turbulence level. Although all the cases tested were determined to be globally stable, variations in the flow velocity change the structure of the flame and flow, which alters the receptivity of the flow to turbulent perturbations. As a result, the strength and regularity of vortex shedding increase with increasing turbulence level and increased receptivity, indicating that the system is a noise-forced globally-stable oscillator.