Impact of PVC Dynamics on Shear Layer Response in a Swirling Jet

Abstract

Combustion instability, or the coupling between flame heat release rate oscillations and combustor acoustics, is a significant issue in the operation of gas turbine combustors. This coupling is often driven by oscillations in the flow field. Shear layer roll-up, in particular, has been shown to drive longitudinal combustion instability in a number of systems, including both laboratory and industrial combustors. One method for suppressing combustion instability would be to suppress the receptivity of the shear layer to acoustic oscillations, severing the coupling mechanism between the acoustics and the flame. Previous work suggested that the existence of a precessing vortex core (PVC) may suppress the receptivity of the shear layer, and the goal of this study is to first, confirm that this suppression is occurring, and second, understand the mechanism by which the PVC suppresses the shear layer receptivity. In this paper, we couple experiment with linear stability analysis to determine whether a PVC can suppress shear layer receptivity to longitudinal acoustic modes in a non-reacting swirling flow at a range of swirl numbers. The shear layer response to the longitudinal acoustic forcing manifests as an m = 0 mode since the acoustic field is axisymmetric. The PVC has been shown both in experiment and linear stability analysis to have m = 1 and m = −1 modal content. By comparing the relative magnitude of the m = 0 and m = −1,1 modes, we quantify the impact that the PVC has on the shear layer response. The mechanism for shear layer response is determined using companion forced response analysis, where the shear layer disturbance growth rates mirror the experimental results. Differences in shear layer thickness and azimuthal velocity profiles drive the suppression of the shear layer receptivity to acoustic forcing.

Excitation of the precessing vortex core by active flow control to suppress thermoacoustic instabilities in swirl flames

“…. Frederick et al 19 showed in their analysis that the PVC leads to a thickening of the shear layers and, hence, reduces the growth rate of the Kelvin-Helmholtz instability, which is also known as a key driver for thermoacoustic instability. 52 .…”

“…The present experiment reveals a significant suppression of the thermoacoustic instability through the actuation of the PVC. An explanation for the suppression of the thermoacoustic instability by the PVC is given by Frederick et al, 19 as described above. However, our experiments conducted at perfectly premixed conditions for different actuation frequencies and amplitudes (compare Figures 10 and 11) do not support this explanation.…”

“…[15][16][17] The interaction of the PVC with thermoacoustic instability is an open question and subject to several studies. Previous studies in isothermal swirling jets suggest that the PVC may modify the mean flow as such that axisymmetric shear-layer instabilities are damped, 18,19 which ultimately leads to a reduction of thermoacoustic instability. 20 Moreover, it was shown by Sto¨hr et al 12 that a thermoacoustic instability may suppress the PVC in a premixed swirl-stabilized flame.…”