This paper describes work to understand the direct response of flames to transverse acoustic velocity disturbances. Prior work by Sattelmayer's group and our group have shown that certain transverse acoustic modes are intrinsically unstable- i.e., they are unstable for all flame configuration parameters. This behavior is quite different from axial instabilities, which show instabilities in "islands" for different flame parameters, some being stable and others unstable. This prior work was performed for axisymmetric flames that were located in the center of the round duct. A key effect of these assumptions is that purely azimuthal velocity disturbances, which dominate the kinetic energy of transverse acoustic modes, do not introduce any flame wrinkling. If the axisymmetric flames are not centered in the duct, then both the radial and azimuthal component of the modal disturbance influences the flame. This effect was explored by the authors in earlier work, showing that such flame offsets have important impacts on stability. In this paper, we evaluate a second mechanism through which the azimuthal motions can influence the flame - mean flame non-axisymmetry effects. Similar to offsets, we show that mean flame asymmetries had very important effects on thermoacoustic stability. For axial, purely-radial modes which are intrinsically unstable when the axisymmetric flame is centered in the duct, can be destabilized.
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