Flame Response to Transverse Acoustic Forcing With Minimal Axial Coupling

Instabilities associated with transverse acoustic modes are an important problem in gas turbines. A number of studies have reported results on the response of flames to transverse excitation, in order to understand the acoustic-velocity-heat release mechanism associated with combustion instabilities. However, all forced and self-excited transverse studies to date have strong coupling between the transverse and axial acoustic fields near the flame. This is significant, as studies suggest that the actual transverse disturbances play a negligible direct role in generating spatially integrated oscillatory heat release. Rather, they suggest that it is the induced axial disturbances that control the bulk of the heat release response. As such, there is a need to control the relative amplitudes of the axial and transverse disturbances exciting the flame, and determine their relative roles in the overall heat release response. This paper presents experimental results to address this issue. The flow field and flame edge were measured using 5kHz simultaneous sPIV and OH-PLIF, and the relative heat release fluctuations were measured through OH* chemiluminescence. The flame was forced with both strong transverse and axial oscillations, with various degrees of coupling between them, showing quite consistently that it is the axial flow disturbances that excite heat release oscillations. These observations demonstrate that the key role of the transverse motions is to set the “clock” for the frequency of the oscillations, but have negligible effect on the actual heat release disturbances exciting the instability. Rather, it is the axial disturbances, induced by inherent multi-dimensional effects that lead to the actual heat release oscillations.