Transformation of transverse acoustic velocity of the burner approach flow into flame dynamics

Modern, large gas turbines for power generation have multiple burners, which are distributed around the circumference of the engine and which generate flames in combustors of either annular or can-annular geometry. In both cases considering only the axial modes has proven to be insufficient for the assessment of the thermoacoustic stability. An adequate analysis requires consideration of the circumferential acoustic coupling, generated by the acoustic field in the upstream and downstream annuli and the open passages between the cans, respectively. As in annular combustors the particularly critical eigenmodes with low frequencies are predominantly of circumferential nature, the stability of annular combustors is often governed by the onset of circumferential acoustic oscillations. In this study one single radial swirl burner is exposed to a transverse velocity fluctuation comparable to a circumferential oscillation in the plenum annulus. The transverse velocity fluctuation is transformed into a rotational flow oscillation through a convective process depending on excitation frequency and mass flow rate. The characteristics of this process are determined and the resulting dynamic flow structure in the burner nozzle is analyzed. Phase plots show that the rotational flow oscillation is transported into the flame causing a rotational flame pulsation. The influence of transverse velocity fluctuation on the global dynamic flame behavior is determined through FTF measurements. It is concluded from the increased FTF amplitude observed for transverse velocity excitation that the modification of the acoustic field at the burner exit due to circumferential acoustic modes has to be taken into account for a reliable prediction of the stability limits of annular gas turbines.