60058 - The Origin of Gain and Phase Modulations in Ch4/h2 and H2 Flame Transfer Functions 

We investigate the origin of modulations in the gain and phase of flame transfer functions (FTF) measured in perfectly premixed, bluff-body stabilised CH₄/H₂ and pure H₂ flames. Modulations in the gain and phase have only previously been observed in swirling flames but we show that this phenomenon can also occur in non-swirling flames. We show that modulations are caused by flow disturbances originating from the upstream geometry, in particular the screws used to center the bluff body, indicative of a more generalised phenomenon of convective wave propagation. Velocity measurements at various locations around the burner exit were performed and it was found that the strength of these modulations depends on the angular position. When placed directly above the a grub screw the velocity spectra showed a peak which scaled linearly with bulk velocity and exhibited twice the level of rms fluctuations. Whereas peaks in the velocity spectra vanished in between the grub screws. To further quantify the role of the convective disturbances produced by the grub screws, hydrodynamic transfer functions of the forced cold flow were measured and similar modulations in both magnitude and phase as the FTFs were found. Flame imaging and PIV measurements showed that both the mean velocity and flame shape were approximately axisymmetric, however, the u_rms-field clearly highlighted the presence of a strong asymmetry due to convective disturbances produced by the grub screws.