The effects of the laminar burning velocity (S_L), on the transfer functions of propane-air and methane-air swirl flames is experimentally investigated. Five equivalence ratios for each fuel are selected, to yield different values of S_L. The flame transfer function (FTF), is obtained by comparing the velocity fluctuations of the incoming flow, measured with a hot wire, to the heat release rate oscillations, collecting OH* chemiluminescence with a photomultiplier tube. Phase-locked images of OH* chemiluminescence are also acquired to analyze the flame dynamics during the forcing cycle. The unforced velocity fields are measured by particle image velocimetry to assess the effects of S_L on the flow fields. Changing the laminar burning velocity affects mainly the gain around 176 Hz and 336 Hz. This paper focuses on 336 Hz. The flame vortex roll-up is recognized as a key parameter controlling the gain of the FTF around 336 Hz. The analysis highlights that Si influences the gain response around 336 Hz in two competing ways: first, it enhances the flame vortex roll-up and second, it affects the stabilization distance of the flame, which influences the vortex generated by acoustic forcing.
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