Flame Transfer Functions for Turbulent, Premixed, Ammonia-Hydrogen-Nitrogen-Air Flames

Ammonia is a promising hydrogen and energy carrier but also a challenging fuel to use in gas turbines, due to its low flame speed, limited flammability range, and the production of NO_x from fuel-bound nitrogen. Previous experimental and theoretical work have demonstrated that partially dissociated ammonia (NH₃/H₂/N₂ mixtures) can match many of the laminar flame properties of methane flames. Among the remaining concerns pertaining to the use of NH₃/H₂/N₂ blends in gas turbines is their thermoacoustic behavior. This paper presents the first measurements of flame transfer functions (FTFs) for turbulent, premixed, and NH₃/H₂/N₂-air flames and compares them to CH₄-air flames that have a similar unstretched laminar flame speed and adiabatic flame temperature. FTFs for NH₃/H₂/N₂ blends were found to have a lower gain than CH₄ FTFs at low frequencies. However, the cutoff frequency was found to be greater, due to a shorter flame length. For both CH₄ flames and NH₃/H₂/N₂ flames, the confinement diameter was found to have a strong influence on peak gain values. Chemiluminescence resolved along the longitudinal direction shows a suppression of fluctuations when the flame first interacts with the wall followed by a subsequent recovery, but with a significant phase shift. Nevertheless, simple Strouhal number scalings based on the flame length and reactant bulk velocity at the dump plane result in a reasonable collapse of the FTF cutoff frequency and phase curves.