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 NOx from fuel-bound nitrogen. Previous experimental and theoretical work has demonstrated that partially-dissociated ammonia (NH3/H-2/N-2 mixtures) can match many of the laminar flame properties of methane flames. Among the remaining concerns pertaining to the use of NH3/H-2/N-2 blends in gas turbines is their thermoacoustic behavior. This paper presents the first measurements of flame transfer functions (FTFs) for turbulent, premixed, NH3/H-2/N-2-air flames and compares them to CH4-air flames that have a similar unstretched laminar flame speed and adiabatic flame temperature. FTFs for NH3/H-2/N-2 blends were found to have a lower gain than CH4 FTFs at low frequencies. However, the cut-off frequency was found to be greater, due to a shorter flame length. The results suggest that NH3/H-2/N-2 blends may excite different thermoacoustic modes in gas turbines. In addition, the dependence of the flame response on forcing amplitude was measured for a forcing frequency of 650 Hz and the linearity of the NH3/H-2/N-2 flame response up to high forcing amplitudes suggests that particularly high-amplitude limit cycles may occur. For both CH4 flames and NH3/H-2/N-2 flames the confinement diameter was found to have a strong influence on peak gain values. The effect on the FTF phase was modest, except in the case of extreme confinement, where almost all of the flame is close to the wall, and in the case of a significant change in the flame stabilisation. 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 cut-off frequency and phase curves.